The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial o...The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean-continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time.Based on zircon U-Pb dating, we determined that the ages of the newly-discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO_2(75.78–79.20 wt%) and a moderate content of Al_2O_3(12.21–13.19 wt%). The total alkali content(K_2O +Na_2O) is 6.14–8.05 wt%, among which K_2O is 2.09–4.72 wt% and the rate of K_2O/Na_2O is 0.38–3.05. Over-based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO_2(0.09–0.24 wt%), CaO(0.15–0.99 wt%) and MgO(0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi-aluminum-weak peraluminous high potassium calc-alkaline and some calc-alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE(Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE(Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2-type granite, similar to the Miaoergou batholith,which indicates they both were affected by post-orogenic extension. Combining petrological, zircon U-Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean-continent transition of the West Junggar is the Late Carboniferous–Early Permian.展开更多
The migration,accumulation and dispersion of elements caused by tectonic dynamics have always been a focus of attention,and become the basis of tectono-geochemistry.However,the effects of faulting,especially strike-sl...The migration,accumulation and dispersion of elements caused by tectonic dynamics have always been a focus of attention,and become the basis of tectono-geochemistry.However,the effects of faulting,especially strike-slip faulting,on the adjustment of geochemical element distribution,are still not clear.In this paper,we select the West Junggar Orogenic Belt(WJOB),NW China,as a case study to test the migration behavior of elements under tectonic dynamics.The WJOB is dominated by NE-trending large-scale sinistral strike-slip faults such as the Darabut Fault,the Mayile Fault,and the Baerluke Fault,which formed during the intracontinental adjustment under N-S compression during ocean-continental conversion in the Late Paleozoic.Geochemical maps of 13 elements,Al,W,Sn,Mo,Cu,Pb,Zn,As,Sb,Hg,Fe,Ni,and Au,are analyzed for the effects of faulting and folding on element distribution at the regional scale.The results show that the element distribution in the WJOB is controlled mainly by two mechanisms during tectonic deformation:first is the material transporting mechanism,where the movement of geological units is consistent with the direction of tectonic movement;second is the diffusion mechanism,especially by tectonic pressure dissolution driven by tectonic dynamics,where the migration of elements is approximately perpendicular or opposite to the direction of tectonic movement.We conclude that the adjustment of element distributions has been determined by the combined actions of transporting and diffusion mechanisms,and that the diffusion mechanism plays an important role in the formation of geochemical Au blocks in the WJOB.展开更多
基金supported by the China Geological Survey (grant numbers DD20160083 and DD20160344-05)the National Key Research and Development Program of China (grant numbers 2018YFC0603701)Fundamental Research Funds for Central Public Welfare Research Institutes (grant numbers CAGS-YWF201706)
文摘The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean-continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time.Based on zircon U-Pb dating, we determined that the ages of the newly-discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO_2(75.78–79.20 wt%) and a moderate content of Al_2O_3(12.21–13.19 wt%). The total alkali content(K_2O +Na_2O) is 6.14–8.05 wt%, among which K_2O is 2.09–4.72 wt% and the rate of K_2O/Na_2O is 0.38–3.05. Over-based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO_2(0.09–0.24 wt%), CaO(0.15–0.99 wt%) and MgO(0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi-aluminum-weak peraluminous high potassium calc-alkaline and some calc-alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE(Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE(Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2-type granite, similar to the Miaoergou batholith,which indicates they both were affected by post-orogenic extension. Combining petrological, zircon U-Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean-continent transition of the West Junggar is the Late Carboniferous–Early Permian.
基金Financial support from the National Key Research and Development Program of China(the DREAM-Deep Resource Exploration and Advanced Mininggrant No.2018YFC0603701)the China Geological Survey(grant Nos.DD20160083 and DD20190011)。
文摘The migration,accumulation and dispersion of elements caused by tectonic dynamics have always been a focus of attention,and become the basis of tectono-geochemistry.However,the effects of faulting,especially strike-slip faulting,on the adjustment of geochemical element distribution,are still not clear.In this paper,we select the West Junggar Orogenic Belt(WJOB),NW China,as a case study to test the migration behavior of elements under tectonic dynamics.The WJOB is dominated by NE-trending large-scale sinistral strike-slip faults such as the Darabut Fault,the Mayile Fault,and the Baerluke Fault,which formed during the intracontinental adjustment under N-S compression during ocean-continental conversion in the Late Paleozoic.Geochemical maps of 13 elements,Al,W,Sn,Mo,Cu,Pb,Zn,As,Sb,Hg,Fe,Ni,and Au,are analyzed for the effects of faulting and folding on element distribution at the regional scale.The results show that the element distribution in the WJOB is controlled mainly by two mechanisms during tectonic deformation:first is the material transporting mechanism,where the movement of geological units is consistent with the direction of tectonic movement;second is the diffusion mechanism,especially by tectonic pressure dissolution driven by tectonic dynamics,where the migration of elements is approximately perpendicular or opposite to the direction of tectonic movement.We conclude that the adjustment of element distributions has been determined by the combined actions of transporting and diffusion mechanisms,and that the diffusion mechanism plays an important role in the formation of geochemical Au blocks in the WJOB.
