Magmatism in the epochs of ural paleocean closure and mobile belt development: Composition, distinctive evolutionary features, and sources

The paper reports major evolutionary trends of Paleozoic magmatism in the Urals in relation to the closure of the oceanic basin and associated subduction, obduction, and collision. An attempt is undertaken to use these data as a basis for reproducing the geodynamic and thermal regime during various stages of the regional geological evolution. Distinctive compositional features of the rocks, the discrete character of magmatism reflecting changes in the geodynamic environments, and the transition from predominantly mantle sources of magmatism (at 460-380 Ma) to mantle-crustal (at 360-310 Ma) and finally crustal (290-250 Ma) ones, alternating epochs of relatively “dry” and “hydrous” mantle magmatism at 460-430 Ma and the “hydrous” type of later magmatism make it possible to reproduce the thermal regime of the orogen. During the intense low-hydrous basite magmatism (460-430 Ma), the temperature in the magma generation region was 1300-1100°C and decreased to 1100-900°C during predominantly hydrous magmatism (425-400 Ma). The P-T parameters of hydrous anatexis (360-310 Ma) that gave rise to tonalites and granodiorites were 7-9 kbar and 800 ± 50°C; they constrain the maximum temperature in the lower crust. The next stage of the hydrous anatexis of tonalites and granodiorites resulted in intense granite-forming processes (290-250 Ma) and occurred at pressures close to 5-6 kbar and temperatures of 700 ± 50°C. In the situation of the adiabatic heat transfer with fluids, the minimum temperatures in the upper mantle in the Ural suprasubduction epioceanic mobile belt at a depth of approximately 80 km can be provisionally estimated as follows: 1300 ± 100°C at 450-425 Ma, 1100 =- 100°C at 425-400 Ma, and 900 ± 100°C at 375-250 Ma. Data reported in the paper testify that the temperature in the upper mantle and crust of the Ural Mobile Belt was 150-200°C lower than in neighboring oceanic and continental areas, and this places the Urals among orogens of the “cold” type (Maruyama et al.,2007).     

Geodynamic settings and history of the Paleozoic intrusive magmatism of the central and southern Urals: Results of zircon dating

The main stages of the Paleozoic intrusive magmatism in the Urals, 460–420, 415–395, 365–355, 345–330, 320–315, and 290–250 Ma, as well as two virtually amagmatic periods, 375–365 Ma (Frasnian-early Famennian) and 315–300 Ma (Late Carboniferous), are recognized. The Cambrian-Early Ordovician pause predated the onset of igneous activity in the Ural Orogen, while the Early Triassic pause followed by an outburst of trap magmatism postdated this activity. The interval from 460 to 420 Ma is characterized by mantle magma sources that produced ultramafic and mafic primary melts. The dunite-clinopyroxenite-gabbro association of the Platinum Belt and miaskite-carbonatite association are specific derivatives of these melts. The rift-related (?) Tagil Synform functioned at that time. The volcanic-plutonic magmatism in this oldest magmatic zone of the Uralides comprises gabbro, gabbro-granitoid, and gabbro-syenite series and comagmatic volcanic rocks. After a break almost 20 Ma long, this magmatism ended in the Early Devonian (405–400 Ma) with the formation of small K-Na gabbro-granitoid plutons. The magmatic intervals of 415–395, 365–355, and 320–315 Ma are characterized by the mantle-crustal nature. The first interval accompanied obduction of the oceanic lithosphere on the continental crust. The subsequent magmatic episodes presumably were related to the subduction of the island-arc (?) lithosphere beneath the continent and to the collision. The intense granitoid magmatism started 365–355 Ma ago. As in the following interval 320–315 Ma, the tonalite-granodiorite complexes, accompanied by hydrous basic magmatism, were formed. Amphibole gabbro and diorite served as a source of heat and material for the predominant tonalite and granodiorite. The Permian granitic magmatism had crustal sources. Thus, the mantle-derived Ordovician-Middle Devonian magmatism gave way to the mantle-crustal Late Devonian-Early Carboniferous plutonic complexes, while the latter were followed by the crustal Permian granites. This sequence was disturbed by rifting and formation of continental arcs accompanied by specific Early Carboniferous Magnitogorsk gabbro-granitoid series and Early Permian Stepnoe monzodiorite-granite series, which deviate from the general evolutional trend.     

