Eclogite-facies vein systems in the Marun-Keu complex (Polar Urals, Russia): textural, chemical and thermal constraints for patterns of fluid flow in the lower crust

Metasomatic amphibole-eclogite sequences grew in selvages of quartz veins from the Marun-Keu complex (Polar Urals, Russia) during high-pressure metamorphism. Relicts of a pre-metasomatic eclogitefacies assemblage are present in the wallrock layers as irregular patches. Wallrock interstitial quartz trails lying at a high angle to reaction fronts provide evidence for grain-scale pore channelisation which may be produced by intergranular-fluid compositional gradients parallel to the quartz trails. Disequilibrium at vein-wallrock scale is inferred from wallrock mineral heterogeneity and from variable initial Sr isotope ratios in mineral separates. Mass-balance calculations between relicts and wallrock assemblages reveal chemical imbalances caused by open system-behaviour with two way mass-transfer. The vein-wallrock system registers a prograde history from 408–434 C (relicts) to 526–668 C (vein precipitates). Vein and metasomatic assemblages formed during a single fluid-rock interaction process, implying high heating rates (‡100 C/Ma), which could result from heat advection by large-scale fluid circulation. Metamorphic fluids in high-pressure regimes Fluid release from subducting crust transfers large amounts of volatiles and incompatible elements to the overlying mantle wedge. High-pressure experiments and calculated phase relations in basaltic systems suggest that aqueous solutions prevail in high-pressure regimes (Pawley and Holloway 1993; Yaxley and Green 1994; Schmidt and Poli 1998, 2003; Molina and Poli 2000; Kerrick and Connolly 2001; Poli and Schmidt 2002). Increasing dissolution of silicates in aqueous fluids with increasing pressure and temperature (Newton and Manning 2000) is responsible for metasomatic processes in a variety of geodynamic systems leading, as an ultimate consequence, to the variably enriched mantle sources responsible for orogenic magmatism. However, the behaviour of fluids under eclogitefacies conditions is not fully understood. At present, evidence is reported for both short and long-range migration. Circulation of high-pressure fluids at kilometre-scale is inferred from fluid-enhanced eclogitisation of dry granulites (e.g. Austrheim 1987; Austrheim and Engvik 1997; Austrheim et al. 1997; Jamtveit et al. 2000; Scambelluri et al. 1998) and from metasomatism of high-pressure rocks from ancient accretionary wedges (e.g. Sorensen and Grossman 1989; Bebout and Barton 1993; Nelson 1995). On the other hand, fluid inclusion and stable-isotope data from Alpine eclogites suggest that fluid flow was restricted under high-P conditions to millimetreto decimetre-scales (e.g. Selverstone et al. 1992; Philippot 1993; Getty and Selverstone 1994; Philippot and Scambelluri 1995; Philippot et al. 1995; Scambelluri and Philippot 2001). In order to reconcile lack of pervasive fluid flow at eclogite-facies conditions with large-scale volatile transfer to the mantle wedge, Barnicoat and Cartwright (1995) also pointed out that fluid migration out of the subducting slab could be strongly channelled. Nevertheless, Zack et al. (2001) invoked pervasive, opensystem flux of high-P fluids to explain homogenisation Editorial responsibility: W. Schreyer J. F. Molina (&) Departamento de Mineralogı́a y Petrologı́a, Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18002 Granada, Spain E-mail: [email protected] Fax: +34-958-243368 J. F. Molina Æ S. Poli Dipartimento Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133 Milan, Italy H. Austrheim PGP and Department of Geosciences, Postboks 1047, Blindern, 0316 Oslo, Norway J. Glodny GeoForschungsZentrum Potsdam, Telegrafenberg C2, 14473 Potsdam, Germany A. Rusin Institute of Geology and Geochemistry, Pochtovy per. 7, Ekaterinburg, Russia Contrib Mineral Petrol (2004) 147: 484–504 DOI 10.1007/s00410-004-0569-z of large ion lithophile elements in different eclogiterock types from Trescolmen (Central Alps). The analysis of vein systems can provide important constraints on fluid circulation in metamorphic terrains (e.g. Yardley and Bottrell 1992; Ague 1994a, 1994b; Cesare 1994; Oliver 1996; Oliver and Bons 2001). Largescale circulation of disequilibrium fluids may cause metasomatic bands adjacent to the fluid channels (e.g. fractures, shear zones, layer boundaries) though such metasomatic effects do not