Zircon Overgrowths Research Articles

Sensitive high-resolution ion microprobe U-Pb dating of baddeleyite and zircon from a monzonite porphyry in the Xiaoshan area, western Henan Province, China: Constraints on baddeleyite and zircon formation process

Baddeleyite and zircon, including zircon overgrowths on baddeleyite, co-occur in a granophyric monzonite porphyry intruding volcanic rocks of the Majiahe Formation of the Xiong’er Group in the Xiaoshan area of the western Henan Province (China). It is inferred that the low silica content of the initial magma resulted in the formation of euhedral baddeleyite. Sensitive high-reso­lution ion microprobe (SHRIMP) U-Pb dating of the baddeleyite yields a weighted mean 207 Pb/ 206 Pb age of 1779 ± 8 Ma, representing the crystallization of the porphyry. Survival of baddeleyite as the magma became silica saturated implies a rapid cooling process. Zircon grains in the porphyry are anhedral to needle shaped, and are often clustered together within K-feldspar stringers or along the interface of quartz and K-feldspar in the granophyric groundmass, suggesting undercooling likely due to rapid emplacement. These observations indicate that zircon did not crystallize until final emplacement. Our U-Pb analyses of zircon yield a weighed mean 207 Pb/ 206 Pb age of 1777 ± 8 Ma, similar to that of baddeleyite. Subsequent medium- to high-temperature hydrothermal alteration affected most minerals at subsolidus conditions. Amphibole-plagioclase thermobarometry indicates pressure of 2.4–4.3 kbar and temperature of 470–580 °C for this alteration stage. Related alteration, such as total albitization of plagioclase, crystallization of secondary apatite, and sporadic secondary zircon overgrowths on baddeleyite, indicates that the fluid phase was enriched in Si, Na, and halogens (e.g., F, Cl). In addition to direct replacement of baddeleyite, zirconium required for the development of zircon overgrowths may also have been available through the alteration of Zr-bearing matrix phases such as amphibole and ilmenite.

Source and mode of the Permian Panjal Trap magmatism: Evidence from zircon U–Pb and Hf isotopes and trace element data from the Himalayan ultrahigh-pressure rocks

We present an integrated study of LA-ICP-MS U–Pb age, Hf isotopes, and trace element geochemistry of zircons from the Himalayan eclogites (mafic rocks) and their host gneisses (felsic rocks) from the Kaghan Valley in Pakistan in order to understand the source and mode of their magmatic protoliths and the effect of metamorphism. Zircons from the so-called Group I (high-pressure) eclogites yielded U–Pb mean ages of 259±10Ma (MSWD=0.74), whereas those of Group II (ultrahigh-pressure) eclogites yielded 48±3Ma (MSWD=0.71). In felsic gneisses the central or core domains of zircons yielded ages similar to those from Group I eclogites but zircon overgrowth domains yielded 47±1Ma (MSWD=1.9). Trace element data suggest a magmatic origin for Group I-derived (having Th/U ratios: >0.5) and metamorphic origin for Group II-derived (Th/U<0.07) zircons, respectively. Zircon Hf isotope data, obtained from the same dated spots, show positive initial 176Hf/177Hf isotopic ratios referred to as “ƐHf(t)” of around +10 in Group I eclogites; +7 in Group II eclogites; and +8 in felsic gneisses zircons, respectively, thus indicate a juvenile mantle source for the protolith rocks (Panjal Traps) with almost no contribution from the ancient crustal material. The similar ƐHf(t) values, identical protolith ages and trace element compositions of zircons in felsic (granites or rhyolites) and mafic (basalt and dolerite) rocks attest to a bimodal magmatism accounting for the Panjal Traps during the Permian. Later, during India–Asia collision in Eocene times, both the felsic and mafic lithologies were subducted to mantle-depths (>90km: coesite-stable) and experienced ultrahigh-pressure metamorphism before their final exhumation.

New age constraints on metamorphism, metasomatism and gold mineralisation at Plutonic Gold Mine, Marymia Inlier, Western Australia

