New Hypothesis for the Assembly of the Catalina Schist

The Catalina Schist is a high P/T terrane situated outboard of SW North America. On Catalina Island, amphibolite facies recrystallization and partial melting occurred at ca. 115 Ma and 40 km depth. Metamorphic grade decreases downward to lawsonite-albite facies across low-angle faults. While the inverted metamorphic zonation on Catalina Island has been widely regarded as a disrupted metamorphic aureole formed during nascent subduction, this hypothesis conflicts with key relationships: (1) Both the Franciscan Complex (north) and the western Baja Terrane (south) record Jurassic thru Cretaceous subduction; (2) Mid Jurassic and Early Cretaceous magmatism occurred in the adjacent, 750 km long, mostly 120-90 Ma Peninsular Ranges batholith (PRB); (3) Sparse, Jurassic blueschist blocks similar in age to those in the Franciscan and Western Baja terranes occur in blueschist melange on Catalina Island. New detrital zircon U-Pb ages (37 samples; N = 750) demonstrate that the Catalina Schist was derived from sucessively accreted sediment shed from the evolving continental margin and rule out the possibility that all elements of the complex formed in an originally contiguous thermal aureole during incipient subduction. Zircons from amphibolite grade metasediments are mostly mid Proterozoic although unrecrystallized grains as young as 120 Ma occur. Taking previously obtained compositional and radiogenic isotopic data into account with along with detrital zircon results from potential source regions, we conclude that the sediments represent early Aptian detritus shed from the early Cretaceous volcanic arc, Jurassic granitoids, and pre-Jurassic flysch wallrocks of the northern PRB. Recrystallized detrital zircon and neoformed grains from pegmatites and leucosomes reveal extensive new zircon growth between 105-120 Ma. Protracted high-temperature conditions are also indicated by hornblende (115-108 Ma) and muscovite (105-100 Ma) Ar cooling ages. Persistence of high-T conditions over 10-15 m.y. is inconsistent with the rapid refrigeration of early accreted materials that is predicted by numerical simulations of nascent subduction that culminates in quasi-steady state, sub-blueschist facies conditions. Detrital zircon populations from epidote amphibolite metagraywackes appear to represent Albian age sediment and are mostly Early Cretaceous and Jurassic with grains as young as 110 Ma. Epidote blueschist results are transitional between the high and low-grade units. The latter (lawsonite blueschist, actinolite-albite, and lawsonite-albite) were derived from Cenomanian-Turonian age detritus from western PRB plutons (125-100 Ma). All yield similar zircon age distributions. The youngest zircons (100-95 Ma) overlap phengite Ar cooling ages. We note that the Catalina Schist structurally underlies the western margin of the northern PRB and that Early Cretaceous forearc rocks are largely absent. We suggest that PRB began overthrusting its forearc basin at 120 Ma and that the gabbroic slab that underlies amphibolite metagraywackes on Catalina Island represents mafic arc basement to the overthrust basin. Epidote amphibolite and epidote blueschist rocks represent successively underthrust duplex sheets. Only the lawsonite blueschist (w/ Jurassic blocks) and lower grade units were accreted within the subduction zone. Overthickening of the wedge triggered extensional denudation that juxtaposed the disparately formed elements of the Catalina Schist in the middle crust by 90 Ma.