Dike swarms and related igneous complexes in the Urals

The dike swarms of the entire Urals are classified for the first time; the related igneous complexes associated with them in space and time are named. The following types and chronological levels of the Uralian dikes are distinguished (proper names are given after type localities). The epicontinental type comprises the Middle Riphean Mashak, Late Riphean Arsha-Serebryanka, Late Cambrian-Early Ordovician Kidryasovo-Lemva, Ordovician-Silurian Ushat, Devoninan Inzer-Timaiz (the most extended of all), Early Carboniferous Magnitogorsk-Mugodzhary, and Triassic Borisovo dike swarms. Many of them are probably related to plume events. The existence of the Early Riphean dike complex remains unclear. Oceanic (spreading or suprasubduction) dike-in-dike type: Ordovician Man’ya oceanic type, Devonian Aktogai backarc and Khabarny suprasubduction types. The igneous complexes associated with dike swarms are rather diverse. In addition to rhyolite dikes, in many cases determining the contrasting character of magmatism, large comagmatic gabbro and gabbro-granite intrusions are noted, as well as minor intrusions of subalkali granitoids, syenites, and, apparently, carbonatites and kimberlites. Flood basalt fields are noted at the periphery of the Urals, implying the occurrence of a feeding dike swarm beneath them.     

Early Devonian suprasubduction ophiolites of the southern Urals

The composition of ophiolites widespread in the southern Urals shows that they were formed in a suprasubduction setting. Low-Ti and high-Mg sheeted dikes and volcanic rocks vary from basalt to andesite, and many varieties belong to boninite series. The rocks of this type extend as a 600-km tract. The volcanic rocks contain chert interbeds with Emsian conodonts. Plagiogranites localized at the level of the sheeted dike complex and related to this complex genetically are dated at 400 Ma. The ophiolites make up a base of thick islandarc volcanic sequence. The composition of the igneous rocks and the parameters of their metamorphism indicate that subduction and ascent of a mantle plume participated in their formation. The nonstationary subduction at the intraoceanic convergent plate boundary developed, at least, from the Middle Ordovician.     

Composition and genesis of the Konevinsky gold deposit,Eastern Sayan,Russia

The Konevinsky gold deposit in southeast Eastern Sayan is distinguished from most known deposits in this region (Zun-Kholba, etc.) by the geological setting and composition of mineralization. To elucidate the cause of the peculiar mineralization, we have studied the composition, formation conditions, and origin of this deposit, which is related to the Ordovician granitoid pluton 445–441 Ma in age cut by intermediate and basic dikes spatially associated with metavolcanic rocks of the Devonian–Carboniferous Ilei Sequence. Four mineral assemblages are recognized: (1) quartz–pyrite–molybdenite, (2) quartz–gold–pyrite, (3) gold–polysulfide, and (4) telluride. Certain indications show that the ore was formed as a result of the superposition of two distinct mineral assemblages differing in age. The first stage dated at ~440 Ma is related to intrusions generating Cu–Mo–Au porphyry mineralization and gold–polysulfide veins. The second stage is controlled by dikes pertaining to the Devonian–Carboniferous volcanic–plutonic association. The second stage is characterized by gain of Hg and Te and formation of gold–mercury–telluride paragenesis.     

Ordovician volcanic and plutonic complexes of the Sakmara allochthon in the southern Urals

The Ordovician terrigenous, volcanic–sedimentary and volcanic sequences that formed in rifts of the active continental margin and igneous complexes of intraoceanic suprasubduction settings structurally related to ophiolites are closely spaced in allochthons of the Sakmara Zone in the southern Urals. The stratigraphic relationships of the Ordovician sequences have been established. Their age and facies features have been specified on the basis of biostratigraphic and geochronological data. The gabbro–tonalite–trondhjemite complex and the basalt–andesite–rhyolite sequence with massive sulfide mineralization make up a volcanic–plutonic association. These rock complexes vary in age from Late Ordovician to Early Silurian in certain structural units of the Sakmara Allochthon and to the east in the southern Urals. The proposed geodynamic model for the Ordovician in Paleozoides of the southern Urals reconstructs the active continental margin, whose complexes formed under extension settings, and the intraoceanic suprasubduction structures. The intraoceanic complexes display the evolution of a volcanic arc, back-, or interarc trough.     

