Oxygen isotope thermometry in carbonatites, Fuerteventura, Canary Islands, Spain

Crystallization temperatures of the oceanic carbonatites of Fuerteventura, Canary Islands, have been determined from oxygen isotope fractionations between calcite, silicate minerals (feldspar, pyroxene, biotite, and zircon) and magnetite. The measured fractionations have been interpreted in the light of late stage interactions with meteoric and/or magmatic water. Cathodoluminescence characteristics were investigated for the carbonatite minerals in order to determine the extent of alteration and to select unaltered samples. Oxygen isotope fractionations of minerals of unaltered samples yield crystallization temperatures between 450 and 960degreesC (average 710degreesC). The highest temperature is obtained from pyroxene-calcite pairs. The above range is in agreement with other carbonatite thermometric Studies. This is the first study that provides oxygen isotope data coupled with a CL study on carbonatite-related zircon. The CL pictures revealed that the zircon is broken and altered in the carbonatites and in associated syenites. Regarding geological field evidences of syenite-carbonatite relationship and the close agreement of published zircon U/Pb and whole rock and biotite K/Ar and Ar-Ar age data, the most probable process is early zircon crystallization from the syenite magma and late-stage reworking during magma evolution and carbonatite segregation. The oxygen isotope fractionations between zircon and other carbonatite minerals (calcite and pyroxene) support the assumption that the zircon would correspond to the early crystallization of syenite-carbonatite magmas.

Nature and origin of ultramafic lamprophyres and carbonatites from the borders of the Labrador Sea

Die Ränder des Labrador Meeres wurden während des späten Neoproterozoikums intensiv von karbonatreichen silikatischen Schmelzen durchsetzt. Diese Schmelzen bildeted sich bei Drucken zwischen ca. 4-6 GPa (ca. 120-180 km Tiefe) an der Basis der kontinentalen Mantel-Lithosphäre. Diese Magmengenerierung steht in zeitlichem und räumlichem Zusammenhang mit kontinentalen Extensionsprozessen, welche zu beiden Seiten des sich öffnenden Iapetus-Ozeans auftraten.

Trace-element modeling of the magmatic evolution of rare-earth-rich carbonatite from the Miaoya deposit, Central China

Carbonatites are known to contain the highest concentrations of rare-earth elements (REE) among all igneous rocks. The REE distribution of carbonatites is commonly believed to be controlled by that of the rock forming Ca minerals (i.e., calcite, dolomite, and ankerite) and apatite because of their high modal content and tolerance for the substitution of Ca by light REE (LREE). Contrary to this conjecture, calcite from the Miaoya carbonatite (China), analyzed in situ by laser-ablation inductively-coupled-plasma mass-spectrometry, is characterized by low REE contents (100–260 ppm) and relatively !at chondrite-normalized REE distribution patterns [average (La/Yb)CN=1.6]. The carbonatite contains abundant REE-rich minerals, including monazite and !uorapatite, both precipitated earlier than the REE-poor calcite, and REE-fluorocarbonates that postdated the calcite. Hydrothermal REE-bearing !uorite and barite veins are not observed at Miaoya. The textural and analytical evidence indicates that the initially high concentrations of REE and P in the carbonatitic magma facilitated early precipitation of REE-rich phosphates. Subsequent crystallization of REE-poor calcite led to enrichment of the residual liquid in REE, particularly LREE. This implies that REE are generally incompatible with respect to calcite and the calcite/melt partition coefficients for heavy REE (HREE) are significantly greater than those for LREE. Precipitation of REE-fluorocarbonates late in the evolutionary history resulted in depletion of the residual liquid in LREE, as manifested by the development of HREE-enriched late-stage calcite [(La/Yb)CN=0.7] in syenites associated with the carbonatite. The observed variations of REE distribution between calcite and whole rocks are interpreted to arise from multistage fractional crystallization (phosphates!calcite!REE-!uorocarbonates) from an initially REE-rich carbonatitic liquid.

