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

Abstract

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 sub-Pacific 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 mélange. 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 mélange within the abyssal realm. This (Kaweah) serpentinite mélange constitutes the majority of the ophiolite belt and encases fragments of both disrupted Early Ordovician oceanic lithosphere and crustal igneous-metamorphic 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 mid- to late Paleozoic Panthalassa regime, when the proto-Pacific basin occupied over half of the Earth's surface. 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 plate motions led to the inception of east-dipping subduction, and the en masse accretion of the ophiolite belt to the hanging wall of the newly established subduction zone. Structural relations and isotopic data on superimposed igneous suites show that the ophiolite belt was not obducted onto the SW Cordilleran continental margin. The accreted ophiolite belt formed the proto-forearc of the newly established active margin. The ophiolite belt never saw high-pressure/temperature (P/T) metamorphic conditions. Rare small blocks of high-pressure metamorphic rocks were entrained from the young subduction zone by serpentinite diapirs and emplaced upward into the ophiolitic mélange within a proto-forearc environment. An Sm/Nd garnet-matrix age on a high-pressure garnet amphibolite block suggests subduction initiation at ca. 255 Ma. This timing corresponds well with the initiation of arc magmatism along the eastern Sierra Nevada region. In Late Triassic to Early Jurassic time proximal submarine mafic eruptions spread across and mingled with hemipelagic and distal volcaniclastic strata that were accumulating above the accreted ophiolite belt. These lavas carry boninitic to arc tholeiitic and primitive calc-alkaline geochemical signatures. By Middle Jurassic time siliciclastic turbidites derived from early Paleozoic passive margin strata and early Mesozoic arc rocks spread across the primitive forearc. In late Middle to Late Jurassic time tabular plutons and dike swarms of calc-alkaline character invaded the ophiolite belt in a transtensional setting. Deformation fabrics that developed in these intrusives, as well as cleavage that developed in the cover strata for the ophiolite belt, imparted components of superimposed finite strain on the ophiolitic mélange structure but did not contribute significantly to mélange mixing. By ca. 125 Ma, copious gabbroic to tonalitic plutonism of the western zone of the Cretaceous Sierra Nevada batholith intruded the ophiolite belt and imparted regional contact metamorphism. Such metamorphism variably disturbed U/Pb systematics in rare felsic intrusives of the ophiolite belt but did not significantly disturb whole rock Sm/Nd systematics. Age constraints gained from the Sm/Nd and U/Pb data in conjunction with Nd, Sr, and Pb isotopic and trace element data clearly define the polygenetic abyssal magmatic history of the ophiolite belt. The variation of Nd and Sr radiogenic isotopes over time from the Paleozoic abyssal assemblages, through early Mesozoic supra-subduction zone volcanism to Early Cretaceous batholithic magmatism, record the geochemical maturation of the underlying mantle wedge without the involvement of SW Cordilleran continental basement.

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