Age and nature of 560–520 Ma calc-alkaline granitoids of Biarjmand,northeast Iran: insights into Cadomian arc magmatism in northern Gondwana

ABSTRACT

The Biarjmand granitoids and granitic gneisses in northeast Iran are part of the Torud–Biarjmand metamorphic complex, where previous zircon U–Pb geochronology show ages of ca. 554–530 Ma for orthogneissic rocks. Our new U–Pb zircon ages confirm a Cadomian age and show that the granitic gneiss is ~30 million years older (561.3 ± 4.7 Ma) than intruding granitoids (522.3 ± 4.2 Ma; 537.7 ± 4.7 Ma). Cadomian magmatism in Iran was part of an approximately 100-million-year-long episode of subduction-related arc and back-arc magmatism, which dominated the whole northern Gondwana margin, from Iberia to Turkey and Iran. Major REE and trace element data show that these granitoids have calc-alkaline signatures. Their zircon O (δ¹⁸O = 6.2–8.9‰) and Hf (–7.9 to +5.5; one point with εHf ~ –17.4) as well as bulk rock Nd isotopes (εNd(t) = –3 to –6.2) show that these magmas were generated via mixing of juvenile magmas with an older crust and/or melting of middle continental crust. Whole-rock Nd and zircon Hf model ages (1.3–1.6 Ga) suggest that this older continental crust was likely to have been Mesoproterozoic or even older. Our results, including variable zircon εHf(t) values, inheritance of old zircons and lack of evidence for juvenile Cadomian igneous rocks anywhere in Iran, suggest that the geotectonic setting during late Ediacaran and early Cambrian time was a continental magmatic arc rather than back-arc for the evolution of northeast Iran Cadomian igneous rocks.     

Long-lived,stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus,Armenia and Nakhitchevan

The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, ²⁰⁶Pb/²⁰⁴Pb vs. Th/Nb, Th/Nb vs. δ¹⁸O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.     

Petrology,geochemistry and U–Pb geochronology of magmatic rocks from the high-sulfidation epithermal Au–Cu Chelopech deposit,Srednogorie zone,Bulgaria

The Chelopech deposit is one of the largest European gold deposits and is located 60 km east of Sofia, within the northern part of the Panagyurishte mineral district. It lies within the Banat–Srednegorie metallogenic belt, which extends from Romania through Serbia to Bulgaria. The magmatic rocks define a typical calc-alkaline suite. The magmatic rocks surrounding the Chelopech deposit have been affected by propylitic, quartz–sericite, and advanced argillic alteration, but the igneous textures have been preserved. Alteration processes have resulted in leaching of Na₂O, CaO, P₂O₅, and Sr and enrichment in K₂O and Rb. Trace element variation diagrams are typical of subduction-related volcanism, with negative anomalies in high field strength elements (HFSE) and light element, lithophile elements. HFSE and rare earth elements were relatively immobile during the hydrothermal alteration related to ore formation. Based on immobile element classification diagrams, the magmatic rocks are andesitic to dacitic in compositions. Single zircon grains, from three different magmatic rocks spanning the time of the Chelopech magmatism, were dated by high-precision U–Pb geochronology. Zircons of an altered andesitic body, which has been thrust over the deposit, yield a concordant ²⁰⁶Pb/²³⁸U age of 92.21 ± 0.21 Ma. This age is interpreted as the crystallization age and the maximum age for magmatism at Chelopech. Zircon analyses of a dacitic dome-like body, which crops out to the north of the Chelopech deposit, give a mean ²⁰⁶Pb/²³⁸U age of 91.95 ± 0.28 Ma. Zircons of the andesitic hypabyssal body hosting the high-sulfidation mineralization and overprinted by hydrothermal alteration give a concordant ²⁰⁶Pb/²³⁸U age of 91.45 ± 0.15 Ma. This age is interpreted as the intrusion age of the andesite and as the maximum age of the Chelopech epithermal high-sulfidation deposit. ¹⁷⁶Hf/¹⁷⁷Hf isotope ratios of zircons from the Chelopech magmatic rocks, together with published data on the Chelopech area and the about 92-Ma-old Elatsite porphyry–Cu deposit, suggest two different magma sources in the Chelopech–Elatsite magmatic area. Magmatic rocks associated with the Elatsite porphyry–Cu deposit and the dacitic dome-like body north of Chelopech are characterized by zircons with ɛHf_T90 values of ∼5, which suggest an important input of mantle-derived magma. Some zircons display lower ɛHf_T90 values, as low as −6, and correlate with increasing ²⁰⁶Pb/²³⁸U ages up to about 350 Ma, suggesting assimilation of basement rocks during magmatism. In contrast, zircon grains in andesitic rocks from Chelopech are characterized by homogeneous ¹⁷⁶Hf/¹⁷⁷Hf isotope ratios with ɛHf_T90 values of ∼1 and suggest a homogeneous mixed crust–mantle magma source. We conclude that the Elatsite porphyry–Cu and the Chelopech high-sulfidation epithermal deposits were formed within a very short time span and could be partly contemporaneous. However, they are related to two distinct upper crustal magmatic reservoirs, and they cannot be considered as a genetically paired porphyry–Cu and high-sulfidation epithermal related to a single magmatic–hydrothermal system centered on the same intrusion.     

