Petrogenesis and Geochronology of Late Devonian Intrusive Rocks in Eastern Tianshan, Xinjiang, China: Subduction Constraints of the North Tianshan Ocean

We conducted a study on the petrology, geochemistry, and zircon U-Pb dating of Late Devonian intrusive rocks in the Tulargen area of the Eastern Tianshan Orogenic Belt, Xinjiang, China. These intrusive rocks primarily consist of gabbro (382 +/- 5 Ma), tonalite (370.9 +/- 2.7 Ma), and biotite monzogranite (362.8 +/- 4.4 Ma). Gabbro belongs to the low-K calc-alkaline series of quasi-aluminous rocks, with a high Al2O3 content (16.46-20.34 wt.%) and Mg# value (64.55-67.73). Tonalite and biotite monzogranite, which belong to the high-K calc-alkaline series, are metaluminous or weakly peraluminous and also exhibit high Al2O3 contents (14.6-15.87 wt.%) and Mg# values (40.12-62.47). These rocks are enriched in light rare-earth and large-ion lithophile elements (Rb, Ba, and K) and depleted in heavy rare-earth and high-field-strength elements (e.g., Ta, Nb, and Ti), characteristics typical of island-arc magmatic rocks. Gabbro melts are primarily derived from the mantle and result from the partial melting of a depleted mantle that has undergone fluid metasomatism due to subducted plates. Tonalite exhibits high 176Hf/177Hf and epsilon Hf(t) values, with a younger two-stage model age (tDM2) derived from partial juvenile crust melting. The source magma of the biotite monzogranite originated from partial metabasalt melting at a medium crustal depth combined with a new lower crustal material. We concluded that the Late Devonian intrusive rocks in this area formed within the island-arc tectonic setting are associated with the subduction of the North Tianshan Ocean.