ABSTRACTThe Plutonic Well Greenstone Belt (PWGB) is located in the Marymia Inlier between the Yilgarn and Pilbara cratons in Western Australia, and hosts a series of major Au deposits. The main episode of Au mineralisation in the PWGB was previously interpreted to have either accompanied, or shortly followed, peak metamorphism in the late Archean at ca 2650 Ma with a later, minor, event associated with the Capricorn Orogeny. Here we present new Pb isotope model ages for sulfides and Rb–Sr ages for mica, as well as a new 207Pb–206Pb age for titanite for samples from the Plutonic Gold Mine (Plutonic) at the southern end of the PWGB. The majority of the sulfides record Proterozoic Pb isotope model ages (2300–2100 Ma), constraining a significant Au mineralising event at Plutonic that occurred >300 Myr later than previously thought. A Rb–Sr age of 2296 ± 99 Ma from muscovite in an Au-bearing sample records resetting or closure of the Rb–Sr system in muscovite at about the same time. A younger Rb–Sr age of 1779 ± 46 Ma from biotite from the same sample may record further cooling, or resetting during a late-stage episode of metasomatism in the PWGB. This could have been associated with the 1820–1770 Ma Capricorn Orogeny, or a late-stage hydrothermal event potentially constrained by a new 207Pb–206Pb age of 1725 ± 26 Ma for titanite in a chlorite–carbonate vein. This titanite age correlates with a pre-existing age for a metasomatic event dated at 1719 ± 14 Ma by U–Pb ages of zircon overgrowths in a sample from the Marymia Deposit. Based on the Pb-isotope data presented here, Au mineralising events in the PWGB are inferred to have occurred at ca 2630, 2300–2100 Ma, during the Glenburgh and Capricorn orogenies, and 1730–1660 Ma. The 2300–2100 Ma event, which appears to have been significant based on the amount of sulfide of this age, correlates with the inferred age for rifting of the Marymia Inlier from the northern margin of the Yilgarn Craton. The texturally-later visible Au may have been deposited during the Glenburgh and Capricorn orogenies.

Argon, oxygen, and boron isotopic evidence documenting 40ArE accumulation in phengite during water-rich high-pressure subduction metasomatism of continental crust

The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40Ar/39Ar, δ18O, and δ11B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ18O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ18O (ca. 14‰) fluids. Heavy δ18O overgrowths of metamorphic zircon over lighter δ18O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ11B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage phengite with δ11B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ11B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon (40ArE) in phengite as a high pressure phenomenon. Phengite 40Ar/39Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40Ar/39Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40ArE concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon (Dphg/fluidAr=10−3to10−5) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40ArE partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10−2 to allow phengites to accumulate large amounts of 40ArE from aqueous fluids under HP to UHP conditions.

Early Miocene subduction in the western Mediterranean: Constraints from Rb-Sr multimineral isochron geochronology

The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high-pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado-Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multimineral isochron 87Rb/86Sr dating on carefully selected mineral samples from high-pressure metamorphic rocks in the NFC to better constrain the timing of high-pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multimineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1 ± 1.1, 16.0 ± 0.3, and 13.3 ± 1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87Rb/86Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu-Hf ages on garnet and U-Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40Ar/39Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid-Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing.

The geology, geochronology and affiliation of the Glenroy Complex and adjacent plutonic rocks, southeast Nelson

ABSTRACTThe Glenroy Complex consists of retrogressed monzodioritic granulites (Woodham Orthogneiss) and heterogeneous metasedimentary rocks (Davis Creek Paragneiss). U–Pb zircon dating shows that the Woodham Orthogneiss comprises rocks ranging in age from 121.8 to 126.6 Ma that are correlatives of the geochemically similar Western Fiordland Orthogneiss Worsley Pluton in northern Fiordland. Permian–Jurassic detrital zircon age peaks in the Davis Creek Paragneiss indicate a maximum depositional age of c. 227 Ma, suggesting correlation with George Sound Paragneiss in northern Fiordland and the paleo-Pacific Gondwana margin cover sequence. Metamorphism of the Davis Creek Paragneiss is constrained to 120.6 ± 0.9 Ma by U–Pb dating of low Th/U zircon overgrowths. The McKnee Intrusives were intruded in two discrete episodes at 225.0 ± 2.7 Ma and 137.8 ± 0.7 Ma which, together with their geochemistry, suggests correlation with northern Fiordland Late Triassic and Early Cretaceous Darran Suite plutons. The HiSY Mt Cann Pluton has an age of 113.8 ± 1.0 Ma and is correlated with the Separation Point Granite. The Glenroy Complex and McKnee Intrusives were likely exhumed during at least two stages: initially in the Cretaceous prior to displacement from their northern Fiordland correlatives by the Alpine Fault; and during emplacement as tectonic blocks in the Matakitaki foreland thrust system at the Big Bend.

