Evolution of a post-batholith dike swarm in central coastal Queensland, Australia: arc-front to backarc?

A swarm of felsic and mafic dikes cuts a Late Carboniferous–Permian batholith called the Urannah Suite in central coastal Queensland. Late Permian–Triassic westward thrusting (Hunter–Bowen Orogeny) exposed this mid-crustal Suite and the crosscutting dikes, thus enabling examination of dikes that range in age from syn- to post-batholithic. Although both mafic and felsic dikes have the same dominant northerly strike, field, geochronologic and geochemical examination reveal that the swarm is composite; felsic dikes are older (285 Ma) and geochemically and isotopically distinct from mafic dikes (273–229 Ma). Dike compositions are compared to those of the host plutonic rocks, and to volcanic rocks the same age as the dikes. Whereas the felsic (older) dikes are compositionally similar to their host granites (initial ⁸⁷Sr/⁸⁶Sr>0.7045), the mafic (younger) dikes are isotopically (Sr, Nd, Pb) less radiogenic. Moreover, several different types of mafic dikes are evident based on geochemistry; most of these have mixed characteristics in terms of tectonic classification. All but two dikes of basalt and basaltic andesite composition classify as ‘with-in plate' on Ti–Zr–Y tectonic classification plots. Many of the basalts have high TiO₂ contents (1.5–3.0 wt.%). Most of these have REE and spider diagram patterns like calc-alkaline tholeiites, the exceptions being a few alkali basalts recognized by their alkali content, and high Ti, Ce, Nb and Zr contents. When put into the context of all plutonic rocks in the area (late Paleozoic and Mesozoic), these dikes record a transition at 280 Ma, after which time, all magmatism in the region is less isotopically evolved (initial ⁸⁷Sr/⁸⁶Sr=0.7033–0.7044). A model of slab retreat and hinge movement to the east in the latest Permian explains the change of geochemical signature from arc-front to backarc at about 280 Ma.     

Age and provenances of sandstones from the Riphean Priozersk and Salmi formations in the eastern Pasha-Ladoga basin (southern margin of the Baltic Shield)

The data on the age of sediments and their sources were first obtained by isotopic methods and lithological-petrographic observations for the Priozersk and Salmi formations in the northeastern Pasha-Ladoga basin. Wide development of coarse-grained terrigenous rocks and peculiar structures point to the proximity of provenances and dominant sedimentation in terrestrial settings. U-Pb dating of 168 detrital zircons revealed that approximately 87% of all grains are Early Riphean in age, while other grains are dated back to the Early Proterozoic. Archean age is documented for only two zircon grains. It is established that detrital material of sandstones from the Priozersk and Salmi formations largely originate from the Salmi Massif (1.55–1.53 Ga) and secondarily from the Svecofennian intrusions of the northern Ladoga region aged approximately 1.88 Ga, which is confirmed also by Sm-Nd data. The occurrence of detrital zircons dated at approximately 1480–1490 Ma indicates development of coeval magmatic bodies in the immediate proximity to the sedimentation basin, which remain undiscovered. The concordant age obtained for the youngest detrital zircon grain points to the onset of sedimentation in the basin after 1477 ± 8 Ma ago. Thus, Riphean sedimentation on the eastern slope of the trough commenced only 20 Ma prior to the emplacement of the Valaam Sill.     

