Abstract
The northwestern section of the Trinity subterrane of northern California has key units that are Ordovician in age, including the 472 ± 32 Ma peridotite and ~470 Ma gabbro. Yet, it also contains tonalite (called plagiogranite by previous authors) dated Precambrian (565 ± 7 Ma), the nature of which has been debated. This thesis presents new structural data, geochemistry, petrologic descriptions, and analysis of existing data in order to better classify the true nature of the Trinity Ophiolite, focusing on the mechanism of formation for the Precambrian plagiogranite. Mapping focused on the structural relationships between the pre-Silurian units, attempting to determine unit relationships during the Ordovician with primary focus on the Precambrian tonalite. Geochemical samples were collected from the tonalite, overlying volcanics, and amphibolitic gabbros for rare earth element (REE) geochemical and discrimination diagram analyses in order to help determine genesis environment and mechanisms. The study found the Precambrian tonalitic unit described as a plagiogranite does not geochemically conform with formation from fractionation of oceanic crust and more closely resembles tonalite-trondhjemite-granodiorite (TTG) rocks, formed from a source with garnet or other refractory mineral present, and has a clear subduction zone / crustal contamination signature. The unit is also observed to be brecciated by and floating in a mafic dike matrix, a feature recognized in all outcrops of the unit. The undated basalts (red/green volcanics) have a strong subduction geochemical signature and the amphibolitic gabbro show a varying geochemical signature ranging from N-MORB to E-MORB, and MORB to volcanic arc. Based on the results of the study, it is hypothesized that the tonalite is part of a previously unrecognized Precambrian dismembered volcanic arc. This arc was juxtaposed to an Ordovician back-arc basin ophiolite via transform faulting prior to the formation of the Silurian forearc ophiolite forming the Trinity subterrane.