Geologic Setting of Eclogite-facies Assemblages in the St. Cyr Klippe, Yukon–Tanana Terrane, Yukon, Canada

The St. Cyr area near Quiet Lake hosts well preserved to variably retrogressed eclogite found as sub-metre to hundreds of metre-long lenses within quartzofeldspathic schist in south-central Yukon, Canada. The St. Cyr klippe consists of structurally imbricated, polydeformed and polymetamorphosed units of continental arc crust and ultramafic–mafic rocks. Eclogite-bearing quartzofeldspathic schist forms thrust slices in a 30 km long by 6 km wide, northwest-striking outcrop belt. The schist unit comprises metasedimentary and felsic intrusive rocks that are intercalated on the metre to tens of metres scale. Ultramafic rocks, serpentinite and associated greenschist-facies metagabbro form imbricated tectonic slices within the eclogite-bearing quartzofeldspathic unit, which led to a previously held hypothesis that eclogite was exhumed within a tectonic mélange. The presence of phengite and Permian zircon crystallized under eclogite-facies metamorphic conditions in the quartzofeldspathic host rocks indicate that the eclogite was metamorphosed in situ together with the schist as a coherent unit that was part of the continental arc crust of the Yukon–Tanana terrane, rather than a mélange associated with the subduction of oceanic crust of the Slide Mountain terrane. Petrological, geochemical, geochronological and structural similarities link St. Cyr eclogite to other high-pressure localities within Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon–Tanana terrane.RÉSUMÉLa région de St-Cyr renferme des éclogites bien conservées à légèrement rétrogradées qui se présentent sous forme de lentilles allant de la fraction de mètre à quelques centaines de mètres de longueur, au sein d’un schiste quartzofeldspathique du centre-sud du Yukon au Canada. La klippe de St-Cyr est structurellement constituée d’unités imbriquées, polydéformées et polymétamorphisées de croûte d’arc continental et de roches ultramafiques à mafiques. Les schistes quartzofeldspathiques à lentilles d’éclogites forment des écailles de chevauchement d’une bande de 30 km de longueur par 6 km de largeur de direction nord-ouest. Les schistes sont constitués de roches métasédimentaires et de roches intrusives felsiques intercalées à des intervalles qui vont du mètre à quelques dizaines de mètres. Les roches ultramafiques, serpentinites et métagabbros au facies à schiste vert forment des écailles tectoniques imbriquées au sein de l’unité quartzofeldspathique à lentilles d’éclogite, d’où une précédente hypothèse voulant que les éclogites soient un produit d’exhumation à partir d’un mélange tectonique. La présence de phengite et de zircon permien cristallisé sous conditions métamorphiques du faciès à éclogite au sein de la roche hôte quartzofeldspathique indiquent que l’éclogite a été métamorphisée en place, avec le schiste comme unité cohérente du terrane de croûte d’arc continental de Yukon–Tanana, plutôt qu’un mélange associé à une subduction de croûte océanique du terrane de Slide Mountain. Des similarités pétrologiques, géochimiques, géochronologiques et structurales lient les éclogites de St-Cyr à d’autres lieux de hautes pressions au Yukon, ce qui indique que les assemblages de hautes pressions forment une unité lithotectonique plus grande au sein du terrane de Yukon–Tanana

Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy

An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied in order to refine the protolith, provenance and metamorphic ages of the Eclogitic Micaschist Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz + phengite + K-feldspar ± garnet + rutile + zircon, whereas the paragneiss hosts garnet + jadeite + phengite ± glaucophane + epidote + rutile + quartz. Phase diagram modeling of two representative samples yields minimum equilibration conditions of 550 ± 50 ∘C and 18 ± 2 kbar. Cathodoluminescence images of zircon from the orthogneiss show oscillatory-zoned cores that are embayed and overgrown by complex, oscillatory-zoned rims. Four concordant secondary ion mass spectrometry analyses from the cores give a weighted mean 206Pb / 238U age of 457 ± 5 Ma. The cores have Th/U = 0.1 and negative Eu anomalies indicative of an igneous protolith, which we interpret to have crystallized in the Ordovician at 780 ∘C, based on Ti-in-zircon measurements. Zircon rims yield a range of 206Pb / 238U dates from 74 to 86 Ma, and four concordant analyses define a weighted mean 206Pb / 238U age of 78 ± 2 Ma. The rims are interpreted to have grown in the eclogite facies based on their lower Th/U (0.01), less negative Eu anomalies and steeper heavy rare earth element (HREE) patterns at &lt;600 ∘C. The paragneiss yielded a detrital zircon population with major peaks at 575–600, 655 and 765 Ma; minor older components; and a maximum depositional age of approximately 570 Ma. The prominent Neoproterozoic zircon population and Ediacaran depositional age suggest derivation from the Gondwana margin. The metamorphic zircon is consistent with the oldest eclogite-facies event in the Sesia Zone; it does not show evidence of multiple periods of rim growth or any pre-Alpine (e.g., Variscan) metamorphism.</p