The age of camptonite dikes of the Agardag alkali-basalt complex (western Sangilen): results of Ar/Ar and U/Pb dating

A camptonite dike swarm (Agardag alkali-basalt complex) in the western part of the Sangilen Upland abounds in mantle xenoliths. Mineralogical, petrographic, and petrochemical studies show that the dikes are composed of lamprophyres of two groups, basic and ultrabasic. Ar/Ar dating of amphibole and phlogopite megacrysts gives an intrusion age for the dikes of 443.0 ± 1.3 Ma. ²⁰⁶Pb/²³⁸U dating of zircon from a glomeroporphyritic intergrowth in camptonite from one of the dikes yielded a core age of 489.0 ± 5.4 Ma. This corresponds to the time of formation of the Chzhargalanta granite–leucogranite complex (489.4 ± 2.6 Ma). The ²⁰⁶Pb/²³⁸U age of the zircon rim is 444.0 ± 7.5 Ma. The ages obtained by Ar/Ar dating of amphibole and biotite megacrysts and by U/Pb dating of the magmatic rim of zircon crystal from the camptonite coincide within the dating error, which indicates that the camptonite dikes formed in the Late Ordovician. These dikes are the oldest-known example of mantle-derived xenoliths in mafic volcanic rocks from an off-craton setting. These are samples of the Upper Ordovician lithospheric mantle.     

北大巴山超基性、基性岩墙和碱质火山杂岩形成时代的新证据及其地质意义

北大巴山早古生代地层广泛出露一套超基性、基性岩墙和碱质火山杂岩(包括碱性玄武岩和粗面岩),为研究北大巴山早古生代构造演化提供了重要的载体。本文通过利用锆石U-Pb定年和金云母~(40)Ar/~(39)Ar同位素定年方法对它们的形成时代进行了系统研究,结合地球化学特征探讨它们的成因及构造意义。通过对岚皋县和镇坪县的两个辉绿岩墙开展锆石U-Pb定年,分别获得它们的年龄为399±1Ma和451±4 Ma,是研究区目前已报道的有关超基性、基性岩墙最年轻和最老的就位时代。地球化学分析结果表明,它们的主量元素与区内其他基性岩墙位于同一演化趋势线上,微量和稀土元素地球化学特征相似,类似于OIB的特征,表明它们来自相同的地幔源区。因此,研究区内基性岩墙侵入事件最早开始于晚奥陶世中期(约450Ma),结束于早泥盆世晚期(约400 Ma),期间经历了多次岩浆侵入活动。对与碱性玄武岩共生的火山碎屑岩中金云母晶屑进行~(40)Ar/~(39)Ar同位素定年,获得年龄446±3Ma,代表了火山喷发时间,表明碱性玄武质岩浆喷发活动与基性岩浆侵入事件大致同时发生。对粗面岩进行SHRIMP锆石U-Pb定年虽未获得准确的岩浆结晶年龄,但较年轻锆石年龄(165±3Ma和229±2Ma)的存在,暗示其形成时代可能属于中生代。系统的年代学研究表明,北大巴山地区早古生代地层中的超基性、基性岩墙和碱性玄武质火山杂岩为同期岩浆活动的产物,最早开始于450Ma,经历了多期火山喷发和岩浆侵入活动(450~400 Ma),结束于早泥盆世晚期(~400Ma)。而粗面岩与上述岩石不是同一期岩浆活动产物,研究区不存在双峰式火成岩组合,它们可能形成于中生代,属于南秦岭中生代岩浆活动的产物,为南秦岭、北大巴山中生代成矿作用提供物源。     

The Late Jurassic Zedong ophiolite: A remnant of subduction initiation within the Yarlung Zangbo Suture Zone (southern Tibet) and its tectonic implications

The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive ε_Nd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.     

Plume magmatism in the northeastern part of the Altai–Sayan region: Stages,source compositions,and geodynamics (exemplified by the Minusinsk Depression)

The results of geochronological (U–Pb, Ar–Ar), geochemical, and isotopic (Sr, Nd) studies of the Ordovician and Devonian mafic volcanic–subvolcanic rock associations of the Minusinsk Depression are presented. The obtained ages of magmatic associations and the basite composition, considering previous studies, witness to the impact of two mantle plumes different in age (Late Cambrian–Ordovician and Devonian) on suprasubduction rock complexes in active continental margin settings.     

