Geochemistry Constraints On The Origin And Evolution Of The Late Paleozoic Ophiolite In The West Junggar

The Central Asian Orogenic Belt (CAOB) is the largest Phanerozoic accretionaryorogens of the world. One of the most important features of accretionary orogens is thatthere are a number of ophiolite belts. As part of CAOB, the West Junggar is located at thesouthwestern margin of the CAOB, and is a key area for understanding the Paleozoictectonic evolution of the CAOB. Several ophiolitic mélanges were formed and preserved inthe West Junggar, including Darbut and Karamay ophiolitic mélange belts in the LatePaleozoic. And the Saertuohai, Baijiantan and Tuzituolangge ophiolite mélange are relativetypical sections of the belts. The Tuzituolangge ophiolitic section is newly discovered, andthe section is the extending of the Karamay ophiolite belt to northeast direction and wascut nearly north-south direction in Darbut fault zone. Recently, the detail studies on thegeochemistry, PGEs and Re-Os isotopic system of the three ophiolitic sections haveallowed us to discuss the genetic relationship of the two ophiolitic mélange belts during theLate Paleozoic in the West Junggar.The three sections of ophiolite are in fault contact with the Carboniferous strata andare locally covered by Quaternary deposits. All of the sections show the typicalanastomosing style and are composed of the serpentinite matrix which suffered fromintense ductile deformation and tectonic lens of relatively weak deformation. The tectonicblocks include ophiolitic members and exotic blocks. The mantle peridotites are mainlyharzburgite and the crust rock assemblages include gabbro, basalt and so on. The chertsand plagioclase amphibolite are also occurred in the mélange zone. Exotic blocks includetuff and oceanic island basalts.LA-ICP-MS zircon U-Pb dating for the Baijiantan ophiolitic mélange and theTuzituolangge ophiolitic mélange yielded the two weighted mean206Pb/238U age of373±5Ma and373±9Ma respectively, suggesting they are all formed during Late Devonian. Andthere also exists two sets of young Carboniferous data, which may be subject to metamictof zircons and can not represent the age of zircon crystallization. There is lack of directgeochronology in tectonic emplacement of the ophiolitic mélange. While acording to theage of the stitching pluton intruded into the Saertuohai ophiolitic mélange, we think the tectonic emplacement age of Saertuohai ophiolitic mélange were no later than309Ma.OIB-type basalts are recognized respectively in Saertuohai, Baijiantan andTuzituolangge ophiolitic mélange, and show the similarity in petrology. The OIB all showLILE and HFSE enrichment, and no obvious Nb and Ta negative anomalies. Although theOIB not belongs to ophiolite, they indicate the hot spots in ancient Junggar Ocean, andmore easily preserved in the subduction process.The members of the ophiolite all display the feature of SSZ ophiolite. The basalt andgabbro are similar to the N-MORB with respect to the LREE depletion patterns and Majoroxides. However, they also show a slight Nb, Ta and Ti negative anomalies in the spiderdiagrams of the trace elements. So they are possesses both of the characteristics of MORBand IAT. The PGEs of chromite rocks and mantle peridotite in Saertuohai show distributionpatterns of IPGE enrichment relative to PPGE. Pt and Pd have a clear differentiation, andmost have positive Pd anomalies. It shows that the chromite rocks and mantle may besubject to the reformation of the melts or fluids from subduction crust. The peridotites ofBaijiantan have the similarly features; meanwhile, their187Re/188Os and187Os/188Os ratiosare0.3731and0.1255respectively, and all are smaller than hose value of0.4346and0.1296in primitive upper mantle. And they also display the feature of SSZ-type ophiolite.PGE and Re-Os isotope characteristics of Baijiantan tuff reveal its PGE are mainly fromthe continental crust. PGEs of the Tuzituolangge ophiolite show an approximately flatpattern, but PGEs of basalts have a slope distribution pattern of PPGE to IPGE. Ingenerally, the rock assemblages of crust from the three ophiolite section all show IPGEdepletion to PPGE with the negative Ru anomalies, but the mantle rocks and the chromitesexhibit IPGE enrichment to PPGE or flat patterns with the positive Pd anomalies. Overall,the geochemical data show they are formed in a spreading center above the subductionzone. And combined with the regional geology, it’s believed that they are formed in amature back arc basin environment.In generally, Durbut, and Karamay ophiolitic mélange belts is very similar in formingage, rock assemblages, composition, tectonic environment and geological features, andsubstantially parallel with each other spatially. The Akebasitao and Tuzituolanggeophiolitic mélange are oblique intersection at Durbut fault. So, we can draw a conclusion that the Darbut and Karamay opholitic mélange zone should belong to a single belt, whichare just faulted into two belts in the latter process of structural transformation because ofthe sinistral strike-slip of the Durbut fault.