Petrology PDF - GEOL 102

GEOL 102 PETROLOGY PETROLOGY 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  PETROLOGY Petros (rock) + logos (study) Study of rocks Deals with the origin, history, occurrence, structure, chemical compos...

GEOL 102 PETROLOGY PETROLOGY 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  PETROLOGY Petros (rock) + logos (study) Study of rocks Deals with the origin, history, occurrence, structure, chemical composition, and classification of rocks. Rocks are classified based on Composition and Texture. Igneous Petrology Sedimentary Petrology Metamorphic Petrology  PETROLOGY COMPOSITION Depends on chemistry of parent material e.g. melt, protolith Percentage of SiO2 (for igneous), Rock Type (for sedimentary), and Lithology of protolith (for metamorphic). TEXTURE Depends on Degree of Crystallinity (for igneous), Size of grain (for sedimentary), and both (for metamorphic).  Igneous Rocks Formed from the crystallization of molten material or melt. Composition depends on Magma chemistry and Temperature. Texture depends on Degree of Crystallinity (due to Rate of Cooling). Classified based on: Silica content (% SiO2) Color (% of dark-colored minerals) Texture (degree of crystallinity) % SiO2 Rock Color  Crystalline Texture Phaneritic Aphanitic Porphyritic  Crystalline Texture Glassy Frothy  Sedimentary Rocks Formed from the deposition, lithification, and diagenesis of sediment grains. Sediments (from any parent rock) are transported via water, wind, or ice and are deposited. Continuous deposition of sediments over earlier layers results to compaction and cementation, forming sedimentary rocks. Strata (beds) are the most diagnostic property Classified into: (1) Clastic, (2) Biochemical, and (3) Chemical. Sedimentary Rocks  Clastic Sedimentary Rocks Composed of fragments (clasts) of pre-existing minerals and rocks, and classified based on Grain Sizes. Conglomerate Breccia  Clastic Sedimentary Rocks Composed of fragments (clasts) of pre-existing minerals and rocks, and classified based on Grain Sizes. Sandstone  Clastic Sedimentary Rocks Composed of fragments (clasts) of pre-existing minerals and rocks, and classified based on Grain Sizes. Siltstone Claystone Mudstone  Biochemical Sedimentary Rocks Composed of fragments (allochems) embedded in a matrix, and classified based on the percent abundance of both. Shelly Limestone Bafflestone Coquina  Chemical Sedimentary Rocks Composed of chemically-precipitated minerals from a mineralized fluid that had undergone evaporation, leaving a residue which later transformed into sedimentary rocks. Evaporite Phosphate rock Coal  Metamorphic Rocks Formed from the recrystallization of pre-existing rocks and minerals due to higher temperature and pressure, forming a new mineral assemblage of that specific P-T range. Rocks are subjected to higher P-T, and mineral components change (either compositionally or texturally). Classified based on Texture: Foliated or Non-foliated Classified based on Composition (dominant minerals) Excellent markers for Temperature and Pressure.  Foliated Metamorphic Rocks Metamorphic rocks characterized with repetitive layering due to either different composition or alignment of phyllosilicate minerals.  Non-Foliated Metamorphic Rocks Metamorphic rocks that have no foliation or sheet-like structures. Marble Eclogite  Why study rocks? Provide industrial and economic mineral resources. Interpret the geologic history of an area. Study the past and origin of a region. Questions?  GEOL 102 PETROLOGY PHASE DIAGRAMS 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  SOLID SOLUTION Minerals with specific atomic sites occupied by two or more different elements in variable proportions.  SOLID SOLUTION TYPES Substitution Interstitial Omission Solid Solution Solid Solution Solid Solution  SUBSTITUTION (IONIC) SOLID SUBSTITUTION Similar Ionic Radius Similar Ionic Charge Temperature  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 1. Simple Ionic Substitution Ions of similar radius and charge substitute for one another in a coordination site in any proportions. Composition of end member is expressed in terms of: Specific formula Mineral name Position on the tie line Proportion of either end member (A% or B%)  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 1. Simple Ionic Substitution Tie Lines are lines that connect mineral phases that coexist with each other at a specific condition. 1,500 oC z 1,000 oC y x 500 oC A B 25% 50% 75% 100%  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 1. Simple Ionic Substitution Olivine (Mg,Fe)2SiO4 = Mg2SiO4 & Fe2SiO4 Fayalite Forsterite Fo0 Fo100 Fe2SiO4 Mg2SiO4 X y z Fe Mg 0 10 20 30 40 50 60 70 80 90 100  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 1. Simple Ionic Substitution (Fe,Mn,Mg)CO3 Siderite (Fe) Rhodochrosite (Mn) Magnesite (Mg)  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 2. Coupled Ionic Substitution Simultaneous substitution of ions of different charges in two different structural sites and preserves the electrical neutrality of the crystal lattice. For ion (B) to replace another ion (A) while maintaining neutral charge, an equal amount of replaced ion (A) is concurrently substituted by another ion (C). 2 A2+ → B3+ + C+  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 2. Coupled Ionic Substitution Plagioclase (Na)(Si)(Al)(Si2O8) (Ca)(Al)(Al)(Si2O8) Albite Anorthite  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 2. Coupled Ionic Substitution (Na)(Si)(Al)(Si2O8) (Ca)(Al)(Al)(Si2O8) Na+ + Si4+ Ca2+ + Al3+  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 2. Coupled Ionic Substitution An0 An100 (Na)(Si) (Ca)(Al) (Al)(Si2O8) (Al)(Si2O8) 0 10 20 30 40 50 60 70 80 90 100 An45 (Na0.55 Ca0.45)(Si0.55 Al0.45) (Al)(Si2O8)  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 3. Limited Ionic Substitution Significant differences in ionic radii or charge of ions limits the substitution, resulting to a Limited Solid Solution. Miscibility Gaps are gaps in solid solution, having immiscible liquids that do not mix with certain proportions.  SUBSTITUTION (IONIC) SOLID SUBSTITUTION 3. Limited Ionic Substitution Miscibility Gap (unknown composition) Low-Mg Calcite High-Mg Dolomite Calcite Calcite Magnesite 4 25 40 Ca Mg 0 10 20 30 40 50 60 70 80 90 100 Ms0 Ms100 %Ms  PHASE Mechanically separable varieties of matter in a system that can be distinguished from other varieties based on their composition, structure, and/or state.  PHASE DIAGRAM Diagrams that display the stability fields for various phases separated by lines representing conditions under which phase changes occur.  TERMS Liquidus Phase boundary separating all-liquid stability field from stability fields with some solids.  TERMS Solidus Phase boundary separating all-solid stability field from stability fields with some liquids.  TERMS Eutectic Condition in which the liquid is in equilibrium with two different solids.  TERMS Peritectic Reaction where a solid and liquid phase together form a new solid phase at a specific temperature and composition.  TERMS Invariant Melting Occurs when melts of same composition are produced by melting rocks of different initial composition.  TERMS Incongruent Melting Occurs when solid mineral phase melts to produce a melt and a different mineral with a different composition from the initial material.  TERMS Discontinuous Reaction Mineral crystals and melt reacts to form a completely different mineral; negligible solid solution exists between the minerals.  TERMS Continuous Reaction Mineral crystals and melt reacts to continuously and incrementally change the composition of both; requires a mineral solid solution series  TERMS Solvus Line separating a homogenous solid solution from a field of several phases formed by exsolution or incongruent melting. It determines the solid solubility limit which changes with temperature. PHASE DIAGRAMS % Composition  GIBB’S PHASE RULE Governs the number of phases that can coexist in equilibrium in any system.  GIBB’S PHASE RULE P=C+2-F P = number of phases in a system C = minimum number of chemical components that define the phases in the system F = number of degrees of freedom or variance.  GIBB’S PHASE RULE Invariant Equilibria = neither P or T can be changed Univariant Equilibria = either P or T is independently changed, which changes the other variable. Divariant Equilibria = both P & T are free to change independently without changed the state of the system PHASE DIAGRAMS Examples  UNARY PHASE DIAGRAMS Silica (SiO2) H2O Carbon CaCO3  BINARY PHASE DIAGRAMS  LEVER RULE Calculates the proportions of melt (liquid) and crystals (solid) in a system. For wt.% solid: For wt.% liquid: 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑙𝑖𝑛𝑒 − 𝑤𝑡. %𝑏𝑎𝑠𝑒 𝑤𝑡. % 𝑏𝑎𝑠𝑒 − 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑙𝑖𝑛𝑒 𝑡𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑙𝑖𝑛𝑒 𝑡𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑙𝑖𝑛𝑒  1,450oC 1,300oC 1,235oC 29 70 82 98 18 70 Plagioclase Olivine Diopside - Anorthite Diopside - Anorthite Diopside - Anorthite Diopside - Anorthite Diopside - Anorthite Albite - Orthoclase Albite - Orthoclase Nepheline - Silica Nepheline - Silica Forsterite - Silica Questions?  