Protolith Zircon Research Articles

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ18O values (9‰–11‰), whereas their apparent 206Pb/238U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-I rims occur in granitic gneisses and jadeite quartzites. They have high δ18O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-II rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ18O values (5‰–8‰) and Th/U ratios (<0.01), compared with zircon cores and Type-I rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ18O values and apparent 206Pb/238U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.

Orogenic eclogites record relative magnitude of deep crustal flow and extent of migmatite-eclogite interaction

In exhumed orogens, refractory mafic rocks have the potential to preserve a record of petrogenesis and high-pressure (H P ) metamorphism that is commonly obliterated in quartzofeldspathic rocks owing to re-equilibration at high-temperature, low-pressure (L P –H T ) conditions. In the Montagne Noire (France) migmatite dome, located in the foreland of the Variscan orogen, eclogite is exposed in both the core and margin of the dome. In this study, we combine in situ U Pb petrochronology and oxygen-isotope analyses of key eclogite phases to demonstrate that eclogites from the two distinct domains had different protoliths and source regions, traveled relatively variable distances in the deep crust, and differentially interacted with surrounding migmatite prior to exhumation. Dome-margin eclogite zircons are small (~40 μm) with well-preserved inherited cores and thin (<15 μm) rims, compared to larger (40–120 μm) neo- and recrystallized dome-core zircons with small relict cores and wide (15–30 μm) recrystallized rims. Protolith and H P metamorphism ages were determined using in situ zircon and rutile petrochronology (LASS-ICP-MS). Both eclogites formed in a continental setting; dome margin protolith zircon cores formed at 442.5 ± 3.4 Ma (steep HREE slope, no Eu-anomaly) whereas zircon cores of the dome-core eclogites yielded scattered dates suggesting protolith crystallization between ~500–400 Ma (steep HREE slope, pronounced Eu-anomaly). Both eclogites experienced H P metamorphism at c. 320–310 Ma in garnet-stable, plagioclase-absent conditions. Most analyzed rutile yielded dates of 307–304 Ma associated with cooling. The record of H P fluid conditions was determined by O-isotope (SIMS) analyses of garnet and zircon. Dome-margin zircon cores and rims have δ 18 O of ~8.2–8.5 ‰, indistinguishable within uncertainty, in isotopic equilibrium with isotopically unzoned garnet (δ 18 O ~ 8.0–8.2 ‰). In contrast, zircons in dome-core eclogites have systematically lower zircon-core δ 18 O values compared to their rims and neocrystallized grains, and zircon cores were in equilibrium with major-cation zoned garnet with respect to oxygen. The two dome-core eclogite samples yielded zircon and garnet δ 18 O values of ~8.6–9.5 ‰ and ~ 9.7–10.5 ‰. Based on these results and existing H P fabric data for these eclogites, we propose that (1) gabbro protoliths for the two eclogites were emplaced at different depths in a Cambro-Ordovician continental crustal package; and (2) dome-core eclogites interacted extensively with surrounding gneiss during burial and foreland-vergent crustal flow, whereas the dome-margin eclogite was sourced proximally to the dome-emplacement location and had minimal chemical interaction with surrounding gneiss. At least parts of the Montagne Noire migmatite dome were deeply sourced, but rocks exhumed in the core had a more extensive and protracted history of deep-crustal flow than deep-crustal rocks exhumed at the margin. • Eclogites in migmatite dome retain memory of deep crust history. • Zircon (re)crystallization extent reveals distinct eclogite source, flow paths. • Petrochronology and O-isotopes record migmatite-eclogite interaction at depth. • Integrated garnet, zircon datasets reveal protolith, peak, exhumation histories. • Domes may exhume crust laterally-sourced >100 s of km from emplacement location.

Zircon as a Recorder of Trace Element Changes during High-Grade Metamorphism of Neoarchean Lower Crust, Shevaroy Block, Eastern Dharwar Craton, India

AbstractSystematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidised granulite-facies rocks near the magnetite–haematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-grade charnockite. Cathodoluminescence and back-scattered electron sub-grain imaging and sensitive high-resolution ion microprobe analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasingly depleted compositions between 2530 and 2500 Ma. With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted metamorphic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.

