Zirconology of miaskites from the Ilmeny Mountains,South Urals

It is shown that the replacement and long evolution of miaskitic zircons led to the formation of two main age groups: 420–380 Ma (I) and 260–240 Ma (II). The age of miaskites is estimated at 440–445 Ma. Zircons I bear traces of fragmentation, dissolution, and replacement; they have “flat” REE patterns typical of metasomatic (hydrothermal) types, which is caused by allochthonous nature of the studied miaskites. Zircons II with differentiated REE patterns are similar to magmatic varieties, but have metamorphic origin. Mineralogical–geochemical and age characteristics of zircons in combination with structural–compositional features of miaskites define their metasomatic nature. The origin of the early zircon generations was related to the Ordovician rifting, while late generations were formed during shear deformations at the final stage of the evolution of the Uralian orogen.     

Zirconology of serpentinites from Nyashevo massif (Southern Urals)

Zircons in serpentinites from Nyashevo massif of the Ilmenogorskii complex were dated for the first time by means of the SHRIMP technique. The maximum date of 1892 ± 23 Ma for the zircons accounts for the minimum age of their mantle substrate probably constituting the restite residue. The date is comparable to those for metamorphic rocks of the Selyankino group, as well as of fenite–sand amphibolites of the Ilmenogorskii complex. The Upper Ordovician age limit of 443 ± 12 Ma is adequate for formation of the massif and conforms to the age of the Buldym massif and miaskites. The Early Permian dates of zircons (275.8 ± 2.1 Ma) represent late shear processes in the Ilmenogorskii complex.     

Geochronological constraints on the evolution of the Embu Complex, São Paulo, Brazil

Petrologic and geochronological work was carried out on a roadside outcrop of amphibolite facies orthogneisses near São Lourenço da Serra, about 50 km southwest of São Paulo City. These orthogneisses belong to the Embu Complex, within the Neoproterozoic Brasiliano Orogenic Cycle mobile belts of SE Brazil. The outcrop consists of predominantly foliated biotite tonalites and granodiorites, which were cut by granitic veins and pegmatites prior to final deformation. SHRIMP U/Pb measurements on zircons from one granodioritic–tonalitic gneiss indicate magmatic crystallization of the protolith at 811±13 Ma (MSWD=1.0). Zircons with dates of ca. 2000 and ca. 1000 Ma in this rock are interpreted as inherited from older crust. One zircon analyzed from the gneiss and three zircons from a discordant pegmatitic vein indicate an event at 650–700 Ma, perhaps related to the intrusion of the pegmatites. A regression of Rb–Sr whole rock data for four biotite gneisses yielded an imperfect isochron, giving an apparent age of 821±68 Ma and an elevated initial ⁸⁷Sr/⁸⁶Sr ratio of 0.719±0.005. The elevated initial ⁸⁷Sr/⁸⁶Sr ratio and the inherited zircons indicate involvement of older crust in the genesis of the gneisses. Rb–Sr feldspar and whole rock pairs yield ca. 560 Ma tielines, giving the time of final cooling below 300–350 °C, and the cessation of medium-grade metamorphism and ductile deformation. These results document a series of tectono-thermal events spanning 250 million years during the Brasiliano Orogenic Cycle. They relate to ca. 800 Ma magmatic arc activity and later allochthonous terrane assembly during closure of the Adamastor Ocean, resulting in the accretion of Western Gondwana.     

Zircons of igneous rocks at the Erdenetuin-Obo porphyry Cu-Mo deposit (Mongolia): U-Pb dating and petrological implications

