Geochemistry and zircon U–Pb–Hf isotopes of granodiorites in the northern Alxa area: implications for the Middle–Late Devonian tectonic evolution of the southern Central Asian Orogenic Belt

The northern Alxa area is the southernmost segment of the southern Central Asian Orogenic Belt. Its tectonic environment during the Middle–Late Devonian is debated: post-collisional or subduction settings. Here, we conducted a combined study associated with the geochemistry, geochronology, and zircon Hf isotope in Middle–Late Devonian (386–375 Ma) granodiorites from the northern Alxa area for understanding their petrogenesis and tectonic setting. Our results show that these granodiorites are peraluminous (ACNK = 1.08–1.12) and (high-potassium) calc-alkaline I-type granites. Moreover, they exhibit light rare-earth elements (LREEs) and large ion lithophile elements (LILEs) enrichment as well as depletions in Nb, Ta, and Ti. The ε Hf (t) values of these granodiorites could be divided into two groups: the granodiorites with ages of 385–380 Ma yield mostly positive ε Hf (t) values (+0.2–+5.1) with young two-stage model ages of 1.78–1.33 Ga and few negative ε Hf (t) values (from − 0.2 to – 9.9) with old two-stage model ages of 2.69–1.80 Ga, suggesting that they were derived from juvenile oceanic crust materials mixed with older continental materials. In contrast, the Late Devonian granodiorite (375 Ma) yielded radiogenic ε Hf (t) values between − 12.6 and − 2.1 with old two-stage model ages of 2.91–1.98 Ga, indicating its origination of the partial melting of ancient crustal materials. Combined with sedimentary evidences, coeval adakites, and arc-related volcanic rocks in the northern Alxa area and the adjacent South Beishan Orogenic Belt, implying the Zhusileng–Hangwula Tectonic Zone underwent a subduction setting during the Middle–Late Devonian and this subduction was possibly associated with the Enger Us Ophiolitic Belt.