Metamorphic Zircon Growth Research Articles

Low-δ18O mantle-derived magma in Panjal Traps overprinted by hydrothermal alteration and Himalayan UHP metamorphism: Revealed by SIMS zircon analysis

We report two generations of low-δ18O zircons from the Himalayan eclogites and their host gneisses. In situ SIMS δ18O analyses on single zircon crystals (with known age and Hf isotope ratios) from two populations of chemically distinct zircons demonstrate a complex history: (1) an early low-δ18O mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-δ18O minerals during the ultrahigh-pressure metamorphism. Magmatic zircon (269Ma) in Group I eclogites yielded δ18O values from 1.9 to 4.6‰ VSMOW with an average value of 4.0±0.2 (n=35, the error is 2SD analytical precision and “n” represents number of analyzed spots), which is lower than the typical mantle values (5.3±0.6, 2SD). In contrast, metamorphic zircons (45Ma) in Group II eclogites preserve unusually low, negative δ18O values from −3.9 to −2.7‰ (average: −3.4±0.4, n=35, 2SD). Zircons in felsic gneiss that surround Group II eclogites have inherited magmatic cores (ca. 260Ma) with δ18O values of ca. 2.9‰, which decrease to −0.1‰ in metamorphic (ca. 45Ma) rims. These zircons preserve lower δ18O values than would be equilibrated with typical mantle. The low-δ18O values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative δ18O values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons. This study reports evidence for melting of subducted low-δ18O ocean crust to form low-δ18O mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578–581) for Proterozoic Scourie Dikes.

Zircon geochemical constraints on the protolith nature and metasomatic process of the Mg-rich whiteschist from the Western Alps

The Mg-rich silicic crustal rocks characterized by high SiO2 and MgO contents are widespread in the Alps, providing rare targets to investigate the origin of Mg-rich fluids in the continental subduction zone. However, it is a prerequisite to elucidate the protolith nature of such Mg-rich rocks, including the possible alteration process experienced before the Alpine metamorphism. For this purpose, we carried out a detailed study of trace elements, O-Hf isotopes and U-Pb ages for zircons from the coesite-bearing whiteschist and its country rock metagranite at Dora-Maria in Western Alps. The whiteschist is characterized by much higher MgO contents than the adjacent metagranite. Relict magmatic zircons from the two types of rocks give essentially the same U-Pb ages of ~262Ma, similar REE contents and patterns, the same range of εHf(t1) values from −6.4 to 2.8 and δ18O values from mostly 9 to 11‰. These features provide compelling evidence that the whiteschist and metagranite share the same protolith, i.e., the Permian granite. All the above geochemical compositions are consistent with the argument that the high MgO content of the whiteschist was acquired from high Mg fluids from dehydration of the serpentinized peridotite at the slab-mantle interface in the continental subduction channel. The hypothesis of a pre-Alpine hydrothermal alteration of the granitic protolith to cause the MgO enrichment is not favorable. Zircon in the metagranite does not show any metamorphic overgrowth during the Alpine orogeny, in contrast to the extensive overgrowth of metamorphic rims at ~35Ma in the whiteschist. The metamorphic rims show relatively low REE contents with variably flattened HREE patterns, different from the relict magmatic zircons that exhibit relative high REE contents with steep HREE patterns. This difference suggests that the breakdown of hydrous minerals plays a crucial role in dictating mineral reactions for zircon growth during the continental subduction-zone metamorphism. This interpretation is also consistent with the local occurrence of garnet coronae along the grain margins of biotite in the metagranite. Therefore, this study provides not only the constraints on the protolith nature and metasomatic process of Mg-rich whiteschist from the Western Alps, but also insights into the role of metamorphic fluids in controlling mineral reactions for the metamorphic growth of both zircon and garnet in the continental subduction zone.

Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet

AbstractThe P–T–t path of high‐P metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P–T path that was marked by post‐Pmax heating–decompression and subsequent isothermal decompression following the attainment of peak high‐P and low‐T conditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water‐unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water‐unsaturated condition relative to water‐saturated model along the prograde path. The high‐P amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a true P–T estimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet‐bearing amphibole schist yield four two‐point garnet–whole‐rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well‐preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism; however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.

Linking the Windmill Islands, east Antarctica and the Albany–Fraser Orogen: Insights from U–Pb zircon geochronology and Hf isotopes

U–Pb and Hf isotopic data from metasedimentary and magmatic rocks from the Windmill Islands in Wilkes Land, East Antarctica, confirm age and crustal evolution links between the Albany–Fraser Orogen and this part of East Antarctica. Detrital zircon age data indicate that the protoliths to the metasedimentary rocks of the Windmill Islands have maximum depositional ages of c. 1350Ma. Metamorphic zircon growth at c. 1300Ma and a crystallisation age of c. 1315Ma for the protoliths to an orthogneiss that intrudes the metasedimentary rocks provide a minimum depositional age. Significant detrital zircon age components are identified at 1790Ma, 1595Ma and 1390Ma. The 1350–1300Ma depositional interval and the detrital age components suggest that the Windmill Islands metasedimentary rocks can be linked to metasedimentary rocks of the Arid Basin in the Albany–Fraser Orogen. The sediment sources were likely to be the West Australian Craton as well as a significant component from the c. 1410Ma Loongana Arc in the Madura Province. This combination of sources suggests a back-arc setting for the Arid Basin, consistent with the short interval between deposition and high thermal gradient metamorphism. The magmatic rocks in the Windmill Islands have intrusive ages of c. 1315Ma, 1250–1210Ma and 1200–1160Ma. The first phase of magmatism was likely to be derived from melting of Arid Basin metasedimentary rocks, based on abundant inherited zircon with similar ages to the surrounding metasedimentary rocks. The final two phases of magmatism have juvenile εHf(t) values consistent with a greater proportion of mantle melt sources.

Zircon Growth during Progressive Recrystallization of Gabbro to Garnet Amphibolite, Eastern Segment, Sveconorwegian Orogen

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Herrestad, in the eastern part of the 1±0 Ga Sveconorwegian Province, involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of Zr-bearing minerals. In this case, textures show that baddeleyite is by far the main source of Zr for metamorphic zircon growth. The amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization, which in turn was controlled by deformation and the presence of a fluid as a transport medium. Zircon in the Herrestad rocks shows a range of morphologies and internal textures at different degrees of metamorphic recrystallization. Igneous zircon occurs together with baddeleyite in coarse-grained olivine-free facies of the gabbro. Metamorphic polycrystalline zircon rims on baddeleyite and minute (<5 μm) bead-like zircon grains at Fe-Ti oxide boundaries characterize the transition to coronitic metagabbro. With increasing metamorphic recrystallization, polycrystalline zircon rims grow at the expense of baddeleyite and the amount of minute bead-like zircon increases, forming strings of zircon beads with increasing distance from Fe-Ti oxide grains. The progressive breakdown of baddeleyite results in polycrystalline zircon aggregates that become denser and finally form single grains in completely recrystallized garnet amphibolite. Late magmatic zircon crystallized at 156765 Ma, whereas metamorphic zircon dates amphibolite-facies metamorphic recrystallization at 97067 Ma. The Herrestad case illustrates a general rule that the bulk Zr budget in originally baddeleyite-bearing rocks will rapidly become locked into metamorphic zircon during the first event of metamorphic recrystallization, when silica and Zr are released from the igneous minerals. Incomplete metamorphic recrystallization and partial preservation of baddeleyite, however, also allows later stages of zircon formation. Thus, in incompletely reacted rocks the final result may be highly complex with microscale zircon of several age generations. (Less)

Multi-stage zircon and monazite growth revealed by depth profiling and in situ U–Pb geochronology: Resolving the Paleoproterozoic tectonics of the Trans-Hudson Orogen on southeastern Baffin Island, Canada

Understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust. We present extensive in situ U–Pb monazite geochronology and U–Pb zircon depth profiling analyses from the mid-crust exposed on southeastern Baffin Island, Canada, in the internal zone of the Paleoproterozoic Trans-Hudson Orogen. In situ LA-ICP-MS U–Pb monazite dating on ten metasedimentary rocks (totalling >300 analyses) from different paleodepths reveals a dominant ca. 1860–1820Ma monazite population that records regional mid-amphibolite- to granulite-facies metamorphism with coeval fabric development. Subordinate monazite with ages between ca. 1800–1750Ma are interpreted to have formed through dissolution-reprecipitation. The SIMS U–Pb depth-profiling technique allowed dating of ca. 1861–1833Ma zircon rims and less abundant ca. 1800–1764Ma zircon rims that would be unresolvable using conventional spot dating. Trace element depth profiling via LA-ICP-MS shows that zircon rims typically have lower Th/U values, higher Hf contents and different REE compositions than zircon interiors. Zircon rim compositions are highly variable, with no apparent link to U–Pb age, and are interpreted as the combined result of metamorphic zircon growth and dissolution-reprecipitation. Our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon, and that they are both sensitive to U–Pb age resetting at lower temperatures during late-orogenic tectonism. Regional mid-amphibolite- to granulite-facies metamorphism and east-directed crustal shortening are bracketed between ca. 1860–1820Ma. These events are interpreted to result from the accretion of cratonic blocks in the internal zone of the Trans-Hudson Orogen. The integrated geochronological approach on regionally-distributed samples enabled us to document ca. 1800–1750Ma monazite domains and zircon rims, which we interpret to result from the terminal collision with the lower-plate Superior craton followed by late- to post-orogenic tectonothermal activity that continued for at least 70 Myr after thermal peak. This prolonged history is characteristic of the northeastern Trans-Hudson Orogen, and implies that orogenic collapse may not be a hallmark of all hot, thickened orogens.

Geochemical constraints on petrogenesis of marble-hosted eclogites from the Sulu orogen in China

Marble-hosted eclogite is volumetrically minor in collisional orogens, but its geochemistry has great bearing on the origin of deeply subducted crustal rocks and the fluid mobility of subduction zones. This paper presents a combined study of whole-rock major-trace elements and SrNd isotopes, mineral O isotopes, carbonate C and O isotopes, and zircon UPb ages and LuHf isotopes for marble-hosted ultrahigh-pressure metamorphic eclogites from Rongcheng and Sanqingge in the Sulu orogen. The results provide insights into the protolith nature of eclogites and the fluid mobility of subduction zones. Zircon UPb dating yields consistent middle Triassic ages for the two occurrences of eclogites, indicating new growth of metamorphic zircon during continental collision. The Sanqingge eclogite shows LREE-enriched patterns and negative εNd(t) of −16.6 to −14.3 for whole-rock and negative εHf(t) of −27.1 to −15.2 for metamorphic zircon. A few relict zircon domains show middle Neoproterozoic UPb ages and negative εHf(t) of −35.2 to −15.5. Thus, the Sangqingge eclogite was metamorphosed from a mafic rock that was derived from partial melting of an anciently enriched mantle source. In contrast, the Rongcheng eclogite exhibits flat or even LREE-depleted patterns with negative εNd(t) values of −12.2 to −1.0 for whole-rock but positive εHf(t) values of 5.4 to 10.4 for zircon. The occurrence of interstitial and highly cuspate plagioclase along grain boundaries indicates the presence of partial melting in the eclogite. Thus, its positive zircon εHf(t) values are ascribed to the eclogite protolith of juvenile origin, whereas the LREE depletion is due to extraction of LREE-rich anatectic melt from the eclogite during the Triassic continental collision. As such, the Rongcheng eclogite was metamorphosed from a mafic rock that was derived from partial melting of a less enriched mantle source. All the eclogites from both areas show variably high δ18O values of 9.4‰ to 19.5‰. Oxygen isotope fractionations between mineral pairs mostly yield eclogite-facies temperature of 600 to 800°C, suggesting that the high δ18O signature was inherited from their protoliths before the Triassic subduction. In combination with the field relation between the eclogite and marble, it is inferred that the eclogite protolith is probably basaltic tuff and its high δ18O value would be acquired together with the marble protolith during their deposition from the surface water. Therefore, there would be the limited isotopic exchange between marble and eclogite during continental collision.

Coupled Lu–Hf and Sm–Nd geochronology constrains blueschist-facies metamorphism and closure timing of the Qilian Ocean in the North Qilian orogen

The Qilian–Qaidam orogenic belt at the northern edge of the Tibetan Plateau has received increasing attention as it recorded a complete history from continental breakup to opening and closure of ocean basin, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. Determining a geochronological framework of the initiation and termination of the fossil Qilian Ocean subduction in the North Qilian orogenic belt plays an essential role in understanding the whole tectonic process. Dating the high-pressure metamorphic rocks in the North Qilian orogenic belt, such as blueschist and eclogite, is the key in this respect. A blueschist from the southern North Qilian orogenic belt was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, and Sm–Nd multichronometric approaches. Pseudosection modeling indicates that the blueschist was metamorphosed under peak P–T conditions of 1.4–1.6GPa and 530–550°C. Zircon U–Pb ages show no constraints on the metamorphism due to the lack of metamorphic growth of zircon. Lu–Hf and Sm–Nd ages of 466.3±2.0Ma and 462.2±5.6Ma were obtained for the blueschist, which is generally consistent with the U–Pb zircon ages of 467–489Ma for adjacent eclogites. Lutetium and Sm zoning profiles in garnet indicate that the Lu–Hf and Sm–Nd ages are biased toward the formation of the garnet inner rim. The ages are thus interpreted to reflect the time of blueschist-facies metamorphism. Previous 40Ar/39Ar ages of phengitic muscovite from blueschist/eclogite in this area likely represent a cooling age due to the higher peak metamorphic temperature than the argon retention temperature. The differences of peak metamorphic conditions and metamorphic ages between the eclogites and adjacent blueschists indicate that this region likely comprises different tectonic slices, which had distinct P–T histories and underwent high-pressure metamorphism at different times. The initial opening of the Qilian Ocean could trace back to the early Paleozoic, and the ultimate closure of the Qilian Ocean was no earlier than c. 466Ma.

Evidence for 1808–1770 Ma bimodal magmatism, sedimentation, high-temperature deformation and metamorphism in the Aileron Province, central Australia

Field relationships and LA-ICP-MS U–Pb geochronology from the Yundurbungu Hills (Aileron Province, central Australia) reveal a record of 1808–1770 Ma bimodal magmatism, sedimentation, high-temperature deformation and metamorphism. Specifically, the data presented here provide the first unequivocal evidence for ca 1774 Ma high-temperature deformation and metamorphism during the 1790–1770 Ma Yambah Event in the southern part of the North Australian Craton. Granitic lithologies were synkinematically emplaced between 1808 and 1770 Ma, with early phases recording D1 deformation and the youngest phase postdating D1 deformation. The protolith to a D1 deformed metasedimentary unit was deposited between 1792 and 1774 Ma, followed by the intrusion and deformation of a composite mafic–felsic magmatic association at ca 1774 Ma. An S1 migmatitic fabric in the composite mafic–felsic gneiss is truncated by the youngest (ca 1770 Ma) phase of granitic magmatism, constraining the timing of S1 deformation. A second period of sedimentation appears to post-date D1 deformation, with deposition occurring sometime after ca 1774 Ma. Subsequent overprinting during the 1590–1550 Ma Chewings Event is recorded by the growth of metamorphic monazite and zircon. This event deformed the ca 1774 Ma S1 gneissic fabric, producing a composite S1/S2 gneissic fabric in early metasedimentary and magmatic lithologies and a simple S2-only fabric in lithologies that were intruded or deposited after ca 1774 Ma. Consistent with previous work, we suggest that localised high-temperature deformation and bimodal magmatism at ca 1774 Ma in the Yundurbungu Hills is consistent with a back-arc setting linked to prolonged north-directed subduction.

The affinity of Archean crust on the Yilgarn—Albany–Fraser Orogen boundary: Implications for gold mineralisation in the Tropicana Zone

