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Granitic Rocks (Chapter 18) Image: http://www.yosemite.ca.us/library/granite_crags/yosemite_watercolors.html Granitoids "Granitoids" (sensu lato): loosely applied to a wide range of felsic plutonic rocks 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 postdate the thickening event by tens of millions of years. Image source: Winter, 2001 Granitoids "Granitoids" (sensu lato): loosely applied to a wide range of felsic plutonic rocks A few broad generalizations: 2) Because the crust is solid in its normal state, a thermal disturbance is required to form granitoids 3) Most granitoids are derived by crustal anatexis, but that the mantle may also be involved as a source of heat for crustal anatexis, or a source of melt. Image source: Winter, 2001 Granitoids Petrographic characteristics of granitoid rocks Image source: Darrell Henry Common minor minerals: apatite, zircon, magnetite, ilmenite, monazite, titanite, tourmaline, allanite, fluorite and pyrite medium-to-coarsed grained rocks reflect slow cooling a volatiles. • dominated by plagioclase (generally first), quartz and Kfeldspar • Hornblende (brown to green) and biotite are the chief mafic minerals, and Al-in-hornblende geobarometer can yield P of crystallization. • Muscovite may be present as melt phase or are a secondary mineral. • Cpx may be found in the more mafic granitoids Granitoids Petrographic characteristics of granitoid rocks Backscattered electron image of a zircon from the Strontian Granite, Scotland. The grain has a rounded, un-zoned core (dark) that is an inherited hightemperature non-melted crystal from the pre-granite source. The core is surrounded by a zoned epitaxial igneous overgrowth rim, crystallized from the cooling granite. Image source: From Paterson et al. (1992), Trans. Royal. Soc. Edinburgh. 83, 459-471. Also Geol. Soc. Amer. Spec. Paper, 272, 459471. Granitoids Textures in granitoid rocks At relatively low PH2O, a single feldspar will crystallize and then undergo further exsolution (a, b) [hypersolvus] Image source: John Winter, 2001 At relatively high PH2O, two feldspars will crystallize with possible further exsolution of each phase (c) [subsolvus] Granitoids Petrographic characteristics of granitoid rocks Myrmekites • intergrowth of dendritic quartz and plagioclase at Kfeldspar/plagioclase interface • texture likely related to subsolidus deformation Myrmekite patch that appears to be replacing microcline. Faint twins in the myrmekite clearly shows that the probably quartz "worms" are in a plagioclase matrix. image source: Kurt Hollocher Granitoids Petrographic characteristics of granitoid rocks Table 18-1. The Various Types of Enclaves Name Nature Margin Shape Features Xenolith piece of country rocks sharp to gradual angular to ovoid contact metamorphic texture and minerals Xenocryst isolated foreign crystal sharp angular corroded reaction rim Surmicaceous Enclave residue of melting (restite) Schlieren disrupted enclave gradual oblate coplanar orientation Felsic Microgranular Enclave disrupted fine-grained margin sharp to gradual ovoid fine-granied igneous texture Mafic Microgranular Enclave Blob of coeval mafic magma mostly sharp ovoid fine-granied igneous texture Cumulate Enclave (Autolith) disrupted cumulate mostly gradual ovoid coarse-grained cumulate texture After Didier and Barbarin (1991, p. 20). sharp, lenticular metamorphic texture biotite rim micas, Al-rich minerals Granitoids Granitoid Chemistry biotite muscovite cordierite andalusite garnet pyroxene hornblende biotite aegirine riebeckite arfvedsonite CaO CaO moles CaO K2O K2O Al2O3 K2O Al2O3 Peraluminous Metaluminous Image source: John Winter, 2001 • most commonly calcalkaline Al2O3 Na2O Na2O • Composition of granitoid controlled by composition of source, pressure, temperature, degree of partial melting, and the nature of differentiation. Na2O Peralkaline • variable aluminum saturation that generally depends on the source of melting Granitoids Granitoid Chemistry The fact that most granitoids plot near the low P ternary minimum melts are most consistent with melting of a quartzofeldspathic crustal parent. Note the effects of increasing pressure and the An, B, and F contents on the position of the thermal minima. Image source: John Winter, 2001 Granitoids Granitoid Chemistry • MORB-normalized spider diagrams for the analyses in Table 18-2 • The subduction zone granitoids display the typical decoupling of the LIL/HFS elements. • The plagiogranite