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）．     

Whole-rock geochemical,U-Pb and Sm-Nd isotope characteristics of the Dongueni Mont nepheline syenite intrusion,Mozambique

The Dongueni Mont nepheline syenite intrudes migmatitic paragneisses and siliciclastic metasediments of the Barue Complex, Mozambique. This study reports the whole-rock geochemical, U-Pb and Nd isotopic data of the nepheline syenite. The ferroan and alkalic geochemical characteristics are typical of alkaline rocks formed in a within-plate setting. The strong depletion in high field strength elements(HFSEs)(e.g. Ba, Nb, P,and Ti) and enrichment in large ion lithophile elements(LILEs)(e.g. Rb, Th, K, and Pb) are consistent with magmatism in a continental alkaline magmatic province associated with intracontinental rifting. Zircon U-Pb data yielded crystallization ages from 498 ± 19 to 562± 14 Ma,consistent with the Pan-African Orogeny and the inherited zircons yield an age of 1040 Ma, which supports the presence of a Mesoproterozoic crust. Theε_(Nd)(t) values from the nepheline syenite samples range from-15.1 to-16.1 and the T_(DM)values from 1.77 to 1.67 Ga, which indicate that the initial nepheline syenite magma formed from a tholeiitic or mantle source in a within-plate setting with crustal assimilation.     

青藏米巴勒地区中新世钠质方沸石(霞石)响岩中单斜辉石环带研究:对岩浆-构造演化的启示

在拉萨地块中部米巴勒地区产出的中新世钠质方沸石(霞石)响岩中的单斜辉石发育大量环带结构,环带结构包括正环带、反环带和韵律环带。其中正环带结构从核部到边部Mg^#、MgO、Cr₂O₃呈逐渐降低趋势,而FeO、Na₂O、Al₂O₃、TiO₂呈逐渐升高的趋势,它反映了岩浆正常结晶的历史。反环带结构中Mg^#、MgO、Cr₂O₃从核部到边部呈升高的趋势,FeO、Na₂O、Al₂O₃的含量总体上呈降低趋势,本文认为响岩中单斜辉石的这种环带结构是受到了和它共存的钾质-超钾质火山岩岩浆混合或混染所致,两种岩石应是各自独立的母岩浆的结晶产物。具韵律环带结构的单斜辉石从核部到边部出现成分规律性波动,Na₂O、Al₂O₃、TiO₂、Cr₂O₃含量的变化在正反环带的范围内,Mg^#值的变化范围较小,在正环带斑晶和基质的范围内,它是岩浆多次混合作用的结果。该地区钠质过碱性岩的出现可能指示青藏高原在这套火山岩喷发时(13~12Ma),构造体制发生了显著的改变,进入到一个典型的陆内伸展阶段。     

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt,west of Khariar,NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.     

Origin of silicate minerals in carbonatites from Alnö Island,Sweden: magmatic crystallization or wall rock assimilation?

Textural, electron microprobe and whole rock geochemical evidence from carbonatites and associated silicate rocks on Alnö Island, Sweden, suggest that the carbonatite, at the time of emplacement, could have been an (almost) pure CaCO₃ liquid with a high volatile (H₂O–CO₂) content and that most silicate minerals, which are ubiquitously present, are either (1) assimilated from the surrounding wall rock, by progressive and coupled fragmentation and corrosion; or (2) by‐products of corrosive interaction between the carbonatite liquid and the wall rock. This interpretation is supported by balancing a reaction to describe interaction between carbonatite and a cpx + ne‐bearing (ijolite) wall rock. Although our analysis does not preclude the possibility that fenitizing agents (e.g. Na, Fe) were transported by the carbonatite liquid, these components are not required to drive the observed mineralogical changes in the carbonatite.     

High-Mg diorites derived from sanukitic HMA magmas, Kyushu Island, southwest Japan arc: evidence from clinopyroxene and whole rock compositions

High-Mg diorites that have similar whole rock composition to high-Mg andesites (HMAs) should not be simply interpreted as rocks solidified from the HMA magmas, because the high-Mg diorites may be mafic cumulates derived from a different magma from the HMAs.

