CRETACEOUS BATHOLITH CONSTRUCTION DURING EPISODIC GARNET GRANULITE-FACIES METAMORPHISM IN THE SAN GABRIEL MOUNTAINS, SOUTHERN CALIFORNIA

The San Gabriel Mountains (SGM) preserve an exhumed section though the crust of a Mesozoic Cordilleran arc. We present new LA-SF-ICPMS and SHRIMP-RG zircon dates from the Cucamonga (CT), San Antonio (SAT), and San Gabriel (SGT) Terranes that allow us to document timescales of magmatism and metamorphism at various structural levels within the Cretaceous arc during batholith construction. The CT exposes the deepest portions of the arc, including migmatitic garnet granulites that are interlayered with quartzofeldspathic gneisses, minor marbles, and calc-silicates; metamorphism occurred at 8-9 kbar and 775–800°C. Metamorphic zircons from a garnet granulite yielded ²⁰⁶Pb/²³⁸U error-weighted average ages of 86.2 ± 0.7 Ma (cores; 2SD) and 83.8 ± 1.5 Ma (rims). Metamorphic zircons from a calc-silicate yielded a similar age of 86.1 ± 2.5 Ma. A nearby charnockite dike yielded an age of 87.3 ± 1.2 Ma.

Mid-crustal, tonalites in the SAT structurally overlie granulites, and locally display migmatitic textures consisting of cm-scale disseminated garnets with plagioclase coronas. Mylonitic fabrics are common in tonalites and are cut by syn- to post-kinematic granodioritic to granitic dikes. Igneous zircons cores from a metatonalite yielded an age of 85.8 ± 0.6 Ma which is interpreted as the timing of igneous crystallization. A minor population of xenocrystic zircons ranging from 1.69-1.74 Ga indicate interaction with Proterozoic wall rock. Metamorphic rims on tonalitic zircons yielded an age of 75.5 ± 5.3 Ma, which is interpreted as the timing of metamorphism in the tonalite unit. This date overlaps with ages from two syn- to post-kinematic granitic dikes at 76.2 ± 0.5 and 74.0 ± 0.7 Ma.

In the SGT, Cretaceous batholitic rocks were emplaced at ~5-7 kbars and include diorites, tonalites, and granodiorites. A tonalite from the Josephine Mountain Intrusion yielded a date of 79.9 ± 1.7 Ma and also contained significant xenocrystic cores with dates of 1.61, 1.69, and 1.75 Ga. A granodiorite from the Waterman Batholith yielded a date of 77.5 ± 1.5 Ma, and the Mt. Wilson quartz diorite gave an age of 73.0 ± 1.9 Ma. Collectively, our zircon dates demonstrate that Cretaceous batholith construction in the SGM occurred over ~14 Myr from 87-73 Ma, and involved two episodes of high grade metamorphism at ~85 Ma and ~76 Ma.