Christopher J. Barnes

A detailed geological background, all analytical methodologies, all acquired data (Tables S1–S4), and one figure highlighting plagioclase chemical features.

A detailed geological background, all analytical methodologies, all acquired data (Tables S1–S4), and one figure highlighting plagioclase chemical features.

The exhumed metamorphic core of the Himalayan orogen comprises meta-sedimentary and meta-igneous rocks that are pervasively deformed, with a tectonic foliation typically defined by the alignment of muscovite/biotite. Three specimens from different lithotectonic units were selected for in situ Rb–Sr and K–Ca geochronology of muscovite and biotite to compare the geological histories recorded therein. Geochronological data and chemistry from the same ablated volumes were integrated with detailed microstructural observations/analyses. In all cases, the two isotopic systems are decoupled, yielding different dates. From higher to lower structural levels, the K–Ca dating of muscovite and biotite yields isochron dates of 26 ± 5 Ma, 33 ± 6 Ma, and 39 ± 10 Ma. In contrast, Rb–Sr geochronology on biotite returned dates of 17 ± 0.2 Ma, ca. 15 Ma, and 6 ± 0.7 Ma, while muscovite Rb–Sr analyses from the same specimens define dates of 46 ± 1 Ma, a range from 1630–5 Ma, and 40 ± 9 Ma, respectively. The late Miocene Rb–Sr biotite dates are consistent with existing records of exhumation, whereas the muscovite Rb–Sr dates record events ranging from early Proterozoic protolith crystallization to late prograde Eocene metamorphism. In contrast, the K–Ca muscovite dates are more concise, interpreted to reflect late prograde to early retrograde Himalayan orogenesis. The decoupling of Rb–Sr and K–Ca systems enable a single mineral phase, and in some cases a single analysis, to provide multi-temporal record of orogenesis, opening new opportunities to resolve the timing and duration of crustal processes in metamorphic belts worldwide.

The mineralogy and textures of several fragments from the Ribbeck aubrite were analyzed using a combination of scanning electron microscopy, electron microprobe analysis, μRaman spectroscopy, and laser ablation inductively coupled reaction cell mass spectrometry (LA‐ICP‐MS/MS). The meteorite fragments are strongly brecciated and show evidence of shock melting. The silicate phases of the fragments predominantly consist of enstatite, albite, and roedderite, with subordinate forsterite, diopside, K‐feldspar. Non‐silicate phases include kamacite, troilite, oldhamite, and Fe‐Ti, Fe‐Mn, and Fe‐Cr rich sulfides. Tridymite and cristobalite, the latter contained in Si‐rich glass, are also present within one enstatite grain. Vesicular fusion crust is apparent in several fragments. The discovery of roedderite is the first reported for the Ribbeck aubrite. In situ Rb/Sr dating combined with chemical analysis of the roedderite was performed via laser ablation ICP‐MS/MS using the single‐spot dating approach. The average chemistry of 20 roedderite analyses is 68.7 wt% of SiO ₂ , 23.1 wt% of MgO, 4.9 wt% of K ₂ O, and 3.3 wt% of Na ₂ O and is −1 , respectively. Single‐spot Rb/Sr dates from the same 20 analyses yielded a weighted average of 4570 ± 27 Ma, interpreted as the formation age of the Ribbeck aubrite parent body. The result highlights advantages of single‐spot Rb/Sr dating compared to short‐lived isotopic systems (e.g., ²⁶ Al‐ ²⁶ Mg, ⁵³ Mn‐ ⁵³ Cr, and ¹²⁹ I‐ ¹²⁹ Xe), long‐lived systems with radiogenic noble gasses (e.g., ⁴⁰ K‐ ⁴⁰ Ar and ^238/235 U‐ ²³² Th‐ ⁴ He), and the conventional Rb/Sr isochron approach for meteorite cosmochronology.