Mineralization,geochronology, and Pb isotope studies of the shoshonitic lava-hosted Nariniya Pb deposit,central Tibet: linking ore formation to post-collisional potassic magmatism

Abstract

A newly discovered, shoshonitic lava-hosted Pb deposit at Nariniya in central Tibet provides an excellent example to help improve our understanding of the linkage between post-collisional potassic magmatism and ore formation in Tibet. The Pb ores exist as veins or veinlets in NWW-striking fracture zones within the potassic lava (trachyte). The veins contain quartz, galena, pyrite, and sericite (muscovite) as well as minor chalcopyrite, sphalerite, calcite, and dolomite with sericitization, pyritization, and minor silicification. The ⁴⁰Ar–³⁹Ar plateau age of the hydrothermal muscovite is 37.95 ± 0.30 Ma, which represents the Pb mineralization age. This obtained age is indistinguishable, within analytical error, from the zircon U–Pb age of 37.88 ± 0.22 Ma for potassic lava. Therefore, the ore formation can be genetically linked to potassic magmatism. Galena has similar Pb isotopic composition to magmatic feldspar from the host lava, suggesting the derivation of Pb from the magmatic system. Previous studies have suggested that S- and ore-forming fluids are of magmatic origin. Published data show that the Nariniya volcanic rocks are acidic, shoshonitic, akakitic, peraluminous, and enriched in Sr–Nd–Pb isotopes. Thus, they are geochemically different from other potassic volcanic rocks (no adakitic affinity) in the North Qiangtang terrane, but similar to the 46–38 Ma high-K calc-alkaline peraluminous adakitic rocks in this terrane and the late Eocene Cu-generating potassic porphyries from the Sanjiang region of eastern Tibet. As such, the Nariniya potassic magma likely originated from melting of subducted continental crust, with or without interaction with the overlying enriched mantle. Such post-collisional potassic rocks in Tibet are thought to be potential targets for prospecting of both Pb–Zn and porphyry Cu ores. Note that other ore styles (in addition to the Nariniya ore style) may exist in the potassic volcanic districts of Tibet.     