The Erdenetuin-Obo porphyry Cu-Mo deposit was formed at the final stage of development of magmatic activity occasionally manifested in the Late Permian-Early Triassic in the period of at least 40 Ma. Early plutonic (host) and late ore-bearing porphyry intrusive complexes were formed in that period. The plutonic complex is multiphase, while the porphyry complex is polyrhythmical and multiphase within rhythms. The obtained data on the U-Pb isotopic composition (SHRIMP II) of zircons from unaltered rocks of the ore field are discussed: gabbro, diorite, and granodiorite of the plutonic complex and granodiorite-porphyry I and II of the first and second rhythms of the ore-bearing complex, respectively. Zircons of different age levels and genotypes were identified in the course of performed investigations. Gabbro are dominated by postmagmatic (superimposed) zircons with the datings of 239–225 Ma. The age of xenogenic zircon brought out from the basement rocks is estimated at 1146 ± 11 Ma. Zircons occur as magmatic and postmagmatic (superimposed) minerals dated 252–247, 244–233 Ma in diorite and 244–242, 239–224 Ma in granodiorite. The ages of postmagmatic zircons from diorite are partially overlapped by datings of magmatic zircons from granodiorite and granodiorite-porphyry. In the porphyry complex, the datings of magmatic zircons are 240–234 and 222–220 Ma in granodiorite-porphyries I and II, respectively. There are also inherited zircons with datings coinciding with those of magmatic zircons from precursor intrusive rocks. Datings of such zircons are 249–241 and 257–231 Ma for granodiorite-porphyries I and II, respectively. As a whole, zircon datings in all studied igneous rocks forming a virtually uninterrupted range in the period of 257–220 Ma allow us to suggest the relation of the ore magmatic system to the long-living constantly active deep source occasionally delivering melt to the upper levels.     

Zirconology of rutile eclogites of the Maksutov Complex (Southern Urals)

The age data (U–Pb, SHRIMP II) of zircons from rutile eclogites of the Maksutov Complex (MC) (village Shubino, Southern Urals) were subdivided into three age groups. The Neoproterozoic zircons (561 ± 10 Ma) recorded the formation stage of eclogites, the protolith of which was diabase and gabbro-diabase bodies. The Neoarchean ancient zircons (2884 ± 36 Ma) belong to the mantle substratum, which was repeatedly transformed (2303 ± 12, 2008 ± 18, 1626 ± 59 Ma). Zircons of early Ordovician–early Silurian age (433–477 ± 6 Ma; 340 ± 40 Ma) recorded superimposed processes, corresponding to high-pressure metamorphism.

     

Age and provenance of the Ergunahe Group and the Wubinaobao Formation,northeastern Inner Mongolia,NE China: implications for tectonic setting of the Erguna Massif

Here, we present the results of U–Pb dating of detrital zircons from the Ergunahe Group and the Wubinaobao Formation, within northeastern Inner Mongolia, NE China, with the aim of constraining the tectonic setting of the Erguna Massif. The majority of detrital zircons from five samples collected from the Ergunahe Group and the Wubinaobao Formation are magmatic, although some zircons have metamorphic growth rims. Zircons in two mica schists and in feldspar–quartz sandstone from the Ergunahe Group yield age populations that cluster around 738, 760, 792, 837, 890, 964, and 1050 Ma, whereas zircons from two quartz sandstones within the Wubinaobao Formation yield age populations that cluster at 466, 484, 515, 738, 795, 837, and 894 Ma. These data, combined with detrital zircon age populations (ca. 712 Ma) from the adjacent Xinghuadukou Group, and the fact that the Ergunahe Group intruded by Caledonian gabbros is overlain by upper Silurian units, indicate that the Ergunahe Group formed at 738–712 Ma (i.e. during the Neoproterozoic). In addition, the Wubinaobao Formation is subdivided into two units: a calcareous siltstone unit within the western part of the study area and a quartz sandstone within the eastern part. The calcareous siltstone formed at 712–795 Ma, similar to the Ergunahe Group, whereas the quartz sandstone formed between the 466 Ma and late Silurian. The age spectra of detrital zircons from the Ergunahe Group and the Wubinaobao Formation indicate that sediment in both of these units was derived from terranes that outcrop around the basin. The widespread occurrence of Neoproterozoic detrital zircons within both the Ergunahe Group and the Wubinaobao Formation suggests that Precambrian terranes are present within the Erguna Massif and that the massif has an affinity to the Siberian Craton.     

