Grenvillian massif-type anorthosite suite in Chiapas, Mexico: Magmatic to polymetamorphic evolution of anorthosites and their Ti-Fe ores

Two meta-anorthosite inliers (similar to 20 km(2) each) were discovered on each side of the Polochic-Tonala fault system within the Chiapas Massif Complex (CMC) in southeastern Mexico. The anorthosites occur commonly associated to subordinate hornblendite, rutile-bearing ilmenitite, oxide-apatite-rich amphibolite and nelsonite. An absolute crystallization age for the anorthosite could not be precisely constrained due to the paucity and apparent resetting of the magmatic zircon. However, they are interpreted as remnants of a Proterozoic massif-type anorthosite complex older than 909 +/- 27 Ma as suggested by U-Pb data and the striking geochemical similitudes (major, trace and REE, Sr-Nd isotopes; T-DM = 1.39-1.45 Ga) to Stenian-Tonian massif-type anorthosites found elsewhere. A complex tectono-thermal history for the anorthosite suite is inferred from the discovery of abundant metamorphic zircon in all samples. Ubiquitous petrographic evidence suggests formation of secondary zircon from reactions involving baddeleyite and srilankite (ZrTi2O(6)) breakdown at different stages of the anorthosite suite evolution. Two generations of baddeleyite are recognized: (1) exsolution from igneous ilmenite and hogbomite ([Mg, Fe2+](2)[Al,Ti](5)O-10) during cooling, and (2) exsolution product from metamorphic rutile and hogbomite during retrogression. U-Pb zircon data (LA-MC-ICPMS) of three comagmatic rocks, including one anorthosite sample, reflect the complex polymetamorphic nature observed in the petrographical evidence; concordant to sub-concordant ages spread in a range from similar to 909 Ma to similar to 242 Ma. Most U-Pb ages cluster around known regional metamorphic events at similar or equal to 450 Ma and similar or equal to 250 Ma. In addition, an age cluster at similar to 600 Ma suggests a Neoproterozoic event, interpreted in terms of reheating of anorthosite during mafic intrusions associated to intra-plate rifting. (C) 2017 Elsevier B.V. All rights reserved.