DETRITAL ZIRCON GEOCHRONOLOGY OF BASEMENT AND COVER ROCKS FROM THE SANTA LUCIA MOUNTAINS, CALIFORNIA; NEW EVIDENCE FOR LIMITED TRANSPORT OF SALINIA

Salinia is an out-of-place granitic terrane in California bounded by strike slip faults and Franciscan rocks. Some 340 km of right lateral displacement are attributable to late Cenozoic motion along the San Andreas system; prior to this, the Salinian block must have been close to the Mojave region in southern California. Paleomagnetic and geologic arguments suggest however, that Salinia could have originated much further south of the Mojave, probably at the latitudes of southern Mexico; after 30 years of research, the controversy of limited vs. significant travel for “orphan” Salinia, has not been settled. In order to further address this problem, we used LA-MC-ICPMS techniques to determine detrital zircon U-Pb ages on two samples from the Coast Ridge Belt (CRB), Santa Lucia Mts.: (1) CONE, a quartzite from the Sur Series exposed in the vicinity of Cone Peak, and (2) ESALEN, a sand-rich layer within the Maastrichtian cover that rests unconfomably over the Salinian basement. The depositional age of the Sur Series is unknown; arc-related granitic rocks in the CRB (92-80 Ma) constrain that age to be pre Late Cretaceous. One hundred zircon grains were measured from each sample, yielding 168 ages that are of sufficient concordance and precision to provide provenance information. CONE yields distinctive age peaks at 360-380, ~425, 1100-1200, ~1430, 1750-1900, and ~2700 Ma. ESALEN has distinctive peaks at ~93, ~135, ~155, 240-280, ~380, and 1400-1440 Ma. From a provenance point of view, the distribution of Precambrian ages is consistent with a paleoposition near the SW US but are inconsistent with an origin near southern Mexico. Other important age peaks in the two analyzed samples are 360-380 Ma in CONE and 240-280 Ma in ESALEN; these are unusual orogenic ages for the southwestern Cordillera. Locations that could have yielded Permian ages are in areas where the eugeocline is present, e.g. in the vicinity of the El Paso Mountains, northern Mojave desert. We suggest that during Late Cretaceous time, mid- and late Paleozoic magmatic assemblages may have been widespread along the continental margin of SW US and NW Mexico.