皮羌盆地新生代基性岩浆活动的年代学及其地质意义

塔里木盆地西北部卷入了西南天山新生代板内造山活动,以发育一系列北东向展布的断层和断层相关褶皱为特征,但该区新生代构造变形的原因尚存在争议。文中以皮羌盆地为例报道了新生代火成岩的地质特征和测年结果,认为构造变形是对幔源岩浆活动的浅部响应,与印度 亚洲大陆碰撞没有必然的联系。皮羌盆地出露有大量的新生代基性岩墙和4个古火山颈,岩墙群整体走向NNW和NNE,产状近于直立,侵入于新生界湖相沉积地层中。火山颈相玄武岩锆石SHRIMP U Pb定年结果为46 Ma,全岩K Ar等时线封闭年龄为(455±613) Ma,表明岩浆活动发生在始新世中期。岩浆活动时间比逆冲推覆构造启动时间(约24 Ma)早约22 Ma。按热传导模式估算,在此期间来自深部热源的热传递距离约为25 km。假定深部热源为底侵或内侵幔源岩浆,新生代岩浆活动可能是塔里木盆地西北部大规模构造变形的触发机制。     

Precambrian mafic magmatism in the Bastar craton,central India

The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.     

藏北阿索地区早白垩世基性岩脉地球化学及年代学特征:对班公湖－怒江洋闭合时限的约束

班公湖－怒江洋闭合时限是中特提斯洋演化争论的焦点之一,而基性岩脉对于识别构造环境转变具有重要意义.本文对阿索地区侵入构造混杂岩带中的基性岩脉进行了1件LA-ICP-MS锆石U-Pb年龄测定和10件全岩地球化学元素分析.辉长岩脉获得了102.90±0.86 Ma（MSWD=1.4）的谐和年龄,全岩地球化学分析显示基性岩脉属于亚碱性系列,钙碱性辉长闪长岩类,样品轻微富集轻稀土元素,富集大离子亲石元素Th、U、Pb等,亏损高场强元素Nb、Ta等.基性岩脉来源于被俯冲沉积物和板片流体改造过的亏损地幔源区,由尖晶石二辉橄榄岩高程度部分熔融形成,未经历明显的地壳混染过程.结合构造环境判别图解和前人研究结果,认为阿索地区的基性岩脉可能形成于早白垩世晚期伸展环境之中.系统收集研究区及附近120~95 Ma之间的岩浆岩数据,发现在110~100 Ma之间班公湖－怒江缝合带及两侧处于伸展环境之中,但不同区域伸展事件的时代有所不同,原因可能与班公湖－怒江洋的不均一闭合相关.班公湖－怒江洋最后的闭合时限可能在103 Ma左右.      

The Yermakovsky deposit,Western Transbaikal Region,Russia: Isotopic and geochemical parameters and sources of beryllium-bearing granitoids and other rocks

An isotopic study of igneous and metamorphic rocks has been carried out at the Yermakovsky bertrandite-phenakite-fluorite deposit. It has been established that the model age of the schists pertaining to the Zun-Morino Formation is 1360–1260 Ma. In Nd and Sr isotopic composition, these schists deviate from the isotopic composition of the continental crust and are close in this respect to the enriched mantle reservoir (EM-II). The model age of carbonate rocks of the Zun-Morino Formation is 1330–1020 Ma. The Middle Riphean model age of the Zun-Morino Formation is interpreted as the age of its protolith. According to the Sr and Nd isotopic data, all preore igneous rocks (granitic dikes, gabbroic rocks, and gneissose granite of the Tsagan Complex) were formed with the participation of continental crustal material. Synore basic dikes, alkali leucogranite stock, and syenite intrusion are considered to be mixtures of mantle components (DM+HIMU) and various continental crustal components (Tsagan gneissose granite, crystalline schists, the mean composition of granitoids of the Angara-Vitim batholith as an estimate of average composition of the regional continental crust). Synore igneous rocks are genetically cognate and related to the magmatic activity in the Western Transbaikal Rift Zone presumably formed in the Triassic under effect of a mantle plume.     

