ISOTOPIC EVIDENCE ON THE NATURE OF PRECAMBRIAN BASEMENT AND OVERLYING SEDIMENTARY ROCKS, CABORCA, SONORA

Combined Nd isotope and U-Pb zircon data have been used to assess the nature of Precambrian basement rocks, and to determine the provenance of overlying Precambrian to Early Cambrian siliciclastic sedimentary rocks, in the Caborca area, Sonora. Paleoproterozoic orthogneiss of the Bamori complex, ~1.4 Ga granitic rocks from Cerro Rajon, and the ~1.0 Ga Aibo granite, have measured eNd values from –18 to –25 and TDM ages from 2.0 Ga to 2.4 Ga. The Nd data suggest that both ~1.4 Ga and ~1.0 Ga rocks contain a significant component of Paleoproterozoic rocks. The highly negative measured eNd values and the 2.4 Ga Nd model age of the Bamori complex suggest that Precambrian crust at Caborca contains components derived from earliest Paleoproterozoic and perhaps Archean continental crust and could represent a segment of “Mojavia” crust similar to that exposed in southern California. Precambrian to Early Cambrian miogeoclinal sedimentary rocks in the Caborca area have whole rock Nd and detrital zircon U-Pb ages consistent with their derivation from erosion of subjacent Paleoproterozoic crust. The primary exception is a sandstone (Unit 1) from the early Cambrian Puerto Blanco Formation, which has higher measured eNd values (up to –4.2) and younger Nd model ages (~1.0 Ga) than the basement rocks in the region, in part reflecting the presence of basaltic volcaniclastic material (Farmer et al., J. S. Amer. Earth Sci, 2001). However, the ~1.1 Ga detrital zircons in this unit have measured eNd values similar to that of the whole rock (-4.8). The zircons were apparently not derived from the highly negative eNd, local ~1.0 Ga Aibo granite and either represent detritus derived from more positive eNd, ~1.0 Ga igneous rocks yet to be identified in the region or from a distinctive source in Grenville-age basement found much further to the east. High eNd, siliciclastic detritus bearing ~1.0-1.1 Ga zircons has been found in a number of Cambrian miogeoclinal sedimentary rocks throughout western North America. It remains unresolved as to whether the existence of such sediment implies that ~1.0 Ga igneous activity was more widespread than previously recognized or that Grenville age rocks were involved in an early Cambrian orogenic event that led to the widespread deposition of Grenville-derived detritus throughout SW North America.