PARENT MAGMAS OF PLUTONIC ROCKS IN THE ELDER CREEK OPHIOLITE, CALIFORNIA: FIRST APPROXIMATION

Plutonic rocks of the Elder Creek ophiolite comprise three distinct intrusive suites that can be distinguished based on their petrology and cross-cutting relationships. These are, from oldest to youngest, (1) the layered gabbro suite, (2) the wehrlite-pyroxenite suite, and (3) the diorite-quartz diorite suite. The diorite-quartz diorite suite appears to represent near-liquid compositions that form a fractionation series from basaltic-andesite to rhyolite. This suite may be projected back to a gabbro/diorite parent with circa 53% SiO2, 0.9% TiO2, and 0.6% K2O. This projected parent resembles low-to-medium K calc-alkaline basaltic andesites of the western Pacific.

The layered gabbro (LG) and wehrlite-pyroxenite (WC) suites are cumulates. The LG suite consists of plg-cpx±ol±opx±hbl cumulates (gabbro, gabbronorite, anorthosite); olivine cumulates (dunite) may also be associated with this suite. Pyroxene-melt partitioning suggests a gabbro parent with mg# 69, <0.9% TiO2, and 16% Al2O3. The WC suite consists of ol-cpx cumulates, with minor plg-cpx cumulates. Pyroxene-melt partitioning suggests a refractory parent magma with mg# 75-82, <0.5% TiO2, and 13% Al2O3.

SIMS analyses of REE in pyroxene show that the LG and WC suites had distinct trace element compositions. Magma compositions calculated for the LG suite are LREE-depleted, with La/Yb 0.5x, La 6x, and the MREE 13x ch, (i.e. about 1x-2x normal MORB component of Sun & McDonough, 1989), similar to MORB from the East Pacific Rise. In contrast, melts calculated for the WC suite are slightly LREE-enriched, with La/Yb 1.2x ch, but with depleted overall concentrations (MREE 7x ch, or 0.4x normal MORB).

The LG parent magma is similar to EPR MORB, but with lower Ti and higher Al and K, analogous to primitive arc tholeiite suites. The WC parent magma is more refractory than the LG parent magma, with very low Ti and high mg#. The combination of depleted total REE with a LREE-rich overprint, and the dominance of high-Ca px, is characteristic of many fore-arc volcanic suites, e.g., high-Ca boninites, and suggests a fore-arc provenance for this suite. These parent magmas (arc tholeiite, boninitic, calc-alkaline) suggest assembly of the Coast Range ophiolite in a supra-subduction zone setting, consistent with conclusions based on lava and phenocrysts compositions.