Inherited Zircon Ages Research Articles

Recurrent rare earth element mineralization in the northwestern Okcheon Metamorphic Belt, Korea: SHRIMP U–Th–Pb geochronology, Nd isotope geochemistry, and tectonic implications

Rare earth element (REE) mineralization is hosted within Neoproterozoic alkaline metaigneous rocks in the northwestern part of the Okcheon Metamorphic Belt (OMB), a polymetamorphosed fold-and-thrust belt transecting the Paleoproterozoic Gyeonggi and Yeongnam Massifs in the southern Korean Peninsula. The principal carrier phase of REEs is allanite. Allanite grains can be subdivided into several types based on the texture and mineral assemblage including quartz, K-feldspar, biotite, britholite, apatite, fergusonite, andradite, magnetite, zircon, titanite and fluorite. Electron microprobe analysis of allanite clearly distinguishes sample-to-sample variations in total REEs, Ca, Al, Fe and Y but the textural varieties from each rock sample do not show marked differences in those elements. Sensitive high-resolution ion microprobe dating of allanite and zircon reveals a complex history of multistage mineralization. Allanite grains from REE ores yielded Late Ordovician (444.6±8.0Ma), Permian to Triassic (ca. 300–220Ma) and Early Jurassic (199–183Ma) 208Pb/232Th ages. These multiple age components often coexist in single grains showing slight differences in backscattered electron brightness. The Ordovician components have distinctly higher Th/U than the younger domains in the same rock sample. The cores and rims of zircon from a syenite hosting REE ore bodies yielded Neoproterozoic (858.2±6.3Ma) and Early Jurassic (ca. 190Ma) 206Pb/238U ages, respectively. The Early Jurassic ages (194–187Ma) also obtained from zircon grains from granites taken from dykes occurring close to the ores and a drill core indicate the correspondence between granitic magmatism and REE mineralization. The Neoproterozoic zircon inheritance (weighted mean=853.9±3.8Ma) in these granites is in sharp contrast to the dominant Paleoproterozoic inherited zircon from the widespread earliest Middle Jurassic granites enclosing the mineralized zone. The geotectonic significance of the Late Ordovician event recorded in the allanite, as well as in detrital zircon from the OMB, is still unclear but its temporal coincidence with intraplate volcanism and arc-related igneous activity, respectively, reported from the southwestern edge of the adjacent Taebaeksan Basin and the southwestern Gyeonggi Massif is noteworthy. The following Permian–Triassic and Early Jurassic mineralization events are probably linked to the continental suturing between the North and South China blocks and subsequent post-orogenic magmatism, and arc magmatism resulting from the paleo-Pacific plate subduction, respectively. Sub-grain Sm–Nd isotopic analyses of allanite by laser ablation multiple collector ICPMS yielded initial εNd values plotting along the Nd isotopic evolution path of the Neoproterozoic metaigneous rocks, indicating that REEs originating from the host rock have been recycled during multistage mineralization events. The profound differences in inherited zircon ages and Nd isotopic compositions between the Early and Middle Jurassic granites may reflect the presence of a major thrust-bounded crustal structure beneath the OMB.

U-Pb zircon geochronology of rocks from west Central Sulawesi, Indonesia: Extension-related metamorphism and magmatism during the early stages of mountain building

Sulawesi has generally been interpreted as the product of convergence in the Cretaceous and Cenozoic, and high mountains in west Central Sulawesi have been considered the product of magmatism and metamorphism related to Neogene collision. New SHRIMP and LA-ICP-MS U-Pb zircon dating of metamorphic and granitoid rocks has identified protoliths and sources of melts, and indicates an important role for extension. Schists, gneisses and granitoids have inherited Proterozoic, Paleozoic, Mesozoic and Paleogene zircons. Mesoproterozoic and Triassic age populations are similar to those from the Bird’s Head region. Their protoliths included sediments and granitoids interpreted as part of an Australian-origin block. We suggest this rifted from the Australian margin of Gondwana in the Jurassic and accreted to Sundaland to form NW Sulawesi in the Late Cretaceous. Some metamorphic rocks have Cretaceous and/or Late Eocene magmatic zircons indicating metamorphism cannot be older than Late Eocene, and were not Australian-origin basement. Instead, they were metamorphosed in the Neogene after Sula Spur collision and subsequent major extension. Associated magmatism in west Central Sulawesi produced a K-rich shoshonitic (HK) suite in the Middle Miocene to Early Pliocene. A later episode of magmatism in the Late Miocene to Pliocene formed mainly shoshonitic to high-K calc-alkaline (CAK) rocks. I-type and silica-rich I-type granitoids and diorites of the CAK suite record a widespread short interval of magmatism between 8.5 and 4Ma. Inherited zircon ages indicate the I-type CAK rocks were the product of partial melting of the HK suite. S-type CAK magmatism between c. 5 and 2.5Ma and zircon rim ages from gneisses record contemporaneous metamorphism that accompanied extension. Despite its position in a convergent setting in Indonesia, NW Sulawesi illustrates the importance of melting and metamorphism in an extensional setting during the early stages of mountain building.

