Large-volume silicic volcanism in Kamchatka: Ar–Ar and U–Pb ages,isotopic, and geochemical characteristics of major pre-Holocene caldera-forming eruptions

The Kamchatka Peninsula in far eastern Russia represents the most volcanically active arc in the world in terms of magma production and the number of explosive eruptions. We investigate large-scale silicic volcanism in the past several million years and present new geochronologic results from major ignimbrite sheets exposed in Kamchatka. These ignimbrites are found in the vicinity of morphologically-preserved rims of partially eroded source calderas with diameters from ～ 2 to ～ 30 km and with estimated volumes of eruptions ranging from 10 to several hundred cubic kilometers of magma. We also identify and date two of the largest ignimbrites: Golygin Ignimbrite in southern Kamchatka (0.45 Ma), and Karymshina River Ignimbrites (1.78 Ma) in south-central Kamchatka. We present whole-rock geochemical analyses that can be used to correlate ignimbrites laterally. These large-volume ignimbrites sample a significant proportion of remelted Kamchatkan crust as constrained by the oxygen isotopes. Oxygen isotope analyses of minerals and matrix span a 3‰ range with a significant proportion of moderately low-δ¹⁸O values. This suggests that the source for these ignimbrites involved a hydrothermally-altered shallow crust, while participation of the Cretaceous siliceous basement is also evidenced by moderately elevated δ¹⁸O and Sr isotopes and xenocryst contamination in two volcanoes. The majority of dates obtained for caldera-forming eruptions coincide with glacial stages in accordance with the sediment record in the NW Pacific, suggesting an increase in explosive volcanic activity since the onset of the last glaciation 2.6 Ma. Rapid changes in ice volume during glacial times and the resulting fluctuation of glacial loading/unloading could have caused volatile saturation in shallow magma chambers and, in combination with availability of low-δ¹⁸O glacial meltwaters, increased the proportion of explosive vs effusive eruptions. The presented results provide new constraints on Pliocene–Pleistocene volcanic activity in Kamchatka, and thus constrain an important component of the Pacific Ring of Fire.     

Numerical model of the supercontinental cycle stages: Integral transfer of the oceanic crust material and mantle viscous shear stresses

We compute the transfer of oceanic lithosphere material from the surface of the model to the inner convective mantle at successive stages of the supercontinental cycle, in the time interval from the beginning of convergence of the continents to their complete dispersal. The sequence of stages of a supercontinental cycle (Wilson cycle) is calculated with a two-dimensional numerical model of assembling and dispersing continents driven by mantle flows; in turn, the flows themselves are forming under thermal and mechanical influence of continents. We obtain that during the time of the order of 300 Myr the complete stirring of oceanic lithosphere through whole mantle does not occur. This agrees with current ideas on the circulation of oceanic crust material. Former oceanic crust material appears again at the Earth’s surface in the areas of mantle upstreams. The numerical simulation demonstrates that the supercontinental cycle is a factor which intensifies stirring of the material, especially in the region beneath the supercontinent. The reasons are a recurring formation of plumes in that region as well as a global restructuring of mantle flow pattern due to the process of joining and separation of continents. The computations of viscous shear stresses are also carried out in the mantle as a function of spatial coordinates and time. With a simplified model of uniform mantle viscosity, the numerical experiment shows that the typical maximal shear stresses in the major portion of the mantle measure about 5 MPa (50 bar). The typical maximal shear stresses located in the uppermost part of mantle downgoing streams (in a zone that measures roughly 200 × 200 km) are approximately 8 times greater and equal to 40 MPa (400 bar).     

