LARAMIDE THRUSTING IN SE CALIFORNIA: NEW AGE CONSTRAINTS FROM LATE SYN-KINEMATIC PEGMATITES AND ACCESSORY MINERAL PETROCHRONOLOGY
We present new data that confirms a Late Cretaceous Laramide deformation age in the Cargo Muchacho Mountains. This deformation entailed N/NW-directed ductile-to-brittle thrusting under conditions that cooled from ca. 500-700°C to below 280°C, at a maximum rate of 24°C/My over a 10 My period. Earlier, high-T deformation is recorded by all units. The Jurassic granites display intense recrystallization in thin section, indicating that deformation post-dated their emplacement. Rare feldspar sigma clasts indicate top-to-N/NW kinematics, and quartz microstructures reflect rapid grain-boundary migration, suggesting deformation temperatures of ca. 500-700°C. Titanite in these granites yield dates of ca. 143-64 Ma, consistent with the timing of granite crystallization and with Late Cretaceous-early Paleocene ductile deformation, as recorded by the strong foliation and deformed pegmatites. These pegmatites form conjugate sets that cross-cut the high-T ductile foliation; pegmatites at low angles to the foliation are intensely boudinaged, whereas those at high angles are folded. This reflects a strain ellipsoid consistent with N-directed thrusting, in line with kinematics recorded in the sheared granite. Pegmatite intrusion is dated at 67-63 Ma by zircon U-Pb analysis; their moderate deformation (folding and boudinage, but no transposition or recrystallization) suggests emplacement during the waning stages of ductile shearing.
Finally, apatite U-Pb ages of ca. 54-47 Ma may indicate cooling through a closure T of ca. 450-500°C, likely reflecting erosion related to crustal thickening during Laramide-age thrusting. Some pegmatites are cut by foliation-parallel brittle faults with a top-N thrust sense, which must have formed after cooling below ca. 280°C, and suggest continued lower-T thrusting.