The prehistory of Suomussalmi, eastern Finland; the first billion years as revealed by isotopes and the composition of granitoid suites

The four papers summarized in this thesis deal with the Archean and earliest Paleoproterozoic granitoid suites observed in the Suomussalmi district, eastern Finland. Geologically, the area belongs to the Kianta Complex of the Western Karelian Terrane in the Karelian Province of the Fennoscandian shield. The inherited zircons up to 3440 Ma old together with Sm Nd and Pb Pb data confirm the existence of previously anticipated Paleoarchean protocrust in Suomussalmi. The general timeline of granitoid magmatism is similar to that of the surrounding areas. TTG magmatism occurred in three distinct phases: ca 2.95 Ga, 2.83 2.78 Ga and 2.76 2.74 Ga. In Suomussalmi the TTGs sensu stricto (K2O/Na2O less than 0.5) belong to the low-HREE type and are interpreted as partial melts of garnet amphibolites, which did not significantly interact with mantle peridotites. Transitional TTGs (K2O/Na2O more than 0.5), present in Suomussalmi and absent from surrounding areas, display higher LILE concentrations, but otherwise closely resemble the TTGs sensu stricto and indicate that recycling of felsic crust commenced in Suomussalmi 200 Ma earlier than in surrounding areas. The youngest TTG phase was coeval with the intrusion of the Likamännikkö quartz alkali feldspar syenite (2741 ± 2 Ma) complex. The complex contains angular fragments of ultrabasic rock, which display considerable compositional heterogeneity and are interpreted as cumulates containing clinopyroxene (generally altered to actinolite), apatite, allanite, epidote, and albite. The quartz alkali feldspar syenite cannot be regarded as alkaline sensu stricto, despite clear alkaline affinities. Within Likamännikkö there are also calcite carbonatite patches, which display mantle-like O- and C-isotope values, as well as trace element characteristics consistent with a magmatic origin, and could thus be among the oldest known carbonatites in the world. Sanukitoid (2.73 2.71 Ga) and quartz diorite suites (2.70 Ga) overlap within error margins and display compositional similarities, but can be differentiated from each other on the basis of higher Ba, K2O and LREE contents of the sanukitoids. The Likamännikkö complex, sanukitoids and quartz diorites are interpreted as originating from the metasomatized mantle and mark the diversification of the granitoid clan after 200 Ma of evolution dominated by the TTG suite. Widespread migmatization and the intrusion of anatectic leucogranitoids as dykes and intrusions of varying size took place at 2.70 2.69 Ga, following collisional thickening of the crust. The leucogranitoids and leucosomes of migmatized TTGs are compositionally alike and characterized by high silica contents and a leucocratic appearance. Due to compositional overlap, definitive discrimination between leucogranitoids and transitional TTGs requires isotope datings and/or knowledge of field relationships. Leucogranitoids represent partial melts of the local TTGs, both the sensu stricto and transitional types, mostly derived under water fluxed conditions, with possible fluid sources being late sanukitoids and quartz diorites as well as dehydrating lower crust. The Paleoproterozoic 2.44 2.39 Ga A-type granitoids of the Kianta Complex emplaced in an extensional environment are linked to the coeval and more widespread mafic intrusions and dykes observed over most of the Archean nucleus of the Fennoscandian shield. The A-type intrusions in the Suomussalmi area are interpreted as partial melts of the Archean lower crust and display differences in composition and magnetite content, which indicate differences in the composition and oxidation state of the source.