is more similar to patterns associated with MORBs. Image source: John Winter, 2001 Granitoids Crustal melting (anatexis) initiated due to increase in mantle-produced heat or thickened crust. two possible ways to produce melts H2O-saturated melting - produces minor amounts of melt due to small amounts of trapped water Migmatite from the Hellroaring Plateau in the Beartooth Mtns (MT) with felsic granitoid melt and restite of quartz + plagioclase + K-feldspar + biotite + garnet + sillimanite. Image source: Darrell Henry Dehydration melting – fluid from breakdown of hydrous minerals: e.g. Mu + Pl + Qtz = Kfs + Al-sil + Melt or Bt + Pl + Al-sil + Qtz = Kfs + Grt + Melt if produced melts are <30% the melt generally stays with the source to produce a migmatite. Granitoids Crustal melting (anatexis) (a) Simplified P-T phase diagram for melting of aluminous quartzofeldspathic materials and (b) quantity of melt generated during the melting of muscovite-biotite-bearing crustal source rocks Shaded areas in (a) indicate melt generation. Granitoids Classifications of Granitoids – (genetic classification) Table 18-3. The S-I-A-M Classification of Granitoids SiO2 K2O/Na2O Type M 46-70% low Ca, Sr high I 53-76% low high in mafic rocks S 65-74% high low A/(C+N+K)* low Fe3+/Fe2+ Cr, Ni low 18O < 9‰ low < 9‰ low high > 9‰ var low var low low: metal- moderate uminous to peraluminous high metaluminous A high 77% Na2O high * molar Al2O3/(CaO+Na2O+K2O) low var peralkaline 87 Sr/86Sr Misc Petrogenesis < 0.705 Low Rb, Th, U Subduction zone Low LIL and HFS or ocean-intraplate Mantle-derived < 0.705 high LIL/HFS Subduction zone med. Rb, Th, U Infracrustal hornblende Mafic to intermed. magnetite igneous source > 0.707 variable LIL/HFS Subduction zone high Rb, Th, U biotite, cordierite Supracrustal Als, Grt, Ilmenite sedimentary source var low LIL/HFS Anorogenic high Fe/Mg Stable craton high Ga/Al Rift zone High REE, Zr High F, Cl Data from White and Chappell (1983), Clarke (1992), Whalen (1985) • I-type granitoids (igneous source) - partial melts of mantle-derived mafic rocks (underplated basaltic melts?); contain abundant hornblende and magnetite Granitoids Classifications of Granitoids – (genetic classification) Table 18-3. The S-I-A-M Classification of Granitoids SiO2 K2O/Na2O Type M 46-70% low Ca, Sr high I 53-76% low high in mafic rocks S 65-74% high low A/(C+N+K)* low Fe3+/Fe2+ Cr, Ni low 18O < 9‰ low < 9‰ low high > 9‰ var low var low low: metal- moderate uminous to peraluminous high metaluminous A high 77% Na2O high * molar Al2O3/(CaO+Na2O+K2O) low var peralkaline 87 Sr/86Sr Misc Petrogenesis < 0.705 Low Rb, Th, U Subduction zone Low LIL and HFS or ocean-intraplate Mantle-derived < 0.705 high LIL/HFS Subduction zone med. Rb, Th, U Infracrustal hornblende Mafic to intermed. magnetite igneous source > 0.707 variable LIL/HFS Subduction zone high Rb, Th, U biotite, cordierite Supracrustal Als, Grt, Ilmenite sedimentary source var low LIL/HFS Anorogenic high Fe/Mg Stable craton high Ga/Al Rift zone High REE, Zr High F, Cl Data from White and Chappell (1983), Clarke (1992), Whalen (1985) • S-type granitoids (sedimentary source) - partial melts of aluminous sedimentary rocks; w/ Al-rich minerals (Alsilicates; cordierite, garnet), biotite (brown) and ilmenite Granitoids Classifications of Granitoids – (genetic classification) Table 18-3. The S-I-A-M Classification of Granitoids SiO2 K2O/Na2O Type M 46-70% low Ca, Sr high I 53-76% low high in mafic rocks S 65-74% high low A/(C+N+K)* low Fe3+/Fe2+ Cr, Ni low 18O < 9‰ low < 9‰ low high > 9‰ var low var low low: metal- moderate uminous to peraluminous high metaluminous A high 77% Na2O high low var peralkaline * molar Al2O3/(CaO+Na2O+K2O) 87 Sr/86Sr Misc Petrogenesis < 0.705 Low Rb, Th, U Subduction zone Low LIL and HFS or ocean-intraplate Mantle-derived < 0.705 high LIL/HFS Subduction zone med. Rb, Th, U Infracrustal hornblende Mafic to intermed. magnetite igneous source > 0.707 variable LIL/HFS Subduction zone high Rb, Th, U biotite, cordierite Supracrustal Als, Grt, Ilmenite sedimentary source var low LIL/HFS Anorogenic high Fe/Mg Stable craton high Ga/Al Rift zone High REE, Zr High F, Cl Data from White and Chappell (1983), Clarke (1992), Whalen (1985) • M-type granitoids (direct mantle source) - e.g. plagiogranites in ophiolites • A-type granitoids (on anorogenic settings) - peralkaline melts in rifts Granitoids Classifications of Granitoids (tectonic setting) Granitoids Classifications of Granitoids (tectonic setting)