TRIASSIC-JURASSIC OPHIOLITE, METAMORPHIC SOLE, AND VOLCANICS AS A HYPEREXTENDED FOREARC FORMED AT THE INITIATION OF SUBDUCTION, SIERRA NEVADA CENTRAL BELT, CALIFORNIA

The Central Belt of the Sierra Nevada consists of an enigmatic mix of ophiolitic, arc, and sedimentary rock that is broadly Triassic-Jurassic in age. Plutonic rocks of the arc, exposed in the Slate Creek, Lake Combie, and Peñon Blanco units, have yielded ages 207 Ma and younger. The ophiolite, which consists of a mix of ultramafic, mafic plutonic and volcanic, and amphibolite, has been described in the literature as disrupted crust or oceanic mélange that may have formed in a fracture zone. Here we identify ophiolite, metamorphic sole, and volcanic rocks present along the length of the Central Belt which may indicate that the ophiolite was formed in a forearc at the initiation of subduction.

The ophiolite, exposed at the Tuolumne and Jarbo Gap ophiolites, consists of coherent ultramafic rocks directly overlain by basalt, chert, or shale. While previously interpreted as being formed in a fracture zone, they are more likely hyperextended oceanic crust analogous to that formed at slow-spreading ridges. The ophiolite is underlain by a thin high-pressure, high-temperature metabasalt and metasediment unit that has been interpreted as the metamorphic sole of the ophiolite. The sole includes eclogite relics of 215-218 Ma in age, overprinted by a later high-temperature event that varies in age along strike from 196 Ma to 185 Ma. This high-temperature event may have generated adakite magmatism present in the Jarbo Gap Ophiolite.

Volcanic rocks associated with the ophiolite are well exposed overlying the Tuolumne ophiolite. The older Jasper Point Formation basalt, which directly overlie ultramafics, have an N-MORB-like chemistry that has been linked to forearc rifting. While the overlying Peñon Blanco Formation volcanics have been interpreted to be nascent arc based on trace-element geochemistry, a re-analyses of published data indicates that the rocks are boninitic, and may be linked to the initiation of subduction.

Together, the ophiolite in the Central Belt may expose the forearc record of Triassic initiation of subduction leading up to generation of the arc starting at about 207 Ma. The co-location of the arc with the ophiolitic material indicates a large degree of slab rollback that occurred during generation of the forearc.