The genesis of the Archaean Welcome Well volcanic complex,Western Australia

The Welcome Well volcanic complex east of Leonora, Western Australia, is interpreted to be the eroded remnant of an Archaean stratovolcano. Andesitic flows and intercalated mudflow deposits comprising the volcanic centre give way to coarse, poorly-sorted lithic wackes that were deposited in alluvial fans skirting the lower slopes or base of the subaerial volcanic edifice. These deposits are succeeded both laterally and vertically by fine-grained, subaqueous, turbiditic sediments that are intercalated with pillowed, tholeiitic basalts.There is a complete petrographic and geochemical gradation from porphyritic basalt through porphyritic andesite to porphyritic dacite. In general, the rocks show calc-alkaline patterns of elemental behaviour, consistent with fractionation of variable proportions of the modal minerals amphibole, plagioclase, clinopyroxene and Ti-magnetite. Among these minerals, amphibole appears to have assumed a major role in producing the geochemical characteristics of the high-Si andesites and dacites as evidenced by the behaviour of Zr, Nb, Y and REE. In order to account for the geochemical variability of the basalts and low-Si andesites, it is proposed that they differentiated from primitive basic parents which had a range of major and LIL element contents. The most plausible origin for the primary magmas involves shallow, hydrous melting of a LIL element-enriched mantle source over a significant pressure range.     

Basic lavas of the Archean La Grande Greenstone belt: Products of polybaric fractionation and crustal contamination

Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.     

Cretaceous complexes of the frontal zone of the Moneron-Samarga Island arc: Geochemical data on the basalts from the deep borehole on Moneron Island, the Sea of Japan

The variations of petrogenic oxides and trace elements have been studied in the Cretaceous volcanic rocks recovered by a deep borehole from the depth interval of 1253–4011 m on Moneron Island. The volcanic section is subdivided into two complexes: the Early Cretaceous and Late Cretaceous. The rocks of the Early Cretaceous Complex occur below 1500 m. Chemically, they belong to low-potassium island arc tholeiites, and their trace element distribution suggests their formation in a suprasubduction mantle wedge under the influence of water fluids that were subsequently released from subducted sediments and oceanic plate during the dehydration of subducted sedimentary rocks and oceanic basalts and, finally, mainly from basalts. The Early Cretaceous basalts from the borehole are interpreted as ascribing to the frontal part of the Moneron-Samarga island arc system. The volcanic rocks of the Late Cretaceous Complex are situated at depths above 1500 m. They also were formed in a suprasubduction setting, but already within the East Sikhote-Alin continental-margin volcanic belt that was initiated after the accretion of the Moneron-Samarga island arc system to the Asian continent. The island-arc section of the Moneron borehole contains basaltic andesite dikes, which are geochemically comparable with the Early-Middle Miocene volcanic rocks of Southwestern Sakhalin.     

滇东南建水地区高镁火山岩包体的成因和构造背景

在滇东南建水地区发现产于峨眉山玄武岩中的高镁火山岩包体,这对于地幔柱的形成演化具有重要研究意义.对这些包体进行了锆石U-Pb年代学、地球化学和岩矿分析.高镁火山岩包体具斑状结构,致密块状构造,斑晶主要为贵橄榄石和透辉石.13颗锆石U-Pb LA-ICP-MS加权平均年龄为259±2Ma(MSWD=1.9),显示与寄主岩石同期形成.包体岩石具有高镁(Mg~#=68～75)、低硅(SiO_2=45.11%～45.93%)特征,轻稀土元素(LREE)、高场强元素(HFSE)富集而重稀土元素(HREE)亏损,属于亚碱性、拉斑玄武岩系列,具有板内玄武岩(IPB)特征.火山岩包体的原始岩浆起源于石榴子石二辉橄榄岩低程度部分熔融的产物,岩浆演化过程中发生了橄榄石和单斜辉石的分离结晶作用,在侵位上升过程中未受明显的地壳混染作用.该高镁火山岩的存在,显示地幔柱除了垂直上升运动外,在地球深部不同的边界还有多次侧向扩展移动,表明滇东南晚二叠世存在峨眉山地幔柱的一个分支-地幔枝活动.     

