Abstract
The Tuolumne Ultramafic Complex is a 30-km-long lens-shaped harzburgite and dunite body exposed in the Central Belt of the Sierra Nevada. It is uncomformably overlain by the dominantly basaltic rocks of the Triassic-Jurassic Penon Blanco volcanic arc. These rocks are intruded by 200 Ma plutonic rocks and are depositionally overlain by clastic rocks of the Mariposa Formation with a 152 Ma maximum depositional age. Both units are at most sub-greenschist in metamorphic grade. At the base of the ultramafic body is a high-strain zone composed of amphibolite-facies mafic garnet amphibolite, serpentinite, metaclastite, and metachert. The mafic amphibolites have abundant rutile, indicating a high-pressure origin, and have yielded 216 Ma U-Pb zircon and 200 Ma K-Ar hornblende ages. Previous workers have considered these rocks as a metamorphic sole, making the ultramafic complex an ophiolite, and linked them to subduction initiation. New mapping presented here shows the sole consists of imbricated slices of ultramafic-basalt-chert-clastic ocean-plate stratigraphy (OPS). Individual imbricates preserve the transition between OPS elements; in places anastomosing shear zones place slices of amphibolite in serpentinite, or serpentinite into metaclastic rocks. Along one transect the sole is made of particularly large volumes of quartz-framework metapsammite with neoblastic garnet, hornblende, and k-feldspar. Locally, weakly deformed leucocratic dikes, composed primarily of plagioclase and hornblende, are present. Zircon separates from the dikes have not yielded igneous ages and are instead entirely detrital; their U-Pb ages broadly match the age distribution present in the surrounding metaclastic rocks. The short, discontinuous geometry of the dikes and the abundant detrital zircon component indicate that they were generated by partial melting of surrounding metaclastic rocks, consistent with partial melt textures common in the mafic amphibolite. The significance of metaclastic rocks in a metamorphic sole is not clear. If they do preserve the rock record of subduction initiation, they may indicate initiation occurred near a continental margin. Alternatively, they may be trench clastic rocks accreted after a period of non-accretion, then affected by a later heating event, such as ridge collision.