LATE CRETACEOUS GRANITOIDS IN THE MIDDLE TO LOWER CRUST, EASTERN TRANSVERSE RANGES, CALIFORNIA: MAGMATIC EVOLUTION IN A CRUSTAL COLUMN

The eastern Transverse Ranges broadly represent a tilted crustal section of a continental arc preserving depths from 0 to ~25 km. The generalized structural column is large, blobby granodiorite to granite plutons at depths shallower than 15 km, a sheeted complex (SC) between 15-22 km composed of meter-thick intrusive sheets, and diorite, tonalite, and granodiorite blobby plutons below the SC.

Two plutons intruding Proterozoic gneiss at or below the depth of the SC have been dated as latest Cretaceous (ICPMS, U-Pb in zircon). The hornblende biotite quartz diorite to tonalite has an earliest magmatic zircon age of 71.9 ± 1.9 Ma, and a range of zircon ages between 70.1-79.5 Ma. Xenocrystic zircon ages are at ~1210, 1380-1456, and 1656-1740 Ma. The granite has an earliest magmatic age of 75.7 ± 3.5 Ma and a range of zircon ages between 73.5-94.8 Ma. Three xenocrysts are 1439, 1635, and 2280 Ma.

These granitoids represent snapshots of silicic melts ascending through the middle crust. They show irregular but conformable margins; field evidence of migmatization and zones of abundant xenoliths of varying composition and orientation suggests local melting and magmatic disaggregation of host rocks. Hornblende-bearing leucocratic dikes in the Proterozoic host gneisses are spatially associated with the granitic pluton and appear to exchange magma with the pluton. While thermobarometric analyses will be performed on the tonalite to better constrain its depth in the crust, relative to the regional structural grain, the tonalite appears to be structurally higher in the crust than the granite and lower than the SC.

The discovery of Cretaceous age granitoids is important for refining models of the SC and the generation of the upper crustal granites. The SC is compositionally diverse, from gabbros to two-mica garnet granites, and very few silicic plutons have been characterized below it. This has suggested that upper crustal granites may be generated in the SC through a combination of magma fractionation processes. However, deeper contaminated felsic melts rising to the level of the SC may contribute more to the creation of upper crustal plutons than previously recognized, either through bypassing the SC or hybridizing within it.