Feature And Tectonic Significance Of The Engerwusu Ophiolitic Mélange Of Alaxa, Inner Mongolia

By petrology, petrochemistry and geochemistry, we can well investigate internal structure, environment, mechanism, tectonic implications in the Engerwusu ophiolitic mélange.The Engerwusu ophiolitic mélange is composed of blocks and matrices.The blocks are lenticular, having light deformation, including ultrabasite, basalt, andesite, siliceous rock, granite, limestone(marble) and sandstone, distributed in the matrices. Then, ultrabasite, basalt, andesite, siliceous rock and granite represent remnants of oceanic crust, and granite, limestone(marble) and sandstone are from continental crust. However, the matrices contains sandy-muddy matrix and volcanic matrix, which have strong cleavage and deformation, expressed as a series of thrust nappe structural rock chips mainly in a north direction in the sections. Moreover, the distrupting and mingling of the rocks in the Engerwusu ophiolitic mélange is obvious.The alteration and carbonatization of the ultrabasites in the Engerwusu ophiolitic mélange are severe, enriched U and strongly depleted Ta, Nb, Ti, showing positive U, La and Sr anomaly and negative Nb and Ti anomaly. Meanwhile,the ∑REE of the ultrabasites is low, displaying no significant fractionation of light rare earths and heavy rare earth, lihgt negative Eu anomaly and negative Ce anomaly, derived from depleted mantle. According to the petrochemistry petrochemical characteristics of the basalts in the Engerwusu ophiolitic mélange, it is indicated that they are oceanic tholeiite of sub-alkaline basalt, formed through differentiation or assimilation of mantle-derived primary magma, selective strong enriched in Rb and Ba of LILE elements, showing positive Hf anomaly and negative Nb, Ti anomaly, corresponding to MORB in REE patterns, Furthermore, by the tectonic environment discriminant analysis of basalt, it is now found that the Engerwusu ophiolitic mélange is MORB-type ophiolite and it has the same characteristic of MOR, formed in the collision environment.In the Engerwusu ophiolitic mélange, it is conformed that the fossils era of limestone is the Late Carboniferous-Early Permian in the blocks, and radiolarians era is the Early Permian and early Late Permian, which are first discovered in the blocks. Meanwhile, we can obtain four isotopic ages by the LA-ICP-MS ziron U-Pb dating: 259.8 ±2.3 Ma of andesite in the blocks, 312.9±1.7Ma of tuff, 267.5 ±3.3Ma of basalt in the blocks and 296.0 ±2.7Ma of basalt in the blocks. Consequently, it is reasonable inferred that the initiate forming time of the Engerwusu ophiolitic mélange is late Late Carboniferous- early Early Permian and the amalgamation of time between the Tarim Plate and the North China Plate is the late Late Permian.As the first-level tectonic boundary in Alaxa, the Engerwusu ophiolitic mélange is a ultimate suture line between the Tarim Plate and the North China Plate. It is composed of typical collisional orogenic belt of continent-arc-continent. We can conclude that collisional orogeny had occurred during the late Hercynian or early Indosinian. Moreover, the dynamics of the collisional orogeny is mainly a southward subduction of oceanic crust and northward obduction of continental crust, followed by movement of shear slip.