Geochronology, geochemistry and Sr–Nd–Pb isotopic constraints on the origin of the Qian’echong porphyry Mo deposit, Dabie orogen, east China

The giant Qian’echong porphyry molybdenum deposit is located in the Dabie orogen, east China. The molybdenum mineralization mainly occurs as molybdenite-bearing quartz veins hosted by the Devonian Nanwan Formation in the external contact zone of the Qian’echong stock. The Qian’echong stock comprises an earlier formed monzogranite and a later formed granite porphyry. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating constrains the timing of crystallization of the monzogranite and granite porphyry to 130±2 and 129±2Ma, respectively. The Re–Os model ages of six molybdenite samples range from 127.9±1.9 to 129.7±1.9Ma with an isochron age of 129.4±1.5Ma, which are all consistent within errors with the zircon U–Pb ages, indicating an Early Cretaceous magmatic and mineralization event. The Qian’echong granites have moderate negative Eu anomalies and are relatively enriched in light rare earth elements (REE), but depleted in heavy REE, Y, and high field strength elements (HFSE; e.g., Nb, Ta, and Ti). The Qian’echong granites are I-type rather than A- or S-type, and they have high (87Sr/86Sr)i (0.706771–0.710326) and low εNd(t) (−25.5 to −16.8). Two-stage Nd model ages (T2DM) vary between 2.29 and 2.99Ga. Sr–Nd–Pb isotopic data suggest that the Qian’echong granites were not derived from the North China Craton (NCC), but rather were generated from the Yangtze Craton (YC) lower crust. Paleoproterozoic inherited zircon age and whole-rock chemical and Sr–Nd–Pb isotopic data suggest that the Qian’echong granites were derived mainly from partial melting of ultrahigh pressure eclogites, with incorporation of some Paleoproterozoic to Archean YC crustal materials at lower crustal levels. Delamination or foundering of eclogitic lower crust, which extensively occurred in the Dabie orogen during the Early Cretaceous, had not taken place beneath the Qian’echong deposit when it formed. The Qian’echong molybdenum deposit formed in an extensional setting in the Dabie orogen, and may have been associated with a change in the subduction direction of the Izanagi (or Paleo-Pacific) Plate after 135Ma.

South China continental margin signature for sandstones and granites from Palawan, Philippines

We report results of heavy mineral analysis and U–Pb dating of detrital zircons from metasediments and Cenozoic sandstones, and U–Pb dating of zircons from Cenozoic granites of the North Palawan Continental Terrane (NPCT) and the South Palawan Terrane (SPT). The NPCT metasediments are derived mainly from granitic and metamorphic rocks of continental character. They contain zircons that indicate a maximum depositional age of Late Cretaceous and other age populations indicating a South China origin. The sediments were deposited on the South China margin before rifting of the continental margin during opening of the South China Sea. Miocene SPT sandstones contain similar heavy mineral assemblages suggesting sources that included NPCT metasediments, metamorphic basement rocks at the contact between the SPT and the NPCT, South China Sea rift volcanic and/or minor intrusive rocks, and the Palawan ophiolite complex. The SPT sandstones are very similar to Lower Miocene Kudat Formation sandstones of northern Borneo suggesting a short-lived episode of sediment transport from Palawan to Borneo in the Early Miocene following arc-continent collision. U–Pb dating of zircons shows that the Central Palawan granite is Eocene (42±0.5Ma). The Capoas granite was intruded during a single pulse, or as two separate pulses, between 13.8±0.2Ma and 13.5±0.2Ma. Inherited zircon ages from the Capoas granite imply melting of continental crust derived from the South China margin with a contribution from Cenozoic rift-related and arc material.

OLDEST U-PB CRYSTALLISATION AGE FOR THE WEST AFRICAN CRATON FROM THE OUDALAN-GOROUOL BELT OF BURKINA FASO

U-Pb geochronological data from the northern and western extensions of the Oudalan-Gorouol volcano-sedimentary belt (OGB) on 3 samples of granodiorite gneiss (sample BF\_1144), migmatitic gneiss (sample BF\_1299), and granite clast from a metaconglomerate (sample BF_1147) gave the oldest U-Pb crystallisation ages for outcropping Palaeoproterozoic rocks yet identified in the West African Craton (WAC). The U-Pb concordant crystallisation ages of 2253 ± 9 Ma and 2253 ± 15 Ma for samples BF\_1144 and BF\_1299, respectively, likely represent the age of crystallisation of parent granodiorite gneiss and may constrain the age of a gneissic protolith on which the OGB basin was deposited, or an early group of granitoids that was emplaced in the hinterland during formation of the OGB basin. A concordant age of 2255 ± 26 Ma for sample BF_1147 is interpreted as the age of crystallisation of a granite pluton that was eroded from a site nearby before becoming part of the Dembam Member, and constrains the maximum age of the Birimian Supergroup in the OGB. The older Pb-Pb ages on zircons from the granodiorite gneiss, migmatitic gneiss and granite clast are in the range ~2.34 to ~2.27 Ga, and are interpreted as inherited from pre-existing zircon cores. They are similar to Pb-Pb ages reported as inherited zircon ages from the Dabakala tonalitic gneiss (in Cote d’Ivoire).

Crustal evolution between 2.0 and 3.5 Ga in the southern Gavião block (Umburanas-Brumado-Aracatu region), São Francisco Craton, Brazil: A 3.5–3.8 Ga proto-crust in the Gavião block?

Abstract The main evolution of the Gaviao block in the Umburanas-Brumado-Aracatu region, in the state of Bahia, is defined by several sets of tonalitic-trondhjemitic and granodioritic gneisses emplaced during the Paleoarchean. The juvenile Bernada gneisses are emplaced at 3386 ± 9 Ma (SHRIMP zircon age). The Aracatu gneisses, probably derived from the partial melting of ca 3.4 Ga gneisses, are emplaced at 3325 ± 16 Ma. They contain inherited zircon dated at 3366 ± 15 Ma in the range of ages obtained for the juvenile Bernada gneisses. Furthermore, one core in these zircons provides an age of 3487 ± 9 Ma, which is the oldest xenocryst found in the Gaviao block. A Neoarchean alkaline granite was emplaced at 2693 ± 5 Ma (Serra de Eixo gneiss) and corresponds to a major crustal reworking stage. All of these rocks were metamorphosed and melted at ca 2.0 Ga, as recorded by monazite ages (EPMA and La-ICPMS) in diatexitic Archean gneisses (Aracatu) and Paleoproterozoic granites (Umburanas). The occurrence of a proto-crust ca 3.5 Ga or older in the Gaviao block is discussed based on inherited zircon ages and Sm–Nd isotope signatures of the Archean gneisses.

The origin of zircon and the significance of U–Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)

U–Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700–500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340–335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of ~16 kbar/850 °C followed by isothermal decompression to 8–10 kbar/820–860 °C. In felsic granulite samples, the assemblage K-feldspar–garnet–kyanite–Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of ~13 kbar/925 °C, and a subsequent P–T decrease to 6.5–8.5 kbar/800–820 °C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P–T paths are used to discuss the significance of the U–Pb ages. In the monotonous unit, inherited zircon ages of 700–500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U–Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.

In situ zircon U–Pb, oxygen and hafnium isotopic compositions of Jurassic granites from the North China craton: Evidence for Triassic subduction of continental crust and subsequent metamorphism-related 18O depletion

Six samples of the Linglong granitic pluton from the southeastern margin of the North China craton yield similar Jurassic magmatic zircon U–Pb ages and evolved whole-rock Sr–Nd isotopic compositions. They are weakly peraluminous to strongly peraluminous, with A/CNK (molar Al2O3/[CaO+Na2O+K2O]) ratios of 1.05 to 1.3 except one sample with an A/CNK of 0.98. However, they have contrasting inherited zircon ages and magmatic zircon Hf and O isotopic compositions. Two samples have ~2.5Ga inherited zircons and high δ18O magmatic zircons and are interpreted to be derived dominantly from the lower crust of the North China craton. Another two samples with dominantly Triassic inherited zircons and low δ18O magmatic zircons are considered to be derived mainly from the subducted continental crust of the South China craton and the source was considered to be significantly depleted in 18O during Triassic metamorphism. The rest two samples have dominantly Neoproterozoic inherited zircons and intermediate δ18O magmatic zircons and are also considered to be derived mainly from the subducted continental crust of the South China craton. But the source was not significantly depleted in 18O during Triassic metamorphism. The different lithologies, whole-rock εNd(t) and zircon εHf(t) between the samples also strongly support that they are derived from different sources. Although magmatic zircons from the six samples have variable δ18O from 0.7 to 8.5‰ and εHf(t) from −27.6 to −11.8, individual samples have more limited εHf(t) and δ18O. The δ18O and εHf(t) of magmatic zircons from the samples indicate no contribution of mantle-derived magmas to the Linglong granites. This is supported by the fact of no coeval mantle-derived rocks in the region. The results suggest that large variations of εHf(t) and δ18O do not necessarily indicate mixing between mantle- and crustal-derived magmas. Instead, they may reflect heterogeneity in the source. The granites are interpreted to result from partial melting of the thickened crust caused by Triassic subduction of the South China craton continental crust beneath the North China craton. Subsequent lithospheric extension and orogenic collapse are the most likely mechanism for the granites.

U–Pb geochronology and zircon composition of late Variscan S- and I-type granitoids from the Spanish Central System batholith

The Spanish Central System (SCS) batholith, located in the Central Iberian Zone, is one of the largest masses of granite in the European Variscan Belt. This batholith is a composite unit of late- and post-kinematic granitoids dominated by S- and I-type series granite, with subordinate leucogranite and granodiorite. Zircon trace element contents, from two representative S-type and three I-type granitoids from the eastern portion of the SCS batholith, indicate a heterogeneous composition due to magma differentiation and co-crystallisation of other trace element-rich accessory phases. In situ, U–Pb dating of these zircons by SHRIMP and LA-ICP-MS shows 479–462-Ma inherited zircon ages in the I-type intrusions, indicating the involvement of an Ordovician metaigneous protolith, while the S-type intrusions exclusively contain Cadomian and older zircon ages. The zircon crystallisation ages show that these granites have been emplaced at ca. 300 Ma with a time span between 303 ± 3 Ma and 298 ± 3 Ma. Precise dating by CA-ID-TIMS reveals a pulse at 305.7 ± 0.4 Ma and confirms the major pulse at 300.7 ± 0.6 Ma. These ages match the Permo-Carboniferous age for granulite-facies metamorphism of the lower crust under the SCS batholith and coincide with a widespread granitic event throughout the Southern Variscides. Ti-in zircon thermometry indicates temperatures between 844 and 784°C for both the S- and I-type granites, reinforcing the hypothesis that these granites are derived from deep crustal sources.

Tectonics of the Strandja Massif, NW Turkey: History of a Long-Lived Arc at the Northern Margin of Palaeo-Tethys

The Strandja Massif, Thrace Peninsula, NW Turkey, forms an important link between the Balkan Zone of Bulgaria, which is usually correlated with Variscan orogen in Central Europe, and the Pontides, where Cimmerian structures are the most prominent. The massif is composed of a Palaeozoic basement and a Triassic metasedimentary cover. The basement is made of various granite gneisses, paragneisses, and schists that are intruded by large plutons of monzonitic metagranites. Detrital zircon studies have revealed Ordovician (433 and 446 Ma) and Carboniferous (305 Ma) ages of the metasedimentary rocks. The isotopic age of the granite gneisses is 308-315 Ma (Carboniferous, Bashkirian-Moscovian) as single zircon evaporation method and conventional U-Pb technique show. The Palaeozoic basement was deformed and metamorphosed before the emplacement of the large monzonitic metagranite plutons yielding zircon ages from 309±24 to 257 Ma (Moscovian-Permian). Geochemical features of the Carboniferous and Permian magmatic rocks indicate a subduction-related tectonic setting similar to coeval rocks exposed in the Balkan zone of Bulgaria. The Triassic metasedimentary cover unconformably overlies the basement with basal conglomerate and arkosic sandstone that pass upward into a thick pile of lithic metasandstones and a metasandstone/pelitic schist alternation. Calcareous metasandstones and black slates appear at the highest structural levels. The Triassic succession reveals obvious orogenic features judged from its great thickness, sedimentary features indicating high-energy currents and the presence of intermediate pillow lavas. Both the basement and the cover units were affected by strong deformation and epidote-amphibolite to greenschist facies metamorphism during the Late Jurassic-Early Cretaceous. This event was terminated by the emplacement of a nappe of unmetamorphosed Jurassic limestones and dolomites occurring at the top of the structural column. Kinematic indicators in mylonites at the base of the nappe suggest its original location in the south. The Strandja Massif shows remarkable similarity to the late Palaeozoic-early Mesozoic Silk Road arc that evolved at the southern margin of Eurasia due to the northward subduction of Palaeo-Tethys (Natal'in & Şengör 2005). The fragments of this arc are exposed in Caucasus, Iran, South Tien Shan, Pamir, and Kunlun. The Precambrian history of the Strandja Massif, as recorded by detrital and inherited zircon ages, reveals many common features with the Baltica-Timanide collage including its fragments distributed in Central Asia. Various sets of data and correlations with surrounding tectonic units show that the Strandja Massif is a fragment of the long-lived, Ordovician to Triassic Silk Road magmatic arc, which evolved on the

Petrology and geochronology of the Namche Barwa Complex in the eastern Himalayan syntaxis, Tibet: Constraints on the origin and evolution of the north-eastern margin of the Indian Craton

The Namche Barwa Complex (NBC) in the eastern Himalayan syntaxis, south Tibet, is generally interpreted as the north-eastern extremity of the exposed Greater Himalayan Sequence, comprising Neoproterozoic to early Paleozoic sedimentary strata along the northern margin of the Indian continent. Field and petrological investigations indicate that the NBC consists mainly of orthogneiss, paragneiss, amphibolites and calc-silicate rocks. U–Pb zircon data demonstrate that the protoliths of the orthogneiss formed during late Paleoproterozoic at ca. 1610 Ma and also in early Paleozoic at ca. 490–500 Ma. The amphibolites were derived from mafic magmatic rocks formed during 1645 to 1590 Ma. Zircons in the paragneisses have highly variable inherited zircon ages ranging from the Neoarchean to early Paleozoic, with four major age populations of 2490 Ma, 1640 Ma, 990 Ma and 480 Ma. The calc-silicate rock has zircons with early Paleozoic metamorphic age of 538 Ma. Almost all the rocks of the NBC have been metamorphosed during Cenozoic with the metamorphic zircon U–Pb ages ranging from 8 to 30 Ma and a peak at 23 Ma. These, together with previous results suggest that the NBC was originally derived from an Andean-type orogeny following the Columbia supercontinent assembly, and experienced multiple reworking during the Grenvillian, Pan-African and Himalayan orogenies. We conclude that the NBC in the eastern Himalayan syntaxis was derived from different provenance and tectonic setting as compared to those of the Greater Himalayan Sequence which constitutes the high-grade metamorphic core of the western and central Himalayan orogenic belt. We thus infer that the NBC was originally part of the eastern segment of the Central Indian Tectonic Zone.

Peri-Amazonian, Avalonian-type and Ganderian-type terranes in the South Carpathians, Romania: The Danubian domain basement

The Danubian domain basement of the South Carpathians, Romania, comprises two Neoproterozoic continental crustal fragments, the Drăgşan and Lainici-Păiuş terranes, which were sutured by the closure of an intervening oceanic domain, the Tişoviţa terrane. Magmatic and detrital zircons extracted from an orthogneiss, four granitoid plutons, two metasedimentary units, and a Liassic sandstone were dated by zircon U/Pb LA-ICP-MS. The Făgeţel augen gneiss from the Drăgşan terrane basement yielded an age of 803.2 ± 4.4 Ma, the oldest well-constrained crystallization age reported from the Romanian Carpathians basement. The Tismana, Şuşiţa, Novaci and Olteţ granitoid plutons, which intrude the Lainici-Păiuş terrane basement, yielded ages of 600.5 ± 4.4, 591.0 ± 3.5, 592.7 ± 4.9, and 588 ± 2.9 Ma, respectively. The Tismana granitoid age of 600 Ma and the youngest detrital zircon ages of 637–622 Ma from a metaquartzite within the Lainici-Paiuş terrane, constrain the deposition of the metaquartzite protolith to ca. 620–600 Ma. The 803 Ma age represents an old Pan-African age, whereas the younger Neoproterozoic ages suggest Pan-African/Cadomian thermotectonic events. Detrital and inherited zircon ages within the Drăgşan and Lainici-Paiuş terranes attest to a peri-Amazonian, Avalonian-type provenance for the Drăgşan terrane and possibly a Ganderian-type provenance for the Lainici-Păiuş terrane. The Lainici-Păiuş terrane rifted off Gondwana before the Drăgşan terrane. Both terranes were attached to Moesia during the Early Paleozoic.

Controls on Devonian–Carboniferous magmatism in Tasmania, based on inherited zircon age patterns, Sr, Nd and Pb isotopes, and major and trace element geochemistry

Devonian–Carboniferous granites are widespread in Tasmania. In eastern Tasmania, Devonian granites intrude Ordovician–Early Devonian quartz-rich turbidites of the Mathinna Supergroup. The earliest (∼400 Ma) I-type granodiorites may be arc-related. Following the Tabberabberan Orogeny (∼389 Ma), more felsic and, finally, strongly fractionated I- and S-type granites were emplaced until ∼373 Ma. In contrast, western Tasmania granites intrude a more diverse terrane of predominantly marine shelf successions, with depositional ages as old as Late Mesoproterozoic. They are mostly felsic and fractionated I- and S-types emplaced from ∼374–351 Ma, possibly in response to post-collisional crustal extension following juxtaposition of the eastern and western Tasmanian terranes. Granites from the two terranes are readily distinguishable by the age spectra of their inherited zircon, which are noticeably similar to those of the detrital zircon from sedimentary successions in their respective terranes. Furthermore, within each terrane, both I and S-types yield similar inheritance patterns. This suggests a pivotal role for the sedimentary successions in the petrogenesis of both types. Western Tasmanian granites are also enriched in ∼1600 Ma zircon, which is essentially unrepresented in the exposed supracrustal succession. Subtle differences between the inheritance and detrital age spectra in eastern Tasmania probably relate to unrepresentative sampling of the supracrustal rocks. Nd, Sr and Pb isotopic characteristics of the granites are consistent with their derivation by mixing of magmas derived from the mantle, possibly the lower crust, and from supracrustal rocks. Systematic isotopic trends in some eastern Tasmanian I-types, particularly in the Scottsdale Batholith, correlate well with major and trace element geochemistry and age. The isotopes are inconsistent with simple restite unmixing or crystal fractionation in a closed magma chamber, and indicate progressive contamination by the Mathinna Supergroup, or similar rocks. The isotopic characteristics of late, strongly fractionated granites, although sometimes obscured by hydrothermal alteration, are also consistent with concurrent assimilation-fractional crystallisation processes. Together with the close association of some strongly fractionated I- and S-types, this suggests that such granites were generated directly in the lower crust, and were not derived from unfractionated parental granite magmas.

Pulsed emplacement of the Mount Kinabalu granite, northern Borneo

Abstract: High-precision U–Pb ion microprobe analyses provide new constraints on the emplacement and origin of the Kinabalu granite in Sabah, northern Borneo. The granite is a sheeted laccolith-like body comprising dyke-fed granitic units that young downwards, each emplaced beneath the previous sheet. Analyses of concentric growth zones in zircons indicate crystallization between 7.85 ± 0.08 and 7.22 ± 0.07 Ma, and show that the entire pluton was emplaced and crystallized within less than 800 ka. Several pulses of magmatism are recognized, each lasting for a maximum of 250 ka, and possibly as briefly as 30 ka. The oldest ages coincide with the highest elevations whereas the youngest ages are found at lower elevations around the edge of the body. Based on these new age data and field observations we identify the biotite granodiorite, hornblende granite and porphyritic facies as the Upper, Middle and Lower Units respectively. Inherited zircon ages indicate different protoliths for the Upper and Middle Units. The Upper Unit is derived from attenuated continental crust of the South China margin subducted beneath Sabah. The Middle Unit is sourced from melting of the crystalline basement in Sabah with little or no contribution from South China crust. Supplementary material: Full U–Pb ion microprobe analytical data, and modal and major element composition data are available at http://www.geolsoc.org.uk/SUP18385 .

Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain

In situ uranium-lead dating (LA-SF-ICPMS and SIMS) and Lu-Hf isotope analyses (LA-MC-ICP-MS) of zircon from eclogite facies rocks from the basal units of the Variscan Belt in Galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement. The samples studied are two felsic gneisses, two eclogites, and one eclogitic gneiss of intermediate composition (metatonalite). Oscillatory-zoned zircon cores from the felsic samples gave a main clustering of U-Pb ages at 493 ± 2 and 494 ± 2 Ma, and some older ages that represent inherited cores. Zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 ± 3 and 498 ± 6 Ma, respectively, interpreted as time of protolith crystallization. Variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 ± 3 Ma, and very tiny zircons of one eclogite gave 350 ± 2 Ma, both of which we interpret as metamorphic ages. The new age data demonstrate that the calc-alkaline magmatic suite described in the basal unit is ca. 20 Ma older than the alkaline to peralkaline plutonic suite of the same unit (dated at 472 ± 2 Ma; Rodriguez et al., 2007), and thus probably represents a distinct geologic event. Overgrowth rims are interpreted as metamorphic on the basis of their Lu/Hf and Th/U ratios. The 372 ± 3 age is considered as dating the high-pressure (high-P) metamorphism, and is essentially in agreement with previous Ar-Ar and Rb-Sr data. This high-P metamorphism marks the initial early-Variscan subduction of the Gondwana margin. The inherited zircon ages and Hf isotopic composition of zircons point to a considerable input of crustal material with West African Craton provenance to the felsic magma.