Extensional collapse of the Gondwana orogen: Evidence from Cambrian mafic magmatism in the Trivandrum Block,southern India

The assembly of Late Neoproterozoice Cambrian supercontinent Gondwana involved prolonged subduction and accretion generating arc magmatic and accretionary complexes, culminating in collision and formation of high grade metamorphic orogens. Here we report evidence for mafic magmatism associated with post-collisional extension from a suite of gabbroic rocks in the Trivandrum Block of southern Indian Gondwana fragment. Our petrological and geochemical data on these gabbroic suite show that they are analogous to high Fe tholeiitic basalts with evolution of the parental melts dominantly controlled by fractional crystallization. They display enrichment of LILE and LREE and depletion of HFSE with negative anomalies at Zre Hf and Ti corresponding to subduction zone magmatic regime. The tectonic affinity of the gabbros coupled with their geochemical features endorse a heterogeneous mantle source with collective melt contributions from sub-slab asthenospheric mantle upwelling through slab break-off and arc-related metasomatized mantle wedge, with magma emplacement in subduction to post-collisional intraplate settings. The high Nb contents and positive Nbe Ta anomalies of the rocks are attributed to inflow of asthenospheric melts containing ancient recycled subducted slab components and/or fusion of subducted slab materials owing to upwelling of hot asthenosphere. Zircon grains from the gabbros show magmatic crystallization texture with low U and Pb content. The LA-ICPMS analyses show ²⁰⁶ Pb/²³⁸ U mean ages in the range of 507-494 Ma suggesting Cambrian mafic magmatism. The post-collisional mafic magmatism identified in our study provides new insights into mantle dynamics during the waning stage of the birth of a supercontinent.     

Composition and Deep Sources of Early Devonian Potassic Rocks of the Sakmar Zone,Southern Urals

The isotope–geochemical composition of the Early Devonian magmatic rocks of the Chanchar potassium basaltoid volcano–plutonic Complex in the Sakmar Zone (Southern Urals) has been studied for the first time. The character of the noncoherent element distribution and their ratios suggest that the melts parental to the rocks of extrusive, subvolcanic and intrusive facies were formed from a single source of the primary magma. The low concentrations of HFSE relative to the MORB composition, relatively low εSr, and high εNd suggest the formation of primary melts from the moderately depleted mantle source. The enrichment of rocks with LILE indicates a fluid mantle addition introduced to the melts during evolution of the primary magma.     

The main features of the Uralian Paleozoic magmatism and the epioceanic nature of the orogen

The 2000 km Uralian Paleozoic orogen is situated on the western flank of the Uralo-Mongolian folded belt. It is characterized by an abundant variety of magmatic rocks and related ore deposits. Uralian Paleozoic magmatism is entirely subduction-related. It is proposed that the Uralian orogen represents a cold mobile belt in which the mantle temperature was 200 to 500 °C cooler than in the adjacent areas; a situation which is similar to the modern West Pacific Triangle Zone including Indonesia, the Philippine Islands, and southern Asia. During the course of the geological evolution of the Uralian orogen, the nature of the magmatism has changed from basic rocks of indisputable mantle origin (460–390 Ma) to mantle-crust gabbro-granitic complexes (370–315 Ma) followed by pure crustal granite magmatism (290–250 Ma). This order in rock type and age reflects the evolution of Paleozoic magmatic complexes from the beginning of subduction to the final stages of the orogen development.     

Age and origin of Miocene gabbroid intrusions in the northern part of the Lesser Caucasus

The results of isotope-geochronological and petrological-geochemical study are reported for Neogene mafic intrusive rocks distributed in the northern part of the Lesser Caucasus (Georgia). It is shown that the young plutonic bodies were formed here in two magmatic stages: in the Middle Miocene (around 15.5 Ma) and in the terminal Miocene (9-7.5 Ma). The first age group includes a microsyenitic massif in Guria (Western Georgia), which was formed in a setting of active continental margin related to the subduction of oceanic part of the Arabian plate beneath the Transcaucasus. The Late Miocene intrusive magmatism already records the incipient within-plate activity: small polyphase bodies of alkaline gabbroids and lamprophyres of Samtskhe (South Georgia) dated around 9-8.5 Ma and teschenite intrusions of Guria dated at 7.5Ma. Petrological-geochemical and isotope-geochemical data indicate that the parental melts of the rocks of all studied Neogene plutonic bodies of the Lesser Caucasus were derived from a single mantle source. Its characteristics are close to those of a Common hypothetical reservoir, which is usually regarded as a source of oceanic and continental hot spot basalts (OIB) but shows some regional peculiarity. The role of crustal assimilation and crystallization differentiation in the genesis of the Miocene rocks of Guria was limited, which is related to the rapid ascent of deep melts to the surface (in a setting of local extension) without intense interaction with host sequences under the absence of consolidated continental lithosphere beneath this part of the Transcaucasus. The parental mantle-derived magmas of the Neogene gabbroids of Samtskhe were strongly contributed by upper crustal material, which caused a change in their isotope (⁸⁷Sr/⁸⁶Sr up to 0.70465, ?_Nd up to + 2.8) and geochemical characteristics relative to the regional mantle source. In addition, the crustal contamination of mantle basic melts during the late phases of the Samtskhe plutonic bodies formation led to their intense fractionation with precipitation of mainly olivine and pyroxene. The larger scale mantle-crustal interaction during formation of the Samtskhe intrusions was probably related to the fact that the upper lithosphere in this sector of the Transcaucasus contained large Paleozoic blocks, which were made up of granite-metamorphic complexes and prevented a rapid ascent of mantle melts to the surface. The rocks of these blocks were presumably assimilated by mantle magmas in the intermediate chambers at the upper crustal levels.     

秦岭造山带古生代岩浆作用及地球动力学意义

秦岭造山带记录了华南华北板块聚合的完整过程.古生代岩浆岩记录了造山过程中的壳幔相互作用和造山带演化的动力学过程.古生代的中基性岩浆岩揭示了俯冲隧道内变质脱水交代岩石圈地幔过程,其中富水基性杂岩为富钾基性岩,地球化学特征显示其地幔源区经历了洋壳沉积物的交代;看丰沟岩体为高镁闪长岩,地球化学特征显示其来自经历俯冲流体交代的地幔源区.通过对古生代岩浆岩的研究发现,其具有明显的时空分布规律,它们对应于原特提斯洋俯冲、后撤、前进和回转等过程.所以壳幔相互作用发生在原特提斯洋俯冲过程中.      

Geochemistry of Early Devonian rocks of the Sakmara zone,South Urals

This paper reports first isotope–geochemical data on the Early Devonian magmatic rocks of the Chanchar potassic mafic volcanoplutonic complex of the Sakmara zone of the South Urals. The incompatible element distribution and ratios indicate that the rocks of the volcanic, subvolcanic, and intrusive facies are comagmatic and were derived from a common source. The low HFSE concentrations relative to MORB and relatively low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd ratios suggest that primary melts were generated from a moderately depleted mantle. The LILE enrichment of the rocks indicates a flux of mantle fluid in the primary magma during its evolution.     

Late Paleozoic alkaline magmatism in Western Transbaikalia,Russia:Implications for magma sources and tectonic settings

This paper presents the results of geochronological(~(40)Ar-~(39)Ar,U-Pb SHRIMP Ⅱ),petrological and geochemical studies of the Late Paleozoic complexes of alkaline rocks(Zimovechinsky,Tuchinsky and Koma) located within the Vitim Plateau(the western part of the Mongol-Okhotsk Orogenic Belt).The rocks were formed at 310-280 Ma.It is coeval with Late Paleozoic magmatism within the Central Asian Orogenic Belt.The ε_(Nd)(T) values show large variations from-2.1 to +3.3 as well as the initial Sr(I) isotopic ratios from 0.7042 to 0.7138,that demonstrate strong isotopic heterogeneity of the magmatic source.The geochemical characteristics of the rocks show pronounced positive Pb and negative Ti,Zr-Hf anomalies that can be explained by involvement of the subducted component in primary melts.The rocks intruded in a setting of extension at the active continental margin of the Siberian Craton during subduction of Mongol-Okhotsk oceanic crust under the Siberian Craton.     