Meta-igneous (non-gneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation

In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events. Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba, Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and 5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented. Received: 1 December 1998 / Accepted: 23 July 1999     

U–Pb SHRIMP ages and tectonic setting of the Munster Suite of the Margate Terrane of the Natal Metamorphic Belt

New U–Pb Zircon SHRIMP ages of 1091 ± 7.1 Ma and 1093.1 ± 5.8 Ma have been determined for two discrete phases of the Munster Suite. The Munster Suite is a calc-alkaline mafic to intermediate suite of intrusive igneous rocks that form part of the southern-most tectonic Terrane of the Mesoproterozoic-aged Natal Metamorphic Belt. Previously published geochemical data indicate that the discrete phases of the Suite are consanguineous and that these rocks originated within an oceanic island arc environment. The new age determinations now show that the different phases are also coeval. Moreover, the ages also indicate that the intrusions were, within statistical error, coeval with S-type granites within the Terrane. This is interpreted to indicate that magmatic underplating provided both the magma by way of a number of progressively more evolved pulses to produce the Munster Suite, as well as the heat necessary for crustal melting to produce the S-type granites within an island arc environment. Therefore, these new age determinations indicate a period of crustal growth at circa 1090 Ma. This moreover, is a maximum constraint on the age of the northward-verging structures within the Margate Terrane.     

Magmatism evolution and carbonatite-granite association in the neoproterozoic active continental margin of the Siberian craton: Thermochronological reconstructions

Neoproterozoic carbonatites and related igneous rocks, including A-type granites in the Tatarka-Ishimba suture zone of the Yenisey Ridge are confined to a horst-anticlinal structure that was formed in a transpression setting during the oblique collision between the Central Angara terrane and the Siberian craton. The carbonatites, associating mafic (including alkaline) dikes as well as the Srednetatarka nepheline syenites are the oldest igneous formations of the Tatarka active continental margin complex. Geochronological data indicate that magmatic evolution continued in the studied anticline for nearly 100 m.y. On the earliest stage carbonatites were formed and on the last stage — the emplacement of mantle-crustal A-type Tatarka granites took place. According to new U/Pb zircon studies, the earliest rocks in the Tatarka pluton are A-type leucogranites aged 646 ± 8 Ma. The younger ⁴⁰Ar/³⁹Ar ages of carbonatites obtained for phlogopites (647 ± 7 and 629 ± 6 Ma) are related to the last tectonic events in the studied region of the Tatarka-Ishimba suture zone, which are coeval with the formation of the A-type granitoids (646–629 Ma).     

A Triassic to Jurassic arc in north Borneo: Geochronology,geochemistry, and genesis of the Segama Valley Felsic Intrusions and the Sabah ophiolite

New field, geochemical, and geochronological data from the Segama Valley Felsic Intrusions (SVFI) of Sabah, north Borneo, shows them to be arc-derived tonalites; not windows or partial melts of a crystalline basement beneath Sabah. U-Pb zircon ages date emplacement in the Triassic and Jurassic: 241.1 ± 2.0 Ma, 250.7 ± 1.9 Ma, 178.7 ± 2.4 Ma, and 178.6 ± 1.3 Ma; contemporaneous with peaks in magmatism and detrital zircons in Sarawak and west Kalimantan (west Borneo). Isotopic data for Sr, Nd, and Pb from whole rocks, and for Hf and O from zircon all show mantle and/or MORB affinities indicating a mantle-derived origin. Enrichment of fluid mobile trace elements and trace element ratios indicate that the most likely setting for this is in a continuation of the Sundaland continental arc. There is no evidence in the field, geochemical, or zircon U-Pb data for continental basement in the Segama Valley region.The intrusive nature of the Segama Valley tonalites constrains the emplacement age of their supra-subduction zone host rocks to at least the Triassic. This new data expands the Triassic and Jurassic extent of Borneo and the Sundaland arc, and challenges models of Borneo's development predominantly through allochthonous terrane accretion in the Cretaceous. Instead, we propose a model of protracted autochthonous growth through supra-subduction zone crustal extension and associated magmatism.     

