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.     

Middle Paleozoic basic magmatism of the northwestern Vilyui Rift: Composition, sources, and geodynamics

Middle Paleozoic magmatism at the eastern Siberian platform was related to riftogenic processes, which were most clearly expressed in the Vilyui Rift and led to the formation of rift depressions filled with sedimentary-volcanogenic rocks and extended basaltic dike belts in rift shoulders. Two fields of diamondiferous kimberlites were found along with basaltic dikes in the Vilyui-Markha dike belt surrounding rift in the northwest. Active subalkali basaltic magmatism predated the emplacement of kimberlite bodies, which occasionally (Nyurba pipe) are cut by dikes of potassium alkali basalts. Based on geochemical and Sr-Nd isotopic characteristics, deep-seated sources were determined for the intrusive and volcanic basalts of the northwestern shoulder of the Vilyui rift. The REE distribution patterns of the studied rocks normalized to the primitive mantle are close to that of OIB, except for somewhat higher HREE. In the diagrams of indicator ratios of trace and rare-earth elements, the basalts are also plotted in the OIB field, being located between the end member of plume composition (FOZO) and enriched mantle sources. The rocks have positive ε_Sr (+3.5 and +28.6) and ε_Nd (+1.3 and +5.3). In a diagram ε_Nd(T)-ε_Sr(T), two fields with distinct content of radiogenic Sr are distinguished, which can be regarded as derived by mixing of the moderately depleted PREMA-type mantle and a source enriched in radiogenic Sr. Available isotope-geochemical data confirm that OIB type basalts of the region were generated by plume activity. The geodynamic setting of Middle Paleozoic magmatism and rifting in the eastern part of the Siberian platform is considered in light of plume-lithosphere interaction. The sequence of tectonomagmatic events during evolution of the Vilyui rift is consistent with the model of plume-lithosphere interaction or the model of active rifting.     

Strike-slip tectonics and subalkaline mafic magmatism in the Early Paleozoic collisional system of the western Baikal region

We discuss strike-slip tectonics as the key agent in the formation of the Early Paleozoic (Caledonian) collisional system of the western Baikal region. This tectonic setting implies existence of local syncompressional extension, with the ensuing conditions for mantle drainage and magmatism. Lower-middle crust collisional complexes exposed in the Olkhon area of the western Baikal region provide a record of synmetamorphic subalkaline-mafic magmatism associated with the early synorogenic collapse of the Olkhon collisional system, a part of the Central Asian collisional-accretionary belt.     

Comparative characteristics of Middle Paleozoic and Late Riphean plume magmatism of the Siberian platform

Based on isotopic and geochemical data for Late Riphean dikes and sills and for Devonian dolerite dikes and basalt covers within the Sette-Daban rift in the western part of the Siberian platform, we proved conceptions about the participation of various deep sources in their formation. The inverse correlation in Devonian basites between concentrations of Nb, light rare earth elements, and a number of other highly incompatible elements on the one hand and Zr, Y, and other moderate incompatible elements, including heavy rare earth elements on the other hand allows us to assume that two sources participated in the formation of melts. The source of dolerites is close to the EMORB type, and the source of basalts is close to the OIB type. The compositions of Riphean rocks correspond to a trend for which magma formation occurred with the participation of a source with characteristics between NMORB and EMORB and also a component typical of subduction zones. The data obtained imply associate formation of basites with the influence of mantle plumes on the lithosphere of the southeastern part of the Siberian craton in the Late Riphean and Middle Paleozoic. In the Riphean the plume mantle was composed of a moderately depleted mantle of the EMODB type and a mantle with the composition close to the above-subduction mantle, which was metosomatically changed under the influence of fluid water, which caused the appearance of a Nb and Ta deficit in melting products. The isotopic characteristics Nd(¹⁴⁷Sm/¹⁴⁴Nd = 0.165 and ɛ_Nd(T) ∼ 2.3–4.7) of rocks show the moderate depleted nature of these sources.     

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.     

