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

Values of εHf_i 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 εHf_i 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) εHf_i 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 εHf_i. 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 εHf_i values. Coupling secular εHf_i 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 εHf_i 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.