汉诺坝麻粒岩捕虏体锆石U-Pb 分析初步结果及讨论

本文报道了汉诺坝玄武岩中麻粒岩捕虏体的锆石 U-Pb 初步分析结果，并讨论了其成因的两种可能性。     

冀北张家口地区汉诺坝玄武岩中麻粒岩捕虏体的硫化物相

捕虏体麻粒岩是了解下地壳形成和演化的重要样品。汉诺坝新生代玄武岩中的二辉麻粒岩捕虏体样品中富含各种硫化物相,主要类型有:①孤立产出的球状出溶硫化物;②矿物颗粒之间或颗粒内的粗晶硫化物;③次生硫化物包裹体群;④裂隙充填硫化物。电子探针分析表明,硫化物的矿物成分均为贫镍磁黄铁矿,(Ni+Co+Cu)/Fe(原子比)远小于0.2;(Fe+Cu+Co+Ni)/S(原子比)比地幔岩的磁黄铁矿小,多小于0.875,反映了一种S过饱和环境。各种产状的磁黄铁矿中Au、Ag都有一定的含量,其平均值分别为0.19%～0.22%(Au)、0.01%～0.02%(Ag),反映下地壳的麻粒岩化与金矿化的成因联系。磁黄铁矿的Ni、Co、Cu含量与S正相关,说明微量重金属元素与S具有同源的关系,由于地幔去气伴随S而进入下地壳。     

冀北张家口地区汉诺坝玄武岩中麻粒岩捕虏体的硫化物相

捕虏体麻粒岩是了解下地壳形成和演化的重要样品.汉诺坝新生代玄武岩中的二辉麻粒岩捕虏体样品中富含各种硫化物相,主要类型有①孤立产出的球状出溶硫化物;②矿物颗粒之间或颗粒内的粗晶硫化物;③次生硫化物包裹体群;④裂隙充填硫化物.电子探针分析表明,硫化物的矿物成分均为贫镍磁黄铁矿,(Ni+Co+Cu)/Fe(原子比)远小于0.2;(Fe+Cu+Co+Ni)/S(原子比)比地幔岩的磁黄铁矿小,多小于0.875,反映了一种S过饱和环境.各种产状的磁黄铁矿中Au、Ag都有一定的含量,其平均值分别为0.19%～0.22%(Au)、0.01%～0.02%(Ag),反映下地壳的麻粒岩化与金矿化的成因联系.磁黄铁矿的Ni、Co、Cu含量与S正相关,说明微量重金属元素与S具有同源的关系,由于地幔去气伴随S而进入下地壳.     

冀北张家口地区汉诺坝玄武岩中麻粒岩捕虏体的硫化物相

捕虏体麻粒岩是了解下地壳形成和演化的重要样品.汉诺坝新生代玄武岩中的二辉麻粒岩捕虏体样品中富含各种硫化物相,主要类型有:①孤立产出的球状出溶硫化物;②矿物颗粒之间或颗粒内的粗晶硫化物;③次生硫化物包裹体群;④裂隙充填硫化物.电子探针分析表明,硫化物的矿物成分均为贫镍磁黄铁矿,(Ni Co Cu)/Fe(原子比)远小于0.2;(Fe Cu Co Ni)/S(原子比)比地幔岩的磁黄铁矿小,多小于0.875,反映了一种S过饱和环境.各种产状的磁黄铁矿中Au、Ag都有一定的含量,其平均值分别为0.19%～0.22%(Au)、0.01%～0.02%(Ag),反映下地壳的麻粒岩化与金矿化的成因联系.磁黄铁矿的Ni、Co、Cu含量与S正相关,说明微量重金属元素与S具有同源的关系,由于地幔去气伴随S而进入下地壳.     

