Geological Characteristics And Tectonic Evolution Of Paleozoic Ophiolitic Mélange In The West Junggar

As part of Central Asian Orogenic Belt （CAOB）, the West Junggar is located atthe southern margin of the CAOB, and is a key area for understanding thePaleozoic tectonic evolution of the CAOB. Several ophiolitic mélanges were formedand preserved in the West Junggar, including Tangbale, Mayle, Durbut, Karamayand Barleik ophiolitic mélange belts. As a result of detailed study on the ophioliticmélanges, combined with data from the literature, I discuss the nature, time andtectonic evolution of the West Junggar Paleozoic ocean basin, and associated withthe structure-magmatism.LA-ICP-MS zircon U-Pb weighted mean206Pb/238U age of the gabbros from theMayile ophiolitic mélange （MOM） in the West Junggar is572±9Ma, belong toEdiacaran, this is oldest age from ophiolitic mélange in the Junggar as well asNorth Xinjiang, indicating the Mayile ophiolitic mélange including Ediacaran andSilurian ocean crust at least. The alkalic pillow basalts from Mayile ophioliticmélange are similar with the ocean island basalt （OIB）, characteristics by high Ti（TiO₂=1.65%³.13%）, Fe （FeOt=8.93%¹8.11%）, P （P₂O₅=0.17%⁰.51%）, andlow Mg （MgO=3.95%⁵.27%）, and high Th/Ta ratios （Th/Ta=1.1¹.9）, and cleardifferentiation between LREE and HREE （（La/Yb）N=2.5⁷.4）. It is assumed that themagma extruded into ocean island or seamount of the ocean floor. However, thegeochemical characteristics of the gabbros different from basalts, may be formsubduction-related setting. The geochemical features of the basalts show theyhave ocean island basalts, derived from EMI type enriched-mantle. Petrogenesis isclosely related to asthenosphere mantle, asthenospheric mantle upwelling led to alarge proportion of spinel lherzolite mantle partial melting to forming the basaltmagam. It is a product of asthenosphere interacted with lithosphere.The time of SSZ-type Tangbale ophiolitic mélange （TOM） belongs to Cambrian.All of the tholeiitic basalt samples bear the signature of subduction related basalt.The rocks were derived from depleted2–5%spinel lherzolite mantle source in aisland-arc or back-arc basin setting. Geochemical data from this study and previous work constrains southward subduction model in the West Junggar for theCambrian–Ordovician.The Darbut ophiolitic mélange （DOM） in the West Junggar is the largest andone of the best-preserved Paleozoic ophiolitic mélanges in the CAOB. However,the time of the Darbut ophiolitic mélange is still disputed. Zircon U–Pb analysesfrom the basalt and gabbro by LA-ICP-MS yielded weighted mean age of391±6Ma368±11Ma and375±2Ma, suggesting a Late Devonian emplacement.Based on detailed studying on the two largest Saertuohai and Suluqiaoke body,geochemically, the basaltic rocks includes at least two distinct magmatic units: oneis tholeiitic basalt, which were produced by ca.2%～5%spinel lherzolite partialmelting in a suprasubduction zone environment （SSZ）, and the other contemporaryalkali lavas, which were characterized by no obvious Nb, Ta and Ti negativeanomalies, suggesting a typical OIB affinity, were derived from5%～10%garnet+minor spinel lherzolite partial melting in an oceanic plateau or a seamountassociated with a mantle plume-related magmatism.The Karamay ophiolitic mélange （KOM）, which is a ophiolitic mélange found bygeological survey and research in recent years, located in the eastern part of theWest Junggar area in NW China. Zircon U-Pb analyses from the basalt and gabbroby laser-ablation ICP-MS yielded weighted mean206Pb/238U age of395±3Ma and387±8Ma, respectively, suggesting a Middle Devonian emplacement, andconsistent with the age of the DOM. The gabbros display tholeiitic basalt features,LILE enrichment and HFSE depletion, and obvious Nb and Ta negative anomalies,derived from a depleted mantle source with addition of fluids from a subducted slabwithin a suprasubduction zone environment. Compared to the gabbros, all basaltsbear the signature of Ocean Island Basalt （OIB） and are characterized by alkalinecomposition, LILE and LREE enrichment and HREE depletion, no obvious Nb, Taand Ti negative anomalies. I propose that these basaltic rocks were derived from amantle plume-related magmatism associated with the evolution of the PaleasianOceanic system, with an enriched mantle source could have contained2%-5%garnet and³%spinel. Basalts from the Barleik ophiolitic mélange in the West Junggar displayalkaline basalt features, and have typical geochemical characteristics to those ofOIB, with high TiO₂, Na2O, and low Al2O3, K2O. The geochemical characteristics ofthe basalts show they have ocean island basalts, petrogenesis is closely related toasthenosphere mantle, asthenospheric mantle upwelling led to a large proportionof spinel lherzolite mantle partial melting to forming the basalt magam.Seamount/oceanic plateau developed in the vast West Junggar Ocean （WJO）in Precambrian, the SSZ-type Tangbale ophiolitic mélange formed following theWJO southward subducted. As subduction move to north, the SSZ-type Mayleophiolitic mélange, including Precambrian seamount from the WJO, formed in thenorth of the Tangbale. In pace with continuous subduction northward retreat, theBarleik, Darbut and Karamay ophiolitic mélange enplaced to the north of the Maylein the Late Devonian. Moreover, some seamount components related to mantleplume found in the Darbut and Karamay ophiolitic mélange. During the period ofCarboniferous, the West Junggar was dominated by normal northwestwardsubduction, and the ridge subduction commenced in the Late Carboniferous, andlead to large-scale magmatic events in the West Junggar. The ridge subdution maybe continued until final ocean basin closed.in the end of the Early Permian.