Mantle structure and rifting processes in the Baikal–Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks

The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P–T- composition lithospheric cross-sections in the region for the depth range of 35–80 km.Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of ^143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle.Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P–T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.     

Low-pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area

ABSTRACT Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°-79°E, 68°-70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south-west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south-west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth. This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre-metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion-driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.     

软流圈

大多数金属矿床的形成直接或间接与岩浆活动有关。岩浆在不同的深度产生,因而带来不同的有用矿物组合。例如,金、银、锡、铜、金刚石、铂族元素等矿床的形成与地幔成因的岩浆有关。有岩浆熔融体产于其中的那一个层称为软流圈。软流圈里发生的过程控制着全球的构造现象、岩浆现象和变质现象。本文用各种方法证实了软流圈的存在。根据最新的观点,软流圈是低粘滞度、低速度、低密度、低质量因数、高塑性、高流动性、高电导性和高地震波能吸收性的一个层。这些特性的异常习性是由于100-300公里深处地幔温度与地幔物质熔化温度互相接近所致。软流圈虽然在全球各地都有记录,但其分布极不均匀。在老构造(如加拿大地盾、波罗的地盾和北美地台)之下,软流圈表现微弱,或者完全缺失。在加拿大地盾和波罗的地盾之下,软流圈厚约加公里。但在现代构造活动区(如贝加尔裂谷带、亚洲-太平洋活动边缘、帕米尔-兴都库什地区)之下,软流圈发育良好,厚约100—200公里。 从贝加尔湖到太平洋的地区是软流圈构造变化最大的地区之一。这里软流圈顶部的深度从,10—20变化到200公里,厚度则达200—400公里。软流圈顶部的最小深度和软流圈的最大厚度见于贝加尔裂谷带和亚洲-太平洋活动边缘。这两处是现代构造活动区。介于其间的地区由?     

http://www.sciencedirect.com/science/article/pii/S1674987110000071

<正>The lithospheric structure of China and its adjacent area is very complex and is marked by several prominent characteristics.Firstly,China's continental crust is thick in the west but thins to the east,and thick in the south but thins to the north.Secondly,the continental crust of the Qinghai—Tibet Plateau has an average thickness of 60—65 km with a maximum thickness of 80 km,whereas in eastern China the average thickness is 30—35 km,with a minimum thickness of only 5 km in the center of the South China Sea.The average thickness of continental crust in China is 47.6 km,which greatly exceeds the global average thickness of 39.2 km.Thirdly,as with the crust,the lithosphere of China and its adjacent areas shows a general pattern of thicker in the west and south,and thinner in the east and north.The lithosphere of the Qinghai—Tibet Plateau and northwestern China has an average thickness of 165 km, with a maximum thickness of 180—200 km in the central and eastern parts of the Tarim Basin,Pamir, and Changdu areas.In contrast,the vast areas to the east of the Da Hinggan Ling—Taihang—Wuling Mountains,including the marginal seas,are characterized by lithospheric thicknesses of only 50—85 km.Fourthly,in western China the lithosphere and asthenosphere behave as a "layered structure", reflecting their dynamic background of plate collision and convergence.The lithosphere and asthenosphere in eastern China display a "block mosaic structure",where the lithosphere is thin and the asthenosphere is very thick,a pattern reflecting the consequences of crustal extension and an upsurge of asthenospheric materials.The latter is responsible for a huge low velocity anomaly at a depth of 85—250 km beneath East Asia and the western Pacific Ocean.Finally,in China there is an age structure of "older in the upper layers and younger in the lower layers" between both the upper and lower crusts and between the crust and the lithospheric mantle.     

大陆岩石圈伸展与斑岩铜矿成矿作用

华南地区自古生代以来一直属于陆内构造演化环境。华南陆内伸展型斑岩铜矿主要形成于早侏罗世、晚侏罗世和早白垩世3个时期,其中晚侏罗世成矿期与华南中生代大规模钨锡成矿作用的形成基本属于一个时期。这些斑岩型矿床的时空分布与同时期的俯冲带在时间上和空间上具有明显不协调的关系,且与俯冲有关的、后俯冲伸展背景以及陆陆碰撞有关的斑岩铜矿的线性分布特点明显不同,尤其是早白垩世斑岩铜矿的分布明显呈面状分布,与华南中生代地壳明显减薄的区域基本一致。虽然这3个时期的斑岩型铜矿在地球化学上显示出弧岩浆岩的特点,但是地质事实证明在这3个时期,华南岩石圈发生了明显的伸展作用,尽管每个时期华南不同地区岩石圈伸展的程度可能不同。因此,我们把华南这种类型的斑岩铜矿归称之为"陆内伸展型"斑岩铜矿。陆内伸展型斑岩铜矿的成矿机制可能是岩石圈伸展背景下软流圈上涌导致陆下岩石圈地幔或者下地壳被改造有关。     

华北克拉通中部过渡带SKS波分裂研究:鄂尔多斯东南角的局部软流圈绕流

本文对布设在华北克拉通东西两块体交界区域的宽频带流动地震观测台阵和部分固定台站的远震波形记录开展了SKS波分裂研究.结果显示,鄂尔多斯块体内部的各向异性比较弱,剪切波分裂导致的时间延迟一般小于0.7s.鄂尔多斯块体东缘的山西断陷带和太行山以及华北平原西部均表现出了比较强的各向异性,时间延迟大于1.0s.特别是在太行山地区观测到的ENE趋向的快波偏振方向明显不同于鄂尔多斯块体和华北平原地区的近E-W和ESE方向的快波偏振方向.在华北克拉通东西两块体交界过渡带的太行山地区观测到的显著上地幔各向异性及变化可能对应于围绕鄂尔多斯块体东南角的局部软流圈绕流,而后者可能起因于鄂尔多斯块体的逆时针旋转以及青藏高原软流圈沿秦岭大别造山带向东的流动.     

东天山黄山西含铜镍矿镁铁-超镁铁岩体岩浆地幔源区特征研究

新疆黄山-镜儿泉铜镍成矿带位于中亚造山带东天山晚古生代造山带,铜镍总储量达百万吨,是我国仅次于金川硫化物矿床的铜镍矿基地.黄山西铜镍矿床是该成矿带内一个大型矿床,Cu平均品位是0.31％,总储量18.8×104t,Ni平均品位是0.49％,总储量32×104t.黄山西岩体岩石地球化学特征与塔里木大火成岩省镁铁-超镁铁侵入岩和玄武岩存在明显差异;较之塔里木大火成岩省镁铁-超镁铁岩体( 269～ 274Ma),黄山-镜儿泉铜镍成矿带镁铁-超镁铁岩体的形成更早(274～298Ma);此外,成矿带内并没有早二叠溢流玄武岩大量出露.黄山西岩体各岩相的MORB标准化微量元素蛛网图显示明显的Nb、Ta、Ti负异常,(87Sr/86Sr)269Ma值较低(0.7034～0.7037),而εNd(269Ma)较高(5.14～ 7.14),这些地球化学特征难以用地壳混染来解释,而显示其原始岩浆来自于交代地幔的部分熔融,表明原始岩浆可能形成于活动大陆边缘.然而,黄山西岩体的岩相学特征与阿拉斯加型岩体存在差别,因此,不能排除交代地幔的部分熔融发生于碰撞造山后的伸展阶段的可能,软流圈的上涌可能起到重要作用.     

18．6年地震轮回及其成因初探

将全球地震高发地区分成１３个研究区，分析各区浅源大地震（１８９７－１９９０年）与１８．６年月亮交点运动周期的关系，发现各区均有明显的１８．６ａ地震轮回存在：活跃期为１２．４ａ，平静期为６．２ａ．提出了软流层岩浆潮的概念，分析１８．６ａ地震轮回的成因是软流层岩浆潮的调制作用，并把这种解释称为岩浆潮假设．全球火山活动的１８．６ａ准周期性和各地震活跃期中心时间分布的规则性是对岩浆潮假设的支持．地震轮回可认为是全球或某一地区对地球内部和外部的非线性相互作用的响应或自组织过程．     

软流圈岩浆潮的调制作用与地震轮回关系初探

本文提出软流圈岩浆潮的假设来解释作者所发现的全球主要地震区的大地震都存在１８．６年的地震轮回。文中阐述了岩浆潮存在的可能性及其影响，认为１８．６年分量是岩浆潮的主要分量，并用全球火山活动的１８．６年准周期性和各主要地震区地震期的中心时间呈规则分布来支持岩浆潮假设。周期性地震轮回是一类混沌现象。地震轮回可以认为是全球或某一地区对地球内部和外部因素的非线性相互作用的响应或自组织过程。     

Temperature and density of the Tyrrhenian lithosphere and slab and new interpretation of gravity field in the Tyrrhenian Basin

The gravity anomaly field of the Tyrrhenian basin and surrounding regions reflects the complex series of geodynamic events active in this area since the Oligocene–Miocene. They can resume in lithospheric thinning and asthenospheric rising beneath the Tyrrhenian Basin, coexisting with the roll-back subduction of the African plate margin westward sinking beneath the Calabrian Arc. The geographic closeness between these processes implies an intense perturbation of the mantle thermal regime and an interference at regional scale between the related gravity effects.A model of the litho-asthenospheric structure of this region is suggested, showing a reasonable agreement with both the evidences in terms of regional gravity anomaly pattern and the results concerning thermal state and petro-physical features of the mantle. The first phase of this study consisted of the computation of the isotherms in the crust–mantle system beneath the Tyrrhenian Basin and, afterwards, of the density distribution within the partially melted upwelling asthenosphere. The second phase consisted of a temperature/density modelling of the slab subducting beneath the Calabrian Arc. Finally, a 2^1 / 2 interpretation of gravity data was carried out by including as constraints the results previously obtained. Thus, the final result depicts a model matching both gravity, thermal and petrographic data. They provide (a) a better definition of the thermal regime of the passive mantle rise beneath the Tyrrhenian basin by means of the estimation of the moderate asthenospheric heating and (b) a model of lithospheric slab subducting with rates that could be smaller than generally suggested in previous works.