Petrology of very high temperature crustal xenoliths in the Puente Negro intrusion: a sapphire-spinel-bearing Oligocene andesite, Mixteco terrane, southern Mexico

Abstract

This study presents petrologic, chemical, geochronological and isotopic data, as well as petrogenetic interpretations about a unique subvolcanic locality in southern Mexico that contains deep-seated xenoliths and xenocrysts (igneous and metamorphic), albeit affected by extreme pyrometamorphism during rapid ascent in a composite andesitic dike. The intrusion has a K-Ar age of 29.2 ± 0.3 Ma on volcanic matrix and 30.5 ± 0.6 Ma on hornblende xenocrysts,and it is part of an arc-related regional magmatic event in southern Mexico. This magma at Puente Negro intruded quartzo-feldspathic gneisses and micaceous schists of the Paleozoic Acatlán Complex. Xenoliths consist of high-grade garnet-bearing gneisses, aluminous metapelites, impure quartzites, and abundant hornblende-rich gabbroic rocks. Garnet, corundum (including purple-blue sapphire), spinel, and aluminous orthopyroxene constitute the main types of deep-seated xenocrysts derived from disaggregation of metamorphic rocks in the andesite. Low pressure assemblages with tridymite, spinel, Al-silicates (mullite and sillimanite), two pyroxenes, Fe-Ti oxides, and high-silica anatectic glasses indicate peak temperatures of pyrometamorphism above 1100 ºC. Decompression coronas of spinel-plagioclase-orthopyroxene ± corundum ± glass about polyphase garnet porphyroblasts in plagioclase-orthopyroxene-spinel restitic gneisses, Al-rich orthopyroxene coring Al-poor orthopyroxene xenocrysts, spinel-plagioclase-corundum xenocrystic pseudomorphs probably after garnet, and local preservation of orthopyroxene-sillimanite and garnet-hypersthene-spinel-quartz assemblages strongly support interaction of original basaltic magmas with the lower crust. Aluminum in orthopyroxene (up to 11.6 Al2O3 wt. %) coexisting with spinel, ilmenite-magnetite pairs, and Fe/Mg partitioning between orthopyroxene and spinel in garnet coronas yield decompression metamorphic temperatures around 990 ºC, whereas coexisting hornblende-plagioclase and two pyroxenes in gabbroic xenoliths yield magmatic temperatures of 800 to 950 ºC. The first basaltic hydrous magma represented by the gabbroic xenoliths differentiated in a magmatic chamber in the middle crust at 4–6 kbar based on Al-in-hornblende barometry. The subsequent injection of this partially to totally crystallized magma chamber by a new basaltic batch apparently caused disaggregation of the hornblende-rich rocks and transported the xenolith-xenocryst load to the surface. Based on major and trace elements and Sr, Pb, and Nd isotope data, we conclude that the Puente Negro “andesite” was the end product of a mantle-derived, relatively long-lived plumbing system of original basaltic composition that in the early Oligocene (29–30 Ma) interacted with the continental crust at lower, middle and shallow levels, which are represented, respectively, by xenoliths of granulite facies quartzites and metapelites, mafic and ultramafic gabbroic rocks, and sanidinite facies quartzo-feldspathic buchites.