中祁连马衔山岩群内基性岩墙群锆石LA-ICP-MSU-Pb年代学及其构造意义

马衔山岩群变质基底岩系中发现大量基性岩墙, 作为中祁连造山带东部一次重要的构造-岩浆事件的标志.根据地质-岩石学特征, 将马衔山岩群中的基性岩墙分为两期.利用LA-ICP-MS(激光剥蚀等离子体质谱)方法, 分别对两期基性岩墙进行单颗粒锆石微区U-Pb同位素测定, 并应用CL图像对所测锆石进行了成因研究.获得早期变辉长岩墙的侵入年龄为(441.1± 1.4)Ma(早志留世早期), 主变质期年龄为(414.3± 1.2)Ma(早泥盆世早期); 晚期辉绿(玢)岩墙群的侵入年龄为(434± 1.0)Ma(早志留世晚期), 并保留有曾遭受马衔山岩群混染的信息(捕获锆石的207Pb/206Pb表面年龄为(2 325± 3)Ma~(2 573± 6)Ma), 以及遭受了加里东晚期构造-热事件的改造的信息(206Pb/238U表面年龄为(400± 2)Ma~ (429± 2)Ma).结合相关研究成果, 认为马衔山岩群中的两期基性岩墙群形成于祁连地区由俯冲造山向碰撞造山的转换时期, 代表了中祁连地块在区域上遭受北东-南西向强烈挤压的过程中派生北西-南东向扩张作用的地质纪录.      

Timing of Back–Arc Spreading of the Paleozoic Island Arc of the Urals: Implication of Sm–Nd and U–Pb Dating of Dolerites and the Composition of Inclusions in Zircon

Doklady Earth Sciences - Dolerites of the parallel dikes formed in a back-arc spreading setting have been dated by Sm–Nd to the Silurian (426 ± 34 Ma) for Mount Azov in the Central...     

华北陆块早元古代基性岩墙群及其构造意义

华北陆块中部带的晋冀蒙地区早元古代未变形变质基性岩脉形成于1781~1765 Ma。东部陆块鲁西地区早元古代类似的基性岩脉形成时间约为1841 Ma。中部带基性岩脉依据其FeOt含量、(Nb/La)N和(Th/Nb)N值的差异能划分为组1、组2和组3三类。它们的元素-同位素组成变化表明,组1样品起源于再循环大陆玄武质组分参与的交代岩石圈地幔,组2样品源于交代富铁岩石圈地幔与MORB组分的混杂源区,组3样品则是受辉长质组分混染的、经俯冲改造而成的岩石圈地幔产物。相反,鲁西地区基性岩脉亏损HFSE,具MORB型钕同位素组成。上述地球化学特征支持华北陆块中部带约1780 Ma的基性岩脉与早期俯冲碰撞作用的关系密切,而东部陆块约1840 Ma基性岩脉类似于弧后盆地构造背景产物。     

Polyphase magmatic pulses along the Northern Gondwana margin: U-Pb zircon geochronology from gneiss domes of the Pyrenees

Alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the Aston and Hospitalet domes of the Axial Zone of the Pyrenees are subjects of a laser ablation ICP-MS U-Pb zircon geochronology study. The age spectra recorded by detrital, magmatic xenocrystic and inherited zircons reveal a more complex, nearly continuous Paleozoic magmatic history of the Variscan basement of the Pyrenees than previously known. Inherited and detrital zircons of Mesoarchean, Paleoproterozoic to Ediacaran ages attest to the Peri-Gondwana location of the Cambrian sediments that later form the metamorphic core of the Variscan Pyrenees. The youngest magmatic zircon ages fall into the late Carboniferous and earliest Permian, ranging from ca. 306–297 Ma, and represent the emplacement ages of the dikes and small granite intrusions. The age spectra of magmatic xenocrystic zircons contain several maxima, middle (475–465 Ma) and late Ordovician (455–445 Ma), early (415–402 Ma) and late Devonian (385–383 Ma), early (356–351 Ma) and middle Carboniferous (ca. 328 Ma). Middle Ordovician and middle Carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded, the Aston and Hospitalet orthogneisses and the Soulcem granite. The presence of early-mid Carboniferous magmatic zircons in several samples lends further support to a wide-spread early Variscan magmatic activity in the central Pyrenees. The other age peaks do not have equivalent igneous or metaigneous rocks in the central Axial Zone, but are thought to be present in the Pyrenean crust, not exposed and yet to be identified. The diversity of Ordovician, Devonian and Carboniferous up to Permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre-Variscan and Variscan orogenies. The source of the heat for the Devonian to early-mid Carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone, lithospheric-scale shearing or a mantle plume (TUZO).     

