Zircon Rims Research Articles

Evolution of the melt source during protracted crustal anatexis: An example from the Bhutan Himalaya

Abstract The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected ɛHf ratios decrease from between −10 and −15 down to values as low as −23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ɛNd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust.

Reworked Precambrian metamorphic basement of the Lhasa terrane, southern Tibet: Zircon/titanite U–Pb geochronology, Hf isotope and geochemistry

Due to the paucity of exposure, the formation and evolution of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon and titanite in situ U–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses from the Dongjiu area of the southern Lhasa subterrane (SLT), southern Tibet. Geochemical data suggest that the protoliths of the biotite-amphibole gneiss and biotite gneiss are granodiorite and granite, respectively. Inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520–1506 Ma, whereas the overgrown zircon rims give metamorphic ages of 605–590 Ma. The Mesoproterozoic granitic rocks have bulk-rock εNd(t) values of −3.6 to +0.1 and zircon core εHf(t) values of −4.5 to +2.6, which give similar TDM2 ages of 2.35–2.05 Ga and 2.54–2.10 Ga respectively, suggesting their derivation from partial melting of Paleoproterozoic crustal material. The granitic rocks are also local provenance for the Mesoproterozoic detrital zircons in the Paleozoic strata in the Lhasa terrane. Titanite in situ U–Pb ages further indicate that the Dongjiu orthogneiss experienced more recent metamorphism at ~26 Ma. The mineral assemblage and thermobarometry calculations indicate that the Oligocene metamorphism occurred under medium-pressure (MP) amphibolite-facies conditions (5.4–7.2 kbar, 691–765 °C). We propose that the Dongjiu gneisses represent the Precambrian metamorphic basement of the Lhasa terrane, but have been intensively reworked by metamorphism in the SLT in response to the continued India-Asia convergence since the collision.

Chronological and geochemical constraints on the pre-variscan tectonic history of the Erzgebirge, Saxothuringian Zone

A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust.

Detrital garnet geochronology: Application in tributaries of the French Broad River, Southern Appalachian Mountains, USA

Abstract Nineteen single-grain detrital garnet ages from a tributary to the French Broad River (North Carolina, USA) establish a novel approach to Sm-Nd detrital garnet geochronology wherein the equilibrium bulk-rock composition lost during weathering and transport is replaced with the composition of inclusions leached from within each garnet grain. Detrital garnet ages were compared to published detrital zircon and monazite ages from the same river tributary system. Results show that 87% of the zircons have inherited Proterozoic ages; only zircon rims give Paleozoic ages. Monazites are exclusively Ordovician (weighted average: 460.9 ± 3.0 Ma). Our new detrital garnet ages (and the detrital zircon rims) record younger ages spanning the Late Ordovician to Early Devonian, likely reflecting prolonged metamorphic heating. The weighted average age of the detrital garnet population is Silurian (430.1 ± 7.2 Ma). Statistical tests confirm that the garnet population is younger than the monazite. The new detrital garnet ages illuminate a previously uninterpreted Silurian tectonometamorphic signal in this region.

Neoarchean arc magmatism and Paleoproterozoic granulite‐facies metamorphism in the Bhavani Suture Zone, South India

The Bhavani Suture Zone in the Southern Granulite Terrane marks the zone of amalgamation of the Neoarchean Nilgiri Block and the northwestern Madurai Block in southern India. Here, we report detailed petrological, geochemical, and geochronological data on the Mettupalayam mafic–ultramafic complex within this suture zone with a view to evaluate the tectonothermal history of the Bhavani Suture Zone and adjoining crustal blocks. The metamorphosed complex includes charnockite, hornblende‐biotite gneiss, mafic granulite, amphibolite, garnet‐bearing mafic granulite, and dioritic gneiss along with metamorphosed banded iron formation. The mafic granulite and the dioritic gneiss occur as concordant layers of varying thickness within the hornblende‐biotite gneiss. The salient geochemical features of the mafic granulite and the dioritic gneiss including the enrichment of large‐ion lithophile elements and depletion of high‐field‐strength elements suggest a subduction‐related arc magmatic setting. However, the amphibolites show MORB‐related affinity, suggesting its formation from a N‐MORB‐related source and their accretion together with the overlying banded iron formation. The peak metamorphic conditions of the garnet‐bearing mafic granulite were estimated using conventional geothermobarometers as 800–820 °C/8.5–9.2 kbar, which we further confirm through phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) system. Magmatic zircons with high Th/U ratio from the amphibolite display a well‐defined discordia with an upper intercept age of 2,600 ± 38 Ma. Zircon grains from the dioritic gneiss show weighted mean 206Pb/207Pb age of 2,524 ± 6 Ma from concordant zircon spots and a comparable upper intercept age of 2,562 ± 34 Ma from discordant zircon spots, indicating protolith emplacement related to Neoarchean arc magmatism as inferred from our geochemical data. The thin overgrowth rims around the magmatic zircon grains in the amphibolite yielded an upper intercept age of 2,520 ± 20 Ma, which is comparable with the protolith crystallization age of the dioritic gneiss. Slightly younger weighted mean 207Pb/208Pb ages of 2,463 ± 27 Ma (from dioritic gneiss) and 2,422 ± 30 Ma (from amphibolite) are also obtained from the metamorphic zircon rims. These ages are correlated to the timing of high‐grade metamorphism associated with final collision of the Nilgiri Block and the northwestern Madurai Block. Similar Neoarchean–Paleoproterozoic magmatism and high‐grade metamorphism were reported from many localities south of the Dharwar Craton. Our study further confirms the previous tectonic model that envisages multiple subduction and collision of magmatic arcs and continental fragments towards the Dharwar Craton during the Archean–Paleoproterozoic transition.

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.

Sr-Nd-Hf isotopic constraints and U-Pb geochronology of the Arroio Pedrado Gneisses, Dom Feliciano Belt, Brazil: A 680 Ma shoshonitic event in the final stages of The Piratini Arc evolution

In this work we present whole-rock and isotopic (Sr-Nd-Hf) chemistry data and zircon U-Pb geochronology from new occurrences of the Arroio Pedrado Gneisses, located near the Brazil/Uruguay border in the southeastern sector of the Dom Feliciano Belt. These monzogranitic orthogneisses are intermediate, metaluminous, with SiO2 concentration ranging from 59.22 to 65.37 wt%, Na2O/K2O ratios < 1, and Ce/Yb and Th/Yb ratios typical of shoshonitic associations. The studied samples are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), resulting in a high LILE/HFSE ratios (LaN/LuN ratios between 17 and 33). These features, together with significant negative Ta and Nb anomalies and low Y and Yb concentrations, fit within the characteristics of subduction-related magmatism. The whole-rock isotopic study demonstrates that the samples have intermediate 87Sr/86Sr(i) ratios (0.7095–0.7112), negative εNd(t) values (−3.7 to −4.2) and Nd model ages of 1.48–1.61 Ga. The studied zircons have a wide range of εHf(t) values (−13.44 to −15.16), suggesting a crustal influence in their generation. The geochemistry and isotope characteristics indicate that these shoshonites are derived from the partial melting of a thickened lower crust, which was affected by subduction-related fluids/melts. This magma underwent significant fractional crystallization – K-feldspar, plagioclase, biotite, and amphibole, and accessory minerals such as allanite and Fe-Ti oxides – in oxidative conditions during magma ascent. The LA-ICP/MS U-Pb zircon dating reveals that the Arroio Pedrado Gneisses were emplaced during the Cryogenian, as suggested by a concordia age of 680 ± 2.9 Ma interpreted as the igneous protolith crystallization age. The metamorphic age of 660 ± 2.8 Ma (zircon rims) is interpreted as the amphibolite-facies peak which marks an early collisional event during the convergence of the Río de la Plata Craton and African-related terranes. The Arroio Pedrado Gneisses are here interpreted as representing the latest magmatic event in the construction of the allochtonous Andean-type Piratini Arc, whose evolution is constrained between ca. 780 (initial magmatism) and 650 Ma (continental collision) during the West Gondwana assembly.

Minta előszűrési vizsgálati módszer cirkonon végzett U–Pb kormeghatározás pontosságának javítására

Habár mind a mai napig az U-Pb geokronológia legmegbízhatóbb ásványa a cirkon, mégis a belső, szöveti heterogenitása a kellő megbízhatóságú és időfelbontású korolás legfőbb akadálya lehet. Kutatási munkánk eredményeképpen e korlát leküzdésére négylépcsős elővizsgálati eljárást alakítottunk ki, mely általánosságban véve bármely olyan esetben hatékonyan alkalmazható, ahol a geológiai kormeghatározás alapját a cirkon ásvány képezi. A négylépcsős elővizsgálati módszer a következő lépéseket tartalmazza: 1. A cirkonkristályok morfológiai típusainak meghatározása. 2. A cirkonkristályokban található egy- és többfázisú ásványzárványok azonosítása elektronsugaras mikronanalízis alapján. 3. A cirkonkristályok (elsődleges és másodlagos) szöveti bélyegeinek nyomozása pásztázó elektronmikroszkóppal (SEM) készített katódlumineszcens (CL) és visszaszórtelektron-képek (BSE) alapján, 4. A cirkonkristályokban lévő zónák szerkezeti állapotának meghatározása Raman-spektroszkópiai vizsgálatok segítségével a ν3(SiO4) rezgési sáv félértékszélessége (FWHM) alapján. Jelen cikkben a módszer hatékonyságát bizonyítandó, hazánk területéről (Mórágy környéke) és Ausztriából (Rastenberg környéke) származó variszkuszi granitoid kőzetek járulékos elegyrészeként megjelenő cirkont vizsgáltunk. Választásunk olyan területekre esett, melyek előzetes megfigyelések alapján geokémiailag és kőzettanilag egyaránt szoros rokonságot mutatnak, így az azonosságok továbberősítése mellett a különbségek kimutatására is lehetőségünk nyílt. A négylépcsős elővizsgálati módszert alkalmazva, a szeparált cirkonkristályok részletes szöveti térképének elkészítése során lehatároltuk az elsődleges (magmás) és másodlagos (utómagmás) geológiai folyamatok által hátrahagyott szöveti bélyegeket, továbbá kizártuk az összes szerkezetileg roncsolt (metamikt) zónát, elkerülve ezzel az ólomvesztés lehetőségét és a felülbélyegzett koradatok esélyét. Végül az összegyűjtött adatok együttes értelmezése nyomán kijelöltük a cirkonkristályokon belül azokat a néhány tíz mikrométeres, kémiailag és szerkezetileg közel homogén területeket, melyek megbízhatóan felhasználhatók az U-Pb korhatározás során, egyúttal megkaptuk a kulcsot is, mellyel a mért koradatokat geológiai folyamatokhoz köthetjük. Ezen négylépcsős módszer segítségével a cirkon magok esetében 80%-os, növekedési és szektorzónáknál 94%-os, konvolút zónák kormeghatározása során 100%-os kihozatallal tudtuk hasznosítani koradatainkat.

Tectono‐magmatic and Stratigraphic Evolution of the Cycladic Basement, Ios Island, Greece

AbstractThe Cycladic Basement (CB) and the overlying Cycladic Blueschist Unit (CBU) are part of the Paleogene Cycladic subduction complex exposed in Miocene metamorphic core complexes in the distended back‐arc of the retreating Hellenic subduction zone of the southern Aegean. While the Cenozoic tectono‐metamorphic evolutions of the CB and the CBU have been the foci of numerous studies, this study presents new laser ablation inductively coupled plasma mass spectrometry bedrock and detrital zircon (DZ) U‐Pb ages that place robust constraints on the presubduction tectonic, magmatic, and paleogeographic evolution of the CB. Zircon U‐Pb ages of crystalline CB are ~306‐330 Ma, demonstrating local plutonism associated with regional voluminous, protracted Carboniferous magmatism related to Paleo‐Tethys subduction. The plutons intruded the CB metasedimentary host‐rock sequence, characterized by distinct Gondwanan DZ U‐Pb provenance, Neoproterozoic to early Paleozoic maximum depositional ages, and synmagmatic, contact metamorphic zircon rims (~300‐330 Ma). DZ U‐Pb dating revealed postmagmatic Permian metasedimentary rocks (~270‐295 Ma) that unconformably overlie the CB and have unimodal DZ spectra that indicate exhumation of the CB prior to Permian deposition within extensional basins, as well as mark the onset of CBU deposition prior to formation of the Pindos rift domain. These U‐Pb results clarify the late Paleozoic‐early Mesozoic evolution of the CB as a peri‐Gondwanan terrane composed of Neoproterozoic and early Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and exhumed in the Permian, prior to Triassic rifting and CBU deposition. Additionally, these data provide a chronostratigraphic framework and illuminate subduction‐related juxtaposition within the CB metasedimentary sequence.

Thickening and partial melting of the Northern Qinling Orogen, China: insights from zircon U–Pb geochronology and Hf isotopic composition of migmatites

The widespread migmatites in the Precambrian basement of the North Qinling Belt are products of partial melting of the Earth's crust and record the tectono-thermal response to Late Paleozoic collision along the Shangdan suture. The morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting. The zircons from leucosome, melanosome and banded migmatites yield U–Pb weighted mean ages of 408 ± 2, 403 ± 2 and 400 ± 2 Ma, respectively, constraining the timing of migmatization. The granitic dyke intruded into the migmatites gives a zircon U–Pb weighted mean age of 396 ± 2 Ma, constraining the minimum time of migmatization. Zircons from the melanosome and banded migmatite yield upper intercept ages of 1088–1061 Ma, representing the protolith ages of the basement of the North Qinling Belt. The zircon rims of the melanosome and banded migmatite predominantly display negative and positive ε Hf ( t ) values, respectively, suggesting the variable input of mantle material. Together with the regional geology and previous geochronological data, we propose that the thickening and partial melting in the North Qinling Belt was induced by the collision along the Shangdan suture between the North and South China Blocks from c. 408 to 400 Ma. Supplementary material: LA-ICP-MS U–Pb data, Lu–Hf isotopic compositions and trace element geochemistry data of the zircons and Ti-in-zircon thermometry are available at https://doi.org/10.6084/m9.figshare.c.4521389

The Neoarchean Uyea Gneiss Complex, Shetland: an onshore fragment of the Rae Craton on the European Plate

A tract of amphibolite-facies granitic gneisses and metagabbros in northern Shetland, UK, is here named the Uyea Gneiss Complex. Zircon U–Pb dating indicates emplacement of the igneous protoliths of the complex c. 2746–2726 Ma, at a later time than most of the Archean protoliths of the Lewisian Gneiss Complex of mainland Scotland. Calc-alkaline geochemistry of the Uyea Gneiss Complex indicates arc-affinity and a strong genetic kinship among the mafic and felsic components. Zircon Hf compositions suggest an enriched mantle source and limited interaction with older crust during emplacement. Ductile fabrics developed soon after emplacement, with zircon rims at c. 2710 Ma, but there was little further deformation until Caledonian reworking east of the Uyea Shear Zone. There is no evidence for the Paleoproterozoic reworking that dominates large tracts of the Lewisian Gneiss Complex and of the Nagssugtoqidian Orogen of East Greenland. The more northerly location of the Uyea Gneiss Complex and extensive offshore basement of similar age implies that, prior to the opening of the North Atlantic Ocean, these rocks were contiguous with the Archean Rae Craton.

New age constraints on the Lan Sang gneiss complex, Thailand, and the timing of activity of the Mae Ping shear zone from in-situ and depth-profile zircon and monazite U-Th-Pb geochronology

The Mae Ping shear zone (MPSZ) is one of four major strike-slip systems in Thailand and a key structural element in some models of the Cenozoic tectonic evolution of Southeast Asia. In northwestern Thailand, the MPSZ exposes a ∼5-km-wide core of intensely deformed high-grade metamorphic and minor undeformed magmatic rocks, the Lan Sang gneiss complex (LSGC). Despite previous efforts to determine the chronology of the LSGC and, ultimately, the MPSZ, the evolution of this gneiss complex remains incompletely resolved. Here we present new zircon and monazite U-(Th-)Pb LA-(MC-)ICP-MS ages and whole-rock geochemical analyses from three peraluminous, high-K (meta)igneous granitic rocks (augen gneiss, sheared granodiorite, undeformed subvolcanic dike) of the LSGC. Our zircon U-Pb dating suggests that the granitic protolith of the augen gneiss intruded metasedimentary basement of Sibumasu as early as ∼235–220 Ma. Zircon rim and monazite U-(Th-)Pb ages from the same sample attest to periods of metamorphic recrystallization and overgrowth in the Late Triassic-Early Jurassic, Early Cretaceous and Eocene. The latter event had started by ∼45 Ma and culminated in partial melting of the basement gneisses and the intrusion of granodiorite magma into the LSGC at ∼35 Ma, contemporaneous with ductile deformation along the MPSZ. A late undeformed subvolcanic dike that crosscuts the mylonitic fabric of the calc-silicate and marble core of the MPSZ, dated in this study at ∼22 Ma, may indicate that ductile shearing had ceased by that time. Thus, our study provides the first evidence for Late Eocene-Early Miocene magmatism in the LSGC.

Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons

High pressure (HP) granulites of regional scale form as a result of tectonic events that lead to crustal thickening or subduction of the crust into the mantle. Most HP granulites are Phanerozoic, a few are Proterozoic, and Archaean HP granulites are even scarcer. Here we present field relationships, mineral chemistry and zircon U-Pb ages, Hf isotope data and trace elements data for the mafic granulite and associated rocks of the Uauá terrane, São Francisco Craton, Brazil, as evidence for the likely existence of a thick continental crust in the Mesoarchaean/Neoarchaean transition. The HP mafic granulite occurs as lensoid bodies within shallow dipping diorite to leucodiorite gneisses. Small scale layering between mafic granulite and diorite gneiss indicate these rocks are cogenetic. Garnet-clinopyroxene pairs with quartz, zircon, ilmenite, plagioclase, and clinopyroxene inclusions in garnet characterize the HP assemblage. Garnet porphyroblasts also show opx-cpx-plag symplectite coronas, which coupled with hornblende and plagioclase define PT conditions to lower grade granulite and amphibolite facies. Microprobe data combined with phase equilibria modelling (pseudosections) indicate 16–18 kbar and 930–960 °C for the peak HP assemblage, and 6.2–7.0 kbar and 660–760 °C for lower pressure granulite to amphibolite facies symplectite coronas. Metamorphic zircon rims in equilibrium with garnet have SHRIMP ages of 2819 ± 14, and igneous zircon cores of 3127 ± 14 Ma. The cores of zircon in associated gneiss samples have U-Pb ages between 3090 ± 13 Ma (LA-ICP-MS) and 3125 ± 15 Ma (SHRIMP) with age cluster at 3120 ± 6 Ma. εHf(t) values on igneous zircon of the HP mafic granulite are slightly positive whereas metamorphic zircon rims are negative. The associated diorite gneiss invariably yielded negative zircon εHf(t) values. Trondhjemite sheets intrusive in the mafic granulite are ca. 20 m.y. younger (2794 ± 13 Ma) than the host granulite. We interpret the igneous protoliths of the mafic granulite and leucodiorite gneiss as a single igneous complex emplaced in the continental crust, later deformed and metamorphosed by contraction and crustal thickening during collision of blocks/cratons to form Earth's first supercratons by the end of the Mesoarchaean.

Testing the fidelity of thermometers at ultrahigh temperatures

AbstractA highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H2O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO2 that is lower than the assumed value of unity.

Disequilibrium REE compositions of garnet and zircon in migmatites reflecting different growth timings during single metamorphism (Aoyama area, Ryoke belt, Japan)

Chemical disequilibrium of coexisting garnet and zircon in pelitic migmatites (Aoyama area, Ryoke belt, SW Japan) is shown by microtextural evidence and their heavy rare earth element (HREE) patterns. In zircon, two stages of metamorphic rim growth is observed under cathodoluminescence image, although their SHRIMP UPb zircon ages are similar at ca. 92 Ma. Inner and outer rims of zircon tend to show steep HREE patterns irrespective of the UPb age. The inner rims tend to give higher U content than the outer rims; some rim analyses give various Th/U ratios of 0.02–0.07 compared to the very low (<0.02) values seen in the rest of rim analyses. The higher-Th/U values are ascribed to the mixed analyses between thin prograde domains and thick retrograde overgrowths. Zircon grains with inclusions similar to previously-reported melt inclusions are further enclosed in garnet, supporting the growth of thin zircon domains coexisting with garnet during the prograde metamorphism.Garnet rims are commonly replaced by biotite-plagioclase intergrowths, indicating a back reaction with partial melts. Garnet exhibits decrease in HREE and Y concentrations towards the rim, pointing to its prograde growth. The garnet cores have prograde xenotime inclusions, show steep HREE patterns, and yield growth temperature of ~530–570 °C by a YAG-xenotime thermometer. On the other hand, the garnet rims have no xenotime inclusion and show flat HREE patterns. Rare garnet domains including sillimanite needles also show flat HREE patterns and low Y concentrations, which is interpreted as a product of dehydration melting consuming biotite and sillimanite at near-peak P-T conditions (~800 °C and ~0.5 GPa). One such garnet domain gives nearly-equilibrium REE distribution pattern when paired with the matrix zircon rims.Retrograde xenotime is present in the cracks in garnet and in the biotite-plagioclase intergrowths, suggesting that retrograde breakdown of garnet released HREE and Y to form it. Considering the availability of HREE and Zr and presence of melt inclusions in zircon rims, most part of the zircon rims with positive HREE patterns likely grew during the melt crystallization stage, meaning that the zircon rims and presently-preserved garnet domains did not grow in equilibrium. The above scenario was tested by the array plot analysis and it gave a result consistent with microtextural and traditional REE distribution constraints. Combination of microtextural and the array plot analyses may become a powerful tool to reliably correlate the zircon ages to the P-T evolution of the high-grade metamorphic rocks.

The timing of migmatization in the northern Arabian–Nubian Shield: Evidence for a juvenile sedimentary component in collision‐related batholiths

