Petrology of high-grade crustal xenoliths in the Chalcatzingo Miocene subvolcanic field, southern Mexico: buried basement of the Guerrero-Morelos platform and tectonostratigraphic implications

Abstract

The Miocene Chalcatzingo trondhjemitic volcanic field, sited along the southern margin of the Trans-Mexican Volcanic Belt, is a newly discovered locality with deep-seated crustal xenoliths that provide fundamental petrologic information on the nature of the unexposed metamorphic basement. The volcanic field lies along the eastern edge of the Cretaceous Guerrero-Morelos platform, which juxtaposes the Guerrero and Mixteco terranes of southern Mexico. Xenoliths consist of high temperature to ultra-high temperature metapelites as well as mafic and quartzofeldspathic gneisses, all of which show evidence of multiple granulite to amphibolite facies metamorphism and ductile deformation. A detailed petrologic study of representative xenoliths indicates a metamorphic evolution that apparently followed a clockwise pressure–temperature path leading from biotite-sillimanite1/kyanite(?)-quartz assemblages (M1) to the assemblage plagioclase-garnet-sillimanite2-rutile/ilmenite (M2) with a peak at ∼9–11 kbar and >870°C. These conditions were followed by rapid uplift to <6 kbar and >800°C, which produced the decompression assemblage spinel-cordierite-sillimanite3-corundum ± orthopyroxene ± quartz (M3) before shallow emplacement of the xenolith-bearing trondhjemitic magma. Three possible sources for the xenoliths are considered: (1) early Mesozoic metasediments buried in the middle crust; (2) Precambrian lower crust; and (3) subducted Cenozoic sediments trapped in the mantle wedge. Based on the deep-seated, polymetamorphic nature of the xenoliths, the Nd depleted mantle model age of an orthogneissic xenolith, and on regional tectonostratigraphic considerations, we suggest that the xenolith source was Proterozoic continental crust. Although old zircon inheritance in the host trondhjemite is minimal, it may be explained by a lack of interaction of the magma with the traversed lithosphere. Studies of Palaeogene shallow intrusions exposed 140 km west of Chalcatzingo in the Guerrero terrane (Pepechuca plug) and 80 km southeast of that place in the Mixteco terrane (Puente Negro dikes) reveal the presence of similar very high-grade aluminous xenoliths. However, these magmas were probably generated by partial melting of Triassic–Jurassic metasediments of the Guerrero terrane underplated by basaltic magmas in Jurassic–earliest Cretaceous times or from Precambrian crust assimilated by underplated mafic magmas of Oligocene age, respectively.