T241 Subduction Zone Magmatism from Source to Eruption, in the Lab and in the Field: In Honor of Christy Till, GSA Mineralogy, Geochemistry, Petrology, and Volcanology Division Early Career Awardee

Wednesday, 25 September 2019
8:00 AM - 12:00 PM
Room 131ABC, North Building

SHIFT FROM SOURCE TO SYSTEM CONTROL ON PLUTON COMPOSITIONS RECORDED IN THE MIGRATED JACK MAIN CANYON INTRUSIVE SUITE IN THE CENTRAL SIERRA NEVADA, CA

Paper number: T241-14
Program: Technical Sessions
Time: 11:40 AM – 11:55 AM

Authors

Abstract

The Cretaceous Sierra Nevada magmatic arc migrated eastward and underwent spatiotemporal magmatic focusing (~105-85 Ma) in the central Sierra Nevada (CSN). The ~210 km2 Jack Main Canyon intrusive suite (JMCIS) formed from ~97.9 to 94.4 Ma by a series of plutons emplaced in the CSN along a ~10 km migration path trending ~295° that is opposite to arc migration and focusing. We synthesize mapping, LA-ICPMS U-Pb zircon ages, petrography, and whole-rock element and isotope chemistry to propose a three-stage physical and petrologic model for this largely unstudied system.

Six plutonic facies young from SE to NW in three stages: (1) Mount Gibson quartz diorite fine (Kgif) and coarse-grained facies (Kgic) and Bearup Lake granodiorite (Kbu), (2) Lake Vernon granodiorite (Klv) and Boundary Lake transitional granodiorite facies (Kblt), and (3) Boundary Lake two-mica granite facies (Kbl). Stage one has sharp contacts and discordant fabrics; stage two and three have gradational contacts and concordant fabrics. U/Pb zircon ages signal incremental growth and increasing antecryst abundance with time. Three Plag populations vary in abundance across all units. Hbl with embayed, brown cores are present at the boundary from stage one to two. Stage one units share few chemical overlaps, have negative (0.7-0.8) Eu/Eu* and lower crustal Dy/Dy* values, and a gently positive La/Yb slope. Stage two units have intersecting compositions, approach unity in Eu/Eu*, have mid/upper crustal Dy/Dy* values, and a steeper La/Yb slope. Stage three has negative Eu/Eu* values (~0.7), upper crustal Dy/Dy* values, and a negative La/Yb slope.

REEs track the onset of Plag suppression and Hbl growth in the source from stage one to stage two. Stage one plutons with sharp contacts reflect no across-unit hybridization and good source compositional control. Stage two plutons represent hybridizations of Hbl-fractionated source melts with JMCIS magmas already in the upper crust. Stage three pluton, Kbl, is a fractionate of Kblt (of stage two) during inward solidification, signaling strong emplacement-level control. A magmatic history spanning source to emplacement level chemical control was preserved in the JMCIS by the migratory emplacement of plutons, offering a natural lab for tracking petrogenesis distinct from common, nested systems.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-341103

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