内蒙古克什克腾旗林西组碎屑锆石LA-ICP-MS年代学及其地质意义

内蒙古克什克腾旗位于西拉木伦河以北,属锡林浩特地块南缘。本文对出露于克什克腾旗北东约5 km的一套变质粉砂岩进行了锆石LA-ICP-MS U-Pb测年,其年龄结果主要分为4个区间:258~298 Ma(峰值为285 Ma)、377~474 Ma(峰值为430 Ma)、1261~1727 Ma、1853~2513 Ma,此外还含有321 Ma和937 Ma的锆石各一颗。锆石CL图像显示:258~298 Ma的锆石以岩浆锆石为主,响应兴蒙造山带的晚古生代岩浆活动;377~474 Ma的锆石中既有岩浆锆石又有变质锆石,表明其源区既有奥陶纪-泥盆纪岩浆岩,又有古生代的变质岩;1261~1727 Ma的锆石以岩浆锆石为主,少数为变质锆石,暗示中元古代的岩浆岩和变质岩也为该组提供物源;1853~2513 Ma的锆石以岩浆锆石为主,反映了华北板块基底的年龄信息。该变质粉砂岩中碎屑锆石的最小谐和年龄是258 Ma,限定了其沉积时代的下限为晚二叠世,应属于林西组。年龄峰值既对应华北板块的重要构造热事件,又有与兴蒙造山带地质事件相关的年龄信息,表明林西组具有南北两个物源区,同时也暗示在其形成时华北板块与西伯利亚板块已经拼合。     

Timing the tectonic mingling of ultramafic rocks and metasediments in the southern section of the coastal accretionary complex of central Chile

Uranium-lead ages are reported for zircons from ultramafic bodies and metamorphic host rocks of the Western Series that outcrop at La Cabaña, in the southern section of the coastal accretionary complex of central Chile. Metasedimentary mica schists hosting the ultramafic bodies contain a main detrital zircon population of Devonian age (365–380 Ma) clustering around ~368 Ma, differing significantly from neighbouring areas where Devonian zircons are scarce. Zircons from the metasomatic reaction zones (albitites and chloritites), formed during the emplacement and alteration of the ultramafic bodies, are mainly Ordovician (~478 Ma) and lack Devonian zircons, resembling a typical detrital zircon pattern from other locations in the Western Series. Zircons from the chloritite reaction zone of the Lavanderos serpentinite, the easternmost ultramafic body in La Cabaña, are in textural equilibrium with metamorphic ilmenite. Some of these zircons yield an average age of 283.4 ± 7 Ma (n = 6) which is identical, within error, to a previously reported K-Ar fuchsite cooling age of 282 ± 6 Ma from the reaction zone. Most zircons extracted from chromitite boulders have euhedral oscillatory-zoned growth patterns with a similar range of ages than those reported for the Western Series (324–1090 Ma; n = 12), except for two zircons with cloudy appearance and high U/Th ratios which yielded an average age of 285.5 ± 7 Ma. The presence of Early Permian zircons (~280–290 Ma) in all studied rocks suggests remobilization of Zr, possibly triggered by metasomatic fluids released during the disequilibrium reaction associated with the tectonic emplacement of the ultramafic rocks into the metasedimentary rock. Simultaneously with the formation of metasomatic zircons, Palaeozoic and Mesoproterozoic zircons from the metasedimentary rocks were mechanically incorporated into the ultramafic rocks, thus providing a record of the timing of crustal emplacement of the ultramafic rocks into the accretionary complex.     

Character of zircon distribution in dunites of the South Urals (Sakharin and East Khabarnin massifs)

Zircons in dunites of the Sakharin and East Khararnin massifs, situated in the South Urals and part of the platinum-bearing Uralian belt, were investigated for the first time. Several types were identified in the polygene-polychronous zircon assemblage of both massifs. The first is represented by Proterozoic (from 1517 ± 12 to 2693 ± 9 Ma) crystals similar to those widely spread in the Riphean and more ancient Uralian deposits. The second type includes dunite varieties of typical magmatic habit and of ages from 377 ± 3.6 Ma to 402 ± 3 Ma. The third contains crystals and crystal fragments of a high degree of crystallinity. The concordant Archean dating (2808 ± 26 Ma) for zircons of this type determines the minimum age of the dunite substrate. Zircons of Type 4 are heterogeneous, consist of the relict (nuclear) part represented by crystals of Types I and III and the newly formed mantle, and are consistent with zircons of Type II.     