The HMAs contain unique clinopyroxenes with higher Mg# and Si than those of other sub-alkaline series igneous rocks. The Mg# and Si are controlled by the source magma composition rather than its crystallized conditions such as pressure and temperature. The chemical composition of clinopyroxenes would present important information for the investigation of the source of high-Mg diorites.

We considered the source of Early Cretaceous high-Mg diorites on Kyushu Island, southwest Japan arc, based on their clinopyroxene and whole rock compositions. The clinopyroxenes have similar chemical characteristics to those in HMAs rather than those in other sub-alkaline rocks. Moreover, the whole rock compositions are equivalent to the sanukitic HMA and do not show features of mafic cumulates. This indicates that the high-Mg diorites solidified from sanukitic HMA magmas. It is generally believed that the sanukitic HMA magmas involve the subduction of a young and/or hot oceanic slab was situated in their genesis. Therefore, the occurrence of the high-Mg diorites suggests that Kyushu was situated in the tectonic setting of young and/or hot slab subduction in the Early Cretaceous.     

Petrology of ultramafic nodules from São Tomé Island, Cameroon Volcanic Line (oceanic sector)

Ultramafic xenoliths were found in recent alkali basalts from São Tomé Island. These include spinel peridotites (lherzolites, harzburgites and dunites) and pyroxenites (orthopyroxenites and clinopyroxenites). Textures and mineral compositions indicate that pyroxenites originated from crystal/liquid separation processes operating on magmas similar to those giving rise to their present host rocks whereas spinel peridotite xenoliths had an accidental origin; Fo (>89) and Ni (>0.36 wt.%) contents in olivines, Mg# (91–95) of orthopyroxenes and low Ti in clinopyroxene (primary crystals: TiO₂<0.06 wt.%) and in spinel (TiO₂<0.1 wt.%) are within the range reported for abyssal peridotites, indicating São Tomé spinel peridotites represent refractory residues of melting. Nevertheless, the lack of correlation between mineral chemistry and modal composition suggests that spinel peridotite xenoliths are not simple residues and were affected by infiltration of fluid/melts within the mantle. The wide temperature range obtained for spinel peridotites (700 to >1150 °C) is compatible with a long period of pre-entrainment cooling supporting Fitton's [Tectonophysics 94 (1983) 473] hypothesis that proposes oceanic lithosphere uprising in the Cameroon Volcanic Line prior to the initiation of the current thermal regime, related to São Tomé magmatism. The association of upper mantle (peridotite) xenoliths with igneous cumulates (pyroxenites) suggests that the spinel peridotite suite originated in the uppermost mantle above the São Tomé magma storage zone(s), probably in a region of high strain rate, near the boundary between the mantle and the overlying oceanic crust.     

Heat capacity and entropy of rutile (TiO<Subscript>2</Subscript>) and nepheline (NaAlSiO<Subscript>4</Subscript>)

 From heat capacities measured adiabatically at low temperatures, the standard entropies at 298.15 K of synthetic rutile (TiO₂) and nepheline (NaAlSiO₄) have been determined to be 50.0 ± 0.1 and 122.8 ± 0.3 J mol⁻¹ K, respectively. These values agree with previous measurements and in particular confirm the higher entropy of nepheline with respect to that of the less dense NaAlSiO₄ polymorph carnegieite. Received: 23 July 2001 / Accepted: 12 October 2001     

Probe and SIMS investigation of clinopyroxene inclusions in chromites from the Troodos chromitites (Cyprus): Implications for dunite–chromitite genesis

Chromite ores formation is still a debated topic and, in this study, detailed analyses of major, minor elements by EMPA and Rare Earth Elements (REE) by SIMS, were performed on silicate inclusions detected in chromite grains of chromitite pods, enveloping dunites and on clinopyroxene of lherzolitic host rock of well known Trodoos chromite ores. Results show the complexity of relationships among lithologies that reflects the subtlety of genetic events and of chromite ore occurrence.Analyses of textural and chemical features have led to a better understanding of the chromite ore genetic process, that is related to a supra-subduction geodynamic setting where partial melting processes were overlapped by metasomatic events.Metasomatism, that marks the general genetic context, is characterized by the presence of a hydrated boninitic melt that favors the precipitation of chromite ores. Chromite ores, typical of ophiolite complexes and usually enveloped by dunite are, in this case, characterized by the presence of two different types of dunites whose geochemical differences are reflected by olivine mineral chemistry and by different REE patterns of analyzed clinopyroxenes. Such geochemical marks, related to different magmatic and metasomatic events, could be a main key for location of fertile or barren dunites in terms of chromite ore occurrence in the field.     

