PROVENANCE AGES OF DETRITAL ZIRCON CORES FROM PARAGNEISS-ANATEXITES: UNRAVELING THE PRE-METAMORPHIC HISTORY OF THE PERMIAN CHIAPAS MASSIF, MEXICO

Medium- to high-grade metasedimentary rocks are exposed as isolated domains in mostly metaigneous crystalline rocks of the Chiapas Massif which is the basement of the southern Maya block in southeastern Mexico. The most important tectonothermal event in the entire massif is of late Permian age, culminating in partial anatexis of the gneiss and intrusion of the Chiapas batholith. Most of the rocks in the region, however, yielded middle Proterozoic (Grenville) protolith ages obtained using U-Pb zircon geochronology (TIMS) as well as Sm-Nd model ages. In our approach to unravel the provenance of zircons from metasediments by both Laser Ablation-Multicollector-ICPMS and SHRIMP, we focused our U-Pb isotope analyses on inherited detrital zircon cores, which are commonly surrounded by metamorphic/anatectic overgrowth (SEM-CL images). Some zircons are partly or completely resetted, yielding a peak in the age spectra at ~250 Ma. After subtracting the metamorphic/anatectic component, inherited detrital cores of the zircons yielded two principal age distributions: (1) the largest peak at 500-600 Ma, medium peaks at ~410 Ma and 1.0-1.2 Ga, some grains having 1.6-2.0 Ga, and a considerable amount of Archean (2.6-3.1 Ga) grains; (2) only ~1.0 Ga zircon cores or the largest peak at 1.0-1.2 Ga, a minor peak at ~1.5 Ga, and insignificant Neoproterozoic or younger grains. The age distribution of zircon cores from the first group (1) of samples is indistinguishable from detrital zircon ages of Pennsylvanian-Permian flysch-type sediments (Santa Rosa Formation) in SE Chiapas, which were shed mostly from either West Africa or from NE South America, where rocks which were formed during the Pan-African-Brasiliano orogenic cycle are common. The second group (2) of protoliths has a different provenance principally from Grenville orogens and similar to those from early Paleozoic sediments in the Maya Mountains of Belize. We suggest that two sedimentary sequences, the Pennsylvanian-Permian Santa Rosa Formation and early Paleozoic sediments were stacked in an orogenic wedge during the late Permian tectonothermal event in the Chiapas massif. These results have implications on the paleogeography of western central Pangea.