VEINS, GEOCHEMISTRY AND THE EVOLUTION OF UPPER ARC CRUST
Major episodes of fluid-rock interaction attended: 1) eruption/hybabyssal emplacement of igneous protoliths on or near the seafloor at ~200 to 150 Ma; and 2) emplacement of the Sierra Nevada batholith at ~100 Ma. Early alteration was primarily manifested by wholesale changes of bulk and mineral compositions relative to likely protoliths. It is layer-to-layer in style, producing metavolcanic rocks that vary between sodic, sodic/calcic, potassic, and calcic bulk compositions, as well as veins and mineral zoning. Early veins are transposed or folded, indicating that deformation attended early alteration. Early alteration effects are seen in units 100s of m thick and km along strike. Late alteration is seen primarily as veins with metasomatic envelopes. Vein envelopes are generally much greater in volume than veins. The geometry of late veins and vein envelopes resembles that of fractures and veins in plutons, indicating that late veins are syn-to-post-plutonic.
In both early and late veins, epidote-group minerals—in particular, pistacite-to-piemontite solid solutions—record further details of vein-forming (and –modifying) fluid-rock interactions. Of particular interest are the abundances of REE, Mn and Sr in veins and envelopes. Overall, the geochemistry of epidote minerals from veins provides evidence for complex fluids of relatively low salinities , in H2O-CO2 systems that fluctuate in composition over time.
This is seen in the products of both early and late alteration. The zoning relationships, microstructures and compositions of epidote-group minerals show that on all scales, at least some mass transport was influenced by fluid properties, including fO2, that varied over scales of μm to km. The geochemistry of metavolcanic host rocks and its veining systems is thus a useful tool in reconstructing the evolution of shallow arc crust throughout its development.