RAPID CRUSTAL THERMAL RESPONSE TO A SHALLOWING SLAB? TEMPORAL CONSTRAINS ON THE LATE CRETACEOUS LARAMIDE TRANSITION IN JOSHUA TREE NP, CALIFORNIA, US

Late Cretaceous igneous rocks in the Little San Bernadino mountains within Joshua Tree National Park, presents a unique opportunity to investigate the conditions and rate of the crustal response of the transition from Sevier- to Laramide-style tectonics, and the final stages of a waning continental margin arc. Canyon walls reveal ductily deformed granodiorite plutons with strong magmatic foliations hosted by migmatized Proterozoic gneiss. Zircon U-Pb ages of the deformed granodiorite yield consistent ages of 74 ± 2 Ma. The ~74 Ma age from the autocrystic zircon suggest these granodioritic bodies are among the final melt products associated with the California Mesozoic arc system.

A swarm of undeformed felsite dikes cross-cut the granodiorite and orthogneiss bodies with an average strike of 230o. In contrast to the ductile deformation and magmatic foliation of the host rocks, the dikes have clearly defined, linear contacts suggesting intrusion conditions above the brittle-ductile transition. The difference in crystallization ages between the ductily deformed granodiorite and these dikes with chilled margins provide opportunity to constrain the rate of crustal cooling at the end of arc magmatism.

Zircon separated from the dikes yield an array of U-Pb ages, including Proterozoic grains interpreted to be inherited from the host gneiss, Mesozoic grains ranging from ~80 to ~100 Ma that reflect the voluminous arc productivity of the region, and ~74 Ma grains likely inherited from the host granodiorites. Proterozoic grains populate apparent discordia projecting toward the age of the granodiorite plutons. Significantly, dikes yield concordant young zircon with ages between ~69 and ~65 Ma; four concordant zircon from one prominent dike yield a weighted mean age of ~66.0 Ma, and indicate cooling of the crust, from ductile conditions to brittle conditions, occurring in under ~8 Ma.

Ongoing geochronologic and isotopic investigations focus on 1) identification of a source material, potentially subduction related schists, melted to generate the Late Cretaceous felsite dikes, and 2) best practices for obtaining robust dates of dikes.