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides preserves a record of HP-UHP metamorphism related to continental subduction along the Baltican margin. Geochronological studies traditionally identified a late Cambrian (c. 490–480 Ma) (U)HP event in the northern SNC, whereas the southern SNC was interpreted to record younger Ordovician metamorphism at c. 470–455 Ma (Gee et al. 2020, and references therein), leading to models of localized late Cambrian subduction restricted to the north.In this contribution, we present a new approach using in situ Lu-Hf geochronology on polymetamorphic garnets to further constrain the subduction histories in the SNC. It is a powerful tool to directly date garnet growth associated with (U)HP conditions and allows recovery of early metamorphic histories in the Scandinavian Caledonides.Preliminary results show ages of 495-480 Ma for a paragneiss in Marsfjället, a garnet schist in Avardo, two garnet schists in Lillfjället, two eclogites in Sjouten, and a schist in EASU. This extends the late Cambrian-early Ordovician subduction record to the central SNC. Furthermore, three Avardo eclogites yield ages of 460-450 Ma, indicating that the central SNC was affected by two metamorphic events, both possibly (U)HP.In the ongoing project, in situ Lu-Hf dating will be applied on garnets farther south in the SNC to constrain the spatial extent of late Cambrian subduction of Baltica. References:Gee, D.G., Klonowska, I., Andréasson, P.G. and Stephens, M.B. 2020. Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion. Geological Society Memoir, 50, 517–548, https://doi.org/10. 1144/M50-2018-73

The western periphery of Baltica has been viewed as a passive continental margin formed during the fragmentation of Rodinia and the opening of the Iapetus and Tornquist Oceans. This view is supported by the Volyn Large Igneous Province (VLIP) of Ediacaran age in Eastern Europe, which may be associated with the opening of the Tornquist Ocean. However, the sedimentary succession overlying the VLIP in western Ukraine contains latest Ediacaran to early Cambrian detrital zircon with mixed ƐHf(t) values that can be interpreted to reflect deposition in a convergent margin setting. To further investigate this issue, we conducted research in the Holy Cross Mts. (HCM), Poland, where tightly folded and slightly metamorphosed middle Cambrian sandstone and slates are unconformably overlain by Lower Ordovician sedimentary rocks. We applied 40Ar/39Ar single grain fusion geochronology on white mica defining cleavage in lower Cambrian rocks and analysed detrital zircons to constrain their age and ƐHf(t) signature. Nine samples of shale and sandstone collected from the HCM showed consistency between stratigraphic age and the calculated maximum depositional age (MDA), ranging between c. 502-538 Ma. All samples have similar Proterozoic and Archean detrital zircon populations with major age peaks at c. 1200, 1500, 1800, and 2100 Ma, suggesting affinity with Baltica-associated sources. New U-Pb detrital zircon ages suggest that the HCM remained a coherent unit throughout the Cambrian and Early Ordovician. Importantly, ƐHf(t) signatures from Ediacaran-Cambrian detrital zircon of the HCM display a wide spread of values from -18 to +12. We interpret these results to reflect a continental magmatic arc setting, where there is significant mixing between mantle derived magmas and evolved crustal material. 40Ar/39Ar geochronology on white mica from one sample yielded two age populations. We interpret a group of ages between 537-640 Ma as detrital, while a younger group of ages yielded a weighted mean age of 510 ± 3 Ma (MSWD = 1.5). Interestingly, this younger age is corroborated by the presence of low-U zircon rims on detrital zircon from the same sample, which have an age of c. 510 Ma. We interpret this c. 510 Ma age population in both muscovite and zircon rims to record docking of a peri-Gondwana terrane, collision with a Baltica-derived terrane or the subduction of a seamount or oceanic plateau and shallowing of the down-going plate triggering deformation. With the present results, none of them can be ruled out. Mixed ƐHf(t) signatures and geochronologic evidence for deformation support the presence of an active margin at the periphery of southwestern Baltica during the Ediacaran and Cambrian. Furthermore, we suggest that this new 40Ar/39Ar geochronologic data may provide a new age constraint for early Gondwanan assembly.This work was funded by the National Science Centre (Poland) project no. 2019/33/B/ST10/01728 to Majka.