Geochronology,geochemistry and petrogenesis of the Laozhaishan dolerite sills in the southeastern margin of the North China Craton and their geological implication

The configuration of the North China craton (NCC) in the Rodinia supercontinent still remains controversial due to few reliable Neoproterozoic magmatism records. Here we report early Neoproterozoic dolerite sill swarms in the Laozhaishan region in the southeastern margin of the NCC. These sills intrude the Neoproterozoic sedimentary rocks (Wangshan, Gouhou and Shijia Formations), and are up to several hundred meters in thickness and >30 km in length. Baddeleyites separated from several representative dolerite samples show SIMS ²⁰⁷Pb-²⁰⁶Pb ages varying from 912 ± 4 Ma to 916 ± 4 Ma, which are consistent with concordant zircons U-Pb age of 913 ± 10 Ma. Geochemically, the Neoproterozoic dolerite samples have relatively low rare earth elements (REEs) contents and show weak positive Eu anomalies. Moreover, these dolerite rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs; e.g., Rb and Ba) and some high field strength elements (HFSEs; e.g., Nb, U, Nd, Ti, Gd) and depleted in heavy rare earth elements (HREEs) and several HFSEs (e.g., Zr, Th), with geochemical affinities to within-plate tholeiitic basalts. The REE patterns, whole-rock ε_Nd(t) (t = 913 Ma) values varying from −0.60 to +3.12, and in-situ ε_Hf (t) (t = 913 Ma) values between +4.50 to +12.85 suggest that these dolerites were generated by mantle plumes, in which some enriched components were involved. In the context of assembly, growth and breakup of the Rodinia supercontinent, together with the recognition of ~913 Ma dolerite swarms in the NCC as well as nearly coeval rift-related magmatism recorded elsewhere on the globe such as the Congo craton and the São Francisco craton, we conclude that the NCC has involved in the evolution of Rodinia and the widespread dolerite swarms indicate the breakup of Rodinia.     

Geochemical and petrological aspects of dike intrusions in the Lycian ophiolites (SW Turkey): a case study for the dike emplacement along the Tauride Belt Ophiolites

Lycian ophiolites located in the Western Taurides, are cut at all structural levels by dolerite and gabbro dikes. The dolerite dikes from this area are both pristine and metamorphosed. The non-metamorphosed dikes are observed both in the peridotites and in the metamorphic sole rocks. Accordingly, the non-metamorphosed dikes cutting the metamorphic sole were generated after cooling of the metamorphic sole rocks. The metamorphosed dolerite dikes are only observed in the peridotites. The physical conditions and timing of the metamorphism for the metamorphosed dolerite dikes are similar to those of the metamorphic sole rocks of the Lycian ophiolites suggesting that the metamorphosed dolerite dikes were metamorphosed together with the metamorphic sole rocks. Therefore, the dike injections in the western part of the Tauride Belt Ophiolites occurred before and after the generation of the metamorphic sole rocks. All metamorphosed and non-metamorphosed dikes are considered to have the same origin and all of them are subduction-related as inferred from whole-rock geochemistry and lead isotopes. Lead isotope compositions of whole rocks of both dike groups cluster in a narrow field in conventional Pb isotope diagrams (²⁰⁶Pb/²⁰⁴Pb = 18.40–18.64; ²⁰⁷Pb/²⁰⁴Pb = 15.56–15.58; ²⁰⁸Pb/²⁰⁴Pb = 38.23–38.56) indicating a derivation from an isotopically homogeneous source. On the ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb diagram, isotope compositions of the dikes plot slightly below the orogen curve suggesting contributions from mantle reservoir enriched by subducted oceanic lithosphere. Such a signature is typical of island arc magmatic rocks and supports the formation of the investigated rocks in a subduction-related environment.     

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

Middle Paleozoic basaltic and kimberlitic magmatism in the northwestern shoulder of the Vilyui Rift,Siberia: relations in space and time

A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian–Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui–Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume–lithosphere interaction.     

