MetPet Lecture 1 (1) PDF

Metamorphic Petrology  APG321  Department of Earth Sciences  University of the Western Cape  Introduction to Metamorphism Definition of metamorphism: “Metamorphism is a subsolidus (no melts/magmas – all solid!) process leading to changes in mineralogy and/or texture (for ex...

Metamorphic Petrology  APG321  Department of Earth Sciences  University of the Western Cape  Introduction to Metamorphism Definition of metamorphism: “Metamorphism is a subsolidus (no melts/magmas – all solid!) process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.” Deformation alone does not count – it requires crystallisation and/or recrystallisation  The Limits of Metamorphism Low-temperature limit grades into diagenesis at low temperatures Metamorphism begins in the range of 100-150oC for the more unstable types of protolith (original rock) Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary The lower limit is marked by the formation of minerals such as laumontite, analcime, heulandite, carpholite, paragonite, prehnite, pumpellyite, lawsonite, glaucophane or stilpnomelane  The Limits of Metamorphism High-temperature limit grades into melting Over the melting range solids and liquids coexist Migmatites (“mixed rocks”) are gradational – consist of products or partial melting and restite (material left behind that did not melt) Migmatite: Pink-coloured portions are leucosomes – portions of the rock produced by partial melting. Dark-coloured portions are the melanosome, typically the portion of the rock that did not melt  Metamorphic Agents and Changes  What brings about metamorphism?  Temperature (T): typically the most important factor in metamorphism The geotherm for oceanic lithosphere is typically much higher than for continental shields  Metamorphic Agents and Changes Increasing temperature has several effects: 1) Promotes recrystallization leading to an increased grain size Shear stresses, however, commonly reduce grain size 2) Drive reactions (endothermic – taking in heat) that consume unstable minerals and produces new minerals that are stable under the new conditions 3) Overcomes kinetic barriers that might otherwise not allow equilibrium to occur  Metamorphic Agents and Changes Pressure (P) Temperature rarely increases without an accompanying increase in pressure Most disturbances are transient (short-lived) and eventually return to “normal” “Normal” gradients perturbed (changed, or disturbed) in several ways, most commonly:  High T/P geotherms (high temperature – low pressure) occur in areas of plutonic activity or rifting  Low T/P geotherms (low temperatures – high pressures) occur in subduction zones Based on P-T estimates for rocks exposed at the surface in these areas along a traverse from lowest to highest metamorphic conditions: metamorphic field gradients – not the same as geotherms. Metamorphic field gradients would be what the typical rise in temperature with change in pressure would be for a particular type of metamorphism or tectonic scenario Figure 1.2. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw- Hill.  Metamorphic Agents and Changes Metamorphic grade: a general increase in degree of metamorphism without specifying the exact relationship between temperature and pressure “high-grade” or “low-grade” Pressure acts as a modifier: High T/P paths (low P – high T) Low T/P paths (high P – low T)  Metamorphic Agents and Changes Lithostatic pressure - uniform stress (similar to hydrostatic pressure) Stress is an applied force acting on a rock (over a particular cross-sectional area) Strain is the response of the rock to an applied stress (which gives rise to the yielding of the rock or deformation) Deviatoric stress - the pressure is unequal in different directions Stress resolved into three mutually perpendicular stress (s) components: s1 - maximum principal (compressive) stress, s2 - intermediate principal stress, and s3 - minimum principal (compressive) stress In hydrostatic situations all three are equal  Metamorphic Agents and Changes Strain leads to deformation Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage, which is dependent on the P-T conditions Strain energy may overcome kinetic barriers to reactions   Foliation is a common result, which allows us to estimate the orientation of s1 s1 Strain ellipsoid  s1 > s2 = s3 foliation and no lineation  s1 = s2 > s3 lineation and no foliation  s1 > s2 > s3 both foliation and lineation Figure 1.3. Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.  Metamorphic Agents and Changes Shear motion occurs along planes at an angle to s1 It may occur as slip along spaced cleavages or as flow Distinguishing between the effects of shearing and flattening may be difficult in certain cases s1 Figure 1.4. The three main types of deviatoric stress with an example of possible resulting structures. c. Shear, causing slip along parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.  