Ra’s Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization, evidenced by zircon morphology and chemistry

Syenogranitic dykes in the north of Egypt’s Eastern Desert are of geological and economic interest because of the presence of magmatic and supergene enrichment of radioactive mineralization. Zircon crystal morphology within the syenogranitic dykes allows precise definition of sub-alkaline series granites and crystallized at mean temperature of about 637 °C. The growth pattern of the zircons suggest magmatic and hydrothermal origins of radioactive mineralization. Hydrothermal processes are responsible for the formation of significant zircon overgrowth; high U-zircon margins might have occurred contemporaneously with the emplacement of syenogranitic dykes which show anomalous uranium (eU) and thorium (eTh) contents of up to 1386 and 7330 ppm, respectively. Zircon chemistry revealed a relative increase of Hf consistent with decreasing Zr content, suggesting the replacement of Zr by Hf during hydrothermal activity. Visible uranium mineralization is present and recognized by the presence of uranophane and autunite.

Petrochronology of the migmatization event of the Xolapa Complex, Mexico, microchemistry and equilibrium growth of zircon and garnet

ABSTRACTThe number of migmatization events in the Xolapa Complex and their absolute age are controversial. U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry was performed on zircon grains from migmatites to investigate the age of different textural domains. Rare-earth element (REE) partition coefficients between zircon and garnet were compared with those established for different temperatures in order to test for equilibrium growth. Two age domains were identified. In one sample where zircon and garnet coexist, the outer zircon overgrowths yield a mean age of 54.16 ± 0.29 Ma (mean square weighted deviation (MSWD) = 3.5), whereas intermediate zones, between the core and outer overgrowths, yield an age of 122.7 ± 1.8 Ma (MSWD = 2.5). Partition coefficients were calculated for REEs between coexisting garnet (two different populations) and zircon using (1) the composition of ca. 54 Ma zircon overgrowths and garnet rims and (2) zircon intermediate zones together with garnet cores. The cores of small garnet grains (garnet A) may have grown in equilibrium with zircon domains of ca. 122 Ma. Both garnet cores and rims of the larger porphyroblasts (garnet B) seem to be in equilibrium with ca. 54 Ma zircon overgrowths. Petrographic observations suggest that crystallization of garnet A occurred during partial melting, placing equilibrium growth and therefore a first migmatitic event during the Early Cretaceous at ca. 122 Ma. This migmatitic event may be related to the collision of the Chortís Block with western Mexico. A second migmatitic event of ca. 54 Ma is suggested by equilibrium growth of large garnets (group B) and the outer zircon overgrowths. The high geothermal gradient necessary for this second migmatitic event might be related to the exhumation of the Xolapa Complex, as a result of the transpression and tectonic transport of the Chortís Block to the southeast from the end of the Mesozoic to most of the Cenozoic.

Zircon xenocryst resorption and magmatic regrowth at El Chichón Volcano, Chiapas, Mexico

El Chichón volcano is the only active volcano located within the Chiapanecan Volcanic Arc in southern Mexico, which lies between the Trans-Mexican Volcanic Belt and the Central American Volcanic Arc. Previous studies have shown that ~12 eruptions have occurred at El Chichón within the last 8000years, forming a complex of lava domes with a central crater and surrounding pyroclastic deposits. Here, we report the discovery of zircon in Holocene El Chichón rocks, which were analyzed by high spatial resolution imaging (color cathodoluminescence CCL) and isotopic (secondary ionization mass spectrometry SIMS) methods to resolve core and rim crystallization ages. Pumice samples from five proximal pyroclastic flow and fall-out deposits were collected based on published stratigraphy. Two of the samples were further (re-)classified by new 14C dates. In addition, we sampled two lavas from the 1982 eruption and from remnants of the older Somma lava complex. Zircon crystals were dated using 230Th/238U disequilibrium (U–Th) and U–Pb geochronology. U–Th zircon ages fall between near eruption ages and ca. 84ka, with overlapping ages in all samples. By contrast, zircon core U–Pb ages range between ca. 290Ma and 1.9Ga. These ages are consistent with xenocrystic origins and their heterogeneity indicates derivation from clastic country rocks. Strong age contrasts between inherited xenocrystic and young magmatic domains in individual zircon crystals are evidence for arrested assimilation of crustal rocks where initially zircon-undersaturated magmas cooled rapidly to form a crystal mush or subsolidus amalgamate as a crustally contaminated boundary layer. This layer contributed zircon crystals to eruptible magma during episodic recharge events followed by partial melt extraction, mixing and homogenization. Zircon overgrowths are significantly older than major minerals whose U-series ages and sharp zonation boundaries suggest crystallization only within a few ka before eruption. This implies that zircon can detect magmatic longevity which is obscured in the major mineral record.

Further evidence of 777 Ma subduction-related continental arc magmatism in Eastern Dom Feliciano Belt, southern Brazil: The Chácara das Pedras Orthogneiss

In this study new SHRIMP U–Pb zircon data for the Chácara das Pedras Gneiss in Porto Alegre, southern Brazil are presented. They represent a small exposure of the crust which was intruded by a large volume of orogenic to anorogenic granitoids at ca. 618-562 m.y. in the Eastern Domain of the Dom Feliciano Belt. The Chácara das Pedras tonalitic orthogneiss has geochemical similarities with subduction-related magmatic rocks of continental arcs. They present high Sr initial ratios (∼0.712), negative ɛNd(t = 777) values (∼−6), TDM varying from 1.8 to 2.0 Ga. The igneous protoliths of these orthogneisses were previously considered to be Paleoproterozoic based on an upper intercept age of discordant zircon analyses. In the present study these orthogneisses were re-sampled and re-analyzed in an attempt to obtain more concordant analytical data. The U–Pb zircon analyses were carried out using the SHRIMP IIe at the Laboratório de Geocronologia de Alta Resolução of the Universidade de São Paulo. The U–Pb concordia age obtained for igneous textural domains of the zircon grains is 777 ± 4 Ma. A few analyses on zircon overgrowths give poorly defined late Cryogenian ages of ca. 650 Ma. Older ages, mostly discordant, were obtained in a few zircon cores, showing an upper intercept age of ca. 1.9 Ga. One sample of the Três Figueiras Granodiorite, which crosscut the orthogneiss in the same outcrop, was also investigated. The zircons of this granodiorite are, however, mostly metamitic, preventing the determination of a reliable age. Some concordant analyses from a few grains define ages ranging in the interval between ca. 603 and 1022 Ma. The youngest (ca. 603 Ma) may represent a maximum age for the granodiorite crystallization. Older ages, with discordance <10%, are of 745, 777, 836 and 1022 Ma. The 777 ± 4 Ma age obtained for the Chácara das Pedras orthogneiss is the first Early Cryogenian magmatic age determined for granitoids in the Porto Alegre region, although records of similar ages (780–800 Ma) are recognized within the Eastern Domain of the Dom Feliciano Belt. Examples are the Piratini Gneisses (Piratini region) and in the Cerro Bori Orthogneisses (Rocha region, Uruguay). All these units show subduction-related continental arc affinities. The identification of a magmatic event of ca. 780–800 Ma in the Porto Alegre region implies that: (i) the collisional magmatism (650 Ma) is ca. 130 Ma younger than the subduction-related magmatism; (ii) the younger orogenic (arc related to post-collisional) and anorogenic granitoids (<618 Ma) of Eastern Domain were emplaced in a hot/weak lithosphere due to the presence of an earlier ca. 780–800 Ma magmatic arc rocks; (iii) the tectonic evolution of this region in West Gondwana suggests a more complex geotectonic scenario than those put forward by previous models involving the amalgamation of the Río de La Plata and Kalahari cratons.

Highly fractionated Late Eocene (~ 35 Ma) leucogranite in the Xiaru Dome, Tethyan Himalaya, South Tibet

The Xiaru dome is located in the middle section of the North Himalayan Gneiss Domes belt in southern Tibet. The leucogranite, which crops out in the core of the Xiaru dome, is a typical medium-grained garnet+tourmaline+muscovite leucogranite. U–(Th)–Pb dating of zircon and monazite from the leucogranite yielded ages of approximately 35Ma. This finding supports a growing body of evidence indicating that an extensive magmatic event occurred during the late Eocene in the Himalayas. This leucogranite is strongly peraluminous with A/CNK values of 1.08–1.52 and characterized by evolved geochemical composition with high contents of SiO2 and alkali elements; low levels of CaO, MgO, TiO2, and FeOT; enriched large-ion lithophile elements (such as Rb); and depleted of high-field-strength elements (such as Nb, Zr, and Hf). The non-CHARAC (CHarge-And-Radius-Controlled) trace element behaviors, which are typical of a highly fractionated granite system, were recorded in the whole rock and the accessory minerals of the Xiaru leucogranite. Furthermore, the magmatic zircon overgrowths have extremely high content of Hf, consistent with those from the highly fractionated aqueous-like fluid system. In addition, whole-rock geochemical fractionation trends were observed, which can be explained by crystal fractionation of biotite, K-feldspar, zircon, xenotime, and monazite. These geochemical features indicate that the Xiaru leucogranite is a typical highly fractionated granite. The geochronological and geochemical features of the inherited zircons from the Xiaru leucogranite show a close affinity to those of the country rocks, suggesting a certain degree of assimilation from the country rocks during melt ascent and emplacement. Although a restricted range of εHf(t) values from −12.8 to −6.6 with Hf TDM2 model ages of 1.2–1.6Ga was obtained from the late Eocene zircons, it is invalid to constrain the source of the parental magma due to the strong fractionation and assimilation processes.

An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprián massif (NW Spain)

Th/U values in zircon are commonly used to discriminate between metamorphic (Th/U &lt; 0.1) and magmatic (Th/U &gt; 0.1) origin for zircon overgrowths. We test this hypothesis in the San Ciprián massif, a late orogenic granitic intrusion in the hinterland of the Variscan orogeny. Zircon grains from this granite have cores with inherited Ediacaran ages and Th/U &gt; 0.1, whereas zircon mantles yield an age of about 287 Ma, interpreted as the time of crystallization of the granite, and have Th/U &lt; 0.1. Hence, the San Ciprián massif presents an uncommon but unambiguous example of magmatic zircon mantles with Th/U values typical of metamorphic zircon. The most likely causes for the unusually low-Th/U zircon values in the San Ciprián massif are a combination of a U-rich magma composition (owing to a fractionation process) and the absence of other U-enriched accessory minerals. Our work in determining Th/U ratios substantiates the warning previously made by some researchers and precludes the use of Th/U values in zircon as an unequivocal indicator of metamorphic origin in the absence of other chemical, zircon morphology or field-based independent criteria. Supplementary materials: Code and data to generate Figures 1 and 5 are available at http://www.geolsoc.org.uk/SUP18885 .

Single-shot laser ablation split stream (SS-LASS) petrochronology deciphers multiple, short-duration metamorphic events

Single-shot laser ablation split stream (SS-LASS) petrochronology offers spatial resolution of <1μm per (surface) analysis. This enables the technique to interrogate metamorphic zircon overgrowths that are very thin, or that preserve a composite age structure. To demonstrate the advantages of SS-LASS, the technique was applied to metamorphic zircon overgrowths in five rocks from the Cordillera de la Costa, Venezuela. In these rocks, SS-LASS was able to decipher discrete, short-duration (<106yr) zircon growth events at c. 33.0, c. 28.3, c. 23.0 and c. 18.2Ma. Comparison with existing geo-/thermochronology suggests that the SS-LASS dates represent (hydro)thermal events that mark distinct episodes of tectonism affecting the northern margin of South America.With an external error of 5% incorporated into isotope ratios (as required to achieve satisfactory analytical precision), SS-LASS dates for reference materials were accurate to within 1.5% of published values. The SS-LASS technique is thus, in favorable cases, capable of providing <106yr resolution for Cenozoic rocks. Methods in ‘geospeedometry’—which employ forward models to reproduce observed diffusion length scales—have obtained anomalously short time scales of 104–106yr for metamorphism. The high spatial and temporal resolution of SS-LASS offers tremendous promise for investigating the veracity of these claims.

The Early Cretaceous evolution of SE China: Insights from the Changle–Nan'ao Metamorphic Belt

The Mesozoic geology in SE China is characterized by widespread Jurassic–Cretaceous magmatism. There appears, however, an episode of “magmatic quiescence” at ca. 130–110Ma in terms of increasing high-precision isotopic age data. The causes and regional tectonic significance of this magmatic quiescence are unclear. In this paper, we report integrated in situ U–Pb, Hf and O isotope analyses of zircons from four mica schist samples along Changle–Nan'ao Metamorphic Belt in SE China coast, and use these data to discuss the provenances of these metasedimentary rocks, the timing of the regional metamorphism and their implications for the regional tectonic evolution. The majority of zircons from the mica schists are euhedral to anhedral crystals showing clear concentric zoning under cathodoluminescence (CL) images. Metamorphic zircon rims with bright luminescence are overgrown on the some of the euhedral to anhedral zoned zircons. The zoned zircons from all mica schist samples are homogeneous in U–Pb age zircon ages (between 131 and 148Ma, peaking at 141Ma), O isotopes (δ18O=5.6±0.8‰, 2SD, n=106), and, to the less extent, Hf isotopes (εHf(T)=−4.7 to −1.4 for the Dongshan mica schists, and −1.4 to 2.2 for the Putian mica schists). They are most likely detrital zircons sourced from the regional Early Cretaceous felsic volcanic rocks (the Nanyuan Formation) and/or coeval intrusions in Fujian Province. The sedimentary protoliths of these mica schists were likely deposited in fore-arc basins. The metamorphic zircon overgrowths have relatively low Th/U ratios; they are dated at ~130–100Ma, with two peaks at ~125Ma and ~108Ma. The first metamorphic age of ~125Ma is coincident with the timing of regional Early Cretaceous magmatic quiescence, while the second metamorphic age of ~108Ma is consistent with the timing of reinitiation of regional Cretaceous magmatism. We use a model of collision between the West Philippines and SE China blocks to interpret our new results. According to this model, the intensive Early Cretaceous (ca. 140–130Ma) magmatism along the coastal region of SE China was attributed to the subduction of the Paleo-Pacific plate beneath SE China. Collision between the West Philippines and SE China blocks at ~130–120Ma resulted in the regional “magmatic quiescence” and formation of the Changle–Nan'ao Metamorphic Belt. Post-collisional extension triggered the reinitiation of magmatism associated with amphibolite-facies metamorphism at ~110Ma.

Geochronology of emplacement and charnockite formation of the Margate Granite Suite, Natal Metamorphic Province, South Africa: Implications for Natal-Maud belt correlations

The Margate Granite Suite underlies much of the Margate Terrane of the Natal Metamorphic Province, SE South Africa. It consists of foliated granites grouped into four main lithotypes: garnet leucogranite, garnet-free leucogranite, charnockite and garnet-biotite augen gneiss. In this study we present new U-Pb (SHRIMP) zircon geochronological data on each of the four lithotypes to constrain the timing of emplacement of the various granite phases and of charnockite formation. Magmatic zircon ages span a period of about 125Ma, indicating that the Margate Suite does not comprise a single coeval group of plutons. The oldest crystallisation age of 1169±14Ma, obtained from the garnet-biotite augen gneiss phase, is statistically similar to that of the Sikombe Granite which is exposed to the south of the Margate Terrane, with which a correlation is made. This implies that the magmatic history of the Margate Terrane is longer and more complex than previously thought. The original granite protolith of a sample from the charnockitised granite in the thermal aureole of the (ca. 1040Ma) Oribi Gorge granite yielded an age of 1135±11Ma, which is statistically similar to the published age of the gneissic Mzimilo Granite in the Mzumbe Terrane. Zircon overgrowths in this sample, dated at 1037±13Ma are coeval with the age of the Oribi Gorge granite and are interpreted to date the secondary charnockitisation of the Margate granite. A sample of a partly charnockitised garnet leucogranite provided an age of 1088±9Ma. This granite contains a pervasive foliation (S2), partly obliterated in charnockitic patches, indicating that both the D2 event and the charnockitisation are younger. This confirms previous work in which the maximum age of the main fabric-forming deformation was constrained by the 1091±9Ma age of the Glenmore Granite. A sample from the garnet leucogranite in the type area of the Margate Granite Suite yielded an age of 1043±4Ma, which is statistically similar to that of the previously dated, garnet-free leucogranite of the Portobello granite, and zircon metamorphic dates of the secondary charnockitisation event associated with emplacement of the Oribi Gorge Suite. Our new data show that the Margate Terrane was subjected to at least four magmatic/thermal events, at ∼1170Ma, ∼1135–1140Ma, ∼1082–1093Ma, and 1025–1050Ma. These events can be correlated with coeval magmatic and thermal episodes in surrounding crustal blocks within Rodinia and Gondwana. In particular the Margate Terrane appears to correlate well with the Vardeklettane Terrane of Dronning Maud Land, East Antarctica and furthermore that the Natal belt may be up to 80Ma older than the Maud belt east of the Heimefront Shear Zone, which is made up of younger crust which was accreted westwards against the Natal belt. In this scenario, the Cape Merdith Complex, West Falkland, which shows no zircon evidence of crust older than ca. 1135 Ma, forms part of the Maud belt, not the older Natal belt.

Cathodoluminescence guided zircon Hf isotope depth profiling: Mobilization of the Lu–Hf system during (U)HP rock exhumation in the Woodlark Rift, Papua New Guinea

Cathodoluminescence image guided Hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses constrains the Lu–Hf system's behavior during rapid exhumation of (U)HP rocks in the Woodlark Rift, Papua New Guinea. Investigation of the depth profiling technique in individual and composite zircon standards demonstrates that it is possible to resolve ~8μm thick domains in which εHf(present) differs by as little as 4units. In a metasedimentary gneiss, 2.89±0.29Ma zircon overgrowths on Cretaceous aged inherited cores have radiogenic εHf(present) indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization (the ‘garnet effect’ of Zheng et al., 2005). In a separate gneiss sample that originated as an exhumation related anatectic melt, 3.66±0.13Ma zircons lacking inheritance contain sub-domains that differ from each other by >15 εHf(present). Some of these sub-domains are radiogenic and can be explained by the ‘garnet effect’, whereas others also contain highly elevated Lu and Yb in addition to their radiogenic Hf compositions, thus necessitating a medium derived from the complete breakdown of garnet. Zircons in this sample also contain non-radiogenic sub-domains that grew in the presence of Hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the Australian continental margin. The results confirm that rapid exhumation of (U)HP rocks can result in the following: 1) transmission of radiogenic Hf (and sometimes Lu and the other HREE) from garnet bearing mafic lithologies into the quartzofeldspathic gneisses, and 2) mobilization and transport of unradiogenic Hf present within the quartzofeldspathic remnants of subducted continental crust.

