Late Paleozoic Subduction Accretion Processes In The Baogutu Tectonomagmatic Belt Of Southern West Junggar And Their Geochemical Constraints

The western Junggar in the southwestern Central Asian orogenic belt is a key area for studying the Junggar oceanic basin and revealing the tectonic evolution before the final closure of the Paleo-Asian Ocean.Late Paleozoic magmatic rocks are widely developed in the southern part of West Junggar（SWJ）,and previous studies have yielded fruitful results.However,there are still obvious controversies about the tectonic background and evolution model of the mid-late stage of Late Paleozoic in the region.On the basis of detailed field work,an integrated study of petrographic,geochronological,geochemical and Sr-Nd-Pb-Hf isotopic compositions has been carried out on the Early Carboniferous-Early Permian volcanic rocks exposed in the Baogutu tectonomagmatic belt in southern West Junggar,aiming to reveal the formation age and genetic mechanism.At the same time,a large number of previously published research results of basic to acid intrusive rocks in the SWJ were collected and integrated to comprehensively analyze and discuss the tectonic development and evolution of West Junggar during the Late Paleozoic era.The results achieved are as follows:1.An Early Carboniferous arc-basin system was ascertained in the SWJ.Early Carboniferous island-arc volcanic rocks,oceanic island basalts and back-arc basin basalts coexisted in the SWJ.The calc-alkaline basic to acid island arc volcanic rocks were formed at344 to 332 Ma,and are distributed in the Miaoergou-Baogutu-Baijiantan area.The alkaline oceanic island basalts were formed at～346 Ma,and are mainly distributed in the Xiaerpu and south of Hongshan regions.The back-arc basin basalts are distributed in the Hatu（324 Ma）and Sulushuoke areas.The newly identified Early Carboniferous tholeiitic back-arc basin basalts in the Sulushuoke area were formed at 347～339 Ma.Geochemical studies show that they have moderate Si O₂and Al₂O₃contents,high TFe₂O₃and MgO contents and Mg^#values.They also possess a near-horizontal REE pattern with enrichment of U,depletions of Nb,Ta,Zr and Hf,and positiveε_Nd（t）（+5.86～+6.59）andε_Hf（t）values（+3.99～+15.6）and high initial Pb isotope ratios,showing dual characteristics of N-MORB and island arc volcanic rocks.These signatures indicate that these tholeiitic basalts were derived from 10%～20%partial melting of a spinel-lherzolitic depleted mantle source modified by fluids released from the subducted lithosphere in a back-arc extensional setting.In combination with the spatial distribution pattern of early Carboniferous volcanic rocks in the SWJ,we believe that the alkaline oceanic island basalts in the Xiaerpu and south of Hongshan regions are more likely to be formed in a back-arc basin closer to the island arc,which may have been generated by partial melting of mantle wedge peridotite metasomatised by hydrous felsic melts with the signatures of LILE,LREE,and HFSE enrichment that were released from the very-low-degree melting of the residual Late Devonian seamounts/ocean islands travelling forward together with the subducting oceanic slab at the postarc depths.2.It is confirmed that the SWJ was still in a subduction dominated environment during the Late Carboniferous to the initial stage of Early Permian.The newly identified Late Carboniferous basalts in the east of Tiechanggou were formed at 313～310 Ma.Geochemically,these basaltic rocks are tholeiitic with low Si O₂and Al₂O₃contents but high TFe₂O₃,MgO contents and Mg^#values.They display slightly flat rare earth element patterns with depletions in Nb,Ta,Ti,Zr and Hf,and also have high positiveε_Nd（t）（+4.21～+4.36）,ε_Hf（t）（+8.06～+15.7）values and initial Pb isotope ratios,exhibiting both N-MORB-like and arc-like signatures,similar to the back-arc basin basalt.At the same time,a set of late Carboniferous basic-acid island arc volcanic rocks with ages of 309～303 Ma was also found in the Hala’alate Mountain.They show enrichments in LILEs and LREEs relative to HFSEs with