Multiple partial melting events in the Ailao Shan–Red River and Gaoligong Shan complex belts, SE Tibetan Plateau: Zircon U–Pb dating of granitic leucosomes within migmatites

Abstract

Thin layers and lenses of granitic leucosomes are widely distributed within migmatites from the Ailao Shan–Red River (ASRR) and Gaoligong Shan (GS) complex belts, SE Tibetan Plateau. They are parallel to, or cross-cut, foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre- to decimetre-scale segregations. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SIMS dates, and in-situ trace element compositions of zircons provides clear evidence on the nature and timing of partial melting in these rocks. Most zircons from the granitic leucosomes occur as distinct overgrowths around inherited (detrital) cores or as new, euhedral crystals. The inherited (detrital) cores are rounded or irregular, contain felsic mineral inclusions of Qtz + Kfs + Ab ± Ap with abundant opaque impurities, and show clear dissolution textures. In contrast, the overgrowths and new crystals commonly show perfectly euhedral shapes, have pronounced oscillatory zoning, contain felsic mineral inclusions, such as Qtz + Kfs + Ab + Ap, and show pronounced high U (1419–6129 ppm) and relatively low Th (58–301 ppm) contents with extremely low Th/U ratios (0.02– 0.07). The chondrite-normalized REE patterns show steep rising slopes from the HREE to LREE with evidently negative Eu anomalies (Eu/Eu* = 0.10–0.31) and pronounced positive Ce anomalies (Ce/Ce* = 35– 145). These data suggest that the new zircons are anatectic in origin and that they grew during partial melting of the metamorphic rocks. Abundant U–Pb spot analyses of the magmatic zircons from granitic leucosomes reveal three discrete and meaningful ages of partial melting within the ASRR and GS complex belts. Anatectic zircons from four granitic leucosomes in weakly deformed migmatites from the GS complex belt yield consistent mean U–Pb ages of 40.6 ± 0.3–38.4 ± 0.2 Ma. In contrast, anatectic zircons from six granitic leucosomes in weakly deformed migmatites from the ASRR complex belt record younger mean U–Pb ages of 33.7 ± 0.3– 30.9 ± 0.3 Ma. However, anatectic zircons from another six granitic leucosomes in mylonitic migmatites from both belts yield the youngest mean U–Pb ages of 27.8 ± 0.2–26.3 ± 0.3 Ma. These data imply that both the ASRR and GS complex belts experienced multiple partial melting events. The differences in the oldest U–Pb ages of zircons from granitic leucosomes indicate that the partial melting event related to the peak amphibolite–granulite facies metamorphism within the GS complex belt occurred 7–10 Ma earlier than that in the ASRR complex belt. The youngest U–Pb ages of zircons from granitic leucosomes within mylonitic migmatites in both the ASRR and GS belts are genetically related to post-peak, isothermal, decompressional retrogression, accompanied by regional high-temperature ductile shearing. The 40Ar/39Ar age data (27.8–22.2 Ma) of biotites and amphiboles from migmatites strongly support this hypothesis.