Neoproterozoic granitic activities in the Xingdi plutons at the Kuluketage block,NW China: Evidence from zircon U–Pb dating,geochemical and Sr–Nd–Hf isotopic analyses

Neoproterozoic igneous rocks are widely distributed in the Kuluketage block along the northern margin of the Tarim Craton. However, the published literature mainly focuses on the ca. 800 Ma adakitic granitoids in the area, with the granites that intrude the 735–760 Ma mafic–ultramafic rocks poorly studied. Here we report the ages, petrography and geochemistry of two granites in the Xingdi mafic–ultramafic rocks, in order to construct a new view of the non-adakitic younger granites. LA-ICP-MS zircon U–Pb dating provided weighted mean ²⁰⁶Pb/²³⁸U ages of 743.0 ± 2.5 Ma for the No.I granite (G1) and 739.0 ± 3.5 Ma for the No.II granite (G2). A clear core-rim texture of similar age and a high zircon saturation temperature of ca. 849 ± 14 °C were observed for the No.I granite; in contrast, G2 has no apparent core-rim texture but rather inherited older zircons and a lower zircon saturation temperature of ca. 763 ± 17 °C. Geochemical analysis revealed that G1 is an alkaline A-type granite and G2 is a high-K calc-alkaline I-type granite. Both granites share similar geochemical characteristics of arc-related magmatic rocks and enriched Sr–Nd–Hf isotopes, likely due to their enriched sources or mixing with enriched magma. Whereas G1 and its host mafic rocks form typical bimodal intrusions of the same age and similar Sr–Nd–Hf isotope compositions, G2 is younger than its host mafic rocks and its Sr–Nd–Hf isotope composition indicates a lower crust origin. Although they exhibit arc-related geochemical features, the two granites likely formed in a rift setting, as inferred from thier petrology, Sr–Nd–Hf isotopes and regional tectonic evolution.     

广东大宝山多金属矿床花岗岩锆石LA-ICP-MSU-Pb定年及其地质意义

广东大宝山矿床位于南岭花岗岩带中带。它是我国著名的大型多金属矿床,开采历史久远。近年来的研究表明大宝山矿床与成矿作用有关的斑岩体为燕山早期岩浆活动的产物,因而人们较多地关注中生代的岩浆活动,而忽视了对其他时代岩浆活动的研究。本文在前人研究的基础上,利用锆石LA-ICP-MS U-Pb定年方法系统地测试了大宝山多金属矿床多个花岗质岩体和辉绿岩脉的形成时代,研究表明徐屋片理化流纹斑岩年龄为426.9±2.2Ma、九曲岭黑云母花岗闪长斑岩、船肚花岗闪长岩和大宝山花岗闪长斑岩形成时代分别为162.2±0.7Ma、160.2±0.9Ma和161.0±0.9Ma。矿区内两条辉绿岩脉的年龄分别为210.4±1.4Ma和163.9±1.8Ma。这些结果证实大宝山矿区内存在加里东期、印支期和燕山期等多个旋回的岩浆活动,中晚侏罗世铁镁质的岩浆活动可能存在对成矿的贡献。     

Detrital and volcanic zircon U–Pb ages from southern Mendoza (Argentina): An insight on the source regions in the northern part of the Neuquén Basin

