IGNEOUS AND METAMORPHIC CHRONOLOGY OF SALINIAN ARC LOWER ARC CRUST, CENTRAL CALIFORNIA COAST

Variation in magmatic flux through time is documented in batholiths throughout the Cordillera, although our understanding of the processes responsible for high-flux events is limited by the scarcity of exposed arc lower crust. The Coast Ridge belt of the Salinian arc exposed along the central California coast consists of Sur Series metamorphic framework rocks intruded by ~0.7-0.8 GPa Late Cretaceous tonalite, and offers a unique opportunity to directly investigate magmagenesis in the arc lower crust. To place additional constraints on the petrogenesis of the Coast Ridge belt, we present new zircon geochronology and geochemistry from three Sur Series paragneisses, two Sur Series orthogneisses, and four samples with variable composition from the orthopyroxene tonalite body at Grimes Point.

Sur Series paragneiss includes abundant Archean, 1.8–1.6, 1.4, and 1.2–1.0 Ga inherited cores, and rims with concordant Cretaceous ages that range in age from 110–80 Ma. In two garnet-bearing paragneisses, 102–92 Ma zircon rims display flat HREE and are interpreted to record the timing of metamorphic zircon growth. Two Sur Series orthogneisses have high Lu cores that yield weighted average ages of 113 and 107 Ma, respectively, and are interpreted to record the timing of garnet-absent igneous crystallization. Zircon rims from the same samples yield flat HREE profiles indicating subsequent garnet-stable zircon overgrowths from 98–90 Ma. Samples from the orthopyroxene tonalite body yield 102–100 Ma zircon cores that are interpreted to represent the timing of its igneous crystallization, and 100–92 Ma metamorphic rims with lower Lu/Dy in garnet-bearing samples, or higher U/Th in samples without garnet.

These results suggest that the Salinian arc lower crust developed during an initial phase of 113–100 Ma garnet-absent igneous intrusion followed by garnet growth and migmatization, potentially either punctuated or protracted, between 100 and 92 Ma. Given the latest Early Cretaceous igneous crystallization ages presented here for Sur Series orthogneiss, we suggest that this metamorphic event may have been triggered by burial and heating associated with eastward underthrusting of older western and axial portions of the batholith beneath the active eastern arc during intra-arc shortening at the onset of the Late Cretaceous.