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Quartz veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid-rock interaction, and thus provide insight into fluid processes in subduction zones. In this paper, we report an integrated study of mineral inclusion, trace-element, U-Pb age, and Lu-Hf isotope compositions of hydrothermal zircon grains from a quartz vein within an UHP eclogite outcrop from the Hong'an area, western Dabie Mountains. These data are used to decipher the age, conditions of formation, and source of fluid for zircon formation during the exhumation of UHP rocks. Zircon grains from the vein have perfect euhedral shape, and show sector zoning or weak zoning, indicating that they precipitated from the aqueous fluid responsible for the vein formation. Raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet, omphacite, rutile, quartz, and [H.sub.2]O, implying that they crystallized from aqueous fluid under HP eclogite-facies conditions. The zircon grains show low Th/U and Lu/Hf ratios, nearly flat HREE patterns, absent Eu anomalies and low LREE contents. These characteristics are consistent with their precipitation in the presence of garnet and epidote, and absence of feldspar, and thus suggest that trace-element concentrations in hydrothermal zircon are controlled by co-precipitation of mineral assemblages. Crystallization temperatures of 670 to 712[degrees]C, which were calculated using the Ti content of zircon, are consistent with their formation under eclogite-facies conditions and may correspond to the temperature of the infiltrating fluid. The weighted mean [sup.206]Pb/[sup.238]U age of 224.7 [+ or -] 1.3 Ma is taken as the best estimate for the age of quartz-vein formation and records aqueous fluid flow during the early exhumation stage of UHP rocks. The zircon grains in the quartz-vein have Hf compositions similar to those in the host eclogite, which demonstrates isotopic equilibrium between fluid and rocks and that the fluid-rock ratio was likely low. Keywords: Hydrothermal zircon, trace element, Lu-Hf isotope, U-Pb age, HP-UHP metamorphism, quartz vein, western Dabie Mountains

Byline: Yuan-Bao Wu (1,2), Shan Gao (1), Hong-Fei Zhang (1), Sai-Hong Yang (1), Wen-Fang Jiao (1), Yong-Sheng Liu (1), Hong-Lin Yuan (2) Keywords: Zircon; Trace element; U--Pb age; Hf isotope; Hong'an area; Western Dabie Mountains Abstract: The Hong'an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U--Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong'an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U--Pb ages, and trace element and Lu--Hf isotope compositions. One group with an average age of 239 +- 2 Ma show relatively high and variable HREE contents (527 aY= Lu.sub.N aY= 14) and 176.sup.Lu/ [.sup.177]Hf ratios (0.00008--0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 +- 2 Ma, and shows consistent low HREE contents and 176.sup.Lu/ [.sup.177]Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP--HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 +- 86 and 752 +- 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (T .sub.DM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation. Author Affiliation: (1) State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China (2) State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an, 710069, China Article History: Registration Date: 09/06/2007 Received Date: 13/02/2007 Accepted Date: 08/06/2007 Online Date: 03/07/2007 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-007-0231-7) contains supplementary material, which is available to authorized users.

The outer medullary descending vasa recta (OMDVR) vasomotor tone is affected by norepinephrine (NE) and acetylcholine (ACh) aiming to modulate the total and regional flow of blood to the renal medulla. The OMDVR constriction by NE and dilation of the NE-preconstricted vessels by ACh are coordinated by a nitric oxide dependent mechanism.

The diffusion of sodium, chloride, inulin and raffinose in the outer medullary descending vasa recta (OMDVR) is through a common pathway while that of urea, water and sodium through the OMDVR wall is by a different mechanism. The permeability of OMDVR to (super 22)Na and ({super 3}H)raffinose increases reversibly with the rate of perfusion of these tracers in the OMDVR. The permeability of Na (PNa) correlates with that of Cl and ({super 3}H)raffinose but not with that of urea. If the PNa is low, the diffusional water and urea permeability is high.

Potassium (K)-rich mafic rocks are viewed as being derived from partial melting of an enriched mantle source, but it is controversial about which processes cause the mantle enrichment. The Fushui intrusive complex is the largest early Paleozoic K-rich intrusive complex in the eastern Qinling orogen. Therefore, detailed studies on the Fushui complex can contribute not only to understanding of the petrogenesis of K-rich mafic rocks, but also to unraveling the Paleozoic evolution of the Qinling orogen. In this study, we provide an integrated investigation of in situ zircon U-Pb dating and Hf-O isotopes, in combination with whole-rock major and trace elements, as well as Sr-Nd-Hf isotopes, for the Fushui intrusive complex. In situ zircon secondary ion mass spectrometry (SIMS)/laser ablation induction coupled plasma mass spectrometry (LA-ICPMS) U-Pb dating reveals that different rock types of the Fushui complex have identical formation ages of 488-484 Ma. The Fushui complex belongs to the shoshonitic series, and is characterized by extreme large ion lithophile element (LILE, e.g., Ba, U, Th and Sr) and Pb enrichment and depletion of high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P and Ti). It shows high initial Sr isotopic ratios (0.7100-0.7151), negative whole-rock [ζ.sub.Nd](0 (-3.97 to -5.68) and negative to slight positive whole-rock (-2.24 to 2.38) and zircon (-2.85 to 0.34) [ζ.sub.Hf](t) values, as well as high zircon δ[sup.18]O values (6.86 ± 0.13 ‰). The Hf-Nd isotopic systems are decoupled with positive Δ[ζ.sub.Hf] values (3.85-5.37). These geochemical features indicate that the mantle source has incorporated subducted zircon-barren oceanic sediments. A simple two-endmembers mixing model constrains the amount of subducted sediments in the Fushui mantle source to 5-8 %. The Fushui complex originated from 1 to 6 % equilibrium melting of a phlogopite-bearing garnet lherzolite by non-modal melting. As shoshonitic magmas have been discovered in modern nascent arcs, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean, when it jumped from the northern to the southern boundary of the North Qinling microcontinent. Keywords Zircon Hf-O isotope * Geochemistry * K-rich mafic rock * Qinling orogen * Subducted sediment, Introduction Potassium (K)-rich mafic rocks are common, albeit minor, in both orogenic belts and cratons (Foley and Peccerillo 1992; Nowell et al. 2004; Prelevic et al. 2010), and are viewed [...]