Late Ordovician back-arc extension in the southeastern margin of the Tarim Craton, NW China: Elemental and Sr-Nd isotopic constraints from the Hongliugou basalts

The North Altyn Tagh subduction-accretionary belt (NASB), in NW China, contains Paleozoic ophiolitic suites, tectonic melanges, and accretionary and metamorphic complexes related to the tectonic evolution of the Hongliugou-Lapeiquan Ocean. These assemblages represent the key to reveal the tectonic evolution of the southeastern margin of the Tarim Craton. New zircon U-Pb dating results indicate the eruption of the Hongliugou basalts in the late Ordovician. These basalts have depleted mantle-like radiogenic Nd (epsilon(Nd)(t) = +6.2 similar to+6.7) and moderately radiogenic Sr isotopic compositions (Sr-87/(86) Sr (i) = 0.7065-0.7068). They also have normalized trace element patterns between N-MORB and E-MORB. The combined geochemical data indicate their derivation from a moderately depleted mantle source enriched by fluids released from subducted sediments in a back-arc environment. Considering the widespread distribution of contemporaneous mafic rocks in the NASB, which show REE patterns lying between E-MORBs and N-MORBs, it is proposed that the Hongliugou-Lapeiquan Ocean remained active in the late Ordovician. The tectonic history of the Hongliugou-Lapeiquan Ocean was longer and more complex than previously recognized. It initially evolved as a marginal oceanic basin during the Cambrian and then began to close through slab subduction at ca. 510 Ma. Before the final closure at 440-410 Ma, the oceanic crust creation was renewed from ca. 460 to ca. 440 Ma in a back-arc setting.