青藏高原东部和周边地区地壳速度结构的背景噪声层析成像

利用连续地震背景噪声记录和互相关技术获得瑞利面波格林函数,进而反演获得了青藏高原东部和周边地区的地壳三维速度结构.地震数据源于北京大学宽频带流动观测地震台阵,国家数字测震台网数据备分中心提供的部分固定台站的连续记录及INDEPTH IV宽频带流动观测地震台阵.首先对观测数据进行处理和分析取得所有可能台站对的面波经验格林函数和瑞利波相速度频散曲线,反演得到了观测台阵下方周期从6~60s的瑞利波相速度异常分布图像.并且进一步反演获得研究区域三维剪切波速度结构和莫霍面深度分布.短周期(6~14s)相速度异常分布与地表地质构造特征吻合较好,在青藏高原和四川盆地之间存在一个明显的南北向转换带.而本文最重要的结果是周期大于25s的相速度异常分布图像显示,以昆仑断裂带为界,柴达木盆地和祁连山脉地区呈现与青藏高原截然不同的中地壳速度结构,反而与青藏高原东缘地区和川滇菱形块体速度结构相似.反演获得的剪切波速度在27.5~45km深度的切片也明显地揭示:青藏高原的松潘—甘孜地块和羌塘地块呈现均一的低速层;然而,柴达木盆地和祁连山脉地区则呈现较强的横向不均匀性,尤其是柴达木盆地的高速异常和四川盆地的高速异常相对应.这些结果为前人提出的青藏高原东北向台阶式增长模式提供了重要的地震学观测证据.与全球一维平均速度模型(AK135)相比较发现,本文测量和反演获得的研究区域内平均相速度和剪切波速度都比AK135模型慢很多,尤其是青藏高原的中地壳(25~40km)剪切波速度显著低于全球平均速度模型.进一步的层析成像反演证实松潘—甘孜和羌塘地块中地壳(27.5~45km)呈现大范围均一的低速层,为青藏高原可能存在大规模中下地壳"层流"提供地震学观测证据.在祁连山脉的27.5~45km深度观测到的明显低速异常体可能对应于该造山带下地幔岩浆活动导致的底侵作用,表明引起该地区地壳增厚的主要机制可能是来自地幔岩浆的底侵作用.     

利用黏弹性有限元模型研究挪威北海北部地区末次冰期以来的冰后回弹和应力演化

挪威北海北部在末次冰期存在较大范围的冰盖,其冰盖的加载和卸载会对地表变形和内部应力调整产生重要影响.本文基于Maxwell黏弹性本构关系,根据初应力法自主开发了一套Maxwell黏弹性体有限元程序,它可以考虑重力和构造加载、地球介质弹性的纵向和横向不均匀性以及黏性的分层性,可以计算冰川载荷变化引起的地球表面变形及内部应力状态的变化.利用它研究了挪威北海北部1.1 Ma以来的冰川载荷变化、特别是两万多年以来冰盖的消退引起的地表冰后回弹.结果表明,自2万年以来,冰后回弹效应在冰盖载荷变化的不同阶段呈现明显的时空变化,现今地表垂直变化速率为几个毫米/年,与观测结果一致.下地壳和上地幔的黏弹性松弛效应明显,上地壳的应力状态在现今海岸线两侧存在差异性,水平和垂直正应力变化可达几十兆帕,剪应力变化有一个先增加后迅速减小至零的过程,与古地震、现今地震时空分布及应力测量结果也比较符合,研究结果有助于加深对冰后回弹的动力学过程的认识.     

A nonstationary model of the thermal regime of axial zones of mid-ocean ridges: Formation of crustal and mantle magma chambers

A nonstationary model of spreading with periodic intrusions of a molten material into an axial zone of a mid-ocean ridge (MOR) is applied to numerical analysis of the thermal state in MOR axial zones and the formation of crustal and mantle magma chambers in them. The model satisfactorily explains the positions, dimensions, and shapes of magma chambers, as well as variations in these parameters depending on the spreading rate, temperature, and composition of crustal and mantle rocks. The release and absorption of the latent heat of rock melting, hydrothermal heating of the crust, and variations in the solidus and liquidus temperatures of crustal and mantle rocks as a function of their composition are factors controlling the shape and position of crustal magma chambers.     

