Håkon Austrheim

Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway

The anorthosites of the Bergen Arcs contain two distinct metamorphic mineral assemblages which are stable under different P—T conditions. A granulite-facies assemblage, which equilibrated at T = 800–900° C, P < 10 kbar, is defined by the minerals plagioclase, Al-rich diopside (Cpx I), garnet (Gnt I) ± scapolite ± orthopyroxene ± dark-brown hornblende. This assemblage is linked by corona structures to a primary magmatic mineralogy, where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to ∼ 8 kbar at 1200° C. The anorthosites are transected by a series of fine-grained shear zones in which the granulite-facies assemblage is replaced by an eclogite mineralogy defined by omphacite (Cpx II) (Jd40–50) + garnet + kyanite + epidote/clinozoisite + phengite + quartz ± amphibole with plagioclase sometimes remaining in the marginal parts. Mineral compositions indicate equilibration conditions for the eclogite paragenesis of T = 700–800° C, P = 16–19 kbar. A series of textural and mineralogical changes, which can be observed as the strain increases into the shear zones, demonstrates that the eclogite mineralogy is formed at the expense of the granulite-facies minerals during the early stages of shear zone development. The eclogite mineralogy defines a strong mylonitic fabric, which again is truncated by centimetres-thick secondary shear zones containing clasts of both granulite and eclogite minerals. These latter fine-grained ultramylonites may be connected to uplift of this deep-seated complex. The eclogite shear zones are highly oblique to the granulite structures, indicating that the eclogite shear zones formed under a new stress-regime. RbSr dating shows that the granulite-facies assemblage formed in Sveconorwegian time or earlier, while minerals from shear zones define ages around 400 Ma, indicating a Caledonian formation of the eclogites. This work demonstrates the critical role of deformation in triggering metamorphic reactions. Whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data.

Fluid controlled eclogitization of granulites in deep crustal shear zones, Bergen arcs, Western Norway

During the Caledonian orogeny large parts of the western margin of the Baltic shield were disrupted, sliced and stacked. Caledonian deformation resulted in a massif thickening of the continental crust. Mafic granulites and granulite facies meta-anorthosites build up a large portion of the Bergen Arcs terrane in southwestern Norway. The rocks represent typical Precambrian continental lower crust. These rocks experienced extensive eclogitization in response to stacking and crustal thickening during the Caledonian orogenic cycle. Eclogite formation resulted from shear deformation and associated infiltration of H2O-rich fluids (XH2O≥0.75). During an early stage, eclogite facies mineralogy formed in extension fractures (veins). The veins are probably related to hydraulic fracture systems which transported the inferred fluid phase. During the main stage, eclogitization occurred along shear zones ranging from centimeters to tens of meters in thickness. Eclogite forming reactions are shown to consume H2O, alkalies and to release SiO2. Much of the SiO2 released by the eclogitization process can be found in late quartz vein systems. The eclogitization took place at a temperature of about 700°C and a pressure between 18 and 21 kbar. Fluid infiltration was supported by a decrease in rock volume during reaction (ΔVsolids<0). The negative volume change of reaction occurs despite that the process of eclogitization involves hydration reactions. The formation of eclogite from granulite produces approximately 15 KJ heat per 100 cm3 original granulite. Numerical modeling of the regional temperature effects associated with partial hydration of the lower crust suggests that these processes may not cause large perturbations on the geotherm. Both, transport of heat and matter by advection of the fluid phase is negligible on a regional scale.

Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization

Abstract Segments of the root to the Caledonian collision zone are exposed along the west coast of Norway, allowing the study of processes and petrophysical conditions at the deepest crustal levels. P-T conditions prevailing in these crustal root zones correspond to eclogite facies. Eclogitization is associated with marked changes in petrophysical properties, notably a density (10–15%) and Vp increase which may give a mantle signature to an eclogitized crust. The observed ductility enhancement may be caused by transformation plasticity, reduction in grain size, formation of new mineral assemblages and presence of fluid. In the Western Gneiss Region and the Bergen Arcs, eclogites form along shear zones, interpreted as fluid pathways with sharp boundaries to their metastable protolith. This makes eclogite shear zones potential deep crustal reflectors. Dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available. Timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the P-T space, but rather on the introduction of fluids into the system. Eclogites are not only passive recorders of P-T-t paths illustrating very deep subduction of crust (> 100 km); rapid changes in petrophysical properties make fluid-induced eclogitization a dynamic process that influences geodynamics. Stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn-eclogitic pseudotachylytes in the metastable protolith. Deformation, including seismicity, enhances fluid infiltration and further eclogitization. Dense and rheologically weak eclogites may delaminate and sink into the mantle. In the absence of fluids, crustal doubling may occur without metamorphic re-equilibration. The evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to P and T.

