Abstract:The role of continent collisional belts in the global carbon budget remains controversial. Collisional orogens have traditionally been considered a net carbon sink, but recent studies have highlighted significant CO2 fluxes. This study carried out comprehensive field mapping, petrography, pressure-temperature determination, geochemistry, and geochronological data along three transects of the South Tibetan Detachment System (STDS) in the Mount Everest and Nyalam regions of the central Himalaya. The results outline a previously unrecognized carbon source in the Himalaya: Buchan-type metamorphic decarbonation of carbonate-bearing lithologies in the migrating hanging-wall of the STDS driven by the juxtaposition against hot migmatite/magma in the footwall of the structure. Specifically, calc-silicates and schists incorporated into the active STDS underwent Buchan-type metamorphic overprinting at P-T conditions of 630–400°C and 5–3 kbar (36–48°C/km) compared to Barrovian-type metamorphism (28°C/km) in the footwall. Monazite and titanite U(-Th)-Pb petrochronology indicate that metamorphism within the STDS occurred between ca. 23 and 19 Ma, which is contemporaneous with deformation along the STDS evidenced by the ages of mylonitized leucogranites. Activity along the STDS sustained to 17–16 Ma, causing resetting of titanite U-Pb ages in some calc-silicates. Detrital zircon geochronology shows that the Yellow Band and North Col Formation in the STDS have an affinity to the Tethyan Himalayan Sequence and were involved in the shear zone during its upward expansion into hanging-wall rocks. Based on decarbonation reactions, protolith restoration, and decarbonation efficiency studies, the metamorphic CO2 degassing from the metamorphism of calc-silicate rocks is quantified to be ~0.8 Mt C/yr during 23–19 Ma. The quantification of upward expansion of the STDS, the resulting juxtaposition of underlying hot migmatites/magma with cold hanging wall-rocks, and the proposed metamorphic decarbonation phenomenon are crucial to understanding the development process of orogen-scale low-angle normal-sense faulting and the resulting carbon sources during Himalayan orogenesis.
Although the Himalayan orogenic belt is dominant by Oligocene to Miocene leucogranites, it initiated magmatic activity from middle Eocene. However, the exact distribution and genesis of early magmatism is yet to be resolved. This study identified new outcrops of middle Eocene magmatism, Haweng granodiorite porphyries, from northwest Langkazi in the northern Tethyan Himalaya, southern Tibet. Identical zircon U–Pb ages were obtained by secondary ion mass spectrometry (SIMS) (17JT13: 45.3±0.4 Ma; 17JT16: 44.5±0.8 Ma) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (17JT15: 44.3±0.8 Ma; 12FW75: 44.4±0.6 Ma) method for different outcrops there. Titanite LA-ICP-MS U-Pb analyses also gave consistent lower intercept ages for samples 17JT13 (45.3 ± 0.5 Ma) and 17JT15 (44.5 ± 0.6 Ma). Zircon metamorphic rims of sample 17JT15 recorded a younger thermal event of 29.9±0.4 Ma. The analyzed samples possess high SiO2 (69.98–73.53 wt.%), Al2O3 (15.07–16.15 wt.%), variable Na2O (3.94–5.81 wt.%) with Na2O/K2O ratios of 1.57–7.88, and A/CNK values of 1.08–1.27 indicative of sodic peraluminous series. They show variable Sr (342–481 ppm), Rb (37.9–133 ppm) concentrations, and low Rb/Sr ratios (0.10–0.39), radiogenic Sr isotopes (87Sr/86Sr(t): 0.7190 to 0.7251) and unradiogenic Nd–Hf isotopes with εNd(t) values of -13.54 to -11.55 and εHf(t) values of -11.97 to -9.37, respectively. The variation of major and trace elements, such increase of Na2O and Sr, and decreases of K2O and Rb, resulted from cumulation of plagioclase and crystal fractionation of K-feldspar during magma evolution. The Haweng granodiorite porphyries were derived from partial melting of dominant amphibolites and variable metasedimentary rocks. The newly identified outcrops help conform the EW trending middle Eocene magmatic belt along the Yarlung Tsangpo suture zone, resulting from breakoff of Neo-Tethyan slab at ca. 45 Ma.
Abstract The continental collision between India and Asia has been ongoing since early Eocene time, but the orogenic record is typically dominated by Miocene and younger deformation and metamorphism that largely overprinted earlier Eocene‐Oligocene events. This hinders our understanding of how crustal thickening responds to initial collision and when the Himalayan mountains initially rise. The advancement of spatially precise petrochronology techniques, however, has provided the means to see through the Miocene overprint and enabled the characterization of Eocene metamorphism in different parts of the Himalaya. The current study presents new monazite petrochronology and paired thermobarometry from the Kathmandu klippe in the central Nepalese Himalaya. These data reveal Eocene prograde metamorphism (44‐38 Ma) and partial melting (38‐35 Ma) under peak P‐T conditions of 730 °C–760 °C and up to 10.5 kbar. The migmatites within the Kathmandu klippe is equivalent to the Upper or Uppermost Greater Himalayan Crystallines and should have been exhumed during Eocene‐Oligocene. The new evidence of Eocene metamorphism and anatexis presented herein adds to a growing body of data detailing initial crustal thickening during the early continent collision. The mid‐Eocene crustal thickening event indicates that the Himalayan felsic crust was thickened to a depth of ∼35 km shortly within 10–20 Myr of the initial collision, which was probably responsible for the initial topographic rise of the Himalayan proto‐mountains. Characterizing the effects of this early orogenesis is critical in understanding the Himalayan architecture prior to the better‐preserved Miocene metamorphism and anatexis record and how the orogen may have been preconditioned for the younger stage.