negative Nb,Ta and Ti anomalies.Among them the basalts and part of basaltic andesites show subduction-related arc magmatic characteristics,some basaltic andesites show Sanukite characteristics,dacites show oceanic crust melting adakite characteristics and rhyolites have I-type granite characteristics.Moreover,the newly identified A₁-type alkali-feldspar granite porphyries in Urho area were formed in the initial stage of Early Permian（295 Ma）,coeval with the A₂-type granites in the SWJ with ages of 308～295 Ma.The alkali-feldspar granite porphyries are geochemically characterized by high silicon,alkaline contents but low magnesium,total iron and aluminum with AR values of 3.78-5.56 and A/CNK values of 0.82～1.01.They show enrichments in Rb,Th,U,K but depletions in Sr,Ba,P,Ti,Eu with highε_Nd（t）value（+7.76～+11.9）and Zr saturation temperature（978～1039°C）as well as low Y/Nb ratio,categorizing them as metaluminous A₁-type granites.The newly identified basic-acid island arc volcanic rocks and back-arc basin basalts indicates that an arc-basin system was still developed in the middle-late stage of Late Carboniferous in the SWJ,and the remnant oceanic basin was under the extensional environment at 313～310 Ma.The contemporaneous A₁and A₂type granites in the region were formed in a back-arc extensional setting triggered probably by roll-back of the northwestern subduction of the Junggar oceanic slab at～310 Ma.3.The down-going of the remnant oceanic slab still proceeded in deep in the middle stage of the Early Permian in SWJ.The studied tholeiitic basaltic volcanic rocks within the Baiyanghe Formation and contemporaneous basic-intermediate diabase and gabbro diorite dykes/stocks in the Hala’alate Mountain were formed in the middle Early Permian with ages of 286-284 Ma,later than the deposition of the continental molasse of Lower Permian Jiamuhe Formation in the region.These tholeiitic basic-intermediate magmatic rocks have medium Si O₂,TFe₂O₃and MgO contents,relatively high Al₂O₃content,Mg^#value but low Ti O₂contents,and are rich in Na₂O but poor in K₂O.They show enrichments in Ba,U,Pb,Sr but depletions in Th,Nb,Ta,Zr,Hf and Ti with highε_Nd（t）value（+5.63 in average）,showing the characteristics of arc magmatic rocks related to subduction.Based on the above analysis,we believe that these late Early Permian magmatic rocks with arc signatures were formed after the extinction of the West Junggar remnant oceanic basin,and trench-arc-basin system had disappeared at that time,which show“lagged”temporal characteristics,resembling those of“Lagged arc magmatic rocks”.Although the marine sedimentation has ended in surface and evolved into intracontinental orogenic stage in the middle Early Permian after the deposition of the Jiamuhe molasse formations in the Hala’alate Mountain,the down-going of the remnant slab after the closure of the oceanic basin still proceeded in deep,which also suggests that the final termination of deep subduction of Late Paleozoic remnant oceanic slab in the West Junggar was later than the mid-late stage of Early Permian.4.Based on the above results and previous achievements,this paper further perfects the multi-stage subduction accretion collage orogenic evolution model in the West Junggar during the Late Paleozoic era.It is considered that the Early Carboniferous,Late Carboniferous to initial stage of Early Permian and middle stage of Early Permian geological bodies exposed in the SWJ are at least three different phases subduction accretion bodies.This model can better explain the special distribution phenomenon that the subduction-related arc volcanism and coeval intrusive rocks possessing intimate relationships with subduction can always be found in the southeast（erupted on the side close to the island arc-back arc basin）and northwest（intruded into the subduction accretion bodies that have finished accretion orogeny）of the regionla fault,respectively in the SWJ,which may reflect an important crustal growth pattern in the southwestern margin of the Central Asian Orogenic Belt during Late Paleozoic.