Origin of the Río Verde Batholith, Southern Mexico, as Inferred from its Geochemical Characteristics

Abstract

The Río Verde batholith, in southern Mexico, represents a pulse of voluminous magmatism occurring over a relatively short time span, followed by uplift and erosion. The batholith consists predominantly of calc-alkaline rocks ranging from tonalites to granites with biotite and hornblende as the main mafic minerals. Intrusion ages for the three recognized plutons range from 29.9 ± 1.1 to 23.5 ± 0.6 Ma, with a younging trend toward the east. Isotopically, these granitoids are characterized by εNd values between −3.0 and +0.9 and 87Sr/86Sr initial ratios ranging from 0.70419 to 0.70532. The small differences between hornblende and biotite ages suggest rapid cooling rates. Geobarometric determinations performed on hornblendes yield a pressure of 5.3 ± 0.6 kbar, which corresponds to a depth of emplacement of 19.7 ± 2.2 km. Using trace-element abundances as well as the Sr- and Nd-isotopic data, we modeled possible magmatic processes of partial fusion, fractional crystallization, and assimilation assuming different crustal contaminants. The results suggest that these plutons originated by fractional crystallization of mantle-derived magmas with a low degree (10 to 30%) of crustal contamination. Cretaceous-Oligocene calc-alkaline magmatism in southern Mexico was dominated by the addition of new magmatic materials of subcrustal origin rather than a recycling process of older crust. The transtensional regime associated with the detachment and subsequent displacement of the Chortis block since the Eocene may have favored decompression melting of the upper mantle, causing the observed voluminous plutonism.