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

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 ± 0.6Ma on hornblende xenocrysts,and it ispart 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 C. Decompression coronas of spinel-plagioclase-orthopyroxene ± corundum ± glass aboutpolyphase garnetporphyroblasts inplagioclase-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 °C, whereas coexisting hornblende-plagioclase and two pyroxenes in gabbroic xenoliths yield magmatic temperatures of 800 to 950 C. 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 an