Mesoarchaean collision of Kapisilik terrane 3070 Ma juvenile arc rocks and >3600 Ma Isukasia terrane continental crust (Greenland)

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ∼2690Ma metamorphic zircon and no 2980–2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ∼2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980–2950Ma metamorphic zircon and also sporadic growth of ∼2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and ∼3070Ma detrital zircons), mafic and ultramafic rocks, with ∼2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ∼3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970–2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ∼3075Ma intermediate volcanic rock have initial ɛHf values of +2 to +5 thus are juvenile crustal additions. ∼3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial ɛNd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ∼3075Ma supracrustal rocks of the Kapisilik terrane have initial ɛHf values of −5 to −10, and must have involved the partial melting of >3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980–2950Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970–2960Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they intruded the overlying allochthonous juvenile ∼3075Ma Ivisârtoq supracrustal belt arc assemblages. The southern edge of the Isukasia terrane is interpreted as an interior nappe of Eoarchaean basement rocks interfolded with a klippe of Mesoarchaean metasedimentary and mafic/ultramafic rocks, both of which are affected by 2980–2950Ma metamorphism. The mixed Eoarchaean–Mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust. The northern part of the Isukasia terrane is interpreted as foreland, free of 2980–2950Ma high-grade metamorphic overprint. This shows that the Isukasia terrane is not a coherent block, but contains ancient rocks that are parautochthonous or allochthonous to each other, with contrasting later metamorphic history.At ∼2690Ma the crustal architecture arisen from Mesoarchaean collision between an older continental block and an island arc was reworked along intra-crustal shear zones, coeval with amphibolite facies metamorphism. This reworking followed on from major terrane assembly at 2710–2700Ma in the southern part of the Nuuk region, when the Eoarchaean Færingehavn terrane was juxtaposed with 2840–2825Ma arc rocks. Thus the 2980–2950Ma assembly of the Isukasia and Kapisilik terranes is distinct from the later 2710–2700Ma terrane assembly further south in the Nuuk region.

Eocene magmatism in the Tethyan Himalaya, southern Tibet

Abstract Recent studies in the Yardoi gneiss dome of the Tethyan Himalaya, southern Tibet demonstrated that the Himalayan orogenic belt experienced a major episode of crustal melting in the Mid-Eocene ( c. 44–40 Ma), represented by the genesis of a suite of high Sr/Y two-mica granites (TMGs) accompanied by leucogranites. Geochemical and U–Pb age data indicate that (1) both the leucogranite and the porphyritic leucogranite formed at c. 44–41 Ma are characterized by similar Sr–Nd–Hf isotope systematics to those in the less evolved TMGs; (2) these three suites of rocks might represent a Mid-Eocene igneous complex derived from partial melting of amphibolite under crustal thickened conditions; and (3) in contrast with the TMGs, leucogranites experienced hydrothermal-magma interactions as well as plagioclase fractional crystallization as revealed from pronounced negative Eu anomalies, much lower Sr concentrations, and elevated common Pb and U concentrations in the latest magmatic zircon overgrowth. Our data suggest that magmatic differentiation of more primitive magmas could have played a critical role in the generation of leucogranites. Documentation of partial melting of deep crustal rocks during the earliest phase of the tectonic evolution of the Himalayan orogen implies that the large-scale collisional orogenic belt could experience intensive partial melting under contraction and thickening conditions, which could profoundly affect the geophysical and geochemical properties as well as rheological properties and facilitate the initiation of movement or flow of deep crustal rocks.

