T111 Tectonic Controls on Volcanism: Geologic Perspectives (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

THE BRECCIA OF FROG LAKES: RECONSTRUCTING TRIASSIC VOLCANISM IN THE EAST-CENTRAL SIERRA NEVADA, CALIFORNIA

Paper number: T111-5
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The western part of the Saddlebag Lake pendant, located in the east-central Sierra Nevada, is composed of Mesozoic sedimentary and extrusive volcanic rocks. The basal part of this volcanic section, which includes welded tuffs and mafic flows and breccias, represents volcanism during the earliest stage of Sierran magmatism. The welded tuffs and mafic breccias therefore provide insight into the diversity of volcanic processes during early evolution of the Sierran arc. The oldest mafic breccia unit stratigraphically overlies the 224 Ma tuff of Saddlebag Lake and is overlain by the 219 Ma tuff of Greenstone Lake. As the welded tuffs limit the depositional age of this breccia to 219-224 Ma, it becomes apparent that the breccia clasts could provide additional insight into mafic volcanism associated with ignimbrite eruptions in this part of the Sierra Nevada during Late Triassic time.

The mafic breccia is a laterally extensive, mostly massive and very poorly sorted, matrix-supported tuff breccia, with up to 60% clasts. The clasts have experienced metamorphism, with alteration of plagioclase into epidote and secondary crystallization of quartz and biotite, leading to the assignment of epidote-hornfels facies. Metasomatism is extensive throughout the region yet it is not severe enough to obscure original igneous textures of the clasts. Petrographic variety is observed among the clasts, which range from aphanitic to porphyritic with up to 15% subhedral to euhedral plagioclase phenocrysts up to 2 mm and recrystallized phenocrysts of mafic minerals. Clast and matrix textures, including potential jigsaw texture and ragged clast edges, support an interpretation of a subaqueous depositional environment. The conglomerate of Clooney Lake, which underlies the tuff of Saddlebag Lake, has sedimentary structures similar to turbites (Schweickert and Lahren 2006), furthering this interpretation, and suggesting that similar to sections farther south in the Triassic arc, the early arc magmatism in Saddlebag Lake pendant was largely subaquatic.