The effect of fluid and deformation on zoning and inclusion patterns in poly-metamorphic garnets

Within the Bergen Arcs of W Norway, Caledonian eclogite facies assemblages (T≥650°C, P≥15 kbar) have formed from Grenvillian granulites (T= 800–900°C, P≥10 kbar) along shear zones and fluid pathways. Garnets in the granulites (grtI: Pyr56–40 Alm45–25Gro19–14) are unzoned or display a weak (ca. 1 wt% FeO over 1000μm) zoning. The eclogite facies rocks contain garnets inherited from their granulite facies protoliths. These relict garnets have certain areas with compositions identical to the garnets in their nearby granulite, but can be crosscut by bands of a more Almrich composition (grtII: Pyr31–41Alm40–47Gro17–21) formed during the eclogite facies event. These bands, orientated preferentially parallel or perpendicular to the eclogite foliation, may contain mineral filled veins or trails of eclogite-facies minerals (omphacite, amphibole, white mica, kyanite, quartz and dolomite). Steep compositional gradients (up to 9 wt% FeO over 40 μm) separate the two generations of garnets, indicating limited volume diffusion. The bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites. The re-equilibration of granulite facies garnets during the eclogite facies event must, therefore, be a function of fracture density (deformation) and fluid availability. The results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re-equilibration of garnets.

Retention of Precambrian Rb/Sr phlogopite ages through Caledonian eclogite facies metamorphism, Bergen Arc Complex, W-Norway

The Lindas Nappe, Caledonides W-Norway was affected by two major tectonometamorphic events. A Precambrian granulite facies event at T=800–900°C, P 15 kbar) and localized amphibolite facies reworking. During the granulite–eclogite facies transition, anorthositic rocks were converted from garnet granulites to kyanite eclogites, while phlogopite-bearing spinel lherzolite reacted to garnet lherzolite. The eclogite and amphibolite facies reequilibration took place along shear zones and fluid pathways. In the unhydrated and undeformed parts, the minerals preserved their granulite facies composition with constant Fe/Mg ratios from core to rim, suggesting diffusional reequilibration. Rb/Sr age dating was carried out on relict granulite facies minerals from three lenses of ultramafites (Alvfjellet, Hundskjeften and Kvamsfjellet). Phlogopite from phlogopite lherzolite at Alvfjellet give 857±9 Ma, while clinopyroxene, amphibole, phlogopite and whole rock from a lherzolite at Hundskjeften yield an age of 842±12 Ma (MSWD=1.9). Clinopyroxene, feldspar, orthopyroxene phlogopite and whole rock from websterite, Kvamsfjellet, yield an age of 835±7 Ma (MSWD<1), while clinopyroxene, phlogopite and whole rock from a lherzolite from the same lens gives a result of 882±9 Ma. These results are interpreted as minimum ages for the granulite facies event and only slightly younger than, or overlap with previous U–Pb zircon ages (929±1 Ma) and Sm–Nd garnet–pyroxene ages (890–923 Ma) interpreted to date the end of the granulite facies event. By contrast, ages obtained for the eclogite and amphibolite facies range from 460 (U–Pb, sphene), 440 (Ar–Ar), 419 (U–Pb, zircon) to 410 Ma (Rb/Sr mineral ages). These results demonstrate that the reopening temperature for the Rb/Sr system in phlogopite–biotite under dry and static high-pressure conditions is, in the given mineral assemblages, at least 650°C, considerably higher than the 300–400°C assumed as the closure temperature of this system. We ascribe this elevated reopening temperature to fluid absent conditions that prevented element transport and rehomogenization.