Seismic data on magma chambers of the large tolbachik eruption

This paper discusses the methods and techniques of observation that can at present be applied to a seismic refraction study of active volcanoes with a view of determining the magma chamber location. A system of magma chambers has been outlined in the earth’s crust and transition layer between the crust and the mantle under the Tolbachik Volcano group. Magma chambers are dynamically related to each other. The feeding magma chambers of the newly-formed Tolbachik volcanoes and Plosky Tolbachik volcano are assumed to be interrelated through the transition zone between the crust and the mantle.     

Structure of the crust and uppermost mantle from broadband seismic array in the western Dabie Mountains,east central China

A broadband seismic array of 7 stations was set up in the western Dabie Mountains (31°20′-31°50′N, 114°30′-115°E). Teleseismic events from May 2001 to November 2001 were collected and analyzed by radial receiver function to determine the S-wave velocity structure of the crust and uppermost mantle. The crustal thickness is 32-38 km beneath the array. The crust-mantle boundary appears as a gently north-dipping velocity discontinuity, but turns to be a velocity gradient beneath a station near the Qiliping shea...     

板块俯冲中水分运移过程及其地球物理含义

流体/水分在地球演化过程中具有重要作用,它的存在可以提高地质体的扩散及蠕变,降低岩石/地幔的固液相限,对地质体的地球物理性质、地质灾害诱发、地质构造演化等都有着重要的作用.在板块俯冲区,流体/水分的影响贯穿了整个动力学过程,但板块深部脱水还存在着争议,且目前系统地研究水分在整个俯冲中的迁移过程及其地球物理意义的工作还较...     

三维数值模拟研究俯冲区火山的时空活动性

俯冲板块的深部脱水使得上覆地幔含水, 从而降低含水地幔的熔点, 导致上覆地幔部分熔融。 部分熔融的地幔柱一旦喷发到地表就是俯冲带火山, 也形成新的地壳。 相对于周围的地幔来讲, 具有较小密度和黏度的部分熔融地幔的时空活动性就控制着俯冲带火山的时空分布特征。 本文主要回顾近年来运用三维热力学岩石力学模型数值模拟研究与板片脱水相关的俯冲带火山活动的时空分布特性。 结果表明, 部分熔融地幔的有效黏度和密度是影响俯冲板片之上的三维地幔柱横向分布特征的主要因素。 高黏度的部分熔融地幔(10²⁰~10²¹ Pa·s )易于形成近平行于海沟的、 长波长(70~100 km)的、 薄的波状地幔柱; 低黏度(10¹⁸~10¹⁹ Pa·s )的熔融地幔易于形成平行于海沟的, 短波长(30~50 km)的蘑菇状地幔柱和垂直于海沟的山脊状地幔柱。 当部分熔融地幔和周围地幔的密度相差小于50 kg/m³时, 在俯冲板片之上只能形成长波长低幅度(宽50~100 km, 高10~15 km)的地幔山丘。 岩浆产率随着时间的变化反映了火山活动的生命周期性。 板块俯冲速度会影响地幔柱形成的深度和范围大小。 高效率熔融提取会增加新地壳增长总量。 低的板块俯冲速度和低的熔融提取效率会增加上地壳(花岗岩质)和中地壳(英安岩质)化学成分的比例。 数值模拟结果可以很好地解释如日本东北、 新西兰、 南阿拉斯加俯冲区火山的横向分布特征。     

Structural control and origin of volcanism in the Taupo volcanic zone,New Zealand

Taupor volcanic zone (TVZ) is the currently active volcanic arc and back-arc basin of the Taupo-Hikurangi arc-trench system, North Island, New Zealand. The volcanic arc is best developed at the southern (Tongariro volcanic centre) end of the TVZ, while on the eastern side of the TVZ it is represented mainly by dacite volcanoes, and in the Bay of Plenty andesite/dacite volcanoes occur on either side of the Whakatane graben. The back-arc basin is best developed in the central part of the TVZ and comprises bimodal rhyolite and high-alumina basalt volcanism. Widespread ignimbrite eruptions have occurred from this area in the past 0.6 Ma. Normal faults occur in both arc and back-arc basin. They are generally steeply dipping (>40°) and strike between 040° and 080°. In the back-arc basin, fault zones are en echelon and have the same trend as alignments of rhyolite domes and basalt vents. Open fissures have formed during historic earthquakes along some of the faults, and geodetic measurements on the north side of Lake Taupo suggest extension of 14±4 mm/year. In the Bay of Plenty and M_L=6.3 earthquake occurred on 2 March 1987. Modelling of known structure in the area together with data derived from this earthquake suggests block faulting with faults dipping 45°±10° NW and a similar dip is suggested by seismic profiling of faults offshore of the Bay of Plenty where extension is estimated to be 5±2 mm/year. To the east of the TVZ, the North Island shear belt (NISB) is a zone of reverse-dextral, strike-slip faults, the surface expression of which terminates at the eastern end of the TVZ. On the opposite side of the TVZ in the offshore western Bay of Plenty and on line with the NISB is the Mayor Island fault belt. If the two fault belts were once continuous, as seems likely, strike-slip faults probably extend through the basement of the TVZ. When extension associated with the arc and back-arc basin is combined with these strike-slip faults, the resulting transtension provides a suitable tectonic environment for caldera formation and voluminous ignimbrite eruptions in the back-arc basin. The types of volcano in the TVZ are considered to be related to the source of magma and overlying crustal structure. Lavas of the arc are probably formed by a multistage process involving (1) subsolidus slab dehydration, (2) anatexis of the mantle wedge, (3) fractionation and minor crustal assimilation and (4) magma mixing. High-alumina basalts of the back-arc basin may be derived by partial melting of peridotite at the top of the mantle wedge, while rhyolitic magmas are thought to come from partial melting of lavas and subvolcanic reservoirs associated with the southern end of the Coromandel volcanic zone. Extreme thinning associated with transtension in the back-arc basin will favour the eruption of large-volume, gas-rich ignimbrites accompanied by caldera formation.     

One-dimensional modelling of mantle flow

A one-dimensional model of flow between a fixed boundary at the bottom and a moving one on top with no net flow through vertical sections is tested for geophysically interesting mantle viscosity-depth functions. Such a model, although simplistic, may help in answering the question to what depth the return flow extends, at least in the case of moving plates measuring many thousand kilometers across, such as the Pacific plate.It the viscosity in the asthenosphere is less than three orders of magnitude smaller than that of the mantle below, the return flow extends to great depth and the asthenosphere is a zone of concentrated shear. If the viscosity contrast is greater, the return flow is concentrated in the asthenosphere. For a wide range of model parameters typical flow velocities below the asthenosphere are about one-tenth of the plate velocity. The pressure gradient required by the mantle flow may be manifest in gravity trends across moving plates, but no excessive gravity anomalies are required by the model if the absolute viscosity values conform to those inferred from post-glacial rebound data. A thinner and lower-viscosity layer is favored over a thicker and more viscous layer if both fit glacial rebound evidence. The present model may not be applicable if down to the core the viscosity is as low as about 10²¹ N s m^–2 with a free-slip bottom boundary.     

U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains——Crust-mantle interaction and LOMU component

Geochronological studies of mafic-ultramafic intrusions occurrence in the northern Dabie zone (NDZ) suggest that these pyroxenite-gabbro intrusions formed 120—130 Ma ago should be post-collisional magmatic rocks[1—4]. These mafic-ultramafic rocks provid…     

The mantle convection model with non-Newtonian rheology and phase transitions: The flow structure and stress fields

The mantle convection model with phase transitions, non-Newtonian viscosity, and internal heat sources is calculated for two-dimensional (2D) Cartesian geometry. The temperature dependence of viscosity is described by the Arrhenius law with a viscosity step of 50 at the boundary between the upper and lower mantle. The viscosity in the model ranges within 4.5 orders of magnitude. The use of the non-Newtonian rheology enabled us to model the processes of softening in the zone of bending and subduction of the oceanic plates. The yield stress in the model is assumed to be 50 MPa. Based on the obtained model, the structure of the mantle flows and the spatial fields of the stresses σ_xz and σ_xx in the Earth’s mantle are studied. The model demonstrates a stepwise migration of the subduction zones and reveals the sharp changes in the stress fields depending on the stage of the slab detachment. In contrast to the previous model (Bobrov and Baranov, 2014), the self-consistent appearance of the rigid moving lithospheric plates on the surface is observed. Here, the intense flows in the upper mantle cause the drift and bending of the top segments of the slabs and the displacement of the plumes. It is established that when the upwelling plume intersects the boundary between the lower and upper mantle, it assumes a characteristic two-level structure: in the upper mantle, the ascending jet of the mantle material gets thinner, whereas its velocity increases. This effect is caused by the jump in the viscosity at the boundary and is enhanced by the effect of the endothermic phase boundary which impedes the penetration of the plume material from the lower mantle to the upper mantle. The values and distribution of the shear stresses σ_xz and superlithostatic horizontal stresses σ_xx are calculated. In the model area of the subducting slabs the stresses are 60–80 MPa, which is by about an order of magnitude higher than in the other mantle regions. The character of the stress fields in the transition region of the phase boundaries and viscosity step by the plumes and slabs is analyzed. It is established that the viscosity step and endothermic phase boundary at a depth of 660 km induce heterogeneities in the stress fields at the upper/lower mantle boundary. With the assumed model parameters, the exothermic phase transition at 410 km barely affects the stress fields. The slab regions manifest themselves in the stress fields much stronger than the plume regions. This numerically demonstrates that it is the slabs, not the plumes that are the main drivers of the convection. The plumes partly drive the convection and are partly passively involved into the convection stirred by the sinking slabs.     

Origin of Icelandic basalts: A review of their petrology and geochemistry

The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low-¹⁸O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.     

剪切生热与花岗岩部分熔融——关于喜马拉雅地区逆冲断层与地壳热结构的分析

本文对影响喜马拉雅逆冲断层地区热结构的三种主要热源（断层的摩擦剪切热,地壳的放射性热和上地幔的传导热）的效应进行了计算分析,在计算中考虑了岩石的流变性造成的剪应力和温度的非线性关系及岩石温度到达熔点时的熔解热.结果表明,尽管地壳的放射性热和上地幔的传导热也可能引起地壳中的部分熔融,但此时熔融将从地壳底部开始发生,这与喜马拉雅地区观测到的地质现象不符;在考虑了剪切断层带的剪切热后,熔融将从断层附近开始发生,因此得到了比较符合实际地质现象的结果.     

剪切生热与花岗岩部分熔融——关于喜马拉雅地区逆冲断层与地壳热结构的分析

本文对影响喜马拉雅逆冲断层地区热结构的三种主要热源(断层的摩擦剪切热,地壳的放射性热和上地幔的传导热)的效应进行了计算分析,在计算中考虑了岩石的流变性造成的剪应力和温度的非线性关系及岩石温度到达熔点时的熔解热.结果表明,尽管地壳的放射性热和上地幔的传导热也可能引起地壳中的部分熔融,但此时熔融将从地壳底部开始发生,这与喜马拉雅地区观测到的地质现象不符;在考虑了剪切断层带的剪切热后,熔融将从断层附近开始发生,因此得到了比较符合实际地质现象的结果.     

Recent volcanism in Masaya-Granada Area,Nicaragua

Masaya-Granada area is located in the middle part of the Central American volcanic zone. A basaltic shield volcano with a caldera, an acidic pyroclastic flow plateau with a caldera, cinder cones, maars, a lava dome and a composite andesitic volcano were formed by recent volcanic activities. Magmas of basic and intermediate ejecta are supposed to be formed by partial melting of the upper mantle material. Most of basalts and andesites was derived from common parental magma after crystallization differentiation history, but some basalts, which have extremely high MgO content and low K₂O content might be derived from primary magma of different type. There is no evidence to deny the possibility of differentiation product of acidic rock from basic magma, but compositional gap on variation diagram suggest the possibility of partial melting origin. Strike-slip fault systems might have been formed in association with plate movement, and fluidal basaltic magma was erupted also along these fault zones.     

Siberian traps: Hypotheses and seismology data

Siberian traps are the result of huge basalt eruptions which took place about 250 Ma ago over a vast territory of Siberia. The genesis of Siberian traps is attributed to a mantle plume with a center in the region of Iceland or beneath the central Urals in terms of their present coordinates. The eruption mechanism is associated with delamination—replacement of the mantle lithosphere by the deep magma material. The receiver function analysis of the records from the Norilsk seismic station (NRIL) allows comparing these hypotheses with the factual data on the depth structure of the region of Siberian traps. The S-wave velocity section place the seismic lithosphere/asthenosphere boundary (LAB) at a depth of 155–190 km, commensurate with the data for the other cratons. The mantle lithosphere has a high S-wave velocity characteristic of cratons (4.6–4.8 km/s instead of the typical value 4.5 km/s). The seismic boundary, which is located at a depth around 410 km beneath the continents is depressed by ~10 km in the region of the NRIL station. The phase diagram of olivine/wadsleyite transformation accounts for this depression by a 50–100°С increase in temperature. At the depths of 350–400 km, the S-wave velocity drops due to partial melting. A new reduction in the S-wave velocities is observed at a depth of 460 km. The similar anomalies (deepening of the 410-km seismic boundary and low shear wave velocity at depths of 350–400 and 460–500 km, respectively) were previously revealed in the other regions of the Meso-Cenozoic volcanism. In the case of a differently directed drift of the Siberian lithosphere and underlying mantle at depths down to 500 km, these anomalies are barely accountable. In particular, if the mantle at a depth ranging from 200 to 500 km is fixed, the anomalies should be observed at the original locations where they emerged 250 Ma ago, i.e. thousands of km from the Siberian traps. Our seismic data suggest that despite the low viscosity of the asthenosphere, the mantle drift at depths ranging from 200 to 500 km is correlated with the drift of the Siberian lithospheric plate. Furthermore, the position of the mantle plume beneath the Urals is easier to reconcile with the seismic data than its position beneath Iceland because of the Siberian traps being less remote from the Urals.     

The existence of a thin low-viscosity layer beneath the lithosphere

The horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone, where plate of different ages is juxtaposed, is expected to drive a local circulation, the characteristics of which can be constrained by the amplitude, wavelength and age-dependence of the geoid. Two-dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone. Only a thin, low-viscosity layer provides a reasonable fit to the data. The best model so far obtained has a fluid layer 150 km thick with viscosity 1.5 × 10¹⁹ Pa s under a 75 km lid. Such a layer, which is incapable of transmitting strong horizontal shear stresses, could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates.     

The activation and cessation of Late Cenozoic extension in the lithosphere at the margin of the Baikal Rift Zone: Alternating sources of volcanism in the Vitim Upland

A comparative analysis of the concentrations of major oxides, trace elements, and the ¹⁴³Nd/¹⁴⁴Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16–14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14–9 Ma time span was caused by “passive” rifting in the west of the upland and by “active” rifting in the east. The “passive” rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the “active” rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1–0.6 Ma time span caused magma effusion with values of ¹⁴³Nd/¹⁴⁴Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material.