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 εHf(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.