GEOL 102 PETROLOGY Common Rock-forming Minerals 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Rock-forming Minerals Group of minerals that make up most of different rock types in the crust. Elements O Si Al Fe Ca Na K Mg x 46.6 27.7 8.1 5 3.6 2.8 2.6 2.1 1.5 Rock-forming Minerals vs Ore-forming Minerals Questions?  GEOL 102 PETROLOGY Magma 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Magma Molten material generated by partial melting of the mantle and crust, which forms igneous rocks.  Magma Mostly silicate in composition. Components have varying proportions depending on Temperature, Pressure, and Magma Chemistry How does a Magma form? Anatexis Partial Melting of the source rock. Liquid melt enriched in low-T components Residual rock enriched in high-T refractory components. Factors: Composition, Texture and Depth of source rock Percent partial melting of source rock Previous melting history of source rock Diversification processes that changes the composition of magma after migrating from source region  Equilibrium Melting Chemicals are neither added nor removed in a closed system. The overall composition of the system remains the same while the composition of melt and solid evolves. Equilibrium Melting 1. Melt becomes enriched in low melting T components. 2. Increased degrees of partial melting dilute the enriched low-T components in the melt; melt becomes progressively less enriched in low-T minerals as higher-T minerals enter the melt. 3. Solid refractory residue is enriched in high melting T components. 4. Solid residue becomes progressively enriched in refractory minerals. Equilibrium Melting Incomplete Equilibrium Melting Large crystal sizes High magma viscosity Low ion migration rates  Fractional Melting Solids and melt separate into isolated fractions that does not continue to react together during the melting process, producing a more evolved melt than the parent source rock. Factors of Anatexis Increasing Temperature Decreasing Pressure Addition of Volatiles Increasing Temperature Temperature increases with depth, represented by the Geothermal Gradient. Geothermal gradient is not uniform vertically, laterally, rock age and tectonic setting. Higher geothermal gradient sites are areas where mantle peridotite may melt due to increased T. Hotspots, magmatic intrusions Decreasing Pressure / Adiabatic Melting Pressure is related to depth (10km = 3.3 kbar) Higher T, higher P (assuming all factors are constant) Lithostatic stress decreases as mantle rises upward in thinning plate, reducing rock melting temperature; lower T can cause melting due to decompression. Primary cause of basaltic magma generation at ocean spreading ridges and continental rifts. Volatile-induced Melting High volatile content under pressure lowers melting T; Flux Increased dissolved water content in magma: Weaken Si-O bonds in silicates Lower rock melting temperature Allow melting over wider T range Decrease amount of FeO & MgO incorporated into partial melt = produce less-mafic magma than source rock Cause of partial melting in Subduction zones.  Melt Composition in Partial Melting 1. Anatectic Melts and Residual Rocks Partial Melting removes incompatible elements (LIL, SiO2, K2O, Na2O) and concentrates to the new magma. Refractory residual rock is rich in high-T compatible constituents i.e., MgO, CaO, High Field Strength elements Both anatectic melts and residual rock remain genetically and chemically related to the original source rock.  Melt Composition in Partial Melting 2. Incompatible element-rich Melts Increasing degree of partial melting results to decreasing enrichment of incompatible elements. Incompatible elements such as Light Rare Earth Elements (La to Sm) and Large Ion Lithophiles (K to Cs) are preferentially incorporated into melts Nature and history of the source region can be inferred based on pattern of REE and LIL in resulting melts and rocks.  Products of Partial Melting 1. Basic rocks (basalt, gabbro) from Ultrabasic peridotite 2. Intermediate rocks (andesite, diorite) from Basic rocks. 3. Silicic rocks (rhyolite, granite) from Intermediate rock. Questions?  GEOL 102 PETROLOGY Magma Diversification Processes 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Magma Diversification Processes that changes the bulk chemistry of the magma after its initial generation from parent rock.  CLOSED – SYSTEM OPEN – SYSTEM Original melt evolves into other Melt interacts with the surrounding melts with different composition country rock or other magmas in the without exchanging materials system, changing the chemical from external sources. composition of resulting melt. Closed-System Diversification No external interaction No exchanging chemical components from outside sources Norman Levi Bowen Igneous Petrologist, Carnegie Institute of Washington (1910) Studied the Palisades Sill in New Jersey, NY Changing mineralogy along the sill’s vertical profile; crystal-liquid “fractionation” process Recognized 8 major igneous mineral groups Bowen’s Reaction Series Zoned Crystals Systematic pattern of chemical variation, recording an incomplete continuous reaction between crystal and melt Reaction Rims Peripheral mineral surrounding a partially-resorbed crystal core of a different mineral, indicating incomplete discontinuous reaction between crystal and melt. Zoned Plagioclase Olivine rimmed by Pyroxene  Fractional Crystallization Separation of crystals and melt from an originally homogenous magma. Liquid – Crystal Fractionation Crystals and Melt are segregated, changing the composition of parent and daughter magma Roof Accretion Marginal Accretion Sidewall Accretion Segregation of crystals from melt along the chamber’s peripheral margins. Floor Accretion Crystal Settling Crystals settle at the base of the chamber due to higher density than melt, resulting to a discrete layer forming banding or layering of cumulate crystals Crystal Flotation Low-density crystals float towards the chamber roof, separating them from the melt. Convective Flow Segregation Liquids and crystals are segregated due to velocity, density, temperature variations. Larger/heavier crystals accumulate in higher velocity regions, or migrate to chamber base by gravitational flow. Filter Pressing Deformation of the chamber while cooling results to the expulsion of melt out from the system, segregating it from the crystals in the chamber. Plagioclase Mafics Rhythmic Bedding in Skaergaard Intrusion Cross-bedding in Skaergaard Intrusion  Stoke’s Law Predicts the settling velocity of high- density crystals in the melt.  Stoke’s Law 2 2𝑔𝑟 ρ𝑠 − ρ𝑙 𝑉= 9η V = settling velocity (cm/sec) g = gravitational acceleration (980 cm/sec2) r = radius of spherical particle (cm) ps = density of solid spherical particle (g/cm3) pl = density of liquid (g/cm3) n = viscosity of liquid (1 c/cm sec = 1 poise)  Stoke’s Law Olivine has a density of 3.3 g/cm3. Basaltic melt has a density of 2.65 g/cm3 and viscosity of 1000 poise. How long and deep would olivine crystals of 0.1 cm radius settle? 2 2𝑔𝑟 ρ𝑠 − ρ𝑙 𝑉= 9η  Liquid Fractionation Parent magma fractionates to produce two or more distinctly different daughter magma with different composition. Differential Diffusion Diffusion of select ions in the magma in response thermal, compositional, density gradients or water content. After Hidreth (1979) Liquid Immiscibility Magma separates into two or more distinct immiscible melts.  Open-System Diversification Chemical reactions due to interaction of magma with surrounding country rock or with other magma in the system. Assimilation Wall rock is intruded and reacts chemically with the magma. Stoping Fracturing of wall rocks due to forceful injection of magma, producing Xenoliths (blocks that is incorporated in the magma but doesn’t completely melt) Magma Replenishment Magma diversification through multiple magma injections over time. Magma Mingling Two or more dissimilar magmas coexist and in contact with each other, but retains the distinctive induvial magma characteristics. Magmas are interjected but do not combine thoroughly possibly due to difference in temperature, density, viscosity, or insufficient convection. Magma Mixing Two or more magmas are thoroughly mixed so that individual characteristics are not recognizable. Questions?  GEOL 102 PETROLOGY Chemical Components of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Igneous Rocks Molten material from which igneous rocks form are rich in Silica (SiO2) Chemical Composition of Igneous Rocks  Igneous Rocks Major Elements Elements with concentration of > 1 wt.% in Earth’s crust. 7 ions bonded with Oxygen = SiO2, Al2O3, FeO, CaO, Na2O, K2O, MgO Minor Elements Elements with concentration of 1.0 wt.% - 0.1 wt.% in crust Cr, Mn, P, H, Ti; Concentration is related to SiO2 concentration Trace Elements Elements with concentration of < 0.1 wt.