Granitoid Rocks

“Granitoids”: loosely applied to a wide range of felsic plutonic rocks Granitoid emplacement may commonly be associated with cogenetic and chemically equivalent felsic volcanism. Associated volcanics are common and have same origin, but are typically eroded away

A few broad generalizations: 1) Most granitoids of significant volume occur in areas where the continental crust has been thickened by orogeny, either continental arc subduction or collision of sialic masses. Many granites, however, may post-date the thickening event by tens of millions of years. 2) Because the crust is solid in its normal state, some thermal disturbance is required to form granitoids 3) Most workers are of the opinion that the majority of granitoids are derived by crustal anatexis, but that the mantle may also be involved. The mantle contribution may range from that of a source of heat for crustal anatexis, or it may be the source of material as well

Petrography: -Medium to coarse grain showing slow cooling and the presence of volatiles. Pl, Q and Alc Feld are the predominant phases, although in some variety of granitoids the variation is common, e.g. tonalite. Hbl and Bi are dominant mafic phases. Mus is common in aluminous granites and occur as both primary igneous crystals and secondary replacement. Cpx is subordinate and present principally in more mafic granitoids. fayalytic olivine occurs in some alkaline granites. Opx occurs in high-temperature anhydrous granitoids (charnockites)

crystallization sequence: is not very simple as mafic rocks and volcanics . Generally first to appear are accessory minerals such as zircon, apatite, pyrite, ilmenite, etc., followed by pl and ferromagnesian minerals. Q and alc feld are commonly late interstitial phases. although variation in P, T, f H2O, fO2 and bulk composition may makes some complexities in crystallization sequence. Corse granophyric or graphic texture reflecting eutectic Q-alc feld co-crystallization are common. Low Ca alkaline granites may have both sodic and potassic feldspar (subsulvus granites) due to high H2O pressure and intersecting solvus to solidus. The alc feld may occur as one single phase (hypersolvus granites) denoting crystallization under low H2O pressure and above the solvus. although only a few percent An component shifts the feld into the ternary solvus so the most granites have two feldspars regardless of PH2O .

Perthites and myrmekite are common. Minor or accessory minerals in granitoids include - apatite (Ca5(PO4)3(F, Cl, OH)) - monazite ((Ce, La, Pr, Nd, Th, Y)PO4) - Zircon( ZrSiO4) - ilmenite ((FeTiO3)) - sphene (CaTiOSiO4), - allanite ((Ce,Ca,Y,La)2(Al,Fe+3)3(SiO4)3(OH)), - tourmaline ((Ca,K,Na)(Al,Fe,Li,Mg,Mn)3(Al, Cr, Fe, V)6 (BO3)3(Si, Al, B)6O18(OH,F)4), - pyrite ( Fe S2), Fluorite (CaF2) Magnetite (Fe3O4), aluminous granites may contain Al-bearing phases such as garnet, cordierite, sillimanite or andalusite. Although these phases may represent, in part at least, minerals from source rock remained unmelted during magma formation (restite mineral).

one clear restite example is the polygenetic zircon xenocrysts. zircons are very hard and refractory. They can thus be eroded from an igneous source, transported, deposited, buried, and metamorphosed. Zircons of purely metamorphic origin are generally unzoned. if the metamorphic rocks are partially melted, their zircons (unmelted) may be carried off by magma so this zircon is not igneous zircon but inherited one (the centre of zircon in image) which will result in high age determination. restite zircon provide nuclei for the epitaxial growth of igneous zircon The image: Backscattered electron image of a zircon from the Strontian Granite, Scotland. The grain has a rounded, un-zoned core (dark) that is an inherited high-temperature non-melted crystal from the pre-granite source. The core is surrounded by a zoned epitaxial igneous overgrowth rim, crystallized from the cooling granite. From Paterson et al. (1992), Trans. Royal. Soc. Edinburgh. 83, 459-471. Also Geol. Soc. Amer. Spec. Paper, 272, 459-471.

Xenoliths are commonly observed in intrusive rocks and have variable origin Table 18-1. Didier, J. and Barbarin (1991) The different type of enclaves in granites: Nomenclature. In J. Didier and B. Barbarin (1991) (eds.), Enclaves in Granite Petrology. Elsevier. Amsterdam, pp. 19-23.

Geochemistry: The chemical composition of a granitoid is controlled by chemical composition of the source, the pressure, temperature, degree of partial melting and the nature and extent of subsequent processes.

- S and A types have more crustal components. -I and M types are believed to be mantle derived either as fractionated mantle partial melts or as remelts of mantle derived gabbroic crustal underplates. - S and A types have more crustal components. Table 18-2. Representative Chemical Analyses of Selected Granitoid Types. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 18-2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid Rocks. Chapman Hall.