金伯利岩中石榴石辉石岩捕虏体的特征及其成因意义

在辽宁复县金伯利岩中发现了石榴石辉石岩的深源捕虏体,它是来自软流层的熔／流体在１００～１２０ｋｍ结晶的产物,其中的石榴石及磷灰石富含流体包裹体,有可能代表了地幔中富流体活动的幔内薄弱带的样品,如果该带普遍发育,则可能是中新生代中国东部活化阶段岩石圈地幔的一个拆沉界面。     

下地壳及壳幔过渡带化学不均一性──河北汉诺坝地区深源捕虏体元素地球化学证据

河北汉诺坝玄武岩中长英质麻粒岩、镁铁质麻粒岩和辉石岩捕虏体的主量元素和微量元素特征,反映了下地壳及壳幔过渡带组成具有高度化学不均一性。不相容元素原始地幔标准化曲线特征的差异表明捕虏体成因复杂。MgO与不相容元素无或仅有很弱的相关性,与相容元素的相关性则相对较明显。这些特征反映了下地壳及壳幔过渡带存在较强和多元的混合作用。捕虏体的加权平均值比基于麻粒岩地体资料估算的下地壳平均组成偏基性,揭示了二者可能的成因差异。     

汉诺坝地幔捕虏体中富硅熔体的成因及其意义

近年来地幔捕虏体中富硅熔体的研究受到广泛关注,富硅熔体不仅具有多种产出状态,其成因来源也具有多样性。本文选取汉诺坝地区地幔捕虏体矿物中的熔体包裹体和斜方辉石反应边中的熔体玻璃为研究对象,探索富硅熔体在岩石圈地幔演化中的作用。通过详细的岩相学观察并结合电子探针分析发现,二者成因不同。熔体包裹体玻璃成分富硅(SiO₂ 61%~65%),相对贫Na₂O(1%~3%)、K₂O(<1%),富含挥发份(约3%~6%),为地幔交代成因;斜方辉石反应边玻璃富硅(SiO₂ 64%~67%)、碱(Na₂O 5%~7%,K₂O 6%~9%),几乎不含挥发份,是地幔捕虏体被寄主玄武岩浆快速携带上升至地表的过程中两者反应的产物。地幔交代作用产生的富硅熔体包裹体深刻影响了岩石圈地幔的地球化学性质;而玄武岩浆在上升过程中与岩石圈地幔捕虏体中斜方辉石的反应,导致了岩石圈地幔由富硅向贫硅转变,为华北克拉通的破坏提供了证据。     

汉诺坝玄武岩中麻粒岩类捕掳体的岩石学特征

根据矿物组合特征,汉诺坝玄武岩中的麻粒岩捕掳体可划分为四种类型：长英质麻粒岩、苏长质麻粒岩、二辉二长麻粒岩和二辉麻粒岩,其中以二辉麻粒岩最为常见。麻粒岩捕掳体均具变质重结晶结构以及块状或条带状构造。不同种类的麻粒岩捕掳体在矿物组成和矿物成分上存在明显差别,但各主要矿物之间存在一定的成分相关性。应用Ｗｏｏｄ和Ｂａｎｎｏ二辉石温度计的计算结果表明,含二辉石麻粒岩捕掳体的平衡温度分布于８００°～９５０℃之间,主要集中于９００°±５０℃范围内,具Ｏｐｘ＋Ｃｐｘ＋Ｐｌ矿物组合的二辉麻粒岩捕掳体的形成压力范围约为０．６～１．１ＧＰａ。二辉二长麻粒岩落在相似的温压范围内。麻粒岩捕掳体在９５０℃处的突然消失指示了第三纪时汉诺坝地区岩石学壳幔边界的温度。     

汉诺坝橄榄岩捕虏体的单斜辉石LAM-ICPMS分析及其岩石圈地幔演化意义

橄榄岩成分及其中矿物(如单斜辉石)微量元素组成可以很好地揭示岩石圈地幔性质．在对汉诺坝新生代玄武岩中橄榄岩捕虏体做详细岩相学和岩石化学研究基础上,重点分析了单斜辉石的激光原位微量元素,并探讨了新生代华北克拉通北缘岩石圈地幔特征及地幔演化．汉诺坝地区岩石圈地幔是相当于原始地幔经过不同程度的部分熔融抽取形成的,除个别样品的部分熔融程度为15％-20％外,多数样品<5％．全岩主元素、单斜辉石成分体现出新生代汉诺坝地区的岩石圈地幔是很不均一的:在主体饱满型中有亏损型地幔的残留．这种共存现象可能是软流圈物质对古老地幔进行侵蚀、混合和改造置换的结果．单斜辉石微量元素组成所体现的碳酸岩岩浆交代作用和硅酸盐熔／流体的交代作用也支持这一认识．     

