Metamorphic Zircon Research Articles

Late Triassic continental eclogite in the central Tibetan Plateau reveals 2500-km-long Paleo-Tethys continental subduction

Permian−Triassic eclogites exposed on the Tibetan Plateau have traditionally been attributed to Paleo-Tethys oceanic subduction. Here we report new data that indicate a continental-subduction origin of the Pianshishan eclogites along the Longmuco-Shuanghu Paleo-Tethys suture in northern Tibet. Phase equilibrium modeling and zircon U-Pb dating constrain eclogite-facies metamorphism of ∼2.1 GPa/530 °C to have occurred ca. 235 Ma. Xenocrysts of ca. 1850 Ma metamorphic zircons and crustal trace element signatures reveal a continental protolith for the Pianshishan eclogites, demonstrating deep subduction of continental passive margins during the early Late Triassic. The Qianmai eclogites along the Changning-Menglian suture in Southwest China have the same petrogenesis, indicating that these two coeval eclogites resulted from continental subduction that extended over 2500 km along the Paleo-Tethys suture. This collision not only marks the end of the Paleo-Tethys Wilson cycle but also could have formed a high-elevation range, comparable in length to the Himalaya, that amplified Tethyan megamonsoons during the Late Triassic Carnian Pluvial Episode.

Metamorphic history of amphibolite facies metapelites from the Southern margin of the Limpopo Belt: In situ U-Pb dating of Zircon, monazite and rutile

The Southern Marginal Zone (SMZ) comprises, in part, Archean granulite facies metapelitic rocks deposited at ∼ 2733 Ma and metamorphosed to granulite facies at ∼ 2713 Ma of the Limpopo Belt in direct contact with the Kaapvaal craton. The SMZ also contains amphibolite facies metapelites within a fragment that is in direct contact with the thrust-faulted tectonic boundary with the Kaapvaal craton. These metapelites are proposed to represent former granulites that were comprehensively rehydrated under amphibolite facies. However, no previous study has directly investigated the conditions of metamorphism or timing of the proposed higher-grade events in these retrograde rocks. Neither has the timing of retrogression in the retrogressed metapelites been well constrained. Here, detailed petrographic analysis, mineral composition and in situ U-Pb dating of zircon, monazite and rutile are presented for the amphibolite facies metapelites. These rocks are characterized by assemblages consisting of garnet, orthoamphibole, biotite, quartz, plagioclase, rutile, kyanite and graphite. Although there is pervasive retrogression to orthoamphiboles (anthophyllite to gedrite series), rare relics of orthopyroxene are preserved. The compositions of the first garnet (Grt 1) generation suggests granulite facies peak metamorphic conditions of 860 ± 10 °C and 11 ± 0.4 kbar. Metamorphic zircon grains and overgrowth rims and monazite, included in Grt 1, indicate that peak metamorphic conditions occurred between 2714 ± 7 and 2713 ± 4 Ma. Dating of rutile inclusions in Grt 1 from granulite yield a Concordia age of 2677 ± 6 Ma. This indicates post-granulite facies cooling and shows that later retrograde metamorphism did not reach temperatures high enough to reset rutile inclusions in Grt 1 garnet. Orthoamphibole and second garnet with kyanite inclusions (Grt 2) compositions suggest that retrogression occurred under amphibolite facies conditions of 600 ± 10 °C and 6.0 ± 0.5 kbar. Rutile included in Grt 2 and as discrete grains equilibrated with the retrograde textures record a range of Paleoproterozoic ages (2212 ± 19, 2148 ± 12 and 2012 ± 10 Ma). Thus, the metamorphic history of the amphibolite facies metapelites of the SMZ is hotter than thought and similar to their Archean granulite facies counterparts, except for younger Paleoproterozoic ages, likely indicating a pervasive retrogression to amphibolite facies conditions.

