Nd-Sr isotope composition of lower crustal xenoliths — Evidence for the origin of mid-tertiary felsic volcanics in Mexico

Isotopic data were collected on lower crustal xenoliths to constrain the Mexican lower crust as source material for the mid-Tertiary Sierra Madre Occidental, which is one of the largest silicic volcanic piles known. The xenoliths are predominantly pelitic gneisses and mafic orthogneisses that were brought to the surface on the eastern edge of the Sierra Madre Occidental by recent alkalic basalts. The pelitic gneisses are uniform in mineral assemblage and contain garnet+quartz+plagioclase+sanidine+rutile +sillimanite/kyanite+graphite. The orthogneisses are plagioclase, garnet and/or spinel bearing two pyroxene granulites. Available geothermometric and geobarometric data show that the xenoliths equilibrated at temperatures and pressures consistent with those of the mantle/crust boundary in those areas.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47347/1/410_2004_Article_BF00399363.pd