ABSTRACT
After two continents collide, plate convergence and orogenesis are sustained because subducted continental lithosphere continues pulling the surface plate. It remains controversial how, why, and when continental plate convergence and collision slow down and eventually cease. We use an unprecedented data coverage and present a regional-scale seismic tomographic image of the mantle structure beneath the Tibetan Plateau. In the mantle transition zone, we identify multiple high-velocity anomalies and interpret them as detached pieces of the Indian continental slab. Facilitated by internal heterogeneity of the continental lithosphere, piecewise slab detachments could reduce the slab pull force, resulting in the Miocene slowdown of the India-Eurasia convergence and coeval diachronous potassic volcanism in southern Tibet. We propose that slab detachment is a mechanism that eventually will lead to the end of the Indo-Eurasian continental collision and the Himalayan orogeny.
ABSTRACT
Porphyry ore deposits, Earth's most important resources of copper, molybdenum, and rhenium, are strongly associated with felsic magmas showing signs of high-pressure differentiation and are usually found in places with thickened crust (>45 kilometers). This pattern is well-known, but unexplained, and remains an outstanding problem in our understanding of porphyry ore deposit formation. We approach this problem by investigating the oxidation state of magmatic sulfur, which controls the behavior of ore-forming metals during magma differentiation and magmatic-hydrothermal transition. We use sulfur in apatite to reconstruct the sulfur oxidation state in the Gangdese batholith, southern Tibet. We find that magma sulfate content increased abruptly after India-Eurasia collision. Apatite sulfur content and the calculated magma S6+/ΣS ratio correlate with whole-rock dysprosium/ytterbium ratio, suggesting that residual garnet, favored in thickened crust, exerts a first-order control on sulfur oxidation in magmatic orogens. Our findings link sulfur oxidation to internal petrogenic processes and imply an intrinsic relationship of magma oxidation with synmagmatic crustal thickening.
ABSTRACT
Plate subduction continuously transports crustal materials with high-δ(18)O values down to the mantle wedge, where mantle peridotites are expected to achieve the high-δ(18)O features. Elevated δ(18)O values relative to the upper mantle value have been reported for magmas from some subduction zones. However, peridotites with δ(18)O values significantly higher than the well-defined upper mantle values have never been observed from modern subduction zones. Here we present in-situ oxygen isotope data of olivine crystals in Sailipu mantle xenoliths from South Tibet, which have been subjected to a long history of Tethyan subduction before the India-Asia collision. Our data identify for the first time a metasomatized mantle that, interpreted as the sub-arc lithospheric mantle, shows anomalously enriched oxygen isotopes (δ(18)O = +8.03 ± 0.28 ). Such a high-δ(18)O mantle commonly does not contribute significantly to typical island arc basalts. However, partial melting or contamination of such a high-δ(18)O mantle is feasible to account for the high-δ(18)O signatures in arc basalts.