Petrogênese das rochas máficas alcalinas do litoral entre São Sebastião (SP) e Parati (RJ)

Lamprófiros e diabásios alcalinos afloram no litoral dos estados de São Paulo e Rio de Janeiro e integram o Enxame de Diques da Serra do Mar (EDSM). Essas rochas ocorrem sob a forma de diques e intrudem o Orógeno Ribeira, de idade Neoproterozóica/Cambro-Ordoviciana, inserindo-se no contexto geodinâmico de abertura do Oceano Atlântico Sul durante o Cretáceo Superior. Essas intrusões são subverticais e orientam-se preferencialmente a NE-SW, seguindo a estruturação das rochas encaixantes. Os lamprófiros são classificados como monchiquitos e camptonitos e exibem, respectivamente, textura hipocristalina e holocristalina. Apresentam também textura panidiomórfica, fenocristais de clinopiroxênio e olivina, imersos em matriz formada essencialmente por esses mesmos minerais, além de biotita, kaersutita e minerais opacos. O camptonito apresenta ainda plagioclásio na matriz. Os diabásios alcalinos são hipocristalinos a holocristalinos, equigranulares a inequigranulares, com fenocristais de olivina e/ou clinopiroxênio e/ou plagioclásio, em uma matriz composta essencialmente por esses minerais. As rochas estudadas caracterizam séries alcalinas miaskíticas, com os lamprófiros sendo tanto sódicos, potássicos e ultrapotássicos e os diabásios alcalinos como predominantemente sódicos. Modelagens petrogenéticas envolvendo possíveis processos evolutivos mostram que é improvável que os lamprófiros sejam cogenéticos por processos evolutivos envolvendo tanto cristalização fracionada, com ou sem assimilação concomitante, quanto hibridização. O mesmo ocorre para os diabásios alcalinos. A discriminação de fontes mantélicas foi feita com base nos teores de elementos traços de amostras representativas de líquidos parentais e indica que esse magmatismo alcalino está relacionado a fontes lherzolíticas com fusão parcial na zona de estabilidade do espinélio, isto é, a poucas profundidades. Os dados litogeoquímicos e isotópicos do sistema Sr-Nd das rochas estudadas sugerem tanto o envolvimento de fontes férteis, associadas ao manto sublitosférico, quanto de fontes enriquecidas, relacionadas ao manto litosférico subcontinental. Modelagens de mistura binária revelam que a petrogênese dos lamprófiros e diabásios alcalinos envolveu uma grande participação de um componente fértil misturado com contribuições menores de um componente enriquecido. Idades TDM (760-557 Ma) obtidas sugerem remobilização do manto litosférico no Neoproterozóico, talvez relacionadas à subducção da Placa São Francisco preteritamente à colisão do Orógeno Ribeira. Altas razões CaO/Al2O3 para os líquidos lamprofíricos menos evoluídos, altos teores de Zr, correlações negativas Zr/Hf e Ti/Eu e associação com carbonatitos indicam condições metassomáticas de alto CO2/H2O. Em escala local, modelos geodinâmicos baseados na astenosfera não isotérmica parecem mais aplicáveis. No entanto, modelos geodinâmicos baseados na astenosfera isotérmica (com o envolvimento de plumas) parecem mais indicados num contexto regional, considerando-se outras províncias alcalinas contemporâneas e correlatas.