The infill of the Neuquén Basin recorded the Meso-Cenozoic geological and tectonic evolution of the southern Central Andes being an excellent site to investigate how the pattern of detrital zircon ages varies trough time. In this work we analyze the U–Pb (LA–MC–ICP–MS) zircon ages from sedimentary and volcanic rocks related to synrift and retroarc stages of the northern part of the Neuquén Basin. These data define the crystallization age of the synrift volcanism at 223 ± 2 Ma (Cerro Negro Andesite) and the maximum depositional age of the original synrift sediments at ca. 204 Ma (El Freno Formation). Two different pulses of rifting could be recognized according to the absolute ages, the oldest developed during the Norian and the younger during the Rhaetian–Sinemurian. The source regions of the El Freno Formation show that the Choiyoi magmatic province was the main source rock of sediment supply. An important amount of detrital zircons with Triassic ages was identified and interpreted as a source area related to the synrift magmatism. The maximum depositional age calculated for the Tordillo Formation in the Atuel-La Valenciana depocenter is at ca. 149 Ma; as well as in other places of the Neuquén Basin, the U–Pb ages calculated in the Late Jurassic Tordillo Formation do not agree with the absolute age of the Kimmeridgian–Tithonian boundary (ca. 152 Ma). The main source region of sediment in the Tordillo Formation was the Andean magmatic arc. Basement regions were also present with age peaks at the Carboniferous, Neoproterozoic, and Mesoproterozoic; these regions were probably located to the east in the San Rafael Block. The pattern of zircon ages summarized for the Late Jurassic Tordillo and Lagunillas formations were interpreted as a record of the magmatic activity during the Triassic and Jurassic in the southern Central Andes. A waning of the magmatism is inferred to have happened during the Triassic. The evident lack of ages observed around ca. 200 Ma suggests cessation of the synrift magmatism. The later increase in magmatic activity during the Early Jurassic is attributed to the onset of Andean subduction, with maximum peaks at ca. 191 and 179 Ma. The trough at ca. 165 Ma and the later increase in the Late Jurassic could be explained by changes in the relative convergence rate in the Andean subduction regime, or by the shift to a more mafic composition of the magmatism with minor zircon fertility.     

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block,China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The ε_Hf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding T_DM1 values are in the range of 919–523 Ma. The ε_Hf(t) values of zircon from the DSC I-type granite are between –1 and 3, with T_DM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ¹⁸O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.     

∼3,850 Ma tonalites in the Nuuk region,Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex

The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660–3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110–140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility–melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo–feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial ɛ_Nd (3,850 Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800–3,600 Ma) rocks of the Itsaq Gneiss Complex. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

小兴安岭-张广才岭铁力地区侏罗纪辉绿岩年代学、地球化学、锆石Hf同位素特征及其构造意义

小兴安岭-张广才岭地区中生代岩浆活动的成因及动力学背景对于揭示古太平洋的构造演化具有重要意义。本文选取小兴安岭-张广才岭铁力地区出露的辉绿岩墙为研究对象,进行锆石LA-ICP-MS U-Pb年代学、全岩地球化学和锆石Hf同位素等分析,对该基性岩的形成时代、岩石成因、岩浆源区以及形成的大地构造背景进行讨论。研究表明:辉绿岩锆石具有高Th/U比值( 0.3),CL图像显示微弱的振荡环带结构,具有岩浆锆石特征,~(206)Pb/~(238)U加权平均年龄为187±2Ma,即形成于早侏罗世;该岩体主要经历了橄榄石和单斜辉石的分离结晶作用,未遭受明显的地壳混染作用,并且具有Rb、Ba、U、Pb、K和Sr等流体活动元素相对富集,Th、Nb和Ta等非流体活动性元素相对亏损的地球化学特征,暗示其形成于被俯冲流体富集交代的亏损地幔部分熔融,源区可能为尖晶石-石榴石二辉橄榄岩,部分熔融程度约为6%～20%。结合该地区同时代的岩浆岩、变形构造、矿床特征和黑龙江蓝片岩的相关报道,本文认为小兴安岭-张广才岭地区在中生代期间处于活动大陆边缘环境,其岩浆岩的形成主要是由于存在于佳木斯地块和松嫩-小兴安岭地块间的牡丹江洋西向俯冲造成的。     

Riftogenic magmatism of western part of the Early Mesozoic Mongolian–Transbaikalian igneous province: Results of geochronological studies

