Petrogenesis and geodynamic implications of Early Cretaceous highly fractionated leucogranites in the northern Lanping–Simao terrane, Eastern Tibetan Plateau

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Highlights

Abstract

A felsic intrusion in the Guaguazi Cu mining district in the northern Lanping–Simao terrane, Eastern Tibetan Plateau was revealed through drill holes and is composed of highly fractionated leucogranites. In this paper, we report zircon U–Pb ages and whole-rock compositions to constrain the petrogenesis and geodynamic setting of this intrusion. Zircon grains separated from two leucogranite samples have identical U–Pb ages of 123.9 ± 2.0 Ma and 123.6 ± 1.9 Ma, indicating that the crystallization age of the intrusion is Early Cretaceous (ca. 124 Ma). Leucogranites from the intrusion have SiO2 contents from 74.3 to 77.6 wt% with differentiation index values of 96.3–97.6, indicative of highly fractionated nature. They are weakly peraluminous (A/CNK = 1.01–1.10) and have light REE enrichment ((La/Yb)N = 5.4–8.5) and negative Eu anomalies (Eu/Eu* = 0.26–0.45). The primitive-mantle-normalized spider diagram indicates strong enrichment in large-ion lithophile elements (Rb, Th, U, K, and Pb) relative to high-field-strength elements (Nb, Ta, P, and Ti). The high initial 87Sr/86Sr ratios (0.7070–0.7094) and negative εNd(t) values (−8.18 to −7.20) and TDM2 ages of 1.58–1.51 Ga of the rocks, suggest that their source was predominantly Mesoproterozoic lower continental crust with insignificant mantle contribution. High initial 207Pb/204Pb (15.74–15.75) and 206Pb/204Pb (18.51–18.63) ratios further confirm their crustal source and highly evolved nature. Considering the petrogenetic similarity of Early Cretaceous granitoids in the Guaguazi area and the Gaoligong tectonic belt, we propose that the eastern Bangong–Nujiang Tethyan Ocean was closed at least no later than the Early Cretaceous, and that the Guaguazi granitic intrusion formed in a crustal thickening setting in response to the Lhasa–Qiangtang collision.

Introduction

The Mesozoic Bangong–Nujiang suture zone (BNSZ) marks the closure of the Bangong–Nujiang Tethyan Ocean (BNTO) which was subducted northward beneath the southern Qiangtang block (Yin and Harrison, 2000, Zhang et al., 2012). The suture zone extends for more than 2000 km and separates the Qiangtang block to the north and the Lhasa terrane to the south (Fig. 1a). Preserved sedimentary and volcanic rocks indicate that the BNTO existed from at least Permian to Early Cretaceous, but the timing of its closure is an unresolved issue, and ranges from the Late Jurassic–Early Cretaceous (e.g., Yin and Harrison, 2000, Kapp et al., 2005, Kapp et al., 2007) to the Late Cretaceous (ca. 100 Ma; e.g., Shi et al., 2008, Baxter et al., 2009). A recently proposed oblique subduction model (Liu et al., 2017) suggests a diachronous Lhasa–Qiangtang collision over the entire Cretaceous.

A discontinuous Jurassic–Cretaceous magmatic belt in the northern Lhasa terrane and southern Qiangtang block stretches from Bangong Co in westernmost Tibet through Gaize and Amdo to Diqing, and continues into the Gaoligong tectonic belt in western Yunnan (Zhang et al., 2012, Liu et al., 2017). The magmatism was related largely to the pre-, syn-, and post-collisional stages between the Lhasa terrane and Qiangtang block (Kapp et al., 2005, Kapp et al., 2007, Zhu et al., 2009a, Zhu et al., 2009b, Zhu et al., 2013, Lin et al., 2012, Zhang et al., 2012, Liu et al., 2017, Li et al., 2017, Zhang et al., 2018, He et al., 2019). The temporal–spatial variations of magmatism remain poorly constrained, particularly in the eastern part of the belt.

