Geochemistry and petrology of mafic volcanic rocks from olistostromes in the basal Great Valley Group, northern California Coast Ranges

The Mysterious Valley formation (MVF) is a 1-km-thick accumulation of chaotic olistostromes, consisting of ophiolitic blocks set in a mudstone and serpentinous mudstone matrix, in the basal Great Valley Group of the northern California Coast Ranges. The MVF overlies serpentinite of the Middle Jurassic Coast Range Ophiolite (CRO). Most igneous blocks from the MVF are mid-ocean-ridge-type tholeiitic basalts (MORB's) with moderate TiO2 contents (1.0%-2.5%) and depletions in light (relative to heavy) rare-earth elements (REE's). Relict clinopyroxenes in these blocks are low-Ti augites. Rare blocks that are light-REE enriched and contain titanaugite probably represent ocean-island-type basalts. Coarse-grained plutonic blocks have not been found; however, abundant blocks of mafic breccia, similar to breccia overlying intact CRO elsewhere in the northern Coast Ranges, contain clasts of ultramafic rock and both cumulus and noncumulus gabbro. Diabase intrusions into the underlying CRO serpentinite are chemically identical to MVF basalts, suggesting a possible relationship between CRO igneous rocks and MVF blocks. Because most MVF basalt blocks are more Ti rich and, in general, more MORB-like than are CRO extrusive basalts, however, the MVF olistostromes are not easily explained as being derived from the uplift and erosion of the ancient CRO. Our chemical data indicate that either (1) MVF basalt blocks were not derived from the CRO; (2) our samples, which are from a geographically limited area, are not representative of the MVF; or (3) the MVF was derived from a more MORB-like part of the CRO that is not well represented in the few analyses presently available from the northern CRO. © 1988 Geological Society of America.