Ultrahigh-pressure to high-pressure eclogite in Cuban ophiolitic mélange reveals proto-Caribbean spreading ridge subduction

Proto-Caribbean oceanic crust produced during ocean-floor spreading between diverging North and South American plates was subsequently subducted beneath the Caribbean plate. However, the timing and spatial configuration of proto-Caribbean spreading ridge subduction remain subjects of debate. High-pressure (HP) basaltic metamorphic rocks, representing relics of the subducted proto-Caribbean oceanic crust, commonly occur in Cuban ophiolitic mélanges. In this study, an integrated set of petrological, geochemical, and geochronological data is presented for eclogite from the Las Villas mélange, central Cuba. The typical geochemical signature of mid-ocean-ridge basalt (MORB) indicates that the protolith of eclogite formed at the proto-Caribbean spreading ridge. Based on pressure-temperature (P-T) estimates obtained by pseudosection analysis as well as Zr-in-rutile and Ti-in-zircon thermometry, the following P-T paths for representative samples can be derived: a prograde path from 24−25 kbar and 510−520 °C to peak conditions of 29−31 kbar and 525−575 °C, and a complex retrograde path initially following almost isothermal exhumation to 25−27 kbar, followed by near-isobaric heating to 610−640 °C before final exhumation. This is the first documentation of prograde oceanic ultrahigh-pressure (UHP) metamorphism in the northern Caribbean area. U-Pb dating of magmatic zircon with steep heavy rare earth element (HREE) patterns and negative Eu anomalies yielded a protolith age of 126.3 ± 0.7 Ma. In contrast, metamorphic zircon with flat HREE patterns and without an Eu anomaly yielded a weighted mean age of 118.6 ± 1.6 Ma. The short time interval of >8 m.y. between MORB magmatism and UHP metamorphism suggests that the oceanic crust was subducted to great depth (∼100 km) shortly after generation in an oceanic ridge, which provides robust evidence for subduction of the proto-Caribbean spreading ridge. Furthermore, this work demonstrates high potential to trace ancient spreading ridge subduction by joint petrological, geochemical, and geochronological study of oceanic eclogite.

Ultraslow cooling of an ultrahot orogen

To constrain the rate of cooling of lower-crustal rocks from an ultrahot orogen, we determined both the age and equilibration temperature of metamorphic zircon from six widely spaced samples of metasedimentary garnet−sillimanite gneiss from the Eastern Ghats Province in eastern India. For the combined data set of metamorphic zircon, concordant dates decrease continuously within 2σ uncertainty from around 950 Ma to 800 Ma, consistent with ∼150 m.y. of zircon crystallization. Ti-in-zircon temperatures for each dated spot during this period decrease with age, corresponding to linear cooling rates ranging from 0.26 to 0.90 °C/m.y. We propose that retention of heat-producing elements in the lower crust of the Eastern Ghats Province and a low net erosion rate were responsible for ∼150 m.y. of ultraslow cooling.

The leaky chronometer: Evidence for systematic cryptic Pb loss in laser ablation U‐Pb dating of zircon relative to CA‐TIMS

AbstractU–Pb dating of zircon via laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has become a primary method for determining the age of crystalline rocks and the provenance and maximum depositional age of (meta)sedimentary rocks. Although chemical abrasion has become a standard approach for mitigating Pb loss in zircon dated via high‐resolution thermal ionization mass spectrometry (CA‐TIMS), laser ablation dating is rarely accompanied by chemical abrasion. We evaluate the accuracy of U–Pb dates acquired via LA‐ICP‐MS relative to CA‐TIMS through analysis of a large database of paired analyses from the same zircon crystals. We show that laser ablation 206Pb/238U dates are systematically younger than their CA‐TIMS counterparts, with a greater shift in detrital zircon (−2.0%) versus igneous or metamorphic zircon (−0.9%). Although systematic biases related to “matrix effects” may be a contributing factor, our analysis suggests that unmitigated cryptic Pb loss is likely widespread in laser ablation U–Pb datasets.

