Partial melting caused by subduction of young, hot oceanic crust in shallow high-temperature and low-pressure environments: Indications from Middle and Late Jurassic oceanic plagiogranite in Shiquanhe, Central Tibet

Abstract

The oceanic plagiogranites that intruded into the ophiolite suite have received a great deal of attention from researchers. Some of these plagiogranites were controlled by mid-ocean ridge spreading and were formed via crystallization of basaltic magma or partial melting of the mafic gabbroic oceanic crust. The other plagiogranites were controlled by subducted oceanic crust slabs, and their formation mechanism is similar to that of the oceanic island arcs. The identification of different types of oceanic plagiogranites is of great significance for understanding the evolution of ancient ocean basins. In the Shiquanhe–Namtso mélange zone in central Tibet, scholars have recently identified many rock sequences related to intra-oceanic arcs, including MORB-like basalts, boninites, high-Mg andesites, and multiple series of granitic rocks. To further explore the formation mechanism of felsic rocks in oceanic island arcs and their role in the growth of continental crustal material, we sampled a series of Middle and Late Jurassic granitoids exposed in the ophiolitic mélange belt in the Shiquanhe area. Using zircon U-Pb dating, zircon Lu-Hf isotopic testing, whole-rock Sr-Nd isotopic testing and whole-rock major and trace geochemical testing, we determined the magmatic genesis and tectonic history of these samples. According to the test results and our field survey, we divided our samples into three groups. The first group consists of low-K plagiogranites (LKGs). These samples have ages that fall between 163 and 162 Ma. With their low K2O/Na2O ratios, low La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, weak negative Eu anomalies and their significantly depleted zircon isotopic compositions (εHf(t) = 10.0–12.5; εNd(t) = 0.64–1.10), we propose that these samples are the products of partial melting of oceanic gabbro and turbidite at shallow depths in low-pressure and high-temperature conditions. The second group consists of samples from dioritic enclaves within the LKGs. These samples, which are slightly older than the LKG samples (175–173 Ma), have isotopic characteristics that are similar to those of the LKG samples (εHf(t) = 10.4–13.5; εNd(t) = 0.51–0.83). The samples in the second group may be homologous captives that formed during the early crystallization of LKG homologous magmas. The third group consists of samples with compositions of high-K granodiorites (HKGs) and ages similar to those of the LKG group (~163 Ma). With high K2O/Na2O ratios, relatively high La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, negative Eu anomalies, significantly enriched isotopic compositions (εHf(t) = −11.1 to −15.4; εNd(t) = −13.29 to −13.43). Considering the basement exposure and known magmatic characteristics in the Shiquanhe area, we propose that HKGs samples are the result of interaction between continental subduction sediment melts and heterogeneous mantle material in high-temperature and low-pressure conditions.By comparing the oceanic plagiogranites in the Shiquanhe–Namtso mélange zone with those developed in the young, hot intra-oceanic subduction zone, we determined that the oceanic plagiogranites in the Shiquanhe–Lhaguotso–Asa area are forearc granites developed in the forearc area. These plagiogranites were controlled by the young, hot subducting plate and were formed by partial melting of a multi-component source area in a high-temperature and low-pressure environment.