山东西部中生代碳酸岩和火山岩的地球化学特征及其成因研究

On account of some very peculiar features, such as extremely high Sr and Nd contents which can buffer their primary isotopic signatures against crustal contamination, deep-seated origin within mantle, and quick ascent in lithosphere, carbonatites are very suitable for deciphering the nature of sub-continental lithospheric mantle(SCLM) and receiving widespread attentions all around the world. The Mesozoic carbonatites located in western Shandong was comprehensively investigated in this dissertation. The extremely high REE concentrations, similar spider diagrams to most other carbonatites around the world and high Sr. low Mn contents of apatite from carbonatites confirm their igneous origin. The K depletion of carbonatites from this studies reflect the co-existing of carbonatite melts with pargasite+phlogopite lherzolite rather than phlogopite lherzolite. Geological characteristics and their occumng without associated silicate rocks argue against their origin of fractionation of or liquid immisibility with carbonated silicate melts. In contrast to the low S7Sr/86Sr and high l43Nd/l44Nd of other carbonatites in the world, carbonatites of this studies show EMU features with high S7Sr/86Sr and low l4jNd/144Nd ratios, which imply that this enriched nature was formed through metasomatism of enriched mantle preexisted beneath the Sino-korean craton by partial melts of subducted middle-lower crust of Yangtze craton. In addition to carbonatites, the coeval Mesozoic volcanic rocks from western Shandong were also studied in this dissertation. Mengyin and Pingyi volcanic rocks, which located in the south parts of western Shandong are shoshonite geochemically. while volcanic rocks cropped out in other places are high-K calc-alkaline series. All these volcanic rocks enriched in LREE and LILE. depleted in HFSE, and show TNT(strong negative anomalies in Ta, Nb. Ti) patterns in spider diagrams which are common phenomena in arc-related volcanic rocks. The Sr-Nd-Pb isotopic systematics reveal that the volcanic rocks decrease gradually in 87Sr/86Sr, 206Pb/204Pb, 20SPb/204Pb and increase in TDM from south to north, suggesting the distinction of SCLM beneath Shandong in Mesozoic is more explicit in south-north trending than in east-west trending. The variable features of SCLM can be attributed to the subduction of Yangtze craton beneath Sino-Korean craton, and subsequent metasomatism of SCLM by partial melts of Yangtze lower crust in different extent.