%; in ppm (1000ppm) Provide information on genesis and history of igneous rocks. REEs, HFSEs, LILs  Compatibility Measure of the ease by which an element fits into a crystal structure. Compatible Elements Incompatible Elements Fit easily into crystal structures and Does not easily fit into crystal structure form long-lasting bonds. and forms easily-breakable bonds. Immobile; do not readily migrate Mobile; readily migrate into melt Tend to remain in Residual Rock Tend to migrate into Anatectic Melts (Restite) Fe, Mn, Zn, Ti, V, Cr, Co, Ni, Cu; HFS REE, LIL Anders & Grevesse (1989)  Trace Elements Rare Earth Elements (REEs) Elements with atomic numbers from 57 (La) to 71 (Lu). Odd-number REE are more abundant, so crustal REE / chondrite REE = Chondrite-normalized Pattern Light REE La, Cs, Pr, Nd, Sm; more incompatible than HREE Heavy REE Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; less incompatible than LREE  Trace Elements High Field Strength (HFS) elements Have relatively high ionic charge (+3, +4) for a given radius. Ionic radius:Valence charge ratio is < 0.2; immobile elements Ti, Ni, Cr, V, Zr, Hf, Nb, Ta, Y; useful for tracing mantle processes Large Ion Lithophile (LIL) elements Elements with radius:valence ratio of > 0.2 for a given radius. Mobile elements; Cs, Ba, Rb, Sr, U, Pb, K, Zr, Th, Ta Determine role of hydrous fluid interaction and parent melt. Mineral Composition of Igneous Rocks Primary Minerals Minerals that crystallize directly from magma at elevated T. Secondary Minerals Minerals that form later due to chemical reactions between melt and country rock. These replace primary minerals or fill in voids through alterations.  Major Minerals Constituents that occur in > 5% abundance. Quartz Quartz, Tridymite, Cristobalite K-feldspar Microcline, Orthoclase, Sanidine Plagioclase Anorthite to Albite Feldspathoid Leucite, Nepheline, Sodalite Mica Muscovite, Biotite, Phlogopite, Lepidolite Amphibole Hornblende, Riebeckite, Richterite Pyroxene Augite, Diopside, Pigeonite, Aegerine, Hypersthene, Enstatite, Bronzite Olivine Forsterite to Fayalite  Accessory Minerals Constituents that occur in < 5% abundance. Magnetite Corundum Pyrope Hematite Apatite Melilite Ilmenite Cassiterite Monazite Spinel Pyrite Epidote Sphene Chalcopyrite Allanite Rutile Molybdenite Tourmaline Fluorite Pentlandite Topaz Chromite Pyrrhotite Columbite Zircon Uvarovite Uraninite Terminologies on Igneous Chemical Composition  Color Index / Mafic Index Proportion of mafic minerals in the total abundance of felsic and mafic minerals in an igneous rock. Does not include dark-colored non-crystalline solids  Color Index / Mafic Index %𝑚𝑎𝑓𝑖𝑐 𝐶𝐼 = x 100 %𝑚𝑎𝑓𝑖𝑐 + %𝑓𝑒𝑙𝑠𝑖𝑐 Color Index / Mafic Index Color Index / Mafic Index  Modal Classification / Mode Actual identification of mineral and its percentage by volume by visual inspection Hand lens for coarse-grain Petrographic microscope for fine-grain  Modal Classification / Mode Quartz Orthoclase (Alkali Feldspar) Biotite Plagioclase Modal Minerals Quartz Plagioclase Alkali Feldspar Mafic Minerals Feldspathoid  Point Count Analysis More accurate modal classification using petrographic microscope. A thin section is moved incrementally on a grid system and tabulates atleast 400 mineral points. Point Count Analysis Point Count Analysis Normative Mineralogy / CIPW Norm Indirect identification scheme using data derived from chemical analysis of the rock sample. Cross, Iddings, Pirsson, & Washington (1902)  Normative Mineralogy / CIPW Norm 1. Chosen hypothetical mineral set closely resembles the actual minerals. 2. Magma crystallized at low-pressure, anhydrous condition near the surface, forming volcanic rocks.  Normative Minerals Quartz Magnetite Olivine Orthoclase Ilmenite Enstatite Albite Apatite Hypersthene Anorthite Corundum Nepheline Diopside Descriptive Terms based on Chemical Composition Abundance of Silica Based on SiO2 content  Silica Saturation Based on quartz, feldspars, and feldspathoids  Silica Saturation Presence/absence of quartz, feldspars, and feldspathoids All available oxides were used, and additional SiO2 Oversaturated remains to form normative quartz (free quartz) All available oxides and SiO2 were used. Saturated Indicated by no quartz or with forsterite/feldspathoid SiO2 is depleted before all oxides are used; insufficient Undersaturated SiO2 to form quartz, feldspars, or orthopyroxenes. Indicated by presence of forsterite or feldspathoids.  Abundance of Aluminum oxide Based on Al2O3 content compared to CaO, Na2O, K2O Minerals with unusually high Al2O3 Peraluminous Muscovite, Corundum, Topaz, Kyanite, Sillimanite, Andalusite Normative or modal minerals with high K2O and/or Na2O Peralkaline Na-rich minerals e.g., Aegerine, Riebeckite, Arvedsonite, Aenigmatite Mafic minerals with average Al2O3 content Metaluminous Lack of Al2O3-rich mineral, Na-pyroxene, Na-amphibole Subaluminous Mafic minerals with low Al2O3 content  Abundance of Aluminum oxide Based on Al2O3 content compared to CaO, Na2O, K2O Questions?  GEOL 102 PETROLOGY Textures of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Igneous Texture Degree of crystallinity of mineral groups comprising igneous rocks, depending on cooling rate, temperature, and pressure.  Degree of Crystallinity Holocrystalline Hypocrystalline Holohyaline All components are All components are All components are partially crystalline, crystalline. glassy. partially glassy.  Crystal Faces Euhedral Subhedral Anhedral Minerals with complete crystal Minerals with partially faces and are not impinged complete crystal faces by Minerals lack any upon by other crystals. which at least one crystal face observable crystal Developed in early phases of is impinged upon by adjacent faces. crystallization. material.  Factors in Crystallization How does crystals form from a liquid phase?  Factors in Crystallization 1. Crystal Nucleation Formation of new crystals large enough to persist and grow into larger crystals. Crystal Nucleation Rate Number of new seed crystals that develop per volume per unit time (nuclei/cm3/s).  Factors in Crystallization 1. Crystal Nucleation Crystal Nucleation Rate Rate of Availability Ease of Ion Undercooling of Ions Migration  Factors in Crystallization 1. Crystal Nucleation Undercooling Occurs when liquids are cooled to sub-liquidus temperatures  Factors in Crystallization 1. Crystal Nucleation Nucleation Rate is low near Liquidus Temperature, and increases at Sub-liquidus (Undercooling) Nucleation Rate is ZERO at very large degrees of Undercooling due to increased magma viscosity, which implies near-Solidus condition.  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) Rate of Availability Magma Undercooling of Ions Viscosity  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) At Liquidus temperature, small number of large crystals forms, ultimately producing phaneritic, porphyritic or pegmatitic texture of euhedral to subhedral crystals.  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) At Sub-Liquidus temperature, a large number of small crystals are formed, resulting to Aphanitic texture.  Factors in Crystallization 2. Crystal Growth Rate Measure of increase in crystal radius over time (cm/s) If temperature decrease is very rapid, nucleation growth rate is prevented, forming non-crystalline texture.  Factors in Crystallization 3. Ion Availability Availability of ions that can fill specific ionic sites in the crystal lattice structure. Readily-available ions enhances crystal growth  Factors in Crystallization 4. Viscosity Resistance of fluid to shear stress; depends on SiO2 Viscosity affects ion diffusion from magma to sites. Low viscosity = high diffusion rate = increased crystal growth rate  Factors in Crystallization 5. Network Formers vs. Network Modifiers Network Formers increase molecular linkages, increasing viscosity. Si, O, Al. Network Modifiers decrease molecular linkages, reducing viscosity. Fe, Mg  Factors in Crystallization 6. Heat Temperature is inversely proportional to viscosity and molecular bonding. Increase Temperature = decrease bonding = reducing viscosity  Factors in Crystallization 7. Gases H2O, CO2, SO2; minor gases include N, H, S, F, Ar, Co, Cl Pressurized dissolved gases are network modifiers. High PH2O decreases viscosity of silicic magmas, increasing diffusion rate and crystal growth rate, forming small numbers of large crystals.  Factors in Crystallization 2. Nucleation Rate Undercooling Occurs when liquids are cooled to sub-liquidus temperatures Crystalline Igneous Texture  Crystalline Texture Pegmatitic Large crystals > 30mm diameter Early-formed, euhedral crystals surrounded by later-formed subhedral crystals.  Crystalline Texture Phaneritic Crystals 30mm – 1mm diameter Early-formed, euhedral crystals surrounded by later-formed subhedral to anhedral crystals  Crystalline Texture Phaneritic Finely phaneritic is 1mm – 3mm Medium phaneritic is 3mm – 10mm Coarsely phaneritic is 10mm – 30mm  Crystalline Texture Aphanitic Small crystals < 1mm diameter Crystals not generally discernible to eye  Crystalline Texture Aphanitic Microcrystalline has microlite crystals that can be identified with a petrographic microscope. Cryptocrystalline has finer-sized crystals that cannot be identified even with petrographic microscope.  Crystalline Texture Porphyritic Two distinctly different-sized crystals Large crystals are Phenocrysts Small crystals are Groundmass  Crystalline Texture Porphyritic Porphyritic-phaneritic has phenocrysts embedded in phaneritic groundmass. Porphyritic-aphanitic has phenocrysts embedded in aphanitic groundmass.  