Petrogenesis and tectonic implications of Devonian granitoids in the central Beishan orogen, NW China: geochemical and Sr-Nd-Hf isotopic constraints

Abstract

ABSTRACT The Devonian granitoids are widespread in the Beishan area of the southern Central Asian Orogenic Belt, which provides key geological records for revealing the middle Palaeozoic evolution of the Paleo-Asian Ocean. This study presents new zircon U–Pb ages, whole-rock major and trace elements, and Sr-Nd-Hf isotopes of four granitic plutons in central Beishan. The Haergen muscovite monzogranites were emplaced in the Early Devonian (401 Ma) and are considered as S-type based on their peraluminous features and high P2O5 contents. They show high 87Sr/86Sr(i) (0.7097 to 0.7141), low ℇNd(t) (−4.7 to −4.3) and variable ℇHf(t) (0 to 6.9), indicating an ancient crust source. The Balegentai, Haerteerdele, and Miaomiaojing plutons were intruded in the Late Devonian with ages of 381 Ma, 379 Ma, and 361 Ma, respectively. They are I-type granites, as indicated by their metaluminous to weakly peraluminous features and low zircon saturation temperatures. Furthermore, they present depleted 87Sr/86Sr(i) (0.7042 to 0.7079), ℇNd(t) (−1.6 to 2.2) and ℇHf(t) (4.0 to 10.6), suggesting that they were derived from the partial melting of juvenile crustal rocks. The Early Devonian granitoids with an age peak of 410 Ma in central Beishan consist of I-S-A type granites with variable Nd-Hf isotopes, implying extensive interactions between juvenile and/or mantle-derived and ancient crust-derived magma. In contrast, the Middle-Late Devonian granitoids with an age peak of 370 Ma consist of I-A type granites with depleted Nd-Hf isotopes, implying the input of more juvenile materials. We conclude that a collisional event between the Mazongshan and Huaniushan units occurred during the Early Devonian. The Middle-Late Devonian magmatism was formed in an extensional setting, and the Huaniushan unit was also influenced by the northwards subduction of the Liuyuan Ocean.