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The Cenozoic collision of India and Eurasia clearly built the high-elevation Tibetan Plateau, but how this collision was accommodated, and the Tibetan Plateau uplifted, remains an area of study. The widespread occurrence of Cenozoic potassic–ultrapotassic lavas provides a valuable opportunity to constrain the relationship between surface deformation and underlying geodynamic processes. In this study, we report sanidine Ar–Ar dating, whole-rock geochemical and clinopyroxene Sr–Nd isotope analyses on pseudoleucite phonolites from the Yulinshan area of south Qiangtang (central Tibet) to determine their petrogenesis and regional tectonic significance. Sanidine Ar–Ar dating yields an Early Oligocene age of ca. 30 Ma, in agreement with previous studies. A high modal abundance of pseudoleucite in the samples indicates their ultrapotassic and silica-undersaturated composition. All of the rocks have typical arc-like geochemical signatures and enriched whole rock and clinopyroxene Sr–Nd isotope signatures. Based on thermodynamic phase equilibria modeling and our detailed study of the textural and in-situ compositional information of clinopyroxene, the intermediate to felsic potassic magmas are interpreted to have been formed through differentiation of primitive, mantle-derived, potassic magmas at shallow crustal levels. The inferred enrichment of the lithospheric mantle is probably related to subduction of the Songpan–Ganze continental lithosphere beneath Qiangtang after India-Asia collision. Voluminous Late Eocene–Early Oligocene magmatism in central Tibet may reflect partial removal of the mantle lithosphere, perhaps triggered by instabilities associated with previous lithospheric thickening. Our work provides additional constraints on the timing of mantle root loss that likely contributed to the surface uplift of central Tibet, which post-dated the cessation of upper crustal shortening and deformation. [ABSTRACT FROM AUTHOR]

High-MgO (>9 wt %) basaltic rocks can be primary magmas and are used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus, the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (>9 wt %) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at c. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt %), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (>3 GPa) in the presence of H2O and CO2. These geochemical features are different from those of the c. 30-Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that a heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (>9 wt %), evidence from mineral chemistry indicates that the whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and the subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of the mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations. [ABSTRACT FROM AUTHOR]

Late Early Oligocene mantle-derived potassium (K)-rich lavas occur widely in the Qiangtang Block, central Tibet. However, it remains ambiguous whether partial melting of crust beneath central Tibet occurred in the Oligocene. Here, we report the newly identified trachytes in the Gemuchaka area of central Qiangtang Block. Zircon U-Pb and biotite 40Ar-39Ar analyses suggest that they were erupted at ca. 30 Ma, coeval with other K-rich lavas in the central Qiangtang Block. They have relatively constant and high SiO2 (62.4–62.9 wt.%), Al2O3 (17.2–17.4 wt.%), K2O (6.0–6.1 wt. %), and Na2O (4.6–4.7 wt.%) contents. They are characterized by enrichments of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and by depletions of heavy REEs (HREEs) and high field strength elements (HFSEs), with negligible Eu but clear negative Ta-Nb-Ti anomalies. They have low εNd(t) (−5.5 to −4.6) but high (87Sr/86Sr)i (0.7076 to 0.7077) values, with variable zircon εHf(t) (−5.78 to 0.92) and δ18O (6.51 to 8.54) values. Except for obviously higher SiO2 contents, the trachytes are geochemically similar to the K-rich rocks in Gemuchaka area. We suggest that the trachytes were probably generated by remelting of the K-rich lower crust, which was possibly formed by the underplating of enriched mantle-derived K-rich lavas during the delamination of the lower part of thickened lithospheric mantle. After the delamination, the upwelling asthenosphere provided heat for partial melting of the lithospheric mantle and newly-formed lower crust. Our study provides the first petrological evidence for the existence of late Early Oligocene crust-derived rocks in the Qiangtang Block. [ABSTRACT FROM AUTHOR]

5-hydroxytryptamine (5-HT) participates in plant growth and development, and can also delay senescence and cope with abiotic stress. To explore the role of 5-HT in regulating the abilities of mangrove in cold resis-tance, we examined the effects of cold acclimation and the spraying of p-chlorophenylalanine (p-CPA, 5-HT synthesis inhibitor) on leaf gas exchange parameters and CO 2 response curves (A/Ca), as well as the endogenous phytohormone content levels in the mangrove species Kandelia obovata seedlings under low temperature stress. The results showed that low temperature stress significantly reduced the contents of 5-HT, chlorophyll, endogenous auxin (IAA), gibberellin (GA), and abscisic acid (ABA). It weakened the CO 2 utilization abilities of plants and reduced net photosynthetic rate, which ultimately reduced carboxylation efficiency (CE). Under low temperature stress, exogenous p-CPA reduced the contents of photosynthetic pigments, endogenous hormones, and 5-HT in the leaves, which aggravated the damages caused by low temperature stress on photosynthesis. By enhancing cold acclimation abilities, the endogenous IAA content in the leaves could was reduced under low temperature stress, promoted the production of 5-HT, improved the contents of photosynthetic pigments, GA, and ABA, as well as enhanced photosynthetic carbon assimilation abilities, which would increase photosynthesis in the K. obovata seedlings. Under cold acclimation conditions, the spraying of p-CPA could significantly inhibit the synthesis of 5-HT, promote the production of IAA, and reduce the contents of photosynthetic pigments, GA, ABA, and CE, which would weaken the effects of cold acclimation by improving the cold resistance of mangroves. In conclusion, cold acclimation could improve the cold resistance abilities of K. obovata seedlings by regulating photosynthetic carbon assimilation capacity and the contents of endogenous phytohormone. 5-HT synthesis is one of the necessary conditions for improving the cold resistance abilities of mangroves.