Zircon Overgrowths Research Articles

Large-scale, short-lived metamorphism, deformation, and magmatism in the Chugach metamorphic complex, southern Alaska: A SHRIMP U-Pb study of zircons

We present sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology and trace-element chemistry of zircons from the Chugach metamorphic complex, southern Alaska, which allow us to precisely constrain the age and duration of peak metamorphism. Zircons separated from migmatitic metasedimentary gneisses show thin (5–30 μm) overgrowths with trace-element compositions indicating growth at or close to the metamorphic peak. Five U-Pb ages of such overgrowths and two U-Pb ages from magmatic zircons from plutons intruding the metasediments from the western and central parts of the complex range between 54.0 ± 0.9 Ma and 52.6 ± 1.0 Ma and are within error of each other. One sample from the southeastern part of the complex yields a zircon overgrowth age of 51.3 ± 0.7 Ma, which is within error of two of the seven samples from the western and central parts. Combined with previous U-Pb zircon ages of magmatic rocks of the Sanak-Baranof belt, our data constrain the age and duration of metamorphism and magmatism to a time span of maximum ∼3 m.y. between ca. 55 and 52 Ma on an ∼200-km-long section in the western and central part of the complex. A time span of ∼2 m.y. between ca. 52 and 50 Ma is constrained from the southeastern part of the complex to Baranof Island. Maximum depositional ages of the sediments in which the complex developed (ca. 60 Ma) limit the time available for prograde metamorphism to a narrow time window of ∼5 m.y. This short time span for prograde metamorphism, followed by near-synchronous peak metamorphism and magmatism for ∼800 km along the margin could be the result of ridge subduction, and tectonic models for the evolution of this region are discussed.

Geochemical characterization of zoned zircon from Wadi Abu Rusheid psammitic gneiss, south Eastern Desert, Egypt

A unique zircon was studied in the gneiss samples collected from the Wadi Abu Rusheid psammitic gneiss using electron scanning microscope and electron probe microanalyses. This zircon can be categorized into two types according to the texture and trace element content: (l) magmatic zircon slightly enriched in HfO2 with ordinary zone. (2) Overgrowths of zircon occur as two species, the first species being highly enriched in HfO2 with irregular zoning. The second species is highly enriched in HfO2 forming a rim around the second species with a very sharp thinner boundary. The first type shows a distinct oscillatory internal zoning pattern without change in shape of this zone and has conspicuous inclusion-free zircon overgrowths with distinct poor concentrations in Y, Hf, Th, U, Nb, and Ta in both rim and core. The second type shows two species, the first one displays distinct irregular interval zoning and irregular overgrowth with abrupt change in composition of these zones with distinct enrichment in Y, Hf, Th, U, Nb, and Ta in the rim relative to the core. The second species is forming a rim around the first species also with distinct enrichment in Y, Hf, Th, U, Nb, and Ta content. These indicate that two events (crystallization environment) have played an important role in the formation of this zircon and largely reflect differences in whole-rock trace element contents between the successive generations of this zircon. The first event is believed to be of magmatic origin giving rise to normal composition of magmatic zircon. The second event shows an intense successive process of metasomatic activity during the formation of the Abu Rusheid radioactive gneiss. Electron microprobe analysis indicates that oscillatory zoned zircon shows poor content of Y, Hf, Th, U, Nb, Ta, and rare earth elements (REE) in the rim and core, while overgrowths of zircon are slightly enriched by these elements. Also, these analyses indicate that the Abu Rusheid psammitic gneiss has been significantly enriched by the thorite mineral (Th content up to 54.72% ThO2) and columbite-bearing minerals (Nb content up to 64.74% Nb2O5, Ta content up to 9.32% Ta2O5). The poor content of REE in overgrowths of zircon indicates mobilization of REE during the metamorphism processes of gneiss.

