Abstract
In Northeast China, oceanic subduction between the Jiamusi and Songliao blocks remains topic of hot debate. The Heilongjiang Complex has been regarded as an accretionary belt resulting from the subduction of an intervening ocean between the two blocks. In this study, we carry out extensive geological, geochemical and geochronological investigations on the sedimentary rocks, amphibolites and blueschists from the Heilongjiang Complex. The detrital zircons from meta-sedimentary rocks yield U-Pb age spams ranging from 268 to 780Ma. Whereas the interlayered amphibolites show negative Nb-Ta-Ti and positive Pb anomalies and have a protolithic age of 188.2±1.0Ma, suggesting a subduction zone or magmatic arc origin in the Jurassic. The elevated initial Sr isotopic ratios (0.708–0.711) and negative εNd(t) values (−4.3 to −1.3) provide further evidence of the modification by upper continental crust during the magma ascending. LREE/HREE and MREE/HREE ratios suggest that the magma was likely derived from the mixing of lithospheric and asthenospheric melts. The presence of low Th/Ce (0.04–0.12) and Hf/Sm (0.35–0.55), but high Zr/Hf (35.4–43.4) and Pb/Ce (0.25–0.49) ratios strongly implies a contribution from subducted sediments. The identification of active continental margin type magmas of Early Jurassic age suggests that the subduction of the oceanic plate between the Jiamusi and Songliao blocks may have started around Early Jurassic time. In addition, the blueschists have a protolithic age of 186±1.1Ma and display geochemical affinities of oceanic island basalts, suggesting that the ocean between the Jiamusi and Songliao blocks closed sometime after 186Ma. Mineral 40Ar/39Ar dating results from the Heilongjiang Complex further indicate that blueschist- to greenschist-facies metamorphism occurred in the Middle to Late Jurassic (158–175Ma), marking the onset of termination of the oceanic subduction.
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