Geochronological studies of rocks from a bimodal high-alkali volcanic–plutonic complex collected in the area of Kharkhorin zone of the Early Mesozoic Mongolian–Transbaikalian igneous province (MTIP) are made. The age of alkali granites from Olziit sum is 211 ± 1 Ma (U–Pb ID-TIMS on zircon) to 209 ± 2 and 217 ± 4 Ma (⁴⁰Ar/³⁹Ar on alkali amphibole); the age of alkali granite-porphyries from the area of Sant sum is 206 ± 1 Ma (U–Pb ID-TIMS on zircon). These rock series formed syncronously to the analogous magmatism episode in the Northern Gobi and Western Transbaikalian rift zones of the MTIP. The similarity of the age and composition of igneous associations of the MTIP suggests a common mechanism of its formation related to the effect of a mantle plume on the continental lithosphere at the base of the entire igneous zone having a zonal structure.     

The significance of U–Pb zircon ages in zoned plutons: the case of the Flamenco pluton,Coastal Range batholith,northern Chile

Compositionally zoned plutons, both layered and concentrically arranged, provide granitic exposures where the mechanisms and timing of the magmatic emplacement processes can be studied. The importance of in-situ geochemical differentiation and the magma replenishment rates are revealed by geochemistry and field relations, together with the increasingly accurate U–Pb geochronology, which has promoted the knowledge about the pluton incremental assembly theories.The Flamenco pluton, located in the Coastal Range of northern Chile, is part of the Upper Triassic to Early Cretaceous Andean intrusives formed in the western active margin of South America, and present a normal zoned structure with mafic magmatic facies (mostly gabbros and Qtz-diorites) close to the contacts with the host metasediments, and tonalites, granodiorites and granites in the inner areas. A combined study of the field relations, geochemistry and zircon geochronology of the magmatic facies was applied to determine the emplacement sequence of the Flamenco pluton, revealing three distinguishable domains separated by metasedimentary septa. The SW area is constituted by mostly homogeneous leucocratic granodiorites that yielded an age of 213 Ma as the best estimation for their emplacement age. Distinctive geochemical characteristics, such as the absence of an Eu anomaly, the depletion in HREE, or the highest Sr, Sr/Y and Ce/Yb values among the granodioritic facies of the pluton, involve lower T and/or higher P conditions at the magmatic source according to experimental studies. These conditions were established during an early stage of the Andean magmatic arc building that is firstly defined here as Upper Triassic. The NW and E domains of the pluton were sequentially emplaced between 194 Ma and 186 Ma and both the field relations and the detailed geochronological results suggest that the mafic facies intruded latter in the emplacement sequence. To the NW, Qtz-dioritic and gabbroic externally emplaced pulses gave a younger crystallization age of 186.3 ± 1.8 Ma, and promoted the granoblastic textures and metamorphic zircon overgrowths that characterize the granodiorites located in the contact with the intermediate and felsic inner magmas, which yielded a best estimation of their emplacement age of 192 ± 1.5 Ma. On the other hand, in the eastern domain, magma-magma relations are observed between gabbros and previously intruded tonalites and granodiorites. Both the mafic and intermediate facies show two main subgroups of ages that yielded 194.7 ± 1.5 Ma to 188.3 ± 2.1 Ma and 193.1 ± 2.2 Ma to 185.5 ± 1.4 Ma respectively. These differences are related to the variations in the magmatic addition rates, which may extend the super-solidus conditions in the eastern domain of the magmatic reservoir as is confirmed by the wider age ranges yielded by these magmatic facies. Zircon overgrowths in the host rocks yield similar ages (around 220 Ma and 205 Ma) than the oldest results obtained in the intrusive facies, indicating that metamorphism correlates with the initial stages of plutonic emplacement.Geochronological results differ between 9 Myr and 41 Myr in the eight studied samples for non-inherited ages and gave very close mean ages (within analytical uncertainty) for all the intrusive units. However, we examine other characteristics such as zircon morphology, internal structure, geochemistry and statistical data to assess if the scattering of the geochronological data may be related to the different processes involved in the construction of the Flamenco pluton. We concluded that this detailed study of U–Pb zircon ages, including individual and significative groups of analyses, is useful to determine accurately the emplacement sequence and the genetic relation between the intrusive units, together with the evidences depicted by the geochemistry and field relations.     

