The Acapulco Granite, Southern Mexico: New Geochemical and Geochronological Data for New Interpretations

The Acapulco intrusive crops out in the neighboring area of Acapulco city, in southern Mexico. It is a composite pluton that belongs to the coastal batholithic belt that intrudes the metamorphic Xolapa Complex. It ranges from granite (s.s), syenite to diorite and gabbro. The most distinctive characteristic of the Acapulco pluton is the rapakivi texture in the granite facies with the presence of biotite, amphibole (riebeckite), allanite and fluorite as distinctive minerals with minor sphene, zircon and apatite. Geochemically, the Acapulco intrusive varies from metaluminous to peraluminous and has the distinctive chemical characteristics of arc magmas (high LILE/HFSE ratios). The studied rocks also show strong negative Sr, Ba and Eu anomalies, incompatible element enrichment and Ga/Al ratios that suggest an A-type granitic nature of this pluton, probably as the result of fractionation of the metaluminous magmas.

Isotopic ratios (⁸⁷Sr/⁸⁶Sr from 0.7034 to 0.7043, and ε Nd from +5.53 to +1.78) indicate relatively small crustal contributions to the magmas. U-Pb geochronology in zircons by LA-ICPMS established crystallization ages of 49.2±1.5 Ma, 48.5±1.2 Ma, 48.3±1.3 Ma and 46.0±1.9 Ma for different lithologies of the Acapulco intrusive. These geochronological data indicate that plutonism between 48 and 50 Ma is widespread in the southern continental margin of Mexico. Finally, new thermobarometric determinations established emplacement conditions of ~700 °C at 8-10 km depth [2.08-2.8 kbar], indicating an exhumation rate of about 0.25 km/Ma between 48 Ma and 20 Ma for the studied area. These calculations call for a review of the models involving fast and/or slow exhumation of the southern Mexico coastal batholitic belt.