UPPER CRUSTAL EVOLUTION DURING AN ARC FLARE-UP: INTEGRATING SEDIMENTARY, VOLCANIC, AND PLUTONIC RECORDS OF THE TRIASSIC-EARLY JURASSIC CONTINENTAL ARC UPPER CRUSTAL SECTION, EASTERN-CENTRAL SIERRA NEVADA, CA

Pendants of the eastern central Sierra Nevada (CSN) preserve a Triassic to Early Jurassic continental arc upper crustal section composed of intra-arc strata, subvolcanic intrusions, and upper crustal plutons spanning the first of three CSN Mesozoic peaks of magmatic addition. We integrate new and published mapping, whole rock geochemistry, zircon geochronology, and zircon trace element and isotope records to characterize this upper crustal arc section. The CSN provides a more complete record of the magmatic flare-up and accompanying volcanism, deformation, shifting magma chemistries, and surface evolution that followed Permo-Triassic arc initiation than other SW Cordilleran arc segments.

~215-235 Ma upper crustal plutons intrude Paleozoic SW Laurentian margin strata and unconformably overlying Triassic intra-arc deposits. Fault bounded stratigraphic sections spanning ~180-240 Ma are dominated by Triassic Koip Sequence volcanics and minor sediments unconformably overlain by Early Jurassic interbedded marine sediments and volcanics. Differences in sequence, lithology, ages, and thickness hinder stratigraphic correlations between sections. Sections comprise three broad tectonostratigraphic groups based on depositional ages and setting. Marine and terrestrial sediments show unimodal detrital zircon distributions with few older ages, in contrast to older and younger regional marine overlap deposits with abundant pre-Mesozoic ages. Major detrital peaks closely track depositional ages, even continuing into the subsequent magmatic lull. Together, these observations indicate that intra-arc sediments dominantly sourced the barely emergent marine arc.

Dominantly intermediate to felsic volcanics and plutons show similar compositional ranges (~ 45 to 75% SiO₂). Whole rock ^87/86Sr_i (~ 0.704 to 0.7065) and eNd (~ -8 to -1) isotopic compositions indicate some crustal input into arc melts. Comparison to isotopic data from coeval arc segments in the oceanic Western Metamorphic Belt and El Paso Terrane, and continental Mojave Desert indicates that the lithosphere underpinning arc segments played an important role in melt sources. CSN volcanic zircon grain eHf values (~ -7 to 7) remains relatively constant until the flare-up wanes suggesting little change in crustal contributions to melt sources.