Early Paleozoic subduction of the Paleo-Asian Ocean: Zircon U-Pb geochronological and geochemical evidence from the Kalatag high-Mg andesites,East Tianshan

The Kalatag area is located on the southern margin of the Turpan-Hami Basin,northern Xinjiang,Northwest China. The Early Paleozoic Daliugou Formation volcanic rocks in this area are widely distributed in Caixia mountains,Tuwu ore and Kalatag districts,which are located to the north of the Dacaotan fault. They consist of basic,intermediate,acidic lavas,and pyroclastic rock.LA-ICP-MS zircon U-Pb dating yield dacite and andesite age of 434. 8 ± 3. 8Ma and 438. 4 ± 4. 9Ma,respectively,which show that the Daliugou Formation volcanic rocks formed in the Early Silurian. Andesite have contents of Si O2( 54. 35% ~ 60. 70%),Al2O3( 11. 8%~ 16. 3%) with A / CNK of 0. 77 ~ 1. 37,Mg O( 4. 94% ~ 8. 27%),Ti O2( 0. 38% ~ 0. 52%),Na2O( 0. 50% ~ 3. 83%) and K2O( 0. 08% ~ 1. 26%) with high Na2 O / K2 O ratios of 3. 63 ~ 38. 3( except one samples),and lower ΣREE( 43 × 10- 6~ 70 × 10- 6).These samples are high-Mg andesites which have the characteristics of depleted source. Dacite have higher contents of Si O2( 63. 74%~ 75. 35%) than andesite,and similar contents of Al2O3( 12. 3% ~ 16. 0%) with A/CNK of 0. 77 ~ 5. 74,Ti O2( 0. 32% ~ 0. 52%),Na2O( 2. 01% ~ 35. 53%) and K2O( 0. 16% ~ 2. 19%),and higher ΣREE( 61 × 10- 6~ 84 × 10- 6). Both andesites and dacites have an obvious differentiation between the light rare earth element( LREE) and heavy rare earth element( HREE),Eu negative anomaly with medium. They are all enriched with large-ion lithophile elements( LILE),but are relatively depleted in high field strength elements( HFSE),and have an obvious negative Nb-Ta-Ti anomaly. These andesite and dacite are likely to come from the same magma chamber based on similar geochemical characteristics. Dacite is likely to be formed after andesitic magma underwent minerals such as hornblende crystallization differentiation. Andesitic magmas were most probably derived from a depleted ocean lithosphere mantle. Based on the volcanic assemblage,geochemical characteristics and regional geology,we speculate that the Early Paleozoic volcanism occurred in an intra-ocean island arc setting. Combined with the regional geological record,comprehensive analysis of the isotope geochronology data indicates that the study area have existed the record of the subduction of the Paleo-Asia Ocean.