Texas and Mexico
Published:January 01, 2004
Mesoproterozoic igneous protoliths that were metamorphosed to granulite facies during the Grenville orogeny are exposed south of the Ouachita suture in eastern and southern Mexico, and clasts of Mesoproterozoic granitoids occur in a Paleozoic conglomerate in northeastern Mexico. These rocks can be divided into two groups, based on crystallization ages. The older group has arc and backarc geochemical signatures and was emplaced between ca. 1235 Ma and ca. 1115 Ma. The younger group, an anorthosite-mangerite-charnockite-granite (AMCG) suite, is not well dated, but present data suggest that it was emplaced between ca. 1035 Ma and ca. 1010 Ma. The Mex-ican terrane was probably a trailing margin from ca. 1115 Ma until the Grenville orogeny at 990 ± 5 Ma that produced widespread granulite-facies metamorphism. There is no structural or metamorphic evidence in Mexico for a major, widespread orogenic event before the emplacement of the AMCG rocks; thus, the model relating the origin of AMCG suites to delamination or convective thinning of subcontinental lithosphere following collision and crustal thickening may not apply to the Mexican Grenville terrane. The Pb isotopic compositions of acid-leached potassium feldspars define a tight linear array on a 207Pb/204Pb versus 206Pb/204Pb diagram. This array is interpreted to reflect mixing between two end-member reservoirs. The radiogenic component was presumably continental in origin (perhaps subducted Paleoproterozic sediments), whereas the nonradiogenic end-member was most likely ca. 1200 Ma mantle. The Pb isotopic compositions of feldspars from eastern and southern Mexico are distinct from those of feldspars from Laurentia as represented by the Grenville Province and Grenville granites of Texas.