Volcanic-sedimentary complex of Otrozhnaya sheet in the Ust-Belaya Terrane,western Koryakia

New data on the structure and composition of the rock complexes making up the Otrozhnaya sheet of the Ust-Belaya Terrane are discussed. This sheet is currently regarded as a complete section of the Early Devonian oceanic crust. The U-Pb age of zircon (547 ± 17 Ma) from plagiogranite porphyry that cuts through the ultramafic-gabbroic complex indicates that the country rocks are much older. Basalts of volcanic complex are similar to MORB-type rocks. The contact between volcanic and volcano-sedimentary complexes is tectonic. No volcanic-cherty association of rocks typical of ophiolites has been established at the base of the sedimentary sequence. The U-Pb age of detrital zircons from the base of this section is 571 Ma. As follows from the composition of terrigenous rocks and the suprasubduction origin of andesitic lavas and tuffs, the rocks of the lower part of volcano-sedimentary complex were deposited at the marginal part of the basin near convergent boundary. The remains of conodont elements make it possible to estimate the age of the upper part of the volcano-sedimentary complex as Late Devonian. The composition of terrigenous rocks shows that the provenance has abruptly changed. Sporadic rock fragments evidence the erosion of ophiolitic sections. Silurian rocks became the main source of clastic material. This is corroborated by redeposited microfauna species and U-Pb age of detrital zircons (～432 Ma). The geochemistry of dacite pebbles from conglomerate indicates that this volcanic rock was formed in a suprasuduction setting. The Ordovician-Silurian island arc could have been a source.     

Devonian supra-subduction zone setting for the Princhester and Northumberland Serpentinites: implications for the tectonic evolution of the northern New England Orogen

The Princhester Serpentinite of the Marlborough terrane of the northern New England Orogen is a remnant of upper mantle peridotite that was partially melted at an oceanic spreading centre at 562 Ma, and subsequently interacted with Late Devonian island arc basalts in an intra-oceanic supra-subduction zone (SSZ) setting. The full range of rare-earth element (REE) contents, including U-shaped patterns, can be explained by a single process of reaction of partially melted, depleted peridotite with Late Devonian calc-alkaline and island arc tholeiite magmas by equilibrium porous flow, fractionating the REE by a chromatographic column effect. The Northumberland Serpentinite on South Island of the Percy Group has similar REE and high field strength element (HFSE) contents to the most depleted samples of the Princhester Serpentinite, supporting a common origin. However, spinel compositions suggest that the Northumberland Serpentinite interacted with boninitic magmas. The REE and mineral geochemistry indicates that the Princhester and Northumberland Serpentinites both represent part of the mantle component of a disrupted SSZ ophiolite. The ophiolite is considered to have formed above an east-dipping subduction zone, based on the geochemistry of Devonian island arc basalts between Mt Morgan and Monto, which include compositions identical to dykes and gabbroic blocks within the Princhester Serpentinite. Blockage of the subduction zone by collision with the Australian continent during the Late Devonian led to slab breakoff and the reversal of subduction direction, trapping the Late Devonian ophiolite in a forearc position. Its location, in a forearc setting above a growing accretionary wedge, conforms to the definition of a Cordilleran-type ophiolite. This interpretation is consistent with current views that most ophiolites are formed from young, hot and thin oceanic lithosphere at forearc, intra-arc and backarc spreading centres in a SSZ setting, and that emplacement follows genesis by 10 million years or less. Late Devonian crustal growth may have been widespread in the New England Orogen, because the disrupted ophiolite assemblage of the Yarras complex in the southern New England Orogen is probably of this age. Extensional tectonism at the end of the Carboniferous dismembered the Princhester – Northumberland ophiolite, removed the crustal section, and produced windows of accretionary wedge rocks within the fragmented ophiolite. The Princhester Serpentinite, together with fault slices of metasedimentary rocks, was thrust westward as a flat sheet over folded strata of the Yarrol Forearc Basin by a Late Permian out-of-sequence thrust during the Hunter – Bowen Orogeny, completing the emplacement of the Marlborough terrane. The Princhester and Northumberland Serpentinites could have been displaced by strike-slip movement along the Stanage Fault Zone or an equivalent structure. There is no record in the northern New England Orogen of SSZ ophiolites and volcanic arc deposits of Cambrian age, as exposed along the Peel Fault. Partial melting of the Princhester Serpentinite at an oceanic spreading centre at 562 Ma, recorded by mafic intrusives displaying N-MORB chemistry, was an earlier event that was outboard of any Early Paleozoic subduction zone along the margin of the Australian continent, and cannot be regarded as representing the early history of the New England Orogen. It is possible that the formation of intra-oceanic arcs in latest Silurian and Devonian time was the first tectonic event common to both the southern and northern New England Orogen.     

