Origin of Awulale adakitic sodium-rich rocks in western Tianshan: Constraints for Nd and Sr isotopic compositions.

The Permian sodium-rich dacites and albite porphyries in Awulale Mountain o f western Tianshan were generated at a post-collisional setting. They have unique major and trace element signatures similar to those bor subduction-r elated adakites. These adakitic rocks are characterized by high Na2O. Al2O3 and Sr contents and high Sr/Y and La/Y ratios (>40 and > 20, respectively) , low Y and Yb contents, strong depletion in HREE, and positive Eu anomaly. The ratio (Nd-143/Nd-144)(i) ranges from 0.512384 to 0.512470 and the epsi lon (Nd) (t=260 Ma) value is positive with a range of + 1.57 similar to + 3 .26; the ratio (Sr-87/Sr-84)i ranges from 0.7051 to 0.7054. These Nd and Sr isotopic compositions are similar to those of the coeval basalts in this a rea, but obviously different from those of subduction-related adakites. The aforesaid Nd and Sr isotopic compositions indicate that the source rocks o f these adakitic rocks come probably from a weakly depleted mantle, or a de pleted mantle, but was contaminated by the crustal materials. These adakiti c rocks were most likely derived from the partial melting of newly underpla ted basaltic rocks under the conditions of amphibolite to eclogite transiti on in the postcollisional setting during the Permian Period. They are proba bly an important indication of the underplating of mantle-derived basaltic magmas and the vertical growth of continental crust in western Tianshan reg ion during the post-collisional setting of late Paleozoic.