Polychronous zirconology of syenites from the Avashla intrusion in the Kurgass anticline (Southern Urals)

Zircons were separated from syenites of the Avashla intrusion in the Kurgass anticline of the Bashkir megaanticlinorium in the Southern Urals. The obtained samples were dated using the U–Pb procedure by means of a SHRIMP II ion microprobe. The integrated mineralogical, geochemical, and isotope studies of zircons resulted in primary data on the origin and current conditions of zircons in syenites of the Avashla intrusion. The relics of early zircon generations (1320–1340 Ma) allowed us to specify the geological position of syenites along with the stratigraphic location of the enclosing sedimentary rocks. The time of the transformation (metamorphism) of the zircons at the Middle–Upper Riphean boundary is represented by a dating of 1097±20 Ma, which is of importance for specifying the boundary age and for revealing a geological event that started a new development stage of the stratoregion in the Upper Riphean.     

华北克拉通桑干地区高压麻粒岩的锆石SHRIMP U-Pb年龄及其地质含义

为了揭示华北克拉通桑干地区古元古代高压麻粒岩变质峰期时限,对选自该区的两个高压麻粒岩样品(DST02,XYS01)进行了锆石SHRIMP U-Pb测年。锆石样品的阴极发光图像显示为球形和无内部结构,Th/U比值变化为0.01~0.93。这些特征表明两样品的锆石应属于变质成因锆石。两样品的SHRIMP测年结果分别给出(1 792±12)Ma和(1 891±46)Ma。根据前人的年代学方面的研究成果,特别是未变质强过铝花岗岩中获得的1 900~1 850 Ma的锆石U-Pb年龄(郭敬辉等,2002)结果来看,本区高压麻粒岩峰期变质时代不会晚于此。因此本文获得的1 850~1 800 Ma的锆石SHRIMP U-Pb年龄应代表退变年龄。而(1 891±46)Ma的年龄限定了峰期高压变质年龄上限。     

冀南西石门矽卡岩型铁矿床成矿年代:热液锆石U-Pb年龄的证据

西石门铁矿床是华北地区最为典型的矽卡岩型铁矿床之一,最新获得金云母40 Ar-39 Ar成矿年龄为(133.1±1.3)Ma、(137.1±1.5)Ma,略老于其成矿母岩(武安杂岩体)形成的年龄(126~136 Ma),这是矛盾的。笔者对角闪正长岩和矽卡岩脉中的锆石进行了LA-ICP-MS U-Pb同位素年代学分析。结果表明,角闪正长岩中锆石呈浅黄色-无色、透明、棱角状、不规则状,振荡环带结构不发育,部分锆石中含棱角状继承锆石内核,206 Pb-238 U年龄加权平均值为(135.6±1.5)Ma,代表了武安杂岩体的结晶年龄。矽卡岩脉中锆石呈棕色,含有少量细小的包裹体,呈透明-半透明,相对于岩浆锆石具有低的Th、U、REE含量及Th/U值(Th=(222.37~1541.11)×10-6,U=(218.44~989.17)×10-6,Th/U=0.90~1.56),并具有强烈的Ce正异常(δCe=4.46~196.22)和Eu负异常(δEu=0.59~0.80),206Pb-238 U年龄介于124.9~133.0Ma,加权平均值为(129.4±2.6)Ma,代表了热液锆石的形成年龄,说明129 Ma是该西石门地区主要成矿期。     

Recycling of continental crust into the mantle as revealed by Kytlym dunite zircons, Ural Mts, Russia