Crystalline Texture Pegmatitic Aphanitic  Crystalline Texture Porphyritic-phaneritic Porphyritic-aphanitic Non-crystalline Igneous Texture  Non-crystalline Texture Glassy Glass-like texture resulting from near-instant solidification of melt, preventing the formation of mineral crystal structure. Has microlites and cryptocrystalline minerals.  Non-crystalline Texture Glassy Quenching occur when melt is in contact with liquid water or air. Rapid Loss of Dissolved Gas is limited to silicic melts; why obsidian is more common than basaltic glass  Non-crystalline Texture Glassy Vitrophyric Texture has recognizable phenocryst in glassy groundmass. Spherulitic Texture has crystals growing outward from existing crystal nuclei  Non-crystalline Texture Glassy Devitrification occurs when glass crystallizes on pre-existing microlites or cryptocrystalline nuclei. Perlitic Texture has cloudy appearance and curved cooling cracks.  Non-crystalline Texture Vesicular Has spherical or ellipsoidal void spaces due to exsolution and entrapment of gas bubbles in lava. Vesicular rock name if > 30% vesicles by volume Vesicular + rock if 5% - 30% vesicles Vesicular-bearing + rock if < 5% vesicles  Non-crystalline Texture Pyroclastic Fragmented particles of various sizes from volcanic eruptions. Classified based on Composition, Size, & Shape  Non-crystalline Texture Pyroclastic Based on Composition: Lithic Pyroclasts are rock fragments Vitric Pyroclasts are glassy fragments Crystals Pyroclasts are minerals  Non-crystalline Texture Pyroclastic Based on Shape and Sizes: Blocks/Bombs if > 64mm pyroclasts Lapilli if 2mm – 64 mm pyroclasts Ash if < 2mm pyroclasts Dust if < 0.0625 mm pyroclasts; Fine Ash  Non-crystalline Texture Pyroclastic Welding Fragments become progressively fused together as porosity decreases during compaction, resulting to elongated, flat, parallel fragments.  Non-crystalline Texture Glassy Vitrophyric  Non-crystalline Texture Spherulitic Perlitic  Non-crystalline Texture Scoria Pumice  Non-crystalline Texture Pyroclastics Welded pyroclastics Questions?  GEOL 102 PETROLOGY Rules of Igneous Rock Nomenclature 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Naming Rules for Igneous Rocks Means “Massige Gesteine” translated as “Igneous or Igneous- looking”. They may have crystallized from magma, or formed by cumulate, deuteric, metasomatic, or metamorphic processes. Primary Classification is based on mineral content (Mode). If a mode is not determined due to presence of glass or fine-grained texture, then other criteria is used.  Naming Rules for Igneous Rocks Plutonic Rocks means igneous rock with Phaneritic Texture in which individual crystals can be distinguished with naked eye and is presumed to have formed by slow cooling. Volcanic Rocks means igneous rocks with Aphanitic Texture in which most of individual crystals cannot be distinguished with naked eye and is presumed to have formed by fast cooling.  Naming Rules for Igneous Rocks Rocks are named according to what they are and not to what they might have been. Any manipulation must be justified. Any useful classification should correspond with natural relationships. It must be simple and easy to use. Classification follows as close as possible to historical tradition so well- established terms are not redefined. All recommendations are English but publication in other languages are encouraged.  Parameters for Naming Igneous Rocks Primary Modal Classification is based on relative proportions below, in which a Volume Modal Data is determined: Quartz Quartz, tridymite, cristobalite Alkali Feldspar Orthoclase, microcline, perthite, anorthoclase, sanidine, albite Plagioclase Anorthite to Oligoclase, Scapolite Feldspathoids / Foids Nepheline, leucite, kalsilite, analcime, sodalite, nosean, hauyne, cancrinite Mafic minerals Mica, amphibole, pyroxene, olivine, opaque and accessory minerals  Parameters for Naming Igneous Rocks Primary Normative Classification is based on weight % Oxides All values are based on CIPW Norm Calculation (or Weight Norm)  Nomenclature Rules Additional qualifiers may be added to a root name. It may be mineral name (e.g. biotite granite), textural (e.g. porphyritic granite), chemical (Sr-rich granite), tectonic (post-orogenic granite), genetic (anatectic granite), or other appropriate terms. Qualifiers must not conflict the definition of root name Qualifiers must be defined if not self-explanatory Multiple qualifiers must be in order of increasing abundance (e.g. hornblende-biotite granite has biotite > hornblende) Suffix “-bearing” is not consistently defined so careful For volcanic rocks with glass, the amount must be indicated; either use special names, or prefix “hyalo-”  Nomenclature Rules Additional qualifiers may be added to a root name.. Prefix “micro-” is only for fine-grained plutonic rock (except diabase or dolerite as these are textural and traditional) Prefix “meta-” is only for metamorphosed igneous rock but still retains the igneous texture and original rock type. Volcanic rocks with no mode or chemical analyses can be named using the phenocryst using the prefix “pheno-” such as pheno-andesite; can also use “Field Classification” Color index M’ (different from modal M) means any mafic mineral minus muscovite, apatite, and primary carbonates (these are colorless); use IUGS color index term  Nomenclature Rules Prefixes for rocks with glass IUGS Color Index terms  Hierarchy of using IUGS Classification Basic principle is “special rock” first before plutonic/volcanic. 1. “Pyroclastic rocks & Tephra”. 7. “Leucite-bearing rocks” 2. “Carbonatites. 8. “Lamprophyre” 3. “Melilite-bearing rocks” 9. “Charnockite 4. “Kalsilite-bearing rocks” 10. “Plutonic rock” 5. “Kimberlite” 11. “Volcanic rock” 6. “Lamproite” 12. Rock is not igneous., or you wrongly identified the rock End  GEOL 102 PETROLOGY Pyroclastic Rocks and Tephra Classification 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Pyroclastic Rocks and Tephra Classification Should only be done if the rock specimen is believed to be pyroclastic in origin i.e. formed by fragmentation due to explosive eruption. Excludes rocks formed by autobrecciation of lava flows. This classification is purely descriptive  Pyroclastic Rocks and Tephra Classification Fragments generated by disruption as a direct result of Pyroclasts volcanic action. In a broad sense, this include air fall, flow, and surge deposits, lahars, subsurface, and vent deposits. Reworked Term used if the pyroclasts have been altered. Pyroclasts Epiclasts Term used if the pyroclastic origin of fragments is uncertain  Pyroclastic Rocks and Tephra Classification Based on fragment composition: Lithic – if fragments are made of rock material Vitric – if fragments are made of glass Crystal – if fragments are made of crystals  Pyroclastic Rocks and Tephra Classification Based on size: Bombs – pyroclasts with mean diameter of > 64 mm. It has a round shape and bread-crust surface indicating a wholly or partially molten condition during formation and transport. Blocks – pyroclasts with mean diameter of > 64 mm. It has an angular shape indicating a solid condition during formation Lapilli – pyroclasts of any shape with a mean diameter of 64 mm to 2 mm Ash – pyroclasts with mean diameter of < 2 mm. Coarse ash grain is 2mm to 1/16 mm, Fine ash/dust is < 1/16 mm diameter Pyroclastic Rocks and Tephra Classification Bomb Pyroclastic Rocks and Tephra Classification Block Pyroclastic Rocks and Tephra Classification Lapilli Pyroclastic Rocks and Tephra Classification Ash  Pyroclastic Deposits An unconsolidated or consolidated assemblage of pyroclasts, containing > 75% pyroclasts by volume and the rest being epiclastic, organic, chemical sedimentary or authigenic origin. Consolidated deposits are called Pyroclastic Rocks Unconsolidated deposits are called Tephra  Pyroclastic Deposits Agglomerate Pyroclastic rock with > 75% bombs Pyroclastic Breccia Pyroclastic rock with > 75% blocks Tuff Breccia Pyroclastic rock with 25% - 75% bombs and/or bombs Lapilli Tuff Pyroclastic rock with < 25% bombs/blocks and < 75% lapilli & ash Lapillistone Pyroclastic rock with > 75% lapilli Pyroclastic rock with > 75% ash. Coarse Tuff if 2mm – 1/16 mm, Tuff / Ash Tuff or Fine/Dust Tuff if < 1/16 mm. Can be subdivided based on fragment composition e.g. Lithic Tuff, Vitric Tuff, Crystal Tuff Pyroclastic Deposits Pyroclastic Deposits Mixed Pyroclastic – Epiclastic Deposit Used for rocks containing both pyroclastic or epiclastic (normal clastic) material, and is named as Tuffites, which can be subdivided based on grain sizes. Mixed Pyroclastic – Epiclastic Deposits Questions?  GEOL 102 PETROLOGY Carbonatites Classification 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Carbonatites Classification Classification is ONLY used if the rock contains more than 50% modal carbonates. They can be plutonic or volcanic in origin and can be distinguished based on its mineralogy.  