The Geochemical Characteristics And Petrogenesis Of Carboniferous Volcanic Rocks In Yining Massif

Carboniferous volcanic rocks are widespread in the Western Tianshan Mountain,Xinjiang.Correctly recognize and identify their tectonic setting is the key to reveal the closure of Paleozoic ocean basin and transformation process of tectonic regime.This study takes the volcanic rocks of Early Carboniferous Dahalajunshan Formation and Late Carboniferous Yishijilike Formation as the objects,which exposed in the research area.On the basis of field investigation and variously previous geological data,through anew drawing of the reginal geological map and reestablishment division of volcanic stratigraphic sequence,getting hold of the time and space distribution state,rock association,area variation regulation of the two suits rocks.The author divided the eruption cycle,established s framework,summarized the geochemical characteristics,based on correct establishment and accurate identification and differentiation the two groups of volcanic rock,we deeply studied of major and trace element geochemical characteristics,Sr-Nd and Lu-Hf isotopes geochemistry characteristics,analysed source characteristics and genetic types.The above aspects provided new evidences for exploration of Late Paleozoic tectonic evolution in the western Tianshan Mountain.By comparative study on volcanic rocks of Dahalajunshan Formation and Yishijilike Formation,made the following understanding:1.The volcanic rocks of Dahalajunshan Formation are widespread in Yining Massif,there are many strong fold structures in the stratum.It composed of basalt,andesite,dacite,rhyplite and homogeneous pyroclastic rocks,and belongs to calc-alkaline series.The largest thickness rocks are andesites,thickness ratio of the basic,intermediate and acid volcanic lava is 1:8:1.Dahalajunshan Formation is given priority to with eruption facies,with less extravasation facies,burst index is generally larger,the highest can be divided into seven eruption cycle.The volcanic rocks of Yishijilike Formation are banding distribution in Yining Massif,most of them are synclinal,anticlinal and monoclinal stratum.The rocks are mainly composed of meso basaltic and acid components,lack or absence of andesite.The rock association has obviously discontinuous compositon,and shows the characteristics of bimodal volcanic rocks,thickness ratio of the basic,intermediate and acid volcanic lava is 4:1:5.This formation gives priority to with the overflow facies,with less eruption phase,the burst index are small with 3 5 eruption cycle.2.This study we collected 7 age samples in volcanic rocks of Dahalajunshan Formation?346.3³36.0Ma?,mainly belongs to Early Carboniferous.By contrast,we collected 5 age samples in volcanic rocks of Yishijilike Formation?313.4²99.4Ma?,mainly belongs to Late Carboniferous.3.The volcanic rocks of Dahalajunshan Formation are mainly classified into calc-alkaline series with SiO₂ range from 45.43% to 79.13%.They are generally enriched in LILE and LREE(LREE=10.93³48.63×10^-6),and relatively depleted in HFSE and HREE(HREE=6.62⁷5.62×10^-6),have obvious Nb(1.02⁴1.58×10^-6),Ta(0.03⁴.47×10^-6)and Ti negative anomalies.However,some samples are characteristic by Nb-enriched arc basalts with high Nb content.The basalts show that?87Sr/86Sr?i=0.7041⁰.7093,?Nd?t?=+0.77⁺7.34,the andesites show?87Sr/86Sr?i =0.7050⁰.7079,?Nd?t?=+0.87⁺3.24,but the rhyolites have wide range with?87Sr/86Sr?i and ?Nd?t?.176Hf/177Hf=0.282352 ⁰.282768,?Hf?t??-8⁺6.9,that display the volcanic rocks from the subduction of fluid/melt replacement of partial melting of the mantle wedge,and affected by different degree of crustal contamination.Combined with the tectonic environment discrimination diagram show the geochemical characteristics of active continental arc volcanic rock.4.The volcanic rocks of Yishijilike Formation are characteristics by bimodel volcanic rock.The basalts of SiO₂ content are range from 42.86 to 53.26%,with average of Rittmann Index is 6.43,rhyolitic of SiO₂ content are range from 70.16 to 70.16,alkalinity rate is an average of 6.51,belong to alkaline series.They are strongly enriched in LILE and LREE(LREE=32.7³63.55×10^-6),and relatively depleted in HFSE and HREE(HREE=3.81⁶5.55×10^-6).The trace elements of rhyolites characteristics similar to A-type granite.Basaltic andesite samples show that?87Sr/86Sr?i=0.7036⁰.7073,?Nd?t?=+2.36⁺4.89,rhyolitic?87Sr/86Sr?i= 0.7022⁰.7056,the rangeof ?Nd?t?is bigger?+2.36⁺4.89?,with an average of 4.52.176Hf/177 Hf is 0.282190⁰.283086,?Hf?t?=-14.1⁺17.4,according to the continental crust material of the neoproterozoic have joined,in combination with geochemical diagrams we believe the Yining Massif has entered the stretching stage after collision in the late Carboniferous.5.Basaltic magma of Dahalajunshan Formation was derived from partly melting mantle wedge,which was metasomatized with fluid?melt?precipitated from subtractive sheet space.Andesitic magma was derived from the root of arc crust and affected by mantle wedge.Acidic magma was derived from the stable lower crust source,mixed with substances from mantle magma.Lithosphere delamination of orogen belt roots caused mantle material upwelling of asthenosphere in Late Carboniferous.Basaltic magma of Yishijilike Formation was derived from partly melting upper mantle substances.Felsic magma was formed by remelting of Precambrian basement,which caused by underplating of basaltic magma to the lower crust.6.This study showed that calc-alkaline volcano-sedimentary rocks were developed during Early Carboniferous in fore-arc-inter-arc-back-arc basin system in Yining Massif from Tarim plate.By contrast,Late Carboniferous bimodal volcanic rocks and sedimentary association formed within regional extension tectonic setting and show affinity with continental rift,a responsible for intra-continental geological processes after the assembling of the Junggar and Tarim blocks.These observations suggest that the ancient oceanic basin should be close in late Early Carboniferous.