大兴安岭南段中生代火山-侵入杂岩带的双层叠置构造体制兼论滇中武定、易门碳酸质火山岩年代、来源及矿库与找矿研究

These are two parts included in this report. In the first part, the zonation of the complexes in its series, lithofacies, the depth of magma source and chambers is discussed in detailed for the first time based on the new data of petrol-chemistry, isotopes, tectono-magma activity of Mesozoic volcano-plutonic complexes in the southern Great Hinggan Mts. Then, the genetic model of the zonality, double overlapped layer system, is proposed. The main conclusions are presented as follows: The Mesozoic volcanic-plutonic complexes in the southern Great Hinggan were formed by four stages of magma activity on the base of the subduction system formed in late Paleozoic. The Mesozoic magmatic activity began in Meso-Jurassic Epoch, flourished in late Jurassic Epoch, and declined in early Cretaceous Epoch. The complexes consist dominantly of acidic rocks with substantial intermediate rocks and a few mefic ones include the series of calc alkaline, high potassium calc alkaline, shoshonite, and a few alkaline. Most of those rocks are characterized by high potassium. The volcano-plutonic complexes is characterized by zonality, and can be divided mainly into there zones. The west zone, located in northwestern side of gneiss zone in Great Xinggan mountains, are dominated of high potassium basalts and basaltic andesite. The middle zone lies on the southeast side of the Proterozoic gneiss zone, and its southeast margin is along Huangganliang, Wushijiazi, and Baitazi. It composed of dominatly calc-alkaline, high potassium calc-alkaline rocks, deep granite and extrusive rhyolite. The east zone, occurring along Kesheketong Qi-Balinyou Qi-Balinzuo Qi, is dominated of shoshonite. In generally, southeastward from the Proterozoic gneiss zone, the Mesozoic plutons show the zones-mica granitites zone, hornblende-mica granitite zone, mica-hornblende granitite zone; the volcanic rocks also display the zones of calc alkaline-high potassium calc alkaline and shoshonites. In the same space, the late Paleozoic plutons also display the same zonality, which zones are combined of binary granite, granodiorite, quartz diorite and diorite southeast wards from the gneiss. Meso-Jurassic Epoch granite plutons almost distribute in the middle zone on the whole. Whereas late Jurassic Epoch volcanic rocks distribute in the west and east zone. This distribution of the volcano-plutonic complexes reveals that the middle zone was uplifted more intensively then the other zones in Meso-Jurassic and late Jurassic Epoches. Whole rock Rb-Sr isochron ages of the high potassium calc-alkaline volcanic rocks in the west zone, the calc-alkaline and high potassium calc-alkaline granite the middle zone, shoshonite in the east zone are 136Ma, 175Ma and 154Ma, respectively. The alkaline rocks close to the shoshonite zone is 143Ma and 126Ma. The isochron ages are comparable well with the K-Ar ages of the rocks obtained previously by other researchers. The compositions of Sr ans Nd isotopes suggest that the source of Mesozoic volcanic-plutonic complexes in Great Hinggan Mts. is mostly Paleo-Asia oceanic volcanic-sedimentary rocks, which probably was mixed by antiquated gneiss. The tectonic setting for Mesozoic magmatism was subductive continental margin. But this it was not directly formed by present west Pacific subduction. It actully was the re-working of the Paleozoic subduction system( which was formed during the Paleo-Asia ocean shortening) controlled by west Pacific subduction. For this reason, Although Great Hinggan Mts. is far away from west Pacific subduction zone, its volcanic arc still occurred echoing to the volcanic activities of east China, it, but the variation trend of potassium content in volcano-plutonic complexes of Great Hinggan is just reverse to ones of west Pacific. The primitive magmas occurred in the southern Great Hinggan Mts. Include high-potassium calc-alkaline basalt, high potassium calc-alkaline rhyolite, high potassium rhyolite, non-Eu negative anomaly trachy-rhyolite et al. Therefore, all of primitive magmas are either mafic or acid, and most of intermediate rocks occurring in the area are the products of Mesozoic acid magma contaminated by the Paleozoic volcanic- sedimentary rocks. The depth of those primitive magma sources and chambers gradually increase from northwest to southeast. This suggests that Paleozoic subduction still controlled the Mesozoic magmatism. In summary, the lithosphere tectonic system of the southern Great Hinggan Mts. controlling Mesozoic magmatism is a double overlapped layer system developing from Paleozoic subduction system. For this reason, the depth of crust of the southern Great Hinggan Mts. is thicker than that of its two sides, and consequently it causes regional negative gravity abnormity. The second part of this report shows the prolongation of the research work carried on in my doctor's period. Author presents new data about Rb-Sr and Sm-Nd isotopic compositions and ages, geochamical features, genesis mineralogy and ore deposit geology of the volcanic rocks in Kunyang rift. On the base of the substantial work, author presents a prospect of copper bearing magnetite ore deposit. The most important conclusions are as follows: 1. It is proved that all of these carbonatites controlled by a ringing structure system in Wuding-Lufeng basin in the central Yunnan were formed in the Mesoproterozoic period. Two stages could be identified as follows: in the first stage, carbonatitic volcanic rocks, such as lavas(Sm-Nd, 1685Ma), basaltic porphyrite dykes(Sm-Nd, 1645Ma), pyroclastic rocks and volcaniclastic sedimentary rocks, formed in the outer ring; in the second stage, carbonatitic breccias and dykes(Rb-Sr, 1048 Ma) did in the middle ring. The metamorphic age of the carbonatitic lavas (Rb-Sr, 893 Ma) in the outer ring was determined. The magma of carbonatitic volcanic rocks derived mainly form enriched mantle whose basement is depleted mantle that had been metasomated by mantle fluid and contaminated by Archaean lower crust. Carbonatitic spheres were discovered in ore bearing layers in Lishi copper mining in Yimen recently, which formed in calcite carbonatitic magma extrusion. This discovery indicates that the formation of copper ore deposit genesis relates to carbonatitic volcanic activity. The iron and copper ore deposits occurring in carbonatitic volcanic- sedimentary rocks in Kunyang rift results from carbonatitic magmatism. Author calls this kind of ore deposits as subaqueous carbonatitic iron-copper deposit. The magnetic anomaly area in the north of Lishi copper mining in Yimen was a depression more lower than its circumference. Iron and copper ores occurrig on the margin of the magnetic anomaly are volcanic hydrothermal deposit. The magnetic body causing the magnetic anomaly must be magnetite ore. Because the anomaly area is wide, it can be sure that there is a large insidious ore deposit embedding there.