Sr–barite droplets associated with sulfide blebs in clinopyroxene megacrysts from basaltic tuff (Szentbékkálla, western Hungary)

A unique occurrence of strontian barite droplets associated with oriented planes of primary sulfide blebs in several clinopyroxene megacrysts was found in the Plio-Pleistocene basaltic tuff of Szentbékkálla (Bakony–Balaton Highland, western Hungary). The barite droplets contain 3.22–14.7 wt.% SrO, variable amount of FeO (up to 3.25 wt.%), minor CaO, CuO and NiO. Textural arrangements of oriented sulfide planes show that they were trapped as immiscible sulfide melt during crystallization of the clinopyroxene. Temperature and pressure estimation (978–1094 °C and 1.0–1.1 GPa) of the host clinopyroxenes indicate that formation of clinopyroxene megacrysts and entrapment of sulfide blebs happened under upper mantle conditions.

A genetic relationship between the sulfate and sulfide blebs is doubtful. Textural features suggest that the barite droplets might have been trapped together with the sulfide blebs from immiscible sulfide melts. However, it cannot be excluded that the barite formed during metasomatic event following the sulfide formation.     

Experimental multipole-refined and theoretical charge density study of LiGaSi<Subscript>2</Subscript>O<Subscript>6</Subscript> clinopyroxene at ambient conditions

The synthetic LiGaSi₂O₆ clinopyroxene is monoclinic C2/c at room-T. Its experimental electron density, ρ(r), has been derived starting from accurate room-T single-crystal diffraction data. Topological analysis confirms an intermediate ionic-covalent character for Si–O bonding, as found by previous electron-density studies on other silicates such as diopside, coesite and stishovite. The non-bridging Si–O bonds have more covalent character than the bridging ones. The Ga–O bonds have different bonding characters, the Ga–O2 bond being more covalent than the two Ga–O1 bonds. Li–O bonds are classified as pure closed-shell ionic interactions. Similar to spodumene (LiAlSi₂O₆), Li has sixfold coordination, but the bond critical points associated to the two longest bonds are characterized by very low electron density values. Similar to what previously found in spodumene and diopside, O···O interactions were detected from the topological analysis of ρ(r), and indicate a cooperative interaction among the lone pairs of neighbouring oxygen atoms. In particular, this kind of interaction has been obtained for the O1···O1 edge shared between two Ga octahedra. Integration over the atomic basins gives net charges of −1.39(10), 2.82(10), 1.91(10) and 0.82(8) e for O (averaged), Si, Ga and Li atoms, respectively. Periodic Hartree–Fock and DFT calculations confirm the results obtained by multipole refinement of the experimental data. Moreover, the theoretical topological properties of the electron density distribution on the Si₂O₆ group are very similar to those calculated for spodumene. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Occurrence and mobility of major and trace elements in groundwater from pristine volcanic aquifers in Jeju Island,Korea

