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The growth and deformation history of the Qilian-Nan Shan thrust belt bounding the NE Qaidam Basin figures importantly in testing models of Tibetan Plateau uplift during the India-Asia collision. However, debate exists about the onset of uplift and exhumation of the Qilian-Nan Shan, with timing estimates ranging from early Paleocene to late Miocene. Here we report integrated analyses of magnetostratigraphy, anisotropy of magnetic susceptibility, sediment provenance, and paleoclimate (using environmental magnetic parameters) for Cenozoic fluvio-lacustrine strata from the Dahonggou section south of the Qilian-Nan Shan. The results are interpreted to demonstrate an early Miocene (ca. 20 Ma) onset of sediment accumulation in this location, with clastic sediment derived initially from the southern Qimen Tagh highland. The sediment source then switched to the northern part of the Qilian Shan region after ca. 18.5 Ma, consistent with initial uplift and exhumation of the Qilian Shan. Thereafter, two additional provenance shifts reveal progressive southward propagation of deformation in the Qilian-Nan Shan. As a result of this southward growth of Qilian-Nan Shan topography, precipitation increased after ca. 11 Ma at the study site due to orographic interception of moisture from the south. This work improves our understanding of the depositional age, sediment provenance, and paleoclimate history of the Qaidam Basin and reveals a prolonged history of Qilian-Nan Shan deformation and uplift, which may have accelerated during the late Miocene.

Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mössbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/ΣFe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

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The Wenquan porphyry moybdenium deposit, Western Qinling, NW China, with a resource of 247 million tonnes at 0.048% Mo, formed during Triassic collision between South China and North China blocks. Ore fluids at Wenquan vary widely in composition from single-phase, low-salinity aqueous to low-salinity vapor, and hypersaline inclusions. Fluid inclusion assemblages in a quartz-molybdenite vein contain >60 vol. % vapor, are equant to negative-crystal shaped, and form clusters or distinct inclusion trails. They yield homogenization temperatures of 285~295 °C, with an estimated trapping temperature of 425 °C, suggesting a paleodepth of about 5 km calculated at a pressure correction of 100~150 MPa. Hydrothermal K-feldspar from early stockwork veins related to potassic alteration have calculated δ 18 O fluid values of-1.9 ‰ to +1.9 ‰. Hydrothermal sericite from an overprinting phyllic alteration associated with late quartz-pyrite veins has calculated δD fluid values between-68 and-60 ‰, and δ 18 O fluid values from-3.7 to +1.4 ‰. These isotopic data suggest that both early-and late-stage fluids are dominated by magmatic fluids, with influx of meteoric water during the late stage. Exsolution of volatiles from magma in a late-stage open system, results in more variable δD values than an earlier closed-system. Molybdenite and pyrite have δ 34 S values ranging from 1.1 ‰ to 6.6 ‰, indicating that sulfur at Wenquan had a magmatic source. A linear relationship between δ 34 S values of sulfides separated from early potassic alteration and late phyllic alteration could reflect incorporation of isotopically heavy evaporate sulfate into source magmas from underlying Devonian sedimentary rocks during late alteration. The δ 56 Fe whole-rock values of altered porphyries range from 0.08 ‰ to 0.26 ‰, similar to δ 56 Fe values of 0.15 ‰ to 0.32 ‰ for pyrite from quartz veins related to both the potassic and phyllic alteration assemblages. The δ 56 Fe values of pyrite are positively correlated to, but in general slightly lighter than, those of altered porphyries, indicating similar metal sources. Moreover, both altered porphyries and the pyrite are progressively enriched in heavy Fe isotopes from biotite, through potassic to late phyllic alteration, consistent with isotopic evolution from an early lithostatic load to a late hydrostatic load. The Triassic intrusive rocks and fluids responsible for mineralization were derived from a deeply-sourced hybrid mantle-crustal magma crystallizing at a paleodepth of 5 km under lithostatic load. External Late Triassic meteoric or Devonian formational fluids became a part of the hydrothermal system during the post-fracturing final stage of ore formation under a hydrostatic regime. The formation of the Wenquan magmatic-hydrothermal systems spans the ductile-brittle transition based on a normal thermal gradient as constrained by the temperature range of 550~300 °C. The early, high-temperature potassic alteration developed under a ductile regime, 3 / 36 whereas the later, low-temperature phyllic alterat...