The Rattlesnake Creek terrane, Klamath Mountains, California: An early Mesozoic volcanic arc and its basement of tectonically disrupted oceanic crust

Abstract

The Rattlesnake Creek terrane in the south-western Klamath Mountains consists of a serpentinite matrix melange basement unconformably overlain by a coherent cover sequence of Upper Triassic and Lower Jurassic volcanic, hemipelagic, and clastic sedimentary rocks. Both the meelange and cover sequence are intruded by a suite of ca. 193-207 Ma (U-Pb zircon) gabbroic to quartz dioritic plutons. These rocks collectively record a complex, multistage evolution through several distinct tectonic environments. Blocks in the meelange basement include peridotite, greenstone, amphibolite, pillow basalt, various generally mafic plutonic rocks, metachert, and limestone, all suspended in a matrix of sheared serpentinite. Trace element chemistry indicates that basalt, amphibolite, and greenstone blocks were derived from normal to enriched mid-oceanic-ridge basalt (N-MORB to E-MORB) and within-plate basalt (WPB) protoliths. There is no geochemical evidence that any blocks were derived from a magmatic arc, nor do any of the sedimentary rocks contained within the meelange reflect deposition near a terrigenous sediment source. Collectively these relations indicate that the basement meelange was derived by disruption of oceanic crust and upper mantle in a setting far removed from either an arc or a terrigenous landmass and are most consistent with an interpretation that meelange formation oc-curred in an oceanic fracture zone. The overlying cover sequence is divisible into a stratigraphically lower Salt Creek assemblage and a stratigraphically higher Dubakella Mountain assemblage, both deposited in a marine setting. Volcanic rocks in the lower assemblage are basaltic pillow lava and pillow breccia with the trace element characteristics of a primitive island arc tholeiite suite, whereas those in the upper assemblage are predominantly chnopyroxene-phyric basalt to basaltic andesite and compositionally similar volcaniclastic rocks, with the geochemical characteristics of a more evolved (cale-alkaline to shoshonitic) volcanic arc suite. The geochemical character of these volcanic rocks is indistinguishable from that of the coeval early Mesozoic plutons, leading to the conclusion that these plutons represent the intrusive roots of the volcanic succession. Collectively, these rocks reflect construction of a Late Triassic to Early Jurassic island arc and indicate that subduction had initiated within or near the under-lying fracture zone assemblage by at least the late Triassic. The latest stages of crustal disruption in the basement thus probably occurred in a subduction zone environment. Both cover sequence units also contain inter-layered chert and argillite, as well as coarse epiclastic rocks with an abundance of quartzose, sedimentary, and metasedimentary grains. These elastic rocks provide the first evidence for terrigenous input into the Rattle-snake Creek terrane and indicate that the early Mesozoic arc developed in proximity to a terrigenous sediment source, probably Paleozoic rocks of the western North American Cordillera. Several lines of evidence suggest that early Mesozoic arc magmatism was accompanied by an episode of extensional tectonism. These include structural evidence for an early Mesozoic phase of high-angle faulting that appears to have localized cover sequence deposition in fault-bounded grabens, sedimentologic evidence indicating that both the basement and terrigenous sediment sources experienced episodic pulses of uplift and erosion during cover sequence deposition, and widespread intrusion of arc plutons into the upper crustal levels of the are carapace. Both the basement complex and the early Mesozoic arc assemblage of the Rattlesnake Creek terrane were then deformed by thrust faulting, folding, and penetrative foliation development during over-thrusting from the east by the western Hayfork terrane in the Middle Jurassic. Regional de-formation was in response to northeast-south-west-directed shortening and was accompanied by metamorphism to subgreenschist grade. Insofar as the western Hayfork terrane represents part of a Middle Jurassic arc whose plutonic roots intrude most Klamath Mountain terranes east of and including the Rattle-snake Creek terrane, it is clear that the Rattlesnake Creek terrane formed part of the Klamath Mountain province by Middle Jurassic time. The paleogeographic and tectonic relation of the Rattlesnake Creek terrane to the rest of the Klamath Mountains prior to the Middle Jurassic, however, is unclear.