Precambrian mafic magmatism in the Singhbhum craton,eastern India

The Singhbhum craton has a chequred history of mafic magmatism spanning from early Archaean to Proterozoic. However, lack of adequate isotopic age data put constraints on accurately establishing the history of spatial growth of the craton in which mafic magmatism played a very significant role. Mafic magmatism in the craton spreads from ca.3.3 Ga (oldest “enclaves” of orthoamphibolites) to about 0.1 Ga (‘Newer dolerite’ dyke swarms). Nearly contemporaneous amphibolite and intimately associated tonalitic orthogneiss may represent Archaean bimodal magmatism. The metabasic enclaves are appreciably enriched and do not fulfill the geochemical characteristics of worldwide known early Archaean (>3.0 Ga) mafic magmatism. The enclaves reveal compositional spectrum from siliceous high-magnesian basalt (SHMB) to andesite. However, the occurrence of minor depleted boninitic type within the assemblage has so far been overlooked. High magnesian basalt with boninitic character of Mesoarchaean age is also reported in association with supracrustals from southern fringe of the granitoid cratonic nucleus. The subcontinental lithospheric mantle (SCLM) below the craton is conjectured to have initiated during the early Archaean. Significantly, recurrence of depleted magma types in the craton is observed during the whole span of mafic igneous activity which has been vaguely related to “mantle heterogeneity”, although the alternative model of sequential mantle melting is also being explored. The Singhbhum craton includes the Banded Iron Formation (BIF) associated mafic lavas, MORB-like basic and komatiitic ultrabasic bimodal volcanism — documented as Dalma volcanics, Dhanjori lavas, and the Proterozoic Newer dolerite dykes. Three different types of REE fractionation patterns are observed in the BIF-associated mafic lavas. These are the REE unfractionated type is more depleted than N-MORB and some lavas with boninitic type of REE distribution. MORB-like basic and komatiitic ultrabasic (Dalma volcanics) are emplaced within the Proterozoic Singhbhum Basin (PSB). The vista of magmatism in the basin was controlled by a miniature spreading centre represented by the mid-basinal Dalma volcanic ridge. The volcano-sedimentary basinal domain of Dhanjori emerged at the interface of two subprovinces (viz. the mobile volcano-sedimentary belt of PSB and rigid granite platform) under unique stress environment related to extensional tectonic regime. Trace element distribution in Dhanjori lavas is remarkably similar to that in PSB minor intrusions and lavas (except a Ta spike in the latter). The Proterozoic Newer dolerite dykes within Singhbhum nucleus manifest an unusually wide spam of intrusive activity (ca 2100 Ma to 1100 Ma) and unexpectedly uniform mantle melting behaviour.     

科克苏-穹库什太古生代构造-岩浆作用及其对西南天山造山时代的约束

天山造山带是古生代多期碰撞增生作用的产物,其确切的造山时代是当前争议较多的热点问题.分布在西南天山的科克苏-穹库什太剖面经历了复杂的构造变形,最明显的两期变形事件分别为朝北的推覆作用和NE-SW韧性走滑作用.本剖面可分为伊犁岩浆岛弧、伊犁结晶基底、高压变质杂岩三个岩石-构造单元.野外可见黑云母花岗闪长岩侵入到绿片岩相变质岩中,岩脉切穿绿片岩中面理构造.通过锆石U-Pb LA-ICPMS测年,科克苏剖面钾长花岗岩的年龄为341±6Ma和338±8Ma,穹库什太黑云母花岗闪长岩的年龄为313±4Ma.其中,花岗闪长岩发生了黑云母定向排列,通过对黑云母进行^40 Ar/^39 Ar测年,获得坪年龄为263.4士0.6Ma,表明该花岗闪长岩受过后期热事件的干扰.地球化学分析表明,黑云母花岗闪长岩属于钙碱性系列,Nb和Ta含量低而Rb,Ba和TH含量很高,与俯冲作用有关的岛弧岩浆岩地球化学组成非常相似.结合前人对该地区高压变质岩、花岗岩和火山岩的研究成果,本文提出,西南天山俯冲-碰撞造山作用发生在晚石炭世之前,研究区后碰撞区域走滑作用标志着西南天山碰撞造山作用在二叠纪之前全部结束.     

