OIB-like magma genesis at the modern Chiapanecan Volcanic Arc, southern Mexico

Abstract

The Chapultenango trachybasalt, located in the modern Chiapanecan Volcanic Arc (CVA) in southern Mexico, is constituted by two mafic dykes. Dyke 1 shows relatively low Th/Nb and Pb/Nb ratios compared to other CVA rocks, and exhibits primitive chemical characteristics (e.g. #Mg >65, Ni > 200, Cr > 600) typical of Ocean Island Basalts (OIB). On the other hand, Dyke 2 is almost in touch with Dyke 1, is calc-alkaline, and more evolved than Dyke 1, similar to other rocks found in El Chichón volcano. Hence this work is mainly focused on Dyke 1 that represents the least differentiated melt observed so far in the Chiapanecan Volcanic Arc, and its composition helps in characterizing the mantle source in this region. Dyke 1 OIB-like magma was likely generated by a slab window resulting from the tearing of the Cocos slab along the Tehuantepec Ridge. This tearing was likely caused by differences in physical properties between the northern and southern segments of the Cocos plate, which can be attributed to their age difference (10 Ma) and velocity convergence. This configuration permits the ascent of OIB-like melts from an enriched and relatively deep mantle, which was metasomatized by subduction components (2–6%) and then underwent partial melting (∼5%), although contamination by crustal rocks (e.g., Chiapas Massif) could not be ruled out. The resulting primitive melt therefore would have ascended rapidly, carrying some phaneritic and gabbroic fragments that exhibit similar chemical characteristics to the Chiapanecan Volcanic Arc rocks.