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.     

The composition and sources of magmas of Changbaishan Tianchi volcano (China-North Korea)

The Changbaishan Tianchi volcano is the greatest stratovolcano within the bounds of the Late Cenozoic intraplate volcanic province of East Asia. Changbaishan Tianchi volcanic cone consists mostly of trachytes and pantellerites. It was found that the lavas composing the shield platform of Changbaishan Tianchi volcano are weakly differentiated basic rocks whose geochemical characteristics are generally similar. All the alkaline salic rocks composing the cone of the volcano are characterized by conformable normalized trace element patterns. The concentrations of rare-earth elements in these rocks are high and amount up to 1000 ppm. The character of the distribution of trace elements in the basic rocks of Changbaishan Tianchi volcano is close to that in the OIB-type basalts. Within the series from basalts to pantellerites, the rocks are enriched in REE and zirconium, but depleted in barium, strontium, and europium. According to the obtained geochemical data, it was shown that the rock series of Changbaishan Tianchi volcano, varying from basalts to trachytes and pantellerites comprises compositions geochemically interrelated by the processes of crystal fractionation. The parental magma for the rocks of the volcano was derived from plume sources of the same type as those of OIB and sources of the Late Cenozoic intraplate province of East Asia.     

冲绳海槽火山岩岩石系列的厘定及构造环境意义

冲绳海槽是一个处于弧后扩张作用早期的年轻的弧后盆地,是研究弧后扩张作用早期盆地演化和壳幔过程的天然实验室.随着调查研究工作的逐步展开和深入,也发现了一些新的、重要的、亟待解决的科学问题,而火山岩岩石系列归属的厘定又是其他研究工作的基础.在系统收集和整理迄今已有冲绳海槽火山岩资料的基础上,结合近期分析测试数据, 对冲绳海槽火山岩的岩石系列归属进行了重新厘定,探讨了火山岩的构造环境指示意义和浮岩与玄武岩之间的成因联系.研究结果表明:冲绳海槽火山岩分布具有以基性玄武岩和酸性(流纹)英安岩为主的双峰式特征,中性火山岩稀少,基性的玄武岩属于亚碱性系列的橄榄拉斑玄武岩,酸性浮岩可归属为亚碱性岩系的流纹英安岩或流纹岩;在构造环境判别上,冲绳海槽玄武岩表现出大洋中脊和岛弧构造环境的特点,既有别于大洋中脊扩张中心,也有别于成熟型弧后盆地,呈现出弧后早期扩张阶段盆地独特的构造环境特征;广泛分布于冲绳海槽的酸性浮岩表现出一定的岛弧环境的特点;酸性浮岩与玄武岩具有同源性,酸性岩是基性的玄武质岩浆经不同程度结晶分异和同化混染作用的产物.      

南秦岭元古宙板内火山作用特征及构造意义

南秦岭元古宙火山岩主要由两大类岩石构成，一类为SiO245%-57%的基性火山岩系，另一类为SiO267%-78%的酸性火山岩系，主要岩石类型为细碧岩、玄武岩和石英角班岩、流纹岩。基性火山岩整体上属拉斑玄武岩系列，酸性火山岩属钙碱系列。火山岩强烈富集稀土元素，尤其是轻稀土元素，酸性火山岩和基性火山岩有相似的稀土元素特征，显示了源区特征的不同。基性火山岩富集强不相容元素，相对亏损Nb和Ti, 成于大陆裂谷环境，具有大陆拉斑玄武岩的特征。同位素特征表明基性火山作用与地幔柱活动密切相关。南秦岭的中、晚元古代大陆拉张及由古地幔柱活动所引发的陆裂火山岩浆活动是古秦岭洋打开的先兆。     

陕西商州地区丹凤变质火山岩的地球化学特征

陕西商州地区丹凤变质火山岩具有洋内岛弧火山岩地球化学特征,它们是分别来自不同源区的拉斑玄武和钙碱性2个系列共存的一套变质火山岩。其Th/Ta比值高及Ni、Ta、Ti、Y和Yb含量低,表明岩石受到消减带组分的影响。种种证据表明,丹凤变质火山岩是早古生代华北地块南缘消减带之上洋内岛弧环境的产物。     

Eclogite xenoliths from stockdale kimberlite,Kansas

Six kimberlite pipes of late Cretaceous or Tertiary age occur in Riley Co., east-central Kansas. Within the pipes xenoliths of local sedimentary and exotic igneous rocks are common, especially in the Stockdale pipe. Igneous rocks which occur as xenoliths include granite, gabbro, metagabbro, pyroxenite and eclogite. In the eclogites omphacitic clinopyroxene (approx. Di₅₂Jd₂₄mol%) and pyropic garnet (approx. Py₄₇Al₃₅Gr₁₂mol%) are the predominant minerals with subordinate amounts of rutile and sulphides (pyrrotite-pentlandite (?)-chalcopyrite). Interstitial kaersutitic amphibole is a minor constituent. The eclogites are chemically equivalent to olivine-basalt. The texture, composition and mineralogy of the eclogites from Kansas are similar to those of eclogites from kimberlite pipes in South Africa and Siberia. Whereas the rocks from these latter localities display a range in composition, those examined to date from Kansas are of fairly restricted composition. Furthermore it seems probable that the eclogites from Stockdale formed under limited P-T conditions within the mantle. This is the first record of such eclogites on the North American continent.     

塔里木盆地北缘新生代碱性玄武岩的地质地球化学

新生代碱性玄武岩是塔里木盆地北缘碱性岩带最晚期形成的,由第三系盆地中陆相中心式喷发的碱性橄榄玄武岩和其下古生代地层中的碱性橄榄辉长岩岩筒和岩脉组成。它们是来自富集地幔的同源碱性岩浆,在不同空间冷凝结晶形成。     

Cenozoic basalts in North China—Their distribution,geochemical characteristics and tectonic implications

Epuptions of alkali and subalkali basalts occurred respectively during Paleogene, Neogene, and Quaternary in North China. They are interplate, non-oceanic basalts. The alkali basalts range from Ne-normative to Hy-normative basalts and the subalkali basalts belong to the continental tholeiites. Based on their distribution, geological ages and petrographic series the magmatic evolution of Cenozoic basaltic rocks in North China can be divided into three stages. Geochemical data from this study indicate that Cenozoic basaltic magmas of North China were derived from two different mantle sources.     

云南香格里拉县洛吉地区基性杂岩的地球化学特征

云南香格里拉县洛吉地区出露由洛吉组玄武岩和基性岩墙群组成的基性杂岩,其母岩浆起源于亚碱性-碱性系列的拉斑玄武系列,地球化学特征表明该基性杂岩体形成于构造变动的复杂环境,并与不同源区岩浆混合及一定程度的壳幔混染有关,其中的碱性玄武岩与基性岩墙属具有相似性OIB玄武岩,是深部地幔石榴石橄榄岩与尖晶石橄榄石小部分熔融的产物,形成于大陆内或陆缘裂谷转变为盆缘造山带的过渡环境,亚碱性玄武岩属于E-MORB并有向OIB过渡趋势,由源区相对浅的尖晶石橄榄石熔融形成,为洋中脊向大陆过渡环境的蛇绿岩质分子,属甘孜-理塘结合带残余产物。     

甘肃走廊南山朱龙关群的时代及其火山岩的岩石化学特征

甘肃走廊南山朱龙关群是一套以发育大量基性火山岩为特征的浅变质火山沉积岩系，分布于托来山北坡刘口峡山至热水大坂一带以及朱龙关河北侧小柳沟和古浪峡等地。本文在前人工作的基础上，对区内地层的分布提出了新的见解，认为沙龙地区和北过龙至金龙河地区出露的一套浅变质火山－沉积岩石应划归来龙关群，并运用同位素年代学方法测定了该地层中火山岩的成岩时代，首次较确切地给出了这套浅变质岩系的时代归属，从而进一步确定了朱龙关群的时空分布。同时根据火山岩岩石化学特征研究，认为朱龙关群火山岩的成岩大地构造背景为陆内裂谷。     

The Mesoproterozoic Midsommersø dolerites and associated high-silica intrusions, North Greenland: crustal melting, contamination and hydrothermal alteration

The Midsommersø dolerites and the flood basalts of the Zig-Zag Dal Basalt Formation, eastern North Greenland, represent a major Mesoproterozoic (∼1,380 Ma) igneous event. The intrusive rocks form large sheets within a succession of feldspathic sandstones which underlie the basalts. The geochemistry of the basalts has recently been re-investigated and reported elsewhere in this Journal; here we present new trace element and Nd-, Sr- and Pb-isotopic data for the intrusive rocks. Unlike the basalts, the intrusions yield evidence of considerable interaction and contamination with upper crustal rocks, especially the sandstones. High-silica rocks (80–90 wt% SiO₂) occur in sheets, up to 60 m thick. They were formed by mobilisation of sandstones, and indicate a very high rate of emplacement of hot basic magma into the sandstones at depth. These mobilised sandstones (‘rheopsammites’) are among the most SiO₂-rich intrusive rocks on earth. Sheets of remobilised granitoid rocks from the crystalline basement (∼70% SiO₂) are also present. Hydrothermal activity, associated with the igneous event, significantly changed the compositions of the silicic rocks as well as that of many dolerites. Sheets of hydrothermally altered (‘red’) dolerites and silicic rocks invariably have borders of dark, fresh dolerite; this is interpreted to be the result of intrusion from zoned magma chambers. Nd isotope data confirm the crustal origin of the silicic rocks as well as the contamination of some dolerites by components derived from crustal sources, while Sr- and Pb-isotopic systems are strongly affected by the hydrothermal alteration, and give little information on the petrogenesis of the rocks. Recent loss of Sr from hydrothermally altered rocks further affected the Sr isotope systems, and earlier age determinations by the Rb–Sr whole-rock isochron method (1,230 Ma) have proved to be in error. The dolerites and the basalts are geochemically very similar, but most dolerites have moderately negative Eu anomalies that are not observed in the basalts. Eu anomalies in the dolerites could be related to contamination by sandstone at depth, but it is not clear why the basalts escaped a similar contamination.     

New data on kimberlite magmatism in southwestern Angola

First data on the geologic and geochemical compositions of kimberlites from nine kimberlite pipes of southwestern Angola are presented. In the north of the study area, there are the Chikolongo and Chicuatite kimberlite pipes; in the south, a bunch of four Galange pipes (I–IV); and in the central part, the Ochinjau, Palue, and Viniaty pipes. By geochemical parameters, these rocks are referred to as classical kimberlites: They bear mantle inclusions of ultrabasites, eclogites, various barophilic minerals (including ones of diamond facies), and diamonds. The kimberlite pipes are composed of petrographically diverse rocks: tuffstones, tuff breccias, kimberlite breccias, autolithic kimberlite breccias, and massive porphyritic kimberlites. In mineralogical, petrographic, and geochemical compositions the studied kimberlites are most similar to group I kimberlites of South Africa and Fe-Ti-kimberlites of the Arkhangel’sk diamondiferous province. Comparison of the mineralogical compositions of kimberlites from southwestern Angola showed that the portion of mantle (including diamondiferous) material of depth facies in kimberlite pipes regularly increases in the S-N direction. The northern diamond-bearing kimberlite pipes are localized in large destructive zones of NE strike, and the central and southern diamond-free pipes, in faults of N-S strike.     

Strontium isotope and alkali element abundances in ultramafic rocks

The concentrations of the trace elements Na, K, Rb and Sr and the isotopic composition of Sr have been measured in a suite of ultramafic rocks, including alpine-type intrusions, inclusions in basalts and kimberlite pipes, zones from stratiform sheets, and a mica peridotite. From these data and those available in the literature the following conclusions can be drawn. Alpine-type ultramafic material appears to be residual in nature and can be neither the source material for the derivation of basalts nor the refractory residue of modern basalts. Alpine-type ultramafic intrusions appear to have no relationship with ultramafic zones in stratiform sheets and were probably derived from the upper mantle. A genetic relationship exists between basalts and their ultramafic inclusions, but it is extremely doubtful that this inclusion material could give rise to basalts by partial fusion. There is a possible genetic relationship between basalts and ultramafic inclusions in kimberlite pipes, and this ultramafic material is a potential source for the derivation of basalts. Ultramafic inclusions in basalts are probably not fragments of an alpine-type ultramafic zone in the mantle. An attempt has been made to synthesize the data and interpretations of this study by way of speculations on the role of ultramafic rocks in the differentiation history of the earth.     

北祁连山元古宙末－寒武纪主动大陆裂谷火山作用

北祁连山元古宙末－寒武纪大陆裂谷火山岩系为双峰式火山岩套，主要由基性与酸性火山岩组成。基性火山岩有磁性玄武岩与拉斑玄武岩两个岩浆系列，且富集ＬＲＥＥ与ＬＩＬ，其岩浆源区为与洋岛玄武岩源相似的富集地幔柱源。软流圈地幔柱上涌导致岩石圈地慢部分熔融，其熔体与地幔柱衍生熔浆混合，形成本区具有中等钕，锶同位素比值特点的基性岩浆。基性岩浆上侵至陆壳，引起下部陆壳深熔，产生长英质岩浆。地幔柱上隆促使大陆扩张，及至形成北祁连山元古宙末－寒武纪大陆裂谷。     

Spatial and temporal geochemical variability in basin-related volcanism, northern Israel

Middle Miocene (17-9 Ma) volcanism in northeast Israel migrated from eastern Lower Galilee (Poria, west of the Sea of Galilee) to the southwest (Yizre'el Valley) in association with the development of an extensional basin in that area. The Yizre'el Valley magmas are highly undersaturated in silica (basanites and nephelinites), while those of Poria are alkali-basaltic. Scarce Middle Miocene basalts from the Golan, further to the east, are also alkali-basaltic. Magmas from Kaukab (southeastern Lower Galilee) show a range of compositions from alkali-basalts to basanites. The patterns defined by ratios versus concentrations of incompatible elements in Kaukab basalts (e.g. versus La), as well as the scatter observed in Sr isotope ratios, are interpreted in terms of binary mixing. It is suggested that the Kaukab magmas were derived from veined lithospheric peridotites, melts of the veins and of the peridotites being the two end-members. Accordingly, alkali-basaltic- and basanite-dominated areas are underlain by vein-poor and vein-rich lithosphere, respectively. It is also suggested that melting during the Middle Miocene mainly occurred in response to extension, and that it followed the propagation of the Yizre'el-Galilee Basin from northeast to southwest.     

金伯利质岩管的认识及其找矿意义

金伯利岩是世界上分布较少和较难寻找的岩石。因其含有金刚石, 具经济价值,含有地幔捕虏体,给科学界带来地壳深部和上地幔信息,一直为地学界所关注。金伯利岩多呈管状产出,岩管中含有来自上地幔的同源包体以及上升通道周边各种岩石的捕虏体,火山爆发所形成的火山碎屑的复杂性造成其岩石命名有多种不同的认识,有关术语及其用法非常多。在研究国内外关于金伯利岩成果的基础之上,本文将术语用法和岩石命名原则进行分析和总结,解剖三种类型金伯利质岩管在纵向上的变化,分析在中国所发现的含金刚石的金伯利质岩管的特征和分析可能存在的金刚石砂矿。     

Geochemistry and Petrogenesis of Three Series of Cenozoic Basalts from Southeastern China

Cenozoic basaltic volcanism in southeastern China was related to the lithospheric extension and asthenospheric upwelling at the eastern Eurasian continental margin. The cenozoic basaltic rocks from this region can be grouped into three different series: tholeiitic basalts, alkali basalts, and picritic-nephelinitic basalts. Each basalt series has distinctive geochemical features and is not derived from a common source rock by different degrees of partial melting or from a common parental magma by fractional crystallization. The mineralogy, petrography, and major and trace-element geochemistry of the tholeiites are similar to oceanic island basalts, implying that the mantle source for these Chinese continental tholeiites was similar to that of the oceanic island basalts—an asthenospheric mantle. The alkali basalts and picritic-nephelinitic basalts are enriched in incompatible trace elements, and their geochemical features can be interpreted as a result of partial melting of an enriched lithospheric mantle, or the mixing products of an asthenospheric magma with a component derived from an enriched lithospheric mantle through thermal erosion at the base of the lithosphere. But the lack of a transitional rock type and continuous variational trends among these basalts suggests that the mixing between asthenospheric magmas and lithospheric magmas probably was not significant in the petrogenesis of the basalts from SE China. Low-degree partial melting of enriched lithospheric mantle alone can account for the observed geochemical data from these basalts.     

The Devonian magmatism in the Kropotkin Ridge (East Sayan) and sources of basites: geological,geochemical, and Sr-Nd isotope data

In the east of the Tuvinian trough within the Kropotkin Ridge, the formation of Devonian volcanic associations was intimately conjugate with rifting on the southwestern framing of the Siberian Platform. The associations include picrite-like basalts, trachybasalts, basaltic trachyandesites, trachyandesites, trachytes, trachyrhyodacites, trachyrhyolites, comendites, and subvolcanic dolerites. The basic and normal-basic rocks are subdivided into two groups by TiO₂ contents: high-Ti (TiO₂ ～ 2.2–4.2 wt.%) and medium-Ti (TiO₂ ～ 1.3–2.0 wt.%). Compared with the high-Ti basites, the medium-Ti ones are depleted in K, Rb, REE, Nb, Ta, Th, and U and have features of magmatic series of active continental margins. The high-Ti rocks are similar in composition to within-plate basalts. But in the isotopic compositions of Sr and Nd the above groups of basites are similar and correspond to mantle sources forming enriched within-plate basalts of the OIB type. This combination of within-plate and continent-marginal geochemical features in the basites localized in the same structure-geologic conditions might indicate the formation of rock associations in the rift zone at the rear of active continental paleomargin during the evolution of their common plume source. Its interaction with the suprasubductional lithospheric mantle determined the geochemistry of rocks.     

“三江”义敦岛弧带玄武岩喷发序列与裂谷—岛弧转化

岩石-构造组合是恢复古板块构造历史的最有效手段之一,同时是表征古板块边界与板内环境的最重要的地质证据。本文拟从岩石-构造组合角度,通过对义敦岛弧带玄武岩,特别是前岛弧期玄武岩喷发序列、岩石组合、地球化学特征和其形成背景的研究,试图从较深层次上揭示岩浆-构造内在联系,探索义敦古岛弧的形成与发展。     

Distinct kimberlite pipe classes with contrasting eruption processes

Field and Scott Smith [Field, M., Scott Smith, B.H., 1999. Contrasting geology and near-surface emplacement of kimberlite pipes in southern Africa and Canada. Proc. 7th Int. Kimb. Conf. (Eds. Gurney et al.) 1, 214–237.] propose that kimberlite pipes can be grouped into three types or classes. Classical or Class 1 pipes are the only class with characteristic low temperature, diatreme-facies kimberlite in addition to hypabyssal- and crater-facies kimberlite. Class 2 and 3 pipes are characterized only by hypabyssal-and crater-facies kimberlite. In an increasing number of Class 1 pipes a new kimberlite facies, transitional-facies kimberlite, is being found. In most cases this facies forms a zone several metres wide at the interface between the hypabyssal- and diatreme-facies. The transitional-facies exhibits textural and mineralogical features, which are continuously gradational between the hypabyssal and the diatreme types. The textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside. Both transitional- and diatreme-facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially CO₂-rich volatiles from the volatile-rich kimberlite magma. The transitional-facies is initiated by volatile exsolution at depths of about 3 km below the original surface. With subsequent cracking through to the surface and resultant rapid decompression, the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies. Thus diatreme-facies kimberlite and Class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism.

Distinctly different petrographic features characterize crater-facies kimberlite in each of the three pipe classes. In crater-facies kimberlites of Class 1 pipes, small pelletal magmaclasts and abundant microlitic diopside are characteristic. These features appear to reflect the derivation of the crater-facies material from the underlying diatreme zone. Most Class 2 pipes have shallow craters and the crater-facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli, which are sometimes welded and have vesicles as well as glass. Possible kimberlite lava also occurs at two Class 2 pipes in N Angola. The possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form Class 2 pipes. Most Class 3 kimberlites have very steep craters and crater-facies rocks are predominantly resedimented volcaniclastic kimberlites, in some cases characterized by the presence of abundant angular magmaclasts, which are petrographically very similar to typical hypabyssal-facies kimberlite found in Class 1 pipes. The differences in crater-facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition, specifically volatile composition. Kimberlite forming pipe Classes 1 and 3 is thought to be relatively water-rich and is emplaced by processes involving magmatic exsolution of volatiles. The kimberlite magma forming Class 2 pipes is CO₂-rich, can rise to shallow levels, and can initiate phreatomagmatic emplacement processes.