The Alamos Metamorphic Complex, evidence of late Paleozoic collision between Laurentia and Gondwanan blocks in northwestern Mexico

The Alamos Complex is a metamorphosed volcano-sedimentary sequence exposed south of the late Paleozoic Ouachita-Marathon-Sonora orogen in northwestern Mexico, which in turn, is regarded as a continuation of the Alleghenian-Variscan mountain chain. Mineral assemblages in metavolcanic rocks indicate greenschist facies metamorphism. Thermobarometric studies yield an average metamorphic temperature of 380 °C, whereas andalusite in metapelites indicates pressure less than 0.4 ± 0.05 GPa. The main deformation phase generated pervasive foliation, which in turn was isoclinal to close folded and a second axial planar cleavage was developed. Metavolcanic rocks of the Alamos Complex can be separated in two suites: (1) an anorogenic suite, similar to enriched middle ocean basalts (E-MORB) in the Tesia region, and (2) an orogenic suite, characteristic of volcanic-arc setting in the Mocuzari region. U-Pb detrital zircon geochronology of one sample from Tesia yielded a maximum depositional age averaging 395 ± 28 Ma. One sample from the Mocuzari region yielded a maximum depositional age averaging 1104 ± 24 Ma, with the largest peak at 1428. A clastic sequence correlated with the Middle Triassic-Lower Jurassic Barranca Group, nonconformably cover the Alamos Complex. The basal conglomerate of that group yields a maximum depositional age averaging 372 ± 23 Ma. Accordingly, the Alamos Complex must be in part deposited after the early Devonian time and before the Middle Triassic. The main sources of detritus for the complex are the Meso- and Paleoproterozoic rocks of southwestern Laurentia, whereas the main source of detritus for the Barranca Group is the Rio Fuerte Formation exposed 100 km south of the study area. The metamorphic event is ascribed to the late Paleozoic collision of Gondwanan blocks against the southern margin of Laurentia.