四川牦牛坪稀土矿床碳酸岩-正长岩成因及成矿流体来源

四川耗牛坪稀土矿床是中国仅次于白云鄂博的第二大原生轻稀土矿床。矿区碳酸岩-正长岩与REE矿化时空上密切共生。本论文系统分析了矿区碳酸岩-正长岩的主要元素、微量元素、REE、PGE和C、O、Sr、Nd、Pb同位素组成，探讨了碳酸岩-正长岩的地慢源区特征及两类岩石的成因联系；结合矿区主要脉石矿物萤石和其它岩浆岩的REE、Sl-Nd-Pb同位素、流体包裹体地球化学，探讨了REE成矿流体的来源，初步建立了REE成矿模式。取得的主要认识有：1，系统研究了耗牛坪REE矿床碳酸岩-正长岩的地球化学特征，揭示了两者的地慢源区特征和成因联系。受陆内俯冲作用的影响，具有EMZ指纹的地壳物质俯冲交代具EMI特征的岩石圈地慢，形成碳酸岩一正长岩的初始地慢，该地慢经低程度部份熔融作用形成富COZ硅酸岩熔体—碳酸岩-正长岩原始岩浆，该熔体在演化过程中发生液态不混溶作用形成碳酸岩和正长岩。2．通过对耗牛坪REE矿床碳酸岩PGE地球化学的研究，发现碳酸岩具有携带PGE的能力，其PGE配分模式与西藏大竹卡方辉橄榄岩相似。初步认为本区碳酸岩中的PGE可能主要来源于俯冲或地鳗交代的流体，除硫化物外，合金或挥发份都可能影响碳酸岩PGE的配分模式。3．萤石是耗牛坪REE矿床重要的脉石矿物之一，其形成贯穿了整个成矿过程。根据REE地球化学参数将萤石分为LREE富集型、LREE平坦埠和LREE亏损型。三种REE类型的萤石具有相似的Sr、Nd、Pb同位素组成，为同源不同阶段的产物，形成顺序为LREE富集型-LREE平坦型-LREE亏损型。4．萤石的Sr、Nd、Pb同位素组成与矿区碳酸岩-正长岩相近，明显不同于花岗岩；早期萤石的REE地球化学特征也与碳酸岩-正长岩相似，与花岗岩、流纹岩和玄武岩存在差别；在萤石中发现了流体-熔融包裹体。这些特征均表明牡牛坪REE矿床成矿流体主要来源于碳酸岩-正长岩熔体，矿床为地幔流体成矿的产物。