Post-UHP tectonic evolution of the East Kunlun orogenic belt, northern tibetan plateau: insight from the regional-scale crustal anatexis

ABSTRACT Extensive crustal anatexis is common during the decompressional exhumation of deeply subducted continental crust and the delamination of a collision‐thickened orogenic lithosphere, indicating that determining the timing of partial melting can provide insights into the possible tectonic evolution during orogenic processes. An integrated study of petrology, zirconology and whole-rock geochemical compositions from migmatites and pegmatites in the Nageng, Langmuri and Wulonggou area of the East Kunlun orogenic belt, is presented in this study. The protoliths of these migmatites include Tonian sedimentary rocks and ca. 440 Ma arc-related dioritic rock. Together with previous studies, an extensive crustal anatexis event recorded by anatectic zircons occurred after ca. 410 Ma. In addition, hafnium isotope analyses indicate that the anatectic zircons may or may not inherit their Hf isotope compositions from protolith zircons during partial melting, depending on whether or not the garnet Hf contributes to the Hf budget of anatectic melt. Thus, the most reasonable method to trace the source of granite is to compare the Hf isotope composition of anatectic zircon in migmatite and magmatic zircon in contemporaneous granite. By such method, anatectic melt produced by migmatites in this study could account for both penecontemporaneous A- and I-type granites with high magmatic zircon εHf(t) values and S-type granites with low magmatic zircon εHf(t) values. As the extensive crustal anatexis is later than both metamorphic peak and amphibole-facies retrogression of high-pressure and ultrahigh-pressure (HP-UHP) metamorphic rocks, it is reasonable that post-collision orogenic collapse of the collision‐thickened orogenic lithosphere as a result of delamination and asthenospheric upwelling occurred after ca. 410 Ma, which is also supported by contemporaneous granulite-facies metamorphism, molasse deposition, and magmatic events including Xiarihamu mafic-ultramafic intrusions, A-type granites, and mafic dykes formed in the continental rifting setting.

Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet

The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher 176Hf/177Hf ratios than their protolith zircons. More interestingly, the 176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated 176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.

Discovery of Early Paleozoic eclogite-facies metamorphic rocks in the western part of North Qinling Orogen and its geological significance

西秦岭地处中国中央造山系东西转换衔接部位，随着东秦岭以及祁连、柴北缘和东昆仑早古生代高压-超高压变质岩石的陆续发现和深入研究，西秦岭造山带变质作用研究吸引了大家的普遍关注。本文在大范围野外地质调查基础上，在北秦岭造山带西段天水南部的秦岭岩群长英质片麻岩中发现了一套石榴子石斜长角闪岩（榴闪岩），并开展了详细的锆石形貌和内部结构、微区微量元素和U-Th-Pb同位素研究。CL图像显示榴闪岩锆石普遍具有核-幔-边或核-边结构，部分存在原岩残留锆石。定年结果得到榴闪岩原岩残留锆石年龄为710±52Ma，同时得到497±3Ma、452±3Ma和423±7Ma三期变质年龄。其中497±3Ma变质年龄来自锆石核部并显示出轻稀土亏损、重稀土平坦且没有明显负Eu异常的稀土配分曲线特征，表明该时期的矿物组合中有石榴子石但没有斜长石，与榴辉岩相变质锆石特征一致；452±3Ma变质年龄来自锆石幔部或边部，对应测点重稀土元素的分异加大，并出现弱的负Eu异常，说明此时石榴子石被消耗且出现少量斜长石；423±7Ma变质年龄来自锆石的最边部，对应稀土配分曲线表现出更明显的负Eu异常和更大的重稀土元素分异特征，指示此时岩石中石榴子石含量更少，斜长石含量更多。上述结果表明该榴闪岩可能经历了~500Ma的榴辉岩相变质作用，并在~450Ma和~420Ma叠加了两期退变质改造。天水地区榴闪岩无论是其野外产状，还是变质锆石的形貌和内部结构、稀土配分曲线特征及其所记录的原岩和三期变质年龄都与北秦岭造山带东段大陆俯冲型高压-超高压变质榴辉岩近乎一致，指示早古生代时期，北秦岭造山带西段与东段经历了相似的大陆（深）俯冲和折返过程，它们共同构成一条统一的早古生代高压-超高压变质岩带。;The western part of North Qinling Orogen (NQO) is located at the conjunction area of several Paleozoic high pressure-ultrahigh pressure (HP-UHP) belts in the central China continent, the North Qinling UHP belt in the east and the North Qaidam and the East Kunlun UHP belts in the west, thus its metamorphism has drawn broad scientific interest. We document here an integrated study of zircon morphology, microstructure, in-situ trace element compositions and U-Th-Pb dating on a newly discovered garnet amphibolite in southern Tianshui, the western part of NQO. The garnet amphibolite occurs as a lensoid block in the felsic gneiss of the Qinling Complex, and mainly consists of amphibolite (50%~60%), garnet (10%~15%), quartz (~10%), plagioclase (5%~10%), muscovite (~5%) and ilmenite (~3%). Zircons from the garnet amphibolite display obvious core-mantle-rim or core-rim structures with a few protolith zircon relicts. LA-ICP-MS U-Pb dating yielded a protolith age of 710±52Ma and three metamorphic ages of 497±3Ma, 452±3Ma and 423±7Ma. The 497±3Ma metamorphic cores display flat heavy rare earth elements (HREE) patterns without obvious Eu anomalies, indicating they were formed in presence of garnet but absence of plagioclase under eclogite-facies condition; The 452±3Ma metamorphic mantles or rims show shallow HREE slopes with moderately negative Eu anomalies, indicating that these zircons grew accompanied by initial appearance of plagioclase and decomposition of garnet; The 423±7Ma metamorphic rims exhibit steep HREE patterns and marked negative Eu anomalies, indicating massive crystallization of plagioclase and decomposition of garnet during their formation. These results suggest that the garnet amphibolite probably experienced eclogite-facies metamorphism at~500Ma and two stages retrograde overprint at ~450Ma and ~420Ma. The occurrence, zircon microstructures, protolith and multiple metamorphic ages of the garnet amphibolite from Tianshui area are basically consistent with the continental-type HP-UHP rocks of the eastern part of NQO. Therefore, we suggest that both western and eastern part of NQO experienced similar continental deep subduction and exhumation processes and thus they form a united Early Paleozoic HP-UHP metamorphic belt.

