Late Neoproterozoic thermal events in the northern Lhasa terrane, south Tibet: Zircon chronology and tectonic implications

Abstract

The high-grade metamorphic rocks from the Lhasa terrane, south Tibet, have been traditionally grouped as the Precambrian metamorphic basement. However, a number of recent studies show that these metamorphic rocks from the southern part of Lhasa terrane, including granulite- and eclogite-facies rocks, were metamorphosed during the Mesozoic and Cenozoic. Based on the LA–ICP–MS in situ zircon U–Pb chronology, we report for the first time late Neoproterozoic metamorphic and magmatic events from the Lhasa terrane. The rocks analyzed in this study occur in the northern part of Lhasa terrane, and include amphibolite, garnet-bearing muscovite schist and marble, with the mineral assemblages of hornblende + andesine + quartz, muscovite + garnet + plagioclase + quartz and calcite + diopside + quartz, indicating an amphibolite-facies metamorphic event. Zircons from a marble and an amphibolite sample show typical features of metamorphic origin, and yield peak-metamorphic ages of ca. 676 Ma and ca. 661 Ma, respectively. Zircons from another amphibolite sample possess inherited magmatic cores and metamorphic overgrown rims, yielding a protolith age of ca. 856 Ma and a metamorphic age of ca. 683 Ma. Zircons from the schist include detrital magmatic cores showing a wide spectrum of Proterozoic ages from ca. 1747 to 789 Ma, representing derivation from multiple sources. However, the metamorphic rims of these zircons yield an age population of ca. 690 Ma. Based on the geochemical and geochronological data presented in this work and from a synthesis of data from previous studies, we propose the location of the Lhasa terrain in a reconstruction of the Neoproterozoic Rodinia supercontinent. We correlate the late Neoproterozoic metamorphic and magmatic events with an Andean-type orogeny that resulted from oceanic subduction beneath the northwestern margin of the Rodinia supercontinent. We correlate the orogeny with a global-scale late Proterozoic arc magmatism widely represented in the Seychelles, Madagascar, northwestern India and South China.