TECTONIC HISTORY OF THE METAMORPHIC BASEMENT ROCKS OF THE SIERRA DEL CARMEN, COAHUILA, MEXICO

The Ouachita interior zone, arching through central Texas into the Big Bend region of the Rio Grande, has been defined chiefly on the basis of well samples and its associated gravity high. The only known exposu re of rocks of the interior zone lies at the base of the Sierra del Ca rmen normal fault escarpment under approximately 1100 m of Lower Creta ceous carbonates in northern Coahuila, Mexico. These lower greenschist facies metamorphic rocks are thinly interlayered graphitic muscovite schist, graphitic marble, and minor quartzite with abundant quartz vei ns. The dominant structures shown by the metamorphic rocks are isoclin al folds (F2) that have a well-defined axial planar foliation (S2). Ea rlier (F1) folds refolded by F2 are locally preserved. At the microsco pic scale, S2 is a crenulation cleavage of the S1 foliation that has b een enhanced by pressure solution. Feldspar porphyroblasts overgrow a folded graphitic S1 foliation. Throughout the area a second crenulatio n cleavage (S3) is present on the S2 foliation surface as an intersect ion lineation (L3). Subsequent Laramide regional-to outcrop-scale fold s and associated thrusts overprint these ductile structures. Lower Cre taceous carbonates form a large, east-verging overturned to recumbent anticline-syncline fold pair with a northwest-trending fold axis. The S2 metamorphic foliations define an antiform that is nearly coincident in trend to that of the fold in the overlying carbonates, indicating that the metamorphic rocks were also folded by this regional structure . A Basin and Range normal fault is interpreted to have reactivated th e Laramide thrust fault responsible for the contractional deformation. One to two normal faults cut the carbonates and metamorphic rocks and truncate the anticline-syncline fold pair. The maximum offset along t he normal fault is 1600 m. Rb/Sr data from the muscovite schist gives a six-point, mineral-whole-rock isochron age of 277 +/- 10 Ma, indicat ing that polyphase deformation and metamorphism occurred during the la te Paleozoic Ouachita orogeny. An initial Sr ratio of 0.721 suggests a n ancient source for these metasediments. Common Pb ratios of the meta morphic whole rock and galena from the Puerto Rico Mine are isotopical ly distinct from those of the North American craton, suggesting that t hey were derived from a non-North American source and thus may be equi valent to unmetamorphosed Ouachita flysch sediments. This indicates th at the metamorphic rocks of the Ouachita interior zone were derived fr om an ancient allochthonous source that was deformed and metamorphosed during the late Paleozoic Ouachita orogeny.