Were South India and North China Craton attached during the Paleoproterozoic (Orosirian) Nuna Assembly? Novel geochemical and isotopic investigations of A2-type granites from the Khammam Schist Belt, Eastern Dharwar Craton, India

The paper presents novel geochemical and geochronological data from granites in Khammam, Eastern Dharwar Craton (EDC), India. The studied granites contain major mineral phases like quartz, alkali-feldspar, plagioclase, biotite, and muscovite in decreasing order of abundance. The accessory phases are epidote, titanite, and zircon. The samples comprise 70 ‐–77 wt% SiO2 and 12–15 wt% Al2O3. The K2O and Na2O concentrations range from 2.57 to 5.65 wt% and 1.17 to 2.69 wt%, respectively. They are enriched in Rb, Th, and Pb and depleted in Nb, Ta, and Ti. On a chondrite-normalized plot, the samples exhibit a rightward trend with a negative Eu anomaly. Zircon saturation in silicate melts yields a temperature (TZr) of 859 ‐ 978 °C. The microstructure and U - Pb isotopic analysis of zircon grains (n = 79) reveals the presence of magmatic and polymetamorphic grains with 207Pb/206Pb age clusters at 1844 Ma (number of analyses, n = 7) - 1858 Ma (n = 8), 1737 Ma (n = 5) -1768 Ma (n = 4), 1619 Ma (n = 7) - 1634 Ma (n = 6), and 1554 Ma (n = 5), respectively. The magmatic zircons exhibit ε(Hf) values between 3 and 18.9 with a two-stage model age of 2.03 Ga. In contrast, the metamorphic zircons exhibit ε(Hf) values between −5.6 and 18, yielding a two-stage model age of 1.97 Ga. The geochemical and geochronological studies indicate that the rocks are A2-type granite emplaced during the accretion of the eastern block of the North China Craton (NCC) and EDC between 1844 Ma and 1858 Ma. The zircons from 1737–1768 Ma and 1620 Ma show the time of metamorphic growth during Antarctica-Nellore Schist Belt (EDC) accretion. Finally, the U-Pb zircon ages from 1554 Ma represent Nuna's final amalgamation. The results of this study posit an association between EDC and NCC during Nuna assembly.

Zircon Internal Deformation and Its Effect on U-Pb Geochronology: A Case Study from the Himalayan High-Pressure Eclogites

Zircon, with a chemical formula of ZrSiO4, is a widely used mineral for determining the crystallization age of igneous rocks. It is also used to constrain the timing of metamorphic events from its overgrowth or recrystallized domains. Furthermore, detrital zircon grains can provide information on the sedimentary provenance. Due to the trace amounts of uranium (parent) which decays into its daughter element (Pb), it is a prime geochronometer for the majority of magmatic and metamorphic rocks. With high-precision analytical instruments, such as TIMS, SIMS, and LA-ICP-MS, huge amounts of geochronological and trace element data from zircon have been generated around the globe to date. Target domains within zircon grains are analyzed to extract geochemical and geochronological records using spatially resolved techniques such as ion probes or laser ablation coupled with mass spectrometry. Before any such analysis, the zircon grains are examined for internal structures, growth zoning, and the presence of tiny inclusions. However, many researchers analyze multiple domains within single zircon grains for U-Pb isotope analysis with little regard for their internal structures, particularly crystallographic orientations. Hence, they may obtain mixed ages with variable discordance, leading to imprecise interpretation especially when the growth domains are not well-identified. Particularly, zircon grains that contain multi-growth domains or have local internal deformations within a single grain may not produce geologically meaningful age results if the analyses are conducted on mixed domains. This study presents a brief review on zircon geochronology, how to identify and visualize micro-deformations in metamorphic zircons through the EBSD analysis, and the effects of micro-deformation on age results. Examples from a case study conducted on zircons hosted in the Himalayan high-pressure eclogites are presented that show intra-grain plastically deformed domains and their effects on the corresponding age results.

