Hot-spots,mantle plumes and a model for the origin of ultramafic xenoliths in alkali basalts

The Wilson-Morgan hypothesis of hot-spots, characterized by high heat flow, positive gravity anomaly and alkalic volcanism, assumes that such hot-spots are surface expressions of mantle plumes rising by thermal convection. Possible evidence of this mantle upwelling is shown here from textural, structural and chemical aspects of ultramafic xenoliths in alkalic basalts. The xenolith-bearing basalts are constanly associated with Wilson-Morgan hot-spots in the ocean basins and with their continental counterparts in the rift valleys which show extensional tectonics. Most of the xenoliths are considered to be accidental fragments of the lithosphere in the host basalts. One remarkable aspect of xenoliths from all parts of the world is their ubiquitous tectonite fabric. The microstructures of these xenoliths are due to plastic deformation. Some of the xenoliths from Baja California show characteristic deformational features which are also found in the marginal parts of diapirically intruded high-temperature peridotite massifs. A model is proposed for the origin of xenoliths in alkalic basalts by mantle upwelling in which the plastic deformation of the xenoliths reflects this dynamic uprise.     

Electrical structure of the upper mantle beneath Central Europe: Results of the CEMES project

In the years 2001–2003, we accomplished the experimental phase of the project CEMES by collecting long-period magnetotelluric data at positions of eleven permanent geomagnetic observatories situated within few hundreds kilometers along the south-west margin of the East European Craton. Five teams were engaged in estimating independently the magnetotelluric responses by using different data processing procedures. The conductance distributions at the depths of the upper mantle have been derived individually beneath each observatory. By averaging the individual cross-sections, we have designed the final model of the geoelectrical structure of the upper mantle beneath the CEMES region. The results indicate systematic trends in the deep electrical structure of the two European tectonic plates and give evidence that the electrical structure of the upper mantle differs between the East European Craton and the Phanerozoic plate of west Europe, with a separating transition zone that generally coincides with the Trans-European Suture Zone.     

Sr-Nd-Pb isotopes of the Early Paleozoic maficultramafic dykes and basalts from South Qinling belt and their implications for mantle composition

Late Early Paleozoic mafic-ultramafic dykes and volcanic rocks from the South Qinling belt are char- acterized by εNd( t ) = 3.28― 5.02, (87Sr/86Sr)i= 0.70341― 0.70555, (206Pb/204Pb)i = 17.256― 18.993, (207Pb/204Pb)i= 15.505―15.642, (208Pb/204Pb)i=37.125―38.968, ?8/4=21.18―774.43, ?7/4=8.11―18.82. These charac- teristics suggest that they derived from a Middle Neoproterozoic mantle with isotopic compositions of mixed HIMU, EMII and minor EMI components. We interpret that these rocks were melting products of depleted mantle modified by subducted ancient oceanic crust and continental margin sediments along the northern margin of Yangtze block during Early Neoproterozoic.     

Correlation and petrogenesis of silicic pyroclastic rocks in the Northern Pannonian Basin,Eastern-Central Europe: In situ trace element data of glass shards and mineral chemical constraints

The Neogene volcanism of the Carpathian–Pannonian Region, Eastern-Central Europe started with repeated explosive eruption of silicic magmas. Volcanic products consist mostly of non-welded and partially to densely welded ignimbrites, which cover large areas in the Pannonian Basin. Since this volcanism occurred during a long time interval, from 21 Ma to 13.5 Ma ago, these pyroclastic deposits have great stratigraphic importance, as well as providing valuable information about petrogenetic processes during the formation of the back-arc basin area. In this study, we used in situ trace element data of glasses (glass shards, pumice and fiamme glass) determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), combined with microprobe geochemical data of the main mineral phases (plagioclase, biotite), to correlate the scattered outcrops of the ignimbrite units. Based on these data, we distinguished four ignimbrite units in the Bükkalja Volcanic Field, Northern Pannonian Basin. Each of these units is characterized by specific geochemical fingerprints. Thorium, Nb, Y and the rare earth elements are the most effective discriminator elements in glasses. The modal composition of mineral phases (occurrence or lack of certain minerals) and chemistry of plagioclases and biotites are also good correlation tools, especially the Fe, Mg and Ti contents of biotites. We suggest that these correlation criteria, particularly the grain-specific LA-ICP-MS data, could be effectively used also in other ignimbrite fields and in wider tephrochronological studies. The in situ trace element composition of glasses, representing the liquid part of the erupted magma, can be also used to constrain the petrogenesis of the rhyolitic magmas. Trace element ratios such as La/Nb, La/Y and Th/Nb suggest the importance of minor (e.g., hornblende and ilmenite) and accessory (e.g., zircon, allanite) minerals controlling the composition of the erupted melt. Rhyolitic magmas probably evolved from metaluminous andesitic parental melts via fractional crystallization. Syn-eruptive magma mingling was detected in the genesis of the Middle Ignimbrite Unit, based on the strong intra-sample geochemical variation both in the glasses and in the phenocrysts.     

Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: Implications for its evolution and recent reactivation

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on “growing” the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years.     

Receiver function study of the crustal structure of Northeast China:Seismic evidence for a mantle upwelling beneath the eastern flank of the Songliao Basin and the Changbaishan region

Northeast China is a unique place to study intra-plate volcanism.We analyzed P-wave receiver function data recorded by 111 permanent broadband seismic stations in Northeast China.The results show that the crustal thickness varies from 27.9 km beneath the eastern flank of the Songliao Basin to 40.7 km beneath the Great Xing'an Range region.The large depth variations of the Moho can be largely but not completely explained by surface topography.The residual Moho depth calculated based on the Airy's isostasy mo...     

新疆准噶尔北缘覆盖区深部地质结构及其地质意义——来自综合地球物理探测的证据

本文利用准噶尔北缘覆盖区的高精度地面重磁数据开展三维约束反演,揭示出准噶尔盆地北缘浅覆盖区内三维密度结构具有横向非均匀性,纵向有向深部延伸的趋势;深部广泛分布石炭纪、泥盆纪火山岩地层等高磁性体,零散分布热液蚀变导致的热退磁的火山岩;北西向分布的低密度低磁性物质,对应无磁性的断裂破碎带.结合多尺度边缘检测方法推断得到研究区隐伏断裂体系以及火山岩地层的空间展布.对贯穿研究区的综合地球物理探测剖面进行人机交互联合反演,结果显示控制扎河坝蛇绿岩带分布的南北界深大断裂在深部构成开口向下的对称扇形构造样式,说明此蛇绿岩带所代表的古洋盆具有双向俯冲的特征;结合区域地质资料,以及岩石地球化学研究成果,认为剖面穿过的3个酸性岩体均为形成于晚石炭世后造山拉张构造环境的A型花岗岩,可能是准噶尔东北缘造山作用结束的标志.最后结合大量地质、地球化学和地球物理资料,对准噶尔东北缘区域构造演化进行了讨论,认为扎河坝—阿尔曼太古洋盆于中奥陶世开始汇聚,古洋盆向南北两侧双向俯冲,在晚志留纪-早泥盆纪洋盆闭合.