Metamorphic Agents and Changes Fluids Evidence for the existence of a metamorphic fluid: Fluid inclusions Fluids are required for the growth of hydrous or carbonate phases Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures A fluid inclusion comprising a liquid phase (liq), a vapour bubble (v), a halite crystal (h), a sylvite crystal (s), and other unknown, unidentified solid phases (x, y)  Metamorphic Agents and Changes Fluids Fluids can be meteoric, juvenile magmatic, fluids derived from dehydration of subducted material, trapped sedimentary brines, or degassing of the mantle The motion of fluids may transport various chemical species over considerable distances – metasomatism Pfluid = S partial pressures (p) of each component (Pfluid = pH2O + pCO2 + …) Mole fractions (X) of components must sum to 1.0 (XH2O + XCO2 + … = 1.0) pH2O = XH2O  Pfluid Gradients in T, P, Xfluid lead to zonation in mineral assemblages  The Types of Metamorphism Different approaches to classification 1. Based on principal process or agent Dynamic Metamorphism Thermal Metamorphism Dynamo-thermal Metamorphism  The Types of Metamorphism Different approaches to classification 2. Based on setting Contact Metamorphism  Pyrometamorphism – “pyro” – fire Regional Metamorphism  Orogenic Metamorphism  Burial Metamorphism  Ocean Floor Metamorphism Hydrothermal Metamorphism Fault-Zone Metamorphism Impact or Shock Metamorphism  The Types of Metamorphism Contact Metamorphism The size and shape of an aureole is controlled by: The nature of the intruding pluton  Size  Temperature  Shape  Composition  Orientation The nature of the country rocks  Composition  Depth and metamorphic grade prior to intrusion  Permeability (allowing for fluid flow)  Contact Metamorphism Different metamorphic rock types will develop depending on which protolith is metamorphosed, e.g. during Adjacent to igneous intrusions contact metamorphism Thermal (± metasomatic) effects of hot magma intruding cooler shallow rocks Occurs over a wide range of pressures, including very low Contact aureole  The Types of Metamorphism Contact Metamorphism Most easily recognized where a pluton is introduced into shallow rocks in a static environment Gives rise to Hornfelses (granofelses) commonly with relict (retained) textures and structures  The Types of Metamorphism Contact Metamorphism Polymetamorphic - usually representing an orogenic event followed by a contact one e.g. spotted phyllite (or slate) The foliated phyllite formed during a regional event, often associated with deformation, and the later ovoid “spots” are minerals that grew during the contact event The later thermal overprint may be due to:  The lag time for magma migration  A separate phase of post-orogenic collapse magmatism  Contact Metamorphism of Pelitic Rocks: The Skiddaw Aureole, UK Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodies Intrusions are shallow Contact effects overprinted an earlier low-grade regional orogenic metamorphism  Contact Metamorphism of Pelitic Rocks: the Skiddaw Aureole, UK The metamorphic aureole around the Skiddaw granite was sub-divided into three zones, principally on the basis of textures: Unaltered slates Increasing Outer zone of spotted slates Metamorphic Grade Middle zone of andalusite slates Inner zone of hornfels Contact Skiddaw granite Figure 1.13. Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. First effects (1-2 km from contact) = 0.2 - 2.0 mm sized black ovoid “spots” in the slates At the same time, recrystallization led to slight coarsening of the grains and degradation (break-down) of the slaty cleavage The spots were probably cordierite or andalusite, subsequently re-hydrated and retrograded back to fine aggregates of mostly muscovite Both cordierite and andalusite occur at higher grades, where they are often partly retrograded, but not farther out Spots in most of the spotted slates are probably pseudomorphs  Contact Metamorphism of Pelitic Rocks: the Skiddaw Aureole, UK Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz Figure 1.14. Cordierite- andalusite slate from the middle zone of the Skiddaw aureole. 1 mm Both cordierite and andalusite are indicators of low-P metamorphism; andalusite is the low-P Al2SiO5 polymorph  Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Inner zone: Figure 1.15. Andalusite-cordierite schist Thoroughly recrystallized from the inner zone of the Skiddaw aureole. Note the chiastolite cross in Both andalusite and cordierite are andalusite. minerals characteristic of low-pressure metamorphism, which is certainly the case in the Skiddaw aureole, where heat is carried up into the shallow crust by the granites The rocks of the inner zone at Skiddaw are characterized by coarser and more thoroughly recrystallized 1 mm textures Same mineral assemblage as the middle zone, but coarser-grained Some rocks are schistose, but in the innermost portions the rock fabric loses the foliation, and the rocks are typical hornfelses  Contact Metamorphism of Pelitic Rocks: the Skiddaw Aureole, UK The zones are determined on a textural basis It is preferable to use the sequential appearance of minerals and isograds to define zones, rather than textures, but here the minerals are similar or the same The first new mineral in most slates is biotite, followed by the approximately simultaneous development of cordierite and andalusite Orthopyroxene occurs in pelitic and quartzo-feldspathic rocks only at the very highest grades of contact and regional metamorphism, grades that may not be reached prior to melting in many instances  The Types of Metamorphism Pyrometamorphism A minor type of contact metamorphism - very high temperatures at low pressures, generated by a volcanic or sub-volcanic body Also developed in xenoliths May be accompanied by various degrees of partial melting  The Types of Metamorphism Regional Metamorphism sensu lato (in a general sense): metamorphism that affects a large body of rock, and thus covers a great lateral extent Three principal types:  Orogenic metamorphism  Burial metamorphism  Ocean-floor metamorphism  The Types of Metamorphism Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress) Foliated rocks are a characteristic product Island arcs, active continental margins, and continental collision zones are the typical environments Most studies focus on orogenic belts, and the term, “regional metamorphism” is often used synonymously with “orogenic metamorphism”  The Types of Metamorphism Orogenic Metamorphism (a) the incipient stages of subduction (b) “orogenic welt” created by compression, crustal thickening, thrust stacking of oceanic slices, and addition of magmatic material from below Underthrusting in the fore-arc migrates trenchward, adding successive slabs to the base of the outer welt (tectonic underplating) Heat added by rising plutons, Figure 1.5. Schematic model for the magmatically underplated sequential (a c) development of a magma, and induced mantle “Cordilleran-type” or active convection continental margin orogen. The dashed and black layers on the right represent the basaltic and gabbroic layers of the oceanic crust.  