In situ geochronology using laser ablation is essential for integrating microstructural and chemical records of minerals with their geochronological information. The in situ method is especially important for potassic mica, which readily deforms, often define rock structures and fabrics, and are susceptible to chemical modification by reactive fluids. In situ 40Ar/39Ar geochronology of mica has been widely used for decades and in situ Rb/Sr geochronology has recently emerged as a prevalent method. In contrast, in situ K/Ca geochronology has been demonstrated, but the method has not been refined or utilized. Combined in situ K/Ca and Rb/Sr geochronology was developed at the Fipke Laboratory for Trace Element Research (University of British Columbia, Okanagan), allowing direct comparison of the two isotopic systems and exploring the potential of K/Ca dating. The methodology was tested using two-mica leucogranites and migmatitic paragneisses from the central Seve Nappe Complex (SNC), comprising remnants of Baltican continental crust in the Scandinavian Caledonides. The central SNC consists of continental crustal rocks hosting volumetrically-subordinate eclogites, peridotites, and pyroxenites. The crustal lithologies record partial melting beginning at c. 484-480 Ma, while the (ultra)mafic lithologies provide evidence for high-pressure metamorphism at c. 460-454 Ma, altogether reflecting subduction-exhumation cycles of the central SNC. Subsequent collision of the Baltican continent with Laurentia led to widespread deformation and metamorphism in the SNC, starting at c. 430-425 Ma. Continental anatexis produced the studied leucogranites and migmatitic paragneisses; these rocks comprise quartz, white mica, biotite, plagioclase, K-feldspar, and clinozoisite, with accessory apatite, monazite, xenotime, and zircon. The accessory phases were dated via in situ U-Pb geochronology to compare with in situ white mica and biotite K/Ca and Rb/Sr geochronology results. The dates yielded by apatite (424 ± 15 Ma, 428 ± 17 Ma), monazite (427 ± 2 Ma), and xenotime (426 ± 2 Ma) are all within uncertainty (pooled age of 426.5 ± 1.4 Ma) and record the timing of continental collision. Zircon provide a wider range of concordant U-Pb dates (473 ± 8 Ma to 419 ± 5 Ma). White mica and biotite Rb/Sr isochron dates from all four examined rocks (438 ± 5 Ma, 430 ± 4 Ma, 433 ± 4 Ma, 431 ± 5 Ma) are slightly older than the apatite/monazite/xenotime and youngest zircon U-Pb dates, but still record continental collision. The same mica volumes yielded older K/Ca isochron dates (484 ± 21 Ma, 482 ± 14 Ma, 486 ± 15 Ma, 486 ± 12 Ma), similar to the oldest concordant zircon U-Pb dates and the overall timing of partial melting in the broader central SNC crustal rocks. The different K/Ca and Rb/Sr dates indicate that these two isotopic systems are decoupled in the same mica volumes, controlled either by interactions of Ca and Sr with the mica lattice or by diffusion gradients influenced by bulk rock composition. The results presented herein demonstrate that the K/Ca isotopic system has the potential for retrieving older geologic histories in polymetamorphic terranes. Further detailed investigations and continued development of the methodology, including improved reference materials, are required.

