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Chapter 23: Metamorphic Textures. Textures are small-scale features of rocks. http://en.wikipedia.org/wiki/List_of_rock_textures. We have already examined the progressive metamorphism of Pelites, mudrocks. Relict Textures Inherited from original rock
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Chapter 23: Metamorphic Textures • Textures are small-scale features of rocks http://en.wikipedia.org/wiki/List_of_rock_textures We have already examined the progressive metamorphism of Pelites, mudrocks.
Relict Textures Inherited from original rock Porphyroclast is a mineral fragment of the original rock before dynamic recrystallization produced the YOUNGER groundmass. A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass.
Example 1 • Mylonitization of Granite • Mylonite is a fine-grained, compact rock produced by recrystallization of the constituent minerals, and resulting in a reduction of the grain size of the rock. Mylonites are crushed rocks, typically in shear zones. http://myweb.facstaff.wwu.edu/~talbot/
Undeformed Granite • b. Mortar texture in which large porphyroclasts of quartz are surrounded by cataclastic matrix • A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a younger groundmass of fine grained crystals. • Cataclastic textures are produced by mechanical crushing without significant recrystallization a Progressive mylonitization of a granite b Quartz strong, not crushed
c,d .Foliation evident and grains elongate and become smaller c Progressive mylonitization of a granite. d
Example 2 • Progressive Thermal Metamorphism of a Diabase (usually a shallow dike, basaltic in composition)
Progressive thermal metamorphism of a Diabase (coarse basaltic) • Stage 1: lots of relict textures from Diabase Augite and Plagioclase • Plag becomes Albite NaAlSi3O8, so it loses Ca and (Ca -> calcite) • Augite • (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 • hydrates -> Chlorite • (Mg,Fe)3(Si,Al)4O10(OH)2.(Mg,Fe)3(OH)6 • Actinolite • Ca2(Mg,Fe)5Si8O22(OH)2. • Color is greener Sphene is CaTiSiO5
Progressive thermal metamorphism of a diabase (coarse basalt) Stage 2- closer to igneous body, thoroughly recrystallized Epidote forms as more Ca released from Calcite Even greener Epidote is Ca2Al2(Fe+3Al)(SiO4)(Si2O7)O(OH)
Progressive thermal metamorphism of a diabase Stage 3: Warmer yet, Actinolite, Chlorite, Epidote, Calcite become unstable as Hornblende and more calcic plagioclase become stable “Amphibolite” Note coarser grain size as well
Progressive thermal metamorphism of a diabase (coarse basalt) Stage 4: Right up against a magma (hot), dehydration at high grade -> Cpx + Plagioclase rock Note that mineralogy is now ~ same as original diabase, but the granoblastic textureis new
Porphyroblasts • A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. • Porphyroblasts have difficulty nucleating, are separated by large distances • Surrounding matrix gets depleted in components • Components must therefore travel long distances by diffusion. • Crystallization occurs via single diffusing ions, not by assembly of clumps as in crystallization from liquids.
Depletion Haloes Figure 23-13. Light colored depletion haloes around cm-sized garnets in amphibolite. Fe and Mg were less plentiful, so that hornblende was consumed to a greater extent than was plagioclase as the garnets grew, leaving hornblende-depleted zones. Sample courtesy of Peter Misch. 3NaAlSi3O8 + CaAl2Si2O8 +3Ca2Mg5Si8O22(OH)2 = 3NaCa2Mg4Al3Si6O22(OH)2 +2Ca3Al2Si3O12 +Mg3Al2Si3O12 +18SiO2