ARC-RELATED INTRUSIONS IN THE OAXACAN COMPLEX: EVIDENCE FOR EARLY TO LATE PERMIAN PACIFIC PLATE SUBDUCTION BENEATH THE WEST CENTRAL MARGIN OF GONDWANA

Several undeformed intrusive bodies dated between 275 and 255 Ma affect the Northern Oaxacan Complex, which in this region consists of minor paragneiss intruded by ≥1140 Ma and 1012±12 Ma (U-Pb zircon ages) within-plate, rift-related orthogneisses (anorthosite, charnockite, metagabbro, and metagranite), metamorphosed in the granulite facies at ~990 Ma.

The age of three of the Phanerozoic, intrusive bodies was adjusted or re-evaluated by means of U-Pb geochronology on zircons using a laser ablation system coupled to a quadrupole inductively coupled plasma mass spectrometer (LA-ICPMS), or a multicollector inductively coupled plasma mass spectrometer (LA-MC-ICPMS) at the Laboratorio de Estudios Isotópicos of the Centro de Geociencias, UNAM. A previously unstudied rhyolitic tabular body yielded a crystallization age (LA-ICPMS) of 275 +6/-5 Ma, with inherited cores ranging in age between 862 to 1090 Ma, as well as two concordant zircons of 440 and 327 Ma with interpreted sources possibly in the Oaxacan and Acatlan Complex, respectively.

Most of the zircons display a typical REE igneous pattern with an increase of HREE and high positive anomaly of Ce and negative anomaly of Eu. La Carbonera intrusive body, previously dated by ID-TIMS at 274±4 Ma (lower intercept), was dated in this study by LA-ICPMS on two samples with different mafic mineral content, yielding ages of 273 +1/-3 and 270 +3/-1 (more and less mafic, respectively). REE’s zircon patterns are typically igneous. Finally, the crystallization age of 255 +2.2/-2.4 obtained by LA-MC-ICPMS from the Etla granite in this work is much younger than the previously known TIMS age of 917 ± 6 Ma. Thus, the assumed Precambrian subduction beneath west central Amazonia and palinspastic reconstructions based on the previously interpreted crystallization age of the Etla granite needs to be re-evaluated.

These new data obtained with modern techniques indicate the contemporaneity of the widely distributed intrusive bodies, which we associate with a Late-Middle Permian subduction event, probably as part of a Permo-Triassic continental arc that extended from southern U.S. to Colombia and representing the eastward subduction of the Pacific plate beneath the western margin of Gondwana.