Carbonatites Classification Carbonatite with Calcite. Coarse grained is Sövite. Calcite-carbonatite If medium to fine-grained is Alvikite. Dolomite-carbonatite Carbonatite with Dolomite. Also called Beforsite Ferrocarbonatite Carbonatite with Ankerite or Fe-rich carbonate Carbonate with Na-, K-, and Ca-carbonates. Natrocarbonatite Only found in Oldoinyo Lengai volcano in Tanzania  Carbonatites Classification Suffix “-bearing” may be used if there is minor ( 20%, the rock is Silicocarbonatite. Carbonatites Classification Bomb Questions?  GEOL 102 PETROLOGY Melilite-bearing Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Melilite-bearing Rocks Classification is ONLY used if the rock contains 10% modal melilite, and if there is feldspathoid present then melilite > feldspathoid. Plutonic melilite-bearing rock is Melilitolite. Volcanic melilite-bearing rock is Melilitite.  Melilitolite Plutonic melilitic rock classified based on its mineral content. Only those with M > 90 and named using dominant mineral content Other principals minerals, giving special names, and are > 10%, are: If perovskite > 10%, then Melilitolite is named Afrikandite If olivine > 10%, then it is Kugdite If haüyne > 10% and melilite > haüyne, then it is Okaite If nepheline > 10% and melitite > nepheline, then it is Turjaite If pyroxene > 10%, then it is Uncompahgrite If a third mineral is present > 10%, then it is used as modifier e.g. magnetite-pyroxene melilitolite  Melilitite If a mode is determined, use the Melilitic Rock Diagram. However if the mode falls in the fodoite field of QAPF, then it is named as such. If mode cannot be determined and a chemical analysis is available, then use the TAS Classification Questions?  GEOL 102 PETROLOGY Kalsilite-bearing Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Kalsilite-bearing Rocks Classification for rocks that contain kalsilite as the principal mineral, along with clinopyroxene, leucite, melilite, olivine, and phlogopite. It can also be applied to rocks with kalsilite but has no leucite or melilite. Only used for volcanic rocks. Plutonic equivalents are appropriately named as Pyroxenite Kalsilite-bearing volcanic rocks  Kalsilite-bearing Rocks The Kamafugitic Series include the rocks Mafurite, Katungite, and Ugandite, which is leucite-bearing but is excluded due to absence of kalsilite. But even with this, the presence of essential melilite and/or leucite is considered petrogenetically distinctive, hence the term Kamafugite for this series is still accepted. A more appropriate name is recommended. Kalsilite-bearing volcanic rocks Questions?  GEOL 102 PETROLOGY Kimberlites 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Kimberlites Dark-colored, dense, altered and brecciated, porphyritic intrusive igneous rock with rounded phenocrysts in fine-grained groundmass. Mineralogically complex but often composed of atleast 35% olivine and other minerals. A variety of mica-rich peridotite with abundant high P-T minerals. Diamond-bearing rocks  Group I Kimberlites Archetype rocks from Kimberley, South Africa; “basaltic” Volatile-rich (CO2), potassic ultrabasic rocks with a distinct inequigranular texture from macrocrysts (0.5mm – 10mm) or megacrysts (1cm – 20cm) in a fine groundmass. Xenocryst of anhedral Mg-ilmenite, pyrope, diopside, phlogopite, enstatite, and Ti-poor chromite, and olivine (characteristic and dominant mineral in all kimberlite types except fractionated) Groundmass is 2nd-gen primary euhedral-subhedral olivine with monticellite, phlogopite, perovskite, Mg-spinel solution, apatite, carbonate, and serpentine. Serpentinite and calcite deuterically replacing olivine, phlogopite, monticellite and apatite is common  Group I Kimberlites Macrocrysts are Cr-rich (>2% Cr2O3),; indicate origin from the disaggregation of mantle-derived lherzolite, harzburgite, eclogite, and metasomatized peridotite xenolith. Megacrysts are Cr-poor Group I Kimberlites are diamond-bearing but is less common. Does not contain pseudoprimary groundmass diopside, except as a product of crystallization induced by assimilation of siliceous xenoliths. Evolved Group I Kimberlites are poor or devoid of macrocrysts  Group II Kimberlites “Micaceous” or “Lamprophyric” kimberlites or “Orangeites” Ultrapotassic, peralkaline volatile-rich (H2O) rocks characterized by phlogopite macro- and microphenocrysts with a phlogopitic groundmass. Olivine is common but not a major constituent. Characteristic primary groundmass is diopside zoned & mantled by Ti-aegirine, Mg- to Ti-spinel, Sr- and REE-rich perovskite, Sr- rich apatite, REE-rich phosphates, K-Ba titanites, Nb-rutile, and Mn-ilmenite set in a mesostasis with rare-earth carbonates. Evolved Group II Kimberlites has groundmass sanidine and K- richterite. Zr-silicates occur as late-stage minerals. Barite is a common deuteric secondary mineral. Questions?  GEOL 102 PETROLOGY Lamproites 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Lamproites Igneous rocks mineralogically characterized by the presence of 5% - 90% of the primary phases: Titanian, Al-poor phenocrystic phlogopite (2-10% TiO2, 5-12% Al2O3) Groundmass poikilitic titanian “Tetraferriphlogopite” (5-10% TiO2) Titanian K-richterite (3-5% TiO2, 4-6% K2O) Forsteritic olivine Al-poor, Na-poor diopside (Al2O3 < 1%, Na2O < 1%) Non-stoichiometric Fe-rich leucite (1-4% Fe2O3) Fe-rich sanidine (1-5% Fe2O3)  Lamproites Presence of any one of the mineral as dominant and with two or three other major minerals is sufficient to be classified as such. Presence of these minerals prohibits a rock’s classification as lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite, or kimzeyite Main petrogenetic factors affecting complexity include (1) variable nature of metasomatized source region in mantle, (2) depth and extent of partial melting, (3) and their extensive differentiation.  Lamproites If there is chemical analysis, Lamproites can also be classified based on the following: If the molar K2O/Na2O > 3; they are Ultrapotassic If the molar K2O/Na2O > 0.8 and often > 1 Molar (K2O+Na2O)/Al2O3 > 1; they are Peralkaline If FeO and Cao are both 2000ppm (commonly >5000ppm), Sr > 1000ppm, Zr > 500ppm, La > 200ppm Lamproites Questions?  GEOL 102 PETROLOGY Leucite-bearing Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Leucite-bearing Rocks Leucite-bearing rocks is named according to the volcanic QAPF with a prefix “Leucite” or “Leucite-bearing”. Rocks with little or no feldspar are called Leucitite.  Leucitite Leucitite is subdivided into: Phonolitic Leucitite if foids are 60-90% felsic minerals and AF > P Tephritic Leucitite if foids are 60-90% felsic minerals and P > AF Leucitite (sensu stricto) if foids are 90-100% felsic minerals and if leucite is the only feldspathoid mineral Leucite-bearing Rocks Questions?  GEOL 102 PETROLOGY Lamprophyres 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Lamprophyres Rocks that cannot be easily chemically separated from other normal igneous rocks, and is distinguished by: Normally occur as dikes and not simply textural varities Porphyritic, mesocratic-melanocratic (M’=35-90) Feldspars and/or foids are restricted to groundmass Has essential biotite (Fe-phlogopite) and/or amphibole or cpyx Hydrothermal alteration of olivine, pyroxene, biotite, plagioclase Calcite, zeolite, and hydrothermal minerals as primary phases K2O and/or Na2O, H2O, CO2, S, P2O5 are high Lamprophyres Questions?  GEOL 102 PETROLOGY Charnockitic Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Charnockitic Rocks Classification is ONLY used if the rock is Charnockitic suite characterized by the presence of orthopyroxene (or fayalite+quartz), perthite, mesoperthite, and antiperthite. Often associated with Norites and Anorthosites closely linked to Precambrian terranes.  Charnockitic Rocks To solve the problem of perthite, it must be distinguished as: Perthite - assign to “A” since major component is alkali feldspar Mesoperthite – assign equally between “A” & “P” Antiperthite – assign to “P” since major component is andesine with minor albite Name must indicate assignment e.g. m-charnockite, p-charnockite Charnockitic Rocks Questions?  GEOL 102 PETROLOGY Plutonic Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Plutonic Igneous Rocks Formed by slow cooling evident by relatively coarse-grained texture with individual crystals visible to naked eye.  Plutonic Igneous Rocks 1. Ultramafic Rock = if M > 90% 2. QAPF Diagram = if M < 90% 3. Field Classification = if no mineral mode yet  Ultramafic Rock Classification Classified according to mafic mineral content Depends on the percentage abundance of: Olivine Pyroxene Hornblende Spinel and/or Garnet Ultramafic Rock Ultramafic Rock  QAPF Diagram Classified according to quartz, alkali feldspar, and plagioclase content Normalization Plagioclase Ratio  QAPF Diagram Normalization Percentage of each modal mineral from the total sum of all modal minerals present in the rock. 𝐴 𝐴′ = 𝑥 100 𝐴+𝐵  QAPF Diagram Plagioclase Ratio Normalized percentages of Alkali Feldspar & Plagioclase are again normalized to know the ratio of both feldspars. 