Paleozoic and early mesozoic magmatism manifestations in the early Precambrian structure of the South Yenisei Ridge

This study characterizes some issues of the Paleozoic and Mesozoic tectonomagmatic evolution of Precambrian structures from the southwestern margin of the Siberian craton. The relationship between the Devonian and Triassic magmatic events is demonstrated from the example of the Severnaya rift-related structure, South Yenisei Ridge. U-Pb SHRIMP dating yielded ages of 387 ± 5 Ma for leucogranites and 240 ± 3 Ma for the overlying alkaline trachytes. These ages show good agreement with Ar-Ar geochronological data (392–387 Ma) obtained for micas from paragneisses and leucogranite dykes of the Yenisei suture zone, the extension of which is superimposed by the studied rift-related structure. The previous geological evidence and the Devonian age estimate first obtained for magmatic rocks of the Yenisei Ridge allow us to interpret the studied leucogranites as products of Devonian continental rifting, similar to volcanic and intrusive rocks of the North Minusa depression and Agul graben. Like other localities within the western margin of Siberian craton, the formation of Triassic alkaline rocks may be related to the Siberian superplume activity.     

Precambrian mafic magmatism in the Western Dharwar Craton,southern India

Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1–3.3 Ga) and Dharwar Supergroup (2.6–2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0–2.2 and 1.6 Ga) constitute the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs. The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0–2.2 and 1.6 Ga) occur around Tiptur and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks, chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the classical greenstone terrains of the world.     

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.     

新疆南天山东段早石炭世——早二叠世花岗岩类及其对南天山洋盆闭合时间的约束

选择南天山东段早石炭世—早二叠世典型花岗岩类,即阿訇开里得南岩体、盲起苏岩体、东泉戈壁岩体和库米什北岩体进行岩石学与地球化学对比研究。结果表明,这些岩体属于准铝质—过铝质高钾钙碱性系列花岗岩,富集Ba、Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Zr、Hf等高场强元素,稀土元素配分模式呈右倾型,富集轻稀土元素、亏损重稀土元素,铕出现中等负异常(δEu=0.52~0.96)。基于上述相似特征,并结合前人研究成果,认为它们形成于陆-陆碰撞环境。基于南天山造山带东西段高压变质岩、岩浆岩、沉积岩和蛇绿混杂岩等方面的岩石成岩年龄对比,认为南天山晚古生代构造演化过程具有区域差异性,碰撞作用在其东段发生于早石炭世,而在其西段发生于晚石炭世。南天山洋东段闭合时间早于西段。     

西南天山晚古生代后碰撞岩浆作用：以阔克萨彦岭地区巴雷公花岗岩为例

西南天山阔克萨彦岭地区巴雷公钾长花岗岩出露于蛇绿混杂岩南侧。地球化学特征显示,该岩石富碱（K2O＋Na2O为8.25～8.72％〉8％）,富钾（K2O／Na2O为1.34～1.56）,准铝质（A／CNK为0.94～1.05）,为高钾钙碱性-钾玄岩系列;岩石富集大离子亲石元素和轻稀土,亏损Sr、P等大离子亲石元素及Nb、Ti等高场强元素,具有中等的负铕异常（δEu=0.49～0.59）,为向A型花岗岩过渡的后碰撞高钾花岗岩特征;岩石的Nd／Th（1.64～3.19）、Th／U（5.95～7.11）、Nb／Ta（7.26～9.17）和高K2O／Na2O比值、低Sr／Ba比值特征表明,巴雷公花岗岩来源于中下地壳物质的部分熔融,残留相为斜长角闪岩。阴极发光图像显示,该岩石中锆石多呈完好的自形晶,具有岩浆锆石特有的韵律环带结构,LA-ICP-MS微区原位U-Pb定年结果表明该花岗岩的结晶侵位年龄为273±2Ma。综合南天山已有研究成果,推测南天山造山带后碰撞花岗质岩浆活动主要发生在282～259Ma之间,具有从高钾钙碱系列（282～266Ma）向碱性系列（266～259Ma）演化的特征,暗示了一个后碰撞阶段的造山带垮塌、陆壳连续伸展减薄的过程。巴雷公高钾花岗岩应为南天山造山带碰撞造山峰期变质后垮塌过程中的第一个阶段或碰撞造山到后造山的一个转折阶段的中下地壳熔融的产物,指示南天山西段古洋盆在中二叠世以前已经闭合,西南天山已进入后碰撞演化阶段,代表了古亚洲洋南部的闭合和中亚南部增生造山作用的结束。这一认识为深入探讨中亚后碰撞岩浆作用的时限和机制提供了新的约束资料。     

Precambrian mafic magmatism of Shillong plateau,Meghalaya and their evolutionary history

The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low-pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im= +57 (α₉₅= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.     

