碳酸岩Sr、Nd、Pb 同位素地球化学研究评述

碳酸岩是出露相对较少的幔源岩石，其中Sr与Nd是研究地幔物质组成的主要对象之一。本文统计了世界上主要碳酸岩的锶、钕、铅同位素组成特征；研究显示，碳酸岩源区主要是洋岛玄武岩高U／Pb的HIMU端员和富集端员(EM1或EM2)的混合作用；此外大部分碳酸岩的锶、钕同位素落在大洋玄武岩范围内；这些均表明其成因与地慢柱有密切联系。碳酸岩及与之共生的硅酸岩的同源或独立源区模式部很难充分解释两者同位素组成特征，逭反映碳酸岩的演化模式涉及更复杂的过程。可能是俯冲作用使碳酸岩源区经历不同时间和程度的富集、亏损过程导致地幔源区成分不均一。     

P–T–fO2 controls on a partly inverse chromite-bearing ultramafic intrusive: an evaluation from the Sukinda Massif, India

The chromites from the alpine type ultramafic intrusive of Sukinda, India, display a typical partly inverse spinel form and occur in two distinct zones: Brown Ore Zone (BOZ) and Grey Ore Zone (GOZ). The host ultramafites are mostly altered and are represented by the serpentinite, tremolite-talc(chlorite) schist, talc-serpentine schist and chlorite rock. The less altered variants are dunite, harzburgite and websterite. A dyke of orthopyroxenite runs through the main ultramafic body.The composition of olivine (Fo₉₂), orthopyroxene (En_92–89) and Al₂O₃ contents of the parental liquid (10.40–11.45%) determined from chromites, suggest that the parent melt is of boninitic affinity. The chemical plot of TiO₂ content against cr# of chromites corroborates a boninitic parental melt. The Fe–Mg partitioning in olivine and chromite depicts the temperature for chromitites as 1200 °C. A compositional plot of mg# and cr# suggests crystallization at high pressure conditions, corresponding to the kimberlite xenolith field. From the P–T diagram of pyrolite melting and mineral assemblage, the pressure of crystallization is stipulated to be ≥1.2 GPa. The f_O2 values estimated from Fe³⁺/Cr+Al+Fe³⁺ ratios range from 10^−8.3 to 10^−9.3 for the GOZ and 10^−7.1 to 10^−7.3 for the BOZ. The f_O2 values together with the pressure range suggest crystallization at upper mantle conditions. The heterogeneity in chemical composition and f_O2 conditions for the GOZ and BOZ could be linked to heterogeneity in the upper mantle.     

Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere

We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=600–1300°C and P= 25–80 kbar in a thick(at least 220–250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 91–93·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton     

Passive Seismic Experiment Mosaic - A Pilot Study of Mantle Lithosphere Anisotropy of the Bohemian Massif

We present results of an array study of seismic anisotropy beneath the Bohemian Massif (BM) showing distinct lateral and directional variations of the fast shear-wave polarization and split-delay time consistent with variations in the P-residual spheres, in which directional means of relative residuals are subtracted. Our analysis of the S- and P-wave anisotropy suggests that the mantle lithosphere of the BM consists of at least three large domains with different orientation of the large-scale fabric separated by sutures cutting most likely the whole lithosphere. Boundaries of the units are characterised by a null or small shear-wave splitting, as well as by smaller values in the P-residual spheres. We present self-consistent 3D anisotropic models of the lithosphere domains of the BM resulting from joint inversion of anisotropic parameters searching for a 3D orientation of mantle fabric. While in the Saxothuringian and Sudetes parts of the BM the (a, c) foliations dip prevailingly to the N-NW, they dip to the S and SW in the southern and eastern parts of the Moldanubian.     

地幔柱构造研究概述

地幔柱构造理论是近年来构造地质学研究的新热点,是当今地球科学——地质学、构造学、矿床学、地球物理学、生物学、环境学和气象学等许多学科关注和研究的前沿领域。它的形成和演化及动力学观点被称为继大陆漂移和板块构造后的第3次地学浪潮,引起了中外地学者的高度重视。本文对地幔柱构造研究现状作了概略介绍,以期在铀矿地质领域内引起关注．起到传递信息和抛砖引玉的作用。     

辽东中生代晚期和古近纪玄武岩及深源捕虏晶--对岩石圈地幔性质的制约

辽东地区玄武岩的K-Ar定年结果表明,曲家屯玄武岩形成于晚白垩世,K-Ar年龄为81．58±2．46Ma;乱石山子玄武岩形成于古近纪,K-Ar年龄为58．36±1．64Ma。本区玄武岩含有丰富的橄榄石、单斜辉石和角闪石捕虏晶。乱石山子玄武岩中橄榄石捕虏晶的Mg^#值（79．5-88．5之间,平均值为84）较曲家屯玄武岩中橄榄石捕虏晶Mg^#值（77．0～79．8之间,平均值为78．4）偏高;单斜辉石捕虏晶为透辉石,其从核部到边部的Mg^#等变化趋势与橄榄石类似;斜方辉石捕虏晶为占铜辉石,其Mg^#值介于85．2-87．6之间,平均值为86．4。捕虏晶发育的环状裂隙、扭折带、矿物成分环带以及捕虏晶与主岩Mg^#值之间的不平衡均暗示它们为玄武质岩浆上升捕获的早期岩浆晶出矿物的堆晶体。玄武岩的岩石地球化学分析结果表明：（1）它们属于碱性系列,为碱性玄武岩,曲家屯玄武岩较乱石山子玄武岩贫硅、镁,富钙、铝,它们均具有原始岩浆的特征;（2）二者具有相似的稀土元素配分模式,但曲家屯玄武岩轻稀土元素总量更高,且轻重稀土元素分离程度高;（3）二者具有相似的Sr-Nd同位素组成,Isr和εNd（t）值分别介于0．7039～0．7045和＋1．60～＋3．69,反映了亏损的岩石圈地幔特征。     

Crustal accretion beneath the Koyna coastal region (India) and Late Cretaceous geodynamics

In 1967 a major earthquake in the Koyna region attracted attention to the hitherto considered stable Indian shield. The region is covered by a thick pile of Deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures. Although deep seismic sounding studies have provided vital information regarding the crustal structure of the Koyna region, much remains unknown. The two available DSS profiles in the region have been combined along the trend of Bouguer gravity anomalies. Unified 2-D density modelling of the Koyna crust/mantle suggests a ca. 3 km thick and 40 km wide high velocity/high density anomalous layer at the base of the crust along the coastline. The thickness of this anomalous layer decreases gradually towards the east and ahead of the Koyna gravity low the layer ceases to be visible. Based on the seismic and gravity data interpretation in the geodynamical/rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust. It is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating Indian plate passed over the Reunion hotspot.