Question

2. The highest grades of regional metamorphism of mica schists are commonly associated with abundant granite. The accompanying map 2 illustrates the distribution of granite and some mineral assemblages common in pelitic schists: e: Qtz + Ms+ Pl+ Bt+ Grt+Sil f: Qtz +Pl + Kfs+ Grt+Sil g: Qtz+ Pl + Kfs +Grt + Ky a: Qtz+ Ms+PI+St+ Bt + And b: Qtz+ Ms+Pl+ St + Bt+ Grt c: Qtz + Ms +Pl+ Bt+Grt d: Qtz + Ms+Pl+St +Bt+ Sil The stability fields of these mineral assemblages are illustrated on the P-T diagram. The upper stability of Qtz + Ms +St extends through all three fields for aluminum silicate polymorphs. The upper stability of Qtz +Ms changes to a reaction limiting Qtz+ Ms + Pl. Assume that St is exhausted first at the low temperature rxn curve, and Ms is exhausted first at the high temperature rxn curve for the bulk composition represented by this phase diagram. 10 Kfs Ms Als P1 liq Ky Sil 5 MsGrt kbar St' Als Ms And Kfs Als 700 500 T (C) Als 500 700 T (C) a). Illustrate the stability fields of assemblages a -g on the P-T diagram. In each P-T field on the diagram, list in alphabetical order the assemblages that are stable. Assume that liquid (liq) crystallized to a granitic rock b). Which of these assemblages are most critical for placing constraints on the metamorphic field gradient? Explain why. c). On the map, draw lines that mark the first appearance of only the critical assemblages with increasing grade. Label clearly the high-grade side of the isograds d). Show on the P-T diagram, the approximate metamorphic field gradient defined by this hypothetical metamorphic map. e). Using the phase diagram, what minimum differences in pressure are implied by this MFG? Which lines on the metamorphic map mark the first appearance of assemblages that define this minimum difference of pressure? g). From your knowledge of the pressure gradient in continental crust, what thickness of crust is required to cause this change in pressure? h). Using the scale of the map, compare the differences of pressure implied by the map with the differences implied by the phase diagram. What can you say about the orientation of this erosion surface at the time of peak metamorphism? Map 2 IN d e C 10 km

Answer 1

l o Ms 2 C a) 70 o T (%) b) a, d, e,f, asing tay