The origin of low-K rhyolites from the mariana frontal arc

Low-K rhyolites and overlying 2-pyroxene andesites, both of Late Eocene age, comprise the oldest volcanic units exposed on Saipan. The mineralogy and geologic setting of these rocks indicate they were erupted in a volcanic arc setting. The presence of andesite and lack of basalts of similar age suggest that the rhyolites are not part of a typical bimodal (basalt/rhyolite) suite.Major and trace element data indicate the Saipan andesites were not parental to the rhyolites. Out of various models evaluated for derivation of the rhyolites, the most reasonable involves crystal fractionation of a boninite series andesite that was very depleted in LIL elements. This andesite probably evolved from more mafic magmas which in turn were derived from the sub-arc mantle. Isotopic data suggest the mantle source for these magmas may have contained a minor seawater component.     

Age relationships in supracrustal sequences of the northern part of the Murchison Terrane,Archaean Yilgarn Craton,Western Australia: A combined field and zircon U–Pb study

Mapping carried out in the northern Murchison Terrane of the Archaean Yilgarn Craton, Western Australia, shows that correlation of units between isolated greenstone belts is very difficult and an informal stratigraphic subdivision is proposed where the greenstone sequences have been divided into a number of assemblages. The assemblages may not necessarily be time equivalent throughout the region. The lower units (Assemblages 1–3) consist of ultramafic, mafic and intermediate volcanic rocks deposited without significant breaks in volcanism. Felsic volcanic packages (Assemblage 4) are conformable with underlying units, but are spatially restricted. Discordant units of graphitic sedimentary rocks are developed along major crustal structures (Assemblage 5). SHRIMP and conventional U–Pb study of zircons reveal that felsic volcanic rocks of Assemblage 4 in the Dalgaranga Greenstone Belt were emplaced at 2747 ± 5 Ma, whereas those in the adjacent Meekatharra — Mt Magnet Greenstone Belt range in age from 2762 ± 6 to 2716 ± 4 Ma. The age of emplacement of a differentiated gabbro sill in the Dalgaranga Greenstone Belt at 2719 ± 6 Ma places a maximum age on major folding in the belt. The presence of 2.9–3.0 Ga inherited zircons in some of the felsic volcanic rocks indicates contamination with, or reworking of, underlying 3 Ga sialic crust. This distinguishes the Murchison Terrane from the central parts of the Eastern Goldfields terranes to the south, where there is no evidence for a 3 Ga imprint in zircons from volcanic or granitic rocks, and also from the Narryer Gneiss Terrane to the north and west, which is composed of older gneisses and granitoids. The ca 2.76–2.71 Ga felsic volcanism in the Murchison Terrane is significantly older than 2.71–2.67 Ga felsic volcanism in the Eastern Goldfields lending support to models advocating assemblage of the craton by terrane accretion.     

天山石炭纪火山岩系中含有富Nb岛弧玄武岩吗?

中国西北部天山石炭纪—早二叠世裂谷火山作用代表了一个新近被认可的大火成岩省。有人认为,在天山石炭纪火山岩系中发育有富Nb岛弧玄武岩、埃达克岩和高镁安山岩组合。然而,该岩石组合具有与典型富Nb岛弧玄武岩、埃达克岩和高镁安山岩不同的化学和同位素特点,表明其并非是岛弧岩石组合。这一推断的岛弧岩石组合实际上是大陆板内火山岩组合,由未遭受地壳混染、受到地壳轻微混染和遭受地壳强烈混染的大陆火山岩组成。     

Early Palaeozoic high-Mg basalt-andesite suite in the Duobaoshan Porphyry Cu deposit,NE China: Constraints on petrogenesis,mineralization, and tectonic setting