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an εNd(0) signature of –18.3 to –12.1, and is intruded by c. 800 Ma and c. 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an εNd(0) signature of –27.7 to –23.4, intruded by c. 1830 Ma granites. The data reveal a c. 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones

Recycling Argon through Metamorphic Reactions: the Record in Symplectites

The 40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20–25 to c. 8–12 kbar at ~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely ‘resets’ the Ar clock, or whether there is some inheritance of 40Ar in the neo-crystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of ~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40Ar recycling can be tracked, thus potentially allowing 40Ar/39Ar dates to be linked more accurately to metamorphic history

Monazite trumps zircon: applying SHRIMP U–Pb geochronology to systematically evaluate emplacement ages of leucocratic, low-temperature granites in a complex Precambrian orogen

Although zircon is the most widely used geochronometer to determine the crystallisation ages of granites, it can be unreliable for low-temperature melts because they may not crystallise new zircon. For leucocratic granites U–Pb zircon dates, therefore, may reflect the ages of the source rocks rather than the igneous crystallisation age. In the Proterozoic Capricorn Orogen of Western Australia, leucocratic granites are associated with several pulses of intracontinental magmatism spanning ~800 million years. In several instances, SHRIMP U–Pb zircon dating of these leucocratic granites either yielded ages that were inconclusive (e.g., multiple concordant ages) or incompatible with other geochronological data. To overcome this we used SHRIMP U–Th–Pb monazite geochronology to obtain igneous crystallisation ages that are consistent with the geological and geochronological framework of the orogen. The U–Th–Pb monazite geochronology has resolved the time interval over which two granitic supersuites were emplaced; a Paleoproterozoic supersuite thought to span ~80 million years was emplaced in less than half that time (1688–1659 Ma) and a small Meso- to Neoproterozoic supersuite considered to have been intruded over ~70 million years was instead assembled over ~130 million years and outlasted associated regional metamorphism by ~100 million years. Both findings have consequences for the duration of associated orogenic events and any estimates for magma generation rates. The monazite geochronology has contributed to a more reliable tectonic history for a complex, long-lived orogen. Our results emphasise the benefit of monazite as a geochronometer for leucocratic granites derived by low-temperature crustal melting and are relevant to other orogens worldwide

Devonian to Carboniferous collision in the Greenland Caledonides: U-Pb zircon and Sm-Nd ages of high-pressure and ultrahigh-pressure metamorphism