The presence of zircons of crustal origin in the dunites of Kytlym, a subduction-related concentrically zoned dunite–clinopyroxenite–gabbro massif of the Urals Platinum-Bearing Belt, may provide the first direct evidence of the recycling of continental crust into the mantle. Zircons were part of subducted sediments that melted to produce silicic magmas with entrained restitic zircons. These melts induced partial melting in the overlying mantle, which later crystallized as the Kytlym massif. Zircons rapidly captured into early formed dunites were prevented from dissolving completely and underwent different degrees of recrystallization. A few crystals still record their original ages, which range from ∼410 Myr to ∼2800 Myr, thus revealing a different origin. The majority, however, recrystallized in the presence of a limited amount of melt and record the diapir formation, 350–370 Ma, which was coeval with the Uralian high-pressure metamorphism. Lastly, several grains record an age of ∼330 Myr, which is identical, within error, to the Rb–Sr age of the tilaitic gabbros, (337 ± 22 Myr), and may, therefore, represent the crystallization age of the last melts formed during the evolution of Kytlym.     

Geochronology and provenance of detrital zircons from late Palaeozoic strata of central Jilin Province,Northeast China: implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

Here we present new U–Pb and Hf isotopic data for detrital zircons obtained from six samples of late Palaeozoic units from central Jilin Province, Northeast China, and use these data and sedimentary formations to constrain the late Palaeozoic tectonic evolution of the eastern segment of the southern margin of the Central Asian Orogenic Belt. The majority of the detrital zircons from the six samples are euhedral–subhedral and exhibit oscillatory zoning, indicating a magmatic origin. Zircons from sandstones in the Devonian Wangjiajie and Xiaosuihe formations yield seven main age populations (399, 440, 921, 1648, 1864, 1911, and 2066 Ma) and two minor age populations (384 and 432 Ma), respectively. Zircons from a quartz sandstone in the Carboniferous Luquantun Formation yield four age populations (~332, 363, 402, and 428 Ma), and zircons from quartz sandstones of the Permian Shoushangou, Fanjiatun, and Yangjiagou formations yield age populations of 265, 369, 463, 503, and 963 Ma; 264, 310, 337, 486, and 529 Ma; and 262, 282, 312, 338, 380, 465, and 492 Ma, respectively. These data, together with the ages of magmatic zircons from interbedded volcanics and biostratigraphic evidence, as well as analysis of formations, give rise to the following conclusions. (1) The Wangjiajie and Xiaosuihe formations were deposited in an extensional environment during Middle and Middle–Late Devonian time, respectively. The former was sourced mainly from ancient continental material of the North China Craton with minor contributions from newly accreted crust, while the latter was sourced mainly from newly accreted crust. (2) The Luquantun Formation formed in an extensional environment during early–late Carboniferous time from material sourced mainly from newly accreted crust. (3) The Shoushangou, Fanjiatun, and Yangjiagou formations formed during a period of rapid uplift in the late Permian, from material sourced mainly from newly accreted crust.     

U-Pb age of zircon from mantle peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau,Transbaikal region

The U-Pb (SHRIMP-II) age of zircons from garnet-spinel peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau, Transbaikal region were determined. Most of the zircons are euhedral and subhedral prismatic crystals with an elongation of 1.5–2.0. Fragments of crystals and nearly equant crystals with rounded edges are present as well. Rounded or irregular cores are observed in some grains. None of the zircons yielded an age that would correspond to the time of basalt eruption (21–2.35 Ma or younger). The youngest dates range from 135.2 ± 2.7 Ma to 141 ± 3 Ma (Early Cretaceous). Both concordant values and the lower intersection of discordia with concordia (138.8 ± 5.7 Ma) are within this age interval. The upper intersection corresponds to 1891 ± 26 Ma. A considerable part of the concordant values are grouped within the intervals (164.6 ± 1.6)–(183.4 ± 2.0) and (264.0 ± 7.3)–(295.7 ± 0.76) Ma (Early-Middle Jurassic and Early Permian, respectively). The older concordant values fall in the interval 1462 ± 19 to 1506 ± 4 Ma (Mesoproterozoic). Proterozoic age was obtained for cores of composite zircon grains. Zircons pertaining to all age intervals are enriched in REE relative to chondrite (except La). The chondrite-normalized REE patterns are positively sloped with an increase in contents from LREE to HREE. The LREE and HREE contents and the depth of the Eu minimum tend to increase with age. In composite zircons of Proterozoic age, cores are somewhat enriched in REE. It has been suggested that crystallization of zircon as a separate phase in peridotites extremely depleted in Zr was related to a low degree of partial melting. The melt that formed in the intergranular space and that was repeatedly enriched in Zr was not extracted from the solid framework of rock and crystallized in situ under the changed thermodynamic conditions in the upper mantle. The occurrence of zircons of several age intervals in peridotites testifies to the multistage evolution of the upper mantle and recurrent partial melting under various physicochemical conditions.     

