Geochemistry, geochronology and Hf isotope of granitoids in the northern Alxa region: Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt

Abstract The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial. The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa. In this paper, we present new zircon U-Pb ages, Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula tectonic belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region. The zircon U-Pb data could be grouped as two phases: Late Devonian granite and diorite (ca. 373–360 Ma), and Late Carboniferous granodiorite (ca. 318 Ma). The Late Devonian granites and diorites are metaluminous to slightly peraluminous, with A/CNK and A/NK ratios of 0.90–1.11 and 0.95–2.19, respectively. The Late Devonian diorites are characterized by high MgO, Cr and Ni contents and MgO# values, together with variable eHf(t) values from –1.0 to +1.3 and old TDM2 ages varied from 1283 Ma to 1426 Ma, indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust. Even though the granites yielded most positive and minor negative eHf(t) values between –1.1 to +5.7 (three grains are negative) with two-stage model ages (TDM2) of 1003–1438, they display low MgO, Cr and Ni contents and MgO# values, suggesting that they were mainly derived from juvenile crustal materials, mixed with a small amount of ancient crust. The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series, with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and 1.75 to 1.90, respectively. These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle, as evidenced by their high eHf(t) values (+11.6 to +14.1) and young TDM2 ages (427 Ma to 586 Ma), as well as high Mg# values, and MgO, Ni and Cr contents. Our data, along with available sedimentary evidence and previous researches, indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting. The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.