Geochemical mapping of the Kings-Kaweah ophiolite belt, California-Evidence for progressive melange formation in a large offset transform-subduction initiation environment

The Kings-Kaweah ophiolite belt of the southwestern Sierra Nevada Foothills was generated in two pulses of mid-oceanic-ridge basalt (MORB) magmatism. The first was in the Early Ordovician, which resulted in the generation of a complete abyssal crust and upper mantle section. The crustal section was rendered from convecting mantle whose Nd, Sr, and Pb isotopic systematics lie at the extreme end of the subPacific mantle regime in terms of time integrated depletions of large ion lithophile (LIL) elements. Semi-intact fragments of this Early Ordovician oceanic lithosphere sequence constitute the Kings River ophiolite. Following -190 m.y. of residence in the Panthalassa abyssal realm, a second pulse of MORB magmatism invaded the Early Ordovician lithosphere sequence in conjunction with intensive ductile shearing and the development of ocean floor melange. This Permo-Carboniferous magmatic and deformational regime produced many of the essential features observed along spreading ridge-large-offset transform fracture zones of the modern ocean basins. During this regime, Early Ordovician upper mantle-lower crustal rocks were deformed in the ductile regime along what appears to have been an oceanic metamorphic core complex, as well as along steeply dipping strike-slip ductile shear zones that broke the ophiolite into semi-intact slabs. Progressive deformation led to the development of serpentinite-matrix ophiolitic melange within the abyssal realm. This (Kaweah) serpentinite melange constitutes the majority of the ophiolite belt and encases fragments of both disrupted Early Ordovician oceanic lithosphere and crustal igneousmetamorphic assemblages that were deformed and disrupted as they formed by diffuse spreading along the fracture zone. An -190 m.y. hiatus in abyssal magmatism cannot be readily accommodated in the current configuration of Earth's ocean basins, but it was possible during the midto late Paleozoic Panthalassa regime, when the proto-Pacific basin occupied over half of the Earth's surface. *jason@ gps.ca1tech.edu The transform history of the ophiolite belt can be directly linked to the late Paleozoic transform truncation of the SW Cordilleran passive margin. Following juxtaposition of the transform ophiolite belt with the truncated margin a change in relative Saleeby, J., 2011, Geochemical mapping of the Kings-Kaweah ophiolite belt, California-Evidence for progressive melange formation in a large offset transformsubduction initiation environment, in Wakabayashi, J., and Dilek, Y., eds., Melanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, p. 31-73, doi:10.1130/2011.2480(02). For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.