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Granitic Rocks (Chapter 18) Image:

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1 Granitic Rocks (Chapter 18) Image:

2 Granitoids Image source: Winter, 2001 "Granitoids" (sensu lato): loosely applied to a wide range of felsic plutonic rocks 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. A few broad generalizations:

3 Granitoids Image source: Winter, 2001 "Granitoids" (sensu lato): loosely applied to a wide range of felsic plutonic rocks 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. A few broad generalizations:

4 Granitoids Image source: Darrell Henry Petrographic characteristics of granitoid rocks medium-to-coarsed grained rocks - reflect slow cooling a volatiles. dominated by plagioclase (generally first), quartz and K- feldspar 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 Common minor minerals: apatite, zircon, magnetite, ilmenite, monazite, titanite, tourmaline, allanite, fluorite and pyrite

5 Granitoids Image source: From Paterson et al. (1992), Trans. Royal. Soc. Edinburgh. 83, Also Geol. Soc. Amer. Spec. Paper, 272, 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 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.

6 Granitoids Textures in granitoid rocks Image source: John Winter, 2001 At relatively low P H2O, a single feldspar will crystallize and then undergo further exsolution (a, b) [hypersolvus] At relatively high P H2O, two feldspars will crystallize with possible further exsolution of each phase (c) [subsolvus]

7 Granitoids 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 Petrographic characteristics of granitoid rocks Myrmekites intergrowth of dendritic quartz and plagioclase at K- feldspar/plagioclase interface texture likely related to subsolidus deformation

8 Granitoids Petrographic characteristics of granitoid rocks

9 Granitoids Granitoid Chemistry Composition of granitoid controlled by composition of source, pressure, temperature, degree of partial melting, and the nature of differentiation. most commonly calc- alkaline variable aluminum saturation that generally depends on the source of melting Image source: John Winter, 2001

10 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

11 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

12 Granitoids Crustal melting (anatexis) two possible ways to produce melts H2O-saturated melting - produces minor amounts of melt due to small amounts of trapped water 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. initiated due to increase in mantle-produced heat or thickened crust. 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

13 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.

14 Granitoids Classifications of Granitoids – (genetic classification) I-type granitoids (igneous source) - partial melts of mantle-derived mafic rocks (underplated basaltic melts?); contain abundant hornblende and magnetite

15 Granitoids Classifications of Granitoids – (genetic classification) S-type granitoids (sedimentary source) - partial melts of aluminous sedimentary rocks; w/ Al-rich minerals (Al- silicates; cordierite, garnet), biotite (brown) and ilmenite

16 Granitoids Classifications of Granitoids – (genetic classification) M-type granitoids (direct mantle source) - e.g. plagiogranites in ophiolites A-type granitoids (on anorogenic settings) - peralkaline melts in rifts

17 Granitoids Classifications of Granitoids (tectonic setting)

18 Granitoids


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