경기육괴 동부 오대산 지역의 구룡층군에 대한 SHRIMP U-Pb 저어콘 연대측정: 새로운 후기 고생대층의 인지와 지체구조적 의의

경기육괴 동부 오대산 지역의 구룡층군 흑운모 편암에 대한 SHRIMP U-Pb 저어콘 연대를 측정하였다. CL 영상에서 저어콘은 변성작용에 의해 원위치에서 성장한 가중평균연령 <TEX>$247{\pm}6Ma$</TEX>의 저어콘 외연부를 갖는다. 쇄설성 저어콘 핵은 대부분 마그마기원을 지시하는 누대구조와 Th/U 비를 보이고, 53점의 분석치에서 46점이 일치연령에 가까운 자료에 해당한다. 이들은 가중평균 <TEX>$378{\pm}10Ma$</TEX>(n=9), <TEX>$420{\pm}4Ma$</TEX>(n=6)와 <TEX>$1845{\pm}9Ma$</TEX> (n=18)의 집중군과 신원생대에서 시생대 최후기에 걸친 <TEX>$687{\pm}9Ma$</TEX>에서 <TEX>$2519{\pm}20Ma$</TEX>의 산발적인 연령을 보인다. 이 연구의 연대자료는 지금까지 시대미상이었던 구룡층군이 경기육괴 동부에서 최초로 보고되는 고생대 후기 지층으로서 최고 퇴적시기가 옥천대 남서부의 퇴적층에 대비될 수 있음을 나타낸다. 또한 이 연구에서 제시하는 중국 중앙부 충돌대의 경우와 같은 구룡층군의 트라이아스기의 변성시기와 고생대 중기(361~425 Ma)의 쇄설성 저어콘 연대는 오대산 지역이 중국 충돌대의 영향을 받은 지역 혹은 그의 연장부에 해당할 가능성을 시사한다. Zircon separated from a biotite schist of the Guryong Group in Odesan area, eastern part of the Gyeonggi Massif in Korea were analysed for SHRIMP U-Pb ages. CL images display composite core-rim structures of the zircon, indicating an in-situ overgrowth of zircon through a high-grade metamorphism. The metamorphic zircon rims give a weighted mean age of <TEX>$247{\pm}6Ma$</TEX>. While the detrital zircon cores have zoning patterns and Th/U ratios indicative of a magmatic origin. Among 53 analyses from the cores, 46 data yield near concordant ages which are concentrated at <TEX>$378{\pm}10Ma$</TEX> (n=9), <TEX>$420{\pm}4Ma$</TEX> (n=6) and <TEX>$1845{\pm}9Ma$</TEX> (n=18) with sporadic Neoproterozoic (<TEX>$687{\pm}9Ma$</TEX>) to late Archean (<TEX>$2519{\pm}20Ma$</TEX>) ages. The age data constraint sedimentation age of protolith of the Guryong Group, so far unknown, as late Paleozoic. The Guryong Group of this study is the first late Paleozoic strata reported from eastern Gyeonggi Massif, and its maximum depositional age (ca 378 Ma) is identical with those of the late Paleozoic strata in the southwestern Ogcheon Belt. The Triassic metamorphic age and abundant middle Paleozoic provenance (361~425 Ma) of the Guryong Group are similar with those reported from the Triassic collisional belt in central China. Thus this study indicates that the Odesan area would be an possible eastward extension of the Triassic collisional belt in central China.

Petrogenesis of Late Permian sodic metagranitoids in southeastern Korea: SHRIMP zircon geochronology and elemental and Nd–Hf isotope geochemistry

One of the striking tectonomagmatic features recently found in southeastern Korea is the occurrence of ca. 250Ma high-silica adakite. Sodic metagranitoids mainly consisting of tonalitic–trondhjemitic–granodioritic gneisses occur in the Andong-Cheongsong area adjacent to the Yeongdeok adakite site. To investigate temporal and petrogenetic relationships of these orthogneisses with the adakite, we conducted SHRIMP zircon U–Pb dating as well as elemental and Nd–Hf isotopic analyses. Zircon core ages of the orthogneisses (ca. 262–251Ma) confirm the widespread occurrence of arc-related Late Permian magmatism in southeastern Korea. The Late Triassic (ca. 230Ma) zircon overgrowths reflect a thermal overprint probably related to the initiation of another subduction system. The analyzed orthogneisses have major element compositions comparable to the Phanerozoic adakites and Archean TTG suite, such as high SiO2 (58.7–65.5wt.%) and Al2O3 (17.1–19.1wt.%) contents and Na2O/K2O ratios (1.83–4.95). However, their moderate Sr/Y (35–43) and La/Yb (14–53) ratios and negative Eu anomalies (Eu/Eu*=0.75–0.95) are incompatible with the key features reported from the Yeongdeok adakite. Moreover, initial whole-rock εNd (−7.9 to −3.3) and zircon εHf (−0.3±2.4) values of the orthogneisses negate a direct derivation from the subducted slab. Our elemental and Nd–Hf isotopic data collectively suggest that the protoliths of the tonalitic–trondhjemitic–granodioritic gneisses were generated by partial melting of mafic lower crust at depths shallower than the garnet stability field. Our Nd and Hf model ages of the gneisses, together with those previously reported from the Mesozoic granitoids indicate a selective involvement of young source materials along the margin of the Yeongnam massif. The Hf isotopic compositions of zircons from a trondhjemitic gneiss attest to the involvement of primitive melts during their crystallization. The ridge subduction and consequent development of a slab window may have facilitated partial melting of the subducted oceanic lithosphere and the lower crust.