新疆阿尔泰凯勒克赛依铁矿地质特征、锆石LA MC ICP MS U Pb年龄及其意义

凯勒克赛依铁矿床是新疆阿尔泰唯一的小型镜铁矿床,赋存于一套变质火山-沉积岩系中,近矿围岩为白云母石英片岩,矿体呈层状,与地层产状一致,矿石具有块状、条带状、条纹状构造,矿石中金属矿物主要为镜铁矿(TFeO=87349%~88988%,TiO2=0~1042%,Al2O3=0036%~0256%),矿化具有沉积特征。近矿围岩镜铁矿白云母石英片岩锆石LA MC ICP MS U Pb谐和年龄为(3756 ± 06)Ma,限定成矿时代在376 Ma左右,即中泥盆世成矿,是阿尔泰为数不多的中泥盆世成矿作用的产物。同时也厘定含矿的变质火山-沉积岩系属中—晚泥盆世阿勒泰镇组,不是前人认为的早泥盆世康布铁堡组。     

The upper geochronological limit for dunites of the East Khabarny gabbro-ultramafic complex (South Urals) by the data of U-Pb age of zircons extracted from vein hornblendites

This paper discusses the results of U-Pb isotopic dating of zircons from hornblendites that cut dunites of the East Khabarny dunite-clinopyroxenite-websterite-gabbro complex in the South Urals. This complex is included into the structure of the large Khabarny mafic-ultramafic allochthon, and the set of rocks and their petrogeochemical peculiarities allow this complex to be compared with the Ural-Alaskan-type complexes. Zircons from hornblendites have been dated on a SHRIMP-II ion microprobe and by the classical thermo-ionization mass spectrometric method by five fractions. The obtained results are comparable between each other within the analysis deviation interval and show that hornblendites were formed in the Early Devonian (408–402 Ma). This age is the upper geochronological limit in estimation of the time when dunites of the East Khabarny complex were formed. The synchronity of intrusive hornblendite manifestation in most of the Urals in the Early Devonian is discussed. The probable Early Devonian stage, in addition to the found Vendian and Ordovician-Silurian ones when dunite-clinopyroxenite-gabbro complexes formed in the Urals, is suggested.     

Petrological and geochemical characteristics of the diabase and metasomatised dikes from the Tekirova ophiolite (SW Anatolia,Turkey): Tectonomagmatic evolution of the southern Neotethys

Ophiolites are widespread across the Anatolian plate and formed during the termination of different branches of the Neotethys ocean. The Tekirova ophiolite, situated in the Antalya ophiolite nappes in Southwest Anatolia, originated in the southern branch of the Neotethys. The former ophiolite consists of mantle and crustal sequences, in which, it is predominantly crosscut by the isolated diabasic dikes. Here, we have distinguished two dike types on the basis of their petrographic features and also degree of alteration. The relatively fresh types are generally named as diabase dikes. While the altered types are called as metasomatised dikes, and these dikes display ophitic to subophitic textures. The diabase and metasomatised dikes show enrichment in LILEs and depletion in HFSEs, indicating characteristics of suprasubduction zone ophiolites. This depletion of mobile elements may have been caused by metasomatism. The HFSEs versus HREEs diagrams show that the dikes were formed from 10 to 30% melting of extracted melts from previous mantle melting in which modified by subduction-derived fluids.     

