Jian-Lin Chen

Sedimentary record of Jurassic northward subduction of the Bangong–Nujiang Ocean: insights from detrital zircons

ABSTRACT The subduction polarity and related arc–magmatic evolutional history of the Bangong–Nujiang Ocean, which separated the South Qiangtang terrane to the north from the North Lhasa terrane to the south during the Mesozoic, remain debated. This study tries to reconstruct the subduction and evolution of the Bangong–Nujiang Ocean on the basis of U–Pb and Hf isotopic analyses of detrital zircons in samples from sedimentary rocks of the middle-western section of the Bangong–Nujiang suture zone in Gerze County, central Tibet. The Middle Jurassic Muggargangri Group in the Bangong–Nujiang suture zone was deposited in a deep-sea basin setting on an active continental margin. The Late Jurassic strata, such as the Sewa Formation, are widely distributed in the South Qiangtang terrane and represent deposition on a shelf. The Early Cretaceous Shamuluo Formation in the Bangong–Nujiang suture zone unconformably overlies the Muggargangri Group and was probably deposited in a residual marine basin setting. The detrital zircons of the Muggargangri Group contain seven U–Pb age populations: 2.6–2.4 Ga, 1.95–1.75 Ga, 950–900 Ma, 850–800 Ma, 650–550 Ma, 480–420 Ma, and 350–250 Ma, which is similar to the age populations in sedimentary rocks of the South Qiangtang terrane. In addition, the age spectra of the Shamuluo Formation are similar to those of the Muggargangri Group, indicating that both had a northern terrane provenance, which is conformed by the north-to-south palaeocurrent. This provenance indicates northward subduction of the Bangong–Nujiang oceanic crust. In contrast, two samples from the Sewa Formation yield variable age distributions: the lower sample has age populations similar to those of the South Qiangtang terrane, whereas the upper possesses only one age cluster with a peak at ca. 156 Ma. Moreover, the majority of the late Mesozoic detrital zircons are characterized by weakly positive εHf(t) values that are similar to those of magmatic zircons from arc magmatic rocks in the South Qiangtang terrane. The findings, together with information from the record of magmatism, indicate that the earliest prevalent arc magmatism occurred during the Early Jurassic (ca. 185 Ma) and that the principal arc–magmatic stage occurred during the Middle–Late Jurassic (ca. 170–150 Ma). The magmatic gap and scarcity of detrital zircons at ca. 140–130 Ma likely indicate collision between the Qiangtang and Lhasa terranes. The late Early Cretaceous (ca. 125–100 Ma) magmatism on both sides of the Bangong–Nujiang suture zone was probably related to slab break-off or lithospheric delamination after closure of the Bangong–Nujiang Ocean.

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.

Re–Os isotope evidence from Mesozoic and Cenozoic basalts for secular evolution of the mantle beneath the North China Craton

The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re–Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re–Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial 187Os/188Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γOs(t) values (+20 to +202). In contrast, the Re–Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial 187Os/188Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γOs(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re–Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts.