西天山达巴特火山岩的形成环境及对斑岩型矿化的制约

本文通过对西天山地区比较典型的达巴特斑岩铜钼矿床矿区范围内出露上的英安岩和花岗斑岩进行了系统的岩石地球化学分析,对英安岩和流纹斑岩中的锆石进行了SHRIMP U-Pb定年研究,分别获得了315.9±5.9Ma和278.7±5.7Ma。岩石化学、微量以及稀土元素特征表明从英安岩到花岗斑岩,岩体具有明显的分异演化特征和很好的继承性。火山岩和次火山岩的精确定年为准确厘定火山岩形成的时限和地球动力学背景提供了依据。结合已有的Re-Os法获得的矿化年龄,表明晚石炭世末-早二叠世初（278.7±5.7Ma）,西天山地区进入板块碰撞-板内伸展阶段,由于板内幔根的部分熔化,造成深源斑岩岩浆侵位,在达巴特矿区形成了由花岗斑岩、流纹斑岩和流纹质凝灰熔岩组成的椭圆形火山机构,并导致相关矿床的形成。     

江西省武功山地区浒坑钨矿床的Re-Os年龄及其地质意义

浒坑钨矿床是位于江西省中部武功山成矿带的大型石英脉型黑钨矿床。为了确定该矿床的成矿时代,笔者选取了浒坑含钨石英脉中与黑钨矿共生的辉钼矿进行了高精度Re-Os同位素定年,并获得5个辉钼矿样品的Re-Os等时线年龄和模式加权平均年龄分别为150.2±2.2Ma和149.82±0.92Ma。测年数据表明浒坑钨矿床的成矿时代为150Ma左右,是华南地区中生代大规模成岩成矿作用高峰期的产物。辉钼矿含铼较低,表明成矿物质可能来自壳源,与形成浒坑花岗岩体的燕山期重熔S型花岗岩岩浆活动有关。该矿床形成于燕山期岩石圈伸展减薄环境。     

Zircon U–Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China

The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751 ± 7 Ma for protolith crystallization, and two group ages of 213 ± 4 to 245 ± 17 Ma and 126 ± 4 to 131 ± 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative ε_Hf(t) values of − 5.1 to − 2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive ε_Hf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ¹⁸O values of − 3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ¹⁸O rocks. North Dabie shares the same age of Neoproterozoic low δ¹⁸O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.     

History of crustal growth and recycling at the Pacific convergent margin of South America at latitudes 29°–36° S revealed by a U–Pb and Lu–Hf isotope study of detrital zircon from late Paleozoic accretionary systems

Detrital zircon provides a powerful archive of continental growth and recycling processes. We have tested this by a combined laser ablation ICP-MS U–Pb and Lu–Hf analysis of homogeneous growth domains in detrital zircon from late Paleozoic coastal accretionary systems in central Chile and the collisional Guarguaráz Complex in W Argentina. Because detritus from a large part of W Gondwana is present here, the data delineate the crustal evolution of southern South America at its Paleopacific margin, consistent with known data in the source regions.Zircon in the Guarguaráz Complex mainly displays an U–Pb age cluster at 0.93–1.46 Ga, similar to zircon in sediments of the adjacent allochthonous Cuyania Terrane. By contrast, zircon from the coastal accretionary systems shows a mixed provenance: Age clusters at 363–722 Ma are typical for zircon grown during the Braziliano, Pampean, Famatinian and post-Famatinian orogenic episodes east of Cuyania. An age spectrum at 1.00–1.39 Ga is interpreted as a mixture of zircon from Cuyania and several sources further east. Minor age clusters between 1.46 and 3.20 Ga suggest recycling of material from cratons within W Gondwana.The youngest age cluster (294–346 Ma) in the coastal accretionary prisms reflects a so far unknown local magmatic event, also represented by rhyolite and leucogranite pebbles. It sets time marks for the accretion history: Maximum depositional ages of most accreted metasediments are Middle to Upper Carboniferous. A change of the accretion mode occurred before 308 Ma, when also a concomitant retrowedge basin formed.Initial Hf-isotope compositions reveal at least three juvenile crust-forming periods in southern South America characterised by three major periods of juvenile magma production at 2.7–3.4 Ga, 1.9–2.3 Ga and 0.8–1.5 Ga. The ¹⁷⁶Hf/¹⁷⁷Hf of Mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile, mantle-derived magma, while that of zircon from the Guarguaráz Complex has a largely juvenile crustal signature. Zircon with Pampean, Famatinian and Braziliano ages (< 660 Ma) originated from recycled crust of variable age, which is, however, mainly Mesoproterozoic. By contrast, the Carboniferous magmatic event shows less variable and more radiogenic ¹⁷⁶Hf/¹⁷⁷Hf, pointing to a mean early Neoproterozoic crustal residence. This zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems, but probably derived from a more juvenile crust in their backstop system.     

Timing of UHP metamorphism in the Hong’an area,western Dabie Mountains,China: evidence from zircon U–Pb age,trace element and Hf isotope composition

The Hong’an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U–Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong’an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U–Pb ages, and trace element and Lu–Hf isotope compositions. One group with an average age of 239 ± 2 Ma show relatively high and variable HREE contents (527 ≥ Lu_N ≥ 14) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.00008–0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 ± 2 Ma, and shows consistent low HREE contents and ¹⁷⁶Lu/¹⁷⁷Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP–HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 ± 86 and 752 ± 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (T _DM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Zircon coronas around Fe–Ti oxides: a physical reference frame for metamorphic and metasomatic reactions

Ilmenite in coronitic gabbros from the Bamble and Kongsberg sectors, southern Norway, is surrounded by zircons ranging in diameters from a fraction of a micrometer to 10 μm across. The zircons are inert during subsequent metamorphism (amphibolite- to pumpellyite–prehnite facies) and metasomatism (scapolitization and albitization) and can be found as trails in silicates (phlogopite, talc, chlorite, amphibole, albite, and tourmaline) in the altered rocks. The trails link up to form polygons outlining the former oxide grain boundary. This 3-dimensional framework of zircons is used to (a) recognize metasomatic origin of rocks, (b) quantify the mobility of elements during mineral replacement, (c) establish the growth direction of reaction fronts and to identify the reaction mechanism as dissolution–reprecipitation. Zircon coronas on Fe–Ti oxides have been described from a number of terrains and appear to be common in mafic rocks (gabbros and granulites) providing a tool for a better understanding of metasomatic and metamorphic reactions.     

