Abstract
The sodium-rich dacites and albite porphyries of Permian in the Awulale Mountain of west Tianshan have unique chemical and trace element signatures identical to adakite. These intermediate-acidic igneous rocks are characterized by high Na2O, Al2O3 and Sr contents and high Sr/Y and La/Y ratios (> 40 and > 20, respectively), and low Y and Yb contents, and strong depletion in HREE, and positive Eu anomaly. The (143Nd/144Nd)i is in the range from 0.51236 to 0.51248 and the eNd(t) is positive value (+0.79+3.11); the (87Sr/86Sr)i is in the range from 0.7052 to 0.7054. These Nd and Sr isotopic composition features indicate that the source rocks of these adakite-type rocks are from a weakly depleted mantle, or a depleted mantle, but was contaminated by the crustal materials. These adakite-type rocks were most likely derived from the partial melting of new underplated basaltic rocks under the conditions of amphibolite to eclogite transition in the postcollisional environment of North Xinjiang during the Permian Period. They are not only the Phanerozoic juvenile crust materials, but also are probably animportant symbol of the underplating of mantlederived basaltic magmas and the vertical growth of continental crust in the west Tianshan area during the postcollision of Late Paleozoic.
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