Age range of formation of sedimentary-volcanogenic complex of the Vetreny Belt (the southeast of the Baltic Shield)

As a result of studying the Vetreny Belt greenstone structure (the southeast of the Baltic Shield), zircons from terrigenous deposits of the Toksha Formation, underlying the section of the sedimentary-volcanogenic complex, and zircons of the Vetreny Belt Formation, deposits of which crown the section, were dated. The results of analysis of age data of detrital zircons from quartzites of the Toksha Formation indicate that Mesoarchean greenstone complexes and paleo-Archean granitogneisses of the Vodlozero Block (Karelia) were the provenance area from which these zircons were derived. The occurrence of the youngest zircons with age of 2654.3 ± 38.5 Ma is evidence that the formation of the Vetreny Belt, including the Toksha Formation, began no earlier than this time. Zircons from volcanic rocks of the Vetreny Belt yielded the age of 2405 ± 5 Ma. Thus, the age interval of the formation of the sedimentary-volcanogenic complex of the Vetreny Belt ranges from 2654.3 ± 38.5 to 2405 ± 5 Ma.     

四川石棉县大水沟岩片绿片岩锆石SHRIMP U-Pb年代学及其地质意义

大水沟岩片位于安宁河深大断裂带中段, 川西松潘甘孜造山带与扬子地台结合部位, 因其特殊的构造位置和赋存碲矿床而备受重视.运用SRHIMP U-Pb定年技术对大水沟绿片岩进行年龄测试, 来确定大水沟岩片的形成时代, 为大水沟周缘石棉-冕宁一带乃至川西扬子地台西缘的构造岩浆演化提供有益信息, 同时为大水沟碲铋矿床的形成时代提供线索.大水沟原岩恢复表明其应当有相当部分为沉积岩, 运用SHRIMP U-Pb定年从中获得的5种不同年龄段具有内部结构、外观特征的锆石差异.2467~2358Ma年龄的残留碎屑岩浆锆石说明扬子地台西缘存在太古代-古元古代的物源搬运; 790.5~762.5Ma的岩浆锆石与新元古代早期Rodinia超大陆裂解、地幔柱上涌时的岩浆事件有关; 696.8~642.9Ma锆石年龄反映了大水沟周缘碰撞造山和后造山期岩浆活动和变质作用; 伴随峨眉地裂大规模基性岩浆活动, 在大水沟近源很可能有262.0~220.0Ma碱性杂岩的侵入; 典型热液增生锆石216.5~167.1Ma年龄揭示了该区中侏罗世大规模的岩浆期后热液活动.大水沟岩片可能为异地推覆系统, 岩浆锆石年龄和热液增生锆石年龄限制了大水沟岩片总体定位于220.0~167.1Ma之间.      

