2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 8
Presentation Time: 8:00 AM-12:00 PM


BERN, A., U.S. Geological Survey, Box 25046, MS973, Denver Federal Center, Denver, CO 80225, HAMMARSTROM, J., U.S. Geological Survey, 974 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, MORTON, D., U.S. Geological Survey, Department of Earth Sciences, University of California, Riverside, CA 92521, PREMO, W., U.S. Geological Survey, Box 25046, MS963, Denver Federal Ctr, Denver, CO 80225 and SNEE, L., U.S. Geological Survey, Box 25046, MS 974, Denver Federal Ctr, Denver, CO 80225, amyjohns@usgs.gov

Magmatism in the northern Peninsular Ranges batholith was marked by the emplacement of plutons from west to east in a progression between 125 and 83 Ma (Premo et al., 1998; Morton et al., 2000). Plutons west of the central part of the Perris block are isotopically primitive and passively intruded at shallow crustal levels into Mesozoic island-arc rocks. To the east, in a zone where high-grade metamorphic rocks of mixed affinity are exposed, high-strain-zone foliated plutons that are more isotopically evolved intruded at ~100-Ma at deeper crustal levels. East of the San Jacinto fault, larger and more evolved 90-100-Ma plutons were emplaced at slightly higher crustal levels than the 100-Ma plutons. Pressures of suture-zone-related 100-Ma metamorphic rocks range from 0.2 GPa in the west through 0.3 GPa to as much as 0.6 GPa in easternmost exposures. Adjacent plutons give Al-in hornblende geobarometry of <0.2 GPa in the west and 0.6 GPa in the east. These pressures are consistent with pressures of the older metamorphic rocks indicating that little uplift of the metamorphic rocks occurred before emplacement of the cross-cutting foliated plutons. Argon thermochronology of these plutons and metamorphic rocks indicate hornblende Ar closure of 550C to K-spar closure of less than 150C from 100 to 85 Ma. The cooling rate and barometry together indicate time averaged uplift rates in the late Cretaceous in the range of 0.3 to 1.0 mm/yr. Al-in hornblende barometry of plutonic rocks on a regional scale is consistent with earlier studies with pressures of <0.2 GPa in the western plutons up to 0.6 GPa in the central block and between 0.5 and 0.7 GPa in the eastern block. Across the Peninsular Ranges mylonite zone in the San Jacinto Mountains, pressure differences of more than 0.1 GPa are preserved in compositionally identical plutonic rocks.