Heterogeneous dacitic magmas and the role of crustal assimilation in the Sierra Chichinautzin Volcanic Field, central Mexico: a perspective from crustal xenoliths

The Sierra Chichinautzin Volcanic Field (SCVF) is constituted by diverse monogenetic volcanic structures with heterogeneous chemical composition. Dacitic magmas are widely distributed in this volcanic field and are the most felsic products. Tabaquillo lava is an example of such felsic rocks, it is porphyritic and highly crystalline, constituted by large phenocrysts of plagioclase and quartz (>1 mm long) + ortho and clinopyroxene + biotite and minor amphibole. Coarse-grained and equigranular clasts classified as ‘crystal-rich enclaves’ are also common in the lava flow, with variable compositions (basaltic andesite, andesite, and dacite) and ages of 0.35–0.36 Ma. The presence of metasandstone crustal xenoliths in the Tabaquillo lava flow, with a maximum sedimentation age of 150 Ma, attests to crustal assimilation of the Guerrero terrain rocks. Isotopic data of Tabaquillo lava and other volcanic rocks from the SCVF show notable variations (⁸⁷Sr/⁸⁶Sr = 0.70379–0.70474; ¹⁴³Nd/¹⁴⁴Nd = 0.51272–0.51288). Most of them lie in the mantle array but trending to the more radiogenic metasandstone xenolith and Guerrero terrain rocks (⁸⁷Sr/⁸⁶Sr = 0.71271 and 0.71627), suggesting that SCVF magmas have partially assimilated these rocks. Interaction between SCVF magmas with metasandstone, quartzite, and skarn rocks took place at depths > 1.3 km, imprinting geochemical and isotopic characteristics on the felsic magmas and produced the diverse compositional lithologies in the entire SCVF. Additionally, crystal-rich enclaves produced during previous magmatic pulses stalled at different depths, were later mixed with the ascending mafic magma, and subsequently incorporated into the crystal cargo that reached the surface. The common presence of quartz crystals (sometimes with reaction rims of clinopyroxene) found in many SCVF lavas could derive from the crystal-rich magmatic enclaves and/or from the metasandstone basement rocks.