MULTIPLE INTRUSIONS AND LOW-PRESSURE METAMORPHISM IN THE CENTRAL OLD WOMAN MOUNTAINS, SOUTH-EASTERN CALIFORNIA - CONSTRAINTS FROM THERMAL MODELING

The Old Woman Mountains in south-eastern California are a Late Cretace ous low-pressure metamorphic terrane where multiple magmatic intrusion s generated broad regions of elevated metamorphic temperatures. In the Scanlon Gulch area, two sheet-like, Late Cretaceous granitoid plutons are in contact with the Scanlon shear zone, a 1-km-thick sheet of iso clinally folded and transposed metamorphic rocks. The metaluminous Old Woman granodiorite underlies the shear zone and the peraluminous Swee twater Wash granite overlies it. Both plutons record emplacement ages of approximate to 74 Ma. Thermobarometry and phase relations in the sh ear zone suggest that peak metamorphism was at 650 +/- 50 degrees C an d 4.3 +/- 0.5 kbar. Late Cretaceous metamorphic temperatures were less elsewhere in the Old Woman Mountains, away from the intrusions. One-d imensional thermal models are used to investigate how differences in t he time between the emplacement of plutons would affect the thermal ev olution of the central Old Woman Mountains. The prediction of a therma l history inferred from petrological and thermochronological data requ ires the rapid emplacement of the two plutons around the shear zone; s imulations with delays of more than 1 Myr in the emplacement of the se cond pluton failed to predict peak metamorphic temperatures. Calculati ons which consider only the emplacement of a single pluton yield metam orphic temperatures that are too low. The time separating the intrusio ns is by far the most sensitive parameter in the calculations; assumpt ions concerning the treatment of the initial geothermal gradient and t he latent heat of crystallization have relatively small effects on the predicted thermal histories. Our results suggest that for certain geo metries, relatively short-lived magmatic events involving rapid emplac ement of multiple intrusions can produce low-pressure metamorphism.