Early Cretaceous granitoid plutons in the northern Lhasa terrane extend to southeast to the Bomi–Chayu batholiths in East Tibet and Gaoligong tectonic belt in western Yunnan (Fig. 1a). They comprise predominantly peraluminous S-type granitoids with minor metaluminous I-type diorites (Fig. 1b). Their widespread distribution is explained to be the melting of various crustal and minor mantle materials in a setting of crustal thickening (Lin et al., 2012, Zhu et al., 2009b, Xu et al., 2012, Zhu et al., 2017). However, Early Cretaceous igneous rocks are rare to the east of the Gaoligong tectonic belt due to the cover of large-scale late Mesozoic to Cenozoic sediments (i.e., Lanping and Jianchuan basins; e.g., Yang et al., 2014a). Investigation of the newly discovered Early Cretaceous granitic intrusion in the Guaguazi Cu mining district of the northern Lanping–Simao terrane (Fig. 2) is important and may provide new insights about the evolution of the eastern BNTO. Here, we present petrological, zircon U–Pb, geochemical, and Sr–Nd–Pb isotopic data for the Guaguazi granitic intrusion to elucidate its crystallization age, petrogenesis, and geodynamic setting.

Section snippets

Regional geology

The Tibetan Plateau is a tectonic collage of the Kunlun, Qiangtang, Lhasa, and Himalaya continental blocks (terranes). These blocks are separated by suture zones that define positions of former Tethyan oceans, namely the Jinsha, Bangong–Nujiang, and Yarlung–Zangbo suture zones from north to south (Yin and Harrison, 2000, Zhu et al., 2013, Metcalfe, 2013) (Fig. 1a).

Scattered granitoid plutons and mafic–felsic volcanic rocks on the southern margin of the Qiangtang block are collectively

Zircon U–Pb analyses

Zircon grains from two samples (ZK5–1-4 and ZK5–5-6; Fig. 2b) were separated from crushed and ground samples by conventional heavy-liquid and magnetic techniques and hand-picked under a binocular microscope. They were mounted in epoxy, polished, coated with carbon, and imaged under cathodoluminescence (CL) light to identify internal textures. CL images were obtained using a HITACHI S-3000N scanning electron microscope fitted with a Gatan Chroma CL system at the Beijing SHRIMP Center, Beijing,

Zircon U–Pb ages

Zircon grains from the two samples are mostly transparent and euhedral–subhedral in CL images with aspect ratios of 1–3 (lengths 65–169 μm; widths 54–94 μm), and display concentric oscillatory zoning (Fig. 4a, b). Trace-element analyses were performed as part of the dating procedure. The zircons have similar chondrite-normalized REE patterns, characterized by heavy REE (HREE) enrichment, light REE (LREE) depletion ((La/Yb)N < 0.031), and negative Eu (Eu/Eu* = 2EuN/(SmN + GdN) = 0.18–0.48) and

Fractional crystallization and granite type

Alteration and metasomatism in felsic intrusions may make rock classification difficult and petrogenetic conclusions problematic, but this does not apply to the leucogranites from Guaguazi, which exhibits insignificant modification during or after emplacement, as indicated by the low abundance of secondary minerals (clay minerals, sericite, and chlorite) and low LOI values (≤1.5 wt%; Table 3). Furthermore, albitization in felsic rocks is also a criterion for metasomatism and is commonly related

Conclusions

The highly fractionated Guaguazi granitic intrusion in the northern Lanping–Simao terrane was generated during the Early Cretaceous (ca. 124 Ma). Its primitive melt was sourced mainly from Mesoproterozoic lower continental crust with minor juvenile crustal or mantle input. The temporal–spatial distributions and petrogenetic similarity of Early Cretaceous magmatism in the Guaguazi area and the Gaoligong tectonic belt indicate that they were triggered by crustal thickening caused by the

CRediT authorship contribution statement

Zhipeng Xie: Conceptualization, Data curation, Writing - original draft, Visualization, Supervision. Chuandong Xue: Resources, Writing - review & editing, Project administration. Tiannan Yang: Validation, Funding acquisition. Kun Xiang: Methodology, Software. Di Xin: Formal analysis, Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Grant number 41603032, 41373049), the Applied Basic Research Foundation of Yunnan Province (Grant number 2017FB075), the Yunnan Key Research and Development Program (Grant number 2015CB452601), and the China Geological Survey (Grant number 12120114064301). Dr. Zhenhui Hou from the University of Science and Technology of China (CAS Key Laboratory of Crust–Mantle Materials and Environments) is acknowledged

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