| The Tianshan orogenic belt is a giant composite orogenic belt which is located in the west of China mainland, the tectonic position surrounded by Siberia plate, Junggar-Kazakhstan plate and Tarim plate, where is the key area of plate convergence and hyperplasia of the crust collage, and also the volcanic-magmatic activity is frequent, the metallogenic area of gold, copper and other metal deposits. At present,the Tianshan orogenic belt is considered as the evolution of the result of the paleo-asian ocean appeared in Sinian, experience complex subduction-collision-orogenetic process, but the final closure deadline of the paleo-Asian ocean and the detailed process of tectonic evolution process during Paleozoic are also a big controversy. East Tianshan region widely development of Ordovician, Carboniferous volcanic and sedimentary rock series, also the granite type, mafic-super and mafic intrusive rocks which is the ideal area to solve these problems.The comprehensive study of Devonian volcanic rock series in the Hami region, east Tianshan Mt. including the petrology, rock geochemistry and zircon U-Pb dating, provide the basis to fully understand the late Paleozoic tectonic evolution of the Tianshan orogenic belt. This papers obtained the following results:1) The volcanic rocks of Dananhu group is a set of neutral-basic lava and homogeneity in the volcanic clastic rock combination, mainly includes basalt, andesite. The pyroclastic rock consists of Andesitic tuff and dacitic tuff, and intruded by the dioritic porphyrite, lava SiO2 content is 53.06wt%~56.79wt%, with high aluminum content(A12O3 content is 15.76wt%~18.27wt%), low potassium content(K2O content is 1.28wt%~2.38wt%), low titanium content(TiO2 content is 0.8wt%~1.31wt%), the characteristics of the rich sodium, Na2O>K2O, Mg#=47.98~62.91, Rittmann index is 1.53~3.38, belong to calc-alkalic rock series; Enrichment of relatively large ion lithophile K, Rb, Sr, Ba, relatively loss of Nb, Ta, Th, Zr, Hf and Ti, have the similar trace elements geochemical characteristics of arc calc-alkaline basalt to island arc tholeiite, formed in the active continental margin environment above the subduction zone. The dioritic porphyrite( subvolcanic rock) have the similar main elements geochemical characteristics with the volcanic lava(high aluminum content, low titanium content, poor, potassium content, rich sodium content); The pyroclastic rock associated with the volcanic lava and subvolcanic rock, its rock chemical composition compared with the volcanic lava; subvolcanic rock and plutonic granitic intrusive rock have consistency, reveals their original magma sources should be homologous.2)granite( diorite) : Intruded into the neutral-basic volcanic lava of Dananhu group, occurrence by large batholith and stock, mainly includes biotite adamellite and granodiorite. SiO2 content is 51.29wt%~69.51wt%, A12O3 content is 15.15wt% ~ 19.67wt%, the alkali ALK(K2O+Na2O) content is 3.5wt%~9.29wt%, with high aluminum, rich sodium, the content of MgO content is 0.83wt%~5.14wt%, Mg#= 40.07~51.48, Rittmann index is 1.48~3.26, which belongs to the series of the alkaline rocks, A/CNK 0.52~0.52, belongs to the quasi-aluminous~peraluminous; Enrichment of relatively large ion lithophile K, Rb, Ba, Th, U, and relatively loss Ti, Nb, Ta, the volcanic magma have homology, formed in the continental margin arc environment.3) biotite adamellite and monzonitic granite zircons LA-ICP-MS U-Pb dating results(361.4 ±4.7Ma, 365.5±2.5Ma) which belong to the late Devonian. The diorite porphyrite in volcanic rocks of Dananhu group Zircons LA-ICP-MS U-Pb dating result(388.8±1.7Ma), which belong to the middle Devonian. Predecessors put it to the Dananhu group, so this Formation should be disintegrated. The age is the lower limit age of Dananhu group. Zircon Hf isotopic results show that the magma source region is mainly derived from depleted mantle, may exist crustal material contamination during the process of magma rising, the second phase pattern age(TDM2) of zircon Hf isotopic is 2561Ma~3511Ma, show that the granite probably from archean remelting of mantle source material. This age roughly similar with the old zircon age(3049±31Ma)of tuffaceous sandstone in Dananhu group, which suggests that the East Tianshan Mountain region has ancient Archean continental crust base.4) the Kanggu’ertage Ocean(the south branch of Paleo-Asian Ocean), formed in the Sinian, subducted beneath to the northern Dzungaria plate in early Ordovician, Devonian subducted to the Dananhu area. At first,formed the calc alkali-tholeiite type basalt in the east and center, then formed large scale of the subvolcanic rock during middle Devonian period, late Devonian development of a series of acidic granitic magma intrusion activities, and then the arc island continued to migrate to the south, closed,collision in the late early Carboniferous, formed widely distributed volcanic and sedimentary rocks since Ordovician and Carboniferous in the East Tianshan region, and accompanying series of intrusive magmatic activities. |
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