Major and trace elements in groundwater from basaltic aquifers in pristine conditions were investigated in a volcanic island to evaluate sources, sinks, and mobility of elements over a wide range of mineralization conditions with total dissolved solids from 50 mg/L to 3400 mg/L. Groundwater was highly undersaturated with respect to primary silicate minerals, indicating that dissolution of basaltic rocks may continue under conditions with precipitation of calcite and secondary silicates. Evolution of B/Cl ratio in groundwater from marine aerosols to basaltic rocks showed that the ratio could be used as a conservative tracer for interactions between water and basaltic rocks. Relative mobility (RM) of elements calculated using the concentrations of elements in the local basaltic rocks and those in groundwater showed that mobility decreased in the order of B > Rb > Na > K > Mg > Ca > Mo > V > Si > Sr > Sc > P > U > Zn > Pb > Cr > Cu > Ba > Ni > Ti > (Mn, Al, Fe, Co, Th) indicating that oxyanion-forming elements and alkali metals had the highest mobility. Compared to average RM, V had decreased mobility, and Fe and Mn had increased mobility in anoxic groundwater while V, Mo, and U had higher mobility in oxic-alkaline water. The sources of V, Cr, Cu, and Zn in rocks were estimated using the partition coefficients between minerals and basaltic melt, and the disparity between sources and mobility indicated that sinks are more important for controlling the concentrations of these elements in groundwater than the contents in the rocks. Principal component analysis (PCA) of hydrogeochemical parameters in groundwater produced three principal components (PC) which represent dissolution of basaltic rocks without significant attenuation of released solutes, higher degree of water–rock interactions resulting in oxic-alkaline conditions, and attenuation of Zn and Cu in higher pH, respectively. Spatial distribution of PCs revealed that groundwater with elevated concentrations of mobile elements was concentrated in the southwestern area and that concentrations of V and Cr were more scattered, which is likely to be controlled by pH and redox states of groundwater as well as degree of water–rock interactions.     

云南金平铜厂斑岩Cu(Mo-Au)矿床含矿石英正长斑岩地球化学特征及成因机制探讨

铜厂斑岩型Cu(Mo-Au)矿床位于金沙江-红河富碱侵入岩带的南段,其含矿的石英正长斑岩侵入体属金沙江-红河富碱侵入岩带的重要组成部分.对铜厂石英正长斑岩开展详细的地球化学研究结果表明,其具有高碱、富钾及准铝质-弱过铝质等特征,属钾玄岩系列岩石;微量和稀土元素分析表明,铜厂石英正长斑岩明显富集Rb、Ba、Th、U、K、La和Sr等大离子亲石元素,相对亏损Nb、Ta和Ti等高场强元素,具有明显的“TNT”负异常,稀土总量高,明显富集轻稀土而亏损重稀土,轻重稀土分馏显著;(87Sr/86Sr)i为0.707097～0.707138,εNd(t)为-7.1 ～ -6.8,在Sr-Nd同位素相关图上,与EMII来源岩石相似.对比金沙江-红河富碱侵入岩带北段的玉龙含矿的二长花岗斑岩和中段的马厂箐含矿的花岗斑岩的地球化学特征,铜厂含矿斑岩具有高的全碱、微量元素和稀土元素含量、高的(87Sr/86Sr)i值和低的εNd(t)值、不具埃达克质岩属性等特征.综合研究表明,铜厂含矿的石英正长斑岩在成因类型上属于A型花岗岩类,形成于～35 Ma的晚碰撞走滑环境且直接起源于EMII地幔的部分熔融铜厂与玉龙及马厂箐斑岩型Cu( Mo-Au)矿床含矿斑岩地球化学特征和成矿规模存在差异,与源区物质的部分熔融程度和地壳物质的混染程度密切相关.     

Compositional characteristics and Existing Forms of Major,Trace and Rare—earth Elements in Cassiterite,Dupangling Tin Ore Field,Guangxi

