Felsic-intermediate magmatism and brittle deformation in Sierra del Tzirate (Michoacán-Guanajuato Volcanic Field)

Abstract

The Tzirate volcanic complex (TVC) is located in the central part of the Michoacán-Guanajuato volcanic field (MGVF), central Mexico. It is characterized by a series of andesitic-dacitic domes and by active faulting of the Morelia-Acambay fault system. Its age and the relationship between the magmatic processes that generated the TVC rocks and the evolution of the local stress field were determined with field work, structural analysis, 40Ar-39Ar dating, geochemistry and petrography of the lavas. The dacitic volcanism of the TVC occurred during the Pleistocene (1 Ma) with the eruption of lavas that formed a series of domes (SiO2 = 63–69 wt.%) aligned in NNE-SSW and NW-SE directions. The area is covered by andesitic-basaltic cinder-lava cones and andesitic lava cones of Pleistocene-Holocene age (2.5 Ma – 0.011 Ma) produced by monogenetic volcanism of the MGVF that surrounds the TVC.The TVC magmas produced rocks that can be classified in four mineral groups. One group contains olivine and pyroxene phenocrysts. The second biotite and amphibole euhedral phenocryst, and the other two contain biotite euhedral phenocryst, amphibole microphenocrysts and biotite subhedral crystals surrounded by a rim of plagioclase microcrysts. The diversity of textures in biotite crystals and the linear trends of major and trace elements between La Huacana granite and the less evolved lavas of the TVC suggest that assimilation of a granitic body in the crust might have influenced the evolution of the TVC rocks.The results of the structural analysis show that when the minimum principal stress was vertically oriented the rise of magma to the surface is hindered, favoring the entrapment of magma in shallow magma chambers. The magmatic assimilation process possibly occurred during the same phase, forming the felsic magmas. Subsequently, the local stress field became extensional, and the vertical principal maximum stress developed normal or extensional faults that affected the magma chamber and served as plumbing system for the rise of felsic lava.