Exhumation of subducted continental crust along the arc region

The Kutná Hora crystalline complex (KHCC) in the Bohemian Massif is a HP/HT complex adjacent to the magmatic arc. It is dominated by migmatite, orthogneiss and granulite with bodies of eclogite and peridotite. The KHCC migmatite consists of K-feldspar, plagioclase, quartz, phengite, biotite, garnet and kyanite. Melting conditions were estimated at 780 °C and >16 kbar and inferred melt volume varies between 1 and 4 vol%. Peak temperature is 865 °C at 18–19 kbar followed by decompression in the presence of melt to 12–13 kbar and 770–800 °C. U-Pb monazite geochronology reveals a spread of ages between 550 Ma and 330 Ma. REE patterns show low Yb/Gd for 550–500 Ma, high Yb/Gd for ages at ~480 Ma, and decreasing Yb/Gd towards ~340 Ma. First monazite in equilibrium with garnet constrains the HP metamorphism to ~350 Ma, which is followed by recrystallization of monazite down to 325 Ma. U-Pb zircon geochronology displays an age range from ~670 Ma to ~430 Ma. The broad age range records a span of protolith crystallization and/or old metamorphism. The presence of HP ky + mu migmatite, their P–T path, protolith zircon and monazite metamorphic ages and whole-rock geochemistry are similar to HP migmatites in the Eger crystalline complex (ECC) in the Saxothuringian domain further in the west. We propose the following geodynamic scenario for subduction-relamination-exhumation mechanism: (i) subduction of the Saxothuringian continental lithosphere at 360 Ma related to early stage of trans-lithospheric diapirism triggered by arc-related magma weakening; (ii) large-scale emplacement of relaminant into the upper plate lithosphere at 350–340 Ma; and (iii) return flow of the relaminant along the subduction interface (the ECC) and emplacement of relaminant in the upper–middle crust in the rear part of the arc system (the KHCC) at 340–330 Ma.

High temperature glacial meltwater–rock reaction in the Neoproterozoic: Evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U–Pb dating for granitic gneiss from the northeastern Sulu orogen, east-central China. Zircon U–Pb dating yields protolith ages of 753±15Ma to 780±13Ma and metamorphic ages of 209±3 to 244±7Ma. The Neoproterozoic cores with concordant U–Pb ages exhibit a wide δ18O range from −11.0‰ to 5.8‰, which is nearly the same as those for cores with discordant U–Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around −10‰ whereas the rims of the other samples show a wider range from −9.8‰ to 5.0‰. The δ18O values as negative as −11.0‰ for zircons with concordant Neoproterozoic U–Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith zircon cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than −9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water–rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

The crust‐mantle interaction in continental subduction channels: Zircon evidence from orogenic peridotite in the Sulu orogen