Zircon U–Pb and Hf isotope constraints from the Ailao Shan–Red River shear zone on the tectonic and crustal evolution of southwestern China

The Ailao Shan–Red River (ASRR) shear zone, one of the most prominent geologic strike-slip shear zones in Southeast Asia, consists of high-grade metamorphic complexes that provide a rare opportunity to sample the mid-crustal rocks along the western margin of the Yangtze Block of South China. Here we report combined, in-situ analyses of zircon U–Pb and Lu–Hf isotopes of eight gneisses from the Diancang Shan and Ailao Shan segments of the ASRR shear zone. Our zircon U–Pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 Ma, with peak ages at ca. 770, 350 and 240 Ma, suggesting three major periods of protolith formation. The 350-Ma zircons, observed only in an orthogneiss from the Diancang Shan, show uniform initial Hf isotopic ratios marked by high and positive ε Hf(T) values from + 16 to + 10. This is in contrast with the other two zircon populations that are more common, occurring in both the Diancang Shan and Ailao Shan, and overall delineate very heterogeneous Hf isotopic compositions, with ε Hf(T) values ranging from + 15 to − 16. Many zircons reveal distinctive core-rim age variations. Zircon rims, formed between ca. 34 and 26 Ma, show significant variations in Th/U ratios (5–0.01) and ε Hf(T) values (+ 14 to − 10) that suggest complicated magmatic and metamorphic zircon overgrowth during the Oligocene. The presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies, corresponding to mid-crustal level P–T conditions, as also suggested by structural and petrologic data. Our data furthermore suggest that the ASRR gneisses were not produced solely by shear heating during the Tertiary strike-slip faulting, but are uplifted, mid-crustal basement rocks that formed essentially during two major stages of magmatism, represented by the Neoproterozoic Kangding Complex and Late Permian Emeishan large igneous province. As a result of the India–Asia collision, these basement rocks underwent regional magmatic and metamorphic overprinting in the Oligocene that, based on our relevant work (Searle et al., 2010, Geosphere, 6, 1–23), predated the initial left-lateral movement along the ASRR shear zone. The 350-Ma protolith, which requires a dominant depleted-mantle input in the petrogenesis, cannot be linked to any major magmatic events in South China but may be interpreted as part of the Paleotethys remnants that were later incorporated into the ASRR shear zone.

SHRIMP zircon chronology of HP-UHP rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan

In this paper we present the first results of sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating obtained for the ultra-high pressure (UHP) metamorphic rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan. Zircon grains from two HP eclogite samples and one sample of UHP garnet-talk-chloritoid schist have similar bright rims identified on cathodoluminescence (CL) images. The rims are interpreted as zircon overgrowth during UHP metamorphism. Dating of zircon rims from the three samples yielded similar ages of 509±7Ma, 502±10Ma and 498±7Ma. The time span of 510–500Ma is considered as the “minimum” age of UHP metamorphism of the Makbal complex. This age is in accordance with Middle Cambrian faunal age of the Kara-Archa ophiolite in the Makbal area. It is suggested that the Makbal rocks were metamorphosed in the Middle Cambrian subduction zone dipping north under the Northern Tien Shan microcontinent. A deformed granodiorite associated with the Makbal metamorphic rocks yielded an age of 514±5Ma, indicating development of a magmatic arc simultaneously with the UHP metamorphism. Cores of zircon grains from the garnet-talk-chloritoid schist yielded a wide spectrum of ages (1850–800Ma) characteristic for detrital zircons of continental origin. A depositional age <800Ma points to a possible derivation from a Riphean passive margin of the Terskey Ocean. Analyses of zircon grains from two eclogite samples fit well-defined discordia lines with upper intercepts at 1446±25Ma and 1447±29Ma, indicating a homogeneous Mezoproterozoic protolith of these rocks. The possible presence of such protoliths in Makbal is illustrated by a similar age of inherited zircon recovered from the granodiorite (1443±25Ma) and by Mesoproterozoic detrital zircons from the garnet-talk-chloritoid schist. Ages of Kokchetav, Makbal, Aktyuz and Anrakhai HP-UHP complexes in the Kokchetav–Northern Tien Shan belt vary from 537 to 475Ma and register subduction–accretion events during the Caledonian orogeny which finally formed the Paleo-Kazakhstan continent prior to Middle Ordovician.

Syn-extensional plutonism and peak metamorphism in the Albion-Raft River-Grouse Creek metamorphic core complex

The Cassia plutonic complex (CPC) is a group of variably deformed, Oligocene granitic plutons exposed in the lower plate of the Albion–Raft River–Grouse Creek (ARG) metamorphic core complex of Idaho and Utah. The plutons range from granodiorite to garnet-bearing, leucogranite, and during intrusion, sillimanite-grade peak metamorphism and ductile attenuation occurred in the country rocks and normal-sense, amphibolite-grade deformation took place along the Middle Mountain shear zone. U-Pb zircon geochronology from three variably deformed plutons exposed in the lower plate of the ARG metamorphic core complex revealed that each zircon is comprised of inherited cores (dominantly late Archean) and Oligocene igneous overgrowths. Within each pluton, a spread of concordant ages from the Oligocene zircon overgrowths is interpreted as zircon recycling within a long-lived magmatic system. The plutons of the CPC have very low negative whole rock εNd values of −26 to −35, and initial Sr values of 0.714 to 0.718, consistent with an ancient, crustal source. Oxygen isotope ratios of the Oligocene zircon overgrowths from the CPC have an average δ18O value of 5.40 ± 0.63 permil (2SD, n = 65) with a slight trend towards higher δ18O values through time. The δ18O values of the inherited cores of the zircons are more variable at 5.93 ± 1.51 permil (2SD, n = 29). Therefore, we interpret the plutons of the CPC as derived, at least in part, from melting Archean crust based on the isotope geochemistry. In situ partial melting of the exposed Archean basement that was intruded by the Oligocene plutons of the CPC is excluded as the source for the CPC based on field relationships, age and geochemistry. Correlations between Ti and Hf concentrations in zircons from the CPC suggest that the magmatic system may have become hotter (higher Ti concentration in zircon) and less evolved (lower Hf in zircon concentration) through time. Therefore, the CPC represents prolonged or episodic magmatism system (32-25 Ma), and the intrusions were each accompanied by sillimanite-grade deformation and extension. The Oligocene magmatism and peak metamorphism preserved in the ARG metamorphic core complex are likely related to regional trends in mantle-derived magmatism that led to protracted heating, melting and mobilization of the deeper crust.

Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai, NW China: Evidence from zircon U–Pb and Hf isotopic study of Paleozoic granitoids

Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Chinese Altai, and zircon U–Pb and Hf isotopic compositions have been determined for samples from eleven granitic plutons/batholiths. The Jiadengyu gneissic granitic pluton yielded a zircon U–Pb age of 479 Ma, and thus it does not represent Precambrian basement as suggested previously. Our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30% area of the Chinese Altai during the period from 447 Ma to 368 Ma with a climax at ca. 400 Ma. Ages for zircon overgrowth rims demonstrate additional thermal events at ca. 360 and 280 Ma, respectively. Positive ε Hf( t) values (0 to +9) of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources. Xenocrystic zircon cores are 543–421 Ma old and also give positive ε Hf( t) values (+2.5 to +12), suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly-accreted meterials. The strong magmatism at ca. 400 Ma significantly changed the Hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period. Geophysical, geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca. 400 Ma and the above mentioned change of Hf isotopic composition in the magma source.

The 390 Ma high-T metamorphic event in the Chinese Altai: A consequence of ridge-subduction?

High-grade rocks occur in the Chinese Altai, but the timing of metamorphism is poorly constrained, which hinders our understanding of the thermo-tectonic history of the region. Representative high-grade samples from the sillimanite zone extending from Hanas to Fuyun were selected for zircon U-Pb dating and temperature estimation. LA-ICP-MS analyses of zircon overgrowth rims and recrystallized domains give consistent ages of ∼390 Ma, which is interpreted to record a regional metamorphic event. Temperature (T) estimations using the amphibole-plagioclase-quartz (Amp-Pl-Qtz) and garnet-biotite (GB) geothermometers give relatively high temperatures ranging from 650 to 700 °C. The zircon metamorphic rims yield temperature estimates of ∼720 °C by using the Ti-in-zircon thermometer. These data suggest that a high-temperature metamorphic event took place in the Chinese Altai in the Middle Devonian, and may imply a tectonic environment involving an unusually elevated heat flux from a deep-seated source. Our data support possible ridge subduction around 390 Ma that caused upwelling of the hot asthenosphere and triggered the high-T metamorphism. This model can also account for coeval volcanic activity with a range of geochemical characteristics, diverse mafic intrusions and extensive hydrothermal mineralization in the Altai orogen.

Tracing the Cadomian magmatism with detrital/inherited zircon ages by in-situ U–Pb SHRIMP geochronology (Ossa-Morena Zone, SW Iberian Massif)

U–Pb SHRIMP geochronology on zircon extracted from a granite, a sandstone and a quartzite of the SW Iberian Massif (Ossa-Morena Zone), was used in order to investigate the contribution of Late Neoproterozoic (Cadomian) tectonothermal history to the crustal growth of northern Gondwana. The analysed Cambrian rocks were sampled along the southern margin of the Coimbra–Cordoba shear zone (Barquete granite and Crato sandstone), and within this shear zone (Ouguela quartzite). The Barquete granite yielded a crystallization age of 526 ± 4 Ma and evidence for Neoproterozoic and Paleoproterozoic inherited ages. The geochemical signature of the Barquete granite matches the igneous activity of the initial magmatic flooding of the Ossa-Morena Zone during the Early Cambrian. The petrography and geochemical signature of the Crato and Ouguela sandstones indicate that these clastic rocks were mainly derived from felsic igneous rocks. The youngest detrital zircon grain extracted from the Crato sandstone, dated at 552.7 ± 5.5 Ma, has a zircon overgrowth that yielded 532 ± 5.6 Ma. In the Ouguela quartzite, the youngest concordant age yielded 556.4 ± 5.7 Ma, but we also found a zircon overgrowth dated at 536.2 ± 5.6 Ma. The ages of the youngest detrital zircon in each sedimentary rock place an upper limit on the deposition age of Early Cambrian (c. 536–532 Ma). Four main Late Neoproterozoic age clusters at c. 640–638 Ma, c. 612–613 Ma, c. 590–585 Ma and c. 560 Ma were recognized in the populations of detrital zircons from both sandstones. These ages of zircon-forming events seem to represent four successive thermal/magmatic pulses that overlap the Cadomian and Pan-African orogenies. These findings match other results published for the OMZ. Our U–Pb results show that detrital zircons in siliciclastic sediments and inherited zircon in granites are dominated by Neoproterozoic ages and few Paleoproterozoic and Archean ages. Those ages reported in the Ossa-Morena Zone rocks, together with a remarkable lack of Mesoproterozoic ages suggest that the clastic rocks in this peri-Gondwana basin were derived from the West African craton. The large population of Late Cryogenian and Ediacaran ages indicates denudation of the Cadomian basement during the Early Cambrian. A potential source for the detrital and inherited zircons found in this study is a long-lived magmatic arc that is only partly exposed in the SW Iberian Massif but is well represented in other peri-Gondwana regions. The large amount of Cadomian ages obtained in this study (c. 640–560 Ma) reinforces the idea that Cadomian magmatism played a significant role in the continental crustal growth history of Late Neoproterozoic uplift and erosion in Western and Central Europe (Cadomia).

Mechanism of metamorphic zircon growth in a granulite-facies quartzite, Adirondack Highlands, Grenville Province, New York

Granulite-facies quartzites from the Adirondack Highlands (Grenville Province, New York) contain accessory zircon with ubiquitous metamorphic overgrowths. Detrital zircon cores are resorbed, preserve various internal zoning styles and inclusions, and have ages of 1.3 to 2.7 Ga. These ages constrain the timing of deposition of the protolith sandstone and suggest that the Adirondack Highlands were proximal or part of Laurentia during deposition. Metamorphic overgrowths formed in the quartzites during the Shawinigan orogeny (1.20-1.14 Ga). The average volume of overgrowths in eight samples ranges from 62-87%, with a positive correlation between zircon growth and feldspar content (melt productivity during metamorphism). Melt volumes and Zr solubility were too small to allow the overgrowths to have formed in one dissolution-precipitation event. Crystal-size distributions suggest zircon coarsening by the dissolution of small crystals and Zr transfer via a partial melt, and thus zircon overgrowths date anatexis. These results have implications for provenance studies, as dissolution of small zircon grains and growth of new zircon could bias age spectra of metasedimentary rocks.

Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature metamorphism as exemplified by Sri Lankan granulites

Ultrahigh-temperature (UHT) granulites of the central Highland Complex, Sri Lanka, underwent some of the highest known peak temperatures of crustal metamorphism. Zircon and monazite U-Pb systems in granulites near Kandy, the highest grade region (∼1050 °C; 0.9 GPa), preserve both a record of the timing of prograde and retrograde phases of UHT metamorphism and evidence for the ages of older protolith components. Zircon grains from a quartz-saturated granulite containing relics of the peak UHT assemblage have remnant detrital cores with dates of ca. 2.5–0.83 Ga. Date clusters of ca. 1.7 and 1.04–0.83 Ga record episodes of zircon growth in the source region of the protolith sediment. Two generations of overgrowths with contrasting Th/U record metamorphic zircon growth at 569 ± 5 and 551 ± 7 Ma, probably in the absence and presence of monazite, respectively. The age of coexisting metamorphic monazite (547 ± 7 Ma) is indistinguishable from that of the younger, low-Th/U zircon overgrowths. Zircon from a quartz-undersaturated monazite-absent UHT granulite with a mainly retrograde assemblage is mostly metamorphic (551 ± 5 Ma). The ca. 570 Ma zircon overgrowths in the quartz-saturated granulite probably record partial melting just before or at the metamorphic peak. The ca. 550 Ma zircon in both rocks, and the ca. 550 Ma monazite in the quartz-saturated sample, record post-peak isothermal decompression. A possible model for this pressure-temperature-time evolution is ultrahot collisional orogeny during the assembly of Gondwana, locally superheated by basaltic underplating, followed by fast extensional exhumation.

SHRIMP Zircon U-Pb Dating of Sapphirine-Bearing Granulite and Biotite-Hornblende Gneiss in the Schirmacher Hills, East Antarctica: Implications for Neoproterozoic Ultrahigh-Temperature Metamorphism Predating the Assembly of Gondwana

Abstract We applied SHRIMP zircon U-Pb age dating to ultrahigh-temperature (UHT) sapphirine-bearing orthopyroxene garnet (SOG) granulite and biotite-hornblende (Bt-Hbl) gneiss in the Schirmacher Hills, East Antarctica. In the Bt-Hbl gneiss, concordant ages of and Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims ( Ma) with low Th/U ratios from the Bt-Hbl gneiss are interpreted as having a metamorphic origin. Oscillatory-zoned and/or irregularly zoned zircon cores may have crystallized during an igneous event at Ma; 800-Ma igneous events have not previously been recognized in central Dronning Maud Land (DML) inland nunatak. Zircons in the SOG granulite yielded a concordant age of Ma, using analyses of sector-zoned and simple-zoned grains. These zircons have relatively high Th/U ratios with a narrow range, and they occur in association with garnet breaking down to form cordierite. The -Ma age obtained from these zir...

A Paleoproterozoic orogeny recorded in a long-lived cratonic remnant (Wuyishan terrane), eastern Cathaysia Block, China

The Precambrian basement of northern Wuyishan (southern Zhejiang Province, eastern Cathaysia Block, South China), consists mainly of Paleoproterozoic granites and metamorphic rocks of the Badu Complex, which are the oldest rocks found in the Cathaysia Block. LA-ICPMS zircon U–Pb ages for a gneiss and five gneissic granites from the Tianhou, Danzhu, Xiaji and Lizhuang plutons indicate that magmatism and metamorphism took place between 1888 and 1855 Ma. The Xiaji (1888 ± 7 Ma) and Lizhuang (1875 ± 9 Ma) granites have high SiO 2, K 2O and Rb contents, high A/CNK (1.09–1.40) and Rb/Sr, and low contents of Sr, REE and mafic components (Mg, Fe, Ti, Mn and other transition metals). They have the geochemical signature of S-type granites, and a sedimentary protolith is confirmed by the presence of abundant inherited zircons with a range of ages and Hf-isotope compositions. The Tianhou and Danzhu granites are metaluminous to weakly peraluminous (A/CNK = 0.80–1.07), and have low SiO 2 contents, high Ga/Al and FeO/(FeO + MgO) ratios, and Zn and HFSE concentrations typical of A-type granites. They also record high crystallization temperatures (885–920 °C), consistent with A-type granites. High Y/Nb ratios (>1.4) indicate that they belong to the A 2 subgroup, suggesting that they probably formed in a post-orogenic tectonic setting. Their ages range from 1867 to 1855 Ma, slightly later than the syn-collisional Lizhuang and Xiaji S-type granites. These granitic rocks and the metamorphic rocks of the Badu Complex define a late Paleoproterozoic orogenic cycle in the area. All the 1.86–1.90 Ga zircons, whether derived from S- or A-type granites, show similar Hf-isotopic compositions, with Hf model ages clustering at ∼2.8 Ga. These model ages, and inherited zircons (ca. 2.5–2.7 Ga) found in some rocks, indicate that the late Paleoproterozoic magmatism and tectonism of the eastern Cathaysia Block represent an overprint on an Archaean basement. This Paleoproterozoic orogeny in the Wuyishan terrane coincides with the assembly of the supercontinent Columbia, suggesting that the Wuyishan terrane was the part of this supercontinent. Zircon ages also record an early Mesozoic (Triassic) tectonothermal overprint that was very intensive in the northern Wuyishan area, leading to high-grade metamorphism of Paleoproterozoic basement, Pb loss from Paleoproterozoic zircons and overgrowth of new zircon. The central and southern parts of Wuyishan and the Chencai area (northern Zhejiang Province) also experienced strong reworking in Neoproterozoic and early Paleozoic times. The Wuyishan terrane (especially in the north) represents a long-lived remnant of the old craton, which has survived for at least one billion years. The compositions of the basement rocks, the Paleoproterozoic orogeny and the Triassic tectonothermal imprint in the Wuyishan terrane are similar to those recognized in the Yeongnam massif of South Korea, suggesting that the two terranes may have been connected from Paleoproterozoic to Triassic time.