Figure 18-3. The Ab-Or-Qtz system with the ternary cotectic curves and eutectic minima from 0.1 to 3 GPa. Included is the locus of most granite compositions from Figure 11-2 (shaded) and the plotted positions of the norms from the analyses in Table 18-2. Note the effects of increasing pressure and the An, B, and F contents on the position of the thermal minima. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

-Subduction zone granitoids show higher LILE/HFSE ratio. - M type granitoid is similar to MORB Figure 18-4. MORB-normalized spider diagrams for the analyses in Table 18-2 . From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Crustal melting: - Some of the granitoids originate from crustal melting. the melting in crust can be start in thickened crusts or the crusts heated by mantle melts. if we consider a Mus-Bi- Als gneissic source, two example of the dehydration reactions are shown in figure. The temperature and nature of melting will depends upon the rock composition, the H2O content, and P-T-t paths.

P-T-t paths can be clockwise: in situation involving continental collision the underthrust crust will experience increased pressure first, and then it will heat up. when crustal rocks being underplated by mantle-derived or involved in post-orogenic collapse the P-T-t paths can be even counterclockwise. crustal anatexis is predominantly a H2O-undersaturated phenomenon because slow heating and temperature of high grade metamorphism will drive off most excess water existing as a free fluid phase. The H2O presented in in the lower crust is mainly contained in the hydrous minerals. The H2O- saturated melting curve is thus of little importance to the melting of most deep-seated rocks. Even if H2O- saturated melting occur the amount of melt is small and well bellow the critical melt fraction required to become mobile.

So the melting in crust is H2O-undersaturated. One of the dehydration reaction is muscovite breakdown which the amount of melt generated by this reaction depends upon the amount of reactants (muscovite in particular). The amount of melt by muscovite breakdown is shown in figure (<10%). The amount of melt is still below the critical fraction for melt mobilization and the result is a migmatite. When biotite begins to breakdown, enough melt can be generate (10 to 60% of the rock). although only two reaction is shown in figure other reactions and other hydrous minerals may breakdown and create melts in different systems.

Figure 18-5. a. Simplified P-T phase diagram and b Figure 18-5. a. Simplified P-T phase diagram and b. quantity of melt generated during the melting of muscovite-biotite-bearing crustal source rocks, after Clarke (1992) Granitoid Rocks. Chapman Hall, London; and Vielzeuf and Holloway (1988) Contrib. Mineral. Petrol., 98, 257-276. Shaded areas in (a) indicate melt generation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Classification of granitoids: - I-type : partial melting of a mafic mantle derived igneous source material (probably of a subcrustal underplate). The common oxide is magnetite, the rock is Hbl-rich and are either metaluminous to weakly peraluminous. S-type: partial melting of peraluminous sediments. The common oxide is ilmenite, and the rock is Bi-rich. They may also contain cordierite, muscovite, andalusite, sillimanite and/or garnet. M-type: direct mantle source. For example in immature arc having no thick sialic crust beneath which to pond underplated magmas and oceanic plagiogranites found in ophiolite- oceanic crust. I-types are two-stage remelts of underplates but M-types are fractionation products of single-stage mantle melts. A –type: anorogenic situations. Higher in SiO2 (mafic and intermediate types are rare), alkalis, Fe/Mg, F and Cl, Ga/Al, Zr, Nb, Ga, Y, and Ce than I-types.

Table 18-4. A Classification of Granitoid Rocks Based on Tectonic Setting. After Pitcher (1983) in K. J. Hsü (ed.), Mountain Building Processes, Academic Press, London; Pitcher (1993), The Nature and Origin of Granite, Blackie, London; and Barbarin (1990) Geol. Journal, 25, 227-238. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 18: Granitoid Rocks Figure 18-6. A simple modification of Figure 16-17 showing the effect of subducting a slab of continental crust, which causes the dip of the subducted plate to shallow as subduction ceases and the isotherms begin to “relax” (return to a steady-state value). Thickened crust, whether created by underthrusting (as shown) or by folding or flow, leads to sialic crust at depths and temperatures sufficient to cause partial melting. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 18: Granitoid Rocks Figure 18-7. Schematic cross section of the Himalayas showing the dehydration and partial melting zones that produced the leucogranites. After France-Lanord and Le Fort (1988) Trans. Roy. Soc. Edinburgh, 79, 183-195. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 18-8. Schematic models for the uplift and extensional collapse of orogenically thickened continental crust. Subduction leads to thickened crust by either continental collision (a1) or compression of the continental arc (a2), each with its characteristic orogenic magmatism. Both mechanisms lead to a thickened crust, and probably thickened mechanical and thermal boundary layers (“MBL” and “TBL”) as in (b) Following the stable situation in (b), either compression ceases (c1) or the thick dense thermal boundary layer is removed by delamination or convective erosion (c2). The result is extension and collapse of the crust, thinning of the lithosphere, and rise of hot asthenosphere (d). The increased heat flux in (d), plus the decompression melting of the rising asthenosphere, results in bimodal post-orogenic magmatism with both mafic mantle and silicic crustal melts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 18: Granitoid Rocks Figure 18-9. Examples of granitoid discrimination diagrams used by Pearce et al. (1984, J. Petrol., 25, 956-983) with the granitoids of Table 18-2 plotted. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.