CABORCAN PROTEROZOIC BASEMENT IN THE RANCHO LA LAMINA REGION, SONORA, MEXICO: U-PB AGES, ND ISOTOPIC COMPOSITION, 40AR/39AR THERMOCHRONOLOGY, AND CLAST PROVENANCE INDICATE PRE-EARLY JURASSIC ARRIVAL AND LARAMIDE EXHUMATION

U-Pb ages and Nd isotope data of Proterozoic rocks in north-central Sonora were collected to test the Mojave-Sonora megashear hypothesis. Proterozoic rocks at Rancho La Lamina near Cucurpe, 40 km north of the postulated megashear, include granitoids with U-Pb zircon ages of 1763 Ma, 1738 Ma, and 1737 Ma. The Escuadra micrographic granite has a U-Pb age of 1076 ± 14 Ma. εNd(t) values in the three Paleoproterozoic rocks are +0.5, +1.4, and -4.3, respectively. The Escuadra granite has εNd(t) of -1.7. These ages and Nd isotopic values are similar to those of the Caborca block at Caborca, Mexico, located south of the megashear. Diffusion and thermal modeling of potassium feldspar for two samples of Proterozoic granite, one (STAM-1) from Rancho Santa Margarita, south of the inferred megashear, and the other (LL-1) from the Escuadra granite at La Lamina, reveal distinct cooling profiles. STAM-1 shows cooling through ~275°C at ~275 Ma. LL-1 shows cooling through ~275°C at ~110 Ma. Both samples experienced final exhumation and cooling through 175°C during the Laramide orogeny, at about 75 Ma.

Mesozoic strata exposed adjacent to the Proterozoic rocks constitute a relatively complete Lower Jurassic-Lower Cretaceous stratigraphic section deposited in arc-related extensional basins bounded by steep normal faults that were inverted during the Laramide to expose the Proterozoic basement. Lower Jurassic strata contain basement clasts with ages of 1730 ± 11 Ma and 1749 ± 6 Ma, and εNd(t) values of –0.7 and –1.6. These clasts share the Caborcan affinity of the local basement. These strata were deposited at about 193 Ma, the age of the youngest detrital zircons in the conglomerate matrix, and before 188 Ma, the age of tuffs higher in the section.

Our data are thus inconsistent with Late Jurassic arrival of the Caborca block in Sonora, and suggest that Late Jurassic sedimentation occurred within a basin floored by Caborcan basement. This basin is adjacent to a similar, parallel rift basin to the north and both are inferred to be part of the Bisbee basin system. The parautochthonous Proterozoic rocks at La Lamina indicate that the Proterozoic crustal boundary between Caborca and the Mazatzal province lies at least 40 km to the north of the currently accepted location of the megashear, suggesting that (1) the megashear does not exist or (2) translation took place to the north of the study area.