AbstractCollision‐related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from metasedimentary sources. However, in the late Neoproterozoic calcalkaline (CA) batholiths of the Arabian–Nubian Shield (ANS), which constitutes the northern half of the East African orogen, any sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here, we use paired in situ LASS‐ICP‐MS measurements of U–Th–Pb isotope ratios and REE contents of monazite and xenotime and SHRIMP‐RG analyses of separated zircon to demonstrate direct linkage between migmatites and granites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth at 640–600 Ma, in metapelites, migmatites and peraluminous granites of three metamorphic suites: Abu‐Barqa (SW Jordan), Roded (S Israel) and Taba–Nuweiba (Sinai, Egypt). The distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions, involving partial melting, prevailed for c. 10 Ma, from 620 to 610 Ma. REE abundances in monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates cluster at 600–580 Ma, recording retrogression to greenschist facies conditions as garnet continued to destabilize. Phase equilibrium modelling and mineral thermobarometry yield P–T conditions of ~650–680°C and 5–7 kbar, consistent with either water‐fluxed or muscovite‐breakdown melting. The expected melt production is 8–10 vol.%, allowing a melt connectivity network to form leading to melt segregation and extraction. U–Pb ages of zircon rims from leucosomes indicate crystallization of melt at 610 ± 10 Ma, coinciding with the emplacement of a vast volume of CA granites throughout the northern ANS, which were previously considered post‐collisional. Similar monazite ages (c. 620 Ma) retrieved from the amphibolite facies Elat schist indicate that migmatites are the result of widespread regional rather than local contact metamorphism, representing the climax of the East African orogenesis.

The Laqiong Sb-Au deposit: Implications for polymetallic mineral systems in the Tethys-Himalayan zone of southern Tibet, China

The Himalayan mineral field includes over 50 quartz-vein type Sb-Au deposits, and placer Au deposits. The poorly documented Laqiong deposit is a typical example of quartz-vein type Sb-Au mineralisation in Tethys Himalayan sequence. The orebody are controlled by shallow north-dipping normal faults and north–south trending faults. Magmatic zircons extracted from muscovitic leucocratic granite from the southern part of the Laqiong mine area yield a Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry U-Pb age of 14 ± 1 Ma (n = 12, MSWD = 0.9) that is similar to the 40Ar/39Ar age of ca. 14 Ma from hydrothermal sericite in auriferous sulphide-quartz veins. The εHf(t) values for the magmatic zircon rims range from −5.4 to −1.9, corresponding to two-stage Hf model ages of 1403–1214 Ma. Quartz from the mineralised veins has δ18OH2O-SMOW values varying from +4.97 to +9.59‰ and δDH2O-SMOW values ranging from −119.7 to −108.1‰. The δ13CV-PDB values for calcite from the ore Stage III range from −6.9 to −5.3‰, and calcite from Stage IV are −3.5 to −1.7‰. The δ18OV-SMOW values for calcite from Stage III are +20.3 to +20.6‰ and for Stage IV are −6.3 to −4.9‰. The stibnite and pyrite samples have 208Pb/204Pb ratios of 38.158 to 39.02, 207Pb/204Pb ratios of 15.554 to 15.698, and 206Pb/204Pb ratios of 17.819 to 18.681, and bulk and in-situ δ34SV-CDT values for stibnite, arsenopyrite and pyrite range from −1.1 to +2.3‰. The calcite from the orebodies are enriched in MREE and depleted in LREE and HREE. Fieldwork, petrological, and geochemical data collected during our study leads to the following salient findings: the mineralising fluid is a mix of magmatic and meteoric fluids; and the deposit is closely related to the emplacement of Miocene granites originating from a thickened continental crust.

Zircon perspectives on the age and origin of evolved S-type granites from the Cornubian Batholith, Southwest England