necessarily require large mass-transport scales (Cesare 1994; Widmer and Thompson 2001). Disequilibrium fluids may not leave much evidence of their circulation under very fast flow rates or very low wallrock permeability. Mobile hydrofractures (fractures moving together with their contained fluid) can ascend rapidly over large distances with negligible effect on the country rocks until they are arrested due to the presence of obstacles, loss of fluid or unfavourable orientation. This is an efficient mechanism for fluid transport since the volume of fluid required to drive mobile hydrofractures is significantly lower than that needed for fluid flow in a connected fracture network (Oliver and Bons 2001). Since the densities of hot fluids are less than those of cold fluids, fluid flow can occur along a temperature gradient if there is good connectivity and permeability. Therefore, large-scale fluid circulation may be an effective transport mechanism not only for mass but also for heat. However, thermo-mechanical models suggest that pervasive fluid flow cannot cause an appreciable thermal perturbation of the conductive geotherm, large fluxes and channelisation, typically related to fractures, shear zones or fold crests, are required (e.g. Bickle and McKenzie 1987; Brady 1988; Chamberlain and Rumble 1988; Oliver 1996; Connolly 1997; Thompson 1997). The eclogite-facies terrain of the Marun-Keu (Polar Urals, Russia) offers a unique example to gain a different perspective on metasomatic processes at lower crustal depths. In this complex, Austrheim (1998) described eclogitisation of dry gabbros along fluid fronts, suggesting fluid migration of, at least, tens to hundred metre scales during the eclogite-facies metamorphism; Molina et al. (2002) report vein systems with well preserved wallrock alteration haloes. In this work, we investigate blocks of amphiboleeclogites (in the sense of Liou et al. 1998) from the Marun-Keu complex in which pre-metasomatic assemblages are still preserved as relicts widespread through the metasomatic bands. Textural evidence for grainscale, pore channelisation as described in experiments by Wark and Watson (2002) is reported for the first time in high-P vein-wallrock systems. Mass-balance calculations are performed for unravelling if chemical imbalances exist in the vein-wallrock system and, hence, for discussing the scale of mass transport during the growth of the amphibole-eclogite wallrock sequences. We also report for the first time very large temperature variations in high-pressure vein-wallrock systems discussing implications for large advective heat transport. Geological setting of the Marun-Keu complex The Marun-Keu complex in the Polar Urals (Russia) (approximately 67 N, 66 E) represents the northernmost high-P metamorphic complex of the Uralian Orogen (Udovkina 1971; Dobretsov and Sobolev 1984; Sobolev et al. 1986; Savelieva and Nesbitt 1996; Lennykh et al. 1997; Austrheim 1998; Molina et al. 2002). This complex is a thrust sheet, with low-grade metasedimentary sequences of the Kharbei complex in the footwall, and oceanic and mantle rocks of the Syum– Keu ophiolite in the hanging wall (e.g. Udovkina 1971; Savelieva and Nesbitt 1996; Scarrow et al. 2001; Molina et al. 2002). The complex consists of Neoproterozoic to Cambrian volcanic-sedimentary sequences with mafic to felsic intrusive rocks (e.g. Udovkina 1971; Molina et al. 2002), which underwent eclogite-facies metamorphism at 356 Ma (Glodny et al. 2003). Our thermobarometric estimates (Molina et al. 2002) for the central and southern parts of the complex give temperatures ranging from 525 to 650 Cand pressures of 14–17 kbar for the high-P event. In the northern part of the complex, lower temperature conditions are suggested by the presence of glaucophane metabasites, garnetcrossite quartzites and metagraywackes (Udovkina 1971; Dobretsov and Sobolev 1984; Sobolev et al. 1986). Vein precipitates and wallrock amphibole-eclogite band sequences: textural analysis A petrological assessment of mafic eclogites from Marun-Keu complex is available in Molina et al. (2002). This study focuses on a systematic mass-balance analysis of two representative samples (J-10 and PU-34) from the amphibole-eclogite sequences (see Table 1 for summary of mineral assemblages and textural features), which appear as decimetre-scale blocks included in a network of centimetre thick quartz-mica veins (Fig. 1a) (see also Fig. 3c in Molina et al. 2002).