NORTHERNMOST EXPRESSION OF THE GONDWANIC CARBONIFEROUS ARC IN MEXICO: ASERRADERO RHYOLITE FROM CIUDAD VICTORIA, TAMAULIPAS

The Huizachal – Peregrina Anticlinorium is a large structure originated during the Cretaceous-Paleogene Mexican Orogeny, and belongs to the Sierra Madre Oriental. It is located near Ciudad Victoria, capital city of the Tamaulipas state. The eroded core of the anticlinorium exposes the basement of NE Mexico, which is composed by a variety of Precambrian and Paleozoic lithologies. One of them is described as the Aserradero Rhyolite that corresponds to a volcanic unit interbedded within the siliciclastics of the Tamatan Paleozoic sedimentary sequence.

Aserradero Rhyolite crop out in the Peregrina and Caballeros Canyons as very irregular and disconnected bodies, without clear contacts. This unit presents hialocrystalline microporphyritic texture, very often fluidal, and a mineralogical arrangement composed mostly of quartz, plagioclase, alkali feldspar and biotite.

The geochemical characteristics of the rhyolite indicate an origin from the upper continental crust, with mixed volcanic arc and collision signatures. The magmas could have been generated over the subduction zone along the NW border of Gondwana, prior its collision against Laurentia and the amalgamation of Pangaea. The collision signature could be explained as an inherited characteristic preserved in the source from former geotectonic processes.

U–Pb laser ablation ICP-MS was used for dating zircon crystals, giving crystallization ages of 340.7±3.6 Ma for a Caballeros Canyon sample and 347.8±2.7 Ma for a Peregrina Canyon sample. The determined ages correspond to the Tournaisian and Visean (Early to Middle Mississippian) respectively.

We propose that the Aserradero Rhyolite represents the northernmost segment of a Carboniferous pre-Andean arc generated in the NW margin of Gondwana. There the older continental crust (comparable to the Novillo Metamorphic Complex, also outcropping in Ciudad Victoria) had already experienced collision processes during the Precambrian (e.g. Zapotecan Orogeny at ca. 990 Ma) giving later this particular signature to the later developed crustal rhyolitic magmas. Since the lavas are interbedded in marine strata, they could have been erupted during a limited exhumation period in the Tamatan basin, not yet reported.