Continental-scale drainage reorganization during Mesoproterozoic orogenesis: Evidence from the Belt Basin of western North America

The Mesoproterozoic Belt Basin of the northwestern United States and southwestern Canada contains a 5–20-km-thick metasedimentary succession deposited during an important transition in the Precambrian development of North America. Key unresolved issues for the Belt Basin include the chronology of deposition, sources of siliciclastic sediment, and regional paleogeography during Laurentian orogenesis. To address these topics, we acquired detrital zircon U-Pb geochronologic data for eastern exposures of the Belt-Purcell Supergroup in the Lewis thrust salient along the USA-Canada border. To define an integrated chronostratigraphic and provenance framework for the Belt Basin, we calculated maximum depositional ages and qualitatively and quantitatively compared our geochronologic data set to a compilation of Laurentian igneous and metamorphic zircon U-Pb ages using multidimensional scaling and an inverse Monte Carlo model. The results suggest a stratigraphic age range of ca. 1495–1380 Ma, constituting a depositional duration of ~115 m.y. with an average sediment accumulation rate of ~40 m/m.y. for the studied locality (extrapolated to ~155 m/m.y. for the basin depocenter). Variations in sediment provenance are expressed by three distinct intervals within the Belt-Purcell Supergroup. The lower Belt Supergroup succession (Waterton to lower Helena Formations; ca. 1495–1440 Ma) is dominated by Paleoproterozoic and Archean grains derived from the northeastern Canadian Shield. The middle Belt Supergroup succession (upper Helena to Sheppard Formations; ca. 1440–1420 Ma) displays mixed early Mesoproterozoic, late Paleoproterozoic, and Archean zircon age groups. The upper Belt Supergroup succession (Gateway to Roosville Formations; ca. 1420–1380 Ma) contains almost entirely late Paleo-proterozoic zircons sourced from the south (Yavapai-Mazatzal and Mojave crustal provinces). We interpret sediment provenance to reflect a continental-scale, fluvial drainage reorganization during middle Belt Supergroup deposition that can be linked to the recently recognized Picuris orogeny.

The Kharbei Amphibolite–Gneiss Complex (Polar Urals): P–T Evolution and Results of U–Pb LA-ICP-MS Isotopic Study of Metamorphic Zircon

The Kharbei Amphibolite–Gneiss Complex (Polar Urals): P–T Evolution and Results of U–Pb LA-ICP-MS Isotopic Study of Metamorphic Zircon

HED zircons as a window into the solar system’s first crust: Decoupling primordial differentiation, metamorphism and impact events through textural and chemical studies

The study of Howardite-Eucrite-Diogenite (HED) meteorites provides unique insights into early planet formation and the impact events that shaped the early Solar System. However, unraveling the complex history of the HED parent body (hypothesized to be the asteroid 4 Vesta) from whole-rock samples is challenging since most HEDs are impact-related breccias comprising mixed lithic and mineral fragments that experienced variable deformation and alteration. Combining U-Pb geochronology, trace element geochemistry, and microstructural analysis of zircon can unravel magmatic, metamorphic and impact processes through time to decipher the HED parent body evolution. Here we present textural (EBSD), geochronological (207Pb/206Pb SIMS dating) and geochemical data (Th/U, REE, Ti-in-zircon thermometry) on 61 zircon grains from melt breccia eucrites, unbrecciated/monomict/polymict eucrites, howardites and diogenites. Diverse textures indicate variable histories of impact deformation and high-temperature recrystallization. Undeformed, fractured zircons preserve primary zoning (CL, Th/U, REE) indicating magmatic and metamorphic origins. At least three magmatic zircon grains (Th/U > 0.3) give 207Pb/206Pb ages of 4558–4565 Ma, suggesting primary differentiation in the parent body first million years. Twenty metamorphic zircon grains (Th/U < 0.3) date to 4420–4568 Ma, indicating prolonged thermal metamorphism from impact heating and/or crustal cooling. Impact-recrystallized granular zircon grains reveal major impacts during and just after the parent body differentiation (4500–4560 Ma), plus later events potentially linked to synchronous impacts in the Solar System (e.g. the Moon). Similarity of metamorphic and shocked zircon ages (circa 4550–4450 Ma) suggests impacts occurred for ≥100 million years after the parent body formed.

Provenance and depositional age of metasedimentary rocks in the Frontenac terrane (Grenville Province, Ontario)

The Mesoproterozoic Frontenac terrane in the southern Grenville Province of Ontario is separated by shear zones from the Composite Arc Belt to the west and the Adirondack Lowlands to the east. The majority of the terrane is made up of granulite-facies metasedimentary rocks that are the oldest lithologies recognized in the terrane. Five samples were selected for U–Pb geochronology to constrain (1) metamorphic age, (2) depositional age of sedimentary protoliths, and (3) source regions of detrital zircon. Two pelitic gneisses and a marble are dominated by metamorphic zircon, whereas two quartzites contain a diverse population of detrital zircon that are surrounded by metamorphic overgrowths. Metamorphic zircon have 206Pb/207Pb ages of 1.19–1.16 Ga, and a small population is 1.22 Ga. These ages correspond to the Shawinigan and Elzevirian orogenies and provide minimum ages for deposition. The youngest detrital grains with ages of 1.25–1.24 Ga provide maximum depositional ages. Quartzites (and pelitic gneisses) have a wide range of detrital zircon ages that reflect local Mesoproterozoic Grenville sources and Paleoproterzoic and Archean sources in the northern Grenville Province and elsewhere in Laurentia. Most notable is a large population of 1.9–1.8 Ga zircon, which points towards derivation from the Penokean orogen in the midcontinent or Makkovikian–Ketilidian orogen of Labrador and Greenland, indicating long sedimentary transport distances. The similarities in depositional ages and detrital zircon ages between Frontenac terrane and Adirondack metasedimentary rocks suggest a shared sedimentary history, which we interpret as deposition in the same Trans-Adirondack backarc basin at ca. 1.25 Ga.

Inside the Ryoke Magmatic Arc: Crustal Deformation, High-T Metamorphism, and Magmatic Pulses

The Ryoke belt represents the root of a volcanic arc exposed across SW Japan. It records successive deformation phases, high-temperature metamorphism, and several magmatic pulses that occurred during the Late Cretaceous. Successive magma intrusions at different crustal levels raised the overall geothermal gradient of the arc crust, and their thermal influence was contrastingly recorded in metamorphic zircon and monazite. Despite a broadly similar duration of magmatic activity (20–30 My) along the belt, the timing and periodicity of magma pulses varied. An along-arc variation in lower crustal magma generation together with a fluctuating crustal stress regime likely controlled the formation and evolution of this magmatic arc section.

From Triassic metamorphism to Early Cretaceous anatexis in the Dabie orogen, central China: Constraints from in-situ U-Pb age and Hf and O isotopes of zircon from migmatites

The Dabie orogen underwent a ‘complete’ orogenic cycle from Triassic collisional orogeny to Early Cretaceous tectonic collapse. Zircons of the migmatite in the North Dabie terrane recorded the processes from Triassic granulite-facies metamorphism to Early Cretaceous anatexis. Three types of zircon are recognized in the migmatite: the Neoproterozoic inherited magmatic zircon, the Triassic metamorphic zircon, and the Late Jurassic–Early Cretaceous anatectic zircon. The Neoproterozoic inherited magmatic zircons suggest the protolith of the migmatite has a tectonic affinity with the Yangtze Craton (YC). The Triassic metamorphic zircon domains have variable εHf(t) values from −4.1 to 3.6 and low δ18O compositions ranging from −4.36‰ to −0.30‰. The anatectic zircon domains have clearly negative εHf(t) values of −19.8 to 0.8 (mostly less than −10.0) and relatively high δ18O values from −4.24‰ to 3.49‰. Based on the chronological and isotopic data of migmatite and the results of previous studies, the Triassic granulite-facies metamorphism was closely related to the asthenosphere upwelling after the slab break-off during the orogenic stage, and the Early Cretaceous anatexis respond to widely heat event during the collapse stage of orogen.

On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt

The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.