Paleoproterozoic sediment recycling during multiphase orogenic evolution in Fennoscandia, the Tampere and Pirkanmaa belts, Finland

Geochronology of detrital zircons and their overgrowths combined with whole-rock geochemical and Sm–Nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution. This approach is applied to the Paleoproterozoic sedimentary rocks of the Tampere and Pirkanmaa belts (southern Finland) in the center of the composite Svecofennian orogen. The lower part of the Tampere belt succession and bulk of the Pirkanmaa belt are characterized by turbidites whereas the upper part of the Tampere belt succession is dominated by 1.90–1.89 Ga mature arc-type volcanic rocks. Detrital zircon geochronology indicates that the Tampere and Pirkanmaa belts have a coeval 1.92–1.89 Ga depositional and tectonic history. Ages of pre-depositional zircon overgrowths vary from 1.91 Ga to 2.0 Ga with clusters at 1.92 Ga and 1.98 Ga. Within the Pirkanmaa belt, post-depositional zircon overgrowths indicate metamorphic culmination at c. 1885 Ma in the Vammala Ni-zone and at c. 1875 Ma in the northern part. The lower conglomerates and graywackes in the Tampere belt and their equivalents in the Pirkanmaa belt are rich both in Neoarchean and Paleoproterozoic grains, the latter ranging in age from 1.9 to 2.1 Ga. Compared to these, a sample from the Vammala Ni-zone has an exotic provenance with at least c. 1.90 Ga, 2.04–2.15 Ga, 2.38–2.48 Ga and 2.57–2.63 Ga components. A sedimentary recycling and tectonic model for the central Fennoscandia is proposed, in which the Paleoproterozoic Keitele + Bergslagen continent was formed during an unnamed orogeny at 1.98–1.97 Ga. The Archean Norrbotten microcontinent was attached to the continent at 1.97–1.93 Ga. Upper Kaleva turbidites, derived from the Lapland-Kola orogen in the north, were deposited before 1.92 Ga on a passive margin of the Archean Karelia craton. The Karelia craton collided with the Keitele + Bergslagen + Norrbotten continent at c. 1.92 Ga forming the Lapland-Savo orogen. Subsequent evolution led to rifting and break-up of the latter continent into two microcontinents in the hinterland. At 1.92–1.91 Ga the rift was developed into a subsiding passive margin of the Keitele microcontinent with voluminous turbidite deposition, now seen as graywackes in the Tampere, Pirkanmaa and Pohjanmaa (western Finland) belts. The turbidite material was derived from the rising Lapland-Savo orogen and included recycled Upper Kaleva, recycled (sandstones) and first-cycle 2.03–1.97 Ga detritus from Keitele, 1.93–1.92 Ga Savo arc material, as well as detritus from the Archean craton and its cover deposits. The collision between Karelia and Keitele caused a subduction reversal and the onset of Tampere arc volcanism at 1.90 Ga. Arc-derived materials started to deposit and were mixed with older sedimentary rocks, and trench-parallel distal turbidites from exotic source were being deposited in the accretionary wedge.

SHRIMP allanite U-Th-Pb dating of bimodal Triassic metamorphism of Neoarchean tonalitic gneisses, Daeijak Island, central Korea

The microstructures, compositions and U-Th-Pb ages of allanite from tonalitic gneisses in a Neoarchean migmatite complex, Daeijak Island, central Korea, have been investigated. Allanite crystals up to ∼300 µm in diameter occur with accessory apatite, ilmenite, magnetite and zircon primarily in the major foliation defined by aggregates of biotite and hornblende. The allanite is commonly rimmed by clinozoisite, and has a range of oscillatory to patchy compositional zoning. Its total LREE + Th content ranges from 0.58 to 0.83 atoms per 12.5 oxygens. The 208Pb/232Th isotopic ages of allanite from two tonalitic gneiss samples measured using the SHRIMP II ion microprobe show the same two age clusters, 229 ± 2 and 215 ± 4 Ma in sample DE28, and 227 ± 7 and 213 ± 4 Ma in sample DE43. The allanite 206Pb/238U ages from sample DE28 are similarly clustered, but those from sample DE43 are consistently younger, ∼185 Ma. These results, indicating a bimodal Triassic metamorphic overprint, are in contrast to the Neoarchean age (∼2.51 Ga) of thick zircon overgrowths in sample DE28. Allanite in both samples has retained its 208Pb/232Th crystallization age(s) through an event that caused major Pb loss from its U-Pb system, recording the later metamorphic history of a Neoarchean migmatite terrane.

The Western Sierras Pampeanas: Protracted Grenville-age history (1330–1030 Ma) of intra-oceanic arcs, subduction–accretion at continental-edge and AMCG intraplate magmatism

New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullun), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ullun. The oldest rocks are the Andino-type granitic orthogneisses of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma. The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites (1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The emplacement of these mantle-derived magmas during an extensional episode produced a widespread thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and back-arc complexes dominated the Sierra de Pie de Palo–Ullun sector, that was fully developed ca. 1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at 1165 Ma was roughly coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are registered in the Pie de Palo–Ullun sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.

Protracted fluid-induced melting during Barrovian metamorphism in the Central Alps

The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites, and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature; that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available. The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling in the order of 100°C/Ma to upper crustal levels.

Eocene arc-continent collision and crustal consolidation in Kamchatka, Russian Far East

The age and origin of high-grade metamorphic rocks of the Sredinnyi Range, Kamchatka have been the subject of a long and controversial debate. Based on geochronologic data and its association with a collage of accreted oceanic terranes, leading interpretations argue that the Sredinnyi Range metamorphic rocks represent an accreted Precambrian or Mesozoic microcontinent. In this contribution, we present new data that indicate that these metamorphic rocks were formed from the Cretaceous-Paleocene sedimentary margin of northeast Russia when it was overridden during Eocene obduction of the Olyutorsky arc, a far-travelled oceanic island arc. Our data include new mapping and structural observations along the northern and eastern flanks of the Sredinnyi Range, and SHRIMP zircon and monazite U-Th-Pb age data from 15 key samples. These new isotopic data demonstrate that paragneissic units were formed from sediments with depositional ages locally no older than Late Cretaceous to Paleocene. Furthermore, the statistical similarity of zircon U-Pb grain-ages from the Kamchatka Schist with very low-grade turbidite sandstones of the Ukelayat and Khozgon Groups indicate that metasediments of the Sredinnyi Range are upgraded stratigraphic equivalents of northeast Russian marginal strata. SHRIMP U-Pb ages of zircon overgrowths and metamorphic monazite extracted from migmatite and gneiss indicate that peak metamorphism occurred at 52 Ma, which is synchronous with the onset of the Olyutorsky arc-continent collision. Heating and cooling occurred rapidly, at rates approaching 80°C/m.y. Rapid heating is attributed to syn-collisional, subduction-related magmatism. Thermochronology and structural observations indicate that exhumation was due to a combination of ductile and brittle thinning of the crust. We speculate that this thinning was caused by diapiric ascent of a low-density low-viscosity continental material beneath a dense structural lid of the obducted island arc.

Polymetamorphism, zircon growth and retention of early assemblages through the dynamic evolution of a continental arc in Fiordland, New Zealand

