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Quartz-feldspar mylonite derived from granite, Santa Catalina Mountains, Tucson, Arizona


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Age & Formation: 
Original granite of Mesoproterozoic age, ~1.4 Ga. Age of mylonitization, ~25-18 Ma.
Geohistory: 
This is a coarse-grained quartz-monzonitic mylonite. The fabric developed through shear zone deformation related to the creation of the Catalina-Rincon metamorphic core complex. The original granite (protolith) was a porphyritic quartz monzonite, with large phenocrysts of feldspar. Mylonitization involved crystal-plastic deformation, during which the feldspar phenocrysts became fractured, broken, and comminuted while the quartz flowed like butter. It is the differential behavior of these two essential minerals that creates the beautiful fabric. The orientation of the mylonitic foliation is essentially parallel to flattening of the rock within the shear zone. In the plane of the foliation is a penetrative mineral lineation, which denotes the direction of shear and the direction of stretching.
Whereabouts: 
There is a mountain of this rock in the foothills of the Catalina Mountains, just a ‘stones throw’ from where I live. I collected this rock in the mid-1970s while mapping and analyzing rocks and structures in the Catalinas and Rincons. This rock ‘takes me back’ to all of the original confusion regarding naming this rock and interpreting its tectonic significance.