Serpentinite mélange member
Serpentinite mélange member
Early Jurassic to Late Triassic and Early Permian to Late Pennsylvanian
Occurs in western slopes of central Sierra Nevada adjacent to the Kaweah River. Roughly equivalent to the Kaweah serpentinite mélange of Saleeby and Sharp (1980). Consists chiefly of foliated serpentinite matrix containing dismembered ophiolitic blocks of a wide range of sizes, shapes, and compositions. Serpentinite matrix derived from highly deformed peridotite, serpentinite diapirs, and serpentinitic debris flows and detrital rocks. The mélange blocks consist mainly of serpentinized peridotite, gabbro, diabase, basalt, chert, and ophicalcite. Mélange interpreted as forming during pillow basalt eruption and tectonic disruption of the seafloor, resulting in extrusion of serpentinite. Pillow basalts, pillow breccias, and hyaloclastites are locally interbedded with ophicalcite and sedimentary serpentinite, but in most localities such flows into serpentinitic sediments were broken into blocks or intergradational lenses with matrix material during subsequent deformation. Pillow lavas erupted over the highly disrupted ophiolitic substrate are commonly interbedded with and overlain by radiolarian chert. Saleeby and Sharp (1980) report a zircon U-Pb age of ~191-225 Ma from a diorite pod within gabbro in ophiolite mélange, as well as discordant zircon U-Pb ages of ~270-305 Ma from metaplagiogranite blocks and metaplagiogranite within gabbro and peridotite blocks. They suggest that the younger ophiolitic rocks formed during disruption of the older ophiolite. Saleeby (2011) reports a multi-rock Sm-Nd isochron age of ~299 Ma for this unit and assigns the Mesozoic rocks to his Kings sequence (included in this area in unit MRCS). Farther north, near the Kings River, small unmapped bodies of this unit are seen to be emplaced into the older Kings River ophiolite belt (unit MRKI). Geochemistry of this unit suggests a mid-ocean ridge basalt (MORB) affinity (Saleeby, 2011). This unit probably formed by ophiolitic extrusion in a transform fault within the older seafloor of the Kings River ophiolite and was subsequently intruded and overlain by Mesozoic volcanic and sedimentary rocks.