A schist and a marble are used to investigate 40Ar and Sr behaviour in white mica that show distinct records of deformation and fluid-mediated chemical exchange. The rocks were obtained from the upper level of the Middle Allochthon in the northern Scandinavian Caledonides. They underwent eclogite-facies metamorphism (2.4-2.6 GPa/590-660°C) at c. 486-481 Ma and were deformed during juxtaposition with the overlying Upper Allochthon in lower amphibolite/greenschist facies conditions at c. 430-420 Ma. Deformation of the schist is recorded by anastomosing shear bands that delimit polymineralic lenses of white mica, quartz, garnet, and apatite. White mica are only locally deformed along shear bands but show irregular zoning indicative of dissolution-reprecipitation, with high-celadonite zones (XCel: 0.22) generally enveloped by low-celadonite zones (XCel: 0.09). Fluid activity during rock deformation is evinced by the presence of chlorite in shear bands and surrounding partially retrogressed garnet, as well as dissolution-reprecipitation of plagioclase. Mobilization of trace elements is evident, with low-celadonite zones enriched in V, Sr, Nb, Ba and depleted in Li, Ti, Co, Zn relative to high-celadonite zones. The former also shows slight enrichment of average B (35.9 µg/g) compared to the latter (27.2 µg/g), but δ11B values are the same for both zones (-13.5 and -13.6 ‰), suggesting locally-derived fluids. Deformation and foliation development of the marble is characterized by shape preferred orientation of calcite and white mica. The mica show variable grain size and are all deformed, highlighted by electron channeling contrast imaging that reveals abundant µm-scale kink bands within individual grains, a feature that is significantly less apparent in the schist mica. The marble mica show uniform high-celadonite content (XCel: 0.28), representing preservation of high-pressure mica chemistry during deformation. They show no trace element variations, except for a decrease B content with δ11B values (36.3 µg/g and -18.1 ‰ to 15.5 µg/g and -22.0 ‰), which may be explained by heterogeneous devolatilization and preferential loss of 11B during high-grade metamorphism. Thus, it is evident the mica remained closed to chemical exchange during deformation. The white mica 40Ar/39Ar dates from the schist are dispersed (491 ± 4 Ma to 427 ± 4 Ma), with the older dates typically provided by high-celadonite zones, and vice versa. The marble mica yielded a similar range of dates (486 ± 4 Ma to 428 ± 4 Ma), with finer-grains yielding younger dates. Weighted averages of single-spot Rb/Sr dates show a similar pattern for the schist (high-celadonite: 485 ± 8 Ma; low-celadonite: 427 ± 15 Ma). However, Rb/Sr dates from the marble mica only reproduce the older population of dates (481 ± 4 Ma) regardless of grain size. These results demonstrate that loss of 40Ar from white mica can be facilitated by either fluid-mediated chemical exchange or deformation, but re-equilibration of Sr isotopes to reset Rb/Sr dates requires fluid-driven processes in lower amphibolite/upper greenschist facies conditions.Research funded by NCN grant no. UMO-2021/40/C/ST10/00264 (C.J. Barnes), supported by “Juan de la Cierva” Fellowship JFJC2021-047505-I by MCIN/AEI/10.13039/501100011033 and CSIC (M. Bukała)

Aqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high‐pressure experiments and natural‐case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high‐pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in‐situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile‐brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite‐blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite‐bearing eclogites imply interaction with an externally‐derived fluid, whereas rare phengite‐free, kyanite‐eclogites only record internally‐derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non‐random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point‐like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak‐pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile‐brittle deformation of eclogite in a subduction setting.

The opening and closure of the Paleozoic Iapetus Ocean, leading to the collision of Laurentia and Baltica forming the Caledonian orogen, is one of the prime examples of a Wilson cycle. In this perspective article, we summarize and discuss the content of 10 new research articles within a 5-stage framework of the Caledonian Wilson cycle from a North Atlantic perspective. Stage 1 covers Neoproterozoic rifting of both the Laurentian and Baltican margins, where the plate tectonic configurations and the timing for the onset of rifting are far from resolved; Stage 2 covers the onset of sea-floor spreading within Iapetus, with several different oceanic basins opening at different times, and with variable geometries of the rifted margins; Stage 3 covers the narrowing of the Iapetus basins along several subduction zones, the number, location and orientation of which are debated; Stage 4 covers the main continent−continent collision, documenting advances in our understanding of (U)HP metamorphism within the Western Gneiss Region; Stage 5 covers post-orogenic extension, transitioning into stage 1 of the subsequent Atlantic Wilson cycle. We review the evolution of the Caledonian Wilson cycle in the light of the recent literature from the past decade and highlight open questions and unresolved issues. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle