Two contrasting magmatic types coexist after the cessation of back-arc spreading

Amongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300 km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC).Chemically, enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that the back-arc seamount chains and the East Shikoku Basin seamounts have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism and distinct from the MORB-like lavas of the Shikoku back-arc basin. ⁴⁰Ar/³⁹Ar ages range from 15.7 to 9.6 Ma for the East Shikoku Basin seamounts, indicating this arc magmatism started immediately after the Shikoku Basin stopped spreading.Although the KSC volcanoes are found to be contemporaneous with the seamount chains and East Shikoku Basin seamounts, their chemical characteristics are very different. Unlike the calc-alkaline seamount chains, the KSC lavas range from medium-K to shoshonitic alkaline basalt. Their trace element characteristics indicate the absence of a subduction influence and their radiogenic isotope systematics reflect a mantle source combining a Philippine Sea MORB composition and an enriched mantle component (EM-1). One of the most remarkable features of the KSC is that their geochemistry has a distinct temporal variation. Element ratios such as Nb/Zr and concentrations of incompatible elements such as K₂O increase with decreasing age and reach a maximum at ca. 7 Ma when the KSC ceased activity.Based on the chemical and temporal information from all the data across the back-arc region, we have identified two contrasting yet contemporaneous magmatic provinces. These share a tectonic platform, but have separate magmatic roots; one stemming from subduction flux and the other from post-spreading asthenospheric melting.     

Tectonics and magmatism of ultraslow spreading ridges

The tectonics, structure-forming processes, and magmatism in rift zones of ultraslow spreading ridges are exemplified in the Reykjanes, Kolbeinsey, Mohns, Knipovich, Gakkel, and Southwest Indian ridges. The thermal state of the mantle, the thickness of the brittle lithospheric layer, and spreading obliquety are the most important factors that control the structural pattern of rift zones. For the Reykjanes and Kolbeinsey ridges, the following are crucial factors: variations in the crust thickness; relationships between the thicknesses of its brittle and ductile layers; width of the rift zone; increase in intensity of magma supply approaching the Iceland thermal anomaly; and spreading obliquety. For the Knipovich Ridge, these are its localization in the transitional zone between the Gakkel and Mohns ridges under conditions of shear and tensile stresses and multiple rearrangements of spreading; nonorthogonal spreading; and structural and compositional barrier of thick continental lithosphere at the Barents Sea shelf and Spitsbergen. The Mohns Ridge is characterized by oblique spreading under conditions of a thick cold lithosphere and narrow stable rift zone. The Gakkel and the Southwest Indian ridges are distinguished by the lowest spreading rate under the settings of the along-strike variations in heating of the mantle and of a variable spreading geometry. The intensity of endogenic structure-forming varies along the strike of the ridges. In addition to the prevalence of tectonic factors in the formation of the topography, magmatism and metamorphism locally play an important role.     

Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin

The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have ⁸⁷Sr/⁸⁶Sr (0.7040–0.7042) and ¹⁴³Nd/¹⁴⁴Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher ⁸⁷Sr/⁸⁶Sr (0.7044–0.7047) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (M_a/M_c) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current.     

滇西大理—剑川地区钾玄质岩浆作用：后碰撞走滑拉伸环境岛弧型岩浆作用的地球化学研究

岩石学、元素地球化学研究表明,在滇西大理－剑川地区分布的富碱斑岩群属钾玄质系列岩石,具有产出于与红河－哀牢山走滑剪切带伴生的早第三纪拉张盆地中,这套钾玄质系列岩石富集大离子亲石元素LILE（Rb,Ba,K,Sr）,轻稀土元素（LREE）和一组相容元素（Cr,Ni,V,Sc）,具有明显的Nb,Ta,Ti,P负异常,反映了后碰撞岛弧型钾玄岩系特征。钾玄质母岩浆主要源于含金云母的尖晶石相地幔橄榄岩低度部分熔融,并在演化过程中经历了充分的分离结晶作用。     

The geochemistry of basalts from a back-arc spreading centre in the East Scotia Sea