High-Mg basalt-andesite suites are extremely rare in the modern Earth but genetically important for indicating essential crust-mantle interactions, ascertaining critical geodynamic settings, and understanding the formation of porphyry copper deposits. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating and geochemistry document an early Palaeozoic high-Mg basalt-andesite suite from the Duobaoshan porphyry Cu deposit, eastern Central Asian Orogenic Belt (CAOB). Dating results reveal that the Duobaoshan high-Mg basalt and andesite erupted ca. 506 Ma and ca. 485 Ma, respectively. These high-Mg samples are typical subduction-related volcanic rocks with SiO₂ contents of 47.73 to 55.16 wt%, high MgO (6.08 to 10.82 wt%), and high Mg# (58 to 67) and feature enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The samples have juvenile whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70272 to 0.70451, ɛ_Nd(t) of 4.8 to 8.7 (average 7.23), and zircon ɛ_Hf(t) of 7.3 to 15.9. Additionally, they have high Sr (average 619.36 ppm) and low Y (average 11.92 ppm) and Yb (average 1.21 ppm) contents that show affinity with adakitic high-Mg basalts and andesites worldwide. These high-Mg, depleted mantle-like isotopic and adakitic geochemical features imply a depleted mantle source variably assimilated by slab-derived melts under a sustained subduction tectonic setting. We also propose that the Duobaoshan high-Mg basalt-andesite suite, as the parental source magma, fertilized the overlying Duobaoshan porphyry Cu deposit by providing water, copper, and sulphur and high oxygen fugacity. A comprehensive comparison of the post-ore volcanic rocks shows that they might have originated from the slab-derived fluid metasomatized depleted mantle wedge, which had different properties from the mantle that produced the pre- and syn-ore volcanic intrusive rocks. The post-ore volcanic rocks underwent little crustal evolution en route to the surface during a reworked subduction event, which indicates a relatively immature island arc environment.     

Formation and emplacement ages of the SSZ‐type Neotethyan ophiolites in Central Anatolia,Turkey: palaeotectonic implications

Isolated outcrops of ophiolitic rocks, termed the Central Anatolian Ophiolites, are found as allochthonous bodies in the Central Anatolian Crystalline Complex, that represent the metamorphosed passive northern edge of the Tauride–Anatolide Platform, central Turkey. In terms of pseudostratigraphic relationships of the magmatic units and their chemical designation, the Central Anatolian Ophiolites exhibit a supra‐subduction zone (fore‐arc) setting within the Vardar–İzmir–Ankara–Erzincan segment of the Neotethys. The epi‐ophiolitic sedimentary cover of the Central Anatolian Ophiolites is generally characterized by epiclastic volcanogenic deep‐sea sediments and debris flows intercalated with pelagic units. The richest and most significant planktonic foraminiferal association recorded from the lowest pelagic members infer a formation age of early–middle Turonian to early Santonian. K/Ar ages of post‐collisional granitoids (81–65 Ma) intruding the basement rocks as well as the Central Anatolian Ophiolites suggest a post‐early Santonian to pre‐middle Campanian emplacement age. The marked high volume of epiclastic volcanogenic sediments intercalated with the pelagics of the Central Anatolian Ophiolite is suggestive of rifting in a marginal sea adjacent to a volcanic arc. Penecontemporaneous tectonism is reflected in repetitions in the stratigraphy and in debris flows, which result from major slides and mass‐gravity reworking of pre‐existing units and of arc‐derived volcanics and sediments. Correlating the rock units and formation/obduction ages of the Central Anatolian Ophiolites with further supra‐subduction zone type ophiolites in the eastern (Turkey) and western (Greece) parts of the Vardar–İzmir–Ankara–Erzincan segment of Neotethys we conclude that the intraoceanic subduction in the east is definitely younger and the closure history of this segment is more complex than previously suggested. Copyright © 2000 John Wiley & Sons, Ltd.     

Nd-isotope and geochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite suite of rocks in the southern Bastar craton,central India: petrogenesis and tectonic significance

ABSTRACT

Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger T_DM model ages than the high-Ca boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean–Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.     

松辽盆地东南隆起区下白垩统营城组三段火山岩岩性、岩相序列

通过大比例尺野外岩性岩相填图、掌子面二维岩性岩相描述和详细岩矿鉴定,研究营城组三段内幕。本区营三段自下而上岩性序列表现为2个中基性到中酸性的火山岩旋回：①下部为石英安山岩、安山岩、安山质集块熔岩、安山质集块岩、安山质角砾岩和安山质角砾凝灰岩,向上过渡为砂质凝灰岩和英安质凝灰熔岩;②上部为玄武安山岩和玄武质集块熔岩,向上过渡为英安岩、珍珠岩、英安岩、英安质凝灰熔岩、英安质沉凝灰岩和英安岩。旋回①岩相纵向序列：溢流相下部亚相、火山通道相火山颈亚相、爆发相空落亚相、火山沉积相再搬运亚相、爆发相热碎屑流亚相。旋回②岩相纵向序列：溢流相上部亚相和下部亚相、火山通道相火山颈亚相、溢流相下部亚相、侵出相内带亚相、溢流相下部亚相、爆发相热碎屑流亚相、火山沉积相再搬运亚相、溢流相下部亚相。营三段火山岩发育于松辽盆地断陷末期,是盆地断陷转为坳陷过程的重要岩石记录。     

Variations in andean andesite compositions and their petrogenetic significance

Three linear zones of active andesite volcanism are present in the Andes — a northern zone (5°N–2°S) in Colombia and Ecuador, a central zone (16°S–28°S) largely in south Peru and north Chile and a southern zone (33°S–52°S) largely in south Chile. The northern zone is characterized by basaltic andesites, the central zone by andesite—dacite lavas and ignimbrites and the southern zone by high-alumina basalts, basaltic andesites and andesites. Shoshonites and volcanic rocks of the alkali basalt—trachyte association occur at scattered localities east of the active volcanic chain,The northern and central volcanic zones are 140 km above an eastward-dipping Benioff zone, while the southern zone lies only 90 km above a Benioff zone. Continental crust is ca. 70 km in thickness below the central zone, but is 30–45 km thick below northern and southern volcanic zones. The correlation between volcanic products and their structural setting is supported by trace element and isotope data. The central zone andesite lavas have higher Si, K, Rb, Sr and Ba, and higher initial Sr isotope ratios than the northern or southern zone lavas. The southern zone high-alumina basalts have lower Ce/Yb ratios than volcanics from the other zones. In addition, the central zone andesite lavas show a well-defined eastward increase in K, Rb and Ba and a decrease in Sr.Andean andesite magmas are a result of a complex interplay of partial melting, fractional crystallization and “contamination” processes at mantle depths, and contamination and fractional crystallization in the crust. Variations in andesite composition across the central Andean chain reflect a diminishing degree of partial melting or an increase in fractional crystallization or an increase in “contamination” passing eastwards. Variations along the Andean chain indicate a significant crustal contribution for andesites in the central zone, and indicate that the high-alumina basalts and basaltic andesites of the southern zone are from a shallower mantle source region than other volcanic rocks. The dacite-rhyolite ignimbrites of the central zone share a common source with the andesites and might result from fractional crystallization of andesite magma during uprise through thick continental crust. The occurrence of shoshonites and alkali basalts eat of the active volcanic chain is attributed to partial melting of mantle peridotite distant from the subduction zone.     

华北克拉通中东部新太古代晚期变质火山岩及动力学体制

热状态和壳幔岩浆作用是理解早期地壳形成演化动力学机制的关键.华北克拉通是世界范围内为数不多的保存有大量新太古代晚期(约26～25亿年)变质火山岩记录的克拉通之一,对揭示全球新太古代晚期壳-幔动力学演化过程具有重要的指示意义.在我们研究组近期关于华北克拉通中东部中新太古代热状态和地壳厚度研究基础上,本文收集并整理了726...     

Oceanic mafic rocks in the Eastern Alps

The Eastern Alps in Austria have been interpreted as a pile of thrust sheets resulting from the collision of two continental masses. The only remains of the ocean-floor which may once have separated these continents could be the highly deformed greenschists, metasediments and serpentinites found in the lower thrust sheets. To test this hypothesis, a total of sixty mafic rocks from the Großglockner, Mooserboden, Fusch, Hochtor, Matrei Zone and Strobl localities have been analysed for the stable trace elements, Ti, Zr, Y, Nb and Cr, and the less stable elements K, Rb, and Sr. Visual and statistical comparison of the stable elements with known magma types reveals that five of the sample groups classify clearly as tholeiitic ocean-floor basalts, while one group, the Fusch locality, classifies as within-plate (probably ocean island) basalts. It is suggested that the tectonic units containing such rocks comprise a mélange of disrupted oceanic crust, upper mantle and seamounts, pelagic sediments and continental margin sediments. The rocks may have formed in a large ocean basin, rather than a marginal basin behind an island arc.     

Gold in the Neoproterozoic juvenile Bossoroca Volcanic Arc of southernmost Brazil: isotopic constraints on timing and sources

The Neoproterozoic Bossoroca juvenile Volcanic Arc of southernmost Brazil contains arc-related gold deposits. The Bossoroca gold deposit consists of veins and stockworks of quartz-gold ores with minor pyrite, chalcopyrite, galena and tellurides. Carbonate, chlorite, sericite and tourmaline are the main gangue minerals. The ore shoots are contained in calc-alkaline pyroclastic andesites and dacites with minor basalts and epiclastic rocks of the Campestre Formation. SHRIMP U/Pb investigations of zircon show that the island-arc volcanogenic sequence was formed ca 757 m.y. ago in the early Brasiliano Cycle and metamorphosed into transitional greenschist/amphibolite facies of low-pressure regional metamorphism at ca 700 Ma. Nearby, the post-tectonic São Sepé Granite was intruded into the volcanic arc at ca 550 Ma. The mineralising fluids have been related either to metamorphism or to solutions derived from post-tectonic intrusive granites. Lead isotopic analyses, carried out on galena from the gold ore, on feldspar and total rock from the associated volcanic pile, and also on feldspar and total rock from the São Sepé Granite, indicate that gold mineralisation is related to the volcanogenic rocks, and that the deposit should be considered to be of an epizonal orogenic type.     

Stratigraphic and structural synthesis of the New England Orogen

The Spring Well volcanic complex in the Eastern Goldfields Province of Western Australia, is a relatively fresh and well exposed Archaean felsic volcanic centre that is preserved in a synclinal structure at the top of the local greenstone succession. Subaerial acid pyroclastic deposits and subordinate lava flows, intruded by anastomosing intermediate‐acid dykes and sills, comprise the near‐vent facies. In the distal regions of the centre, subaqueous crystal tuff and other tuff units are intercalated with epiclastic sediments.

Geochemical modelling indicates that the acid rocks are unlikely to have been derived by batch partial melting of probable crustal sources. However, differentiation from intermediate parents is compatible with the available geochemical data. The intermediate rocks, in turn, have critical geochemical characteristics comparable with all other studied intermediate calc‐alkaline rocks in the Yilgarn Block. Since it can be demonstrated that many of these rocks have an ultimate mantle source (through differentiation of LIL element enriched mafic primary magmas) it follows that such an origin is applicable in the Spring Well rocks. Therefore, it is concluded that the Spring Well volcanic complex represents a mantle‐derived, calc‐alkaline differentiation series, in which the more silicic members of the suite predominate. Apart from the diagnostic geochemical characteristics of these acid volcanic rocks, their spatial association with intermediate rocks distinguishes them from anatectic acid volcanic rocks that also occur in the greenstone sequences of the Yilgarn Block.     

Middle to late Archaean geology of the eastern Baltic shield,with a note on its similarity and contrast with the Archaean of southern India

The middle to late Archaean rocks of Kola and Karelia in the eastern Baltic shield consist of the Infracomplex overlain by the Saamian complex, and the Lopian greenstone belts. The Infracomplex which forms the basement is a polymigmatite, parts of which are at least 3100 Ma old. The Saamian in the central Belomorian region comprises granite gneiss, amphibolite, garnet-kyanite gneiss and high alumina gneisses which belong to the Keret, Hetolombina and Chupa suites. The Lopian greenstone belts ranging in age from 3000 to 2700 Ma are composed of peridotitic, pyroxenitic and basaltic komatiites, tholeiitic basalts, andesites, dacites and rhyolites, together with tuffs, graywackes and iron formations. Whereas there is a dominance of volcanic over sedimentary rocks in the greenstone belts of the Baltic shield, a significant proportion of detrital and chemogenic sedimentary rocks characterizes the Dharwar succession of approximately the same time span in the southern Indian shield. Association of mature and immature detrital sedimentary rocks with bimodal volcanic assemblages points to a back-arc setting for the Dharwar belts. This contrasts with the association of immature sediments with calc-alkaline volcanic rocks in the greenstone belts of the eastern Baltic shield, suggesting an island arc environment there.     

Stratigraphy and petrology of the Archaean Nsuze Group,northern Natal and southeastern Transvaal,South Africa

The Nsuze Group is the lower, dominantly volcanic, division of the Pongola Supergroup that accumulated on a sialic basement between 3.1 and 2.9 Ga. The Nsuze Group is subdivided into a lower sedimentary unit (800 m thick), a middle volcanic unit (± 7500 m thick) and an upper volcaniclastic-sedimentary unit (5–600 m thick). The predominant sediments in the lower unit are immature, medium- to very coarse-grained quartz wackes with thin intercalated lenses of quartz and arkosic arenites, and minor conglomeratic wackes. These sediments were deposited in a distal braided stream environment.There followed a major period of volcanism during which lavas showing a continuous spectrum of compositions from basalt to rhyolite were extruded subaerially. Flows of both different and similar compositions are complexly interfingered on both regional and local scales. As volcanism waned, pyroclastic and sedimentary rocks became dominant in the upper unit. The Nsuze Group is gently dipping and is metamorphosed to low greenschist grade.The Nsuze Group is significant in that it provides evidence for the existence of high-standing continental sialic crust in the southeastern part of the Kaapvaal province at ca. 3.0 Ga. Volcanism and sedimentation in the Pongola Supergroup are more typical of Proterozoic basins than of Archaean environments, despite their age. Komatiitic and high-Mg basalts were, however, being extruded in Zimbabwe contemporaneously with the Nsuze lavas.     

辽西早白垩世义县组火山夺的起源及壳幔相互作用

对燕山造山带辽西早白垩世义县组火山岩的Nd,Sr,Pb同位素分析，作者认为义县组火山岩起源于岩石圈地幔的部分熔融，岩浆在上侵过程中发生了结晶分异和同化混染作用，即AFC过程。与新生代汉诺坝玄武岩中的中生代镁铁质麻粒岩捕虏体和太古代片麻岩对比研究，发现义县组火山岩与这些镁铁质麻粒岩捕虏体有许多地球化学相似之处，而与长英质麻粒岩捕虏体和太古代各种片麻岩差别较大。作者认为早白垩世燕山板内造山带发生了强烈的岩石圈伸展作用，辽西义县组火山岩和汉坝新生代玄武岩中的镁铁质麻粒岩捕虏体均为这一构造背景下的产物，它们属于幔源岩浆喷发与大规模玄武zh质岩浆底侵作用形成的“同质异相体”。     

Geology and geochemistry of metabasalts of Shimoga schist belt,Dharwar Craton: implications for the late Archean basin development

The late Archaean Shimoga schist belt in the Western Dharwar Craton, with its huge dimensions and varied lithological associations of different age groups, is an ideal terrane to study Archean crustal evolution. The rock types in this belt are divided into Bababudhan Group and Chitradurga Group. The Bababudhan Group is dominated by mafic volcanic rocks followed by shallow marine sedimentary rocks while the Chitradurga Group is dominated by greywackes, pillowed basalts, and deep marine sedimentary rocks with occasional felsic volcanics. The Nb/Th and Nb/La ratios of the studied metabasalts of the Bababudhan Group indicate crustal contamination. They were extruded onto the vast Peninsular Gneisses through the rifting of the basement gneiss. The Nb/Yb ratios of high-magnesium basalts and tholeiitic basalts of Chitradurga Group suggest the enrichment of their source magma. Based on the flat primitive mantle-normalized multi-element plot with negative Nb anomalies and Th/Ta-La/Yb ratios, the high-magnesium basalts and tholeiitic basalts are considered to have erupted in an oceanic plateau setting with minor crustal contamination. The high-magnesium basalts and tholeiitic basalts formed two different pulses of same magma type, in which the first pulse of magma gave rise to high-magnesium basalts which were derived from deep mantle sources and underwent minor crustal contamination en route to the surface, while the second pulse of magma gave rise to tholeiitic basalts formed at similar depths to that of high-magnesium basalts and escaped crustal contamination. The associated lithological units found with the studied metavolcanic rock types of Bababudan and Chitradurga Groups of Dharwar Supergroup of rocks in Shimoga schist belt of Western Dharwar Craton confirm the mixed-mode basin development with a transition from shallow marine to deep marine settings.