Bao-Di Wang

Late Paleozoic Woniusi basaltic province from Sibumasu terrane: Implications for the breakup of eastern Gondwana's northern margin

The tectonic mechanism responsible for detachment of the Sibumasu terrane in the eastern Cimmerian continent from the Australian Gondwana margin remains poorly understood and debated. Our study on the late Paleozoic Woniusi basaltic province in the northern Sibumasu terrane may provide a new perspective on this problem, and further, on the rifting dynamics of eastern Gondwanas northern margin. In this study, we revealed that the Woniusi basaltic province is spread over an area of similar to 12,000 km(2) with a thickness of similar to 300-500 m for the eruptive phase. Direct zircon U-Pb dating of mafic dikes indicates that the Woniusi basaltic province was emplaced between the latest Carboniferous and late Early Permian (301 Ma to 282 Ma), synchronous with basaltic rocks from the Panjal Traps, Tethyan Himalaya (Bhote Kosi, Selong, Abor), Lhasa, and southern Qiangtang. Geochemically, the Woniusi basaltic rocks show subalkaline tholeiitic affinity and relatively homogeneous features of enriched light rare earth elements (LREEs) and large ion lithophile elements (LILEs) but depleted Nb and Ta patterns. They were most likely derived from an enriched subcontinental lithospheric mantle source and triggered by the mantle plume-induced rifting process. The Woniusi basaltic province, together with other coeval basaltic provinces from the Panjal Traps, Tethyan Hima-laya, Lhasa, and southern Qiangtang, belongs to a fragmented large igneous province that may have had an original area of over 2 x 10(6) km(2), which is comparable to that of the Ontong-Java, Deccan, or Siberian Traps. These Tethyan basaltic provinces share a common mantle plume centered in northern Greater India. It was this mantle plume that may have triggered the initial rifting of the eastern Cimmerian terranes (e.g., Sibumasu, southern Qiangtang) from both the northern Indian and Australian Gondwana margins, as well as the opening of Meso-Tethys Ocean. This study, moreover, sheds light on the paleopositions of the Lhasa and Tengchong terranes before their breakups.

Geochemistry of Eocene high-Mg# adakitic rocks in the northern Qiangtang terrane, central Tibet: Implications for early uplift of the plateau

It is generally believed that the Tibetan Plateau is the result of crustal thickening in response to the collision of the Asian and Indian plates. However, the specific timing and uplift mechanism remain controversial. The widespread occurrence of Cenozoic lavas in the northern Qiangtang terrane provides a unique opportunity to constrain the dynamic processes that resulted in uplift of the northern Tibetan Plateau. Eocene lavas from the northern Qiangtang terrane display adakitic geochemical characteristics, such as high SiO 2 and Al 2 O 3 contents, low Y and Yb contents, positive Sr anomalies, and high Sr/Y and La/Yb ratios, in combination with high Mg# (43–69) and negative anomalies for Nb and Ta, which suggest a garnet + rutile-in and plagioclase-free source residue. The same samples also have high K 2 O and Th contents, high Th/Ce ratios, and low Nb/U, Ce/Pb, Ti/Eu, and Nd/Sm ratios, as well as high 87 Sr/ 86 Sr (i) (0.7062–0.7075) and low e Nd(t) (−6.3 to −2.9), which show a clear continental crust affinity. These high-Mg# adakitic rocks, combined with other characteristics of Tibetan Cenozoic lavas, indicate that they were derived from partial melting of delaminated lower continental crust, which subsequently reacted with surrounding mantle peridotites during ascent to crustal depths. The Eocene high-Mg# adakitic rocks (46–38 Ma), north-south–trending shoshonitic dikes (47–38 Ma), and contemporaneous mantle-derived Mg-rich potassic and shoshonitic lavas indicate that the thickness of the crust was at least 50 km before ca. 46 Ma, at which time rapid uplift and extension occurred, most likely caused by small-scale delamination of the lithospheric mantle at 46–38 Ma (Eocene) in central Tibet.