华北陆块北缘崇礼-赤城地区晚古生代花岗岩类的锆石年龄和Sr-Nd-Hf同位素组成

本文报道冀北崇礼-赤城地区晚古生代花岗岩类岩石的锆石U-Pb年龄和Sr-Nd-Hf同位素组成特征.它们出露在华北陆块北缘的构造单元内,侵位于中高级变质基底岩石红旗营子群中.锆石LA-ICP-MS定年结果表明,海流图花岗岩岩体记录了两期岩浆作用,即299±3Ma和254±11Ma;镇宁堡片麻状二长花岗岩和白花沟片麻状黑云母石英二长闪长岩分别形成于287±1Ma和252±3Ma.这些晚古生代花岗岩类岩石具有较低的初始~(87)Sr/~(86)Sr值(0.7062～0.7076)、低的ε_(Nd)(t)值(-18.1至-9.6)和古老的Nd亏损地幔模式年龄(2.49～1.87Ga).其锆石的ε~(Hf)(t)值变化在-13.2至-7.4之间,Hf平均地壳模式年龄值(T_(DM)~C)在2.15Ga至1.79Ga之间.锆石Hf同位素特征与全岩Nd同位素特征指示古老的华北陆块地壳物质是花岗岩浆的主要物源.在形成时代和地球化学特征上,崇礼-赤城地区晚古生代花岗岩与出露在东部丰宁-承德地区的花岗岩类岩石既有相似性,又有不同之处,可能代表华北陆块北缘不同构造背景下岩浆作用的产物.     

Geochronology and geochemistry of early Paleozoic granitic and coeval mafic rocks from the Tannuola terrane (Tuva,Russia): Implications for transition from a subduction to post-collisional setting in the northern part of the Central Asian Orogenic Belt

Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks.     

Paleozoic tholeiitic magmatism of the Kola province: Spatial distribution,age, and relation to alkaline magmatism

This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380–390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume–lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.     

Sources of basaltoid magmas in rift settings of an active continental margin: Example from the bimodal association of the Noen and Tost ranges of the Late Paleozoic Gobi-Tien Shan rift zone, southern Mongolia

The bimodal volcanoplutonic (basalt-peralkaline rhyolite with peralkaline granites) association of the Noen and Tost ranges was formed 318 Ma ago in the Gobi-Tien Shan rift zone of the Late Paleozoic-Early Mesozoic central Asian rift system, the development of which was related to the movement of the continental lithosphere over a mantle hot spot. A specific feature of the Late Paleozoic rifting was that it occurred within the Middle-Late Paleozoic active continental margin of the northern Asian paleocontinent. Continental margin magmatism was followed after a short time delay by the magmatism of the Gobi-Tien Shan rift zone, which was located directly in the margin of the paleocontinent. Such a geodynamic setting of the rift zone was reflected in the geochemical characteristics of rift-related rocks. The distribution of major elements and compatible trace elements in the rift-related basic and intermediate rocks corresponds to a crystallization differentiation series. The distribution of incompatible trace elements suggests contributions from several sources. This is also supported by the heterogeneity of Sr and Nd isotopic compositions of the rift-related basaltoids: ε_Nd(T) ranges from 4.4 to 6.7, and (⁸⁷Sr/⁸⁶Sr)₀, from 0.70360 to 0.70427. The geochemical characteristics of the rift-related basaltoids of the Noen and Tost ranges are not typical of rift settings (negative anomalies in Nb and Ta and positive anomalies in K and Pb) and suggest a significant role of the rocks of a metasomatized mantle wedge in their source. In addition, there are high-titanium rocks among the rift-related basaltoids, whose geochemical characteristics approach those of the basalts of mid-ocean ridges and ocean islands. This allowed us to conclude that the compositional variations of the rift-related basaltoids of the Noen and Tost ranges were controlled by three magma sources: the enriched mantle, depleted mantle (high-titanium basaltoids), and metasomatized mantle wedge (medium-Ti basaltoids). The medium-titanium basaltoids were formed in equilibrium with spinel peridotites, whereas the high-titanium magmas were formed at deeper levels both in the spinel and garnet zones. It terms of geodynamics, the occurrence of three sources of the rift-related basaltoids of the Noen and Tost ranges was related to the ascent of a mantle plume with enriched geochemical characteristics beneath a continental margin, where its influence caused melting in the overlying depleted mantle and the metasomatized mantle wedge. The formation of rift-related andesites in the Noen and Tost ranges was explained by the contamination of mantle-derived basaltoid melts with sialic (mainly sedimentary) continental crustal materials or the assimilation of anatectic granitoid melts.     

