Polygenetic ophiolite belt of the California Sierra Nevada: geochronological and tectonostratigraphic development.

Abstract

New Pb/U zircon ages are presented for key locations along a 375km segment of the W Sierra Nevada ophiolite belt. These age data are combined with structural-stratigraphic observations and published ages, and significant tectonic implications for the ophiolite belt emerge. Three different ophiolitic assemblages are recognized with igneous ages of abut 300, 200 and 160Ma B.P. Rocks of the 300Ma assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange. Fragments of upper Paleozoic seamounts occur in association with the ophiolitic melange, and together these assemblages constitute the basement framework for the W Sierra. Pb/U and K/Ar isotopic systematics are complex within this framework and indicate a polymetamorphic history. Systematics in the 200 and 160Ma assemblages are less complex and give tighter igneous age constraints. Both the 200 and 160Ma assemblages have equivalent age basinal volcanic-sedimentary sequences that lie unconformably above the ophiolitic melange basement. In each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age-equivalent ophiolite sections.-from Author