ALBERT

Description: This dataset reports major and trace elements, Sr and Nd isotopic ratios, carbonate C and O isotopic ratios, and mineral chemistry of Eocene (~35 Ma) magmatic rocks from the Nangqian basin, Eastern Qiangtang, Central Tibet. Samples have microlithic to microphaneritic porphyritic textures. Trachydacites show K-feldspar, plagioclase and amphibole phenocrysts in a matrix of feldspar + amphibole + biotite + quartz + oxides; tranchyandesites show clinopyroxene, apatite and resorbed biotite phenocrysts in a matrix of feldspar + clinopyroxene + oxides. One intrusive outcrop of porphyritic syenite was also sampled, composed of clinopyroxene and biotite phenocrysts in a matrix of feldspar + clinopyroxene + biotite + apatite + oxides. Whole-rock major and trace elements were measured at ISTerre, University Grenoble Alpes. The SARM-CRPG in Nancy and SEDISOR in Brest performed the whole-rock Sr and Nd isotope analyses. In-situ major-element compositions of mineral phases were obtained using the JEOL JXA-8230 Electron Microprobe at ISTerre, University Grenoble Alpes. Stable isotope analysis of carbonates was carried out in the stable isotope laboratory of Geoscience Rennes, CNRS-University of Rennes I. These geochemical data suggest that the source of the Eocene magmas in Nangqian was a H2O- and CO2-enriched lithospheric mantle. A full discussion of the results can be found in the related article.

Description: Representative major element compositions (in wt%) of clinopyroxenes and phlogopites from Nangqian ultrapotassic rocks. In-situ major-element compositions of mineral phases were obtained using the JEOL JXA-8230 Electron Microprobe at ISTerre, University Grenoble Alpes. Analytical conditions were 15 kV accelerating voltage and 12 nA beam current. The ZAF procedure was applied to reduce the raw data. The microprobe was calibrated using natural and synthetic standards. An X-ray element map of a calcite-bearing aggregate was acquired using 15 kV accelerating voltage and 10 nA beam current.

Description: Whole-rock major and trace elements data, and whole-rock and carbonate isotope data, for the Nangqian potassic and ultrapotassic rocks. Whole-rock major and trace elements were measured at ISTerre, University Grenoble Alpes. For major elements, 50 mg of rock powder were digested in HF/HNO3 mixture at 90 during five days. Excess HF was neutralized using boric acid and solutions were diluted with 250 mL of Milli-Q water. Major element contents were measured by Inductively Coupled Plasma - Atomic Spectrometry (ICP-AES) in Grenoble using the method given in Chauvel et al. (2011, doi:10.1111/j.1751-908X.2010.00086.x). For trace elements, 100mg of rock powder were digested with a mixture of concentrated HF and HNO3 at 150 for at least two weeks in steer Spar bombs. Excess Hf was neutralized with HNO3, using cycles of acid addition/evaporation. 300 mg of a spike containing Be, Ge, In, Tm and Bi were added to an aliquot of the rock solution corresponding to 8 mg of the initial powder. The solutions were then evaporated, diluted with 2% HNO3 (+ 1 drop of HF), and analysed by Inductively Coupled Plasma - Mass Spectrometry. During measurement, the signal was calibrated using the reference material BR24 (Chauvel et al., 2011, doi:10.1111/j.1751-908X.2010.00086.x), which was run every 4 or 5 analyses. Quality of the analytical procedure was checked by analysing blanks, international reference materials (BHVO2, BEN, BCR2), duplicate solutions and multiple runs of solutions. Only elements with external reproducibility 〈 15% are given. The SARM-CRPG in Nancy and SEDISOR in Brest performed the whole-rock Sr and Nd isotope analyses. Results were normalized to values of 143Nd/144Nd = 0.512110 for JNd-I reference material and 0.511850 for LaJolla, and to 87Sr/86Sr = 0.710250 for the reference material NIST SRM 987. Blanks were 74 pg for Nd and 137 pg for Sr. ε-Nd(T) ratios were calculated using the CHUR isotopic composition of Bouvier et al. (2008, doi:10.1016/j.epsl.2008.06.010). Stable isotope analysis of carbonates was carried out in the stable isotope laboratory of Geoscience Rennes, CNRS-University of Rennes I. Carbonates in whole-rock powders were selectively dissolved at 50 with anhydrous phosphoric acid H3PO4. The released CO2 gases were collected using a cryogenic extraction line, and their isotopic compositions were analyzed by a VG Optima triple collector mass spectrometer. Results were normalized to the values of the laboratory in-house standard Prolabo Rennes and the international standard NBS18. The analytical uncertainty is ±0.2 for δ18O carb, and ±0.1 for δ13C carb.