Variscan basic dykes in the Pelagonian (Northern Greece and south FYROM): Geodynamic significance based on petrological, geochemical and geochronological studies

Dolerite dykes intruding Variscan plutonites were studied in terms of mineralogy, petrology, geochemistry and geochronology. The main mineral constituents were studied and the sequence of crystallization has been derived. The geochemical characteristic indicate mantle origin of the dolerites and magma sources different from the hosting granitoids. From SHRIMP analyses of five spots on four different zircon crystals, resulted a 292.0±4.1 Ma age that is interpreted as the time of crystallization of the dolerite. The hosting granitoids are probably the result of mixing between two possible end-members: enriched mantle and acid metaigneous or lower crustal metasediments.

The Variscan age of the dolerites, in combination with the geochemical characteristics, indicated that the enriched mantle basaltic material should be the source of the dolerite veins. These mantle-derived basaltic melts may represent the underplated material, which probably provided the necessary thermal input to the dehydration melting in the lower crust. The dolerites should have intruded the newly formed batholiths before or at the first stages of their uplift, recording the last events of the Variscan subduction.     

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals

Establishing the petrogenesis of volcanic and plutonic rocksis a key issue in unraveling the evolution of distinct subduction-relatedtectonic phases occurring along the South American margin. Thisis particularly true for Cenozoic times when large volumes ofmagma were produced in the Andean belt. In this study we havefocused on Oligo-Miocene magmatism in central Chile at 33°S.Our data include field and petrographic observations, whole-rockmajor and trace element analyses, U–Pb zircon dating,and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene,and zircon mineral separates. Combined with earlier dating resultsthe new zircon ages define a 28·8–5·2 Maperiod of plutonic and volcanic activity that ceased as a consequenceof flattening subduction of the Nazca–Farallon plate.Rare earth elements patterns are variable, with up to 92 timeschondrite concentrations for light rare earth elements yielding(La/Yb)_N between 3·6 and 7·0, and an absence ofEu anomalies. Initial Pb isotope signatures are in the rangeof 18·358–19·023 for ²⁰⁶Pb/ ²⁰⁴Pb, 15·567–15·700for ²⁰⁷Pb/ ²⁰⁴Pb and 38·249–39·084 for ²⁰⁸Pb/²⁰⁴Pb. Initial ⁸⁷Sr/ ⁸⁶Sr are mostly in the range of 0·70369–0·70505,with two more radiogenic values at 0·7066. Initial Hfisotopic compositions of zircons yield exclusively positiveHf_i ranging between + 6·9 and + 9·6. The newlydetermined initial isotope characteristics of the Oligo-Miocenemagmas suggest that the mantle source lithologies are differentfrom both those of Pacific mid-ocean ridge basalt and oceanisland basalt, plotting in the field of reference values forsubcontinental lithospheric mantle, characterized by moderatelarge ion lithophile element–high field strengh elementdepletion and high ²³⁸U/ ²⁰⁴Pb. A Hf model age of 2 Ga is estimatedfor the formation of the subcontinental mantle–continentalcrust assemblage in the region, suggesting that the initialSr and Pb isotope ratios inferred for the source of the Oligo-Mioceneparental magmas are the result of later Rb and U enrichmentcaused by mantle metasomatism. A time-integrated model Rb/Srof 0·039 and µ 16 are estimated for the sourceof the parental magmas, consistent with ratios measured in peridotitexenoliths from continental areas. Evolution from predominant(>90%) basaltic–gabbroic to andesitic–dioriticmagmas seems to involve a combination of (1) original traceelement differences in the metasomatized subcontinental mantle,(2) different degrees of partial melting and (3) fractionalcrystallization in the garnet- and spinel-peridotite stabilityfields. The genesis of more differentiated magmas reaching rhyolitic–graniticcompositions most probably also includes additional crystalfractionation at both shallow mantle depths and within the crust,possibly leading to some very minor assimilation of crustalmaterial. KEY WORDS: calc-alkaline magmatism; Oligo-Miocene; U–Pb dating; Sr–Pb–Hf isotopes; central Chile     

Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 1: Western part of the Dzhavakheti Highland

Isotopic-geochronological study of the Pliocene magmatic activity in western part of the Dzhavakheti Highland (northwestern region of the Lesser Caucasus) is carried out. The results obtained imply that the Pliocene magmatic activity lasted in this part of the highland approximately 2 million years from 3.75 to 1.75–1.55 Ma. As is established, the studied volcanic rocks correspond in composition mostly to K-Na subalkaline and more abundant normal basalts. Time constraints of main phases in development of basic volcanism within the study region are figured out. We assume that individual pulses of silicic to moderately silicic volcanism presumably took place in the Dzhavakheti Highland about 3.2 and 2.5 Ma ago.     

First U–Pb SHRIMP age of the Hauterivian stage, Neuquén Basin, Argentina

A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuquén Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuquén Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France.