AMPHIBOLE, CLINOPYROXENE AND TITANITE AS PETROGENETIC TRACERS IN THE EJB PLUTON, WHITE-INYO RANGE, EAST-CENTRAL CALIFORNIA, USA

The monzonitic Eureka Valley–Joshua Flat–Beer Creek (EJB) composite arc pluton in the White-Inyo Range of eastern California represents a unique composition among Mesozoic North American arc terranes. The EJB pluton is among the oldest of about twenty intrusions in the range. The Eureka Valley facies (EVF) is a monzonite, the Joshua Flat facies (JFF) is a monzonite to granodiorite, and the Beer Creek facies (BCF) is a monzonite to granite. These facies were emplaced over a span of ca. 10 Ma. Earlier studies by Calvin Miller and colleagues showed that the EJB monzonites were sourced by partial melting of a garnet-bearing eclogitized island arc and are inherently different from the Sierra Nevada batholith. Here, we use amphibole, clinopyroxene, and titanite to investigate the petrogenesis of the EJB magmas from their source to magma chamber. Despite the broad range in major element compositions of the three facies, the slopes of their chondrite-normalized REE patterns are similar (Sm/Yb ratios are >4.5). The REE patterns suggest that the three facies shared a similar garnet pyroxenite source.

Petrography and modeling of trace element compositions show that in the EVF clinopyroxene and plagioclase were early phases, and clinopyroxene was followed by amphibole. In the JFF and BCF, titanite and amphibole appear to have crystallized first. Amphibole compositions in the EVF suggest crystallization between 1040 and 1130 MPa. This pressure range is interpreted to indicate the approximate depth of the source, ~40 km, and it supports an eclogite facies source. In the JFF and BCF, amphibole indicates crystallization between 450 and 710 MPa, in the pressure range of pluton emplacement near the brittle-ductile transition within the crust. Although enclaves within the JFF and BCF facies are compositionally similar to the EVF, amphibole in them displays pressures and temperature of crystallization similar to amphibole in the host granitoids. Because whole rock REE slopes are similar, it is likely that all three facies shared a similar source, but the presence of mafic and felsic counterparts during the entire emplacement history of the EJB pluton show that the source was inherently heterogeneous.