Nb/Ta-U/Pb and Th/Pb fractionation in rutile-bearing veins from the Dabie Shan ultrahigh pressure metamorphic belt
Zusammenfassung der Projektergebnisse
The present study provides a comprehensive geochemical dataset on UHP/HP eclogites and enclosed metamorphic veins, targeting to reconstruct fluid origin, fluid evolution and fluid-assisted element fractionation in deep subduction environments. The occurrence of different vein generations demonstrates multiple stages of fluid flow during deep subduction and exhumation of continental basaltic rocks. Representing remnants of prograde, “dry” eclogite dehydration, the fluids that formed 1st generation quartz-rutile veins expectedly solidified in a closed system without remarkable compositional evolution. Fluids that precipitated mineralogical complex 2nd generation veins, in contrast, evolved and altered from the onset of exhumation towards a late retrograde metamorphic stage. Their source is determined to be water that was initially derived from prograde dehydration of bedrock gneisses that experienced LT-hydrothermal alteration by ancient glaciation waters prior to subduction. Intercalated basalts that interact with these gneissic fluids during conjoint subduction experienced Ca-metasomatism, resulting in the formation of lawsonite. Later evolving eclogites show both chemical (Ca↑) and isotopic (87Sr/86Sr↑, d’18O↑) fingerprints of the gneiss-derived fluids, and contain 2nd generation mineral veins. In contrast, fluid-unaffected pristine eclogites are unaltered compared to their assumed continental basalt precursors, and lack 2nd generation veins. The compositional evolution of the post-peak, vein-forming fluids proceeds in three stages: (i) The initial fluid is derived from lawsonite breakdown and is rich in Ca, Al, REE and most trace elements. It is diluted by partitioning of these elements into newly forming allanite/clinozoisite during the early retrograde stage, and trapped in primary and possibly early pseudosecondary fluid inclusions found in eclogite and vein minerals. (ii) The initial fluid is successively diluted towards the amphibolite stage due to element partitioning into newly forming, retrograde clinozoisite. This fluid is trapped at variable stages in pseudo-secondary fluid inclusions. (iii) Secondary fluid inclusions and late epidote record the introduction of low salinity fluids that are most likely diluted by meteoric waters, and adopted REE trace element characteristics of decomposed garnet. Nb/Ta is elevated in retrogressively affected eclogites relative to pristine samples. Fluid-affected eclogites are a potential high-Nb-high Nb/Ta candidate to balance the global “missing Nb”. However, wether or not the crust-mantle vs. eclogite mass balance can be closed depends on the mass of such a reservoir, which is unknown. U/Pb is lowered in retrogressed eclogites relative to pristine samples, resulting in shifting of retrogressed eclogites towards unradiogenic 206Pb/204Pb compositions. High pressure retrogressed eclogites, as described in this study, are thus a potential candidate to balancing the global lead-paradox. The residuum after fluid-rock interaction provides a conceptual explanation for the enigmatic HIMU source.
Projektbezogene Publikationen (Auswahl)
- (2012): Trace Element Mobility and Compositional Evolution in (U)HP Fluids - A Study on High-Grade Metamorphic Veins from Dabieshan, China. European Mineralogical Conference, Frankfurt
Albrecht, N., Wörner, G., and Xiao, Y.
- (2013): Fluid inclusions and epidote zoning in UHP eclogite-hosted veins (Dabieshan, China). DMG/GV Joint Annual Meeting, Tübingen
Albrecht, N., Wörner, G., Xiao, Y. and van den Kerkhof, A.M.
- (2013): High pressure fluid evolution derived from veins in UHP eclogites (Dabieshan, China). Goldschmidt Conference, Florence
Albrecht, N., Wörner, G., Xiao, Y. and van den Kerkhof, A.M.
- (2014): Evolution of high pressure fluids deduced from fluid inclusions and epidote zoning in metamorphic veins (Dabieshan UHP Orogenic Belt, China) ECROFI XXII, Antalya
Albrecht, N., Wörner, G., Xiao, Y. and van den Kerkhof, A.M.