AbstractThe Marguerite Amphibolite and associated rocks in northern Fiordland, New Zealand, contain evidence for retention of Carboniferous metamorphic assemblages through Cretaceous collision of an arc, emplacement of large volumes of mafic magma, high‐P metamorphism and then extensional exhumation. The amphibolite occurs as five dismembered aluminous meta‐gabbroic xenoliths up to 2 km wide that are enclosed within meta‐leucotonalite of the Lake Hankinson Complex. A first metamorphic event (M1) is manifest in the amphibolite as a pervasively lineated pargasite–anorthite–kyanite or corundum ± rutile assemblage, and as diffusion‐zoned garnet in pelitic schist xenoliths within the amphibolite. Thin zones of metasomatically Al‐enriched leucotonalite directly at the margins of each amphibolite xenolith indicate element redistribution during M1 and equilibration at 6.6 ± 0.8 kbar and 618 ± 25 °C. A second phase of recrystallization (M2) formed patchy and static margarite ± kyanite–staurolite–chlorite–plagioclase–epidote assemblages in the amphibolite, pseudomorphs of coronas in gabbronorite, and thin high‐grossular garnet rims in the pelitic schists. Conditions of M2, 8.8 ± 0.6 kbar and 643 ± 27 °C, are recorded from the rims of garnet in the pelitic schists. Cathodoluminescence imaging and simultaneous acquisition of U‐Th‐Pb isotopes and trace elements by depth‐profiling zircon grains from one pelitic schist reveals four stages of growth, two of which are metamorphic. The first metamorphic stage, dated as 340.2 ± 2.2 Ma, is correlated with M1 on the basis that the unusual zircon trace element compositions indicate growth from a metasomatic fluid derived from the surrounding amphibolite during penetrative deformation. A second phase of zircon overgrowth coupled with crosscutting relationships date M2 to between 119 and 117 Ma. The Early Carboniferous event has not previously been recognized in northern Fiordland, whereas the latter event, which has been identified in Early Cretaceous batholiths, their xenoliths, and rocks directly at batholith margins, is here shown to have also affected the country rock. However, the effects of M2 are fragmentary due to limited element mobility, lack of deformation, distance from a heat source and short residence time in the lower crust during peak P and T. It is possible that many parts of the Fiordland continental arc achieved high‐P conditions in the Early Cretaceous but retain earlier metamorphic or igneous assemblages.

Neoproterozoic magmatism and high-grade metamorphism in the Goiás Massif: New LA-MC-ICMPS U–Pb and Sm–Nd data and implications for collisional history of the Brasília Belt

This paper presents the first U–Pb and Sm–Nd data for the gneisses of the high-grade Uruaçu Complex, exposed in the northern part of the Neoproterozoic Brasília Belt, central Brazil. Previous studies have interpreted this complex as the exposure of Archaean or Paleoproterozoic continental crust, included, therefore, in the so-called Goiás Massif, a small Archaean/Paleoproterozoic allochthonous (?) block. The complex is located in the northern Brasília Belt, adjacent to the eastern limit of the Goiás Magmatic Arc, and comprises a variety of para- and orthogneisses metamorphosed under amphibolite to granulite facies. New U–Pb LA-MC-ICPMS data for zircon grains from felsic and mafic orthogneisses reveal ages ranging from ca. 690 to 650 Ma, interpreted as representative of the original igneous crystallization. Zircon overgrowths indicate metamorphic ages around 650 Ma, suggesting that the metamorphism was partly concomitant or only slightly younger than the intrusion of granitoids and mafic bodies, as well as with the closure of the ocean between the Amazon and São Francisco continents at the end of the Neoproterozoic. The main geological characteristics of the Uruaçu Complex, as well as the new U–Pb crystallization and provenance ages discussed in this work, are analogous to those observed in the Anápolis–Itauçu Complex, exposed in the southern Brasília Belt. This may suggests the correlation between both complexes, resulting in a long granulitic belt developed to the east of the Goiás Magmatic Arc during the main collisional stage of evolution of the Brasília Belt. High-grade metamorphism and associated igneous intrusions represented mainly by metagabbros and metagranites are, therefore, a conspicuous feature of the final stages of evolution of the Brasília Belt and might be related to delamination, uplift and extension following ocean closure and continental collision.

Metamorphic zircon, trace elements and Neoarchean metamorphism in the ca. 3.75 Ga Nuvvuagittuq supracrustal belt, Québec (Canada)

The Nuvvuagittuq supracrustal belt (NSB) in northwestern Québec ranks as one of the oldest granitoid gneiss complexes thus far discovered. Emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz–magnetite schists at the Porpoise Cove outcrops of the NSB cluster at 3.75 Ga. As with all other pre-3.7 Ga terranes thus far documented, the NSB has been metamorphosed and multiply deformed. In a combined approach to explore the thermal history of the NSB, conventional garnet–biotite and plagioclase–hornblende geothermometry was coupled with zircon U–Th–Pb depth profiles, REE partitioning and Ti-in-zircon thermometry to show that Neoarchean metamorphism in the belt reached upper amphibolite facies conditions (~ 640 °C). This metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the Northeast Superior Province and the initiation of widespread igneous activity in the vicinity of the NSB. An earlier (resolvable) metamorphic episode from a zircon depth-profile, recorded at 3.62 Ga, probably corresponds to the intrusion of the granitoids which envelope the NSB.