ALBERT

Description: Identification of advancing and retreating modes in fossil orogenic belts is not always straightforward. Such issue is addressed in the submitted paper via the case study of the northwestern Chinese Altai where the suprasubduction structures are well preserved. Combined with detailed mapping, structural/petrological observations, seven samples were collected from the key lithological units of Jiadengyu and Chonghuer region in the Xinjiang Uygur Autonomous Region for zircon and monazite U-Pb dating to provide critical geochronological constraints on the individual deformational episodes, including two zircon samples (19CA36 and 17CA105-7) and five monazite samples (17CA107, 17CA105-4, 19CA41, 19CA45-1, and 19CA45-3). Migmatite samples 19CA41 and 17CA107 are characterized by sub-horizontal S1 foliations associated with extensional shear bands. Sample 19CA45-1 collected from the migmatitic Habahe Group domain shows nearly complete transposition of S1 foliation by S2 fabric. Samples 19CA36 and 19CA105-7 were collected from syn-D2 granite intrusions. Leucogranite dyke (sample 17CA105-4) and pegmatite (sample 19CA45-3) were emplaced as tensional fractures that formed in response to D2 shortening. Zircon and monazite grains were separated after rock crushing using conventional heavy liquid and magnetic properties and then selected under a binocular microscope. These grains were mounted in epoxy resin, polished to approximately one-third of their thickness. U-Pb dating of zircon and monazite were analyzed by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Co., Ltd. U-Pb dating of zircon samples were conducted by a COMPexPro 102 ArF excimer laser and a MicroLas optical system coupled with an Agilent 7700e ICP-MS. Most analyses were performed with a beam diameter and frequency of 32 μm and 5 Hz. GJ-1 standard zircon was also determined for monitoring the accuracy of U-Pb dating. Zircon 91500 was used as an external standard for U-Pb dating calibration. U-Pb dating of monazite samples was performed using the same operating processes and instruments. In this work, the spot size and frequency of the laser were set to 16 μm and 2 Hz, respectively. Monazite standard 44069 was used as an external standard for U-Pb dating. Monazite standard Trebilcock was used as a secondary standard to assess the accuracy of analyses. Each analysis of zircon/monazite was performed using a background acquisition of approximately 25 s followed by 65 s of data acquisition.

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: Flow of partially molten crust is a key contributor to mass and heat redistribution within orogenic systems, however, this process has not yet been fully understood in accretionary orogens. This issue is addressed in a Devonian migmatite-granite complex from the Chinese Altai through structural, petrological, and geochronological investigations presented in this study. The migmatite-granite complex records a gradual evolution from metatexite, diatexite to granite and preserves a record of two main Devonian phases of deformation designated D1 and D2. The D1 phase was subdivided into an early crustal thickening episode (D1B) and a later extensional episode (D1M) followed by D2 upright folding. The D1M episode is associated with anatexis in the deep crust. Vertical shortening, associated with D1M, gave rise to the segregation of melt and formation of a sub-horizontal layering of stromatic metatexite. This fabric was reworked by the D2 deformation associated with the migration of anatectic magma in the cores of F2 antiforms. Geochronological investigations combined with petro-structural analysis reveal that: (1) D1M partial melting started probably at 420–410 Ma and formed sub-horizontal stromatic metatexites at ∼30 km depth; (2) The anatectic magma accumulated and migrated when a drainage network developed, as attested by the pervasive formation of massive diatexite migmatites, at 410–400 Ma; (3) Soon after, massive flow of the partially molten crust from orogenic lower to orogenic upper crustal levels, assisted by the interplay between D2 upright folding and magma diapirism, led to migmatite-granite emplacement in the cores of regional F2 antiforms that lasted until at least 390 Ma; (4) a terminal stage was manifested by the emplacement of 370–360 Ma granite dykes into the surrounding metamorphic envelope. We propose that Devonian anatexis assisted by deformation governed first the horizontal and then the vertical flow of partially molten orogenic lower crust, which drove crustal flow, mass redistribution, and crustal differentiation in the accretionary system of the Chinese Altai.