DETRITAL PROVENANCE OF THE GRENVILLIAN OAXACAN COMPLEX, SOUTHERN MEXICO: A ZIRCON PERSPECTIVE

The Oaxacan Complex is the largest exposure of Grenvillian-age rocks in Mexico, constituting the backbone of the Oaxaquia microcontinent. Whereas the main rock-forming events were previously established at 1150-1200 Ma (charnockite-syenite-gabbros), 1020 Ma (AMCG suite), 990 Ma (granulite-facies metamorphism), and ca. 970 Ma postectonic pegmatites, no data are yet available to establish provenance link with other Grenville-age terranes. In this work we studied 11 metasedimentary samples, all metamorphosed under granulite facies but variably affected by retrogression. LA-ICPMS U-Pb geochronology was employed on separated zircons to determine their crystallization age and geochemistry. The results of the analysis of about 100 core crystals per sample show that the studied zircons range between ca. 940 to 1400 Ma, with only three samples having zircons between 1400 and 1600 Ma, and only one showing older zircons up to ca. 1775 Ma. Whereas some of the slightly discordant (1-5%) zircons in several samples show ages younger than the granulite metamorphism (probably as a result of Pb loss), and thus a disturbed geochemical pattern (abnormal enrichment in LREE, decreasing HREE), a few metamorphic zircons show flat and depleted HREE patterns, contrasting with the igneous-like pattern of older zircons (positive Ce anomaly, negative Eu anomaly, enriched HREE pattern).

The main distribution peaks in probability density plots fall in the range 975-995 Ma (six samples), 1100 Ma (four samples), 1120-1170 Ma (six samples). Only the southernmost sample, shows a marked peak at ca. 1400 Ma.

The application of K-S statistical test to the studied samples, and particularly the comparison of obtained P-values, yields interesting similarities with many of the studied samples. In particular, three samples are very dissimilar, whereas four others have a 95% probability to not be dissimilar, i.e. they may belong to the same source area. Analogies of these data with the Grenville Appalachian massifs, some of the Sveconorwegian orogens such as Telemarkia, the Sunsas and Rondonia-San Ignacio belts of Amazonia, and some of the Precambrian massifs cropping out in the Andes suggest possible conjugate margins of Oaxaquia.