NEW SHRIMP-RG U-PB ZIRCON AND SR ANALYSES FROM JURASSIC AND LATE CRETACEOUS PLUTONIC SHEETS IN THE EAST-CENTRAL SAN GABRIEL MOUNTAINS, CALIFORNIA

We report SHRIMP-RG U-Pb zircon and whole rock Sr analyses from nine Jurassic and ten Cretaceous intrusive rocks of the east-central San Gabriel Mountains. These samples aid correlation of several plutonic sheets intruded into a framework of Proterozoic gneisses previously described by Premo et al. (2007). Distinct host rocks (1.66-1.70 Ga augen gneiss, 1.4 Ga megacrystic granite, and 220 Ma Mt. Lowe intrusive suite) provide markers for reconstruction along Late Cenozoic faults.

Jurassic units define a gently east-plunging synform centered on Glendora Ridge. Three samples of diorite-qtz diorite at lower structural levels yield mean ²⁰⁶Pb/²³⁸U ages between 165 and 157 Ma. Porphyritic bio ± hbld granodiorite and monzogranite (3 samples each) hosted by the diorite are 166 to 159 Ma and 160 to 150 Ma, respectively. The Jurassic samples contain inherited zircon that is especially abundant in the granites; significant ²⁰⁶Pb/²⁰⁷Pb peaks occur at ~1.7 Ga, ~1.4 Ga and ~220 Ma. Sheets of qtz diorite that intrude the Proterozoic-Jurassic complex occur at four structural levels: Marshall Canyon (deepest), Glendora-Sunset Ridge, Clamshell Canyon, and Cogswell Reservoir (shallowest) that likely represent eastward extensions of the Mt. Wilson batholith. Mean ²⁰⁶Pb/²³⁸U ages of five samples suggest a single plutonic episode at 75 ± 2 Ma.

Correlative plutons occur in the hanging wall of the Vincent thrust north of the San Gabriel Fault between Mt. San Antonio and Middle Fork Lytle Creek. Three samples of qtz diorite yield mean ²⁰⁶Pb/²³⁸U ages between 78 and 72 Ma. A mylonitic leucogranite intrusion (70 ± 2 Ma) in qtz diorite constrains the maximum age of Vincent thrust displacement.

Initial Sr values of .7081-.7092 and .7076 -.7013 for the Jurassic and Cretaceous plutons, respectively, reflect the character of basement host rocks. Pronounced crustal assimilation is recorded in a granite dike of Cobal Canyon (~70Ma) that yielded >50% inherited zircon with peaks at 1.75, 1.68, and 1.4 Ga.

Mid Miocene restoration along the San Gabriel-San Andreas faults places the study area adjacent to the Cargo Muchacho and Chocolate Mountains of southeast California. This reconstruction expands the extent of basement units mapped by Dillon (1976) and establishes continuity to Mesozoic arcs and country rocks previously described in northwest Sonora.