Underplating along the SE Margin of the Tibetan Plateau and Its Implications for the Reworking of the Lower Crust in SW South China: Evidence from Mafic Granulite Xenoliths in Maguan, Yunnan Province

The Maguan area in Yunnan Province is situated at the southeastern edge of the Tibetan Plateau and the southwestern margin of the South China Block, representing a significant junction between these tectonic units. The structural characteristics of the deep lithosphere, particularly the lower crust, remain unclear. Deep-Seated xenoliths, which are rapidly transported to the surface by igneous rocks, reflect in-situ information about the deep lithosphere and serve as direct samples for studying lower crustal growth and evolution.This report focuses on the discovery of mafic granulite xenoliths within Cenozoic basalt in the Maguan area, located at the southwestern margin of the South China Block. We conducted a comprehensive analysis including mineralogical and geochemical characterization, zircon U-Pb geochronology, and in-situ Hf isotopic analysis.

Eequilibrium temperature and pressure estimates of the xenoliths suggest that they have a balance temperature exceeding 950 °C and a pressure of approximately 1.2 GPa, showing that these xenoliths originate from the lower crust. The SiO₂ content of the xenoliths ranges from 50.3 to 51.7 wt.%, with Mg# values of 67 to 68. The whole rock exhibits slight enrichment of light rare earth elements (LREE) and shows weak negative europium anomalies (δEu = 0.82 to 0.86). Additionally, the Sr-Nd isotopic composition is enriched and resembles EM II (⁸⁷Sr/⁸⁶Sr = 0.7081 to 0.7088, εNd = -3.8 to -4.7), while the Hf isotopic ratios of the magmatic zircon vary between -4.5 and +4.8.The concordia and near-concordia zircon U-Pb ages display several significant events, including 62 Ma, 41 Ma, 34 Ma, 26 Ma, 20 Ma, and 15 Ma, likely corresponding to collision and post-collision magmatic events related to the Tibetan Plateau and its southeastern margin, as well as the extensive left-lateral strike-slip movements in the Ailao Shan Belt. Inherited zircon concordia U-Pb ages include 874 Ma, 461 Ma, 338 Ma, 271 Ma, 131 Ma, and 102 Ma. Together with the zircon Hf isotopic data, the mafic xenoliths from Maguan record the lower crustal growth associated with the early Paleozoic orogeny in the Jiangnan Orogen, linked to the South China Sea (εHf = +1.4 to +4.5, T_DM1 = 1.0 to 0.9 Ga), late Paleozoic events (εHf = -5.7 to +6.0, T_DM2 = 1.9 to 1.0 Ga), and early to mid-Mesozoic growth and reworking related to the closure of the Paleo-Tethys Ocean (εHf = +0.4, T_DM1 = 0.8 Ga). Additionally, the late Mesozoic tectonic activities post-Indochina collision (εHf = -7.1 to +2.3, T_DM2 = 1.5 to 1.0 Ga) and Cenozoic reworking events (εHf = -4.5 to +4.8, T_DM1 = 0.9 to 0.5 Ga) are also recorded.

Considering the tectonic position of the Maguan area, this study suggests that the mafic granulite xenoliths represent ancient material from the Paleoproterozoic, reflecting the complex evolutionary history of the South China Block. Furthermore, they have been influenced by multiple magmatic and metamorphic events related to the Cenozoic collision of the Tibetan Plateau and the Indo-Asian region.

 

Keywords: Mafic granulite xenoliths; magmatic underplating; continental lower crust growth and reworking; southeastern Tibetan Plateau.