Granite stocks across southwest England have played a significant role in the genesis of world-class polymetallic mineralisation. This study presents the first geochemical and geochronological dataset for the composite Crownhill stock, placing it into the newly emerging geochronological framework for the Cornubian Batholith. The Crownhill stock comprises kaolinised two-mica granite in the north and variably-grained biotite granite in the south that encloses pods of tourmaline granite. All granites are peraluminous (A/CNK > 1) and the biotite (BG) and tourmaline granites (TG) are related by the replacement of biotite by tourmaline and secondary muscovitization. Integrated LA-ICP-MS and CA-ID-TIMS geochronology indicate two-phase magmatism, where zircon cores yield 288.9 ± 5 Ma and 286.4 ± 5 Ma and rims yield 277.74 ± 0.33 Ma and 278.35 ± 0.35 Ma, for BG and TG respectively. The zircon cores crystallised during initial magmatism, that formed the two-mica and muscovite granites (e.g., Carnmenellis, Bodmin, and Hemerdon) exposed in the north of the Crownhill stock. The zircon rims crystallised from the second phase of magmatism that formed the biotite and tourmaline granites (e.g., Dartmoor and St. Austell). This indicates that zircon crystals were assimilated from older two-mica and muscovite granites and entrained in the second phase of magmatism. Trace element compositions of zircon grains suggest that the rims crystallised from a more evolved magma, where zircon grains hosted in tourmaline granites are broadly more evolved than those from biotite granites. This is likely a result of elevated volatile concentrations delaying zircon fractionation. Trace cassiterite has been observed within interstitial tourmaline in the tourmaline granites, where crystallisation was likely induced by the removal of boron through tourmaline fractionation, coupled with the addition of Sn sourced from the alteration of biotite. The assimilation and over-printing of older granites by second-stage magmatism suggests that the initial phase of magmatism could be more widespread than initially thought and that tourmalinisation may have been responsible for leaching and remobilising Sn from the biotite-rich granites.

P–T evolution and episodic zircon growth in barroisite eclogites of the Lanterman Range, northern Victoria Land, Antarctica

AbstractPrograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M1–2 and M3. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M1) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M2 assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M1, whereas the inclusion‐poor outer part (M2) is typified by the outward decrease in Ca/Mg and XFe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M1) to ~22–25 kbar and 630–650°C (M2). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M3A garnet, ubiquitous overgrowth of the M3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean 206Pb/238U age of 515 ± 4 Ma (tσ), representing the time of the M2 stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M3. Average burial rates estimated from the M2 and M3 ages are too low (&lt;2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.

Archean crustal evolution of the Aravalli Banded Gneissic Complex, NW India: Constraints from zircon U-Pb ages, Lu-Hf isotope systematics, and whole-rock geochemistry of granitoids

The Aravalli Banded Gneissic Complex forms a block of Archean gneisses and granitoids in the southern domain of the Aravalli orogen in NW India. Results of U-Pb-Hf isotope analyses on zircon grains combined with whole-rock geochemical data reveal that the gneissic complex is made up of TTGs, which intruded during the Paleoarchean (3310 Ma) and Neoarchean (2563–2548 Ma), and granite-granodiorite-quartz diorite emplaced between 2545 and 2485 Ma. Zircon xenocrysts additionally point to Meso- and Neoarchean magmatic episodes at 3100–3000, 2880–2800, 2690 and 2660–2630 Ma. Ages from zircon rims in conjunction with previously published Sm-Nd isotope data suggest metamorphic events at ca. 2450 and 520 Ma. Medium-HREE characteristics and near chondritic εHf3.31 Ga of −0.4 suggest formation of the Paleoarchean TTGs by partial melting of a deep-seated mafic crust, derived from CHUR-like mantle immediately prior to the TTG formation. In contrast, the high-HREE characteristics and subchondritic εHf2.56-2.55 Ga from −3 to −8 of the Neoarchean TTGs suggest partial melting at relatively shallow depth of a younger oceanic crust, which was derived from a depleted mantle source at <3.1–3.4 Ga. The Neoarchean granitoids (2540–2485 Ma) with εHft from −8 to +2 in magmatic and xenocrystic zircon were likely originated by internal reworking of a heterogeneous felsic crust, consisting of Paleo-, Meso- and Neoarchean rocks. The Neoarchean quartz diorite (ca. 2545 Ma) represents a hybrid rock, which resulted from mixing of enriched mantle-derived mafic melts (εHf2.55Ga = −3.4) with felsic melts formed by anatexis of Paleoarchean crust (εHf2.55Ga = −11.7). Comparison of our new data from the Aravalli Banded Gneissic Complex with those of the adjacent Bundelkhand Craton indicate that both cratonic nuclei underwent different evolutionary histories during the Archean.