ARC-RELATED, PERMO-CARBONIFEROUS PLUTONISM IN A SYNCHRONOUS PULL-APART BASIN ALONG THE WESTERN MARGIN OF PANGEA IN SOUTHERN MEXICO

The Totoltepec pluton (TP) in the eastern part of the Acatlán Complex of southern Mexico forms part of a belt of Permo-Carboniferous arc magmatism extending from western USA to southernmost Mexico. We present new LA-ICP-MS U/Pb geochronological, geochemical and Sm-Nd isotopic data that constrain the timing and magma sources of the TP, as well as the age range and depositional setting of the low-grade, polydeformed, Permian Tecomate Formation (TF) metasediments, which are in fault-contact with the pluton.

The TP consists of a gabbroic-dioritic phase, occurring as dikes (previously dated at 289±1 Ma) and as marginal bodies (306±2 Ma), intruded by a 289±2 Ma trondhjemitic phase. Both phases exhibit an enrichment in large-ion lithophile elements (Cs, Ba, Pb, Sr), a depletion in high-field strength elements (Th, Nb, Ta) and flat HREE patterns, consistent with their formation in an arc-environment. Though cogenetic, field evidence, contrasting geochemical and Sm/Nd isotopic characteristics suggest that the two components have different sources. Whereas the mafic-intermediate phase displays εNd(t) values of +1.3 to +2.6 ( t = 306 Ma) and T_DM model ages between 0.9 and 1.7 Ga, the felsic phase is less radiogenic (εNd(t) values of -0.8 to +0.6, t = 289 Ma) and is probably derived from a ca. 1 Ga old crust (T_DM model age = 1.0 to 1.2 Ga). The petrogenetic evolution of the felsic rocks may have involved accessory phase fractionation.

The TF in the study area exhibits an arc-related geochemistry, a range of εNd(t) values (-5.5 to +0.3, t = 292 Ma) overlapping with those of the TP as well as detrital zircons of Permo-Carboniferous age (youngest cluster 292±8 Ma), indicative of its derivation from the contemporaneous arc. By contrast, detrital zircons in the basal TF outside the study area have yielded only Proterozoic ages, and Permo-Carboniferous detrital zircons are also absent in the neighbouring type area of the TF, suggesting the deposition in separate basins. Published kinematics of Permian structures in the TF indicate a dextral transtensional regime, which might have facilitated the intrusion of the TP and lead to several pull-apart basins developing in response to oblique subduction of the paleo-Pacific ocean at the western margin of Pangea, analogous to present-day oblique subduction of the Cocos Plate beneath Central America.