A Late Cretaceous felsic magmatic suite from the Tengchong Block, western Yunnan: integrated geochemical and isotopic investigation and implications for Sn mineralization

Abstract

The Tengchong Block within the Sanjiang Tethys belt in the southeastern part of the Tibetan plateau experienced a widespread intrusion of a felsic magmatic suite of granites in its central domain during Late Cretaceous times. Here, we investigate the Guyong and Xiaolonghe plutons from this suite in terms of their petrological, geochemical, and Sr–Nd, zircon U–Pb and Lu–Hf–O isotopic features to gain insights into the evolution of the Neo-Tethys. The Guyong pluton (76 Ma) is composed of metaluminous monzogranites, and the Xiaolonghe pluton (76 Ma) is composed of metaluminous to peraluminous medium- and fine-grained syenogranite. A systematic decrease in Eu, Ba, Sr, P and Ti concentrations; a decrease in Zr/Hf and LREE/HREE ratios; and an increase in the Rb/Ba and Ta/Nb ratios from the Guyong to Xiaolonghe plutons suggest fractional crystallization of biotite, plagioclase, K-feldspar, apatite, ilmenite and titanite. They also show the characteristics of I-type granites. The negative zircon εHf(t) isotopic values (−10.04 to −5.22) and high δ18O values (6.69 to 8.58 ‰) and the negative whole-rock εNd(t) isotopic values (−9.7 to −10.1) and high initial 87Sr/86Sr ratios (0.7098–0.7099) of the Guyong monzogranite suggest that these rocks were generated by partial melting of the Precambrian basement without mantle input. The zircon εHf(t) isotopic values (−10.63 to −3.04) and δ18O values (6.54 to 8.69 ‰) of the Xiaolonghe syenogranite are similar to the features of the Guyong monzogranite, and this similarity suggests a cogenetic nature and magma derivation from the lower crust that is composed of both metasedimentary and meta-igneous rocks. The Xiaolonghe fine-grained syenogranite shows an obvious rare earth element tetrad effect and lower Nb/Ta ratios, which indicate its productive nature with respect to ore formation. In fact, we discuss that the Sn mineralization in the region was possible due to Sn being scavenged from these rocks by exsolved hydrothermal fluids. We correlate the Late Cretaceous magmatism in the central Tengchong Block with the northward subduction of the Neo-Tethys beneath the Burma–Tengchong Block.

The genesis of Eocene granite‐related Lailishan tin deposit in western Yunnan, China: Constraints from geochronology, geochemistry, and S–Pb–H–O isotopes

“…The Lailishan monzogranites and syenogranites contain relatively high Sn contents (6.08–49.50 ppm), which are similar with those of Sn specialized granites (Sn ≥15; Neiva, 1984) and the Sn‐bearing Xiaolonghe granites in Tengchong Block (8.14–20.2 ppm; Sun et al, 2020), instead of normal granites without Sn mineralization (4.3 ppm; Tischendorf, 1977). Hence, the granitic melts produced by the partial melting of the ancient crustal basement from the Tengchong Block are enriched in Sn, and the crystallization differentiation of silicate melts made the residual melts fertile and generated volatiles through exsolution (Halter & Webster, 2004; Lehmann, 1990).…”

“…The Late Cretaceous granites (Xiaolonghe and Yunfengshan) were mainly emplaced during 76–68 Ma (Figure 1b; Jiang, Gong, Zhang, & Ma, 2012; Ma et al, 2013; Xu et al, 2012). The Xiaolonghe tin deposit is associated with the Late Cretaceous fine‐grained syenogranite in the Xiaolonghe granite unit (76 Ma;Sun et al, 2020). The Early Tertiary granites consist of the Xinqi, Lailishan, and Baihuanao granitic units, and exhibit zircon U–Pb ages ranging from 66 to 52 Ma (Booth et al, 2004; Liang et al, 2008; Chiu et al, 2009; Xie et al, 2010;Xu et al, 2012).…”

Alkaline magmatism on Neo‐Tethyan extensional domains: Evidences from the Gejiu complex in Yunnan, China

“…The fractional crystallization of K‐feldspar could reduce the Ba and Eu content in the melt without affecting other trace elements (Zhu et al, 2020); therefore, the decreasing Ba and Eu contents from ASs to FSs also confirmed the fractional crystallization of K‐feldspar. In addition, the ratios of the trace elements such as Rb/Sr, Rb/Ba and Nb/Ta are good indicators to evaluate the degree of fractional crystallization (Sun et al, 2020). As shown in Figure 13, the increase in Rb/Sr and Rb/Ba ratios and the decrease in Ba content from ASs to FSs, suggesting fractional crystallization of K‐feldspar and biotite.…”

Petrogenesis of microgranitoid enclaves and their host granites: The example of Late Cretaceous Guyong pluton in Tengchong Block, SW China

“…Besides, the magmatism in Tengchong Block shows a western younging trend and can be divided into three granite belts (Z. Sun et al, 2020): the Early Cretaceous Donghe granite belt in the eastern Tengchong Block, the Late Cretaceous Guyong granite belt in the middle Tengchong Block, and the Palaeogene Binglangjiang granite belt in the western Tengchong Block (Figure 1b).…”

“…The Tengchong Block in SW China hosts abundant Mesozoic-Cenozoic granitoids with various compositions, recording complex processes of magmatic evolution (Z. Sun et al, 2020;Xie et al, 2016).…”