滇西哀牢山岩群变沉积岩碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

出露于扬子板块与印支板块之间的哀牢山岩群中包含大量副片麻岩、大理岩、云母片岩和石英岩等变沉积岩,这些岩石对研究扬子板块西南缘构造演化具有重要意义。但有关哀牢山岩群的沉积时代、物质来源以及形成的构造背景等一系列问题长期以来存在着较大争议。本文选取哀牢山岩群内石榴黑云斜长片麻岩、石榴夕线黑云斜长/二长片麻岩和大理岩4件变沉积岩进行了锆石阴极发光和LA-ICP-MS U-Pb年代学研究。哀牢山岩群碎屑锆石具有明显或弱的振荡环带和较高的Th/U比值,指示其岩浆成因。年代学分析结果表明石榴黑云斜长片麻岩记录始新世末-渐新世(36.3~25.0Ma)哀牢山岩群部分熔融-岩浆活动,其余3件变沉积岩碎屑锆石均得到550~450Ma与900~700Ma两组峰值,其中片麻岩与大理岩中最年轻的碎屑锆石峰值分别为452Ma、461Ma和458Ma,指示哀牢山岩群沉积时代应不早于中奥陶世,而非原先认为的古元古代。碎屑锆石年龄分布特征表明哀牢山岩群物源区包含扬子板块西缘新元古代花岗岩和印支板块内泛非期花岗岩,支持前人提出的哀牢山岩群沉积于古特提斯分支洋盆——金沙江-哀牢山-马江洋拉张过程中沉积的观点。     

Continuous zircon growth during long-lived granulite facies metamorphism: a microtextural,U–Pb,Lu–Hf and trace element study of Caledonian rocks from the Arctic

Microtextural, U–Pb, trace element and Lu–Hf analyses of zircons from gneisses dredged from the Chukchi Borderland indicate a long-lived, Cambrian–Ordovician, granulite facies metamorphism. These results reveal a complete prograde, peak and cooling history of zircon growth during anatexis. Early increasing temperatures caused modification and Pb-loss of Precambrian zircons by recrystallization and dissolution/re-precipitation of existing grains. Small variations in initial ¹⁷⁶Hf/¹⁷⁷Hf results (0.282325–0.282042) and flat HREE patterns of these zircons indicate that they grew by dissolution/re-precipitation in the presence of garnet. Zircons subsequently crystallized from a partial melt during peak to post-peak metamorphism from 530 to 485 Ma. A broad range of initial ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282693–0.282050) and mineral inclusions within zircons suggest that this phase of growth incorporated Zr and Hf obtained from the breakdown of Zr-enriched phases. Microtextural evidence along with trace element and isotopic data suggests that final growth of metamorphic rims on zircon occurred during slow cooling and crystallization of residual partial melts during the early Ordovician (485–470 Ma). Younger, late Ordovician–Silurian (420–450 Ma) euhedral, oscillatory-zoned, trace element-enriched zircons crystallized within leucocratic veins that intrude the gneisses. Their age corresponds to granitoids dated from this same dredge. The intrusives and veins provide evidence that the Chukchi Borderland rifted from a position near Pearya and northwest Svalbard, which represent the northern continuation of the Caledonian orogen. Evidence for earlier Cambrian metamorphism has not been reported from this region. The age of granulite facies metamorphism reported here represents the earliest phase of deformation in the Arctic Caledonides.     

松辽盆地南缘基性岩脉中捕获锆石U-Pb年龄及其对基底岩浆事件的制约

团山子基性脉岩为松辽盆地南缘晚中生代最后一期岩浆活动,在其中获得了较多的捕获锆石。为了反映松辽盆地基底岩浆活动事件,对基性脉岩中捕获锆石进行了锆石LA-ICP-MS U-Pb定年。捕获锆石阴极发光(CL)图像和Th/U比值显示具有岩浆成因的特点。从锆石定年结果得到8组年龄,反映出早侏罗世晚期(176Ma)、海西期(291Ma)、加里东期(467Ma)、新元古代(942Ma)、中元古代(1368Ma)、古元古代(1886Ma和2165Ma)、新太古代(2458Ma)岩浆事件记录。松辽盆地南部基底组成以古生代和早中生代岩浆岩为主,并可能存在西保安群前寒武纪结晶基底(942Ma),并经历了复杂的岩浆演化。中元古代、古元古代、新太古代年龄的锆石具有磨圆和反应边,可能为与华北克拉通岩浆事件形成的锆石被搬运到松辽盆地南部,并被再次捕获的结果,反映松辽盆地南部与华北大陆具有一定的联系,但不确定存在该期的结晶基底。