Conditions of metamorphism in an early Paleozoic blueschist,schist of Skookum Gulch,northern California |
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| Authors: | Spencer J Cotkin |
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| Institution: | (1) Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton Street, 53706 Madison, WI, USA |
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| Abstract: | The Late Ordovician schist of Skookum Gulch, northern California, is one of only a few pre-Mesozoic blueschist localities in North America. Among these, the Skookum Gulch occurrence is noteworthy because it contains lawsonite and has had a relatively simple metamorphic and structural history.Numerous assemblages and a wide variation in the modal proportion of minerals are present due to a spectrum of bulk compositions. This is reflected in the composition of Na-amphibole, which varies from ferro-glaucophane, glaucophane, and crossite (±magnetite) to magnesio-riebeckite (+hematite). Application of the available experiments and empirically calibrated equilibria to the assemblages glaucophane+lawsonite+chlorite+quartz+albite and glaucophane+actinolite+epidote+chlorite+quartz+albite yield estimates of temperature and pressure near T= 275° C and P=7.0 kbar. Estimates of uncertainty are difficult to assess, but are no more than ±100° C and ±3.0 kbar, and are probably considerably smaller. Calcite +quartz+sphene and calcite+quartz+actinolite indicate an extremely H2O-rich fluid (X(CO2)<0.003).The absence of a greenschist facies overprint indicates that the schist of Skookum Gulch was uplifted soon after metamorphism. However, it was not exposed until the recent geologic past, having resided at shallow crustal levels for approximately 400 Ma. |
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