Ane K. Engvik

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.

Structural, mineralogical and petrophysical effects on deep crustal rocks of fluid‐limited polymetamorphism, Western Gneiss Region, Norway

The Proterozoic banded granulite facies complex of BÅrdsholmen, Western Gneiss Region, Norway (T=815–845°C) was locally transformed to eclogite (T=455–510°C, P>12 kbar) and amphibolite facies rocks (T=460°C) during the Caledonian continental collision. The granulite complex consists of mafic two‐pyroxene granulite and leucocratic orthopyroxene+garnet‐bearing layers alternating on a scale from 1 cm to 10 m. The granulite facies rocks change to eclogite facies rocks over centimetre‐scale distances along well defined fluid‐infiltration fronts. The mafic granulite was transformed to omphacite+garnet‐rich eclogites and the leucocratic rocks were converted to quartz+phengite‐rich assemblages with minor garnet and local omphacite. Melange‐like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies. During amphibolitization this melange‐like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss, a lithology typical of the Western Gneiss Region. The BÅrdsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism, the structural make up and petrophysical properties such as density and rheology of crustal root zones. Fluid‐induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and Moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust. We suggest that orogens may develop differently depending on the fluid budget.

Lead and bromine enrichment in eclogite-facies fluids: Extreme fractionation during lower-crustal hydration

Hydration reactions associated with eclogite-facies metamorphism of granulites in the Norwegian Caledonides led to the formation of saline brines with exceptional compositions. Primary omphacite- and garnet-hosted fluid inclusions from Norwegian eclogites formed by hydration of granulites contain a plethora of solid phases. A total of 18 different minerals have been identified in multiphase brines, and 12 of these occur at the Badsholmen locality in Sunnfjord. Fluid inclusions from this locality are characterized by a high Br/Cl mass ratio (0.03) and extreme Pb enrichment as demonstrated by the presence of Pb-bearing daughter minerals including galena, a Pb-Cl–bearing phase, and a Pb-Cl-Br–bearing phase. The fluid compositions may reflect enrichment of elements incompatible with the silicate structure during consumption of H 2 O by eclogite-forming hydration reactions. Pb is believed to have been scavenged from the parent granulite by the increasingly saline brine during K-feldspar breakdown.

Fluid Transport, Deformation and Metamorphism at Depth in a Collision Zone

According to geological maps, the exposed continental crust contains abundant anhydrous granulite facies and igneous rocks. Exposed crustal sections (Fountain and Salisbury, 1981) suggest that such rocks are even more abundant in the deep continental crust. During continental collisions, anhydrous granulite facies and igneous rocks can be explaced into deep crustal root zones and become subjected to P—T conditions where their original mineral assemblage is not stable. Numerous field and petrological investigations have shown that anhydrous rocks can survive even eclogite facies conditions with their original mineralogy and structure intact (Wayte et al., 1989; Rubie, 1990). Experimental work and field examples suggest that overstepping of reaction boundaries by at least 5 kbar at temperatures as high as 700°C is possible without re-equilibrium.

Pan-African extension and near-isothermal exhumation of a granulite facies terrain, Dronning Maud Land, Antarctica

T he M¨ uhlig-Hofmann- and Filchnerfjella in central Dronning Maud Land, Antarctica, consist of series of granitoid igneous rocks emplaced in granulite and upper amphibolite facies meta- morphic rocks. The area has experienced high-temperature metamorphism followed by near-isothermal decompression, partial crustal melting, voluminous magmatism and extensional exhumation during the later phase of the late Neoproterozoic to Cambrian Pan-African event. Remnants of kyanite- garnet-ferritschermakite-rutile assemblages indicate an early higher-pressure metamorphism and crustal overthickening. The gneisses experienced peak granulite facies temperatures of 800-900 ◦ Ca t intermediate pressures. Breakdown of garnet + sillimanite + spinel-bearing assemblages to cordierite shows subsequent re-equilibration to lower pressures. An E-W foliation dominating the gneisses illustrates transposition of migmatites and leucocratic melts which evolved during the near-isothermal decompression. Occurrence of extensional shear bands and shear zones evolving from the ductile partial melting stage through semiductile towards brittle conditions, shows that the uplift persisted towards brittle crustal conditions under tectonic W/SW-vergent extension. Late-orogenic Pan-African quartz syenites intruded after formation of the main gneiss fabric contain narrow semiductile to brittle shear zones, illustrating that the extensional exhumation continued also after their emplacement. The latest record of the Pan-African event is late-magmatic fluid infiltration around 350-400 ◦ C and 2 kbar. At this stage the Pan-African crust had undergone 15-20 km exhumation from the peak granulite facies conditions. We conclude that the later phase of the Pan-African event in central Dronning Maud Land is characterized by a near-isothermal decompression P-T path and extensional structures indicating tectonic exhumation, which is most likely related to a late-orogenic collapsing phase of the Pan-African orogen.

