ABSTRACT

The North Altyn Tagh subduction-accretionary belt (NASB), in NW China, contains Paleozoic ophiolitic suites, tectonic mélanges, 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 (ε_Nd(t) = +6.2~+6.7) and moderately radiogenic Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr (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.