GRANITOID ROCKS The rock granite (or its volcanic equivalent rhyolite) has a limited range of composition that results in crystallization of equivalent amounts of quartz, plagioclase, and K-feldspar. Granites (sensu stricto) constitute only a limited portion of quartz and plagioclase-rich magma series that may include tonalites, granodiorites, diorites, quartz monzonites, and granites. Therefore, the term granitoid is more useful. It includes all rocks that are rich in quartz and plagioclase. Granitoid rocks include: plutonic equivalents of subduction-related calc-alkaline volcanics that involve partial melting of the mantle rocks that formed by partial melting of the mafic lower continental crust rocks that formed by partial melting of silicic crustal material (major) The continental crust is involved somehow in the generation of the vast majority of granitoid magmas. I. Occurrence Orogenic associations mountain belts associated with Andean type subductions (e.g, the Andes, Sierra Nevada) orogenic belts that formed by continent-continent collision (e.g., Himalayas, Trans-Hudson orogen) extensional “anorogenic” terranes (e.g., Basin and Range Province, St. Francois Mountains). Style of occurrence Large batholiths – The batholiths include numerous individual granitoid plutons. They primarily occur in mountain belts associated with Andean type subductions. Individual distributed plutons – This style of occurrence is common for granites that formed during extension. Volcanic rocks are also commonly associated. Composite plutons formed from numerous dikes – This style is common for granites that formed by partial melting of metasedimentary rocks during continental convergence. Pegmatites are also associated with this style of granite plutonism. II. Mineralogy Major defining minerals: quartz, plagioclase, alkali feldspar Other major minerals: Biotite, hornblende, muscovite and tourmaline are common, depending on the chemical composition of the granitoid. Garnet and orthopyroxene are less common. Ubiquitous trace minerals: zircon, apatite Other trace minerals: sphene, oxides, allanite, monazite, xenotime III. Classification “Alphabet soup” classification There have been many classifications proposed for granitoids. However, the one most often used is the S-I classification devised by two Australians Bruce Chappel and Alan White. Their classification is based on the presumed source rocks – sedimentary (pelitic) and igneous (mafic) - that impart characteristic composition on the granitoids. However, people found additional granitoids with distinct characteristics - anorogenic (A-type) and those derived by fractional crystallization of mafic magma (M-type). So now we have the S-I-A-M classification. The problem is that this classification is hybrid, because it mixes composition types (S, I), with tectonic setting (A-type) and petrogenetic process (M-type). A much more useful classification is one based on tectonic setting. Fortunately, one can relate the two classifications to have a sensible view of the meaning of different types of granitoids. Because the “alphabet soup” classification is used as a means of communication, we will discuss it first. Characteristic S-type I-type A-type M-type (minor) SiO2 65-79% 53-76% 60-80% 54-73% Al2O3/(Na2O+K2O+CaO) >1.1 <1.1 variable to <1 <1.2 Na20 in silicic rocks variable >3.2% >2.8% >3.2% K2O/ Na2O high low usually high very low Fe3+/Fe2++Fe3+ low medium-high medium high Eu low? low-high low ? Cr, Ni high low low ? normally >0.708 normally <0.708 0.703-0.712 usually <0.704 Nd -4 to -17 -4 to -9 ? ? Main Enclaves metasediments, amphibolite, diorite enclaves rare Hornblende Key CIPW Minerals >1% C <1% C di common ? Key Modal Minerals muscovite, Hornblende, biotite, biotite, sphene, sillimanite, magnetite ilmenite, garnet biotite, alkali Hornblende, pyrox., alkali Clinopyroxene amph.,magnetite, biotite, magnetite fluorite Common Range of Rock types leucogranite to granodiorite alkali granite to anorthosite Associated Volcanic Rocks siliceous ash flows 87Sr/86Sr initial granite to gabbro rhyollite ash flows, dacite, andesite alkalic rhyolites quartz diorite to gabbro Calc-alkalic Andesite, dacite, Rhyolite; tholeiite Sources: Chappell and White (1974), White and Chappell (1977, 1983), Loiselle and Wones (1979), White (1979), Collins et al. (1982), Didier, Duthon, and Larneyre (1982), W. S. Pitcher (1982), McHone and Butler (1984), A. J. R. White et al. (1986), J. B. Whalen, Chappell, and Chappell (1987), Barbarin (1990a), Eby (1990). Alumina Saturation Index The ratio Al2O3/(Na2O + K2O + CaO) is in terms of molar proportion. The related terminology is: peraluminous: Al2O3 > (Na2O + K2O + CaO) or Al2O3/(Na2O + K2O + CaO) > 1.1; Rock will probably have muscovite and may have garnet; will be corundum-normative. metaluminous: Al2O3 < (Na2O + K2O + CaO) but Al2O3 > (Na2O + K2O) or Al2O3/(Na2O + K2O + CaO) 1.0; rock may have