LATE JURASSIC TO LATE CRETACEOUS LASS TITANITE AGES FROM CALC-SILICATE ROCKS FROM THE BIG MARIA MOUNTAINS, SE CALIFORNIA: CONSTRAINTS ON THE TIMING OF HIGH-VOLUME MID-CRUSTAL FLUID INTERACTION

Metamorphosed and deformed Paleozoic platform facies strata are exposed in the Big Maria Mountains in SE California. Of particular interest is the timing of crystallization of massive wollastonite in the metamorphosed Supai Fm, which was previously determined to have required interaction with large volumes of mid-crustal water-rich fluids at upper greenschist to amphibolite facies conditions. Pressures of metamorphism are constrained by the presence of kyanite in the lower grade rocks to be >4 kb. We dated metamorphic titanite from three calc-silicate samples, one of Supai Formation (SW2-2-17) from the western part of the range, and two of Kaibab Formation (N3-2-7 and N-1-19) from the NW part of the range. SW2-2-17 comprises wollastonite (66.9%), calcite (19.0%), microcline (4.0%), diopside (1.0%), grossular (8.5%) and titanite (0.5%). N3-2-7 comprises tremolite (39.2%), diopside (17.7%), microcline (21.5%), quartz (17.9%), calcite (2.45%) and titanite (1.09%). N3-1-19 comprises tremolite (21.9%), diopside (2.24%), phlogopite (2.37%), microcline (38.9%), quartz (32.8), and titanite (1.75%). A T-X(CO)₂ isochemical plot for SW2-2-7 predicts titanite crystallized from a rutile consuming reaction at temperatures ~80 °C lower than the crystallization of wollastonite. In the other two samples, titanite may have crystallized from rutile at or below the temperature of the stability of the rocks’ mineral assemblage, depending on pressure and value of X(CO)₂. Titanite U-Pb ages were determined for all three samples using the laser ablation split-stream (LASS) method. Both SW-2-2-17 and N3-1-19 yielded single discordia arrays on inverse concordia plots with ages of 158.5 ± 3.7 Ma (MSWD = 2.3) and 126.7 ± 7.2 Ma (MSDW=1.07), respectively. Data for N3-2-7 were more scattered; 74 spots yielded two populations of 207-corrected ²⁰⁶Pb/²³⁸U dates: 66-76 Ma (n=72) and 84 Ma (n=2). Collectively, the ages date metamorphism from the Late Jurassic through the Late Cretaceous. An Ar-Ar step-heating age on hornblende from the western part of the range indicates that cooling was underway by ~75 Ma. Wollastonite formed post-158 Ma, possibly in the Late Cretaceous, and may have resulted from fluids passing vertically upward from the shallowly subducting dewatering Farallon slab.