Post-orogenic and anorogenic A-type fluorite-bearing granitoids, Eastern Desert, Egypt: Petrogenetic and geotectonic implications

The present study focuses on four A-type fluorite-bearing granitic plutons in the Eastern Desert of Egypt which are classified into post-orogenic subsolvus (Homrit Waggat, 535 Ma; Homer Akarem, 541 Ma and Ineigi, 571 Ma) and anorogenic hypersolvus (Gabal Gharib, 476 Ma) granites. All the granites are Si- and alkali-rich and MgCaTi poor. Whereas both granite types appear relatively homogeneous in terms of most of their major and trace elements, they differ in that the subsolvus granites are depleted in TiO₂, FeO^*, Ba, Sr and Zr and enriched in Rb and Y with respect to the hypersolvus granites. The two granite types, however, can be distinguished more easily by their rare-earth element (REE) patterns. Chondrite-normalized REE patterns of the hypersolvus granite display a gull-wing shape, characterized by a large negative Eu anomaly and moderate-to-high LREE contents. Relative to the hypersolvus granite, subsolvus granite is depleted in LREE and more enriched in HREE contents. The increase of HREE in the subsolvus granite is presumably caused by F complexing during the late stage of its evolution. This is supported by the abundance of fluorite veins cross-cutting the subsolvus granite. The negative Eu anomalies in the subsolvus granite point to the role of feldspars as residual phase in the source, and as a crystallizing phase during magmatic differentiation.Field relations, textural, mineralogical and geochemical data of the post-orogenic subsolvus granite are consistent with its derivation from a parental basic magma through crystal-liquid fractionation of alkali feldspar, plagioclase, amphibole, FeTi oxides, titanite, zircon, monazite and allanite. Crystallization occurred in a water-enriched and rather oxidizing environment, as a result of which the entire suite has a transitional character between that of a post-orogenic and an anorogenic setting. On the other hand, the most credible mechanism for the origin of the anorogenic hypersolvus granite is partial melting of I-type granodiorite-monzogranite source rocks in the study area.     

Geology and geochemistry of the granitic rocks and associated dykes, East Gabal Nuqra, South Eastern Desert, Egypt

A crescent-shape granitic stock and associated dykes is located to the East Gabal Nuqra at the extreme western part of Wadi Natash,South Eastern Desert,Egypt.The examined granites are classified as alkali-feldspar granites and mainly consist of quartz,potash feldspars,plagioclases,and aegirine-augite.Xenotime,zircon,apatite and allanite are accessories representing the source of Y,U,Th and REEs in these rocks.These granites are characterized by high K2O,Na2O and Zn contents and Rb/Sr ratio.Also,they are highly enriched in high field strength elements(HFSE),especially Zr(1529×10-6),Nb(100×10-6),Hf(91×10-6) and Y(624×10-6) and light rare-earth elements(LREE,141×10-6) concentrations and strongly depleted in Ca,Mg,Sr and Eu contents.These features suggest that they are similar to A-type granites(type-2).The rhyolite dykes and granites have similar geochemical characteristics whereas the chondrite-normalized REE patterns show a LREE enriched feature with strong negative Eu-anomaly,whereas the REE pattern of trachydacites show slightly fractionated pattern with no Eu-anomaly.It is suggested that the trachydacites were generated by small degree of partial-melting deep-seated basic source.Such liquid,when subjected to fractional crystallization involving separation of plagioclases as residue,generated the alkali-feldspar granites.And further fractional crystallization gave rise to the alkali rhyolites.The igneous rock suite originated from metaluminous to alkaline trachytic magma,and was developed in a within-plate tectonic environment.The extension caused by NW-SE right-lateral shear in area led to the emplacement of the alkali-feldspar granites.The later extrusion of the alkali rhyolite and trachydacite dykes was due to cauldron subsidence.     