Mesozoic Bimodal Volcanic Suite in Zhalantun of the Da Hinggan Range and Its Geological Significance： Zircon U-Pb Age and Hf Isotopic Constraints

1 Introduction Mesozoic volcano-intrusive rocks are widely distributed in the Da Hinggan Range of northeastern China, and are considered as one of the most spectacular geological sights in eastern Asia. Recently, studies on granites with high εNd(t) values and Phanerozoic crustal growth in the Centra Asian Orogenic Belt have greatly promoted fundamental research into the geology of this area (Jahn et al., 2000, 2001, 2004; Wu et al., 2000, 2002, 2003). However, work on the eruption time,…     

黄陵野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成

采用激光剥蚀-等离子质谱(LA-ICP-MS)分析技术测定野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成,以探讨黄陵地区TTG片麻岩原岩的形成及变质时间、是否存在比崆岭群更古老的地壳等问题。野马洞辉绿岩脉(1850 Ma)侵入TTG片麻岩,并从TTG片麻岩中捕获了大量捕掳晶锆石。捕掳晶锆石岩浆结晶核部的U-Pb年龄分别为2842 Ma、2900 Ma和2949 Ma,指示TTG花岗岩体为复式岩体,其至少经历了2949 Ma、2900 Ma和2842 Ma三期岩浆作用。捕掳晶锆石变质边部的U-Pb年龄为2557 Ma,指示TTG花岗岩体转变为TTG片麻岩,是"水月寺运动"及其构造热事件共同作用的结果,其变形变质的时间为2557～2511 Ma。捕掳晶锆石的εHf(t)为-9.85～0.89、平均值为-4.07,亏损地幔模式年龄TDM为3.6～3.2 Ga,指示黄陵地区存在比崆岭群(3.2 Ga)更古老的陆壳。     

Timescales and significance of high-pressure, high-temperature metamorphism and mafic dike anatexis, Snowbird tectonic zone, Canada

New geochronological, isotopic and geochemical data for a spectacular swarm of deep crustal migmatitic mafic dikes offer important insight into processes operative during 1.9 Ga high pressure, high temperature metamorphism along the Snowbird tectonic zone in northern Saskatchewan. High-precision U–Pb zircon dates reveal anatexis of Chipman mafic dikes at 1,896.2 ± 0.3 Ma during syntectonic and synmetamorphic intrusion at conditions of 1.0–1.2 GPa, >750°C. U–Pb zircon dates of 1,894–1,891 Ma for cross-cutting pegmatites place a lower bound on major metamorphism and deformation at the currently exposed crustal levels. The persistence of elevated temperatures for ~14 m.y. following peak conditions is implied by younger U–Pb titanite dates, and by Sm–Nd whole rock isotopic data that suggest the derivation of the pegmatites by melting of a mafic source. Limited melting of the host felsic gneiss at 1.9 Ga despite high temperature is consistent with evidence for their previous dehydration by granulite facies metamorphism in the Archean. Spatial heterogeneity in patterns of mafic dike and tonalitic gneiss anatexis can be attributed to lateral peak temperature and compositional variability. We correlate 1,896 Ma Chipman mafic dike emplacement and metamorphism with substantial 1.9 Ga mafic magmatism over a minimum along-strike extent of 1,200 km of the Snowbird tectonic zone. This suggests a significant, continent-wide period of asthenospheric upwelling that induced incipient continental rifting. Extension was subsequently terminated by hinterland contraction associated with Trans-Hudson accretion and orogenesis. Little activity in the lower crust for ca. 650 m.y. prior to Proterozoic metamorphism and mafic magmatism implies an extended interval of cratonic stability that was disrupted at 1.9 Ga. This episode of destabilization contrasts with the record of long-term stability in most preserved cratons, and is important for understanding the lithospheric characteristics and tectonic circumstances that control the destruction or survival of continents.     