汉诺坝辉石岩包体中单斜辉石的微区微量元素组成特征及其地球动力学意义

本文对来自汉诺坝玄武岩中的三类辉石岩包体的单斜辉石（Cpx）利用LA—ICP—MS进行了原位微区微量元素分析研究。金云母单斜辉石岩中的单斜辉石具有低Cr,Co和Ni，高CaO、Zr、Hf、Ta和LREE特征，并且具有较低的Mg^#，反映金云母单斜辉石岩可能是经受了后期交代作用的地慢岩浆堆晶产物尖晶石二辉橄榄岩中的石榴辉石岩脉体中的Cpx具有高Na2O低CaO特征，其Ti、Zr、Hf、Y含量明显高于其它石榴辉石岩Cpx中的含量，反映了石榴辉石岩脉体可能是富Na硅酸盐熔体与地幔尖晶石二辉橄榄岩反应的成因，部分石榴辉石岩包体的Cpx具有特征的La富集或微量元素（如Zr、Hf、Ta、Li、P、REE等）环状对称分布特征，暗示华北克拉通北缘可能存在多期次地幔交代富集作用．通过对同一Cpx颗粒中的流体包裹体密集区和干净区域的对比研究，发现Cpx中的流体包襄体不仅富集LILE和LREE，而且HFSE中的Nb、Ta、Zr和P也具有明显的富集特征，LREE和HFSE未发生明显分异，这可能暗示地幔交代流体中含有少量的络阴离子（如F^-和Cl^-等）．     

汉诺坝长英质麻粒岩包体同位素年代学及其地球动力学意义

汉诺坝地区周坝长英质麻粒岩包体的岩石学、地球化学特征指示它们部分为变沉积岩。从周坝麻粒岩相变沉积岩包体的 Sm- Nd同位素分析结果获得了 (424± 10) Ma的全岩-单矿物 Sm- Nd同位素等时线年龄。全岩 Sm- Nd同位素组成指示这些包体可能与地表出露的太古宙麻粒岩有亲缘关系，但二者的 Pb- Pb与 U- Pb锆石年龄又反映二者成因不同。周坝长英质麻粒岩包体可能为残留于大陆下地壳的古老物质。 424 Ma的 Sm- Nd年龄可能反映了与加里东运动时期蒙古板块向华北地台俯冲而发生的变质与再就位抬升作用。     

Granulite and pyroxenite xenoliths from the Deccan Trap: insight into the nature and composition of the lower lithosphere beneath cratonic India

Granulite and pyroxenite xenoliths in lamprophyre dykes intruded during the waning stage of Deccan Trap volcanism are derived from the lower crust beneath the Dharwar craton of Western India. The xenolith suite consists of plagioclase-poor mafic granulites (55% of the total volume of xenoliths), plagioclase-rich felsic granulites (25%), and ultramafic pyroxenites and websterites (20%) with subordinate wehrlites. Rare spinel peridotite xenoliths are also present, representing mantle lithosphere. The high Mg #, low SiO₂/Al₂O₃ and low Nb/La (<1) ratios suggest that the protoliths of the mafic granulites broadly represent cumulates of sub-alkaline magmas. All of the granulites are peraluminous and light rare-earth element-enriched. The felsic granulites may have resulted from anatexis of the mafic lower crustal rocks; thus, the mafic granulites are enriched in Sr whereas the felsic ones are depleted. Composite xenoliths consisting of mafic granulites traversed by veins of pyroxenite indicate intrusion of the granulitic lower crust by younger pyroxenites. Petrography and geochemistry of the latter (e.g. presence of phlogopite) indicate the metasomatised nature of the deep crust in this region.Thermobarometric estimates from phase equilibria indicate equilibration conditions between 650 and 1200 °C, 0.7-1.2 GPa suggestive of lower crustal environments. These estimates provide a spatial context for the sampled lithologies thereby placing constraints on the interpretation of geophysical data. Integration of xenolith-derived P-T results with Deep Seismic Soundings (DSS) data suggests that the pyroxenites and websterites are transitional between the lower crust and the upper mantle. A three-layer model for the crust in western India, derived from the xenoliths, is consistent with DSS data. The mafic nature of this hybrid lower crust contrasts with the felsic lower crustal composition of the south Indian granulite terrain.     