Mapping Structurally Controlled Alterations Sparked by Hydrothermal Activity in the Fatira–Abu Zawal Area, Eastern Desert, Egypt

The Eastern Desert of Egypt suffered a protracted period of deformation triggered by cratonization of the new juvenile crust known as the Arabian Nubian Shield (ANS), which has been proposed for potential gold discoveries associated with the corresponding tectonic event. The Fatira area, on the border of Egypt''s Northern and Central Eastern Deserts, is covered with metavolcanic rocks twisted by a dextral relocation of the Fatira Shear Zone (FSZ) relative to the Barud magmatic body. The recent study evaluated many deformed post-orogenic granitic intrusions and felsite dikes associated with promising mineralization localities, notably orogenic gold deposits. The combination of various field observations and remote sensing data, followed by the analysis of aeromagnetic enhanced maps, allowed the differentiation of distinct lithologies, structural features, and hydrothermal alterations in the study area. Additionally, the integrated results obtained from the different interpretation techniques are utilized to identify and confirm the previously supposed mineralized localities in the Fatira and Abu Zawal areas and predict other matched localities. The final ASTER, Sentinel 2 hydrothermal alteration, and orientation entropy heat maps demonstrate the association between these mineralized regions and major structures related to the FSZ late stage of deformation rather than other structures studied throughout the area of interest.     

新疆吉木乃县恰其海A型花岗岩的地球化学特征、年代学及构造意义

新疆西准噶尔北部发育大量碱性花岗岩, 其形成时代为早二叠世, 岩石类型为钾长花岗岩和石英钾长正长岩。恰其海岩体的形成年龄为280 Ma, SiO2为66.83%～68.44%, 高铝(14.46%～15.41%);富含碱质, 里特曼指数(δ)均大于3.3, 介于4.47～4.61之间; 富钾贫钠, K2O/Na2O为1.10～1.25;铁高而镁低, FeOt/MgO为4.06～7.85。其稀土配分模式右倾, 轻重稀土分馏明显, 具有中等-强烈铕负异常。地球化学特征表明: 岩石形成于造山晚期伸展体制的构造环境中, 应为后碰撞演化的晚期阶段, 早于东准噶尔后碰撞的时间(二叠纪末)。构造图解判别岩体为A2型花岗岩, 可能受到与俯冲有关的流体交代的影响, 为探索区域晚古生代地壳演化提供了约束。     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Determination of the Neoproterozoic Shicaogou Syn-collisional Granite in the Eastern Qinling Mountains and Its Geological Implications

The Shicaogou granite has been identified as a magnesian (Fe-number=0.71-0.76), calcic to calc-alkalic (MALI=3.84-5.76) and peraluminous (ASI=1.06-1.13) granite of the syn-collisional S-type, with high SiO2(>71%), A12O3 (>13%) and Na2O+K2O (6.28%-7.33%, equal for NaO2 and K2O). Trace element and REE analyses show that the granite is rich in LILE such as of Rb, Sr, Ba and Th, and poor in HFSE like Yb, Y, Zr and Hf. Its Rb/Sr ratio is greater than 1; the contents of Nb and Ta, and the ratio of Nb/Ta as well as the REE geochemical features (e.g. REE abundance, visible fractionation of LREE and HREE and medium to pronounced negative Eu anomalies) are all similar to those of crust-origin, continent-continent syn-collisional granite. Moreover, the granite exhibits almost the same pattern as that of the typical continent-continent syn-collisional granite on the spider diagram and all samples fall within the syn-collisional granite field.The cathodoluminescence (CL) investigations have revealed that the zircon f     

江西大岭上钨矿含斑细粒花岗岩锆石成因及成岩成矿启示

锆石成因研究成果是锆石定年、源区性质示踪和岩浆演化过程分析的基础。本文利用阴极发光和LA-ICP-MS技术发现研究区含斑细粒花岗岩包含了无继承核锆石和有继承核锆石。无继承核锆石阴极发光暗黑,其Th/U值低,LREE相对富集,Ce弱正异常,Ti温度计得到地质温度为801℃,是形成于富含热液的岩浆环境的锆石。有继承核锆石残留核具有均匀的灰白阴极发光,边缘阴极发光暗黑,其Th/U值较高,LREE相对较低、Ce正异常明显,Ti温度计得到地质温度为771℃,是形成于正常岩浆环境的中、深成岩浆岩中的锆石。结合区域地质情况,推断残留核锆石很可能是晋宁期花岗闪长岩中的锆石,说明矿区晋宁期花岗闪长岩可能是含斑细粒花岗岩的一种源岩。无继承核锆石的结晶环境反映了当时岩浆已经演化到晚期,而且其结晶时的富含热液的岩浆可能是一种良好的成矿物质的载体,值得进一步研究。     

Geochemistry and Radioactive Potentiality of Um Naggat Apogranite, Central Eastern Desert, Egypt

Abstract: The northern part of Um Naggat granite massif (UNGM) has suffered extensive post-magmatic metasomatic reworking which results into the development of (Zr, Hf, Nb, Ta, U, Th, F)– and albite-enriched and greisenized apogranite body (UNAP) of 600 m thick, and more than 3 km in the strike length.
Albitization produced an enrichment in Zr (av. 2384 ppm), Hf (61), Nb (419), and U (43). The Th/U ratio ranges between 1. 33 and 1. 90. Extreme albitization (i. e. the albitite rock) is characterized by sharp decrease in the rare metals contents. However, extreme greisenization (i. e. endogreisen bodies) is characterized by a considerable enrichment in Zr (av. 5464 ppm), Hf (143), Nb (2329), Ta (152), U (66) and Th (178). The Th/U ratio ranges between 1. 57 and 3. 60. In contrast to extreme greisenization, it seems that extreme albitization does not apparently change the fluid pH and therefore poor amounts of rare metals are localized in the albitites.
It is suggested that the presence of Na⁺, H⁺ and F^- in the ore fluids was essential to stablize complexes of Zr, Hf, Nb, Ta, U, Th, and HREE during extraction and transportation. In contrast, contemporaneous decrease of temperature and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr and Nb in the apical parts of the UNAP. The enhanced uranium content in the alteration facies of UNAP coupled with the absence of significant uranium mineralization may indicate the metalliferous rather than mineralized nature for the UNAP. The high uranium contents are stabilized in refractory accessory minerals. However, with repect to Zr and Nb, the UNAP especially the albitized and greisen facies, can be categorized as a mineralized productive granite.     

The nature of gold-bearing fluids in Atud gold deposit,Central Eastern Desert,Egypt

Electron microprobe analyses of gold and associated ore minerals as well as stable isotope analyses of sulphide and carbonate minerals were performed in order to determine the metal and fluid sources and temperature of the mineralizing systems to better understand the genesis of the Atud gold deposit hosted in the metagabbro–diorite complex of Gabal Atud (Central Eastern Desert, Egypt). The gold can be classified as electrum (63.6–74.3 wt.% Au and 24.6–26.6 wt.% Ag) and is associated with arsenopyrite and As-bearing pyrite in the main mineralization (gold-sulphides) phase within the main mineralized quartz veins and altered host rocks. Based on the arsenopyrite geothermometer, As-contents (29.3–32.7 atom%) in arsenopyrite point to deposition in the Log ?_S2 and T ranges of ?10.5 to ?5.5 and 305–450°C, respectively, during the main mineralizing phase. Based on the δ³⁴S isotopic compositions of the sulphides, they are originated from magmatic fluids in which the sulphur is either sourced directly from magma or remobilized from the magmatic rocks (gabbroic rocks). On the other hand, calcite formed from fluids having mainly magmatic mixed with variable metamorphic signatures based on its δ¹³C and δ¹⁸O values. This work concluded that the gold-bearing ores at Atud deposit have magmatic sources leaching from the country intrusive rocks during water/rock interactions then remobilized during a metamorphic event. Therefore, the Atud gold deposit is classified as an intrusion-related gold deposit, in which the gabbro–diorite host intrusion acted as the source of metals which were mobilized and deposited as a result of the effects of NW–SE shearing.     