During the Late Cretaceous, a 2700 km long magmatic arc extended from the Lesser Caucasus through the Pontides into Srednogorie, Timok, Banat, and Apuseni (ABTS) in the Balkans. We studied the arc volcanic rocks in three regions of the Western Pontides, and compared them to the other arc magmatic rocks from the Lesser Caucasus, Eastern Pontides and Balkans. Prior to the onset of the arc magmatism, the region underwent uplift and erosion. New and published geochronologic and biostratigraphic data indicate that magmatism in the Lesser Caucasus, Pontides and Balkans started during the Turonian (ca. 93 Ma), peaked in the middle Campanian (80–78 Ma), and subsequently became rare and sporadic after the late Campanian (ca. 75 Ma). The arc magmatism, characterized by typical subduction signatures, was mainly of middle to high-K calc-alkaline affinity. Late Cretaceous volcanism occurred in a submarine and extensional environment. Along the whole belt, the arc volcanic rocks are overlain by Maastrichtian to Paleocene marine limestones and sandstones, marking the end of the main phase of arc magmatism. However, in the Western Pontides, Maastrichtian limestone sequence includes a volcanic horizon with a U-Pb zircon age of ca. 71 Ma. The geochemistry of the Maastrichtian volcanic rocks is more diverse compared to the older arc volcanic rocks, including alkaline and calc-alkaline basalts, as well as adakitic dacites. The coeval initiation of arc magmatism along the 2700-km-long magmatic arc is associated with the acceleration of Africa-Eurasia convergence at ca. 96 Ma, which is also independently indicated by the beginning of intra-oceanic subduction, inferred from the ages of suprasubduction-zone ophiolites and sub-ophiolite metamorphic rocks in Anatolia. The end of the magmatic activity in the arc is associated with a marked decrease in the convergence rate during the Campanian.     
A double sodium layer (DSL) structure was observed during the night of June 13, 2000 over Wuhan, China (31°N, 114°E) by our Na lidar. The unique feature of this DSL is that a normal sodium layer at altitudes of 80 ∼ 105 km was accompanied by a secondary sodium layer at altitudes of 105 ∼ 125 km for about 2 hours. The lidar observation result together with that obtained from the nearby ionosonde and geomagnetic equipments are presented. While the exact mechanism responsible for this DSL formation is still unclear, some possible explanations and corresponding observation evidences are discussed.
目前研究已经显示，喜马拉雅淡色花岗岩具有良好的铍-铌钽-锂等稀有金属成矿潜力。其中珠穆朗玛峰（后文简称珠峰）西侧的普士拉一带，是喜马拉雅地区锂辉石伟晶岩集中的区域。本文报道在普士拉东北的珠峰北侧热曲地区，发现有含锂辉石伟晶岩脉，这些伟晶岩呈透镜体状集中赋存于肉切村群黄带层大理岩与北坳组钙质硅酸岩的接触界线部位，同围岩一起经历了强烈的变形，且未出现明显内部分带结构，矿物组成中包含锂辉石、透锂长石、绿柱石、铌钽铁矿、锡石等锂-铍-铌钽-锡稀有金属矿物，其Li2O含量达1.30%~2.15%，显示经历过高程度分异演化的岩浆结晶特征。热曲含锂辉石伟晶岩的发现表明珠峰地区具有锂成矿的良好前景，是未来锂矿产勘查的重点靶区，而藏南拆离系韧性剪切带中的肉切村群黄带层下部与北坳组顶部位置，是锂辉石伟晶岩的重要富集层位，值得今后在锂资源寻找过程中予以充分关注。;Present studies have revealed the leucogranites from Himalayan orogen are vital potential for Be-Nb-Ta-Li rare-metal mineralization, and spodumene-bearing pegmatites dominantly concentrated in the Pusi La of the Mount Qomolangma region. In this study, the first spodumene-bearing pegmatite is discovered in the Ra Chu transect, north of the Pusi La. The foliated pegmatites present as lenses concordant with the prominent mylonitic fabric and mainly concentrated in the contact boundary between the calc-silicate of the North Col Formation and the marble in the Yellow Band of the Rouqiecun Group. Internal zonation is not observed from the pegmatites. The mineral assemblages of the pegmatite contain Li-Be-Nb(Ta)-Sn minerals including spodumene, petalite, beryl, columbite and cassiterite. Li2O concentrations of the pegmatite samples from the transect are 1.30%~2.15%. Mineralogy and geochemistry results suggest the pegmatites have experienced a high degree of crystal fractionation, which indicates that there is a sufficient prospect for lithium mineralization in the Mount Qomolangma region. The bottom of the Yellow Band and top of the North Col Formation corresponding to the high-strain shear zone of the South Tibetan Detachment System is probably the enrichment layer of spodumene-bearing pegmatites and it is an important target zone for lithium mineral exploration in the future.
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