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Description
When continental plates collide with each other, then the margin of one continent dunks temporarily underneath the other. The maximum depth, from which crustal rocks (gneiss) returned to the surface, increased as more as our planet matured. Exhumation depths of 90 km are documented in many orogens worldwide since about 630 million years ago and of 120 km since about 530 million years ago. One problem relates to the transformation of these ultra-high pressure (UHP) metamorphic rocks on their way back to the surface. Another complication is that evidence for UHP is preferentially preserved in a minor rock type (eclogite) enclosed in gneiss, which dominantly lacks such evidence. The proposed study aims to investigate different types of eclogite exposed in the Western Gneiss Region (WGR) of Norway, which is one of the best studied and largest UHP terrains. These eclogites occur inside and outside of distinct UHP areas recognised to bear UHP metamorphic rocks. This study will compare the eclogites’ mineral and fluid inclusions, mineral chemical zoning and the water content of nominally water-free minerals. Evaluation of these data is expected to provide answers on whether different types of eclogite had different or similar evolutionary stages. The degree of retrogression is particularly important. A systematically higher degree of retrogression in normal eclogite than in UHP eclogite would imply that a much larger area of exposed continental crust was buried to minimum depth of 90 km than recognised so far. A similar conclusion has recently been drawn from associated but exotic mantle fragments enclosed in the same gneiss. If confirmed then a direct consequence is that retrogression was spatially variable and may have influenced metamorphic and isotopic age data of eclogite used for tectonic models to explain the exhumation of UHP metamorphic rocks in the WGR. It follows that applicable models need to be revised.
Summary of project results
The secular cooling of our planet allowed the emergence of plate tectonics more than 2.5 billion
years ago. These plate movements will inevitably come to a halt in the future, an event expected in
about 0.5 billion years. In the meantime, the nature of plate tectonics has changed. This is evidenced
by surface rocks that have formed since the late Proterozoic (about 0.6 billion years ago) at depths
of more than 90 km in zones of formerly colliding plates (orogens). Surface exposures of such rocks
have not been found earlier in Earth’s history. The project investigated such so‑called ultrahigh pressure
(UHP) metamorphic rocks from western Norway. The aim was to better understand the preservation
of these high‑grade rocks during their long journey to the surface, which is a prerequisite for
a viable reconstruction of the underlying mechanisms in a maturing Earth.
Over more than two decades, intensive research on mafic rocks (eclogite) enclosed as isolated
rock bodies in continental gneiss in western Norway has suggested a striking pattern where UHP
eclogite is exposed alternating with high pressure (HP) eclogite (formed at depths of less than 90 km).
This zebra‑stripe‑like pattern has been incorporated into tectonic models to explain the orogenic
collapse in the Scandinavian Caledonides. The eclogites are closely associated with other isolated
rock bodies: garnet‑bearing ultramafic mantle rocks (peridotite and pyroxenite). Recently, it has
been shown that all of these ultramafic rocks contain evidence of an origin at UHP conditions, i.e.
regardless of whether they are associated with HP or UHP eclogite (Spengler et al., 2003, Journal of
Petrology, 62, egab008). This finding directly challenges the significance of the alternating HP‑UHP
metamorphic pattern and thus the validity of the corresponding tectonic models in the region. It
also questions the ability to identify the maximum metamorphic conditions in rocks with extreme
metamorphism in plate collision zones.
The aim of the project was to answer the following questions:
• What is the difference between eclogite occurring in HP and UHP metamorphic zones, apart
from the inferred (preserved) metamorphic conditions?
• Does the retrogression history of both eclogite types differ?
• Is there really no evidence of possible UHP metamorphism in HP eclogite?
• Is there evidence of obliteration of a UHP metamorphism?
The eclogites studied were assembled from a region that stretches from Moldefjord to Storfjord in
western Norway and covers two UHP areas and one HP area in between. Samples were collected
from pre‑existing rock collections and during field work. After initial microscopy, the sample selection
was refined to ten samples of which five contain orthopyroxene and the other five are without.