hornblende subaluminous: Al2O3 < (Na2O + K2O + CaO) but Al2O3 = (Na2O + K2O); Al2O3/(Na2O + K2O + CaO) < 1.0; rock will probably have hornblende; will have Di in the norm. peralkaline: Al2O3 < (Na2O + K2O) or Al2O3/(Na2O + K2O + CaO) << 1.0; rock will probably have lot of K-feldspar in the norm, probably a feldspathoid or very little, if any quartz. Tectonic Classification A tectonic classification relates chemical and intrusive style of granitoids to periods of orogenic cycles or periods of extension. Some notes on Table above: M-type granites of island arcs are rare. They form by fractional crystallization of calc-alkaline or tholeiitic magmas. M-type granites of ocean ridges are extremely rare. They form by fractional crystallization of calc-alkaline or tholeiitic magmas. The Transitional Granitoids are also called Post-orogenic. Note that some granitologists also refer to them as Anorogenic, because they form during extension. IV. Petrogenesis The granite problem (or, not really): During the first half of the 20th century, most people believed, following Bowen’s idea of fractional crystallization, that granitoids represent the residual melts formed by fractional crystallization of mafic magmas. This remains true of the M-type granites. However, many mountain belts contain huge volumes of granitoids. To get that volume by fractional crystallization would require >10 times greater volume of the mafic magmas. The huge volumes of mafic magmas are not observed. Therefore, the general belief now is that granitoids are generated primarily by the partial melting of various source rocks. Relationship between tectonic setting, source rocks, and granitoid types 1) Active continental margin - Most prevalent are calc-alkaline I-type granitoids, including tonalites and granodiorites. These are thought to form by partial melting of amphibolitic lower continental crust or gabbroic rocks crystallized below the Moho. The mineralogy of the parent rocks is dominated by amphibole and plagioclase. Magma mixing between the granitoids and basalts intruded into them is common. 2) Continental collision - The most prevalent granitoids are S-type. They form by partial melting of shales and graywackes. Some magmas bring up large amount of restite, pieces of the parent rocks. In some cases they do not. The melts without restite usually contain <1% MgO, FeO, and CaO; they are called leucogranites. Pegmatites are also common. 3) Post-orogenic extension - The source rocks for the granitoids are usually granulitic silicic rocks in the lower crust. Formation of the silicic magma involves very high temperatures (~1000° C). The high temperatures are achieved by intrusion of mafic magma from the mantle. Chemically bimodal volcanism is common, as is magma mixing in magma chambers. Melt generation (anatexis) Anatexis is a name given to the process that generates partial melts in a parent rock. 1) tonalites and granodiorites of active continental margins The generation of these granitoids involves the following anatectic reactions in amphibolites or gabbros in the lower crust or below the Moho: 33 hornblende + 5 plagioclase + 9 qtz 6 opx + 20 cpx + 19 tonalite melt or cpx + opx + 13 plagioclase 18 tonalite melt These reactions produce melts high in Ca, Al, and Na that upon crystallization lead to rocks rich in plagioclase with some quartz. 2) true granites in continental collisional orogens The compositions of true granites is expressed well by the ternary NaAlSi3O8-KAlSi3O8-SiO2 system: Note from the diagram on the right, that formation of true granites near a eutectic at high pressure requires the presence of plagioclase, alkali feldspar and quartz. Plagioclase and quartz are usually found in pelitic schists or metagraywackes. However, the required KAlSi3O8 component is found in muscovite or biotite. Thus, generation of true granites involves the following incongruent melting reactions: KAl2AlSi3O10(OH)2 + SiO2 + NaAlSi3O8 granite melt with H2O + Al2SiO5 K(Mg,Fe)3AlSi3O10(OH)2 + NaAlSi3O8 granite melt with H2O + 3(Fe,Mg)SiO3 Water is an important component in granite systems, because as shown in the above reactions, the hydrous minerals contribute it to the melt. The above and similar reactions can be illustrated in a P-T diagram appropriate for the conditions in the continental crust: The diagram also shows the granite solidus when free H2O is present (ab + ksp + qtz = L) and a typical pressure-temperature-time (P-T-t) path of a portion of a deep crust during crustal thickening associated with continental collision. A pelitic rock following this and similar paths will undergo partial melting to generate an S-type granite when the melting reactions are intersected. Migmatites The process of granite formation is evident in migmatites. Migmatites are rocks from which granitoid melts were not removed. The light parts of migmatites contain the solidified granitoid melts. The dark parts of