Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 39-6
Presentation Time: 8:30 AM-6:00 PM


PITSILLIDES, Iason, NRES, Cal Poly San Luis Obispo, 1 Grand Ave, San Luis Obispo, CA 93407, JOHNSTON, Scott, Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407 and KYLANDER-CLARK, Andrew R.C., Earth Science, University of California, Santa Barbara, CA 93106

Voluminous magma that forms in arcs above subduction zones at convergent margins is a first-order tectonic process that has implications for the formation of continental crust. Magma bodies evolve during ascent, cooling, and interaction with crust in the overriding plate. However, the timing of deformation and metamorphism in the overriding plate with respect to intrusion are poorly constrained, and degree to which rising arc magmas are contaminated by lower and middle crustal rocks remains uncertain. In this study, we investigate Salinian block arc exposures from the Santa Lucia Mountains where granitic arc rocks and their high-grade metamorphic framework present a unique opportunity to investigate lower- and middle-crustal arc processes and magma evolution. To place constraints on the timing of deformation, metamorphism, and igneous intrusion within the Salinian block, we collected garnet-bearing samples of orthogneiss and paragneiss from a transect across the central portion of the Salinian block where prior studies have indicated metamorphism and crystallization at middle-crustal depths. Here, we present the field and petrographic observations as well as coupled zircon geochronology and geochemistry from these rocks. Our results indicate a protracted intrusive history from ~120–85 Ma followed by high-temperature metamorphism at 90–85 Ma. Outcrop and microstructural relations suggest that the main phase of deformation occurred after 95 Ma, but that lithostatic conditions were dominant during metamorphic zircon crystallization in pelitic migmatites and during intrusion of late dikes from 90–85 Ma. Our future work will supplement this existing dataset with samples collected from an excellently exposed transect along the Arroyo Seco River gorge. Preliminary observations indicate the presence of foliated migmatitic, garnet–sillimanite paragneisses interlayered with cm- to 10s-of-m-scale garnet-bearing orthogneisses cut by late pegmatites. The continuous exposures in Arroyo Seco, when coupled with petrographic observations and detailed zircon geochronology and geochemistry, will help to provide context between the numerous rock types within the metamorphic host, and ultimately, to constrain the timing of intrusion, deformation, and metamorphism in the Salinian block middle crust.