内蒙古大井锡多金属矿床岩脉LA-ICP-MS锆石 U-Pb定年及其地质意义

内蒙古大井矿床是我国北方一个重要的锡多金属矿床,矿区浅成-超浅成岩脉发育,并与矿脉密切共生。前人的年代学研究结果显示矿区岩脉与矿脉同属于燕山中晚期岩浆热液活动的产物,因此该矿床成因被定为与矿区次火山岩脉有关的热液脉型矿床。本文通过选取矿区分布最广,且与矿脉相伴出现的霏细岩脉和英安斑岩脉进行LA-ICP-MS锆石U-Pb定年,结果表明两个英安斑岩脉样品DJ-7和YX-20的形成年龄分别为240±1Ma和239±1Ma,而霏细岩脉样品DJ-1的形成年龄为162±1Ma。因此,矿区岩脉至少形成于两个不同时代,即印支早期和燕山中期。结合前人在区域上的研究成果分析,认为英安斑岩脉和霏细岩脉形成的构造背景分别为同造山-后造山转换阶段和造山后的伸展阶段。因为该矿床的成矿年龄可以确定在140Ma左右,所以本文认为矿区岩脉的形成与矿床的形成并无直接关系,只是为后者提供了有利的就位空间。     

Age paradoxes of Devonian magmatism of the northeastern part of the Kola Peninsula

A structural-geochemical study has been conducted on the dikes of presumably Devonian mafic rocks confined to a small graben filled in with Riphean sedimentary rocks hosted by Early Precambrian granite-gneiss of the Murmansk block. It has been demonstrated that the dolerite dikes of this region can be considered as manifestations of trap magmatism whose products fill in the foundation of the East Barents riftogenic downfold. In turn, manifestations of alkaline and kimberlite rocks of the White Sea region are confined to the peripheral portion of the trap magnetism area. Zircons from dolerite transecting Late Riphean sediments examined in two laboratories have a concordant age of 2.74–2.72 Ba, while zircons from a similar dike located in granite-gneiss of the basement are characterized by an age range of 2700–155 Ma, and the concordant age based on 4 points is 790 Ma. All these factors indicate that the age determinations of the mafic rocks are ambiguous, particularly in the zone of transition from the center of the trap province to its periphery, where alkaline magmatism is observed.     

Geochronology and geochemistry of the Devonian Gumbardo Formation (Adavale Basin): evidence for cratonisation of the Central Thomson Orogen by the Early Devonian

Abstract

The Devonian subsurface Adavale Basin occupies a central position in the Paleozoic central Thomson Orogen of eastern Australia and records its tectonic setting during this time interval. Here, we have focussed on the basal volcanics of the Gumbardo Formation to clarify the tectonic setting of the basin. The approach has been to undertake stratigraphic logging, LA-ICP-MS U–Pb zircon geochronology and whole-rock geochemical analysis. The data indicate that basin initiation was rapid occurring at ca 401?Ma. The volcanic rocks are dominated by K-feldspar phyric rhyodacitic ignimbrites. The whole-rock geochemical data indicate little evidence for extensive fractional crystallisation, with the volcanic suite resembling the composition of the upper continental crust and exhibiting transitional I- to A-type tectonomagmatic affinities. One new U–Pb zircon age revealed an Early Ordovician emplacement age for a volcanic rock previously interpreted to be part of the Early Devonian Gumbardo Formation, and older basement age is consistent with seismic interpretations of uplifted basement in this region of the western Adavale Basin. Five ignimbrites dated from different stratigraphic levels within the formation yield similar emplacement ages with a pooled weighted age of 398.2?±?1.9?Ma (mean square weighted deviation?=?0.94, n?=?93). Significant zircon inheritance in the volcanic rocks records reworking of Ordovician and Silurian silicic igneous basement from the Thomson Orogen and provides insight into the crustal make-up of the Thomson Orogen. Collectively, the new data presented here suggest the Adavale Basin is a cover-type basin that developed on a stabilised Thomson Orogen after the major Bindian deformation event in the late Silurian.