Petrogenesis of granitic unit of Naqadeh complex, Sanandaj–Sirjan Zone, NW Iran

The granitic unit is a component of the Naqadeh plutonic complex, NW of Sanandaj–Sirjan Zone (NW Iran). This unit is composed of high-K calc-alkaline, slightly peraluminous (ASI = 1.12–1.17) evolved monzogranites. These monzogranites have 41.85 ± 0.81 Ma (zircon U–Pb sensitive, high-resolution ion microprobe (SHRIMP) age) with two inherited zircon ages of 98.5 ± 1.7 and 586.6 ± 13.1 Ma, respectively. The only enclave type consists of quartz-amphibolite enclaves indicating residual parental rocks. Chemical and isotopic (87Sr/86Sr40Ma = 0.708638; eNd40Ma = −4.26) characteristics of monzogranites suggest that they could be derived by partial melting of crustal mafic rocks followed by some assimilation of metasedimentary rocks. With regards to inherited zircon age and quartz-amphibolite composition of Naqadeh granite, the old mafic rocks of this complex (Naqadeh dioritic rocks with ~100 Ma) can be considered as parental rocks, and their partial melting under high water content, and assimilation of produced melt by metasedimentary rocks, would lead to the generation of a Naqadeh granitic unit.

U–Th–Pb SHRIMP ages and oxygen isotope composition of zircon from two contrasting late Variscan granitoids, Nisa-Albuquerque batholith, SW Iberian Massif: Petrologic and regional implications

The late Variscan Nisa-Albuquerque batholith in the SW Iberian Massif, consists of a dominant very coarse-grained porphyritic S-type monzogranite to syenogranite (Nisa monzogranite) surrounding a discontinuous central core that includes contrasting very fine-grained I-type tonalite–granodiorite (Aldeia da Mata tonalite). The batholith is located at the boundary between the Central Iberian and Ossa-Morena Zones, a complex segment of crust that was subject to both Cadomian and Variscan tectonism. Variscan zircons in the Nisa monzogranite can be broadly classified into three texturally and chemically distinct types with mutually indistinguishable SHRIMP 206Pb/ 238U ages: 1) high-U, low-Th/U (< 0.1) outermost overgrowths (307.4 ± 4.0 Ma); 2) moderate U and Th/U zircon with concentric zoning occurring both as inner overgrowths and whole grains (305.4 ± 6.2 Ma)";; and 3) texturally discordant cores (309.0 ± 4.6 Ma). Many other cores have ages in the ranges 2.56–1.85 and 0.66–0.51 Ga. The overgrowths and Variscan cores with low Th/U have uniformly high δ 18O (9.5 ± 0.2‰). Variscan cores with moderate Th/U have a wide range of δ 18O (6.7–10.9‰). Cores older than 500 Ma have an even wider range of composition (4.4–10.0‰). Zircon from the central Aldeia da Mata tonalite, in contrast, contains no inherited cores, has moderate to high Th/U (0.5–1.8), and is uniform in 206Pb/ 238U age (306.2 ± 3.0 Ma) and δ 18O (7.4 ± 0.3‰). The zircon in the Nisa monzogranite records a history of magma genesis involving mixing between 1) a metaluminous magma progressively contaminated by a small sedimentary component, and 2) a more voluminous peraluminous magma originating from a largely metasedimentary source. The inherited zircon age pattern closely matches the age pattern of detrital zircon in early Paleozoic sediments from North Africa. The zircon in the Aldeia da Mata tonalite records nothing of the age of the magma's source rocks, but the moderately high δ 18O does preclude derivation of the magma directly from the mantle. Both the chemical and isotopic compositions of the tonalite zircon make it highly unlikely that the tonalite magma was a component in the monzogranite magma mixture.

Tectonic Evolution of Metasediments from the Gangdise Terrane, Asian Plate, Eastern Himalayan Syntaxis, Tibet

The Eastern Himalayan Syntaxis of Namche Barwa carries critical information for understanding the geodynamics of the Indian-Asian collision. In this syntaxis, the Nyingchi Group is a sequence of medium- to high-grade metasediments, located on the north side of the Indus-Yarlung suture zone. Results from zircon U-Pb ages coupled with their Hf isotope systematics for the Nyingchi Group and a two-mica granite intruding the paragneisses reveal strong contrasts in provenance and depositional age within the sequence. Detrital zircons from a garnet paragneiss of the lower Nyingchi Group display 206Pb/238U ages from 493 Ma to 2466 Ma, with peaks at 506 Ma, 821 Ma, and 1032 Ma. The minimum detrital zircon age of 493 Ma constrains the maximum depositional age of the lower Nyingchi Group. Detrital zircons from a paragneiss of the middle Nyingchi Group yield 206Pb/238U ages ranging from 55 Ma to 1362 Ma, with peaks at 60 Ma, 95 Ma, 138 Ma, and 171 Ma. This indicates that the maximum depositional age is less than 55 Ma. Zircons from a two-mica granite, intruding the upper Nyingchi Group, yield a magma crystallization age of 22 ± 1 Ma and inherited zircon ages of 52 Ma to 911 Ma. Because the Hf isotopic ratios together with the ages of detrital zircons suggest an origin from anatexis of the middle Nyingchi Group sediments, these data reduce the maximum depositional age of the middle Nyingchi Group to less than 52 Ma. Results preclude the Nyingchi Group from representing Precambrian basement of the Gangdise terrane. The detrital zircon and inherited zircon age distributions, combined with their Hf isotopic compositions, reveal that whereas the lower Nyingchi Group may be correlated with crystalline units from the Himalayan terrane, the middle-upper Nyingchi Group is a much younger deposit derived from the Gangdise magmatic arc. In order to explain the medium- to high-grade metamorphism of the middle-upper Nyingchi Group, we suggest that following the Indian-Asian collision, a thrust slice from the leading edge of the Asian continental slab (the middle-upper Nyingchi Group) became detached and subducted, together with Himalayan lithologies from the Indian continental slab. These results provide new insight into the tectonic evolution of the Eastern Himalayan Syntaxis.