Cold deep subduction of Indian continental crust and release of ultrahigh‐pressure fluid during initial exhumation: Insights from coesite‐bearing eclogite‐vein systems in Kaghan Valley, Pakistan

AbstractThe ultrahigh‐pressure (UHP) eclogites from the Kaghan Valley in Pakistan, which formed by the deep subduction of the Indian plate beneath the Asian plate in the Eocene, contain complex metamorphic vein systems (including both isolated veins and vein networks), with mineral assemblages of epidote + quartz + kyanite + phengite ± omphacite ± garnet. The investigations on the Kaghan UHP eclogite‐vein systems provide important insights into the mechanism and timing of metamorphic dehydration, fluid flow, and fluid–rock interaction in the deeply subducted Indian continental slab as well as the chemical characteristics of slab‐derived, aqueous fluids. Abundant lawsonite pseudomorphs, characterized by prismatic aggregates of epidote, kyanite, and quartz porphyroblasts, are first recognized in the Kaghan eclogites. This observation, in combination with the occurrence of coesite pseudomorphs in epidote porphyroblasts as well as the coexistence of epidote and coesite in the eclogite zircon, indicates the previous existence of UHP lawsonite in these eclogites. Petrological studies and phase equilibrium modelling reveal clockwise P–T trajectories for the Kaghan eclogites that are featured by prograde vectors in lawsonite‐stability regions with peak conditions of 3.0–3.4 GPa/650–690°C, followed by isothermal decompression and lawsonite breakdown under UHP conditions during the initial exhumation stage. The results of metamorphic evolution, together with in situ epidote and bulk Sr isotopic analyses, indicate that the fluids responsible for vein systems are most likely derived from the breakdown of UHP lawsonite in the eclogites. SIMS U–Pb dating of metamorphic zircons from the eclogites, integrated with the Raman analysis of inclusions in zircons, indicates that the UHP dehydration of eclogites occurred at 46.4 ± 1.2 and 46.8 ± 0.9 Ma. Analyses of hydrothermal zircons from the veins yielded slightly younger ages of 44.7 ± 1.0 and 44.9 ± 1.4 Ma, which represent the timing of fluid flow and/or vein crystallization during exhumation of the UHP rocks. Mass‐balance calculation results, in combination with the vein compositions, show that the fluid flow and fluid‐eclogite interaction led to the transfer of Si, Al, Ca, K, and incompatible trace elements from the eclogites into the fluids, from which the vein systems crystallized. This study indicates cold deep subduction of Indian continental crust along low geothermal gradients (6–7°C/km). The UHP fluid liberation and channelized fluid flow occurred during the initial exhumation of the cold Indian slab and are expected to induce the transfer of H2O and incompatible trace elements from the Indian slab to the Asian lithosphere, which potentially contributes to the formation of post‐collisional magmas. Moreover, we suggest that metamorphic vein systems in UHP lawsonite eclogites offer important constraints on the occurrence and timing of fast slab exhumation in continental subduction‐collision zones.

Late Paleozoic magmatism and foreland deformation associated with opening and closing of marginal basins in the North Patagonian Andes

Remnants of coeval Devonian oceanic and continental foreland rocks are preserved in the basement of the North Patagonian Andes. Our previous studies of the igneous rocks here have shown that the primitive oceanic and continental igneous rocks are coeval and belong to a marginal basin that opened and closed over 50 Myr. A structural study and four new U–Pb SHRIMP ages and zircon Hf–O determinations allowed the identification of three metamorphic episodes, the first one of which, M 1 (D 1 -S 1 ), is coeval with Andean-type granite intrusion in the foreland (405–380 Ma). This activity was simultaneous with the development of an oceanic ridge and a marginal basin, at the outer edge of which a primitive granitic oceanic arc formed (380–385 Ma; zircon δ 18 O 3.6–5.2‰). The extinction of this ridge initiated under-thrusting of the oceanic crust below the continent and an important mid- to high-grade metamorphic event, M 2 (D 2 -S 2 ) at 375–360 Ma, its peak dated by metamorphic zircon rims in migmatite at 365 ± 3 Ma. Basin closure occurred after the intrusion of S-type granites (357 ± 2 Ma; zircon δ 18 O 7.4–10.4‰) in the foreland and the accretion of gabbros, cumulate gabbros and trondhjemites at the proto-Pacific margin. Compression prevailed for 20 Myr in the foreland, causing a mylonitic medium-grade M 3 (D 3 -S 3 ) event. Supplementary material: Datasets are available at https://doi.org/10.6084/m9.figshare.c.6989873

