FIELD INVESTIGATION OF PALEOPROTEROZOIC(?) METAMORPHIC ROCKS AND JURASSIC(?) INTRUSIONS ON CENTRAL GLENDORA RIDGE, SAN GABRIEL MOUNTAINS, CALIFORNIA

We present geological mapping and thin section descriptions from a 500 m-thick section of Proterozoic(?) gneisses and likely Jurassic meta-plutonic rocks exposed along a hairpin on the Glendora Ridge Road. This section is important because the left lateral San Dimas Canyon fault cuts through a saddle directly east of the study site. We aim to compare the study area to a group of rocks previously mapped and dated farther east (Nourse, 1998; Premo et al., 2007) to constrain total displacement across the San Dimas fault. U-Pb zircon ages for the key rock units are pending from the ICP-MS lab at CSUN.

The focus area reveals a south-dipping section of metamorphic rocks composed of 1) Jurassic(?) fine grained foliated hbld-bio diorite, 2) medium grained biotite granite augen gneiss with rapakivi texture, similar to a large pluton of 1690±12 Ma porphyritic granite gneiss dated 5 km to the west (Premo et al., 2007), and 3) fine grained Paleoproterozoic(?) banded gneiss with isoclinally folded qtz-feldsp and bio±hbld-qtz-feldsp components, interlayered with mylonitized granite sills. Units 2) and 3) preserve amphibolite facies metamorphic textures in thin section, where primary mylonitic fabrics are statically annealed. Foliated sills of Jurassic diorite that intrude these rocks are also recrystallized.

Reconnaissance mapping to the southwest shows that the section is folded into a tight map-scale southwest trending synform with Precambrian rocks in the core and Jurassic meta-plutonic rocks on both limbs. The south-dipping limb is not accessible to the north due to heavy brush cover. However, the north-dipping limb is well-exposed along fire access roads and trails south of Glendora Ridge. Here the diorite unit is underlain by Jurassic biotite granite and bio-hbld quartz diorite containing large xenoliths of very coarsely porphyritic granite augen gneiss with 2-6 cm Kspar phenocrysts. This distinct rock resembles a 1398±5 Ma biotite granite exposed to the east across the San Dimas Canyon fault.

Ongoing research will constrain details of the metamorphic stratigraphy in areas south of the focus site. Distinct rock units and map-scale folds offer great promise to establish piercing lines across the San Dimas Canyon fault. U-Pb ages should provide important new information about this folded section of presumed Proterozoic crust.