Pelona-Rand Schist and related units
Pelona-Rand Schist and related units
Paleocene to Late Cretaceous
Consists of thin-layered, light to dark schist containing varying amounts of quartz, muscovite, biotite, and plagioclase. Local thin-layered quartzite (derived from meta-chert) with garnet, mica, and feldspar. Local mafic metavolcanic layers composed of albite, actinolite, epidote, and sphene. Local layers of schistose hornblende amphibolite and antigorite serpentinite, and chlorite schist. Local calcareous, silicic, and pelitic schist, and gneiss. Local hornfels adjacent to granitic plutons. Pelitic schist locally contains andalusite, lesser sillimanite and garnet. In the map area, is located in San Emigdio Mountains [there named the San Emigdio schist by Chapman and others (2011) and Rand Schist by Kellogg and others (2008)], and within the Garlock fault zone in the southwestern Tehachapi Mountains (mapped as Pelona Schist by Dibblee (1967), but more likely Rand Schist or San Emigdio schist based on its position along the fault separating those two parts of this unit). Part of the larger Pelona-Orocopia-Rand schist terrane which extends from southwest Arizona to the Sierra de Salinas near Monterey Bay in central California. Terrane is an exhumed, shallow, subduction-zone complex or underthrust forearc. Detrital zircons suggest that the depositional age of the protolith varies from Late Cretaceous (early Turonian) for the schist in the San Emigdio Mountains to Late Cretaceous (Santonian-Campanian) for the Rand Schist south of the map area to Late Cretaceous (late Campanian to Maastrichtian) for the Pelona Schist to latest Cretaceous (Maastrichtian) to Paleocene (possibly early Eocene in Arizona) for the Orocopia Schist (Jacobson and others, 2011). Ar-Ar dating of metamorphic minerals (Chapman, 2017) suggests a similar southward-younging trend in timing of metamorphism, with muscovite and hornblende Ar-Ar ages ranging from late Turonian for the schist in the San Emigdio Mountains to late Eocene in Arizona. Uplift from lower crustal levels may have occurred during subduction of one or more oceanic plateaus (Chapman and others, 2012). Down-thrusting occurred beneath Sierra Nevada and southern California batholith to 9 Kbar or more pressure (~30 km?) and subsequently elevated to upper crustal depths. Terrane exhibits inverted regional metamorphism caused by shallow subduction beneath hot roots of the Sierra Nevada arc (Kidder and Ducea, 2006). The timing and nature of uplift varies across the terrane, with latest Cretaceous to Eocene uplift of both upper plate (Tehachapi-San Emigdio intrusive suite or unit Gte) and lower plate consisting of the San Emigdio schist in the San Emigdio Mountains, whereas emplacement pressure and thermochronologic constraints in the Rand Mountains require significant unroofing via detachment reactivation of the Rand thrust (Chapman and others, 2010) that continued into Miocene time (Shulaker and Grove, 2015). Terrane structurally overlain by Tehachapi-San Emigdio terrane (unit TSS) and the Tehachapi-San Emigdio intrusive suite (unit Gte). Terrane occurs in lower plate of Rand thrust.