The Mejillonia suspect terrane (Northern Chile): Late Triassic fast burial and metamorphism of sediments in a magmatic arc environment extending into the Early Jurassic

The Mejillonia terrane, named after the Mejillones Peninsula (northern Chile), has been traditionally considered an early Paleozoic block of metamorphic and igneous rocks displaced along the northern Andean margin in the Mesozoic. However, U–Pb SHRIMP zircon dating of metasedimentary and igneous rocks shows that the sedimentary protoliths were Triassic, and that metamorphism and magmatism took place in the Late Triassic (Norian). Field evidence combined with zircon dating (detrital and metamorphic) further suggests that the sedimentary protoliths were buried, deformed (foliated and folded) and metamorphosed very rapidly, probably within few million years, at ca. 210 Ma. The metasedimentary wedge was then uplifted and intruded by a late arc-related tonalite body (Morro Mejillones) at 208 ± 2 Ma, only a short time after the peak of metamorphism. The Mejillones metamorphic and igneous basement represents an accretionary wedge or marginal basin that underwent contractional deformation and metamorphism at the end of a Late Permian to Late Triassic anorogenic episode that is well known in Chile and Argentina. Renewal of subduction along the pre-Andean continental margin in the Late Triassic and the development of new subduction-related magmatism are probably represented by the Early Jurassic Bólfin–Punta Tetas magmatic arc in the southern part of the peninsula, for which an age of 184 ± 1 Ma was determined. We suggest retaining the classification of Mejillonia as a tectonostratigraphic terrane, albeit in this new context.     

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new ⁴⁰Ar/³⁹Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.     

Geochemistry and early Palaeogene SHRIMP zircon ages for island arc granitoids of the Sierra Maestra, southeastern Cuba

The Palaeogene volcanic arc successions of the Sierra Maestra, southeastern Cuba, were intruded by calc-alkaline, low- to medium-K tonalites and trondhjemites during the final stages of subduction and subsequent collision of the Caribbean oceanic plate with the North American continental plate. U-Pb SHRIMP zircon dating of five granitoids yielded ²⁰⁶Pb/²³⁸U emplacement ages between 60.5±2.2 and 48.3±0.5 Ma. The granitoids are the result of subduction-related magmatism and have geochemical characteristics similar to those of magmas from intra-oceanic island-arcs such as the Izu Bonin-Mariana arc and the New Britain island arc, Lesser Antilles. Major and trace element patterns suggest evolution of these rocks from a single magmatic source. Geochemical features characterize these rocks as typical subduction-related granitoids as found worldwide in intra-oceanic arcs, and they probably formed through fractional crystallization of mantle-derived low- to medium-K basalt.     

1.94–1.93 Ga charnockitic magmatism from the central part of the Guyana Shield, Roraima, Brazil: Single-zircon evaporation data and tectonic implications

Age and origin of the charnockitic rocks of the central part of the Guyana Shield have been a matter of discussion. These rocks have been interpreted either as Transamazonian granulites metamorphosed around 2.02 Ga or as 1.56 Ga old igneous charnockites. Recently, most of the Roraima charnockitic rocks have been recognized as igneous rocks and included into the Serra da Prata Suite (SPS). Five Pb–Pb single-zircon evaporation ages were obtained for samples representative of different facies of the SPS and these constrained the age of the charnockitic magmatism between 1943 ± 5 Ma and 1933 ± 2 Ma. This charnockitic magmatism may be related to a post-collisional setting after the evolution of the Cauarane-Coeroeni Belt (～2.00 Ga), or may represent a post-collisional (or intracontinental?) magmatism related to orogenic activities along the plate margins around 1.95–1.94 Ga.