The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous 40Ar/39Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision.
The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions. 相似文献
Experimental phase equilibrium and trace element partitioningdata are reported for H2O-saturated mid-ocean ridge basalt at2·5 GPa, 750–900°C and oxygen fugacities atthe nickel–nickel oxide buffer. Garnet, omphacite andrutile are present at all temperatures. Amphibole and epidotedisappear as residual phases above 800°C; allanite appearsabove 750°C. The Na–Al-rich silicate glass presentin all run products is likely to have quenched from a supercriticalliquid. Trace element analyses of glasses demonstrate the importantcontrol exerted by residual minerals on liquid chemistry. Inaddition to garnet, which controls heavy rare earth elements(HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanitebuffers the light REE (LREE; La–Sm) contents of liquidsto relatively low levels and preferentially holds back Th relativeto U. In agreement with previous experimental and metamorphicstudies we propose that residual allanite plays a key role inselectively retaining trace elements in the slab during subduction.Experimental data and analyses of allanite-bearing volcanicrocks are used to derive a model for allanite solubility inliquids as a function of pressure, temperature, anhydrous liquidcomposition and LREE content. The large temperature dependenceof allanite solubility is very similar to that previously determinedfor monazite. Our model, fitted to 48 datapoints, retrievesLREE solubility (in ppm) to within a factor of 1· 40over a pressure range of 0–4 GPa, temperature range of700–1200°C and for liquids with anhydrous SiO2 contentsof 50–84 wt %. This uncertainty in LREE content is equivalentto a temperature uncertainty of only ± 27°C at 1000K, indicating the potential of allanite as a geothermometer.Silicic liquids from either basaltic or sedimentary protolithswill be saturated in allanite except for Ca-poor protolithsor at very high temperatures. For conventional subduction geothermsthe low solubility of LREE (+ Th) in liquids raises questionsabout the mechanism of LREE + Th transport from slab to wedge.It is suggested either that, locally, temperatures experiencedby the slab are high enough to eliminate allanite in the residueor that substantial volumes of H2O-rich fluids must pass throughthe mantle wedge prior to melting. The solubility of accessoryphases in fluids derived from subducted rocks can provide importantconstraints on subduction zone thermal structure. KEY WORDS: subduction; experimental petrology; allanite; solubility; supercritical liquid; eclogite相似文献