Abstract
The Pampean Ranges of northwest Argentina are a basin-and-range tectonic province with a late Precambrian to Paleozoic basement and extensive Miocene-Recent calc-alkaline volcanism. The volcanoes include the large resurgent Cerro Galan caldera, and Recent scoria cones and lava flows. Miocene-Recent volcanic rocks of basalt to dacite composition from the Cerro Galan area exhibit a range of Rb/Sr ratios of 0.043–1.092 and initial 87Sr/ 86Sr ratios of 0.7057–0.7115 with a clear positive correlation between 87Sr/ 86Sr and 87Rb/ 86Sr, indicating an apparent age of ca. 130 Ma. This relationship is interpreted to indicate that the Sr isotope variation in the Cerro Galan volcanic rocks results from mixing of a mantle-derived component with low 87Sr/ 86Sr (<0.7057) and high Sr (>700 ppm) with a crustal component characterized by higher 87Sr/ 86Sr (>0.7115) and lower Sr (<240 ppm). It is concluded that the mixing is best explained as a result of a small degree of selective crustal Sr contamination (ca. 10%) of a range of subsequently erupted magmas produced largely by fractional crystallization within the continental crust. We propose that the mantle-derived end-member is derived by partial melting of sub-Andean mantle with an 87Sr/ 86Sr ratio of ca. 0.704, and that such an Sr isotope ratio characterizes the source region for calc-alkaline volcanic rocks throughout the Andes.
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