CONTRASTING ORIGINS OF SERPENTINITES IN THE DIABLO RANGE SEGMENT OF THE CALIFORNIA COAST RANGES

In order to constrain their original tectonic setting and subsequent evolution, serpentinites in the Diablo Range of California were analyzed for their relict mineral chemistry (spinel and olivine) using the JEOL JXA-8230 electron microprobe at Stanford University. Sample locations include (from north to south): 1) Sunol Regional Wilderness (SRW); 2) the Gilroy-Yamato Hot Springs (GYHS) and Steer Ridge (SR) areas of Henry W. Coe State Park; and 3) Panoche Pass Road (PPR). All samples are from serpentinite matrix mélange. Based on the presence of bastite pseudomorphs after orthopyroxene and mesh textures, the serpentinites represent altered harzburgite and dunite. Exotic blocks (eclogite; amphibolite) are found with serpentinite at the SRW and PPR locations; none were observed at the Coe Park sample sites. The SRW and PPR serpentinites are clearly intra-Franciscan in both map view and in structural position. The tectonic setting of the Coe Park samples is less clear. Some of these crop out in the Madrone Springs fault between the Great Valley Group and the Franciscan and may represent Coast Range Ophiolite.

Spinel compositions for the SR and GYHS samples (Cr# [Cr/[Cr+Al] = 22.5–52.8; Mg# [Mg/Mg +Fe]= 41.6-66.5) plot in the abyssal and abyssal-suprasubduction zone (SSZ) overlap fields on discrimination diagrams for spinels from mantle peridotite. In contrast, spinels in the SRW and PPR serpentinites have high Cr# (82.1-92.5) and low Mg# (25.0-36.2), consistent with a SSZ origin. Spinels in most of the Diablo Range serpentinite samples have low TiO2 values (< 0.07 wt. %) , but some of the SR and GYHS samples are enriched in TiO2 (0.36-0.59 wt. %). The latter plot in the abyssal peridotite field on the TiO2 vs. Cr# discrimination diagram. Olivine (Fo = 89.9-92.0%; NiO = 0.19-0.28) is only found in the GYHS samples and plots in the abyssal peridotite field on a Fo- NiO discrimination diagram.

Based on spinel and olivine compositions, serpentinites with both abyssal and SSZ affinities are present in the Diablo Range. Intra-Franciscan serpentinite samples from Sunol Regional Wilderness and Panoche Pass Road are of SSZ origin, while the Coe Park samples fall in both the abyssal and abyssal-SSZ overlap fields. Some of the Coe Park samples are enriched in TiO2 suggesting interaction with a MORB-like melt, possibly in a back-arc setting.