migmatites contain the residual minerals. For partial melting of amphibolites or gabbros to produce the tonalites-diorite-granodiorite series (I-type granites), the residual minerals are usually hornblende, cpx, opx, (± garnet). For partial melting of metapelites or metagraywackes to produce S-type granites, the residual minerals are usually biotite, aluminosilicate, garnet, Ca-rich plagioclase. V. Magma Ascent Style of Ascent Up to approximately the late 1970’s, most people believed that granitic magmas ascended through the crust as diapirs. Now, probably the majority of people believe that granitic magmas ascent through dikes, as the magmas fracture the overlying rocks. Another possible mechanism is stoping, in which the magmas ascent by breaking-up the overlying rocks. Level of Emplacement in the Crust The level of magma emplacement in the crust is to a large extent controlled by the amount of water in the magma. Consider the diagram below that shows the amount of water that a granite melt must have as a function of P and T: The diagram shows the liquidi of a granite melt as a function of the amount of water dissolved in the melt (thin lines) and the granite solidus. The liquidi essentially show how much water must be in the melt at the P-T conditions defined by a given liquidus. If the magma has less water than indicated, it will become solid. The diagram shows that a magma that is generated at a higher temperature will have lower H2O content than a magma that is generated at a lower temperature. Upon adiabatic ascent through the crust, the high-temperature, dry magma can reach the surface, whereas the lowertemperature magma with more H2O will hit the water-saturated solidus before reaching the surface. Therefore, only relatively dry, high-temperature granitic magmas can lead to volcanism. Magmas that initially contained a high concentration of H2O will be deep-seated. Note that S-type magmas, which often form by relatively low-T, muscovite melting of metapelites are almost never volcanic. On the other hand, high-T magmas that form during extension very often have volcanic centers. VI. Magma Chamber Process Granitoid magmas are generally too viscous to undergo significant accumulation of crystals, as often happens in mafic magmas. However, two important mechanisms by which compositions of granitoid magmas change occur in magma chambers – magma mixing, and side-wall fractional crystallization. This diagram shows processes that typically occur in calc-alkaline granitoid magma chambers. Magma mixing In convergent orogens and during extension, mafic magmas from the mantle may go up the same conduits as the granitoid magmas. Very often, the two types of melts end-up in the same magma chamber. Usually, the mafic melt pool at the bottom of the magma chamber, because they are denser than the silicic melts. However, because of convection in magma chambers, two contrasting melts may mix, sometimes completely. It is possible that many diorites and granodiorites may be the products of mixing mafic and silicic melts. A good example of the magma mixing process is the Tuolumne Meadows magma series in the Yosemite National Park. Side-wall (boundary layer) crystallization This mechanism was proposed by Alexander McBirney in the 1980’s to explain the commonly inferred chemical stratification in silicic magma chambers. For example, in the Bishop tuff, a thick pile of extrusive rocks from the Long Valley volcanic complex, more evolved (silicic) compositions occur at the bottom, whereas the more mafic varieties occur at the top of the erupted pile. The rocks lowest in the pile crystallized from magma from the top of the underlying magma chamber and the top rocks represent deeper parts of the magma chamber. The chemical stratification that is thought to occur in magma chambers develops when magma begins to crystallize from the margins inward. There, the local interstitial residual liquid becomes silica- and H2Orich. Because it is more buoyant than the crystals and melt further inside the chamber, it migrates along the sidewall to the top of the magma chamber. The high H2O content of the magma at the top of the magma chamber promotes explosive eruption, sometimes large enough to partially empty the chamber and create a caldera. VII. Pegmatites Pegmatites are very low-viscosity hydrous melts. They can be residual melts resulting from fractional crystallization of granites or partial melts from metapelites. They typically contain extremely large crystals and are rich in rare elements such as Li, B, P, Be, Ta, Cs. These elements permit pegmatites to crystallize at extremely low temperatures, down to ~350° C. Some minerals that occur in pegmatites (aside from quartz, microcline, and albite, include: spodumene: LiAlSi2O6 (pyroxene) lepidolite: K(Li,Al)2-3AlSi3O10(OH)2 (mica) beryl: Be3Al2Si6O18 tourmaline: (Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)6(BO3)3Si6O18(OH)4