Silurian Tectonic Setting Analysis In Qiada Area Of West Junggar

Mayile ophiolitic mélange is one of the best-preserved Paleozoicophiolitic mélanges in southwest area of Mayile mountain in West Junggar. Itis unconformably overlained by the Silurian Mayilashan Formationvolcanic-sedimentary strata. This provided favorable conditions for the studymagmatic-tectonic evolution of the Silurian in West Junggar. This dissertationis object on the stratum, volcanic rocks, ophiolite of Silurian System in theQiada area of West Junggar, Xinjiang. Based on petrology, rock assemblages,geochemistry and tectonic setting, this study focuses on evolution of theSilurian ocean basin.1. Based on the Predecessors, the Silurian Mayila formation is dividedinto four parts. The first part(S_2－3m¹)is mainly composed of gray-greenmuddy siltstone and tuffaceous fine-grained sandstone, the second part（S2－23m）is mainly composed of gray-green and gray purple tuffaceous siltstone,they occured as interbedded or mezzanine, and also found volcanic rocksmezzanine in this lithologic. That is basaltic andesite, andesite, volcanicbreccia. the third part(S_2－3m³)is mainly composed of tuffaceous fine-grainedsandstone and muddy siltstone, and the fourth part(S_2－3m⁴)is mainlycomposed of chert and siliceous siltstone.2. According to analyzing depositional environments of the Silurianstrata, this study suggest that the four parts of the Mayilashan Formation formed in different settings, shoreface, foreshore, shoreface and shallow sea,respectively.3. We report here geochemical data obtained for basalt embedded in theSilurian strata. All of the samples bear island arc geochemical signatures: theyare tholeiitic in composition with low concentrations of Rare Earth Element（REE）, clear differentiation between LREE and HREE, weak or no Euanomalies, typical geochemical characteristics to those of E-MORB, enrichedin LILE, depleted in HFSE, and obvious weak Nb, Ta negative anomalies.4. Zircon U-Pb analyses from one representative gabbro sample fromMayile ophiolitic mélange by LA-ICP-MS yielded a weighted mean age of431±24Ma. The basalts from mélange display tholeiitic in composition withenrichment in LILEs, depletion in HFSEs, depletion Nb，Ta generally,indicate from subduction-related setting, may be formed in back-arc basinsetting.5. Data in this study, in combination with previous work constrainstrench-arc-basin model in the West Junggar, and the oceanic plate southwardsubducted in the Mayilashan area.