45 Tectonics I: Cordilleran Terranes, Arc Tectonics

Sunday, 2 November 2003
1:00 PM - 3:45 PM

DEPOSITION, ERUPTION AND METAMORPHISM OF THE GOLDSTEIN PEAK PENDANT PROTOLITHS, WESTERN SIERRA NEVADA BATHOLITH, CA

Paper number: 45-9
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

Located at the boundary between the Kings and Kings-Kaweah Terranes, the Goldstein Peak pendant preserves a record of non-marine siliciclastic sedimentation and subaqueous-to-subaerial volcanism in the western Sierra Nevada arc. The 2-km-thick, subvertically foliated section is composed of the following: (1) Clast-supported quartz-pebble metaconglomerates—rarely crossbedded, and interlayered with crossbedded quartzites—that were deposited in a braided fluvial environment. (2) Sillimanite-quartz-biotite schists that contain multiple resistant, <1-m-thick whiteschists and were deposited in a lacustrine or shallow marine environment. Chemical compositions and lateral continuity of the whiteschists suggest their origin as ash beds. (3) A lens of basaltic-to-dacitic metavolcanic rocks interrupts the fluvial-lacustrine/shallow marine section. Preserved volcanic structures indicate subaqueous extrusion of the thickest part of the section and fluvial reworking of ash flows in the thinnest part of the section. (4) Matrix-supported polymictic pebble metaconglomerates were probably deposited in an alluvial fan environment. Quartzites and amphibolites, including distinctive peperites and plagioclase-porphyritic mafic metavolcanics, compose more than 97% of the clasts and presumably were derived from rocks similar to the underlying volcanic and sedimentary units. Hornblende-biotite granitoid clasts and a single anorthosite clast, however, indicate exposure of differentiated arc plutons prior to conglomerate deposition.

Hornblende-hornfels-facies metamorphism by the 123-to-115-Mya ring dikes of the Early Cretaceous batholith may have occurred prior to complete lithification of the volcano-sedimentary section. Granite dikes intruding the upper conglomerate have cuspate margins and irregular, curvilinear apophyses, strongly suggestive of intrusion into wet sediment. Sets of evenly spaced, epidote-quartz tubes cut across protolith structures and are found in virtually all protolith types. These structures are interpreted as fluid escape tubes resulting from contact metamorphism of wet sediments. If correct, this reconstruction implies that the Goldstein Peak pendant preserves information about Early Cretaceous nonmarine environments of the newly emergent continental margin arc.