Late Mesozoic magmatism in the Jiaodong Peninsula,East China:Implications for crust-mantle interactions and lithospheric thinning of the eastern North China Craton

A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula,East China,containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions,was chosen to investigate the lithospheric evolution of the eastern North China Craton(NCC).Zircon U-Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic(～145 Ma) and late Early Cretaceous(112-107 Ma),respectively;high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at～139 Ma and ～118 Ma,and 125-145 Ma and 115-120 Ma,respectively.The geochemical data,including whole-rock major and trace element compositions and Sr-Nd-Pb isotopes,imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab.Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC,while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts.Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous.We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic.Slab rollback of the plate from ～160 Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.     

The 1501 Ma Kuonamka Large Igneous Province of northern Siberia: U-Pb geochronology,geochemistry, and links with coeval magmatism on other crustal blocks

A new large igneous province (LIP), the 1501 ± 3 Ma Kuonamka LIP, extends across 700 km of northern Siberia and is linked with coeval dikes and sills in the formerly attached Sao Francisco craton (SFC)-Congo craton to yield a short-duration event 2000 km across. The age of the Kuonamka LIP can be summarized as 1501 ± 3 Ma (95% confidence), based on 7 U-Pb ID-TIMS ages (6 new herein) from dolerite dikes and sills across the Anabar shield and within western Riphean cover rocks for a distance of 270 km. An additional sill yielded a SIMS (CAMECA) age of 1483 ± 17 Ma and sill in the Olenek uplift several hundred kilometers farther east, a previous SIMS (SHRIMP) age of ca. 1473 Ma was obtained on a sill; both SIMS ages are within the age uncertainty of the ID-TIMS ages. Geochemical data indicate a tholeiitic basalt composition with low MgO (4–7 wt%) within-plate character based on trace element classification diagrams and source between E-MORB and OIB with only minor contamination from crust or metasomatized lithospheric mantle. Two subgroups are distinguished: Group 1 has gently sloping LREE ((La/Sm)_PM = 1.9) and HREE ((Gd/Yb)_PM = 1.8) patterns, slightly negative Sr and moderate TiO₂ (2.2 wt%), and Group 2 has steeper LREE ((La/Sm)_PM = 2.3) and HREE ((Gd/Yb)_PM = 2.3), strong negative Sr anomaly, is higher in TiO₂ (2.7 wt%), and is transitional from tholeiitic to weakly alkaline in composition. The slight differences in REE slopes are consistent with Group 2 on average melting at deeper levels. Proposed reconstructions of the Kuonamka LIP with 1500 Ma magmatism of the SFC-Congo craton are supported by a geochemical comparison. Specifically, the chemistry of the Chapada Diamantina and Curaga dikes of the SFC can be linked to that of Groups 1 and 2, respectively, of the Kuonamka LIP and are consistent with a common mantle source between EMORB and OIB and subsequent differentiation history. However, the coeval Humpata sills and dikes of the Angola block of the Congo craton represent a different magma batch.     

Middle Paleozoic and Early Mesozoic anorogenic magmatism of the South Yenisei Ridge: first geochemical and geochronological data

In this paper we present complex geological, petrographic and geochronological data of the study of intermediate and acid composition intrusive and volcanogenic rocks from the Porozhnaya massif of the South Yenisei Ridge. For the first time in the Yenisei Ridge Devonian and Triassic U-Pb age values (SHRIMP method) have been obtained for leucogranites—387 ± 5 Ma and alkaline trachytes—240 ± 3 Ma, which allows us to attribute them to two different complexes, despite the fact that these rocks were formed within the same Severnaya riftogenic structure. Geochronological Ar-Ar data (392–387 Ma) for micas from paragneisses and leucogranitic dikes of the Yenisei suture zone on whose extension the Severnaya riftogenic structure is located are also given in this study. These data on Devonian tectonic-magmatic events in the South Yenisei Ridge agree well with coeval events of continental rifting—the formation of intrusive and volcanogenic rocks of the Agul graben in the Prisayan region and the Minusa basin in the Altai-Sayan folded area. The forming of alkaline trachytes and alkaline syenites of the Severnaya riftogenic structure, for which an age of 240 ± 3 Ma has been established, is related to the trap magmatism of the Siberian platform.     

