Low Radiogenic, Young and Mafic to Intermediate Crust Beneath Colima Volcano: Xenoliths and O-Sr-Nd-Pb Isotopic Evidence.

Systematic Sr-Nd-Pb-O isotopic analyses focused on plutonic basement rocks, andesitic lavas and gabbroic-granodioritic xenoliths from Colima Volcano (CV), western Mexican Volcanic Belt (MVB) were performed for a better understanding of the nature and the importance of the crust involved in CV magma generation. CV constitutes isotopically the less evolved stratovolcano in the MVB. Petrographic studies show that granodioritic xenoliths (SiO2: 59.73-62.37 wt %) found in CV lavas were partially melted, producing melted quartz and pink-pale glass. In the gabbroic xenoliths (SiO2: 49.9-51.50 wt %), highly melted zones were identified showing brown glass and partially melted pyroxenes. This gives clear evidence for interaction of these xenoliths with the andesitic host magmas. Our new isotopic results show a narrow range for CV lavas: 87Sr/86Sr: 0.70352-0.70361, eNd: +4.6 to +6.9; 206Pb/204Pb: 18.56-18.61, 207Pb/204Pb: 15.56-15.59, 208Pb/204Pb: 38.25-38.41, and delta18 O: 5.7-6.5 per mil. Xenoliths display 87Sr/86Sr: 0.70387-0.70393, eNd: +4.7 to +6, 206Pb/204Pb: 18.57-18.59, 207Pb/204Pb: 15.58-15.60, 208Pb/204Pb: 38.29-38.35, and delta18 O: 6.4 to 7.0 per mil. Basement rocks show 87Sr/86Sr: 0.70351-0.70489, eNd: from + 2.9 to + 7.0, 206Pb/204Pb: 18.67, 207Pb/204Pb: 15.58, 208Pb/204Pb: 38.27, and delta18 O: 5.1 to 9.1 per mil. Geochemical and isotopic results emphasize the importance of partial melting processes in the mantle wedge involved in CV magma generation. During magma ascent magma batches were stored at several depths, but only at upper crustal levels AFC processes were important. Isotopic results show that the gabbroic local basement, nearly identical to CV lavas, was not detectable as contaminant source in Sr-Nd diagrams but can be clearly seen in eNd vs. delta18 O plots.