Magmatic sources of dikes and veins in the Moncha Tundra Massif,Baltic Shield: Isotopic-geochronologic and geochemical evidence

The dike-vein complex of the Moncha Tundra Massif comprises dolerites, gabbro-pegmatites, and aplites. The dolerite dikes are classified into three groups: high-Ti ferrodolerites, ferrodolerites, low-Ti and low-Fe gabbro-dolerites. The U-Pb age of the ferrodolerites is 2505 ± 8 Ma, and the amphibole-plagioclase metagabbroids hosting a ferrodolerite dike are dated at 2516 ± 12 Ma. Data on the U-Pb isotopic system of zircon from the gabbro-pegmatites and titanite from the aplites indicate that the late magmatic evolution of the Moncha Tundra Massif proceeded at 2445 ± 1.7 Ma, and the youngest magmatic events in the massif related to the Svecofennian orogeny occurred at 1900 ± 9 Ma. The data obtained on the Sm-Nd and Rb-Sr isotopic systems and the distribution of trace elements and REE in rocks of the dike-vein complex of the massifs provide insight into the composition of the sources from which the parental magmas were derived. The high-Ti ferrodolerites were melted out of a deep-sitting plume source that contained an asthenospheric component. The ferrodolerites were derived from a mantle MORB-type source that contained a crustal component. The parental melts of the gabbro-dolerites were melted out of the lithospheric mantle depleted in incompatible elements after Archean crust-forming processes above an ascending mantle plume, with the participation of a crustal component. The gabbro-dolerites and the rocks of the layered complex of the Moncha Tundra Massif exhibit similar geochemical characteristics, which suggest that their parental melts could be derived from similar sources but with more clearly pronounced crustal contamination of the parental melts of the rocks of the massif itself. The geochemical traits of the gabbro-pegmatites are thought to be explained not only by the enrichment of the residual magmas in trace elements and a contribution of a crustal component but also by the uneven effect of sublithospheric mantle sources. The aplites were derived from a sialic crustal source.     

藏北羌塘早古生代岩浆作用及其地质意义

藏北羌塘早古生代岩浆作用及其构造演化对研究青藏高原早期演化历史以及羌塘盆地基底性质结构等具有重要科学意义.本文在综述前人研究基础上,系统总结了藏北羌塘地区早古生代岩浆岩的时空分布特征及年代学格架,初步探讨了青藏高原早古生代构造-岩浆事件对冈瓦纳大陆北缘构造演化以及羌塘盆地基底属性的约束.羌塘地区早古生代岩浆岩主要分布在...     

地壳根、造山热与岩浆作用

简要讨论了近年来造山带及其岩浆作用研究的主要进展。造山带流变学结构与造山热和岩浆作用有着密切的耦合关系。年轻的山带往往存在地壳根,但古老的山带地壳根是否存在,取决于造山带的热状态和榴辉岩化的强度,只有缺乏流体和冷的造山带才保留有地壳根,例如古生代的南乌拉尔山和北美前寒武纪的南Trans-Hudson造山带。造山带的伸展塌陷往往伴随着幔源岩浆底侵、地壳软化、隆升和强烈岩浆作用。由于地幔浮力和造山热的作用,一些山带具有高的海拔和薄而热的地壳,属于具有长期活动性的构造带。研究表明,这些具有长期活动性的构造带,是建立在以前形成的热的、软化了的弧后区内。中国昆仑—秦岭—大别造山系北缘,古生代时期发育了与俯冲有关的弧岩浆带,而南缘发育了相近时代的与弧后伸展有关的双峰式岩浆带,构成古生代双岩浆带。该造山系早中生代的造山作用,就是在南缘的古生代弧后岩浆带基础上发展起来的。因此,该双岩浆带提供了造山热控制复合造山作用的实例。     

Chronology and tectonic implications of Neoproterozoic blocks in the South Qinling Orogenic Belt,Central China

The Qinling Orogenic Belt (QOB) located between the North China Craton (NCC) and the Yangtze Craton (YZC) is composed of the North Qinling Belt (NQB), the South Qinling Belt (SQB) and the northern margin of the YZC. Detailed geological and geochronological investigations have revealed distinct Neoproterozoic blocks of various scales in the middle and western segments of the SQB, including the Madao block (MDB), Mihunzhen intrusion (MHI), Zhenggou block (ZGB), and Lengshuigou block (LSB) which constitute an east-west trending Neoproterozoic uplift zone of the basement continental blocks. These blocks are mainly composed of four lithological groups. Group #1 consists mainly of diorites in the LSB, the zircons from which yield a weighted mean ²⁰⁶Pb/ ²³⁸U age of ca. 941 Ma. Group #2 is chiefly composed of hornblende gabbros and diorites in the MHI and LSB, which were formed at ca. 885 Ma. Group #3 comprises massive diorites, quartz diorite, tonalites, granodiorites, and monzogranites in the MDB, MHI, ZGB and LSB, which were emplaced during ca. 785–740 Ma. Group #4 is composed of hornblende gabbros with an emplacement age of ca. 667 Ma in the ZGB.Detailed whole-rock geochemical and zircon Hf isotopic studies reveal the following: (1) The diorites of Group #1 were produced by partial melting of depleted mantle which was enriched by slab-derived melts, with the parental magmas contaminated by crustal materials. (2) The gabbros of Group #2 were derived from the partial melting of depleted mantle enriched by slab-derived melts and the diorites are the fractional crystallization products of the gabbroic magmas. (3) Group #3 which can be further sub-divided based on lithological assemblages and zircon Hf isotopic features into two subgroups, one representing massive diorites, quartz diorite, tonalites, granodiorites, and monzogranites (DTGMs) and the other composed of gneissic quartz diorites and granodiorites. Among these, the DTGMs were derived through magma mixing between melts derived from the depleted mantle wedge altered by slab-derived fluids and melts from juvenile sources, which subsequently underwent amphibole-dominated fractionation, whereas the gneissic granitoids formed through partial melting of thickened lower crust contaminated by depleted mantle melts. (4) The gabbros of Group #4 originated from a depleted lithospheric mantle that was enriched by slab-derived melts and fluids with contribution of asthenospheric mantle-derived materials. In conjunction with data from previous studies on the Neoproterozoic blocks in the SQB and basement blocks in the northern margin of the YZC, our new geological, geochronological and geochemical data suggest a large Neoproterozoic uplift zone in the SQB, which was destructed by Paleozoic to Mesozoic magmatism and deformation. The Neoproterozoic uplift zone of the SQB might have been separated from the northern margin of the YZC during the formation of the Mianlue Ocean, and might have evolved under an active continental margin setting and subsequent continental rift setting accompanied by significant crustal growth. The magmatism also resulted in the formation of important Neoproterozoic ore deposits and supplied the material sources for some of the major Mesozoic ore deposits.     

Age and geological position of the Okunyovo rare-metal ore magmatic complex (Western Sayan Mountains)

Geological, geochemical, and geochronological studies of rocks have been performed at the Okunyovo rare-metal ore magmatic complex (Western Sayan Mountains) composed of alkali granites and related F–Be mineralization. The geological data obtained and the features of the geochemical similarity of granite and ore mineralization identified are indicative of their genetic relation. Riebeckite from two granite samples has been subjected to ⁴⁰Ar/³⁹Ar analyses. The estimated age values of 481.5 ± 2.7 Ma and 486.5 ± 5.8 Ma are consistent within the limits of error and determine the age of Okunyovo alkali granite within 481–486 Ma ago. These data have made it possible to relate the studied rock and ore formation to the development of the Early Paleozoic ASFR controlled by the mantle plume. Along with the Okunyovo rare-metal granite massif, within the Sayan Region of the magmatic province, there are a number of Early Paleozoic alkali massifs related to the Aryskan, Raduga, and Kazyr rare-metal deposits. They are located in the conjugation area of the Western and Eastern Sayan ridges, in the Early Paleozoic Eastern Sayan rare-metal magmatism zone specialized in Be, W, Mo, Zr, Nb, and REEs.     

Age and source material of the Kingash ultramafic-mafic massif,East Sayan

The REE distribution patterns and Nd whole-rock and mineral isotope ratios of the Kingash ultramafic-mafic massif enabled us to propose a multistage history for its evolution at 1410 and 875 Ma. These stages reflect the magmatic evolution of the Siberian paleocontinent margin during the Late Precambrian. The age of metamorphism of the massif during collision and accretion in the Early Paleozoic (∼500 Ma) was obtained based on a Sm-Nd mineral isochron from rheomorphic veined albitite. The Nd and Sr isotopic compositions of rocks from the Kingash massif suggest mantle sources for picritic and basic magmas, which are thought to have originated by mixing of different proportions of depleted (PREMA or DM) and enriched (EM) melts. The initial isotope ratios of the parental melts transformed during interaction with Sr-rich material from the host metasedimentary complexes.     

Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica

The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively. Received: 31 March 1998 / Accepted: 3 May 1999