Trace elements in cassiterite,including Ta,W,Fe,Mn,Ti,Zr,V,Sc,Si,Al,In,Ga,Ge,Be,Bi,Ag,Sb,As,Cu,Pb,Zn,Co and REE,have been studied by many workers (Shan Zhenhua etal.,1998;Huang Zhou Tianren et al.,1987;Wu Qingsheng et al.,1988;Hu Zening,1988,Li Zhong-qing 1988 Mingzhei et al.,1988;Wang Lihua et al.,1988;Liu Kanghuai,1990).Up to now,however,most of the previous studies are concerned with trace-element variations in cassiterites of different occurrences and colors from different types of ore deposits,Data concerning the modes of occurrence of these trace elements are rare,except for the contention that Nb-Ta,Fe^2 -Mn-Fe^3 and W-Fe^3 may substitute isomorphously for Sn as pointed out by Zhou Tianren et al.(1987) and Moller et al.(1988).In this paper we are concerned with the compositional characteristics as well as the modes of occurrence of trace elements in cassiterites from quartz veins and greisens in the Dupangling tin field,Guangxi,based on multivariate statistical analyses.Tin mineralization in the Dupangling area is found associated with the medium-to fine-grained protolithionite-albite granite(γ5^2b) and its outer contacts.Cassiterite occurs,with wolframite,both in quartz veins in the contact and in greisens within the granite.^1) Spatially,greisens become dominant over quartz veins in the contact andin greisens with the granite.^1)Spatialy,gresens become dominant over quartz veins in going from the contact to the interior of the granite and with increasing depth.The greisens are of various shapes.The vein-shaped and the sheet-shaped greisens at the top of the granie are rich in quartz and the chambered greiens always constitute rich ores and contain abundant topaz or mica.Genetically,Sn,W mineralizations associated with the protolithionite-albite granite(γ5^2b) are considered to have been formed from fluid melt derived from the ore-forming magma responsible for the granite(γ5^2b).     

Time-dependent geochemistry of clinopyroxene from the Alban Hills (Central Italy): Clues to the source and evolution of ultrapotassic magmas

We investigated chemical and isotopic compositions of clinopyroxene crystals from well age-constrained juvenile scoria clasts, lava flows, and hypoabyssal magmatic ejecta representative of the whole eruptive history of the Alban Hills Volcanic District. The Alban Hills is a Quaternary ultra-potassic district that was emplaced into thick limestone units along the Tyrrhenian margin of Italy. Alban Hills volcanic products, even the most differentiated, are characterised by low SiO₂ content. We suggest that the low silica activity in evolving magmas can be ultimately due to a decarbonation process occurring at the magma/limestone interface. According to the liquid line of descent we propose, the differentiation process is driven by crystallisation of clinopyroxene + leucite ± apatite ± magnetite coupled with assimilation of a small amount of calcite and/or with interaction with crustal CO₂. By combining age, chemical data, strontium and oxygen isotopic compositions, and REE content of clinopyroxene, we give insights into the evolution of primitive ultrapotassic magmas of the Alban Hills Volcanic District over an elapsed period of about 600 kyr. Geochemical features of clinopyroxene crystals, consistent with data coming from other Italian ultrapotassic magmas, indicate that Alban Hills primary magmas were generated from a metasomatized lithospheric mantle source. In addition, our study shows that the ⁸⁷Sr / ⁸⁶Sr and LREE/HREE of Alban Hills magmas continuously diminished during the 600–35 ka time interval of the Alban Hills eruptive history, possibly reflecting the progressive depletion of the metasomatized mantle source of magmas.     

Mobilization and re-distribution of major and trace elements during extreme weathering of basalt in Hainan Island, South China

Major, trace and organic elements of a laterite profile developed on Neogene basalts in northern Hainan Island, South China were reported in this paper, the aim of which was to investigate element mobilization and re-distribution during extreme weathering. The results indicate that most of the elements have been mobilized and transferred downwards along the profile by aqueous solution. Organic matter (OM) can significantly improve the transport of insoluble elements. Among all the elements, Th is the least mobile. As for the general conservative elements during incipient chemical weathering, such as Fe, Ti, Zr, Hf, Nb and Ta, the removals are up to 20-40% in the upper profile. However, these elements behave as conservatively as Th in the lower profile. In the middle profile, oxic environment occurs, accompanied with significant OM decomposition. The Mn and Ce transferred downward are readily oxidized into insoluble Mn(IV) and Ce(IV) and precipitate in the oxic front. Important OM decomposition decreases the capacity of transfer of insoluble elements in aqueous solution. Consequently, Al significantly precipitates in the oxic front, and REEs, with the exception of Ce, precipitate largely in the OM-depleted layers. Co and U are also concentrated in the oxic front in association with Mn and Ce, respectively. However, Cr shows a negative correlation with Mn because its response to redox condition changes is reversed from that of Mn. Mn oxides/hydroxides, Fe oxides/hydroxides and secondary phosphate minerals other than clay minerals are potential hosts for REEs except for Ce in the profile; REEs with high concentrations in the profile seem closely associated with Mn oxides/hydroxides. Remarkable, highly correlated, Ce and Gd anomalies are observed in the profile. Ce anomalies are caused by Ce precipitation in the oxic environment and successive decomposition of organic matter. Gd anomalies are likely to have resulted from lower stability constants of Gd-OM complexes compared to those of neighboring REEs. The overall elemental behaviors in this profile suggest that organic matter plays a very important role in the mobilization and re-distribution of the elements during extreme weathering.     