Fluid-Induced Metamorphism and Deformation at the Eastern Boundary of the Sveconorwegian Province

Abstract The Sveconorwegian orogen in Scandinavia and the Grenville orogen in Canada are both remnants of large and hot orogens that formed part of the supercontinent Rodinia around 1 billion years ago. Formerly deeply buried portions of crust in these orogens are exposed and offer insights into the tectonic dynamics of the basement within large orogens. The Eastern Segment of the Sveconorwegian Province hosts a ~ 30 000 km2 crustal portion that was buried to c. 40 km depth at a late stage of the orogeny, 980–960 Ma ago, and is bound towards the foreland in the east by a ~ 25 km wide zone of step anastomosing deformation, the Frontal wedge. This zone represents the outermost ductile deformation that developed within the crystalline basement in the orogen. We investigated a heterogeneously deformed and recrystallised syenodiorite with the aim to understand the character of the deformation-related metamorphism within the Frontal wedge. Field relations, microtextures, and mineral reactions show that the metamorphic recrystallisation was governed by hydrous fluid infiltration along the ductile deformation zones. Equilibrium was attained on a millimetre scale only and metamorphic recrystallisation was dependent on the introduction of hydrous fluid. The metamorphism reached high-pressure epidote-amphibolite-facies; geothermobarometric estimates suggest 540°C to 600°C and 9 to 12 kbar. Metamorphic zircon formed during the breakdown of Zr-bearing igneous phases, primarily baddeleyite. SIMS U–Pb analyses of igneous zircon and baddeleyite date the igneous crystallisation of the syenodiorite at 1230 ± 6 Ma. Metamorphic zircon grains are &amp;lt;20 μm and too small for precise dating, but yielded ages around 1 Ga. Collectively, the metamorphic data indicate that subvertical movements along steep planes within the Frontal wedge allowed for the regional-scale tectonic burial to ~40 km depth of the Eastern Segment to the west. Some of the same steep deformation structures were re-utilised as discrete movement planes during later exhumation.

New age constraints for metasedimentary rocks in southern Finland

Metasedimentary rocks from nine sampling sites in southern Finland and an intermediate dyke in one of these sites were sampled for analysis of U–Pb zircon age and geochemical composition. The zircons of the metasedimentary rock samples yield 207Pb/206Pb dates ranging from 3281 to 1810 Ma. The nearly concordant dates from apparent detrital zircon cores indicate a prominent source with age around 2.1–2.0 Ga. The maximum depositional ages estimated for six of the metasedimentary rock samples span from 1.96 to 1.89 Ga. The data from zircon rims and overgrowths and metamorphic zircons in the metasedimentary rock samples indicate regional metamorphic events in the study area at least at ca. 1.89 and 1.84 Ga, and possibly also at ca. 1.87 Ga. The minimum age of the deposition (≤1.89 Ga) was estimated based on the ages of the regional metamorphism and the intrusive rocks. Some zircon rims and overgrowths yield ≥1.91 Ga 207Pb/206Pb dates, which are considered to possibly represent metamorphic events in the source areas.

An Iapetus origin for a layered eclogite complex in the northern Western Gneiss Region, Scandinavian Caledonides