𝐴′ 𝐴′′ = 𝑥 100 𝐴′ + 𝐵′  QAPF Diagram For example: Igneous rock with 20% orthoclase, 20% plagioclase, 10% albite, 11% quartz, and 5% accessory ilmenite. Alkali feldspar granite Alaskite if light-colored due to M < 10% Granite Based on Feldspar: Syenogranite if 35% - 65% A Monzogranite if 65% - 90% A Alkali Granite if >90% alkali feldspar Based on chemistry: Peraluminous Metaluminous Subaluminous Peralkaline Based on genesis: I-type if metaluminous granite derived from igneous rocks S-type if peraluminous granite derived from sedimentary rocks M-type if mantle-derived A-type if peralkaline Granodiorite Commonly has Oligoclase Granodiorite has An0 – An50 Granogabbro has AN50 – An100 Tonalite Tonalite or Plagiogranite for light colored due to M < 10% Has mineral modifier, usually hornblende (Hbl tonalite) Monzodiorite Monzogabbro Separated depending on the average composition of plagioclase Monzodiorite if An0 – An50 Monzogabbro if An50 – An100 Syenodiorite & Syenogabbro for rocks between syenite and diorite/gabbro Diorite Gabbro Anorthosite Separated depending on Color Index & Plagioclase composition Anorthosite if M10%, An0-An50 Gabbro if M>10%, An50-An100 Dolerite/Diabase for medium- grained gabbro rather than “Microgabbro” Gabbro Subdivided by abundance of Opx, Cpx, Hbl and Olivine Gabbro (ss) if Plag + Cpx Norite if Plag + Opx Troctolite if Plag + Ol Gabbronorite if Plag + equal Opx+Cpx Opx Gabbro if Plag + Cpx + Opx Cpx Gabbro if Plag + Opx + Cpx Hbl Gabbro if Plag + Hbl + 10% Ol Phonolitic Tephrite if < 10% Ol Phonotephrite is not used as it is in TAS Classification Phonobasanite can be used as a synonym as it is not in TAS Basanite Tephrite Separated based on amount of Olivine in the CIPW Norm: Basanite if > 10% Ol Tephrite if < 10% Ol Dominant foid mineral must be indicated as prefix in the name Phonolitic Foidite Replace the term “foidite” with the dominant foid mineral, subject to naming rules, such as Phonolitic nephilinite Tephritic Foidite Basanitic Foidite Separated by abundance of Olivine in the CIPW Norm. Replace “foidite” with dominant foid mineral name i.e., Tephritic nephilinite Foidite (sensu stricto) Named based on predominant feldspathoid mineral e.g. Nephelinite, Leucitite, Analcimite, Hauynite Field Classification of Volcanic Rocks Only used if neither an accurate mineral more nor a chemical analyses is yet available.  Field Classification of Volcanic Rocks Total Alkali – Silica Classification Only used if the rock is: 1. Volcanic in origin 2. Mineral mode cannot be determined due to presence of glass or fine-grained nature 3. Chemical analysis of rock is available Total Alkali – Silica Classification Notes: 1. Purely descriptive terms 2. No implied genetic significance 3. Only analyses with H2O+ < 2% and CO2 < 0.5% are used, except in high-Mg rocks  Total Alkali – Silica Classification Reminders: 1. Analyses are re-calculated to 100% on an H2O and CO2-free basis 2. Only analyses of rocks with H2O+ < 2% and CO2 < 0.5% are used, unless high-Mg rocks. 3. Application to altered rocks must be satisfactorily satisfied. 4. Caution for weathered, altered, metasomatized, metamorphosed rocks or rocks undergone crystal accumulation  Total Alkali – Silica Classification High-Mg volcanic rocks SiO2 MgO TiO2 Na2O + K2O ROCK > 52% > 8% < 0.5% Boninite 30% - 52% > 18% < 1% < 2% Komatiite 30% - 52% > 18% > 1% < 2% Meimechite 30% - 52% > 12% < 3% Picrite  Total Alkali – Silica Classification 1. High-Mg volcanic rocks If rock falls on shaded region, check the lower diagram if not Komatiite, Meimechite, or Picrite, before naming it as Foidite, Picrobasalt, or Basalt. Also check for Boninite.  Total Alkali – Silica Classification 2. Nephelinites, Melanephelinites, and certain Leucitites Nephelinite if ne > 20% Melanephilinite if ne < 20% and ab < 5% SiO2 TAS  Total Alkali – Silica Classification 1. Alkali Basalt and Sub-alkali Basalt is separated based on presence of normative ne. 2. Basalt, Basaltic Andesite, Andesite, and Dacite are qualified as Low-K, Medium-K, and High-K (but is not synonymous to Potassic as High-K rocks can have more Na2O than K2O. 3. Rhyolite is divided into Peralkaline Rhyolite based on Peralkaline Index (Na2O+K2O / Al2O3 = > 1). 4. Trachyte and Trachydacite is divided based on normative Q (100 x Q/Q+an+ab+ar); Trachyte if < 20% (Peralkaline if PI > 1)  Total Alkali – Silica Classification 1. Peralkaline Rhyolite and Trachyte are divided into Comendite, Comenditic trachyte, Pantellerite, & Pantelleritic trachyte based on relative amount of Al2O3 vs. Total Iron (FeO) 2. Before deciding if Foidite, check if it is Melilitite using: 1. If no Kalsilite but has normative cs (larnite) > 10% and K2O 10%) 2. If normative cs > 10%, K2O>Na2O and K2O>2%, then Potassic Melilitite or Potassic Olivine Melilitite (or Katungite) 3. If normative cs is present but < 10%, then Melilite Nephelinite or Melilite Leucitite depending on dominant feldspathoid mineral. Questions?  GEOL 102 PETROLOGY CIPW Norm Calculations 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  CIPW Norm Calculation To begin the calculation: 1. Wt.% oxides in rock analysis is converted to Mole Proportions by dividing each oxide wt.% by its molecular weight. (MnO & NiO are added to FeO, BaO & SrO are added to CaO) 2. Using the formula in T.1., the mole proportions are distributed among the normative minerals according to the Rules below. 3. Wt.% are calculated by multiplying the oxide by the factor in T.1. 4. To check: the sum of normative minerals must be the same as the sum of the original wt.% oxides; (H2O is ignored)  T.1. Formula for Mole to Wt.% factors Minerals Abbrev. Formula Mol. To Wt.% Factor Quartz Q SiO2 SiO2 x 60.09 Corundum C Al2O3 Al2O3 x 101.96 Zircon Z ZrO2SiO2 ZrO2 x 183.31 Orthoclase or K2O. Al2O3. 6SiO2 K2O x 556.67 Albite ab Na2O. Al2O3. 6SiO2 Na2O x 524.46 Anorthite an Ca2O. Al2O3. 2SiO2 CaO x 278.21 Leucite lc K2O. Al2O3. 6SiO2 K2O x 436.50 Nepheline ne Na2O. Al2O3. 6SiO2 Na2O x 284.11 Kaliophilite kp Ca2O. Al2O3. 2SiO2 K2O x 316.33 Acmite ac Na2O. Fe2O3. 4SiO2 Na2O x 462.02 Na-metacilicate ns Na2O. SiO2 Na2O x 122.07  T.1. Formula for Mole to Wt.% factors Minerals Abbrev. Formula Mol. To Wt.% Factor wo CaO. SiO2 CaO x 116.17 Diopside en MgO. SiO2 MgO x 100.39 fs FeO. SiO2 FeO x 131.93 Wollastonite wo CaO. SiO2 CaO x 116.17 en MgO. SiO2 MgO x 100.39 Hypersthene fs FeO. SiO2 FeO x 131.93 fo 2MgO. SiO2 MgO x 70.35 Olivine fa 2FeO. SiO2 FeO x 101.89 Ca-orthosilicate cs 2CaO. SiO2 CaO x 86.12 Magnetite mt FeO. Fe2O3 FeO x 231.54 Chromite cm FeO. Cr2O3 Cr2O3 x 223.84  T.1. Formula for Mole to Wt.% factors Minerals Abbrev. Formula Mol. To Wt.% Factor Hematite hm Fe2O3 Fe2O3 x 159.69 Ilmenite il FeO. TiO2 TiO2 x 151.75 Sphene tn CaO. TiO2. SiO2 TiO2 x 196.07 Perovskite pf CaO. TiO2 TiO2 x 135.98 Rutile ru TiO2 TiO2 x 79.90 Apatite ap 3 (3CaO. P2O5). CaF2 P2O5 x 336.21 Fluorite fr CaF2 F x 39.04 Pyrite Pr FeS2 S x 59.98 Calcite Cc CaO. CO2 CO2 x 100.09  Rules to follow in CIPW Norm 1. P2O5 is allotted to apatite, and CaO is reduced by 3.33 x P2O5 2. S is allotted to pyrite, and FeO is reduced by 0.5 x S 3. Cr2O3 is allotted to chromite, and FeO is reduced by Cr2O3 4. TiO2 is allotted to ilmenite, and FeO is reduced by TiO2. If TiO2 exceeds FeO, the excess is allotted to provisional sphene (tn’), and CaO and SiO2 are both reduced by an amount equal to the excess of TiO2; this step is ONLY for if CaO remains after the formation of anorthite. If TiO2 still remains, it is calculated as rutile.  Rules to follow in CIPW Norm 5. F is allotted to fluorite, and CaO is reduced by 0.5 x F. 6. CO2 is allotted to calcite, and CaO is reduced by CO2 7. ZrO2 is allotted to zircon, and SiO2 is reduced by ZrO2 8. K2O is allotted to provisional orthoclase (or’), and Al2O3 is reduced by K2O, and SiO2 is reduced by 6 x K2O 9. Al2O3 remaining from step 8 is combined with an equal amount of Na2O to form provisional albite (ab’), and silica is decreased by 6 times this amount. If there is insufficient Al2O3, proceed to step 11.  Rules to follow in CIPW Norm 10. Al2O3 remaining from step 9 is combined with an equal amount of CaO to form provisional Anorthite (an’), and silica is decreased by 2x this amount. If Al2O3 exceed CaO, it is calculated as corundum 11. If Na2O exceeds Al2O3 in step 9, an amount of Fe2O3 equal to the excess is allotted to acmite, and silica is decreased by 4x this amount. 12. If Na2O still remains after step 11, the remaining Na2O forms sodium metasilicate, and silica is reduced by the amount of the remaining Na2O (this is extremely rare)  Rules to follow in CIPW Norm 13. All remaining Fe2O3 is allotted to magnetite, and the FeO is decreased by Fe2O3. If Fe2O3 exceeds FeO, the excess is Hematite 14. All remaining MgO & FeO forms pyroxenes & olivines. At this point, MgO & FeO are added together but the proportions are maintained in calculating the amounts of Mg & Fe end-member components of the pyroxenes and olivines. 15. The CaO remaining from step 10 forms provisional diopside (di’), which decreases MgO+FeO by an amount equal to CaO, and silica by twice this amount  Rules to follow in CIPW Norm 16. If CaO exceed MgO+FeO, the excess form provisional wollastonite (wo’), and silica is decreased by the excess CaO. 17. If the MgO+FeO in step 15 exceeds the CaO, the excess forms provisional hypersthene, and silica is decreased by excess MgO+FeO 18. If SiO2 is still positive, remaining SiO2 is calculated as quartz. 