Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

The Sveconorwegian orogeny

Recently it has been argued that the Sveconorwegian orogeny in southwest Fennoscandia comprised a series of accretionary events between 1140 and 920 Ma, behind a long-lived, active continental margin characterized by voluminous magmatism and high-grade metamorphism. Voluminous magnesian granitic magmatism is recorded between 1070 and 1010 Ma (Sirdal Magmatic Belt, SMB), with an apparent drop in activity ca. 1010-1000 Ma. Granitic magmatism resumed ca. 1000-990 Ma, but with more ferroan (A type) compositions (hornblende-biotite granites). This ferroan granitic magmatism was continuous until 920 Ma, and included emplacement of an AMCG (anorthosite-mangerite-charnockite-granite) complex (Rogaland Igneous Complex). Mafic rocks with ages corresponding to the spatially associated granites suggest that heat from underplated mafic magma was the main driving force for lower crustal melting and long-lived granitic magmatism. The change from magnesian to ferroan compositions may reflect an increasingly depleted and dehydrated lower crustal source. High-grade metamorphic rocks more than similar to 20 km away from the Rogaland Igneous Complex yield meta-morphic ages of 1070-1015 Ma, corresponding to SMB magmatism, whereas similar rocks closer to the Rogaland Igneous Complex yield ages between 1100 and 920 Ma, with an apparent age peak ca. 1000 Ma. Ti-in zircon temperatures from these rocks increase from similar to 760 to 820 degrees C ca. 970 Ma, well before the inferred emplacement age of the Rogaland Igneous Complex (930 Ma), suggesting that long-lived, high-grade metamorphism was not directly linked to the emplacement of the latter, but rather to the same mafic underplating that was driving lower crustal melting. Structural data suggest that the present-day regional distribution of high-and low-grade rocks reflects late-stage orogenic doming.

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Magma-driven, high-grade metamorphism in the Sveconorwegian Province, southwest Norway, during the terminal stages of Fennoscandian Shield evolution

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

Conditions of the prograde, peak-pressure and part of the decompressional P–T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe–Ti-rich eclogite record a first prograde and syn-deformational stage along a Barrovian gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5–19 kbar at ~850–900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite-bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near-isothermal decompression and partial exhumation produced plagioclase-bearing symplectites replacing kyanite and clinopyroxene at an estimated 850–870 °C and 10–11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P–T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite-bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrological relationships, which demonstrate foreland-vergent partial exhumation of the eclogite-bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian-aged tectonics. (Less)

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P–T evolution of Precambrian eclogite in the Sveconorwegian orogen, SW Sweden

A Fe–Ti-rich garnet, clinopyroxene, and quartz eclogite sample from the 1.0 Ga Sveconorwegian orogen, SW Sweden, contains abundant quartz, rutile, and zircon in distinct micro-textural sites: garnet core, garnet rim, and matrix, constituting an ideal case for investigation of the behavior of Zr-in-rutile and Ti-in-quartz at high-pressure and temperature. A P–T path, peaking at 16.5–19 kbar and 850–900 °C, has been constrained independently for the same rock by pseudosection modelling; input pressures from this model were used for trace element geothermometry of each garnet micro-textural domain. Trace element thermo(baro)metry, based on in situ Secondary Ion Mass Spectrometry analyses of Ti contents in quartz and Zr contents in rutile, yields P–T estimates of progressive crystallization of quartz and rutile along the prograde metamorphic path. For inclusions in the garnet cores, Zr-in-rutile geothermometry yields 700–715 °C and Ti-in-quartz 620–640 °C at 7 kbar. For inclusions in the garnet rims, temperature estimates are 760–790 °C (Zr-in-rutile) and 740–920 °C (Ti-in-quartz) at 12–18 kbar. Finally, matrix rutile records 775–800 °C and locally ~ 900 °C, and quartz records temperatures up to 900 °C at 18 kbar. Ti-in-quartz estimates for the metamorphic peak (inclusions in the garnet rims and matrix) conform to the pseudosection, but appear too low for the early prograde stage (garnet cores), possibly due to lack of equilibrium at T < 700 °C. The pseudosection shows that rutile was produced by continuous ilmenite breakdown during the early stages of prograde metamorphism, a reaction that was completed at ~ 730 °C. Rutile grains in the garnet rims and the matrix grew subsequently larger by recrystallization of previously produced rutile. However, recrystallized rutile does not predominantly record peak temperatures, but instead yield 745–840 °C between 12 and 18 kbar. In the pseudosection, this temperature range broadly coincides with a stage during which (Ti-bearing) hornblende was consumed and clinopyroxene produced (i.e., dehydration); the Zr contents thus appear to reflect the last stage of efficient rutile recrystallization, catalysed by fluids released by the dehydration of hornblende preceding the metamorphic peak. Concurrently, combination of the isopleths for Ti content in quartz and Zr content in rutile (i.e. independent from pseudosection modelling) yields pressure and temperature conditions in almost perfect agreement with the P–T path as deduced from the pseudosection. The variation in Ti concentration in quartz is small regardless of crystal size, and the Ti-in-quartz geothermometer provides both precise and accurate peak temperatures of 875–920 °C, without a significant diffusional reequilibration. The lack of significant Ti diffusion in quartz is consistent with an inferred short residence time at high temperature. This study illustrates that Zr-in-rutile and Ti-in-quartz geothermobarometry can robustly constrain prograde P–T conditions and yield further insights into recrystallization processes at high temperature. The combination of these methods and integration of the results with pseudosection modelling is a versatile tool for investigating the petrologic history of high-grade rocks.