Description: Deng et al . (2015b) propose that a couple of kilometers of Palaeocene-Eocene strata were deposited across most of the Sichuan basin and later eroded, mainly based on their low-temperature thermochronological data. They further suppose that such a geological process would influence the datum plane of our isostatic model and the Cenozoic elevation history of the Sichuan, Tarim and Qaidam basins. Indeed, we selected the modern state of Suining as the datum plane for all our isostatic calculations (Yu et al ., 2015). This article is protected by copyright. All rights reserved.

Description: Several solutions have been proposed to explain the long-standing kinematic observation that post-collisional upper crustal shortening within the Himalaya and Asia is much less than the magnitude of India-Asia convergence. Here we implement these hypotheses in global plate reconstructions and test paleolatitudes predicted by the global apparent polar wander path against independent, and the most robust paleomagnetic data. Our tests demonstrate that 1) reconstructed 600–750 km post-collisional intra-Asian shortening is a minimum value; 2) a 52 Ma collision age is only consistent with paleomagnetic data if intra-Asian shortening was ~900 km; a ~56-58 Ma collision age requires greater intra-Asian shortening; 3) collision ages of 34 or 65 Ma incorrectly predict Late Cretaceous and Paleogene paleolatitudes of the Tibetan Himalaya (TH); and 4) Cretaceous counterclockwise rotation of India cannot explain the paleolatitudinal divergence between the TH and India. All hypotheses, regardless of collision age, require major Cretaceous extension within Greater India.

Description: Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to constrasting paleogeographic interpretations. Here, we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U-Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of post-tilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° [35.7°, 40.5°] (95% significance) and a secondary remanence with inclination of 42.9° [41.5°,44.4°] (95% significance). The paleolatitude defined by the modeled primary remanence—21°N [19.8°N, 23.1°N]— is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination-shallowing. The start of the Tibetan Himalaya-Asia collision was situated at ~20°N, and took place by ~50 Ma.

Description: Abstract The Bayin River, the largest river in the northeastern Qaidam basin, plays an important role in the source‐to‐sink system and landscape evolution at the basin‐mountain boundary between the Qilian Mountains and the Qaidam basin in the northern Tibetan Plateau. In this study, we conduct field observation, topographic analysis, zircon U‐Pb dating and apatite (U‐Th)/He dating to constrain the landscape and tectonic evolution of the Bayin River watershed. Bedrock zircon U‐Pb dating indicates the age group of 420‐450 Ma for far‐source sediments and the age group of 〉1700 Ma for near‐source sediments in the Bayin River watershed. Detrital zircon U‐Pb dating results from the Mesozoic and Cenozoic strata in the Bayin River watershed reveal that the most important source transition occurred during the Cretaceous. The Zongwulong Mountains gradually uplifted throughout the Cenozoic, along with decreasing far‐source and increasing near‐source based on detrital zircon U‐Pb dating. Rapid uplift occurred across the Qilian Mountains during the late Cenozoic, leading to high normalized steepness indices, young apatite (U‐Th)/He ages and deep incised valleys at the basin‐mountain transition zone. The knickpoints caused by the latest headwards erosion just reach an elevation of ~3800 m on the river longitudinal profiles, indicating that the latest uplift magnitude is ~300‐400 m relative to the basin surface of the Qaidam basin. Elevation distribution and apatite (U‐Th)/He ages reveal that river incision leads to high relief in the Zongwulong Mountains and influences its tectonic evolution.

Description: Based on isostasy, this paper elegantly explains the differential modern elevations of the large-scale cratonic basins around the Tibetan Plateau. The Qaidam and Tarim basins are endorheic regions with thick Cenozoic deposits, and they uplifted dramatically during the Cenozoic. In contrast, the Sichuan basin is exorheic and most Cenozoic sediments flowed out of the basin; no visible Cenozoic uplift is observed. We suggest that whether a basin is endorheic or exorheic significantly influences its modern elevation, illustrating how exogenic geological processes impact endogenic geological processes. In addition, a new method of estimating paleoelevation is proposed, which predicts successively accelerating uplift of the Qaidam basin during the Cenozoic and provides good constraints on the geological evolution of the northern Tibetan Plateau. This article is protected by copyright. All rights reserved.

Description: The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou Basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40 Ar/ 39 Ar and U-Pb zircon dates of ~52 Ma for the upper Linzizong, and 40 Ar/ 39 Ar dates (~51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of ~5°N, compared to the upper part (~20°N) and to underlying Cretaceous strata (~20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by ~52 Ma at ~20°N.

Description: The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of ‘Greater India', the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here, we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a pre-folding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses, rock magnetic tests, end-member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°S, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the Indian plate at that time. The size of ‘Greater India' during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8°S [21.8°S, 26.1°S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103-83 Ma or 77-67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya. This article is protected by copyright. All rights reserved.