T71 The Changing Cordilleran Margin: Permian through Eocene Tectonism, Magmatism, and General Geology from Western Mexico through Northern California and Eastward

Sunday, 22 September 2024
1:30 PM - 5:30 PM
304A

SECULAR EVOLUTION OF PLUTONIC ZIRCON ΕHFI VALUES FROM THE SOUTHERN SIERRA NEVADA ARC: A RECORD OF THE CHANGING ORGANIZATION OF MESOZOIC SUBDUCTION ZONES

Paper number: T71-4
Program: Technical Sessions
Time: 2:10 PM – 2:25 PM

Authors

Abstract

Values of εHfi in concordant zircon (n=563) separated from Permian through Cretaceous plutons of the Kern Plateau define a 5-stage secular evolution curve and support reconstruction of Mesozoic subduction zones beneath the southern Sierra Nevada arc. Stages include: (1) A Permian-Early Triassic isotopic pull-up (ca. 290 – 240 Ma) previously interpreted as recording extension driven by slab roll-back. (2) Following an apparent magmatic gap, zircon xenocrysts (ca. 230 – 200 Ma) record an isotopic draw-down connecting Early Triassic (+8) to Middle Jurassic (–10) zircon εHfi values. (3) A nearly 40-m.y. period (ca. 200 – 160 Ma) in which differentiation produced voluminous, isotopically enriched granitoid magmas of the Sacatar Intrusive Complex. Signals of punctuated extension include zircon isotopic excursions to both high (+15) and low (–40) εHfi at ca. 190 and 160 Ma, and intrusion of a 10-m-wide, NW-trending andesitic dike (162 ± 2 Ma) along the NE margin of the Sacatar Intrusive Complex (pluton ages ca. 185 – 163 Ma). (4) An abrupt, 40-ε-unit isotopic pull-up (ca. 155 – 145 Ma zircon) and concomitant emplacement of the bimodal Summit Igneous Complex (148 ± 2 Ma) along the Kern Plateau shear zone. Intrusion of the Summit and the sub-parallel Independence dike swarm (ca. 149 ± 1 Ma) have previously been interpreted as a failed rift event. (5) A Cretaceous isotopic draw-down from +10 to –10 εHfi. Age gaps in the Kern Plateau plutonic record (ca. 230 – 200, 140 – 100 and 90 – 80 Ma) are filled by zircon xenocrysts presumably plucked from plutons underlying the Plateau. Plutonic zircon (n=116) of the nearby Late Triassic Scheelite, Middle Jurassic Palisades Crest, and Early Cretaceous Stokes Mountain suites corroborate xenocryst data as representative of regional arc εHfi values. Coupling secular εHfi trends with geologic constraints supports a model of arc magmatism and transpressive accretion by multiple, eastward-subducting slabs and westward-stepping trenches. Interpreted with the plutonic record, Nb and εHfi excursions recorded by Middle to Late Jurassic MHREE-rich detrital zircon support progressive evacuation of the arc’s mid-to-deep crust magmatic column; record an isotopically primitive Middle Jurassic signature previously seen in metavolcanic strata; and reveal intrusion of Nb-rich asthenospheric magmas suggesting slab tear during latest Jurassic failed rifting.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-404261

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