19. If SiO2 is negative, the rock has insufficient silica for provisionally formed silicates, and these must be converted to minerals with less silica until the silica deficiency is eliminated.  Rules to follow in CIPW Norm 19. The order is as follows: first, hypersthene converted to olivine, then sphene to perovskite, albite to nepheline, orthoclase to leucite, wollastonite and diopside to Ca-orthosilicate & olivine, and finally leucite to kaliophilite. Let silica-deficiency be “D” 20. If D < hy/2’, set ol = D, and hy = hy’ – 2D. If D > hy’/2, all provisional hypersthene is converted to olivine (ol = hy’), and the new silica deficiency D1 is D – hy’/2 19. If Dtn’, all provisional sphene is converted to perovskite (pf = tn’), and the new silica deficiency D2 is D1 – tn’  Rules to follow in CIPW Norm 22. If D2 < 4ab’, some of provisional albite is converted to nepheline, such that ne = D2/4 and ab = ab’ – D2/4. If D > 4ab’, all provisional albite is converted to nepheline (ne=ab’), and the new silica deficiency D3 is D2 – 4ab’ 23. If D3 < 2or’, some of provisional orthoclase is converted to leucite, such that lc = D3/2 and or = or’ – D3/2 If D3 > 2or’, some of provisional orthoclase is converted to leucite, (lc’ = or’), and the new silica deficiency D4 is D3 – 2or’  Rules to follow in CIPW Norm 25. If D5 < di’, some of diopside is converted to Ca-orthosilicate and olivine, which are added to that previously formed; set cs = D5/2, ol = D5/2, and di = di’ – D5, remembering to add the amounts of cs and ol to those already formed in steps 24 and 20, respectively. If D5 > di’, all provisional diopside is converted to Ca-orthosilicate and olivine, such that cs = di’/2 and ol = di’/2 (add to amounts formed in steps 24 and 20), and new silica deficiency D6 is D5 – di’ 26. Finally, if there is still a deficiency in silica, leucite is converted to kaliophilite, set kp = D6/2, and lc = lc’ – D6/2  Rules to follow in CIPW Norm Once the silica deficiency is eliminated (steps 20-26), the norm calculation is completed by multiplying the mole proportion of the first oxide in the formula of each normative mineral formed by the weight-conversion factor in T.1. Only rare rock types have low enough silica contents to cause the norm calculation to proceed beyond step 22. For most rocks, the calculation is short. If large numbers of norms is determined and the calculations become tedious, they are done with a computer (using NORRRM program) Questions?  GEOL 102 PETROLOGY Petrographic Textures of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Crystallinity Degree of structural order of a solid evident in the arrangement of atoms or molecules in a consistent and repetitive manner.  Degree of Crystallinity Holocrystalline Hypocrystalline Holohyaline/Vitric Consist entirely of Consists of both Consist entirely of crystals crystals and glass glass  Grain Size Size of grains determined by rate of crystallization of the melt.  Grain Size Aphanitic Phaneritic Cryptocrystalline Grains too fine-grained Grains are coarse Grains too fine-grained to see with naked eye enough to see with even with microscope naked eye  Grain Size Fine grained Medium grained Coarse grained Crystals are < 1 mm Crystals is 1mm to Crystals are > 3 mm 3mm  Grain Size Pegmatitic Aplitic Very coarse grained Fine- to Medium-grained crystals xenomorphic and equigranular; sugar-like  Grain Size Equigranular Inequigranular Grains are equally the Grains are of varying same size sizes  Porphyritic Large crystals (phenocrysts) are set in a finer-grained or glassy groundmass  Porphyritic Porphyritic Megacryst Oikocryst Bimodal size Unusually large crystal Host phenocryst in distribution either as phenocryst or Pokilitic texture xenocryst  Porphyritic Poikilitic Cumulophyric Glomeroporphyritic Host phenocryst has Clusters of Clusters of many inclusions of phenocrysts phenocrysts other minerals  Porphyritic Seriate Hiatal Aphyric Continuous range of Sizes are not continuous Aphanitic groundmass sizes from phenocryst with sizes markedly with no phenocrysts to groundmass different from one another  Textures of Individual Grains More often individual crystal grains exhibit different textures depending on various factors of their formation  Individual Grains Idiomorphic Hypidiomorphic Allotriomorphic Euhedral crystal Dominantly subhedral Anhedral crystal grains grains; minor euhedral grains and anhedral faces  Individual Grains Embayed Equant Laths Corroded grains due to Grains having Elongated prismatic resorption of crystal by boundaries of equal crystal grains the melt lengths  Grains in the Rock as a Whole Sutured Mosaic Intersertal Articulation along highly Polygonal equigranular Glass in between irregular interpenetrating crystals plagioclase laths boundaries  Grains in the Rock as a Whole Ophitic Sub-ophitic Pilotaxitic Large pyroxene grains Plagioclase laths are Feldspar microlites in enclose small random larger and only partially groundmass arranged plagioclase laths enclosed by pyroxene in sub-parallel mode  Grains in the Rock as a Whole Trachytic Rapakivi Miarolitic Aligned microlites due to Plagioclase rims around Void spaces or pockets flow K-feldspar with outlines shaped by neighboring crystals  Grains in the Rock as a Whole Spinifex Xenocryst Extremely acicular olivine Crystals not derived from phenocrysts original magma  Pyroclastic Pele’s Hair Pele’s Tear Fiamme Fibrous volcanic glass Teardrop-shaped Small dark glassy lens volcanic glass softened & compressed during welding Questions?  GEOL 102 PETROLOGY Magma Series 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Magma Series Genetically related magma that have changed composition from a common original parental magma  Alkaline Magma Series Rocks with high Na2O and K2O compared to SiO2 and are silica undersaturated  Alkaline Magma Series Never contain equilibrium Quartz High concentrations of Alkali (Li, Na, K, Rb, Cs, Fr) Has feldspathoid, alkali feldspar, and Na-mafics Diverse chemical composition of 0-65% SiO2 and 0-0.88 MgO/MgO+FeO Geochemical signature of a Mantle source  Alkaline Magma Series Feldspathoids include nepheline, leucite, analcite, sodalite, cancrinite. SiO2-saturated alkaline rocks are enriched in alkali feldspar minerals sanidine, anorthoclase, perthitic K-felds, microcline, orthoclase, albite. Amphibole include riebeckite and richterite. Pyroxene include aegirine, augite, spodumene  Alkaline Magma Series Volcanic rocks include alkali basalts, hawaiite, benmoreite, mugaerite, trachyte, phonolite, lamprophyre, carbonatite, komatiite, kimberlite, etc. Plutonic rocks include syenite, nepheline syenite, Monzosyenite, Monzogabbro, nephelinite, etc. Occur at hotspots, ocean island, rift environments where limited partial melting (10%) of undepleted alkali-rich mantle rocks at high-P conditions  Sub-alkaline Magma Series Rocks with low to moderate Na2O and K2O compared to SiO2 or high FeO and CaO concentration.  Calc-alkaline Magma Series Progressive decrease in Fe and Mg with increasing SiO2 and Alkali concentration. Olivine, pyroxene, amphibole, biotite, and Fe-oxides are crystallized from melt, enriching the melt in Na2O, K2O and SiO2. Andesite, Dacite, Rhyolite, High-AL2O3 Basalt Convergent margins; known as BADR Series.  Tholeiitic Magma Series Increasing fractional enriches melt w/ Iron at low to moderate SiO2 Forsterite and Anorthite depletes MgO & Cao, causing FeO, SiO2, alkali enrichment. Then, FeO-rich minerals deplete FeO, and enriching melt in SiO2 and alkali; this results to Basalt with subtle compositional variations. Produce small amounts of silicic magmas; dominate extensional settings and occur over hotspot and front of immature volcanic arcs.  Bimodal Magma Suites Voluminous occurrence of silicic and basic rocks with few intermediate rocks, associated with continental rifts.  Bimodal Magma Suites Basic rocks derived from partial melting of mantle, while silicic rocks from partial melting of the continental crust heated by rising basic magmas.  Magma Series Calc-alkaline exclusively at convergent margins. Tholeiitic at ocean spreading centers, in continental rifts, backarc basins, ocean islands, hotspots. Alkaline primarily in hotspots, ocean islands, rifts Bimodal volcanism at continental rifts  Variation Diagrams Concisely and clearly display rock chemistry variations between samples to establish their origins and relationships.  Variation Diagrams Linear or curvilinear data trend Random data trend indicate indicate rocks derived from different sources or severe common source. alteration Bivariate Variation Diagrams Consists of two variables plotted in the x and y coordinates.  Harker Diagrams Plots major or minor oxide compounds against SiO2, displaying variations of their abundance with regards to silica. Permits overall trends of major element variation to be deduced.  