Chemical and Isotopic Characteristics of Gabbroic Xenoliths from Hannuoba,China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd,Sr and Pb isotopes.The results show that the gab-broic xenoliths are of more mafic basaltic composition .Their abundances show narrow variations in major elements.The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths.The gabbroic xenoliths are rich in Nd(0.51159-0.51249),Sr(0.70491-0.70768) and low in radiogenic Pb(16.283-17.046, 15.191-15.381 and 36.999-37.476),significantly different from basalts and lherzolites in isotopic space.The calculated Nd and Pb model ages are about 3.0-3.5 Ga.The rocks have relatively low equilibrium T(-850℃) and P(0.8-0.9 Gpa).They could be inter-preted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle.Their chemical and isotopic variations can be ascribed to different degrees of melting,segregation and long-term evolution.     

Thermal and petrological structure of the lithosphere beneath Hannuoba, Sino-Korean Craton, China: evidence from xenoliths

Deep-seated xenoliths entrained in the Hannuoba basalts of the northern Sino-Korean Craton include mafic and felsic granulites, mantle wall-rock from spinel– and garnet–spinel peridotite facies, and basaltic crystallisation products from the spinel-pyroxenite and garnet-pyroxenite stability fields. The mineral compositions of the xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the upper mantle and lower crustal rock-type sequences for the region. The xenolith-derived paleogeotherm is constrained in the depth interval of 45–65 km and like others from areas of young basalt magmatism, is elevated and strongly convex toward the temperature axis. Two-pyroxene granulites give the lowest temperatures and garnet pyroxenites the highest, while the spinel lherzolites fall between these two groups. The present-day Moho beneath the Hannuoba area is defined at 42 km by seismic data, and coincides with the deepest occurrence of granulite. Above this boundary, there is a lower crust–upper mantle transition zone about 10-km thick, in which spinel lherzolites and mafic granulites (with variable plagioclase contents) are intermixed. It is inferred that this underplating has resulted in a lowering of the original pre-Cenozoic Moho (then coinciding with the crust–mantle boundary, CMB) from about 30 km to its present-day position and was due to intrusions of basaltic magmas that displaced peridotite mantle wall-rock and equilibrated to mafic granulites. Trace element patterns of the diopsides (analysed by laser ablation-ICPMS) from the Cr-diopside series spinel lherzolites and associated layered xenoliths (spinel lherzolites and pyroxenites) indicate a fertile uppermost mantle with moderate depletion by low degrees of partial melting and little evidence of metasomatic activity. The similarity in major and trace element compositions of the minerals in both rock types suggests that the layered ultramafic xenoliths formed by mantle deformation processes (metamorphic segregation), rather than by melt veining or metasomatism.     

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.     

河北汉诺坝玄武岩中橄榄岩包体矿物的含水性研究——微区红外光谱分析

对12个来自河北汉诺坝玄武岩的橄榄岩包体中的单斜辉石和斜方辉石进行了详细的微区傅立叶变换红外光谱(Micro-FTIR)分析。结果显示,所有的单斜辉石和斜方辉石颗粒都含有以OH形式存在的结构水。对部分粒径较大的辉石颗粒内部的多点分析表明,结构水含量表现出中心高边缘低的不均一分布。这种不均一分布的特征应该来自于包体上升过程中由于压力降低而引起的H扩散。如果用每个样品多个测定颗粒的中心部位的平均值来代表该样品,12个样品的单斜辉石水含量为48×10-6~152×10-6,斜方辉石水含量20×10-6~55×10-6。根据矿物水含量(假设橄榄石的水含量为2×10-6)和它们的体积分数计算的全岩水含量为11×10-6~48×10-6。结合已经发表的橄榄岩包体数据来看,在岩石圈地幔的物理化学条件下,单斜辉石与斜方辉石之间水的平衡分配系数大约为2.2±0.6;岩石圈中水的分布可能具有纵向和横向上的不均一性。