Rapid sea floor spreading has taken place over the last 8 Myr behind the South Sandwich island arc, producing a regular set of magnetic lineations. Suites of fresh basalts have been dredged from four widely separated localities along the spreading axis. Dredges 20 and 23 yielded sub-alkaline olivine tholeiites, dredge 22 recovered vesicular tholeiites with minor normative olivine, while dredge 24 contained a fractionated suite of highly vesicular quartz-normative basalts with higher $\text{Fe}\text{Mg}$. The concentrations of the incompatible elements Ti, P, Zr, Hf, Nb, Ta, Y and the REE increase systematically from dredge 24 through dredges 22 and 20 to dredge 23 and there is a comparable increase in $\text{Ce}{}_{\mathrm{N}}\text{Yb}{}_{\mathrm{N}}$. Quantitative modelling suggests that all the basalts can be derived from an essentially similar mantle source (with respect to these elements) through varying degrees of partial melting, but involving some residual clinopyroxene. Basalts from dredge 24 have unusually low concentrations of Ti, P, Zr, Nb, Y, REE and Ni, similar to the values in arc tholeiites, and the more primitive dredge 24 liquids seem to have been generated through high degrees of partial melting (~ 30%) leaving a dunitic residue. Transitional arc tholeiite characteristics are also apparent in the relatively high K, Rb, Ba contents and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of dredge 24 and 22 basalts, though Nd isotope ratios are uniform. It is considered that fluids derived from the dehydrating subducted slab may have locally penetrated the source regions of the back-arc basalts, carrying K, Rb, Ba and seawater-enriched ⁸⁷Sr, and producing conditions of magma generation similar to that of arc tholeiites. However, it is unlikely that the sources for these and other marginal basin basalts differ fundamentally from the range of mantle sources feeding normal mid-ocean ridges.     

The belt of metagabbros of La Pampa: Lower Paleozoic back-arc magmatism in south-central Argentina

Combined geological, geochronological, geochemical and geophysical studies have led to identification of a large (∼300 km long, ∼5 km wide) N–S trending belt of metagabbros in the province of La Pampa, south-central Argentina. This belt, though only poorly exposed in the localities of Valle Daza and Sierra de Lonco Vaca, stands out in the geophysical data (aeromagnetics and gravity). Modeling of the aeromagnetic data permits estimation of the geometry of the belt of metagabbros and surrounding rocks.The main rock type exposed is metagabbros with relict magmatic nucleii where layering is preserved. A counterclockwise P–T evolution affected these rocks, i.e., during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). During the Upper Devonian, a retrograde, greenschist facies metamorphism (M3) partially affected the metagabbros.The whole-rock Sm–Nd data suggest a juvenile source from a depleted mantle, with model ages ranging from 552 to 574 Ma, and positive Epsilon values of 6.51–6.82. A crystallization age of 480 Ma is based on geological considerations, i.e. geochronological data of the host rocks as well as comparisons with the Las Aguilas mafic–ultramafic belt of Sierra de San Luis (central Argentina).The geochemical studies indicate an enriched MORB and back-arc signature.The La Pampa metagabbros are interpreted to be originated as a result of the extension that took place in a back-arc setting coevally with the Famatinian magmatic arc (very poorly exposed in the western part of the study area). The extensional event was ´aborted´ by the collision of the Cuyania terrane with Pampia-Gondwana in the Middle Ordovician, causing deformation and metamorphism throughout the arc–back-arc region.The similarities between the La Pampa metagabbros and the mafic–ultramafic Las Aguilas belt of the Sierra de San Luis are very conspicuous, for example, the age (Lower Paleozoic), geochemical signature and timing of metamorphism (dated at ca. 465 Ma in the study area), which allow definition of a single, mafic back-arc belt in central Argentina, from San Luis to La Pampa.     

Compositional data analysis in the study of integrated geochemical anomalies associated with mineralization

Geochemical data are typical compositional data which should be opened prior to univariate and multivariate data analysis. In this study, a frequency-based method (robust principal component analysis, RPCA) and a frequency-space-based method (spectrum–area fractal model, S–A) are applied to explore the effects of the data closure problem and to study the integrated geochemical anomalies associated with polymetallic Cu mineralization using a stream sediment geochemical dataset collected from the Zhongteng district, Fujian Province (China). The results show that: (1) geochemical data should be opened prior to RPCA to avoid spurious correlation between variables; (2) geochemical pattern is a superimposition of multi-processes and should be decomposed; and (3) the S–A fractal model is a powerful tool for decomposing the mixed geochemical pattern.     

Origin of the alkali tonsteins from southwest China: Implications for alkaline magmatism associated with the waning stages of the Emeishan Large Igneous Province

A unique type of Nb–Zr–REE–Ga-enriched alkali tonstein of pyroclastic origin occurs exclusively within the late Permian coal measures of southwest China. The alkali tonsteins are located within the lowest Xuanwei or Longtan formations of Wuchiapingian age, indicating that their age is later than the main episode of Emeishan Large Igneous Province (ELIP) magmatism. The alkali tonsteins have intermediate–felsic Al₂O₃/TiO₂ values (12.6–34.2, mean 22.0), light rare earth element-enriched chondrite-normalised patterns, negative δEu and incompatible element ratios similar to those of ELIP alkaline Nb–Ta-enriched syenites. All available evidence shows that the alkali tonsteins from southwest China originated from coeval ELIP alkaline magmatism. The enrichment of Nb–Zr–REE–Ga in alkali tonsteins is derived from the ELIP alkaline Nb–Ta-enriched volcanic ashes and may represent the last stage of mineralisation associated with the Emeishan mantle plume activity.     

