Petrology, Geochemistry and Petrogenesis of the Kalpin Basalts and Their Nd, Sr and Pb Isotopic Compositions

Abstract

The Kalpin basalts, located on the western margin of the Tarim plate, occur in the Lower Permian Kupukuziman Formation and Kaipaizileike Formation. Their petrochemical compositions are characterized by remarkably enriched TFeO, TiO2, P2O5 and unsaturated SiO2. Most of the samples belong to the alkali.basalt series and only one sample belongs to tholeiite series. The REE and trace element geochemical features show that the Kalpin basalts occur in a within-plate extensional setting. eNd(t) - - 1. 73 to -3. 69, esr(t) = + 27. 56 to +56.87, = 17.87 - 18.02, 207Pb/204Pb =15.45 - 15.53, 208Pb/204Pb = 38.22 - 38.49. These Nd, Sr and Pb isotopic compositions show that the Kalpin basalts are derived from the Precambrian enriched-type continental lithospheric mantle. And this case is representative among the well-developed mafic magmatic rocks in the Tarim Basin and its peripheral areas. Accordingly, the northern and southern parts of Xinjiang can be divided into two obviously different isotopic geochemical provinces: the northern province is characterized by isotopic compositions of the depleted mantle, whereas the southern province by isotopic compositions of the enriched mantle. The Late Paleozoic magmatic rocks separately belonging to the two provinces cannot be oroducts of the same p-eolopical orocess.