Contact Metamorphosed Ultramafic Rocks in the Western Sierra Nevada Foothills, California

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Abstract
Within the western Sierra Nevada metamorphic belt, linear bodies of alpine-type ultramafic rock, now composed largely of serpentine minerals, parallel the regional strike and commonly coincide with major fault zones. Within this metamorphic belt, east of Sacramento, California, ultramafic rocks near a large mafic intrusion, the Pine Hill Intrusive Complex, have been emplaced during at least two separate episodes. Those ultramafic rocks, evidently unaffected by the Pine Hill Intrusive Complex and composed largely of serpentine minerals, were emplaced along a major fault zone after emplacement of the Pine Hill Intrusive Complex. Those ultramafic rocks, contact metamorphosed by the Pine Hill Intrusive Complex, show a zonation of mineral assemblages as the igneous contact is approached: olivine+antigorite+chlorite +tremolite+Fe-Cr spinel↑ olivine+talc+chlorite+tremolite+Fe-Cr spinel ↑ olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel ↑ olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel. Superimposed on these mineral assemblages are abundant secondary minerals (serpentine minerals, talc, chlorite, magnetite) which formed after contact metamorphism. Correlation of observed mineral assemblages with the experimental systems, MgO-SiO2-H2O and MgO-Al2O3-SiO2-H2O suggests an initial contact temperature of 775±25 °C for the Pine Hill Intrusive Complex assuming Ptotal ≃Pfluid ≃PH2O. The pressure acting on the metamorphic rock during emplacement of the intrusion is estimated to be a minimum of 1.5 kb.

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