AbstractThe Western Gneiss Region (WGR) is a Precambrian basement domain in the Scandinavian Caledonides and one of the world's largest high‐ and ultrahigh‐pressure terranes. The south–central WGR underwent regional eclogite facies metamorphism 415–400 Ma ago when Baltica subducted beneath Laurentia, during the Scandian orogeny. Eclogites in the WGR group into two traditional types: (1) Precambrian mafic intrusions metamorphosed in situ during Scandian continental subduction and (2) eclogites, garnet peridotites and garnet pyroxenites within ultramafic complexes derived from the subcontinental mantle beneath Laurentia. We document, using field relations, petrography, whole‐rock geochemistry and secondary ion mass spectrometry (SIMS) zircon geochronology, a hitherto unrecognized third type of eclogite in the WGR that places new constraints on its tectonic architecture: an eclogitized fragment of oceanic crust from the Iapetus Ocean. The Kråkfjord eclogite complex is a km2‐sized body with an interior consisting of kyanite eclogite (meta‐troctolite) and subordinate layers and lenses of garnet peridotite, garnet websterite and kyanite–garnet leucotonalite. This interior is capped by Fe–Ti‐rich eclogite, which locally contains subordinate pockets of migmatitic aluminous gneiss. The elemental abundances and isotopic compositions of the Fe–Ti‐rich eclogites resemble those of mid‐ocean ridge basalt (MORB). In contrast, the interior kyanite eclogites, peridotites and pyroxenites have compositions similar to the gabbroic cumulates in the lower oceanic crust of slow‐spreading ridges. U–Pb SIMS dating of igneous zircon cores from a leucotonalite pod in the interior of the Kråkfjord complex yields Cambro‐Ordovician igneous ages of 500–440 Ma, with the ~500 Ma age interpreted as the isotopically undisturbed age. This age matches those of Iapetan oceanic rocks exposed elsewhere in the mountain belt. Metamorphic zircon from an Fe–Ti‐rich eclogite in the carapace of the Kråkfjord complex dates the eclogite facies metamorphism at 421.9 ± 2.2 Ma, synchronous with the continental collision. Zircon from a leucosome in Fe–Ti‐rich retro‐eclogite indicates an age of 408.5 ± 2 Ma for the crystallization of partial melt following the decompression. Detrital zircon core ages from a pocket of aluminous migmatitic gneiss in the carapace indicate derivation of sediment from the Baltic crust. Collectively, the data show that the eclogite complex (1) originated at an Iapetus spreading centre near the continent Baltica, (2) subducted to eclogite conditions during Scandian continental collision and (3) was tectonically intercalated with the Precambrian Baltica basement of the WGR.

Implications of high-grade metamorphism on detrital zircon data sets: A case study from the Fraser Zone, Western Australia

Detrital zircon grains provide a useful tool to determine the maximum age of sedimentary strata and can elucidate tectonic setting and basin evolution. However, high-grade metamorphism can overprint the mineral chemistry of the detrital cargo and reset both primary age and compositional information. Incomplete resetting of the U–Pb isotopic system, fluid alteration, and complex internal structures due to granulite-facies metamorphism makes dating and interpretation of the obtained zircon ages challenging. Here, we use trace element chemistry, U–Pb geochronology (LA-ICP-MS and SHRIMP), textural observations, and inclusion characterization from detrital zircon crystals in the Snowys Dam Formation (Albany–Fraser Orogen; AFO) to deconvolve its strongly overprinted sedimentary history, and to evaluate the likely geodynamic setting for basin genesis. Detrital zircons that survived metamorphic overprinting show that the main detrital load was sourced from Paleoproterozoic lithologies from neighbouring regions, with rare Archean Yilgarn crystals. The youngest detrital grains correlate with the age of plagiogranites in the Madura Province to the east of the AFO and imply a maximum depositional age of c. 1390 Ma, indicating that the Fraser Zone temporally correlates to an oceanic arc and thus best fits a back-arc basin setting. High-grade metamorphism during Stage I of the orogeny (c. 1330–1260 Ma) promoted recrystallization of pre-existing detrital zircons together with neoblastic growth of metamorphic zircon rims. At c. 1245 Ma, during early Stage II of the AFO, anatexis of the metasedimentary rocks produced rare neocrystallized magmatic zircon. Late-stage fluids are indicated by elevated LREE for these grains, which also contain a hydrous inclusion assemblage. High volume sampling techniques such as LA-ICP-MS often drilled through multiple age domains that can hinder geological interpretation. Thus, a lower volume sampling technique (e.g., SHRIMP) and verification with mineral chemistry is highly instructive in accurately resolving primary age groups in high-grade terrains.