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 gabb