A variety of eclogites from an east-west transect across the North-East Greenland eclogite province have been studied to establish the timing of high pressure (HP) and ultrahigh-pressure (UHP) metamorphism in this northern segment of the Laurentian margin. Garnet + omphacite ± amphibole + whole rock Sm-Nd isochrons from a quartz eclogite, a garnet + omphacite + rutile eclogite and a partially melted zoisite eclogite in the western HP belt are 401 ± 2, 402 ± 9 and 414 ± 18 Ma, respectively. Corresponding sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U ages of metamorphic zircon in the same samples are 401 ± 7, 414 ± 13, and 393 ± 10 Ma. Metamorphic zircon domains were identified using morphology, cathodoluminescence (CL) imaging, U, Th, Th/U and trace element contents. Zircon from the quartz eclogite and the garnet + omphacite + rutile eclogite are typical of eclogite facies zircon with rounded to subhedral shapes, patchy to homogenous CL domains, low U, and very low Th and Th/U. The partially melted eclogite contains euhedral zircons with dark, sector-zoned, higher U, Th and Th/U inherited cores. Three cores give a Paleoproterozoic 207Pb/206Pb age of 1,962 ± 27 Ma, interpreted as the age of the leucogabbroic protolith. CL images of the bright overgrowths show faint oscillatory zoning next to homogenous areas that indicate zircon growth in the presence of a HP melt and later recrystallization. Additional evidence that zircon grew during eclogite facies conditions is the lack of a Eu anomaly in the trace element data for all the samples. These results, combined with additional less precise Sm-Nd ages and our earlier work, point to a Devonian age of HP metamorphism in the western and central portions of the eclogite province. An UHP kyanite eclogite from the eastern part of the transect contains equant metamorphic zircon with homogeneous to patchy zoning in CL and HP inclusions of garnet, omphacite and kyanite. These zircons have slightly higher U, Th and Th/U values than the HP ones, no Eu anomaly, and are thus comparable to UHP zircons in the literature. The 206Pb/238U age of these zircons is 360±5 Ma, much younger than the HP eclogites. The same sample gives a Sm-Nd age of 342 ± 6 Ma. Unlike the HP eclogites, the Sm-Nd age of the UHP rock is ca. 20 Ma younger than the U-Pb zircon age and most likely records slow cooling through the closure temperature, since peak temperatures were in excess of 900°C. Widespread HP metamorphism of both the Laurentian and Baltica continental margins marks the culmination of this continent-continent collision in the Devonian. Carboniferous UHP conditions, though localized in the east, suggest a prolonged collisional history rather than a short-lived Scandian orogeny. The traditional Silurian Scandian orogeny should thus be extended through the Devonian.20 page(s

Forward modeling corona growth in a partially eclogitized leucogabbro, Bourbon Island, North-East Greenland

Abstract Coronitic textures are common in partially eclogitized igneous bodies, such as gabbros, leucogabbros, and anorthosites, east of the Germania Land Deformation Zone in North-East Greenland. Coronas formed by prograde metamorphic processes that transformed the gabbroic bodies to eclogite facies, and record frozen stages of the prograde metamorphic evolution of these rocks. A metaleucogabbro-norite body on Bourbon Island in Jøkelbugt is characterized by three concentric areas: a coronitic core, a mottled inner rim with areas of completely eclogitized material surrounded by a matrix of coronitic metaleucogabbro, and an outer rim of strongly foliated and completely retrogressed amphibolite. The Bourbon body preserves four stages of the prograde metamorphic history: Stage I, Stage II, Stage III, and Eclogite Stage. Stage I coronas are found only in the core of the body, which is the least reacted part of the leucogabbro-norite and the closest to the protolith, and is characterized by the corona sequence Pl rim / Grt + Kfs + Amp/Grt + Amp/Cpx rim . The typical corona sequence for Stage II is Pl rim /Grt + Pl + Zo/Cpx rim /Amp rim . Stage III samples show a Pl rim + Ky + Scp/Grt + Pl + Qtz/Qtz + Pl sequence, with the relict clinopyroxene being replaced in part by microcrystalline aggregates of Cpx + Amp + Pl. The Eclogite Stage shows relict Pl completely replaced by Grt, and the relict Cpx completely replaced by aggregates of Omp + Pl + Kfs + Amp. We tested open-system grain boundary diffusion (OSGBD) theories to model the prograde Stage I symplectitic coronas. The observed ratio of the thickness of the different layers is Pl rim :Grt + Kfs + Amp:Grt + Amp:Cpx rim equal to 3:1.3:0.95:0.5. These ratios are very close to the modeled ones of 2.7:1.1:1:0.5. Furthermore, subtle textural changes within the Grt + Kfs + Amp corona were also reproduced by the model. The model gave us insight into the conditions of the metamorphic system in which the coronas formed. The sequence Pl rim /Grt + Kfs + Amp/Grt + Amp/Cpx rim formed by diffusion driven reactions in an open system involving gain of Fe, K, and Na, and loss of Ca and Mg at the original clinopyroxene-plagioclase boundary. Relative mobilities of the different components within the corona layers were Fluid circulation was active to some degree during the transformation to eclogite. The differences between core, inner rim, and the two domains within the inner rim of the metaleucogabbro-norite can be explained by different degrees of fluid circulation in different portions of the rock. The presence of phases containing Cl and P, such as scapolite, in completely eclogitized samples supports the presence of fluid circulation in the system. Another possible explanation for the mottled appearance of the inner rim is protolith heterogeneity