GEOLOGY AND PRELIMINARY GEOCHEMISTRY OF TERTIARY VOLCANIC ROCKS FROM THE DIABLO RANGE, WEST-CENTRAL CALIFORNIA

Several small dikes and plugs of mafic to intermediate Tertiary volcanics are exposed in the Diablo Range of west-central California. According to Nakata et al. (1993), these isolated occurences of Miocene (~ 8-10.5 Ma) volcanics, which we refer to as the Diablo Range Volcanics (DRV), are broadly coeval with and may be erosional remnants of the more extensive Quien Sabe Field located to their south and east. Like the Quien Sabe rocks, the DRV are inferred to be part of a northward younging sequence of volcanic fields that may be related to northward migration of the Mendicino Triple Junction (MTJ).

Preliminary chemical analysis(24 samples)indicates that the DRV vary from basalt to andesite; andesite is most abundant. The suite is dominantly calc-alkaline, although one basalt sample is alkaline. DRV samples show abundant petrographic evidence for magma mixing, including reversely zoned plagioclase with sieve or spongy textures and quartz xenocrysts rimmed by clinopyroxene. Similar features have been observed in the Quien Sabe and Clear Lake volcanic centers. The DRV are highly porphyritic, with up to ~ 40 percent phenocrysts. The most abundant phenocrystic phase is glomeroporphyritic plagioclase; clinopyroxene is the most commonly encountered mafic phenocryst. The DRV enclose xenoliths of two types which may help to constrain the nature of basement rocks underlying the DRV: 1)inclusions of gabbro, diorite, and granite; and 2)granulite facies metamorphic rocks, including biotite and sillimanite-corundum schists. No ultramafic xenoliths have been found.

We interpret the geochemical characteristics (pronounced Nb anomalies and relative HFSE depletion) seen in mafic and intermediate rocks of the DRV suite as due primarily to crustal contamination of subalkaline basalts; this trace element pattern may also reflect modification by slab-derived fluids, but the petrographic observations suggest that crustal contamination was the dominant process. In spite of their association with the northward passage of the MTJ, none of the DRV mafic rocks have the OIB-like trace element signatures that characterize magmas generated in some slab windows.

Additional data are being gathered with the goals of: 1) obtaining thermobarometric estimates for the xenoliths; and 2) identifying any chemical clues to triple junction magmatism.