Granite Formation and Classification

Study Notes

Granite Formation and Classification

Granites form from magma produced by partial melting of basic to intermediate igneous rocks in subduction zones, specifically at ocean-ocean or ocean-continent convergent margins.

S-type Granites

Commonly found at continent-continent (C-C) collision zones.
Peraluminous; the sedimentary component is derived from phyllosilicate minerals in graywackes and mudstones from continental crust.
Typically consist of two-mica granites, which include biotite and mica.

General Characteristics of Granites

Plutonic rocks; contain 20-60% quartz and 35-90% alkali to plagioclase feldspar.
Essential mineral groups include quartz and feldspars.

Geological Settings

Associated with Precambrian cratons and convergent margins.
Phanerozoic granitic plutonic belts are located along continent-ocean (C-O) subduction zones or at continent-continent (C-C) suture zones.

Granite Types

M-I-S types: Orogenic granites related to subduction zones.
A-type: Anorogenic granites that originate from processes not involving lithospheric plate interactions; enriched in alkaline elements and exhibit high K/Na ratios.

M-type Granites

Derived from mantle-derived parental magma; marked by low Sr87/Sr86 ratios (0.704).
Associated with calc-alkaline tonalites, quartz diorites, and gabbroic rocks containing major elements like quartz, feldspar, hornblende, clinopyroxene, biotite, and magnetite.
Commonly develop in island arc settings, often showing copper (Cu) and gold (Au) mineralizations.
Chemical composition includes SiO2 = 54-73%, low Na2O (2.8%), and high K2O/Na2O ratios.

A-type Granites

Formed from volcanic and tectonic activities unrelated to subduction and collision processes.
Characterized by a high concentration of alkaline elements and may have high concentrations of high-field strength (HFS) elements and rare earth elements (REE).
Typically depleted in magnesium (Mg), calcium (Ca), aluminum (Al), chromium (Cr), and nickel (Ni), with lower water content.
Often peralkaline, containing minerals like biotite, alkali pyroxene, and alkali amphiboles, with associates including alkali granite, syenite, and quartz syenite.
Commonly formed during the mid-Proterozoic era (1.1-1.4 billion years ago) with links to the assembly of the Columbia Supercontinent.

Al-type Granites

Result from the overthickening of continental lithosphere and involve the partial melting of residual silicic granulite rocks, previously generating I-type granites.

Description

Explore the fascinating world of granite formation and its classification through different geological settings. This quiz covers S-type granites, their characteristics, and the environments in which they form, emphasizing their significance in geology.