Metamorphic constraints on the Caledonian Upper Allochthon of Central Norway: the Gula Complex staurolite–garnet–kyanite mica schist

Abstract Petrological studies of staurolite–garnet–kyanite–biotite schist and garnet–muscovite schist of the Gula Complex, central Norway, provide constraints on metamorphic evolution during Scandian continent–continent collision, burial and exhumation of the Caledonian Upper Allochthon. The biotite schist contains conspicuous porphyroblasts of Fe-rich staurolite, garnet and kyanite, set in a fine-grained, well-foliated matrix of biotite, quartz, minor plagioclase and muscovite. The muscovite schist is fine- to medium-grained with a muscovite–quartz-dominated matrix, including garnet, biotite, minor plagioclase and clinozoisite. Pressure–temperature (P–T) modelling based on thermobarometric calculations and construction of P–T pseudo-sections illustrate that metamorphism reached 680 °C with pressures estimated up to 1.01±0.11 GPa. Retrogression and decompression are constrained by secondary mineral reactions: local replacement of kyanite to fibrous sillimanite indicates decompression below 0.7 GPa. Growth of foliation-parallel chlorite reflects cooling below 640 °C and the chlorite formation proceeded during cooling and decompression towards 550 °C and 0.4 GPa. The metamorphism is associated with a strong north–south-trending regional foliation, and retrogression and decompression apparently continued within the same strain regime. The P–T modelling shows that even small variations in whole-rock chemistry and P–T conditions can explain heterogeneity and significant shifts in mineralogy and modal concentration of the index minerals of metapelites.

Dynamic Metasomatism: Stable Isotopes, Fluid Evolution, and Deformation of Albitite and Scapolite Metagabbro (Bamble Lithotectonic Domain, South Norway)

New stable isotopic data from mineral separates of albite, scapolite, amphibole, quartz, and calcite of metasomatic rocks (Bamble lithotectonic domain) give increased knowledge on fluid type, source, and evolution duringmetamorphism. Albite from a variety of albitites givesδ18OSMOW values of 5.1–11.1‰,while quartz fromclinopyroxene-bearing albitite gives 11.5–11.6‰. δ 18OSMOW values for calcite samples varies between 3.4 and 12.4‰ and shows more consistent δC values of −4.6 to −6.0‰. Amphibole from scapolite metagabbro yields a δ18OSMOW value of 4.3 to 6.7‰and δDSMOW value of−84 to−50‰,while the scapolite gives δ 18OSMOW values in the range of 7.4 to 10.6‰.These results support the interpretation that the originalmagmatic rocks weremetasomatised by seawater solutions with a possible involvement from magmatic fluids. Scapolitisation and albitisation led to contrasting chemical evolution with respect to elements like P, Ti, V, Fe, and halogens. The halogens deposited as Cl-scapolite were dissolved by albitisation fluid and reused as a ligand for metal transport. Many of the metal deposits in the Bamble lithotectonic domain, including Fe-ores, rutile, and apatite deposits formed during metasomatism. Brittle to ductile deformation concurrent with metasomatic infiltration illustrates the dynamics and importance of metasomatic processes during crustal evolution.

Metasomatic Formation and Replacement of Apatite (Bamble Sector, South Norway)

The Odegarden verk apatite deposit is hosted in scapolitised metagabbro. Chlorapatite (Cl < 6.8 wt%) occurs dispersed in the metagabbro and as coarse crystals in apatite–phlogopite veins, crystallised and deposited during scapolitisation. Secondary pseudomorphic replacement related to albitisation transformed chlorapatite to porous hydroxy-fluor-apatite. Mass balance calculations show that scapolitisation of the Odegarden gabbro caused extensive depletion in Fe2O3, progressive depletion in CaO and Al2O3 and increase in MgO and Na2O. The apatite–phlogopite veins are markedly enriched in P2O5, K2O and MgO. Albitisation shows extensive depletion in Fe2O3, depletion in TiO2, MgO and CaO, while an increase in Na2O, Al2O3 and SiO2. Bamble sector is characteristically affected by metasomatism along a 100-km long and 25-km wide area, resulting in chemical and mineralogical transformation of continental crust. Metasomatic formation and replacement of apatite at Odegarden verk is in accordance with the regional distribution of Ap-deposits occurring related to scapolitised metagabbros in the area.

Late Pan-African Fluid Infiltration in the Mühlig-Hofmann- and Filchnerfjella of Central Dronning Maud Land, East Antarctica

The nunataks of Muhlig-Hofmannfjella and Filchnerfjella in central Dronning Maud Land, East Antarctica, comprise a deep-seated metamorphic-plutonic rock complex, dominated by a dark colour due to dark feldspar and containing granulite facies minerals including perthite, plagioclase, orthopyroxene and garnet. The area was affected by a late Pan-African fluid infiltration outcropping as conspicuous light alteration zones restricted to halos around thin granitoid veins. The veins were formed during infiltration of volatile-rich melts, probably originating from underlying magma-chambers. The alteration halos were formed by CO2-H2O-volatiles emanating from the veins into the host rock causing hydration of the granulite facies assemblages. The alteration involves a breakdown of orthopyroxene to biotite and sericitisation of plagioclase at crustal conditions around 350–400°C and 2 kbar. The marked colour change is caused by transformation of feldspars, spread of dusty micas, opaques and fluid inclusions in addition to replacement of coarse to finer grains. The process is locally penetrative indicating that fluid infiltration can affect large rock volumes. The frequent distribution of alteration zones throughout the mountain range independent of lithological variations shows that the fluid infiltration is regionally extensive.