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Tracking the prograde P–T path of Precambrian eclogite using Ti-in-quartz and Zr-in-rutile geothermobarometry

High-temperature deformation in the basal shear zone of an eclogite-bearing fold nappe, Sveconorwegian orogen, Sweden

Rare earth element (REE) signatures of high‐U/Pb and high‐Th/Pb accessory minerals are typically used to link their ages to specific petrological processes (‘petrochronology’)—most notably the growth or breakdown of garnet. Although this approach is powerful, gaps in our understandings of REE systematics in high‐grade rocks exist, particularly regarding the degree to which these minerals chemically equilibrate at extreme conditions. To investigate this, we performed comparative chronology and REE analysis of garnet, zircon, and monazite in (1) a fluid‐rich, ultrahigh‐pressure (UHP) migmatite from the Western Gneiss Region, Norway, and (2) dry felsic granulite xenoliths from the Pamir, Tajikistan. Zircon and monazite from the hydrous migmatite provided ages of 450–370 Ma and a range of Gd/Yb values across this time span. A Lu‐Hf garnet age of 422 ± 2 Ma coincides with the age at which zircon and monazite exhibit the highest Gd/Yb values, as well as the largest range therein. The degree of dispersion in these values is substantial, especially for monazite. Zircon and monazite in the dry xenoliths provide age clusters between 50 and 11 Ma, recording pulsed growth and recrystallization. The Lu/Hf garnet ages for these samples are 41–38 Ma. The accessory minerals of that age are texturally associated with garnet, yet have the lowest, not the highest, Gd/Yb values. In both cases, there is evidence that co‐genetic garnet and accessory minerals achieved REE equilibrium during growth. However, the age and compositional record of chronometric accessory minerals that were co‐genetic with garnet are distinctly different between the two case examples. In the hydrous migmatite, supply‐limited garnet growth in the presence of fluid phase resulted in strongly zoned garnet and a correspondingly large range in Gd/Yb values among co‐genetic zircon and monazite. The range in Gd/Yb values match among these phases and collectively capture the strong fluctuations in the REE composition of matrix micro‐domains caused by garnet growth. In this case, it is the range in Gd/Yb values, rather than a specific composition that is diagnostic for garnet growth. In the felsic granulites, garnet and accessory minerals growth occurred in a fluid‐limited regime, in which short fluid pulses triggered reactions that had been likely significantly overstepped. The seemingly contradictory pattern of increasing Gd/Yb after garnet growth in these samples is the result of garnet having high Gd/Yb and continued zircon growth forcing further HREE depletion of the matrix with time. The situation that high, rather than low, Gd/Yb values in zircon and monazite indicate equilibrium with garnet may be common in dry granulites and other anhydrous high‐temperature rocks. Our findings provide an improved framework for the reliable identification of the accessory minerals that equilibrated with garnet, even when the REE‐age record of these minerals is complex, dispersed, and seemingly contradictory. In addition, the combined zircon‐, monazite‐, and garnet‐age and REE‐record refines the regional tectonic interpretation of our samples.

Lorraine Tual

Papers

P–T evolution of Precambrian eclogite in the Sveconorwegian orogen, SW Sweden

Tracking the prograde P–T path of Precambrian eclogite using Ti-in-quartz and Zr-in-rutile geothermobarometry

High-temperature deformation in the basal shear zone of an eclogite-bearing fold nappe, Sveconorwegian orogen, Sweden

Garnet, zircon, and monazite age and REE signatures in (ultra)high‐temperature and high‐pressure rocks: Examples from the Caledonides and the Pamir

Rapid, paced metamorphism of blueschists (Syros, Greece) from laser-based zoned Lu-Hf garnet chronology and LA-ICPMS trace element mapping