The granulite-eclogite facies transition: A comparison of experimental work and a natural occurrence in the Bergen Arcs, western Norway

A unique segment of the deeper parts of the Caledonian continent-continent collision zone is exposed in one of the thrust sheets in the Bergen Arcs of western Norway. It demonstrates that Precambrian granulite-facies rocks underwent fluid-controlled eclogitization on a regional scale. The transition from granulite to eclogite facies is sharp. It occurs over distances of centimetres to metres and follows shear zones and infiltration fronts. Major anastomosing shear zones, typically 50 to 100 m thick, with well-banded eclogite-facies rocks crosscut the lithological boundaries. They can be followed continuously for several kilometres. Breccias, where metre-size blocks of granulite float in an eclogite matrix, outcrop over areas of 0.5 km2. Areas comprising more than 80% eclogite occupy several square kilometres. This mixture of metamorphic facies indicates the metastable occurrence of granulite-facies rocks in the lower crust of modern and ancient orogenic belts. In contrast to earlier experimental work, such field relations imply that the granulite-eclogite facies boundary in nature can be sharp and independent of lithological boundaries. A partly eclogitized lower crust, of the kind exposed in the Bergen arcs can explain the 7.2–7.8 km/s Vp layer present in many crust-mantle transition zones. Anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors.

Eclogite-facies shear zones—deep crustal reflectors?

Strongly foliated eclogite-facies rocks in 30- 150 m thick shear zones of Caledonian age occur within a Grenvillian garnet granulite-facies gabbro-anorthosite terrain in the Bergen Arcs of Norway. The predominant eclogite-facies mineral assemblages in the shear zones are omphacite + garnet + zoisite + kyanite in gabbroic anorthosite and omphacite + garnet in gabbro. Eclogite-facies rocks in shear zones are generally fine-grained; alternating omphacite/ garnet- and kyanite/ clinozoisite-rich layers define gneissic layering. A strong shape preferred orientation of omphacite, kyanite, and white mica (phengitic muscovite and/or paragonite) define the foliation. The anorthositic eclogites show omphacite b-axis maxima approximately normal to the foliation and c-axis girdles within the foliation plane. P-wave velocities (V,) determined at confining pressures to 600 MPa for samples from eciogite-facies shear zones range from 8.3 to 8.5 km s-l and anisotropy ranges from 1 to 7%. The few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation. The fast propagation direction lies within the c-axis girdles (parallel to foliation) and the slow propagation direction is parallel to the b-axis concentration (normal to foliation) in samples for which omphacite crystallographic preferred orientation was determined, V, for the granulite-facies protoliths average about 7.5 km s - ‘. High calculated reflection coefficients for these shear zones, 0.04-0.14, indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high-pressure conditions but escaped thermal reactivation.

Precise eclogitization ages deduced from Rb/Sr mineral systematics: The Maksyutov complex, Southern Urals, Russia

Abstract The Maksyutov complex (Southern Urals, Russia) is a well-preserved example of subduction-related high-pressure metamorphism. One of its two litho-tectonic units consists of rocks that experienced eclogite-facies conditions. Published 40Ar/39Ar data on phengite, U/Pb data on rutile, and Sm/Nd mineral data define a cluster of ages around 370 to 380 Ma. Nevertheless, no consensus exists as to the detailed interpretation of data and the exact age of eclogitization. We present new, high-precision internal mineral Rb/Sr isochrons for eclogite-facies metabasites, felsic eclogites, and eclogite-facies quartz veins. Nine isochrons, mainly controlled by omphacite and white mica phases, give concordant ages with an average value of 375 ± 2 Ma (2σ). Microtextural features, such as prograde growth zoning in eclogite-facies phases, suggest that the assemblages dated formed at a stage of prograde metamorphism. Sr-isotopic equilibria among eclogite-facies phases, and among eclogite-facies fluid veins and the host rocks, indicate that our ages reflect crystallization ages, related to the prograde-metamorphic, probably fluid-mediated eclogitization reactions. This interpretation is reinforced by data from fluid-precipitated quartzitic eclogites, whose modal composition, together with intergrowth relationships, conclusively imply closed-system behavior after crystallization. The possible occurrence of a pre-375 Ma event of ultra-high-pressure metamorphism (UHPM) in the Maksyutov complex is disproved by isotope systematics, microtextures, and mineral zoning patterns.

Nitrogen-bearing, aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides

AbstractMinerals in eclogites from different localities in the Western Gneiss Region of the Norwegian Caledonides (age ≈425 Ma) contain a variety of fluid inclusions. The earliest inclusions recognized are contained in undeformed quartz grains, protected by garnet, and consist of H2O+N2 (with