Reconstructing the evolutionary history of the Paleo‐Tethys Ocean remains at the center of debates over the linkage between Gondwana dispersion and Asian accretion. Identifying the remnants of oceanic lithosphere (ophiolites) has very important implications for identifying suture zones, unveiling the evolutionary history of fossil oceans, and reconstructing the amalgamation history between different blocks. Here we report newly documented ophiolite suites from the Longmu Co‐Shuanghu Suture zone (LSSZ) in the Xiangtaohu area, central Qiangtang block, Tibet. Detailed geological investigations and zircon U‐Pb dating reveal that the Xiangtaohu ophiolites are composed of a suite of Permian (281–275 Ma) ophiolites with a nearly complete Penrose sequence and a suite of Early Carboniferous (circa 350 Ma) ophiolite remnants containing only part of the lower oceanic crust. Geochemical and Sr‐Nd‐O isotopic data show that the Permian and Carboniferous ophiolites in this study were derived from an N‐mid‐ocean ridge basalts‐like mantle source with varied suprasubduction‐zone (SSZ) signatures and were characterized by crystallization sequences from wet magmas, suggesting typical SSZ‐affinity ophiolites. Permian and Carboniferous SSZ ophiolites in the central Qiangtang provide robust evidence for the existence and evolution of an ancient ocean basin. Combining with previous studies on high‐pressure metamorphic rocks and pelagic radiolarian cherts, and with tectonostratigraphic and paleontological data, we support the LSSZ as representing the main suture of the Paleo‐Tethys Ocean which probably existed and evolved from Devonian to Triassic. The opening and demise of the Paleo‐Tethys Ocean dominated the formation of the major framework for the East and/or Southeast Asia.
Abstract:Two new monoterpenoids, 8,10-dihydroxy-9(2)-methylbutyryloxythymol (1) and 10-hydroxy-8,9-dioxyisopropylidene-thymol (2), together with five known thymol derivatives: 8,9,10-trihydroxythymol (3), thymol-β-glucopyranoside (4), 9-hydroxythymol (5), 8,10-dihydroxy-9-isobutyryloxythymol (6), and 8-hydroxy-9,10-diisobutyryloxythymol (7), were isolated from Centipeda minima. Their structures were identified by means of spectroscopic analyses. Interestingly, compound 2 is not an extraction artifact according to a close HPLC examination of material after extraction by analytical MeOH at ambient temperature. The antibacterial activities of compounds 1-7 were evaluated against eight microbial strains by the agar dilution method.
The time and mechanism of crustal thickening and initial surface uplift of the Tibetan Plateau remain highly controversial. Here we report on zircon U‐Pb age, and mineral, element, and Sr‐Nd isotope composition data for Eocene adakitic porphyries in the Laorite Co area of central‐northern Qiangtang Block, central Tibet. Laser ablation‐inductively coupled plasma‐mass spectrometry zircon U‐Pb age dating shows that the porphyries were generated at 42–41 Ma. All samples display adakitic geochemical characteristics, such as high SiO2 and Al2O3, and low Y and Yb contents, and high Sr/Y and La/Yb ratios with positive Sr and negligible Eu anomalies and Nb‐Ta depletion. They also have low magnesium number (Mg #) (16–45) and high K2O (4.0–4.6 wt %) values, as well as high (87Sr/86Sr)i (0.7076–0.7080) and low εNd(t) (−5.9 to −4.4) values. These adakitic rocks were most probably generated by partial melting of thickened lower crust in the stability field of garnet, amphibole, and rutile but with relatively minor plagioclase. Moreover, such a crustal thickening process beneath this block likely occurred no later than the middle Eocene and was most probably caused by the southward subduction of the Songpan‐Ganzi Block in the early Cenozoic. In addition, the occurrences of exposed adakitic porphyries unambiguously indicate that exhumation process occurred after their formation. Thus, these ~42 Ma adakitic porphyries indicate the occurrence of significant exhumation since 42 Ma. Integrated with the thermochronological and paleoelevation data, we suggest that the initial surface uplift of the Qiangtang Block, triggered by crustal thickening, was underway by the middle Eocene (~42 Ma).
One new monoterpene glucoside minimaoside A (1) and one new sesquiterpene glucoside minimaoside B (2), together with four known terpenoids, were isolated from the whole plants of Centipeda minima (L.) A. Braun et Ashers. Their structures were determined by spectroscopic methods. Compounds 5 and 6 showed weak or moderate cytotoxic activity toward several tumor cell lines.
Investigation of the N-BuOH extract of the roots of Rumex nepalensis afforded two new seco-anthraquinone glucosides, nepalensides A and B, along with nine known compounds, torachrysone ( 3), rumexoside, orientaloside, orcinol glucoside, aloesin, lyoniresinol 3 alpha- O- beta- D-glucopyranoside, (-)-epicatechin-3- O-gallate, (3,5-dimethoxy-4-hydroxyphenol)-1- O- beta- D-(6- O-galloyl) glucose, and (-)-epicatechin. Their structures were elucidated on the basis of spectroscopic methods. The possible formation of nepalensides A and B is briefly discussed.
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