Craton margins can be subject to a wide array of gold genesis and redistribution processes, although high-grade terrains on craton margins are frequently viewed as less prospective than lower-grade counterparts. In contrast to this, the high-grade Tropicana Zone, a newly defined Archean crustal component on the eastern margin of the Yilgarn Craton within the Albany–Fraser Orogen (AFO), contains a significant Proterozoic gold deposit. This deposit and zone comprise mid-amphibolite to granulite-facies gneissic rocks with evidence of partial melting and granite injection. The Tropicana Zone contains significant low-Si, LILE-enriched, granites classed as sanukitoids. Along with the distinctive compositions, the rarity of these rocks within any Archean craton suggests that the granitoid protoliths represent a single suite, emplaced during one event.Due to the intense granulite-facies overprinting of the Tropicana Zone rocks, determination of the magmatic protolith age for these sanukitoids is challenging. Nonetheless, the best age estimate for magmatism is 2692±16 Ma, based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt. This age is older than compositionally similar magmatism found within the Yilgarn Craton, although a sanukitoid in the Northern Foreland of the AFO has a similar age. Furthermore, the granulite-facies metamorphic zircon growth in the Tropicana Zone at 2718–2554 Ma was prolonged compared to that in the Yilgarn Craton. Nonetheless, the Hf isotopic signature of the Tropicana Zone zircon shares strong similarity to that from the Eastern Goldfields Superterrane of the Yilgarn Craton. This implies that the Tropicana Zone reflects a deeper crustal level of the Yilgarn Craton, exhumed and thrust NW an unknown distance over the craton edge. In addition, we observe that the granulite-facies zircons have a less radiogenic Hf-isotope signature than the preserved pre-metamorphic zircon cores. Based on correlations with alpha dose, U and Th content and 176Hf/177Hf we suggest this reflects the preferential destruction and release of unradiogenic Hf from inherited zircon whereas the protolith sanukitoid zircon, with lower U and Th content, was more resistant to mobilisation during high-grade metamorphism. We note this situation may be a more general response of the Hf isotopic system, in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic Hf reservoir than its felsic counterpart.Juvenile granitic veins dated at c. 1780 Ma intruded into the Tropicana Zone indicate that the Tropicana Zone was structurally emplaced at or before c. 1780 Ma, given similar Proterozoic magmatic events are well documented from (para)autochthonous adjacent units. Re–Os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c. 2100 Ma, supportive of a Palaeoproterozoic age of mineralisation. This mineralisation event is distinct from major Proterozoic tectonothermal events elsewhere in the AFO. Sanukitoid magmas are well-known for gold fertility and were likely the original source of gold in the Tropicana Zone, which was subsequently concentrated into brittle structures during several episodes. Gold mineralisation post-dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining Yilgarn Craton.

Zircon dissolution and growth during metamorphism

Kohn et al. 2015 (this volume) present models of zircon dissolution and growth in hydrous MORB and metapelite bulk compositions under metamorphic conditions. They evaluate the propensity metamorphic minerals in equilibrium with zircon, to assess what minerals (besides zircon) affect whole rock Zr mass balance. Results indicate that mineral reactions over a range of metamorphic grades could play an important role in the dissolution and growth of metamorphic zircon, with Zr preferentially incorporated into melt, rutile, garnet, and hornblende with increasing temperatures and decreasing pressures.

Zircon geochronology of the Koraput alkaline complex: Insights from combined geochemical and U–Pb–Hf isotope analyses, and implications for the timing of alkaline magmatism in the Eastern Ghats Belt, India

Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. Investigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex patterns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869±7Ma and 690±1Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM=1.5 to 1.0Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860Ma have most probably experienced partial resetting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP-MS and CA-TIMS U–Pb ages of 700–690Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries.

Evidence for a far-traveled thrust sheet in the Greater Himalayan thrust system, and an alternative model to building the Himalaya

The Galchhi shear zone underlies the Kathmandu klippe in central Nepal and has emerged as a key structure for discriminating competing models for the formation of the Himalayan orogenic wedge. New chronologic data from the Galchhi area suggest a new structural and orogenic interpretation. Zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 + 7/ − 10 Ma and ~570 Ma, with metamorphic zircon growth at 23–29 Ma. Comparable data from the Greater Himalayan Sequence (GHS) at the intra-GHS Langtang thrust, north of Galchhi, similarly restrict GHS deposition to between 475 + 7/ − 3 and ~660 Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to ≥22.5 ± 2.3 Ma, long before slip of the Main Central Thrust in the region (≤17 Ma). Shear sense indicators in the Galchhi area indicate both top-to-the-south and top-to-the-north shears. The old age of movement, Neoproterozoic youngest detrital zircons, occurrence of top-to-the-south shear sense indicators, and intrusive Paleozoic granites, all suggest that the Galchhi shear zone is an intra-GHS top-to-the-south thrust, rather than either a thrust involving Lesser Himalayan rocks, or a top-to-the-north shear zone that juxtaposed Tethyan and GHS rocks during coeval movement of the Main Central Thrust. The GHS in Nepal was not emplaced as a single body of rock but consists of at least two ductile “thrust sheets,” present in both the hinterland at Langtang and toward the foreland at Galchhi. GHS thrust sheets sequentially underplated during southward propagation of the thrust belt.

Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6–8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U–Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P–T–t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

Post-peak, fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block, southern India

The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH2O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.

Neoarchean and Paleoproterozoic crust formation in the Ubendian Belt of Tanzania: Insights from zircon geochronology and geochemistry

LA-ICP-MS U–Pb zircon geochronological and geochemical data of meta-igneous and metasedimentary rock types of the Katuma Block of the Paleoproterozoic Ubendian Belt in Tanzania are used to unravel the crustal evolution of this metalliferous terrain. The protoliths of the metabasites and orthogneisses previously considered to be Paleoproterozoic are in fact mostly Neoarchean in age (2713±11Ma to 2638±5Ma), from which the oldest rocks experienced their first metamorphism during the same Neoarchean orogenic cycle at ca. 2650Ma. A second event of mafic magmatism (2021±11Ma) was concomitant with the migmatization of the Neoarchean orthogneisses and was succeeded by granitic intrusions at 1990–1940Ma. All rocks of the Katuma Block experienced their main metamorphic reworking during several Paleoproterozoic orogenic events, which were recognized by dating of various metamorphic zircon growth zones and the age of magmatic events dated at ca. 2050, 1960 and 1880Ma. The detritus of the high-grade metasedimentary rocks derived from Neoarchean (Katuma Block or Tanzania Craton?) and Paleoproterozoic provenances and the minimum age for the deposition is constrained by its first metamorphism at ca. 1960Ma. The Neoarchean and Paleoproterozoic metabasites, gabbronorites and orthogneisses are sub-alkaline in composition displaying a REE and trace element geochemistry akin to those of rocks formed in modern-arc settings. On the basis of the geochemical data, the presence of eclogites, deformation and metamorphic ages, we suggest that in Paleoproterozoic time the Katuma Block was again at an active continental margin, below which a Paleoproterozoic oceanic lithosphere was subducting.

Hf and O isotopic evidence for metamorphic crystallization of zircon during contact metamorphism of Fenniaolin metabasalts, Tananao complex, Taiwan

Abstract The eHf and δ18O compositions of zircons separated from an amphibolite in the Tananao complex of northeast Taiwan have been analyzed in order to test the hypothesis that these zircons are metamorphic in origin. A previous study found the U–Pb age of 18 zircons to be 86 ± 1 Ma. The 176Hf/177Hf ratio of 13 of these zircons is rather constant with an average value of 0.283263 ± 0.000037 (2σ SD) and a weighted mean of initial eHf(t) value of + 19.2. The δ18O compositions of these same 13 grains and of 7 other grains vary from + 1.2 to + 2.0‰ with a weighted mean of + 1.5 ± 0.43‰ (2σ SD). The very positive eHf(t) values show that the protolith of this amphibolite had a primitive composition, consistent with N-MORB. The low δ18O values are not consistent with typical mantle values of ~ 5.3; rather we interpreted them to reflect metamorphic growth of zircon at greenschist facies conditions in a basalt already hydrothermally altered by seawater. In keeping with other studies, we conclude [1] that the host amphibolite was basalt of MORB composition extensively altered by seawater shortly after extrusion, [2] that the crystallization of the metamorphic zircons was caused by contact metamorphism of the host amphibolite by the 87 Ma intrusion of the adjacent granodiorite, and [3] that this metamorphism was not caused by the Neogene collision of the Philippine Sea plate with the south China margin.

Metamorphic zircon formation at the transition from gabbro to eclogite in Trollheimen–Surnadalen, Norwegian Caledonides

Abstract A transition from gabbro to eclogite has been investigated at Vinddøldalen in south-central Norway, with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct U–Pb zircon age of the metamorphic process. In the different rocks of the transition zone zircon occurs as (I) igneous prismatic grains, (II) metamorphic polycrystalline rims and pseudomorphs after baddeleyite, and (III) as tiny (&lt;10 µm) bead-like zircon grains. Textural relations suggest that type II zircon formed by breakdown of baddeleyite in the presence of silica, whereas Fe–Ti oxides were the main Zr source for the type III zircon. Subsolidus liberation of Zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid-assisted metamorphism, and by resorption of Fe–Ti oxide in rock domains that were completely recrystallized to eclogite. SIMS (secondary ion mass spectrometry) and TIMS (thermal ionization mass spectrometry) dating provides comparable U–Pb ages of magmatic zircon and baddeleyite. Baddeleyite (TIMS) yielded an age of 1457±11 Ma for the gabbro emplacement. Bead-type metamorphic zircon from eclogite gave 425±10 Ma (TIMS) dating the metamorphic transition from gabbro to eclogite in the upper basement of the Lower Allochthon in the south-central Scandinavian Caledonides.

U–Pb detrital zircon geochronology of the Dalradian Supergroup, Shetland Islands, Scotland: implications for regional correlations and Neoproterozoic–Palaeozoic basin development

U–Pb detrital zircon data show that the East Mainland Succession, the presumed correlative of the Dalradian Supergroup in the Shetland Islands, Scotland, is dominated by Mesoproterozoic and Archaean material, with some Palaeoproterozoic detritus. The data are most consistent with derivation from eastern Laurentia, although western Baltica sources cannot be excluded. A magmatic event at c . 576 Ma supplied detritus to the Clift Hills Group, and was the source of high-temperature fluids that resulted in growth of new metamorphic zircon, and altered old detrital grains within the underlying sedimentary pile. This provides a constraint on the age of the global Shuram–Wonaka event recognized within the Whiteness Group, which underlies the Clift Hills Group. The presence of common Archaean detritus is compatible with broad time-correlation of the East Mainland Succession with the Dalradian Supergroup. However, differences in age, thickness and basin evolution are consistent with deposition in separate basins along the extending Laurentian margin during supercontinent break-up and development of the Iapetus Ocean. Similarities in the detrital zircon records of the East Mainland Succession and the offshore Devonian–Carboniferous Clair Group permit derivation, at least in part, of the latter from the Shetland Islands and proximal sources on the adjacent continental shelf. Supplementary material: SIMS and LA-ICP-MS analytical data, and statistical analyses are available at www.geolsoc.org.uk/SUP18674 .

Geologically constraining India in Columbia: The age, isotopic provenance and geochemistry of the protoliths of the Ongole Domain, Southern Eastern Ghats, India

The Ongole Domain in the southern Eastern Ghats Belt of India formed during the final stages of Columbia amalgamation at ca. 1600Ma. Yet very little is known about the protolith ages, tectonic evolution or geographic affinity of the region. We present new detrital and igneous U–Pb–Hf zircon data and in-situ monazite data to further understand the tectonic evolution of this Columbia-forming orogen.Detrital zircon patterns from the metasedimentary rocks are dominated by major populations of Palaeoproterozoic grains (ca. 2460, 2320, 2260, 2200–2100, 2080–2010, 1980–1920, 1850 and 1750Ma), and minor Archaean grains (ca. 2850, 2740, 2600 and 2550Ma). Combined U–Pb ages and Lu–Hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent Dharwar Craton. Instead they were likely derived from East Antarctica, possibly the same source as parts of Proterozoic Australia. Magmatism occurred episodically between 1.64 and 1.57Ga in the Ongole Domain, forming felsic orthopyroxene-bearing granitoids. Isotopically, the granitoids are evolved, producing εHf values between −2 and −12. The magmatism is interpreted to have been derived from the reworking of Archaean crust with only a minor juvenile input. Metamorphism between 1.68 and 1.60Ga resulted in the partial to complete resetting of detrital zircon grains, as well as the growth of new metamorphic zircon at 1.67 and 1.63Ga. In-situ monazite geochronology indicates metamorphism occurred between 1.68 and 1.59Ga.The Ongole Domain is interpreted to represent part of an exotic terrane, which was transferred to proto-India in the late Palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south-west Baltica and south-eastern Laurentia. Given the isotopic, geological and geochemical similarities, the proposed exotic terrane is interpreted to be an extension of the Napier Complex, Antarctica, and may also have been connected to Proterozoic Australia (North Australian Craton and Gawler Craton).