TRACE-ELEMENT GEOCHEMISTRY AND GEOBAROMETRY FROM OPHIOLITIC ROCKS NEAR GRASS VALLEY, CALIFORNIA

Framework rocks of the northern Sierra Nevada (California) consists of north-south trending belts of accreted rock ranging in age from Ordovician to Jurassic. One of these, the Triassic-Jurassic metamorphic belt, extends about 300 km along the western foothills and consists of several discontinuous ophiolitic fragments associated with sedimentary rock. The largest of these ophiolitic units, the Lake Combie Complex, is exposed near the town of Grass Valley, and has been interpreted as an arc-trench system related to the Jurassic-aged Smartville Complex to the west. Previous work has focused mostly on structural and lithological relationships, but has lacked geochemical and geobarometric data. Here, we present whole-rock trace-element geochemistry and mineral electron microprobe data to help determine the tectonic setting of these rocks.

Samples were collected from previously-defined units within the Lake Combie Complex including serpentinite, gabbro, diabase, and overlying volcanic rocks. Thin section petrography was used to define rock type and assess degrees of alteration. Whole-rock samples were sent to a commercial lab for trace-element geochemistry collected by ICP-MS. Hornblende in polished thin sections of gabbro and diabase units were analyzed by electron microprobe, with pressure and temperature then estimated using a standard Al-in-hornblende formula.

Gabbro units consisted of plagioclase, clinopyroxene, and hornblende retrograded to actinolite. Normalized trace-element diagrams show fractionated patterns with high Th/Nb ratios and steep negative slopes due to strong enrichment in incompatible elements. Diabase and volcanic units also exhibit high Th/Nb ratios and strong enrichment in incompatible elements. Al-in-hbl geothermobarometry yields very scattered data centered around 500° C and 6 kbar. The geochemical results suggest the gabbro, diabase, and volcanic rocks share a similar origin in a suprasubduction zone setting, consistent with previous published interpretations. However, the strongly enriched incompatible elements are unusual and suggest the incorporation of an enriched source, possibly due to ridge subduction or addition of fertile mantle to the subduction system.