AbstractA combined secondary ion mass spectrometer and laser ablation‐(multicollector)‐inductively coupled plasma mass spectrometer study of zircon U‐Pb ages, trace elements, and O and Hf isotopes was carried out for orogenic peridotite and its host gneiss in the Sulu orogen. Newly grown zircon domains exhibit weak zoning or no zoning, relatively low Th/U ratios (&lt;0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element‐HREE patterns, negative Eu anomalies, and negative to low δ18O values of −11.3 to 0.9‰ and U‐Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from metasomatic fluids during the early exhumation of deeply subducted continental crust. The infiltration of metasomatic fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole, serpentine, and chlorite. In contrast, relict zircon domains exhibit magmatic zircon characteristics. Their U‐Pb ages and trace element and Hf‐O isotope compositions are similar to those for protolith zircons from ultrahigh‐pressure metamorphic rocks in the Dabie‐Sulu orogenic belt. Thus, these relict magmatic zircons would be physically transported into the peridotite by metasomatic fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab‐mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust‐mantle interaction at the deep continental subduction zone.

The tectonic transition from oceanic subduction to continental subduction: Zirconological constraints from two types of eclogites in the North Qaidam orogen, northern Tibet

In the plate tectonics theory, continental subduction is pulled by subduction of dense oceanic crust. In practice, however, it is not easy to demonstrate that preceding oceanic crust exposes as oceanic-type eclogite together with continental-type eclogite in collisional orogens. The North Qaidam orogen in northern Tibet is an ultrahigh-pressure (UHP) metamorphic belt that contains the two types of eclogites, providing us with an excellent opportunity to study the tectonic transition from oceanic subduction to continental subduction. In order to constrain the protolith nature and metamorphic evolution of eclogites, we performed a combined study of zircon U–Pb ages, trace elements, mineral inclusions and O–Hf isotopes for various eclogites from the orogen. We discriminate the two types of eclogites by their differences in zircon U–Pb ages and O–Hf isotopes. CL-dark zircon domains exhibit high Th/U ratios, steep HREE patterns and significantly negative Eu anomalies, indicating that they are protolith zircons of magmatic origin with different extents of metamorphic recrystallization. Relict magmatic zircon domains in Type I eclogites yield Neoproterozoic protolith ages of >830Ma and Hf model ages of 850–1100Ma, whereas those in Type II eclogites yield Cambrian protolith U–Pb ages of >489Ma and Hf model ages of 500–650Ma. Most of the CL-bright zircon domains show low Th/U ratios, flat HREE patterns and no negative Eu anomalies, and contain mineral inclusions of garnet, omphacite and rutile, indicating their growth under eclogite-facies metamorphic conditions. These metamorphic domains have consistent eclogite-facies metamorphic ages of 433–440Ma throughout the North Qaidam orogen, regardless of the eclogite types and locations. The metamorphic zircon domains in Type I eclogites mostly exhibit δ18O values higher than normal mantle values, whereas Type II eclogites mostly have δ18O values lower than the normal mantle values. The difference in the δ18O values indicates that their protoliths underwent different temperatures of hydrothermal alteration at different tectonic settings. Combining zircon U–Pb ages and O–Hf isotope compositions with local tectonics, it is inferred that Type I eclogites were metamorphosed from Neoproterozoic continental mafic rocks, whereas Type II eclogites were metamorphosed from oceanic mafic rocks that were subducted prior to the continental subduction. The consistent eclogite-facies metamorphic ages for the two types of eclogites indicate that the exhumed oceanic-type eclogite was detached from the subducted oceanic crust and then entrained by the exhuming continental crust. Therefore, the coexistence of oceanic- and continental-type eclogites in the North Qaidam orogen demonstrates the tectonic transition from oceanic subduction to continental collision in the early Paleozoic.

Zircon U–Pb and trace element zoning characteristics in an anatectic granulite domain: Insights from LASS-ICP-MS depth profiling

Understanding the geochemical characteristics of metamorphic zircon, and how they may be modified by recrystallization processes, is fundamental to defining the timescales of tectonic processes affecting continental lithosphere. We utilize laser ablation split-stream (LASS)-ICP-MS depth-profiling analysis to obtain a continuous rim-to-core record of the U–Pb ages and trace-element composition preserved within variably recrystallized zircon from different rock types within a well-studied granulite domain in the western Grenville Province, Canada. Detailed analysis of the depth-resolved signal enables definition of chemically distinct (homogeneous) internal domains and heterogeneous intervening zones that can generally be correlated with textural features observed in CL. Three age populations have been distinguished within the ~35μm deep profiles that correlate well with the established timing of protolith formation, granulite-facies metamorphism, and amphibolite-facies shearing, respectively. The U–Pb isotopic system and Th/U ratios in much of the crystal interiors have undergone considerable modification, as evidenced by a linear correlation between 207Pb/206Pb age and Th/U ratio. Interior and rim domains commonly contain blurred or faded oscillatory zoning patterns, suggesting that solid-state recrystallization is at least partially responsible for the modified U–Th–Pb composition. A number of systematic trends in trace element composition are also observed between interior domains and recrystallized rims, including 1) decreased Th/U (to ~0.1), 2) tighter clustering of Hf concentrations, 3) decreased total REE, 4) unchanged Eu anomalies, and 5) a widened spread of HREE enrichment values (YbN/GdN). Both YbN/GdN vs. Th/U and U/Ce vs. Th plots show increasing degree of compositional differentiation from protolith zircon as a function of metamorphic reworking processes (i.e. sample type). The transition zones between interior and rim domains exhibit textural features consistent with solid-state recrystallization and migration of a transgressive recrystallization front. The main compositional characteristics of the transition zones are 1) spikes in U and Ta, 2) steady to abrupt increases in Hf toward maximum values in the rims, 3) steady to abrupt decreases in Th toward minimum values in the rims, and 4) variable REE patterns that include spikes, troughs, and montonic decreases. Most of these characteristics are consistent with the purging of larger ions from the crystal lattice during recrystallization, with local bands of high concentration associated with recrystallization fronts.

Zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh-pressure jadeite quartzite in the Dabie orogen

A combined study of mineral inclusions, U–Pb ages, trace elements and Hf-O isotopes was carried out for zircons from a coesite-bearing jadeite quartzite in the Dabie orogen. The results provide insights into the action of ultrahigh-pressure (UHP) metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone. The zircons mostly show core-rim structures in cathodoluminescence images. The overgrown rims contain rare mineral inclusions, and exhibit concordant U–Pb ages of 225 to 246Ma and flat HREE patterns with negligible Eu anomalies. In contrast, the relict cores contain UHP metamorphic mineral inclusions such as coesite, jadeite and rutile, and show discordant U–Pb ages ranging from 983 to 2045Ma and steep REE patterns with significant negative Eu anomalies. The U–Pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000±43Ma and 234±18Ma, respectively. We interpret the rims with Triassic ages as the new growth of metamorphic origin and the cores with Precambrian ages as the protolith relics of magmatic origin. The relict magmatic zircons underwent two subtypes of metamorphic recrystallization, i.e., solid-state transformation and metasomatic alteration. The solid-state recrystallized zircons exhibit slightly discordant U–Pb ages close to the protolith age, steep MREE–HREE patterns, and almost unchanged Hf isotope ratios. These observations point to the lowest degree of resetting to the geochemical composition of protolith zircons. In contrast, the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon REE composition and U–Pb and Lu–Hf isotopic systems. All of the zircon domains, regardless of the rims and cores, show relatively consistent δ18O values of 4.0±0.2‰. Such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the UHP metamorphic fluids of Triassic age. The metasomatic recrystallization of protolith zircons is indicated by the occurrence of UHP inclusion minerals such as coesite, rutile and jadeite in sealed microcracks. In this regard, the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the UHP conditions. As such, the metasomatic recrystallization has heterogeneously reset the U–Pb and Lu–Hf isotope systems of protolith zircons. The composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in Si, Ti, Na and Al. These elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss. Therefore, the jadeite quartzite would be precipitated from the UHP metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction-zone metamorphism.

The anatectic effect on the zircon Hf isotope composition of migmatites and associated granites

Zircon Hf isotope composition is widely used to trace the growth and evolution of continental crust. However, it is controversial whether the Hf isotope composition of magmatic zircons can faithfully reflect that of their sources, especially for S-type granites. In order to provide an insight into this issue, we have revisited the published Lu–Hf isotope data of zircons from well-studied migmatites and associated granites in the Sulu orogen and the Cathaysian terrane, respectively. The results show greatly elevated 176Hf/177Hf ratios (by more than 10ε units) for newly grown zircon domains compared to the relict zircon domains. This indicates considerable contributions from non-zircon Hf to anatectic melts during crustal anatexis and subsequent magmatism. Furthermore, this more radiogenic Hf isotope signature was not erased during magmatic processes such as crystal fractionation during melt ascent and emplacement. The budget of Hf isotopes in source rocks with respect to mineral Lu/Hf ratios suggests the involvement of Hf-bearing major minerals in anatectic reactions by dissolving Hf-bearing major minerals into the anatectic melts. The significant Hf isotope variations in some anatectic and magmatic zircon domains from the migmatites and granites suggest not only the source heterogeneity but also the variable non-zircon Hf contributions. As such, the Hf isotope compositions of anatectic and magmatic zircons are substantially dictated by the mass balance between the non-zircon Hf from anatectic reactions and the zircon-Hf from the dissolution of protolith zircons into the anatectic melts. They are primarily controlled by P-T conditions and mechanism of crustal anatexis, and the magmatic processes during melt evolution. The present study highlights the important contribution of non-zircon Hf to the anatectic and magmatic zircon domains. In this regard, the greatly elevated 176Hf/177Hf ratios for newly grown zircon domains in the migmatites and granites cannot reflect the Hf isotope compositions of their source rocks. Such a deviation must be kept in mind when linking relatively depleted Hf isotope compositions to the nature of source rocks in petrogenetic interpretation and to the growth of continental crust. The contribution of non-zircon Hf can also better account for the variable Hf isotope compositions often shown by zircons from S-type granites.

Zircon (re)crystallization during short-lived, high-P granulite facies metamorphism (Eger Complex, NW Bohemian Massif)

The Eger Complex in the northwestern Bohemian Massif consists mainly of amphibolite facies granitic gneisses containing a subordinate volume of felsic granulites. Microstructural changes and modelling of metamorphic conditions for both rock types suggest a short-lived static heating from ~760 to ~850 °C at a constant pressure of ~16 kbar, which led to the partial granulitization of the granitoid rocks. Detailed study of the protolith zircon modifications and modelling of the Zr re-distribution during the transition from amphibolite to granulite facies suggests that the development of c. 340 Ma old zircon rims in the granulite facies sample is the result of recrystallization of older (c. 475 Ma) protolith zircon. This study suggests that the partial granulitization is a result of a short exposure of the Eger Complex metagranitoids to a temperature of ~850 °C at the base of an arc/fore-arc domain and their subsequent rapid exhumation during the Lower Carboniferous collision along the western margin of the Bohemian Massif.

Syn-exhumation partial melting and melt segregation in the Sulu UHP terrane: Evidences from leucosome and pegmatitic vein of migmatite

Abstract In order to investigate partial melting and melt evolution during exhumation of deeply subducted continental crust, we carried out a combined study on layered leucosomes and pegmatitic veins from the Weihai–Rongcheng migmatites in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The leucosomes are millimetric to centimetric in thickness and mainly consists of K-feldspar + quartz + plagioclase. The pegmatitic veins emplace along the compositional layers or crosscut them and are mainly composed of K-feldspar + quartz. CL images show that most zircon grains of the leucosomes have a core–rim zoning structure, whereas zircons of the pegmatitic veins are mostly new growth grains with rarely preserved relict domains. The inherited zircon domains (ca. 750 Ma) are of a magmatic origin with zircon eHf(t) values between − 5.7 and 5.3 (mean = − 0.8 ± 1.5), suggesting that the protolith rock is a Mid-Neoproterozoic juvenile crust. The zircon overgrowth rims (223 ± 3 Ma) of the leucosomes and the new zircon grains (217 ± 2 Ma) of the pegmatitic veins formed in equilibrium with melt and are similar to magmatic zircons in terms of their CL images and trace element compositions, e.g., heavy-enriched REE patterns (i.e., very low (Gd/Lu)N ratios) with positive Ce and variably negative Eu anomalies, high Y contents and low Hf/Y ratios. Compared with the inherited protolith zircon domains, the Triassic zircon domains of the leucosomes and the pegmatitic veins have obviously high U but low Th contents, resulting in very low Th/U ratios (

Petrogenesis of metatexite and diatexite migmatites determined using zircon U–Pb age, trace element and Hf isotope data, Higo metamorphic terrane, central Kyushu, Japan

AbstractMetatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the Higo low‐P/high‐Tmetamorphic terrane. Here, we report data from an outcrop in the highest grade part of the granulite facies zone, in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic‐structured metatexite within pelitic gneiss. The migmatites and gneiss contain the same peak mineral assemblage of biotite + plagioclase + quartz + garnet + K‐feldspar with retrograde chlorite ± muscovite and some accessory minerals of ilmenite ± rutile ± titanite + apatite + zircon + monazite ± pyrite ± zinc sulphide ± calcite. Calculated metamorphicP–Tconditions are 800–900 °C and 9–12 kbar. Zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning, but no core–rim structure. Zircon from the gneiss and metatexite forms euhedral–subhedral grains comprising inherited cores overgrown by thin rims. The overgrowth rims in the metatexite have lower Th/U ratios than zircon in the diatexite and yield a206Pb/238U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma206Pb/238U age derived from zircon in the diatexite. Zircon from the diatexite has variableREEcontents with convex upward patterns and flat normalizedHREE, whereas the overgrowth rims in the metatexite and gneiss have steepHREE‐enriched patterns; however, both types have similar positive Ce and negative Eu anomalies.176Hf/177Hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite. Based on U–Pb ages, trace element and Hf isotope data, the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt, following partial dissolution of inherited protolith zircon during anatexis, whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component. By integrating zircon and petrographic data for the migmatites and pelitic gneiss, the metatexite migmatite is interpreted to have formed byin situpartial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived juvenile component. The Cretaceous high‐temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle‐derived basalts under a volcanic arc along the eastern margin of the Eurasian continent and advection of heat via hybrid silicic melts from the lower crust. Post‐peak crystallization of anatectic melts in a high‐Tregion at mid‐crustal depths occurred in the intervalc. 116–110 Ma, as indicated by the difference in zircon ages from the metatexite and diatexite migmatites.

Contrasting Lu–Hf isotopes in zircon from Precambrian metamorphic rocks in the Jiaodong Peninsula: Constraints on the tectonic suture between North China and South China

Zircon Lu–Hf isotopes, together with zircon U–Pb ages, trace elements and O isotopes were analyzed for various types of metamorphic rocks from the Jiaodong and Jiaobei terranes in the Jiaodong Peninsula, east-central China. The results show contrasts in the crustal nature between the two terranes and thus provide constraints on the tectonic suture between the North China Craton and the South China Block in this region. Zircon U–Pb dating for the Jiaodong terrane indicates protolith emplacement mainly in the middle Neoproterozoic and subordinately in the middle Paleoproterozoic. The protolith zircon of Neoproterozoic U–Pb ages exhibits a wide range of ɛHf(t) values from −16.8±1.0 to 5.9±1.0, indicating reworking of both ancient and juvenile crustal rocks during the Neoproterozoic magmatism. Low zircon δ18O values (<5‰) are a diagnostic feature of the Neoproterozoic protolith in the Jiaodong terrane. Growth of the juvenile crust in the middle Paleoproterozoic is also recorded by a few samples. In contrast, zircon U–Pb dating for the Jiaobei terrane yields three episodes of magmatism at ∼2.9, ∼2.7 and ∼2.5Ga, respectively. Zircon Hf isotope compositions suggest growth of the juvenile crust at the two earlier episodes but reworking at the later episode. The crustal reworking is particularly intensive in the middle Paleoproterozoic, resulting in the diagnostic high-pressure granulite-facies metamorphism at 1.90–1.85Ga and migmatization at 1.75Ga. At the same time, remelting of the Archean crust gave rise to the 1.90–1.85Ga S-type granitic rocks. The zircon of Paleoproterozoic U–Pb ages usually exhibits high δ18O values (7–10‰). Taken together, the crustal nature of the Jiaodong and Jiaobei terranes are generally similar to that of the South China Block and the North China Craton, respectively. This lends support to the contention that the suture between the two collided continents is located along the Wulian–Yantai fault marking the boundary between the Jiaodong and Jiaobei terranes.

Fluid-rock interaction and geochemical transport during protolith emplacement and continental collision: A tale from Qinglongshan ultrahigh-pressure metamorphic rocks in the Sulu orogen