中天山东段古生代淡色花岗岩的发现及其构造意义

在吐鲁番地区底坎尔南的中天山构造带,新发现含电气石和石榴石的淡色花岗岩侵入于富铝泥质片岩系中。SHRIMP U-Pb锆石年代学分析,表明淡色花岗岩的形成不晚于354±16Ma,同时还给出了600～1200Ma的原岩继承锆石年龄。淡色花岗岩的发现及其形成年龄的确定,表明中天山在354Ma前具有与陆缘弧背景相似的构造环境,并在354Ma前完成碰撞过程,为进一步研究古生代天山构造演化提供了新的证据。     

An uncommon episode of mafic magmatism at 1347 Ma in the Mesoproterozoic Telemark supracrustals, Sveconorwegian orogen—Implications for stratigraphy and tectonic evolution

The Mesoproterozoic Telemark supracrustals in southern Norway comprise two major assemblages of bimodal volcanic and clastic metasedimentary rocks. The older Vestfjorddalen supergroup evolved from A-type, ca. 1500 Ma continental felsic volcanism, via within-plate type basaltic volcanism, into open sea siliciclastic sedimentation, and produced an at least 5 km thick, quartzite-dominated sequence, the Vindeggen group. It overlies a basement formed by just slightly older, 1550–1500 Ma mature arc rocks. The younger, 1170–1140 Ma Sveconorwegian supergroup was characterized by bimodal volcanism, associated with plutonism, and with several intervening periods of clastic sedimentation. The metadiabase dated in this study cuts the Vindeggen group at the top of the older supergroup and is itself delimited by an unconformity at the bottom of the younger supergroup. The 1347 ± 4 Ma age, obtained by ID-TIMS analysis of zircon, defines a mimimum age for deposition of the Vindeggen group. The age is unique in the regional context but in general terms it fits a pattern of episodic and locally intense magmatism that characterized the Mesoproterozoic development of the margins of Proto-Baltica and -Laurentia and has been related to the evolution of a long-lived convergent margin. The similarities between some of these terranes and distinctiveness from others, in both orogens, may indicate outboard evolution of the Telemarkia and Frontenac terranes before their aggregation within the Sveconorwegian–Grenvillian orogen.     

南天山北缘榆树沟变质基性超基性岩的地球化学及其成因机制

榆树沟变质基性－超基性岩带出露于塔里木板块与哈萨克斯坦板块之间的南天山北缘，主要由变质橄榄岩和变质基性岩组成。变质橄榄岩富相容元素Cr、Co和Ni，贫不相容元素，太离子亲石元素Ba、Rb和Sr含量较低，与世界典型蛇绿岩相似，代表了地幔残留物特征。REE分布模式为LREE亏损型，REE含量小于或等于2．5倍球粒陨石，类似于阿尔卑斯型变质橄榄岩，显示榆树沟的变质橄榄岩是原始地幔岩部分熔融萃取出玄武岩后的残留物。变质基性岩绝大部分为LREE亏损型，类似于N－MORB。所有样品均以富集Nb和Ta、高场强元素不分异，以及微量元素含量低为特征，批示岩浆源区总体上类似于MORB，Nb、Ta富集可能与OIB型源区有关，Nd、Sr同位素特征也显示其具有OIB型源区特征。综合分析认为，榆对沟变质基性岩石的岩浆可能经历了两个阶段的演化过程，即上地幔底部或下地幔顶部的OIB型原始岩浆形成阶段和软流圈地幔亏损阶段。     

Type sections,stratigraphy and problems relating to structure and magmatism of Karelides

The karelides are Karelian volcanic and sedimentary rocks in various stages of metamorphism which form broad northwesterly trending belts. These rocks occur in synclinoria between which are areas of the old granite-gneiss basement of the extensive Karelian massif. Details of the stratigraphic sequence, structure, magmatic and metamorphic events, and geologic history are described. Radioactive age determinations are interpreted within the framework of the inferred geologic history. This is a timely supplement to the recently published Volume 1 of The Precambrian, edited by K. Rankama. — C. G. Tillman.