The Petrogenesis and Geological Significance of the Hornblende Gabbro in Western Huangshan of East Tianshan: Evidence from Zircon U-Pb Chronology and Geochemistry

The Carboniferous-Permian magmatic rocks are widely distributed in the Jueluotage tectonic belt in East Tianshan. Previous researchers have made researches into its tectonic background but there is still considerable controversy.This paper carried out studies on the hornblende gabbro, which were newly delineated in the 1:50,000 regional geological survey,in such aspects as petrology, geochemistry,and zircon U-Pb chronology.The rock geochemical analysis shows that the SiO₂ content of hornblende gabbro is 48.72%~53.09%, the Al₂O₃ content is 16.75%~19.48%,the MgO content is 5.67%~7.87%, and the Mg# value is 53.83~66.85.The Ritman index σ is 2.37~2.95,belonging to the calc-alkaline rock series.The distribution of rare earth elements is a right-leaning pattern, LREE/HREE=4.78~5.69, and δEu is 0.95~0.98, (La/Yb)N=4.66~5.80).Rb, Ba, Th, U, K, Sr and other large ion lithophile elements (LILE)are obviously enriched,and Nb, Ta are depleted.Based on the above characteristics,it is held that the magma source of the hornblende gabbro is affected by fluid metasomatism in the subduction zone. The zircon age (319.8±2.6) Ma of the hornblende gabbro obtained corresponds to the magmatic activity of the early Late Carboniferous in the region, which formed in an extension setting after the closure of the north Tianshan Ocean.