湘赣地区中生代镁铁质岩浆作用与岩石圈伸展

综合分析了华南内部中生代 (178～ 80Ma)镁铁质岩石的年代学和元素同位素地球化学特征。研究表明区内主要发育 4期镁铁质岩浆活动 :2 2 0Ma± ,175Ma± ,12 0～ 15 0Ma ,80～ 90Ma ,其中 2 2 0Ma仅在道县发育辉长岩包体。地球化学上区内 175Ma左右的宁远太阳山和赣中项家碱性玄武岩和 80～ 90Ma的镁铁质岩石主要表现为Hawaii OIB玄武岩的元素同位素地球化学特征 ,而175Ma的其他镁铁质岩石表现为岩石圈地幔属性。 12 0～ 15 0Ma的镁铁质岩石则介于岩石圈地幔端员与软流圈地幔端员之间。时、空上表现为以郴州—临武断裂为界 ,西侧自 170Ma左右的EMI型为主向 80～ 90Ma的OIB型为主迁移演化 ,而东侧则自老而新自EMII型 (175Ma)为主演变为以OIB型 (80～ 90Ma)为主。上述资料暗示区内至少存在 4期强烈的岩石圈减薄作用 ,软流圈物质上涌和岩石圈伸展减薄是华南中生代岩浆作用形成的主要机制。但 2 2 0Ma左右的伸展减薄范围局限 ,而较大规模的岩石圈伸展减薄和软流圈上涌始于 178Ma ,其形成可能与华南印支造山作用的后造山 (或后碰撞 )拉张裂解地球动力学背景有关。同时也暗示郴州—临武断裂可能是界定中生代EMI型扬子岩石圈和EMII型华夏岩石圈地幔的重要边界。     

Age and geochemical constraints for partial melting of granulites in Estonia

Summary The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.     

Geochemistry and geodynamic implications of the Triassic bimodal magmatism from Western Kunlun Orogen,northwest China

The Western Kunlun Orogen occupies a key tectonic position at the junction between the Tarim block and the Tethyan domain. However, the late Paleozoic to early Mesozoic, especially the middle to late Triassic tectonic evolution history of the Western Kunlun Orogen remains controversial. This study reports SHRIMP zircon U–Pb ages and geochemical as well as Sr–Nd–Hf isotopic data for middle to late Triassic Taer pluton in Western Kunlun Orogen, Northwest China. The Taer pluton shows a strong bimodal distribution of compositions, with the felsic rocks dominant and the mafic rocks subordinate. Zircon U–Pb dating reveals that the coexisting mafic and felsic rocks are coeval, both emplacing in a period between 234 and 225 Ma. Most of the studied rocks are potassium rich and can be classified into high-K calc-alkaline to shoshonitic series. They are also strongly enriched in LREE, LILE and depleted in HFSE with strong negative Ti and Nb anomalies, and characterized by enriched Sr–Nd–Hf isotopic signatures. Detailed geochemical and isotopic studies indicate that the Taer pluton was emplaced in a post-collisional extensional setting, with the mafic rocks derived from partial melting of the enriched continental lithospheric mantle in the spinel facies field, and the felsic rocks formed by anatexis of newly underplated basaltic rocks. The existence of middle to late Triassic post-collisional magmas in Western Kunlun region suggests that the final closure of Paleo-Tethys and the initial collision between the Western Kunlun and the Qiangtang terranes may have happened before ~234 Ma, most probably in late Permian, rather than in late Triassic or early Jurassic. In assistance with other geological evidences, such as the presence of early Triassic to late Triassic/early Jurassic S-type magmatism, terrestrial molasse depositions, regional unconformities, and strong deformation, we propose that the Western Kunlun Orogen may have undergone a long post-collisional intracontinental process from early Triassic to late Triassic/early Jurassic.