岩溶上覆风化壳的粒度分布特征及其对物源和形成过程的指示意义

以贵州为中心的中国西南岩溶区是世界上连片分布面积最大的岩溶区，其上分布着厚度不一的红色土层。由于碳酸盐岩易溶蚀、酸不溶物含量极低（一般＜5％）、成土慢，同时在风化过程中伴随着巨大的体积缩小变化，原岩结构和半风化带无法保留，岩-土界面呈突变接触关系，缺失过渡层，宏观上缺乏直接的野外地质证据支持二者之间存在着明确的继承关系，因此对于岩溶上覆风化壳的物质来源，长期以来一直存在着争议。对岩溶区风化壳成因存在的不同认识，成为利用风化壳进行一系列科学研究的障碍，因此正确理解岩溶上覆风化壳的物质来源和成因显得十分紧迫和重要。本文在已有成果的基础上，选择灰岩、白云岩与碎屑岩呈镶嵌产出的贵州岩溶台地（包括湘西吉首的一个剖面）作为研究区域，尝试利用粒度分析这一反映沉积若（物）的结构组成的研究手段，通过对20条剖面的精细采样分析．系统地探讨了不同基岩与仁覆风化壳的粒度分布特征，并以此为主线，结合磁化率、pH值、地球化学和矿物学资料，对贵州岩溶上覆风化壳的物质来源和剖面演化过程进行了探讨，取得了以下几点认识：第一，在碳酸盐岩上覆风化壳的成因研究中，粒度分析是一个有效而直观的物源示踪方法。不同沉积背景下形成的沉积岩其粒度分布特征是不同的，而在此基础上发育的风化壳就继承了母岩的粒度分布特征，粒度频率分布曲线表现出和母岩的一致性和渐变过渡性，在风化程度不是特别强烈的条件下（排除如铝土矿化的红土化阶段），风化壳仍保留了“源”的信息。而由碳酸盐岩发育的风化壳就继承了基岩酸不溶物的粒度分布特征。由不同粒度组成的沉积岩发育的风化壳，其粒度组成也存在着明显的浪异。第二，通过对贵州岩溶区不同基岩（包括碎屑岩）及其上覆风化壳的粒度分析表明，各风化剖面的粒度分布特征与下伏基岩有明显的继承性，而各剖面之间的粒度分布存在着明显的差异，说明岩溶区上覆风化壳没有共同的物质来源，碳酸盐岩上覆风化壳是碳酸盐矿物溶蚀、残余酸不溶物长期积累的结果。第三，在非等体积风化过程中，风化前锋即“岩一土界，”是一个重要的地球化学作用场所，在这一狭窄的界面上，不仅碳酸盐矿物充分淋失，而且残余酸不溶物也开始了分解，同时岩一土界面的风化梯度明显强于已成风化剖而后期的演化强度。即从基岩酸不溶物到风化壳底部，风化强度突变性增大，而风化壳的后期演化则是一个缓慢的过程。第四，由于充沛的水热条件，风化壳的淋溶淀积作用普遍存在，粘粒含量在剖面七部由下向上表现为逐渐降低的“倒置”现象。在风化壳发育浅薄的石灰土剖面，淀积层甚至可以直接淀积在剖面底部。第五，石灰土尽管发育程度较低，可以看作碳酸盐岩风化壳演化的早期阶段，但是在形成石灰土的过程中，各风化指标已表现出显著的变化，具有了红色风化壳的发育特征。因此从严格意义上讲，石灰土已不具有从基岩到红色风化壳的“过渡层”身份。第六，石灰土剖面普遍具有典型残积风化的特征，粒度和地球化学指标表现为单调变化的趋势。而红色风化壳的粒度参数在整体上具有风化壳正向演化特征的基础上，在剖面上表现为强烈的波动，地球化学指标及矿物学组成在剖面上也呈现相应的波动。造成这种现象的原因可解释为：一是风化前锋向下拓展过程中风化条件的变化，二是后期古地下水位的波动对风化剖面的改造。石灰土剖面形成时间短，在浅薄的风化壳发育过程中，风化条件和水文状况稳定，风化壳的发育完全是在气下由大气降水形成的风化溶液对一剖面由浅入深的风化作用形成的，未受到后期地下水的改造。而厚层红色风化壳，形成时间长，在其长期的地质演化过程中，风化条件的变化和古地下水位的波动将会频繁的发生。尤其岩一土界面是一个重要的地球化学风化界面，在风化前锋向下拓展过程中，风化条件的变化必然影响到相应层位风化程度的差异性，因此造成剖面上风化指标的波动性。在风化前锋，由风化条件的差异性导致粒度及地球化学指标的波动性，在这一过程中，粒度及地球化学指标的变化是可预测的，即强烈的风化条件可以导致粒度变细，粘粒含量增加，CIA增大，反之亦然，风化过程表现为活动元素的净带出。然而，由古地下水位的波动对风化剖面的改造作用是可变的，不可预测的，既可使剖面的物质被带出，也可以带入一些活动元素，如K、 Na等的交代。同时，地下水对剖面粒度组成的影响也很复杂，物质的带入不一定就会使粒度变粗，粘粒含量降低，而物质的带出也不一定就导致粒度变细，粘粒含量增加。在两种机理的相互叠加影响下，使风化壳的演化趋势更趋复杂化。风化剖面中，粘粒含量和CIA之间既有同步变化的层段，也有呈强烈反相关的层段，还存在没有明显相关肋层段。作为等体积变化的典型剖面-吉首剖面，其风化过程具有碎屑岩的发育特征，“粘粒含量从下向上表现为顺次增大的趋势，如果也存在古地下水对剖面的改造作用，那么说明对粒度的影响是微弱的。第七，对于红色风化壳，不管是碳酸盐岩风化过程中由于风化条件的差异造成的风化指标波动还是受后期地下水的改造作用引起的，剖面整体上仍具有向上风化程度增强的趋势。PH向上逐渐降低、磁化率的增大均表明了风化强度的增强，“这与剖面愈向上经历的风化时间愈长所对应的。第八，碳酸盐岩风化壳和结晶岩风化壳的形成过程相比，前者母岩的风化起点高，即母岩的成熟度高。由于碳酸盐岩风化壳的风化母质是基岩酸不溶物，而酸不溶物本身就是表生风化条件下的产物，因此可以说风化母岩就是风化壳，基本上不含易风化的斜长石类矿物。在A-CN-K风化趋势三角图解上，已表现为风化壳发育的第二演化阶段，即沿着A-K端线向A点逼近。第九，在结晶岩及碎屑岩类风化过程中所证实的守恒元素，在碳酸盐岩非等体积风化过程中其比值发生了显著变化，这是碳酸盐岩风化成土作用过程中的一个特点，可能会引起某些所谓的守恒元素的地球化学分异，但其成因尚不清楚。在风化剖面中守恒元素的比值保持稳定，具有其它岩类风化的特点。

