Geochemical Characteristics and Geological Significance of Gabbro from the South Tianshan Hongliugou in Xinjiang

The Hongliugou gabbro is located in the eastern part of the Halke Saarming Late Paleozoic arc belt on the northern edge of the South Tianshan Mountains. It is one of the few intrusive rock masses exposed in the Early Paleozoic arc magmatic belt on the northern edge of the South Tianshan Mountains and the southern edge of the Central Tianshan Mountains. Gabbro is mainly composed of plagioclase (±48%), common pyroxene (±42%), biotite (±5%), and hornblende (±5%). The chemical composition of the rock shows that the Na₂O content of gabbro ranges from 2.49% to 3.08%, with an average of 2.83%; The K₂O content ranges from 1.6% to 2.4%, with an average of 2.02%. The Rittman index is 1.78 to 2.09, both of which are less than 3.3%; The differentiation index ranges from 50.3 to 57.06, and the main trace diagram shows that it belongs to the plaque calcium alkaline series; The standardized distribution pattern of rare earth elements in gabbro chondrite meteorites is similar to that of E-MORB, with enrichment of light rare earth elements and depletion of heavy rare earth elements, and no significant difference observed δ Eu anomaly, trace elements with enrichment of large ion lithophilic elements such as Rb, Ba, Th, U, and depletion of high field strength elements such as Ta, Nb, Ti; Zircon in-situ the εHf (t) value ranges from 1.27 to 8.75, indicating that the gabbro originated from the depleted mantle and was significantly contaminated by the crust during its ascent. The w (Na₂O+K₂O)/w (TiO₂) discrimination diagram of the basic rocks shows partial melting of carbonated peridotite in the Hongliugou gabbro source area. The zircon U-Pb dating shows that the age of gabbro formation is (410.4±2.3)Ma (MSWD=0.51, n=25). This study supplemented the dating and geochemical evidence of gabbro in the South Tianshan tectonic belt, and combined with regional geological data, we proposed that in the Early Devonian, there was oceanic continental subduction in the South Tianshan Ocean, accompanied by the intrusion of basic rocks.