Origin of Cenozoic Syenitic Porphyries and Enclaves in Liuhe, Yunnan Province, SW China, and its Tectonic and Metallogenic Significance

Since the Cenozoic, the India-Eurasia continental collision has caused large-scale uplift and lithospheric extrusion of the Tibetan Plateau. As the response to this collision, two Cenozoic alkaline magmatic belts developed along the Jinshajiang-Ailanshan and Himalayan tectonic belts which have been considered to be two boundaries for adjusting the compressional stress in the interior of the plateau. The genesis and rock-forming mechanism of these alkali-rich magmatic rocks are the key to explore the deep geodynamics during the India-Eurasia collision. In this paper, we present zircon U-Pb geochronology and Lu-Hf isotopic compositions, whole-rock major and trace elemental, and Sr-Nd isotopic results of the Liuhe syenite porphyries and their enclaves. Our dating results together with previous data indicate that the syenite porphyries formed at ca. 37 Ma, which is slightly earlier than the peak alkali-rich magmatism (36 Ma) along the Jinshajiang-Ailaoshan belt. The Liuhe syenite porphyries are geochemically rich in alkali and potassium, and low in magnesium, coupled with the enrichment of Light Rare Earth Elements (LREE) and Large Ion Lithophile Elements (Rb, Ba, Sr), and depletion of Heavy Rare Earth Elements (HREE) and High Field Strength Elements (Ta, Nb, Ti). Their high Sr contents and Sr/Y ratios along with weak negative Eu anomalies (δEu=0.84–0.85) show an affinity to adakitic rocks. Their overall geochemical signatures and the similarities in Hf-Nd isotopic compositions to those of the hosted garnet-bearing amphibolite enclaves indicate that the Liuhe syenite porphyries were derived from partial melting of the thickened lower crust rather than the enriched lithospheric mantle. Moreover, the Liuhe syenite porphyries featured by low oxygen fugacity and high water content have not the potential to form porphyry deposits. As for the low-grade metamorphic enclaves hosted in the Liuhe syenite porphyries, they have significantly different zircon U-Pb age, major-trace element and Sr-Nd isotopic characteristics from the host rocks, indicating that they were captured during the ascent of the host porphyric magmas and their involvement in the genesis of the host porphyries were negligible if any. Collectively, our results combined with previous data reflect that the distribution of the early Eocene alkaline magmatic rocks along the Jinshajiang-Ailaoshan tectonic belt does not show a younging trend from north to south as proposed before, and that this stage of magmatism was related to the lithospheric extrusion of the Tibetan Plateau in response to the India-Eurasian continental collision rather than the Eocene lithospheric delamination. © 2023 Science Press. All rights reserved.