Four samples were from known UHP areas, the other six from the interjacent HP‑area and all cover
a region that is approximately 40 × 70 km2 in size. Detailed petrography revealed that all eclogite samples contain clinopyroxene with elongated mineral inclusions that are oriented parallel to each other. Raman spectroscopy identified these inclusions
to be of either quartz or quartz + pargasite or albite. Such inclusions were described before
to occur in very few eclogites with UHP metamorphic history in western Norway. The finding in all
studied samples was surprising. All samples were investigated for mineral chemistry including traces of water. The mineral chemistry
revealed that oriented lamellae of quartz formed by a chemical reaction in which a precursor
clinopyroxene with vacancies in the crystal structure broke down during decompression from UHP.
Part of the quartz lamellae subsequently suffered alteration by another, pressure‑sensitive chemical
reaction that transformed these lamellar quartz inclusions finally to albite. This sequence demonstrates
that the oriented mineral inclusion microstructure formed in eclogite early during its way up
to the surface. Low water contents imply that the decompression environment was initially relatively
dry. In consequence, lamellar pargasite, which is a hydrous mineral, likely formed concurrently with
lamellar quartz in those samples that had slighly more hydroxyl in the precursor clinopyroxene.
Samples that contain orthopyroxene allowed directly for the calculation of metamorphic conditions
from the mineral chemistry. They gave overlapping estimates of maximum metamorphic
pressures and temperatures deep in the stability field of diamond for eclogites from both HP and
UHP areas. Unless the samples suffered intense retrogression that partially replaced the primary
mineral assemblage by secondary (late), often hydrous minerals. These late hydrous conditions are
mirrored by an inverse relationship between metamorphic pressure preserved in the mineral chemistry
and trace amounts of water in these minerals.
The project shows that the maximum metamorphic conditions found in many eclogites from western
Norway over decades are not those that the rocks have ever experienced. Mineral microstructures
were shown to be robust to survive chemical overprints during retrogession. The project results
unite different UHP areas into one, making the UHP area in western Norway one of the largest
in the world. It follows that the zebra‑stripe‑like pattern of different metamorphic grades (HP and
UHP) in western Norway is unlikely suitable to constrain tectonic models of exhumation of UHP metamorphic
rocks.
The merging of different UHP areas into a single one means that the full spatial extent of exposure
of rocks with UHP history in western Norway is currently unknown and needs to be reassessed. The
realisation that the size and shape of previously well‑studied UHP areas in western Norway, and even
the intensity of metamorphism, have been underestimated, raises questions about the validity of the
results in other orogens. Therefore, the results of this project are expected to fundamentally impact
the UHP metamorphic community.
The finding that eclogite with most hydrous clinopyroxenes occurs in HP areas (for which there is
independent evidence of UHP metamorphism) suggests that the alternating pattern of HP and UHP
areas may reflect spatial differences in the degree of late retrogression. If confirmed, then metamorphic
isogrades obtained from eclogite mapped over decades and considered representative of
the highest metamorphic conditions in the region represent post‑peak metamorphic information.
This might be of interest to studies that use 3D numerical modelling to investigate the interaction between fluid pathways, metamorphism and rheology during orogenic collapse. The few samples
from this project cannot trace this retrogression pattern in detail, which will require a follow‑up
study.
The origin of aligned hydrous mineral inclusions in nominally anhydrous minerals is controversially
discussed in the literature. The constraints on the isochemical origin of aligned quartz + pargasite
lamellae in clinopyroxene from this project imply that dehydroxylation of clinopyroxene occurs
upon decompression, provided that the precursor clinopyroxene had sufficient structural water in
the crystal lattice. This project is an example of how petrology can be used to determine whether an
external hydrous source is required to explain such mineral inclusion microstructures.