SHRIMP zircon and EPMA monazite dating of granitic rocks from the Maizuru terrane, southwest Japan: Correlation with East Asian Paleozoic terranes and geological implications

Abstract The Maizuru terrane, distributed in the Inner Zone of southwest Japan, is divided into three subzones (Northern, Central and Southern), each with distinct lithological associations. In clear contrast with the Southern zone consisting of the Yakuno ophiolite, the Northern zone is subdivided into the western and eastern bodies by a high‐angle fault, recognized mainly by the presence of deformed granitic rocks and pelitic gneiss. This association suggests an affinity with a mature continental block; this is supported by the mode of occurrence, and petrological and isotopic data. Newly obtained sensitive high mass‐resolution ion microprobe (SHRIMP) zircon U–Pb ages reveal the intrusion ages of 424 ± 16 and 405 ± 18 Ma (Siluro–Devonian) for the granites from the western body, and 249 ± 10 and 243 ± 19 Ma (Permo–Triassic) for the granodiorites from the eastern body. The granites in the western body also show inherited zircon ages of around 580 and 765 Ma. In addition, electron probe microanalysis (EPMA) monazite U–Th–total Pb dating gives around 475–460 Ma. The age of intrusion, inherited ages, mode of occurrence, and geological setting of the Siluro–Devonian granites of the Northern zone all show similarities with those of the Khanka Massif, southern Primoye, Russia, and the Hikami granitic rocks of the South Kitakami terrane, Northeast Japan. We propose that both the Siluro–Devonian and Permo–Triassic granitic rocks of the Northern zone are likely to have been juxtaposed through the Triassic–Late Jurassic dextral strike‐slip movement, and to have originated from the Khanka Massif and the Hida terrane, respectively. This study strongly supports the importance of the strike‐slip movement as a mechanism causing the structural rearrangement of the Paleozoic–Mesozoic terranes in the Japanese Islands, as well as in East Asia.

Crustal Contributions to Late Hercynian Peraluminous Magmatism in the Southern Calabria-Peloritani Orogen, Southern Italy: Petrogenetic Inferences and the Gondwana Connection

Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircon from granites of the medium-high grade Aspromonte^ Peloritani Unit, Calabria^Peloritani Orogen (CPO), southern Italy, show that one of the minor trondhjemites (313 7 3 5Ma) represents the earliest identified occurrence of Late Hercynian peraluminous igneous rocks in the CPO, predating the emplacement of the more common peraluminous leucogranodiorites by about 14 Myr. Some of the trondhjemite zircon grains contain small cores with ages of about 2 45 Ga, 625Ma and 490Ma, consistent with the presence of a sediment component in the magma. A newly dated leucogranodiorite (300 2 3 8Ma) is rich in inherited zircon. Cores with ages of about 2 36 Ga, 870Ma, 630Ma, 545Ma and 460Ma are overgrown by two generations of Hercynian igneous zircon, the first with moderate to highTh/U (up to 1 67), and the second with low Th/U (50 1).The overgrowths probably crystallized from magmas of two compositions, the first metaluminous and the second peraluminous. This could indicate either magma mixing or, more probably, crystallization in a single, evolving magma. In either case, the leucogranodiorite magma is considered to have been the product of anatexis of a metasedimentary source. Differences in the inherited zircon age spectra, and the relatively small amount of inheritance in the trondhjemite, indicate that the trondhjemite and leucogranodiorite are unlikely to be genetically related.The ages of the inherited zircons are consistent with the sedimentary component in both magmas being derived from North Africa, with a possible contribution from Pan-African granitoids similar to those exposed in southern Calabria.