冀西北姚家庄超镁铁岩-正长岩杂岩体中辉石的环带特征及意义

冀西北姚家庄存在一套晚三叠世的超镁铁岩-正长岩杂岩体,岩体内发育具有环带特征的单斜辉石。辉石的环带有两种:简单环带和复杂环带。简单环带一般为正环带,辉石核部的MgO和Cr2O3含量高,Si O2、Fe O和Na2O含量低;边部的主要氧化物含量与核部刚好相反。简单正环带可以分为两类,其中核边接触带平滑、由核到边化学成分具有渐变特征的正环带辉石可能是岩浆在分离结晶或地壳混染过程中形成。而核边接触带有熔蚀结构、由核到边化学成分突变的正环带辉石可能是早期结晶的辉石颗粒受到晚期低镁岩浆的溶蚀改造而成的。复杂环带具有核-幔-边结构,其中,核部低镁高铁、幔部高镁低铁、边部与核部相似,但其Mg#更低,这些特征暗示了岩浆混合作用的存在,形成辉石核部的母岩浆可能来自富集的岩石圈地幔,幔部高Mg#的特征指示了软流圈地幔物质的贡献,其边部低Mg#的特征则指示了地壳物质的加入。具有韵律环带的复杂辉石是在岩浆多期侵入的过程中形成的。辉石环带的组成特征表明,姚家庄杂岩体是由岩浆多期侵位形成的,后期侵入的岩浆与前期就位的岩浆不断反应,形成了具有多种不同环带特征的辉石,并最终形成了空间上由外到内依次为辉石岩、辉石正长岩和正长岩的环状杂岩体。结合前人的研究成果,推测形成姚家庄岩体的岩浆主要来源于富集的岩石圈地幔,并由少量地壳组分和软流圈物质的贡献。     

Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia)

Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain (Central Asian Orogenic Belt, Northern Mongolia). The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals, calculations of closure temperatures and composition of equilibrium melt. The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho- and clinopyroxene due to melt-rock interaction. Relics of peridotite olivines are found in pyroxenites, Cr^# of spinel increases from peridotites to pyroxenites, and compositions of ortho- and clinopyroxene change from peridotite to pyroxenite. The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain. Therefore, formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites. Orthopyroxenite veins formed at first, followed by websterite veins. Thus, the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.     

Geochemical variations of major and trace elements in recent sediments,off the Gulf of Mannar,the southeast coast of India

The Gulf of Mannar along the Tuticorin coast is a coral base of the southeast coast of India. To obtain a preliminary view of its environmental conditions, geochemical distribution of major elements (Si, Al, Fe, Ca, Mg, Na, K, P), trace elements (Mn, Cr, Cu, Ni, Co, Pb, Zn, Cd) and acid leachable elements (Fe, Mn, Cr, Cu, Ni, Co, Pb, Zn, Cd) were analyzed in surface sediment samples from two seasons. Geochemical fractionation confirmed the lithogenic origin of metals, which were mainly associated with the detrital phase. The sediments in the gulf are sandy with abundant calcareous debris, which controls the distribution of total and acid leachable elements. Enrichment factors relative to crust vary by a magnitude of two to three and the presence of trace metals indicates the input of Cr, Pb, Cd, Cu and Zn in both forms through industrial activities. Factor analysis supports the above observation with higher loadings on acid leachable elements and its association with CaCO₃. The increase in concentration of trace metals (Cr, Pb, Cd, Cu, Co, Ni, Zn) along the Gulf of Mannar indicates that the area has been contaminated by the input from riverine sources and the industries nearby. The present study indicates that other sources should be evaluated in the long-term monitoring program.