Sr-Nd-Pb isotopic compositions of the lower crust beneath northern Tarim: insights from igneous rocks in the Kuluketage area, NW China

Abstract

The composition of lower crust of the Tarim Craton in NW China is essential to understand the petrogenesis of the ~290–275 Ma Tarim basalts and associated intermediate-felsic rocks. However, it remains poorly constrained because extremely sparse granulite terrains or granulite xenoliths have been found in the Tarim Craton. New trace element and Sr-Nd-Pb isotopic data are reported for the Neoarchean and Neoproterozoic igneous rocks widely distributed in the northern margin of the Tarim Craton. The Neoarchean granitic gneisses show fractionated REE (rare earth element) patterns [(La/Yb) N = 12–58, YbN = 10.6–36] with pronounced negative Nb-Ta and Ti anomalies. These features, together with negative eNdi (−0.7 to −3.2) suggest that they were derived from melting of mafic lower crust. The Neoproterozoic biotite granodiorites are strongly depleted in HREE with (La/Yb) N up to 55. They are characterized by high Sr (671–789 ppm) but very low Y (7.10–8.06 ppm) and Yb contents (0.47–0.58 ppm), showing typical features of adakitic rocks. The samples with different SiO2 contents display identical 87Sr/86Sri (0.7101–0.7103), eNdi (−14.1 to −15.7) and Pb isotopes (208Pb/204Pbi = 36.94–37.07). These features together with arc-like trace element patterns suggest that they were derived from melting of thickened lower crust. In comparison, the Neoproterozoic hornblende-biotite granodiorites have similar trace element compositions except for weaker depletion in HREE and have lower 87Sr/86Sri (0.7078) and initial Pb isotopes, and higher eNdi (−12.3 to −12.7). This suggests that they were formed by melting of old lower continental crust at a shallower depth than the biotite granodiorites. These rocks were derived from the lower crust, thus providing valuable information on the nature of the lower crust beneath northern Tarim. Combined with published data, the 87Sr/86Sri, eNdi, 206Pb/204Pbi and eHfi of the northern Tarim lower crust ranges from 0.7055 to 0.7103, from −12 to −17, from 16.20 to 16.65, and from −7 to −19, respectively, at ~785 Ma. These data also suggest vertical compositional heterogeneity of the northern Tarim lower crust.