Mesoproterozoic continental breakup in NW China: Evidence from gray gneisses from the North Wulan terrane

The North Wulan terrane forms part of the micro-continental blocks in the early Paleozoic Qilian and North Qaidam orogenic belts in NW China. LA-ICP-MS dating of magmatic zircons from two gray gneisses from this terrane yielded U-Pb ages of 1519 ± 5 Ma and 1497 ± 8 Ma, suggesting emplacement of their precursor magmas at ∼1.5 Ga. The rocks have high SiO2 (70.6-75.6 wt.%) and Na2O (3.96-4.84 wt.%), but relatively low Al2O3 (mostly <15 wt.%) and K2O (0.90-2.52 wt.%) contents. They possess low contents of Ni, Cr, Sc, Sr, Rb and moderate Y, with slightly enriched LREE ((La/Yb)N = 3.46-11.3) and flat HREE ((Gd/Yb)N = 0.95-2.06) patterns and strongly negative Eu anomalies (Eu/Eu∗ = 0.22-0.38). Thus, these rocks have trondhjemite compositions and are comparable to the low-Al or low-pressure TTG gneisses worldwide. Zircons from these rocks yielded εHf(t) and TDM2 values of −2.2-5.1 and 1.93-2.39 Ga, respectively. Our data suggest that the precursor low-Al trondhjemite magma was generated from partial melting of the early Paleoproterozoic mafic rocks at shallow crust level through possible heat input from upwelling mantle in a continental rifting setting. We suggest that these ∼1.5 Ga gneissic rocks were formed in a tectonic setting identical to that of coeval mafic sills and dykes in the northern Tarim Craton and western Yangtze Craton, and can be broadly correlated with the anorogenic magmatism associated with the initial fragmentation of the supercontinent Columbia.