The 275 Ma arc-related La Carbonera stock in the northern Oaxacan Complex of southern Mexico: U-Pb geochronology and geochemistry
The 300 m wide La Carbonera stock intrudes the northern ~1 Ga Oaxacan Complex, and ranges in composition from diorite through granodiorite to granophyric granite. The associated brecciation of the host rock and the zoned nature of amphibole and plagioclase suggest an explosive mechanism of intrusion. U-Pb analyses of abraded zircons yield variably discordant data that plot on a chord with intercepts at ~1,400 Ma and 275 ± 4 Ma, the latter interpreted as the time of intrusion based on the nearly concordant analysis of euhedral crystals. The upper intercept may be interpreted in terms of a mixed crustal source in the Oaxacan Complex that may include subducted sediments and enriched mantle sources. Variations in the major elements with silica (55->75%) suggest that the rocks represent a consanguineous series with crystallization of amphibole, Fe-Ti oxides and apatite up to ~60% SiO2, above which plagioclase crystallized. Their chondrite-normalized REE patterns are enriched in LREE, with this enrichment decreasing with increasing SiO2 content. Their arc magmatic signature is recorded by various chemical discriminants: (1) mantle-normalized trace element patterns which show depletion in Nb and Ti; (2) their locations on Nb+Y versus Rb tectonic discrimination diagram; and (3) their locations in the pre-collisional field in the Batchelor and Bowden’s chemical classification scheme. La Carbonera stock appears to form part of the Permo- Triassic magmatic arc that extends from the southern U.S.A through Mexico to Colombia, which has been related to subduction of Pacific plates beneath the western margin of Gondwana.
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