Abstract:

The formation and evolution of the Ailaoshan ocean (AO), a branch of the Paleo-Tethyan ocean, has been precisely depicted via deciphering the magmatic records in the Sanjiang area, western Yunnan. However, regional sedimentary records that responded to the subduction and closure of the AO have still not been fully understood. Therefore, an integrated study has been carried out on 255 detrital zircon grains including U-Pb dating and analyses of trace element concentrations and Lu-Hf isotopic compositions from three sedimentary samples in the Diancangshan metamorphic terrane (DMT) in the Ailaoshan tectonic belt (ATB). Together with previously study, regional sedimentary records during the last stage evolution of the AO has been delineated. The weighted mean ages of the youngest igneous zircon groups of metamorphic sandstone DC1801, schist DC1703, and carbonate DC1702 are ~180 Ma, ~247 Ma, and ~254 Ma, respectively. Together with their field contact relationships and regional sedimentary correlation, the oldest depositional ages of these sedimentary rocks are restricted to the Early Jurassic (DC1801) and Early Triassic (DC1702 and DC1703). Provenance analyses show that different igneous rocks with ages of ~0.8 Ga, ~0.45 Ga, and ~0.25 Ga in the ATB and the late Grevillian (~0.95 Ga) and Pan-African (~0.6 Ga) igneous materials in the Indian continent before dispersion of the Indochina block from Gondwana were the main inputs for the Triassic clastic sedimentary rock (DC1703). In contrast, the Indosinian igneous rocks in the Simao block were the main inputs for the Jurassic clastic sedimentary rock (DC1801), while contributions of the ~0.45 Ga, ~0.8 Ga and ~0.95 Ga materials were largely depressed. Carbonates and coeval clastic sediments in the same depositional basin show similar detrital zircon U-Pb age distribution patterns, but signals that represent for far sourced materials notably decreased in carbonates. Evolution of the Hf isotopic compositions through time verified that the closure of the AO was at ~247 Ma. The sedimentary sequences of marine carbonate and continental clastic rocks (i.e., schist) documented the last stage evolution and closure of the AO. The Eu/Eu^* values of these detrital zircon grains unravel regional crustal thickening from the closure of AO at ~247 Ma to ~235 Ma. The shift of the source areas from the Triassic to the Jurassic probably resulted from the uplift of the depositional areas due to the closure of the AO.

Key words: Tibetan Plateau, Ailaoshan tectonic belt, detrital zircons, Paleo-Tethyan ocean, geochemistry