Chronology And Formation Mechanisms For The Heilongjiang Complex In Yilan Aear, Heilongjiang

It is assumed that the Heilongjiang complex was tectonic melange forming with the oceanic crust subduction between the Jiamusi massif and Songnen massif, subsequently of the continental collision. It is mainly exposed in the Mudanjiang-Muling, Yilan-Huanan and Luobei-Jiayin areas, East of Heilongjiang Province. These were generally north-south lay along the western edge of the Jiamusi massif and controlled by Mudanjiang Fracture. In this paper, we focus on the Heilongjiang Complex in Yilan and Xinxing Formation in Hailin City. Heilongjiaang Province, and do a study of state, phase, bit mode, tense and metamorphism characteristics of it using geology, petrology, tectonic geology, geochemistry, geochronology and other methods, comparing with the previous data and research results. The formation and evolution of the Mudanjiang branch of the Paleo-Asian Ocean are further discussed in the end.The Heilongjiang Complex in Yilan is characterized by "block-in-matrix"structure. The Matrix is composed of felsic mylonite and greenschist, which account for 70% of the entire terrane.The main block type includes serpentinite, galucophane-bearing metamafic rocks, blueschist, greenschist, amphibolite. marble and cherts. Pillow structure is well exposed in the metabasalts outcrop. Sodic amphiboles are mainly represented in three type rocks as follows:(1) a kind of glaucophanite, in which sodic amphiboles perform non-direction to weak-direction from core to rim of the rock, displaying pinch-and-swell structure and extreme necking and even boudinage in greenschist;(2) chlorite-blueschist or glaucophane-greenschist, in which glaucophane that included in matrix have the same orientation with chlorite;(3) galucophane-bearing metabasalt rocks, in which sodic amphiboles survive in albite and remain almost the same schistosity with that in matrix.The geochemical analysis for the major rocks of the Heilongjiang Complex, including glaucophanite, greenschist, granulite and metabasalts, suggest that the nature of the protolith derived from sub-alkaline-alkaline basalts, with REE characteristics of OIB-type basalts. The metabasalts are enriched in incompatible elements and have high TiO2(up to 5.96%), Nb positive anomalies in the (Nb/Th)pm(0.8-1.3) and negative anomalies in the (Nb/La)pm(0.4-0.7). all of which indicate that the protolith of metabasalts are OIB-type basalt. We speculate that the OIB-type protolith together with the marble, which lies near the metabasalt and contains less terrigenous debris, compose the typical " basalt basement plus cap carbonate rocks" bilayer structure that formed in oceanic islands-seamounts environment.Analysis of tectonic deformation and metamorphism suggested that the Heilongjiang melange is North-East imbricate or duplex thrust nappe structure and is strongly affected by ductile shearing. Petrographical characteristics of metamorphic minerals and electron microprobe analysis indicate that the Heilongjiang Complex should have been experienced at least two stage metamorphism.The early phase is epidote-blueschist facies metamorphism characterized by typical mineral assemblage including epidote+phengite+sodic amphibole+sphene. The later phase is greenschist facies with mineral assemblage including actinolite+albite+chlorite+ magnetite+sphene. The metamorphic P-T conditions are estimated to be T=250～340℃, P=0.34～0.61GPa for epidote-blueschist facies,T=280～430℃, P= 0.14-0.34GPa for greenschist facies, respectively. We therefore purpose that the Heilongjiang Complex is represented by a clockwise type P-T-t path.Large amount of ～260Ma U-Pb isotopic age data from zircons that derived from granulite and mainly represented with a magmatic features, is considered to be an indicative age of arc volcanic eruption and tuffite deposition during the peak of the subduction. Zircons with-307Ma age display a similar feature with magmatic zircon and should indicate a magmatic event during subduction between the Jiamusi massif and Songnen block. We obtain a microfossil assemblage characterized by Asteridium sp., Ceratophyton vernicosum, Lophosphaeridium truncatum from Xinxing Formation, which suggests that Xinxing Formation formed in the late Ealy Cambrian and the Mudanjiang branch of the Paleo-Asian Ocean has been accepting deposits since that time.Coupled with the Ordovician chitinozoa and Ealy Paloezoic isotopic data, we assume that the protolith of the Heilongjiang Complex formed during the whole Paleozoic Period, limited between the late Ealy Cambrian and Late Permain.Combined with the reported Paleontological and isotopic data in Jiamusi massif and other related areas, we suppose that the branch of the Paleo-Asian Ocean between the Jiamusi massif and Songnen block has existed at least since the late Ealy Cambrian and has no closure until late Permian. Subduction occurred in 260Ma-317Ma (the lower age limit may be earlier) and transferred to continental collision after the arc volcanic activity reaching a peak at ～260Ma. The blueschist facies metamorphism accordingly should occur during this period. The continental collision occurred during 220Ma-260Ma then turned into post-orogenic collapse from 220Ma to 180Ma.～180Ma, orogeny basically completed and the Heilongjiang Complex was influenced by the northern segment of Tanlu slip fault leading to the occurrence of greenschist facies metamorphism and intense ductile shearing. The evolution of the branching ocean between the Jiamusi massif and Songnen block is essentially consistent with the Paleo-Asian Ocean, indicating that it was mainly controlled by the ancient Asian Ocean tectonic domain before the Late Permian and the ancient Pacific tectonic domain became dominate in the Mesozoic.