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

The formation of arc magmas above subduction zones, and their evolution during ascent and interaction with wall rocks in the overriding plate of convergent margins, represents a first-order tectonic process with implications for the composition of continental crust. However, the timing of deformation and metamorphism in wall rocks with respect to the timing of intrusion are poorly constrained, and the degree to which rising arc magmas are contaminated by wall rocks in the lower–middle crust remains uncertain. In this study, we present new zircon geochronology and geochemistry from a suite of samples collected from a transect across the Salinian arc in the central California coast ranges where previous work has documented an arc section that exposes a coastal belt of lower-crustal arc rocks overlain by middle-crustal rocks farther east.

Results from six orthogneisses from the lower-crustal Coast Ridge belt yield high Lu zircon cores that define weighted average ages from 113–100 Ma and garnet-stable zircon rims from 100–92 Ma that are interpreted to represent the timing of igneous crystallization and metamorphism, respectively. Three paragneisses from the Coast Ridge belt yield Proterozoic cores with garnet-stable rims that range from 100–80 Ma. Farther east in middle-crustal arc exposures, high Lu cores from five orthogneisses define igneous ages ranging from 118–98 Ma with garnet-stable metamorphic rims between 90–80 Ma, whereas zircon from a cross-cutting dike and garnet-stable rims in three paragneisses yield ages from 90–80 Ma. Outcrop and microstructural relations suggest that deformation across the region continued after 98 Ma, but that lithostatic conditions were dominant during metamorphic zircon crystallization in pelitic migmatites and intrusion of late dikes from 90–80 Ma. Together, these results document a protracted 35 Myr history of magmatism in the central Salian arc from ca. 120–85 Ma that contrasts with more voluminous and migrating magmatism in the Sierra Nevada and Peninsular Range batholiths. Furthermore, the timing of high-temperature metamorphism and migmatization, which consistently post-dates igneous crystallization of adjacent gneisses, suggests that contamination of Salinian arc magmas with evolved sources occurred in the lower crust and prior to significant downward flow of the supracrustal rocks sampled in this study.