U-Pb Neoproterozoic-Ordovician protolith age constraints for high- to medium-pressure rocks thrust over low-grade metamorphic rocks in the Ixcamilpa area, Acatlan Complex, southern Mexico

High-grade and high-pressure rocks in Acatlan Complex (southern Mexico) are inferred to have been emplaced either during the convergence and collision between Laurentia and Gondwana or during subduction on the western margin of Pangea. In the Ixcamilpa area, such rocks occur in a synformal nappe and are subdivided into (1) the Neoproterozoic-Ordovician Piaxtla Suite (metapsammite, meta-pelite, and amphibolite) that passes structurally upwards from blueschist through eclogite to amphibolite facies; intruded by (2) Cambro-Ordovician megacrystic granitoids; both of which were thrust westwards over (3) the Carboniferous Zumpango Unit consisting of clastic and meta-volcanic rocks. Laser ablation--inductively coupled plasma--mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology yielded age population peaks at (i) 435-490 Ma, probably derived from Acatlan granitoids; (ii) 500-700 Ma, likely derived from the Yucatan Peninsula and Brasiliano Orogens; (iii) 800-900 Ma, with provenance in the Goias Arc of eastern Amazonia; and (iv) 950-1300 Ma, sourced either from Oaxaquia, Amazonia, or Laurentia: the younger ca. 310-360 Ma ages are limited to the Zumpango Unit and likely have a local provenance. The overall similarity of the Piaxtla rocks in the Ixcamilpa area and those in the Piaxtla-Mimilulco median belt suggests that Ixcamilpa nappe roots in the median belt, which is interpreted as an extrusion zone within the Acatlan Complex. Since neither high-pressure belt represents a closed ocean, deposition of the Neoproterozoic-Ordovician rocks is inferred to have taken place on the southern margin of Rheic Ocean adjacent to Oaxaquia-Amazonia, whereas the Carboniferous rocks were deposited on the western margin of Pangea synchronous with extrusion of the high-pressure rocks.