Melt Enrichment of Shallow Depleted Mantle: a Detailed Petrological, Trace Element and Isotopic Study of Mantle-Derived Xenoliths and Megacrysts from the Cameroon Line

Major element, trace element and Sr–Nd–Pb isotopiccompositions of ultramafic xenoliths and megacrysts from thecontinental Cameroon line provide evidence for metasomatismof the upper most lithospheric mantle by enriched melts duringthe Mesozoic The megacrysts probably crystallized within thelower continental crust from melts similar to the host magmas.All the xenoliths originated as depleted residues after theextraction of basaltic melts, but some indicate evidence ofinteraction with enriched partial melts before entrainment.The U–Pb isotopic data on garnet are consistent with coolingthrough >900C at >300 Ma. The Sm–Nd isotope systematicsin constituent phases appear to have been in equilibrium ona xenolith scale at the time of entrainment, indicating derivationfrom mantle that remained at temperatures >600C until eruption.Spinel therzolies that show simple light rare earth element(LREE) depletions are characterized by isotopic compositionsthat are comparable with, but slightly more depleted than AtlanticN-MORB, suggesting that the unmetasomatized sub-continentallithosphere of the Cameroon line may be isotopically similarto that of sub-oceanic lithosphere. The Nd-depleted mantle modelages of these xenoliths are consistent with late Proterozoicdepletion, similar in age to much of the overlying continentalcrust. In contrast, samples that have LREE-enriched clinopyr-oxenes(La/Yb =4.7–9.4) contain trace amounts of amphibole, areenriched in U and have more radiogenic Pb and Sr. These xenolithsyield U–Pb and Sm–Nd model ages consistent withMesozoic enrichment, in agreement with the age of enrichmentof the source regions of the basalts, as deduced from Pb isotopiccompositions. Clinopyroxenes record three orders of magnitudeenrichment in U and LREE accompanied by progressive K depletionassociated with the growth of trace amphibole, with K/U ratiosthat range from 12000 to 1. The ratios of the trace elementsthought to have similar bulk D in mantle melting, Ce/Pb, Ba/Rband Nd/Sr ratios, display regional variations related to thetime integrated history of enrichments indicated by Nd isotopiccompositions. Mass balance calculations suggest that the meltsresponsible for the most recent enrichment of the lithospherehad higher La/Yb and U/Pb than Cameroon line host magmas, andwere probably the product of small degrees of partial meltingassociated with the earliest stages of the breakup of Pangea. KEY WORDS: Cameroon line; mantle xenoliths; megacrysts; REE; isotopic composition; trace element     

THE DEVELOPMENT OF THE UPPER YANGTZE VALLEY

GENERAL FEATURES. The Upper Yangtze valley includes the valleys of the main river and its chief tributaries. Chinshactiang (金沙江) and Yalungchiang (雅龍江) flow in north-south courses through Hsikang, Eastern Tibet, and turn to NEE     

QUATERNARY GLACIATION IN THE YANGTZE VALLEY

Eleven years ago when this society held its Third General Meeting I had the opportunity of calling your attention to certain strange deposits and topographical features that came under my notice in certain parts of North China. I showed you a number of st     

矿物热电效应测试

1965年笔者曾简要地介绍过有关热电效应的基本概念,国外有关矿物热电效应研究及其在地质学中的应用。 近年笔者装配了一套热电效应测试仪器,先后对33种矿物共103块标本作了测试。现将测试设备及结果简介如下。     

大气折射率起伏对雷达波的散射

研究大气折射率起伏对雷达波的散射时,得到的主要結果有以下两点: 1.在适当的大气条件下,折射率起伏构成的后向散射波能够超出一般雷达的灵敏度之上,它的量級正好和雷达观測到的气象“仙波”相同.根据这一結果和其他事实,作者认为起伏的后向散射可能是形成气象“仙波”的重要机制。 2.推导了表示雷达波在折射率有起伏的介貭中传播时,强度随距离变化的方程式,結果发現:一般情况下,散射效应不致引起波的强度的显著变化.但是,当大气折射率起伏的尺度长較小、輻度較大、并且雷达波瓣較窄时,散射效应会引起波瓣的加寬。     

桂北新元古代两类过铝花岗岩的地球化学研究

广西北部新元古代花岗岩类岩石包括黑云母花岗闪长岩和黑云母花岗岩。地球化学特征表明，黑云母花岗闪长岩与含堇青石的过铝花岗岩（CPG）相当，而黑云母花岗岩则类似于白云母二长花岗岩（MPG）。黑云母花岗岩类是成熟地宙岩石部分熔融作用的产物，而黑云母花岗闪长岩类的形成与地幔柱起源的镁铁质岩浆和地壳起源的过铝质黑云母花岗岩浆之间的混合作用有关。这两类新元古代过铝花岗岩的形成与碰撞造山导致地壳加厚的挤压性构造无关，而与导致Rodinia超大陆裂解的地幔柱上升诱发岩石圈伸展的张性构造相联系。     

遥感、GIS结合与区域天然滑坡调查

以香港大屿山岛中部为试验研究区，探讨了遥感与GIS结合在区域滑坡调查中应用的可能性。在详细分析滑坡与环境因子关系后，将岩土类型、地貌、侵蚀、坡度等环境因子引入到影像中，与影像组成多源复合图像，可使地物的空间属性信息得到补充。同时采用适合高维复合数据的BP神经元网络对研究区天然滑坡进行识别，结果表明，区域滑坡的识别能力有较大提高。该方法的应用，将为大区域滑坡调查与监测奠定基础。     

“Red Beds”of Hunan

INTRODUCTION The term"Red Beds"has never been used in a restricted sense,because it could include formations of different ages and sometimes theyare not distinctly red in color.The"Red Beds"of Hunan were first     

Glaucophane-bearing Metamorphic Rock Types of the Cazadero Area, California

A detailed field and laboratory study has been made of a well-exposedglaucophane schist sequence within the Jurassic and CretaceousFranciscan Formation of northern California. Three types ofglaucophane-bearing metamorphic rocks have been distinguishedin and around the area of the detailed study. Each is characterizedby distinctive textures and mineral assemblages that are interpretedto represent different grades of metamorphism within the glaucophaneschist facies. From the combination of small- and large-scale mapping in thearea described it is clear that coarsely schistose blocks, tensof feet in diameter, commonly rest directly upon and withinless intensely metamorphosed terrain. In the Cazadero area theseisolated blocks of coarsely crystalline rocks are concentratedin a band that is roughly concordant with some of the majorfaulting, and their metamorphic fabric shows no consistent relationto local or regional structures. It is tentatively suggestedthat these blocks have been transported upward tectonicallyand that they are not stratigraphically equivalent to the othertypes of glaucophane-bearing metamorphic rocks in the area. Chemical and petrographic evidence indicates that basalt andsediments have been converted to glaucophane-bearing rocks underconditions of metamorphism that were essentially isochemical,except for fugitive components and some minor elements.