Plume-lithosphere interaction at Santiago Island (Cape Verde) : implications for ocean island magma genesis models

Tese de doutoramento, Geologia (Geoquímica), Universidade de Lisboa, Faculdade de Ciências, 2014

Magnetotelluric deep soundings, gravity and geoid in the south Sao Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

In the south Sao Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an Archean-Paleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 Omega m) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 Omega m) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m(3)) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (similar to 0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO(2), H(2)O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a high-degree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the Sao Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Parana and Espinhaco (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (similar to 3%) carbonatites in their composition. The occurrence of a positive geoid anomaly (+ 10 m) and pre-tholeiites (age > 138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the Sao Francisco-Congo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous. (C) 2010 Elsevier B.V. All rights reserved.

HYDROTHERMAL REE FLUOROCARBONATE MINERALIZATION AT BARRA DO ITAPIRAPUA, A MULTIPLE STOCKWORK CARBONATITE, SOUTHERN BRAZIL

The Barra do Itapirapua ( BIT) carbonatites in southern Brazil belong to the final stages of the Early Cretaceous alkaline rock - carbonatite magmatism of the Ponta Grossa Arch Province. The BIT complex is a dyke and vein stockwork in which four main carbonatitic phases are recognized, mainly magnesiocarbonatites and ferrocarbonatites. These carbonatites are generally overprinted by pervasive hydrothermal events. The C-O stable isotopic data indicate re-equilibration under hydrothermal conditions at temperatures between 375 and 80 degrees C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited. Syntaxy between synchysite-(Ce) and parisite-(Ce) is very common owing to the similarity in structures, with alternating (001) layers of (CeF), (CO3) and (Ca). However, bastnasite-(Ce) occurs as individual crystals, overgrown by the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into the mobility of rare-earth elements during fluid-rock interaction. The BIT complex is considered to be of potential economic interest for production of the rare-earth concentrates.