User:ObliqueFault/sandbox-Granitoid-Adding content draft

RE: Evaluating the Original Granitoid article :

The original article is below: A granitoid or granitic rock is a variety of coarse grained plutonic rock — granite or similar — which mineralogically is composed predominantly of feldspar, quartz and mica. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite, tonalite and trondhjemite. Many are created by continental volcanic arc subduction or the collision of sialic masses. Volcanic rocks are common with granitoids and typically have the same origins. However, they are normally worn away after years of erosion.
 * a distinctive definition of Granitoid is missing; there is no clear distinction between granitoid and granites & granitic rock.
 * there is no clear structure to common features of granitoids, and yet the characteristic features that are exceptions or not as common are listed.
 * Aside from this draft, but ideally in the next adding content exercise, the various Systems of Granitoid Classification and what they convey to the understanding of particular Granitoids will be developed. However in this draft, they are only briefly described.

Many granitoid rocks are located in areas that have experienced crustal thickening during orogenies but others, known as anorogenic granitoids, are unrelated to convergent boundaries or subduction zones. These anorogenic granitoids may represent the deep sources for rift volcanism exposed where erosion has removed the volcanic rocks and other evidence of rifting. These A-type granitoids may have been produced by hotspots or mantle plumes.

NEW DRAFT of article: A Granitoid is a generic term for a diverse collection of coarse-grained igneous rocks that consist predominately of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz-poor monzonites to quartz-rich quartzolites. As only 2 of the 3 aforementioned defining mineral groups need to be presence to be a Granitoid, foid-bearing or feldspathoid rocks, meaning rocks predominately containing feldspars but no quartz, are also granitoids. The terms ‘granite’ and ‘granitic rock’ are commonly used interchangeably for granitoids; however granite is just a particular type of granitoid.

Granitoids are diverse and no one classification system for Granitoids can give a complete, unique and absolute characterization of the origin, compositional evolution and geodynamic environment for the genesis of a granitoid. Accordingly, multiple granitoid classification systems have been developed such as those based on: geochemistry, modal content, emplacement depth, and tectonic regime.

Granitoid generalizations
There are several generalizations that apply to majority of granitoids. Typically, granitoids occur where orogeny thickens continental crust either by subduction yielding a continental arc or by convergence yielding continental collisions. Generally, the evolution to granitoid magmas requires a thermal disturbance to ascent though continental crust. Most granitoids are generated from crustal anatexis, the partial melting of the crust;  however the mantle may contribute both heat and material. Granitoids can occur coeval with volcanic rocks that have equivalent chemical composition ( granites-rhyolites, syenite-trachyte, dacite-granodiorite etc.) however, these extrusive rocks are often eroded so just the plutonic rocks outcrop. Granitoids can form in all tectonic environments.

There are numerous exceptions to these generalizations. For example, granitoids can form in anorogenic  environments, a granitoid source rock can be from the mantle (ex. intraplate hotspots) and the melting mechanism can be radiogenic crustal heat.

Granitoid Classifications
** see Word document for each of these sections and their individual citations to merge with the outline

Modal content diagram and Granitoid vs. Granites
QAPF diagram → names and defines modal composition of granitoids. Granites are formally defined by IUGS as granitoids having a volume composition of  between 20-to-60% quartz and a plagioclase- to-total feldspar ratio percentage of between 10-to-65%. Hence true granites are only 2 fields on the QAPF diagram: syeno-granites(field 3a) and monzo-granites (field 3b). Some petrologists include up to 20% foid on the APF ternary of the QAPF diagram under the term granitoid; others limit granitoid to the complete QAP ternary.

Alphabet or S-I-A-M
I-type; S-type; M-type;  A-type;

Geochemistry
Alumina saturation index: Availability of Al for feldspars;

isotopic signatures identifying tectonic regime,.

Emplacement depth
Tabular intrusive bodies(sills, dikes, veins); Non-tabular bodies ( plutons);    contact relationship;  timing of intrusion;  all leading to qualify how Depth of Intrusion:  epizonal, mesozonal, and catazonal classification  describes granitoid attributes

Tectonic regime
granitoids are predominately formed by partial melting of silicic crust and minorly by partial meting of the mafic lower continental crust, and plutonic equivalents of calc-alkaline volcanics. These plutonic calc-alkalines form from subductive-related partial melting of the mantle. OIBs, Continental Arc, Continental Collision, post-orogenic uplift/collapse, continental rifting, hotspots, MORs and oceanic islands.

Composites
Fe#, MALI and ASI to define 16 composite types of granitoids.