Synnøve Elvevold

N2 and CO2 in deep crustal fluids: evidence from the Caledonides of Norway

Abstract High-pressure metamorphic rocks (eclogites and high-pressure granulites) occur along the entire length of the Norwegian Caledonides, and have formed from a variety of protoliths. In some cases, the relationship between protoliths, high-pressure rocks and their later retrogression products have been preserved in-situ. Fluid-inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition: (1) Eclogites and high-pressure granulites contain N 2 -bearing fluids (pure N 2 , or mixtures with CO 2 or H 2 O, with X N 2 > 5%). In some eclogite-facies rocks, CO 2 /1bN 2 inclusions are associated with aqueous brine inclusions (ca. 30 wt% NaCl), the two compositions representing immiscible fluids at peak metamorphic conditions. (2) Granulite -facies protoliths and eclogites reworked in the granulite-facies contain pure CO 2 or CO 2 -dominated fluids with less than 2.5% N 2 . (3) Rocks retrograded in the amphibolite facies contain H 2 O/1bNaCl fluids. Immiscibility between brine and anhydrous N 2 /1bCO 2 fluid, and between anhydrous fluid and waterbearing aluminosilicate-melt have taken place in some eclogites. During high-grade metamorphism, nitrogen may be incorporated in minerals, as NH 4 substituting for K, or it may occur as N 2 in the free fluid phase. The partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism, low a H 2 O and/or high f O 2 partitioning nitrogen to the fluid phase. A rock interacting with a carbonic fluid at granulite-facies PT conditions will be depleted in mineralogically bound nitrogen. In cases where the protoliths of high-pressure rocks have been through a previous, granulite-facies event, a local source for the nitrogen contained in high-pressure fluid is therefore unlikely.

Discovery of Paleozoic Fe-Mg carpholite in Motalafjella, Svalbard Caledonides: A milestone for subduction-zone gradients

Paleozoic blueschist facies rocks are relatively scarce on Earth due to warmer geother- mal gradients at that time and/or later reequilibration. Ferro-magnesiocarpholite (Fe-Mg carpholite), the typical low-temperature blueschist facies index mineral in metapelites, was discovered 30 yr ago and is known only in Tethyan belts metamorphosed ,80 m.y. ago. Herein we report the discovery of Paleozoic Fe-Mg carpholite in the ca. 470 Ma blueschists of Motalafjella, Svalbard Caledonides, the oldest known occurrence on Earth. The car- pholite-bearing rocks reached pressure-temperature (P-T) conditions of 15-16 kbar and 380-400 8C and followed a nearly isothermal exhumation path. In the cooling Earth per- spective, these P-T estimates for Motalafjella blueschists demonstrate the existence of cold subduction-zone gradients (;7 8C/km) from the middle Paleozoic onward.

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.

Calculated phase equilibria for phengite-bearing eclogites from NW Spitsbergen, Svalbard Caledonides

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature (P–T) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.

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.