Harker Diagrams If it displays smooth, curvilinear trend then: 1. Rocks are genetically related 2. Major element variation reflect liquid descent from common source in which diversification caused their variations progressively with respect to varying SiO2 3. Parent magma has composition near that of sample with the least SiO2 (basaltic)  Harker Diagram With increase SiO2 K2O and Na2O increases Fe2O3, MgO, and CaO decreases Al2O3 increases then decreases  Spider Diagrams Trace elements are plotted related to a standard reference for each element. Useful in demonstrating enrichment vs. depletion indicating the diversification trends of magmatic source. Also yield information on fractionation of particular minerals. Spider Diagram v Ternary Variation Diagrams Displays the relationship of samples in terms of three variables.  Trivariate Variation Diagrams Tholeiitic Magma shows significant Fe enrichment with increasing fractionation Calc-alkaline Magma does not Questions?  GEOL 102 PETROLOGY Magma Series 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Petrotectonic Associations Suites of rocks that formed in response to similar geological conditions. These commonly develop at divergent & convergent boundaries, and in hotspots. 1. Divergent Boundaries Regions where partial melting of ultramafic rock results to basaltic magma Oceanic Crust Ultramafic Rock (Basalt) (Peridotite) Oceanic Lithosphere (Residual ultramafic rock)  1. Divergent Boundaries (b) Cross-section of Divergent boundary, (c) oceanic crust and lithosphere, (d) mid-oceanic ridge systems on Earth  Ocean Crust Cross-Section 1 Layer 1 = well-stratified marine pelagic sediments 2 Layer 2 = upper pillow basalt, and lower sheeted diabase dike complex Layer 3 = upper massive isotropic 3 gabbro, middle cumulate (layered) gabbro, and lower layered peridotite Layer 4 = non-cumulate metamorphosed depleted mantle peridotite 4 refractory residue A. Mid-oceanic Ridge Basalts (MORB) - most abundant volcanic rock - Tholeiites with low SiO2 & K2O but high in MgO, Al2O3, and compatible elements (Ni, Cr) - From partial melting of depleted mantle source (Mantle Lherzolite) from low 87Sr/86Sr ratio, low volatile and incompatible element concentration. - Divided into Normal MORB and Enriched MORB based on minor and trace element abundance A. Mid-oceanic Ridge Basalts (MORB) Normal MORB (N-MORB) Enriched MORB (E-MORB) Strongly depleted in highly Highly abundant in incompatible incompatible elements; elements (LREE). Chondritic La/Sm < 1.8 Chondritic La/Sm > 0.7 MgO > 65%, K2O < 0.10, TiO2 < 1.0 MgO > 65%, K2O > 0.10, TiO2 > 1.0 partial melting of well-mixed depleted partial melting of more fertile mantle mantle source source A. Mid-oceanic Ridge Basalts (MORB) Where is the “fertile” source of E-MORB? 1. Deeper, more fertile mantle 2. Depleted upper mantle locally fertilized by prior intrusions of magma from deeper mantle 3. Lateral mixing along mid-oceanic ridges with materials from nearby plumes A. Mid-oceanic Ridge Basalts (MORB) How does MORB form? - Separation of plates in divergent regions - Upward motion of mantle into the extended region - Decompression partial melting associated with near- adiabatic rise - N-MORB melting initiated at 60-80km depth in upper depleted mantle where it inherits depleted trace element and isotopic character. - Melt blobs separate at 25-35km depth A. Mid-oceanic Ridge Basalts (MORB) Pillow lava How does MORB form? Dikes Crust Gabbro Axial Magma Chamber Primitive (Original model) Melt Ol + Cpx Cumulates Ol + Sp Mantle Mantle Harzburgite Harzburgite A. Mid-oceanic Ridge Basalts (MORB) How does MORB form? - Small sill-like magma bodies surrounded by Mush and Transition Zones - Mush Zones ( 16%) - High TiO2 (>1.3%) andesitic to rhyolitic volcanic rocks TRIVIA: Divergent margins generate the bulk of ocean floor rocks (70% of Earth’s area) which are all < 200 MYO. 2. Convergent Boundaries Magmatism along the trench resulting to both plutonic rocks and volcanic arc rocks 2. Convergent Boundaries Factors: Thin overlying oceanic crust produces metaluminous mafic to Composition and intermediate rocks. Thickness of overlying Thick overlying continental crust produces peraluminous, plate potassic intermediate to silicic rocks. Composition of resulting plutonic and volcanic rocks are affected Composition of rock if the partially melted rocks are the overlying ultrabasic mantle undergoing anatexis wedge, basic to silicic forearc basement, subducted basic- ultrabasic ocean lithosphere or marine pelagic sediments. Volatile-rich minerals (mica, amphibole, serpentine, talc, Flux Melting carbonates, clays, brucite) release volatile vapor that lowers melting temperature of mantle peridotite  2. Convergent Boundaries Factors: Diversification Fractionation, Assimilation, Magma Mixing, and Metamorphism Process and strongly alters the magma composition generated in overlying Metamorphic wedge of arc system Reactions Old, cold, dense lithosphere = steep subduction Dip Angle of Young, warm, buoyant lithosphere = shallow subduction Subduction Zone Steeply-inclined subduction = melting of thick wedge mantle slab Shallow-dipping subduction = melting of thin wedge mantle slab 2. Convergent Boundaries Phanerozoic convergent margins dominated by Calc-alkaline suites; enriched in SiO2, Alkali (Na2O, K2O), LIL, LREE, and volatiles Presence of hydrous minerals hornblende and biotite indicate that arc magma has > 3% H2O Arc rocks display variations in K2O (low in tholeiite, medium in calc-alkaline, and high in calc-alkaline to shoshonite; reflects increasing K2O and K2O/Na2O and decreasing Fe enrichment. 2. Convergent Boundaries Low-K2O tholeiites in with thin slab 0km – 20km Medium- to High-K2O calc-alkaline andesite in thick slab 20km – 40km High-K2O shoshonite in very thick slab > 40km Ocean-ocean = young island arc volcanic complex Ocean-continent = mature continental arc complex Continent-continent = ceased subduction & collision B. BADR Calc-alkaline group of Basalt, Andesite, Dacite, Rhyolite is signature of volcanic rock suite of convergent margin. One of most voluminous rock assemblages next to MORB Linear trend in Harker plots indicate descent from common source (parent basaltic magma) Andesite is most common calc-alkaline volcanic rock B. BADR BASALT Basalt in convergent margin are aphanitic to aphanitic-porphyritic Arc Tholeiites (low K2O) and Calc-alkaline Basalts (mod K2O). Arc Tholeiites have higher Al2O3 (>16%) than tholeiitic basalts (MORB, OIB); High Alumina Basalt Calc-alkaline basalts have higher alkali (K2O) than tholeiites and has no Fe enrichment. Plagioclase phenocrysts is common B. BADR ANDESITE Volcanic rock with 52% - 63% SiO2. Divided based on SiO2: Basaltic Andesite = 52% - 57% SiO2 in young island arc Silicic Andesite = 57% - 63% SiO2 in mature continental arc Gray, porphyritic-aphanitic with phenocryst of plagioclase (zone euhedral), hornblende (reaction rims), and pyroxene, and biotite. 25o subduction angle, anatexis of thick continental hanging wall plates and partial melting of subducted slab at 70-200km depth B. BADR DACITE 63% - 68% SiO2; TAS extends to 77% Quartz-phyric volcanic rock between andesite & rhyolite Enriched in plagioclase. Phenocrysts are subhedral to euhedral zoned oligoclase and labradorite or sanidine. Minor minerals are biotite, hornblende, augite, hypersthene, and enstatite B. BADR TRACHYANDESITE (Latite and Shoshonite) 66% - 69% SiO2; TAS extends starting limit at 57% SiO2 Phenocryst of andesine to oligoclase in a groundmass of orthoclase and augite. B. BADR Rhyolite and Rhyodacite > 69% SiO2and 68% - 73% SiO2 respectively Explosive silicic eruption producing fragmental, glassy, and aphanitic to aphanitic-porphyritic texture. Occur as glass (obsidian or pumice), pyroclastic tuffs & breccia, or as aphanitic to aphanitic-porphyritic crystalline rocks Phenocryst of alkali feldspar or quartz, with minor hornblende and biotite 3. Island Arcs Magmatism along the trench resulting to both plutonic rocks and volcanic arc rocks 3. Island Arcs Underlain by intermediate to mafic plutonic suits dominated by diorite, quartz diorite, granodiorite, tonalite, and gabbro. Island arc granodiorites are metaluminous, with hornblende, biotite, and minor muscovite. Young island arcs produce andesite and basaltic andesite, low-potassium arc tholeiite basalts, Boninites, and Adakites 3. Island Arcs Low-potassium arc tholeiites occur in oceanward side nearest the trench Tholeiite island arc basalt form at subduction zones with thin overlying plates Major element concentration in tholeiite is similar to MORB (low K2O & Fe enrichment) indicating similar depleted mantle source by flux melting; but has higher potassium & LIL and lower HFS Tholeiitic island arcs are basalt, basaltic andesite, and andesite in the volcanic arc  C. Island Arc Basalts BONINITES High-Mg intermediate volcanic rock w/ SiO2-saturated groundmass Phenocryst of orthopyroxene and no plagioclase phenocrysts. Enriched in Cr, Ni, volatiles, LEE, Zr, Ba, Sr; depleted in HREE & HFS Occur proximal to trench with geochemical signature of primitive mantle-derived magma produced early in the subduction cycle Product of subduction-related melting in the forearc of young island arc system  C. Island Arc Basalts ADAKITES SiO2-saturated w/ high Sr/Y and L/Yb ratio (LREE enriched) and low HFS (Nb, Ta) Form at continent-continent collision site due to shallow slab subduction of continental lithosphere Questions?