Paleomagnetic reconstruction of Coahuila, Mexico: The Late Triassic Acatita intrusives

Abstract

The Acatita plutons intrude the upper Paleozoic of Las Delicias, southern Coahuila. We collected 96 samples from 15 sites in two plutons from Valle El Sobaco and Sierra Los Remedios for paleomagnetic and ⁴⁰Ar-³⁹Ar analyses. Separates of hornblende (215.9 ±1.9 Ma, 2a confidence level), biotite (217.3 ±1.2 Ma), and K-feldspar (205.6 ±1.4 Ma) yield nearly flat age spectra although none of the data satisfy strict plateau criteria, possibly reflecting slight alteration. The preferred ages are interpreted as fairly precise records of Late Triassic cooling of the plutonic suite, which has yielded concordant U-Pb Zircon ages ca. 220 Ma. Most samples yield north-directed and steep positive magnetizations similar to the late Cenozoic expected field direction. Some samples, mostly from mafic enclaves, show northwest magnetization of shallow inclination (southeast at one site) with alternating fields above ~30 mT and thermal demagnetization above ~400°C. The overall mean direction, corrected for 15° northeast dip of the overlying lower Cretaceous strata (dec=342.7°, inc=+4.2°; k=35.4, a95=9.4°, n=8 sites) indicates moderate counterclockwise (14°±8°) rotation with respect to the Late Triassic cratonic reference (reference pole: 57.5°N / 84°E), and moderate southward displacement (7°±5°). The Acatita pole falls within a cluster of Late Triassic poles from Africa, South America, and North America as restored in North American coordinates. A major difficulty is distinguishing local, small-scale rotations, such as a gentle tilt of the pluton after magnetization, from regional events involving large-scale displacements. Rotations such as observed at Acatita are suggested by synfolding magnetizations (Eocene in age) in the transverse sector of the Sierra Madre Oriental and by primary magnetizations in Eocene volcanic rocks near Chihuahua. Therefore rotations of Coahuila Island may be a Cenozoic attribute related to late Laramide deformation. Reconstructions of western equatorial Pangea should place the Coahuiltecano terrane at or near its present position with respect to North America.