The Types of Metamorphism Orogenic Metamorphism Temperature increases both downward and toward the axial portion of the welt where plutons are concentrated Hottest portions occur toward the central axis of the orogenic belt grading to lower temperatures and the axial metamorphic grades further way and to either side Temperature increases both downward and toward portion of the welt where plutons concentrated Temperature increases both downward and toward the axial portion of the welt where plutons concentrated  Orogenic metamorphism Uplift and erosion results in exposure of the metamorphic rocks Heat dissipates slowly, so the metamorphism often continues long after major deformation ceases When this occurs, the metamorphic pattern is simpler than the structural one Folding and thrusting are often complex, but the metamorphic pattern may be a simple thermal dome, centering on the metamorphic/igneous core where heat input, thickening, and uplift are the greatest Exposed surface pattern of increasing metamorphic grade from both directions toward the core area  The Types of Metamorphism Orogenic Metamorphism Uplift and erosion Metamorphism often continues after major deformation ceases  Metamorphic pattern is simpler than the structural one Pattern of increasing metamorphic grade from both directions toward the core area Example from the north- eastern US From Understanding Earth, Press and Siever. Freeman.  The Types of Metamorphism Orogenic Metamorphism Polymetamorphic patterns are common Often associated with continental collision The metamorphism is not considered contact metamorphism because it develops regionally, the pattern of metamorphic grade does not relate directly to the proximity of the igneous contacts Regional contact metamorphism - If there are numerous plutons which are closely spaced, and the metamorphism is spread over a large area  Regional Metamorphism: the Scottish Highlands George Barrow (1893, 1912): one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphism SE Highlands of Scotland – subjected to the Caledonian Orogeny at ~ 500 Ma Deformation was intense: rocks folded into a series of nappes Numerous granites also intruded toward the end of the orogeny, after the main regional metamorphism  Barrow’s Area Figure 1.7. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.  Regional Metamorphism: the Scottish Highlands Barrow noted significant and systematic mineralogical changes in the pelitic rocks He subdivided the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased (which correlates with an increase in grain size) The new mineral that characterizes a zone is termed an index mineral The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are: Chlorite zone: Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite (Na-rich end-member of plagioclase) Biotite zone: Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite Garnet zone: Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase (more intermediate composition plagioclase) Staurolite zone: Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist Kyanite zone: Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite Sillimanite zone: Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite. Some kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5)  This sequence of zones is now recognized in other orogenic belts around the world Now so well established that the zones are often referred to as Barrovian zones (after Barrow) The P-T conditions are referred to as “Barrovian-type” metamorphism (fairly typical of many belts) The Barrovian zones are now extended to a much larger area of the Scottish Highlands Isograd = line that separates the zones (a line in the field of constant metamorphic grade). It separates one zone characterized by a certain mineral assemblage and distinctive, characteristic mineral, from the next one containing an additional, new, characteristic mineral in addition to the rest Figure 1.8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London. To summarize: An isograd - first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade When one crosses an isograd, such as the biotite isograd, one enters the biotite zone Zones thus have the same name as the isograd that forms the low-grade boundary of that zone An index mineral may still be stable in higher grade zones A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district Pelitic compositions are similar, but the sequence of isograds is:  chlorite  biotite  cordierite  andalusite  sillimanite  The stability field of andalusite occurs at pressures less than 0.37 GPa / 3.7 kbars (~10 km), while kyanite sillimanite at the sillimanite isograd only above this pressure Metamorphism in the Buchan area thus occurred at lower P than in the classic Barrovian area Figure 1.9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).