Ultrahigh-pressure (UHP) metamorphic rocks from the Qinglongshan region of the Sulu orogen are comprehensively studied for their whole-rock geochemistry, mineral O isotopes and zirconology. The metamorphic minerals, which experienced eclogite- to amphibolite-facies metamorphism, exhibit low to negative δ18O values, suggesting that the 18O-depletion of UHP rocks was acquired from their igneous protolith due to high-T meteoric-hydrothermal alteration during the Neoproterozoic. The O isotope heterogeneity in the protolith was not homogenized during the Triassic UHP metamorphism, indicating very limited fluid flow during orogenesis. However, the fluid flow is locally significant during exhumation of the UHP rocks, resulting in the formation of quartz veins, symplectites and coronas. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low 87Sr/86Sr ratios at t1 = 750 Ma but much radiogenic Sr isotopes at t2 = 230 Ma, suggesting the mobility of water-soluble elements due to hydrothermal alteration during protolith emplacement and metamorphic dehydration during continental collision. Fluid-rock interaction during exhumation would also have mobilized Al, Si, Ca and LREE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to differential redistribution of trace elements and O-Hf isotopes. Newly grown zircons of metamorphic origin exhibit negative δ18O values, indicating precipitation from negative δ18O fluids that were likely generated by metamorphic dehydration of the hydrothermally altered negative δ18O rock-forming minerals during the Triassic. The metamorphic zircons exhibit relatively homogeneous Hf isotope compositions, suggesting that fluid Hf isotopes originated from the same Hf isotope composition of the protolith. Relict zircon domains of magmatic origin exhibit both positive eHf(t) and negative eHf(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.

Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: Petrological evidence from granitic gneiss in the Sulu orogen

Petrological evidence is provided for partial melting of ultrahigh-pressure (UHP) metamorphic granitic gneiss in the Sulu orogen. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss. Zircon domains grown from these melts were identified based on CL images, mineral inclusions and REE patterns. Some large zircon grains (>100μm) contain small relict domains of magmatic origin, suggesting nearly complete dissolution of the protolith zircon during the anatexis. All newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions. One group of domains contain no coesite inclusion and exhibit high U contents but low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and UPb ages of 217±2 to 224±2Ma. The other group of domains contain coesite inclusions and exhibit low U contents and very low Th/U ratios (<0.01), steep REE patterns with strong negative Eu anomalies, and UPb ages of 221±5 to 226±3Ma. The two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative Eu anomalies that are absent for metamorphic zircons grown from aqueous fluids. The zircon UPb ages of 217±2 to 226±3Ma are close to, but slightly younger than, known ages for major UHP metamorphism in the Sulu orogen. Therefore, the UHP gneiss experienced incipient melting during the initial exhumation subsequent to the peak UHP metamorphism and extensive anatexis later at lower pressures. Muscovite relicts coexist with cuspate feldspars, suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the UHP granitic gneiss. As the degree of partial melting increases with temperature, significant fractions of the anatectic melts would be produced in the regional gneiss provided that the UHP rocks experienced “hot” exhumation. Such melts can be gathered together, being eventually emplaced as synexhumation granitic intrusions.

Zircon U–Pb and garnet Lu–Hf geochronology of eclogites from the Lhasa Block, Tibet

The Tibetan Plateau is the result of several collision events between a series of Gondwana-derived blocks (e.g., Qiangtang, Lhasa and India) and the Asian continent since the early Paleozoic. The occurrence of high-pressure eclogites has a significant bearing on the Paleo-Tethys subduction and plate suturing processes in this area. A quartzite, together with its host eclogite from the Jilang County in the Lhasa Block of northern Tibet, was investigated in terms of petrology, geochemistry, and combined Lu–Hf and U–Pb geochronology. The composition of the Jilang eclogite is similar to supra-subduction-zone basalts found in back-arc basins (LREE depleted, negative Nb–Ta anomalies, positive Pb anomalies, εNd(t)=~+8). The observations of coesite/coesite pseudomorph in the eclogites and the estimated P–T conditions of 3.4–3.8GPa and 753–790°C for the eclogites imply that the Jilang eclogites have undergone ultra-high-pressure metamorphism. The Lu–Hf age of 265.9±1.1Ma (2σ, 14 points, MSWD=1.8) of the Jilang eclogite most likely reflects garnet growth and is interpreted to reflect eclogite–facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. U–Pb analyses of the metamorphic rims of zircons from eclogites indicate that two episodes of zircon growth occurred at ~261Ma and ~238Ma. The identical zircon 206Pb/238U age of 291±4Ma (2σ, MSWD=2.2, n=12) for the quartzite and 290.6±6.2Ma (2σ, MSWD=1.1, n=11) for the magmatic cores of the eclogite suggest that the detrital zircon in quartzite has the same origin as the protolith zircon in the eclogite. The initial opening of the back-arc basin in the Lhasa Block was no later than ca. 290Ma; the final closure of the back-arc basin in the Lhasa Block, led by the arc–continent collision between the Gangdese island arc and the northern Lhasa Block, was no earlier than ca. 266Ma.