The Stratigraphic Sequanence, Petrogenesis And Tectonic Setting Of The Volcanic Rocks Of The Dahalajunshan Formation, Western Tianshan Mountain, China

The volcanic rocks of the Dahalajunshan Formation are widespread in the Western Tianshan Mountain,and mainly exposed at the north, the south edge and the inside of the Yili Block. They are closely relatedwith mineralization, also are one of the focus on clousure of Paleozoic ocean basin and transformation forthe Western Tianshan tectonic system. Therefore, some meticulous research on the volcanic rocks of theDahalajunshan Formation has important scientific meaning for better understanding the ocean-continenttransformation process of the South Tianshan Ocean, which are represented by Ophiolitic melang belt at thesouthern margin of Middle Tianshan. At the same time, it also provides basic geologic information on LatePaleozoic volcanic metallogenic background. On the basis of previous work, this paper take the volcanicrocks of Dahalajunshan Formation exposed in the whole region as an object of study, by systematicallystudying the aspects of petrography, geochemical and geochronology, we determined the rock association,stratigraphic sequence and its variation, established a regional geochronological framewock, and thendiscussed the petrogenesis of the volcanic rocks and its tectonic setting, constrained on the closure time ofthe South Tianshan Ocean. Made the following understanding:1. The volcanic rocks of the Dahalajunshan Formation are overall composed of basalt, andesite, dacite,rhyolite and intermediate-acid pyroclastic rock.. The thickness ratio of respective lava is1:4:2:2, the lavaand the pyroclastic rock is7:3. The distribution of volcanic rocks of the whole region has the regularity: Atthe north of the Yili block, volcanic rock assemble is mainly basaltic andesite and andesite, and graduallybecome mainly dacite and rhyolite from west to east in Boluokenu area. At the central of the Yili block,volcanic rocks assemble is mainly andesite, dacite and a small amount of rhyolite in Awulale area. At thesouth of the Yili block, volcanic rocks are mainly andesite up to50%, with a similar proportion of basalt,dacite and rhyolite in Wusunshan-Tasibashan area.2. In the aspects of lava’s eruption sequence: In Boluokenu area firstly erupt intermediate-basic magma andthen intermediate-acid magma; In Wusunshan-Tasibashan area firstly erupt acid magma, and then basic andacid magma in turn. The activities of volcano have a character of fissure-central eruption and the scalegradually become large.3. Geochronology study showing that, the age of volcanic rocks of the Dahalajunshan Formation is369Mato332Ma, mainly belongs to Early Carboniferous Tournaisian period. A small amount may be formed inthe late Late Devonian and Early Carboniferous Visean period. Regional Volcanism gradually becameyounger from the south and north edge to the inside, from the west to east.4. The volcanic rocks of the Dahalajunshan Formation are mainly classified into ca-alkaline magma series,few tholeiitic series. They are generally enriched in LREE and LILE, and relatively depleted in HREE andHFSE, have obvious Nb, Ta and Ti negative anomalies. Therefore, they show the geochemisticalcharacteristics of island arc or active continental arc rock..5. The remarkable differences in REE and trace element normalized pattern of basalt-basaltic andesite,andesite, dacite-rhyolite suggest that they derived from different magma source. Basalt and basaltic andesite have characters of depleted mantle with enrichment metasomatism, obviously contained somecomponents of continental crust and subdution sediments. Andesite magma was derived from the arc crustroots and affected by mantle wedge. Acid magma have origin of Lower crust source, where the plagioclaseis stable, it also contains a small amount of mantle material.6. The volcanic rocks of the Dahalajunshan Formation may be formed at active continental margin, createdby the subduction of the South Tishan Ocean towards north beneath the Yili Block. This constrained on thetime of final closure of the South Tianshan Ocean later than the Early Carboniferous.