Magmatism In Tibet Research Articles

India-Asia collision as a driver of atmospheric CO2 in the Cenozoic

Deep Earth degassing is a critical forcing factor for atmospheric CO2 variations and palaeoclimate changes in Earth’s history. For the Cenozoic, the key driving mechanism of atmospheric CO2 variations remains controversial. Here we analyse three stages of collision-related magmatism in Tibet, which correspond temporally with the three major stages of atmospheric CO2 variations in the Cenozoic and explore the possibility of a causal link between these phenomena. To this end we present geochemical data for the three stages of magmatic rocks in Tibet, which we use to inform a model calculating the continental collision-induced CO2 emission flux associated with the evolving Neo-Tethyan to continental subduction over the Cenozoic. The correlation between our modelled CO2 emission rates and the global atmospheric CO2 curve is consistent with the hypothesis that the India-Asia collision was the primary driver of changes in atmospheric CO2 over the Cenozoic.

Late Oligocene–early Miocene transformation of postcollisional magmatism in Tibet

Abstract Uplift of the Tibetan Plateau is thought to be one of the most important orogenic and climate forcing events of the Cenozoic Era, associated with geodynamic changes related to India-Asia collision and subsequent continental lithosphere subduction. However, the fate and scale of the subducted continental lithosphere segments remain highly controversial. Using a comprehensive compilation of the spatiotemporal distribution of postcollisional magmatic rocks across Tibet, together with new geochemical and Sr-Nd-Pb isotopic data and modeling simulations, we propose a holistic, two-stage evolutionary model to explain the link between genesis of the magmas and continental subduction. The magmatism prior to 25 Ma resulted from continuous upwelling of a carbonate-rich upper-mantle plume induced by northward underthrusting of Indian oceanic and continental lithosphere with its cover of Tethyan platform carbonate sediments, whereas magmatism after 25 Ma was related to opposing north-directed and south-directed continental subduction. Our model indicates a transformation in the distribution and nature of the magmatism in Tibet at ca. 25 Ma, which reflects a significant change in the Himalayan-Tibetan orogen and associated mantle dynamic processes in the early Miocene. Understanding this transformation could have important implications for the utility of the Himalayan-Tibetan system as a modern analogue for ancient orogens.

Nd isotopes of siliciclastic rocks from Tibet, western China: Constraints on provenance and pre-Cenozoic tectonic evolution

Nd isotope data from pre-Cenozoic siliciclastic rocks from Tibet are used to determine variations of source regions with time and hence tectonic history. On average, the T DM and T CHUR ages of the Tibetan samples are up to 1.83 ± 0.36 Ga and 1.25 ± 0.45 Ga, respectively, indicating that Tibet is underlain by Lower Proterozoic basement and has close affinity with the Himalayas. An Nd isotope crisis occurred at the Early Permian, with older samples defining a trend of slight decrease in ε Nd values with time while the Lower Permian rocks sharply increasing ∼ 6.5 ε Nd units. This is related to the coeval widespread basaltic magmatism in Tibet, which could be responsible for Tibet rifting away from northern Gondwanaland. The central Qiangtang meta-sedimentary samples have distinct Nd isotope characteristics ( ε Nd(0) − 19.87 ± 1.51; T DM 2.07 ± 0.13 Ga; T CHUR 1.78 ± 0.21 Ga) from the Songpan–Ganzi samples ( ε Nd(0) − 12.77 ± 2.87; T DM 1.49 ± 0.18 Ga; T CHUR 1.08 ± 0.23 Ga), suggestive that these two groups were derived from different sources and that the former were not the equivalents of the latter that was underthrust from the Jinsa suture and may represent subducted continental crust. The Mid-Jurassic flysch rocks in northern Lhasa have typical upper crust Nd isotopic signatures and could have formed under a passive margin environment. Their Bangong counterparts, averagely 6.35 units higher in ε Nd values, unveil derivations from a likely magmatic arc along the northern Bangong margin. Basaltic pebble with low Nd content and ε Nd value from the Berriasian–Valanginian rocks in central Tibet, along with the samples with generally high ε Nd values, shows that the Bangong ophiolites could have been a main source of sediments during the earliest Cretaceous time. A significant transition of sediment sources occurred at the beginning of the Hauterivian Stage (∼ 120 Ma). Sedimentary basaltic tuff ( ε Nd( t)2.27) in the Hauterivian–Lower Cenomanian samples in central Tibet, and common Nd signatures for input of mantle-like materials in these rocks, indicate that mantle-derived magmatic rocks could have constituted a significant source. The huge Hauterivian–Lower Cenomanian sediments in central Tibet are thus inferred to have been accumulated under a (back-arc) rifting setting.

Cretaceous–Tertiary geology of the Gangdese Arc in the Linzhou area, southern Tibet

Knowledge of the Cretaceous–Tertiary history of upper crustal shortening and magmatism in Tibet is fundamental to placing constraints on when and how the Tibetan plateau formed. In the Lhasa terrane of southern Tibet, the widely exposed angular unconformity beneath uppermost Cretaceous–lower Tertiary volcanic-bearing strata of the Linzizong Formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs. during the Indo-Asian collision. In the Linzhou area, located ∼ 30 km north of the city of Lhasa, a > 3-km-thick section of the Linzizong Formation lies unconformably on Cretaceous and older rocks that were shortened by both northward- and southward-verging structures during the Late Cretaceous. The Linzizong Formation dips northward in the footwall of a north-dipping thrust system that involves Triassic–Jurassic strata and a granite intrusion in the hanging wall. U–Pb zircon geochronologic studies show that the Linzizong Formation ranges in age from 69 Ma to at least 47 Ma and that the hanging wall granite intrusion crystallized at ∼ 52 Ma, coeval with dike emplacement into footwall Cretaceous strata. 40Ar/ 39Ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 Ma, followed by an episode of accelerated cooling to upper crustal levels beginning at ∼ 42 Ma. The onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern Tibet and is interpreted be a consequence of either (1) Tertiary thrusting in this region or (2) regional rock uplift and erosion following removal of overthickened Gangdese arc lower crust and upper mantle or break-off of the Neo-Tethyan oceanic slab.

High-P metamorphic rocks from the Himalaya and their tectonic implication — a review

The suture zones bordering the Indian subcontinent on the E, N and W are characterized in several places by the occurrence of ophiolitic complexes and tectonic melanges. High-P metamorphic rocks have recently been discovered in the melanges in Burma, Naga Hills, southern Tibet, eastern and western Ladakh, Kohistan (Jijal, Allai, Shangla) and Khost (Afghanistan). The development of these rocks has an important bearing on the plate tectonics of the Himalaya. The High-P metamorphic rocks belong to prehnite-pumpellyite, blueschist and high-P greenschist facies but extensive garnet-granulites have developed at 35 km depth in Jijal. In the Indus-Zangbo suture zone (IZS) the high-P metamorphism is complemented to the N by low- or medium-P metamorphism and calc-alkaline magmatism in Tibet, Ladakh as well as Kohistan. High-P metamorphism in Jijal has been dated at 104 Ma, in Shangla at 70–100 Ma and in western Ladakh during mid-Cretaceous. Elsewhere, the timing of the high-P metamorphism is not known but a Cretaceous age is inferred. Since collision along the IZS occurred during Eocene, the high-P metamorphism is therefore related to the northwards subduction of the neo-Tethyan lithosphere under Tibet or late Mesozoic magmatic arcs. The timing of high-P metamorphism coincides with the breakup of India from Gondwanaland and its rapid northwards movement, whereas the tectonic melanges may principally have formed during Eocene collision and obduction.