西南印度洋洋盆演化和岩浆地球化学印迹

西南印度洋中脊（SWIR）平均扩张速率约为14 mm/yr,是全球洋中脊系统的重要组成端元,因其具有慢速-超慢速扩张特征,引起全球科学家的广泛关注。基于前人对SWIR的综合研究成果,从构造和岩浆作用两个角度出发,系统地回顾了SWIR的形成和演化历史,探讨了岩浆的分布特征和地幔不均一性成因。SWIR的形成始于冈瓦纳大陆的裂解,中段洋脊区域（26~42°E）是印度洋最早开启的窗口,历经多次洋脊跃迁和扩展作用形成了斜向扩张展布,多分段的构造格局。地幔热点在冈瓦纳大陆裂解过程中扮演了关键角色,并对SWIR的洋底地貌和岩浆作用具有显著影响,其中Bouvet和 Marion热点在SWIR的西段和中段岩浆均留下了地球化学印迹。SWIR西段岩浆除却Bouvet热点影响之外表现出与大西洋-太平洋型玄武岩相似的同位素地球化学特征。在SWIR中段,39~41°E附近的岩浆具有显著的DUPAL异常特征,与冈瓦纳大陆的初始形成、裂解紧密相关。受俯冲改造的中—新元古代的造山带岩石圈地幔拆沉是造成SWIR中段地幔不均一性的重要根源。在SWIR东段,46~52°E区域内的局部岩浆组成异常,推测具有大陆地壳物质混染的成因。而在Melville转换断层以东,洋脊形成时间最晚,玄武岩的地幔源区受到了富集组分的交代作用,其同位素组成与相邻的中印度洋中脊（CIR）和东南印度洋中脊（SEIR）地幔源区具有亲缘性。     

吉中-延边地区燕山期岩浆作用与矿床成矿系列研究

运用矿床的成矿系列理论,以构造岩浆演化为基础,对吉中-延边成矿带与燕山期岩浆作用有关的矿床成矿规律进行了分析总结。研究表明,受古太平洋板块俯冲方向、速度的制约,吉中—延边地区燕山期构造岩浆活动复杂,总体上可分为5次成岩成矿事件:①晚三叠世初基性超基性岩侵入,形成红旗岭铜镍矿床等岩浆型矿床;②晚三叠世—早侏罗世初,吉中地区高钾钙碱性岩浆活动,形成兰家金矿床等岩浆热液型矿床,延边地区中低钾钙碱性—高钾钙碱性岩浆活动,形成前山金矿床等火山-潜火山岩型矿床;③早侏罗世末—中侏罗世初,富铝酸性岩浆活动,形成大黑山钼矿床等斑岩型钼矿床;④中侏罗世末—晚侏罗世初,研究区以抬升剥蚀为主,成岩成矿作用较弱;⑤晚侏罗世末—早白垩世,吉中地区高钾钙碱性岩浆活动,形成二道甸子金矿床等岩浆热液型矿床,延边地区中低钾钙碱性—高钾钙碱性岩浆活动,形成小西南岔金铜矿床等火山-斑岩型矿床。在建立吉中—延边地区与燕山期岩浆作用有关的矿床成矿系列的基础上,进一步划分为6个矿床成矿亚系列,各亚系列在时间、空间上既有联系,又有区别,显示了成矿作用的复杂性。     

Distinctive petrological, geochemical, and geodynamic features of subduction-related magmatism

Geological-petrological and geochemical data on subduction-related magmatism (including the volumes and compositions of the corresponding magmatic series) are compared to the results of experiments and numerical simulation. The subduction zone is subdivided into five depth sectors and volcanic zones I, II, and III: 1 is the accretionary wedge that controls the geodynamic stability of subduction; 2 is the sector of dehydration and fluid filtration; 3 is the zone of eclogitization and initial partial melting in the slab above which boninite volcanic zone I is formed during early stages; 4 is the main zone of melting of the sedimentary-basite layer and the development of volcanic zone II with the predominance of andesites; and 5 is the zone of higher degree melting, above which volcanic zone III (basaltic andesite and alkali basalt) is formed. The criterion of volcanism intensity, which was obtained within the scope of the melting model, is proportional to the subduction velocity and the thickness of the melting zone, and the distance between the groups of volcanics along the subduction zone is 75–100 km, at a thickness of the melting zone of 15–20 km. The calculated isotherm of 600°C, which controls the stability of serpentine and chlorite, is not identified at depth above 150 km, and this is confirmed by the composition and P-T conditions of the high-pressure rocks (containing diamond and coesite), which were brought from depths of 150–200 km in subduction zones. Seismic sections constructed with regard for the amplitude characteristics of seismic waves show two melting zones (“wet” melting at a depth of 100–200 km and “dry” melting at a depth of 150–200 km) and a complicated thermal structure of the suprasthenospheric wedge, which can include slant magma conduits. The mineralogical and geochemical features of arc magmatic series are formed at a decisive role of an H₂O-CO₂ fluid and an elevated oxidation potential. The predominant buffer minerals are as follows: garnet in the slab melting zone; magnetite, Ca-pyroxene, and amphibole in intermediate magmatic chambers; and amphibole, protoenstatite-bronzite (in place of olivine), and Cr-spinel (in place of magnetite) for boninite series generated in a “hot” asthenospheric wedge at interaction with fluids or water-rich melts. Actively disputable problems are the interactions scale of melts and fluids generated in a subduction zone with a “hot” mantle wedge, the possibility of transporting water-rich minerals deep into the mantle (to depths greater than 150 km), and the evolution of the scale at which young continental crust is generated by subduction melts.     

化探找金几个阶段中的方法探讨

目前，化探找金逐步被人们重视，在地质找矿中的效果也逐渐明显，成为寻找各种类型金矿床比较快速、经济、有效的重要手段．在区域普查中，通过查明区域地球化学异常，可迅速指出找矿远景区；在详查及勘探阶段，通过岩石地球化学异常的研究，可直接发现金矿床或矿体，更好地发挥化探在地质找矿工作中的作用。     

中酸性岩浆体系成矿流体及微量元素地球化学特征

从流体成矿作用角度出发，与酸性岩浆体系有关的成矿流体可以分为：酸性岩浆硅酸盐熔融体，岩浆一热液过渡阶段硅酸盐熔融体及其分异的流体，酸性岩浆熔体分异形成的热水成矿溶液。酸性岩浆体系主要提供热源和部分矿质，其提供的热源驱动地下水淋滤、萃取围岩中的成矿物质形成地下水热液成矿流体。变质岩混合岩化形成花岗质岩浆过程中所形成的混合岩化成矿流体。在此基础上，讨论了上述不同成矿流体的微量元素地球化学特征及其对成矿的控制作用。     

A Cretaceous dike swarm provides evidence of a spreading axis in the back-arc basin of the Kohistan paleo-island arc,northwestern Himalaya,Pakistan

A 100–4000 m wide and 15 km long dike swarm, consisting of basalt and dolerite, occurs at the base of the Thelichi Formation in the Kohistan paleo-island arc terrane, north Pakistan. The dikes contain hornblende (altered from diopsidic-augite), diopsidic-augite (relics; ophitic to subophitic texture), chlorite, epidote, sphene, apatite, zircon, ilmenite, titanomagnetite and magnetite. The geochemistry reveals two groups of dikes: (1) Higher TiO₂ (2.74–3.50 wt%), Na₂O, Fe₂O₃ and lower Al₂O₃ (12.65–14.16 wt%) and MgO (3.73–5.04 wt%); (2) Lower TiO₂ (1.24–2.05 wt%), Na₂O, Fe₂O₃ and higher Al₂O₃ (14.02–16.52 wt%) and MgO (3.98–7.52 wt%). The MgO contents (3.73–7.52-wt%) show a variation in the dikes from relatively primitive to more evolved compositions. The dikes contain high amounts of both LILE and HFSE. The major, trace and rare-earth elements data confirm the MORB affinity and the back-arc basin origin of the dike swarm. The NW–SE orientation of the dike swarm and its 134 ± 3 Ma K–Ar age suggest the spreading axis of the back-arc basin in the Early Cretaceous.     

A trace-element geochemical model for imperfect fractional crystallization associated with the development of crystal zoning

Although many petrological studies of volcanic rocks have suggested that crystallization proceeds within magma bodies, highly compatible trace elements do not display the marked variations and extreme depletions predicted to result from perfect fractional crystallization. Imperfect crystal-liquid separation is a key process in explaining this paradox. The presence of suspended crystals greatly affects variations in highly compatible elements, and has been quantitatively modeled by assuming perfect equilibrium between the suspended crystals and the liquid (equilibrium crystallization and imperfect separation; ECIS); however, volcanic rocks generally contain zoned phenocrysts that reflect the absence of solid-state equilibration. The present study develops a mass-balance model for zoned crystallization and imperfect separation (ZCIS). The ZCIS process is more efficient than the conventional ECIS process in generating depleted compatible elements. These two end-member models are able to explain the compositional range of igneous rocks that experienced imperfect fractional crystallization under natural conditions. The predicted compositional regions in bivariate trace-element diagrams successfully account for the sizes and shapes of the regions defined by whole-rock and melt-inclusion data from the Bishop Tuff, CA, USA.     

A fragment of back-arc paleospreading in the Tunka terrane

High magnesium metavolcanites, which are found for the first time in the Tunka terrane (the Baikal-Khubsogul region), are described. By the content of MgO (12–16%), SiO₂ (up to 52%), and alkali oxides, they are classified as metamorphosed picrites. There are two groups of them, divided by the TiO₂ content and by the TiO₂/Al₂O₃ ratio. When making a distinction between geochemical peculiarities of two groups, their rare-element composition is intermediate between picrites and boninites. The comparison between metavulcanites and their nonmetamorphosed analogues is made, some aspects of their genesis are considered, and the conclusion is made that these metavulcanites mark paleospreading of the back-arc basin.     

Mantle derived lherzolite nodules associated with kimberlite,carbonatite and basalt magmatism: A review

Occurrences, petrography and major element chemistry of lherzolite nodules are reviewed. Spinel-garnet stability relationships in these nodules are discussed and are shown to be controlled by chemical as well as physical (P/T) considerations. On the basis of a survey of spinel compositions it is proposed that three classes of spinel bearing lherzolite nodules should be recognised: namely Al-spinel lherzolites [spinel 100 Cr/(Cr + Al)<25], Cr-spinel lherzolites [spinel 100 Cr/(Cr + Al) 25–65] and chromite lherzolites [spinel 100 Cr/(Cr + Al)>65].All lherzolite nodules yield sub-solidus P/T equilibration estimates and are interpreted as fragments of upper mantle wall rocks incorporated during the volatile charged eruptions. Depths of derivation increase from < 60 kms for most nodules in alkali basaltic magmas to > 150 kms for some nodules in kimberlites. The fact that Al-spinel lherzolites are the most common nodule type in magmas of the alkali basalt suite whilst garnet lherzolites are dominant in kimberlites is attributed to the combined effects of a typically steeper geotherm in the mantle beneath areas of alkali basaltic volcanism and a shallower depth of origin for this type of magmatism. Al-spinel lherzolites do, however, occur in the kimberlites of the western U.S.A. and south-west Greenland in contrast to their apparent absence in the kimberlites of southern Africa and Yakutia, U.S.S.R. This suggests that the uppermost mantle beneath these latter regions (as represented by nodules of chromite lherzolites and chromite or Cr-spinel harzburgites) has a more refractory residual type composition with a higher Cr/(Cr + Al) ratio—although the evidence indicates an overall decrease in the level of depletion in ‘basaltic’ magma yielding constituents with depth. Lherzolite nodules generally have chemical compositions which are depleted in such constituents relative to the pyrolite model compositions for primitive or pristine mantle; nevertheless their composition range is thought to encompass both highly ‘depleted’ and essentially ‘undepleted’ upper mantle compositions.The fact that consistent temperature estimates can be obtained from the various calibrated element exchange reaction geothermometers for some lherzolite nodules but not for others (notably those with strikingly porphyroclastic textures) may indicate that some lherzolite nodules comprise mantle derived rocks which failed to totally re-equilibrate following the diapiric movements which immediately preceded their incorporation and rapid transportation to the surface.     

Proterozoic basic magmatism of the Siberian Craton: Main stages and their geodynamic interpretation

Geological data on the Precambrian basic complexes of the Siberian Craton and their isotopic age are considered. The three main episodes of Precambrian basic magmatism of Siberia correspond to certain stages of the geodynamic evolution of the craton and the Earth as a whole. In the Late Paleoproterozoic, largely in the south and the north of the craton, the basic rocks were emplaced against the background of post-collision extension, which followed the preceding collision-accretion stage responsible for the formation of the craton. In the Mesoproterozoic, primarily in the north of the craton, basic magmatism was controlled by dispersed within-plate extension apparently caused by the impact of a mantle plume. Neoproterozoic basic magmatism widespread in the southern and southeastern parts of the craton was caused by rifting, which accompanied breakdown of the Rodinia supercontinent and opening of the Paleoasian ocean along the southern margin of the Siberian Craton.