武夷山中段司前花岗岩体形成时代、地球化学特征及地质意义

本文对武夷山中段的司前岩体进行了系统的岩石学、地球化学、年代学及Lu-Hf同位素研究。结果表明,司前岩体形成于(140±1)Ma,主要由黑云母二长花岗岩组成,富碱(K_2O+Na_2O=6.67 wt%~8.25 wt%),富钾(K_2O/Na_2O=1.16~2.41),A/CNK值介于1.01~1.25之间。岩体具有较高的∑REE(177.73×10~(-6)~427.88×10~(-6))、Zr+Nb+Y+Ce含量(262.6×10~(-6)~581.5×10~(-6))和Zr饱和温度(平均824°C),FeOt/MgO(3.06~3.93)和10 000×Ga/Al(2.64~3.28)比值均较高,属典型的铝质-过铝质A型花岗岩。岩体的锆石εHf(t)值均为负值(–18.6~–7.9),暗示其源于古老的地壳物质重熔。综合上述结果和区域背景推测,司前岩体的源岩为新元古代麻源群变质火山-碎屑岩,源岩可能经历早古生代和白垩纪两期熔融事件,地幔岩浆为花岗岩的形成提供了热源,但并未贡献物质,岩体的形成与古太平洋板块俯冲过程中因俯冲板片后撤诱发的弧后扩张作用有关。     

Columbia超大陆裂解：来自塔里木克拉通东南缘大红山A型花岗岩的证据

笔者对塔里木克拉通东南缘大红山花岗岩体进行了岩石学、地球化学和年代学研究。通过大红山花岗岩锆石LA-ICP-MS测年,获得207Pb/206Pb加权平均年龄为1732.3±7.4Ma,全岩δ18 OV-SMOW为7.6‰~8.6‰。岩体富硅(SiO2=71.14%~75.85%)、富碱(Na2O+K2O=8.32%~9.94%);稀土配分曲线呈显略右倾的"海鸥型"分布特征,显示强烈的Eu负异常(δEu=0.08~0.30);微量元素特征显示具有较高Rb(80.9×10-6~129×10-6)、Th(17.3×10-6~28.6×10-6)、Y(62×10-6~220×10-6)、Ga(28.1×10-6~44.6×10-6)和Nb(52.6×10-6~145×10-6)含量,较低的Sr(12.2×10-6~79.3×10-6)含量,Zr+Nb+Ce+Y和104×Ga/Al值均较高,具有A型花岗岩的地球化学特征。结合Sc/Nb-Y/Nb图解,大红山花岗岩为A1型花岗岩,是低压条件下形成。通过计算18个花岗岩样品的锆石饱和温度,TZr为732.45~1128.33℃,平均为916.3℃,属高温花岗岩。大红山花岗岩是由挤压体制向伸展转变过程的地质记录,是伸展构造背景下软流圈上涌或幔源岩浆底侵导致下地壳在高温低压条件下发生部分熔融形成,记录了塔里木克拉通在Columbia超大陆聚合后的高热事件,是克拉通大规模裂解初期阶段的痕迹。     

云南保山东缘早古生代高Si花岗岩的成因及构造意义

对保山地块东缘高Si花岗岩开展矿物化学、岩石地球化学及锆石U Pb Hf系统研究,结果表明该高Si花岗岩为具钙碱性、强过铝质特征的S型花岗岩。锆石U Pb同位素分析表明,高Si花岗岩侵位于454 Ma,并含有800~1 100 Ma的继承锆石。锆石Hf同位素分析表明其岩浆锆石具有与青藏高原及东南缘同时代长英质侵入体相似的Hf同位素组成,暗示其相似的岩浆起源。矿物化学、同位素组成及Melts模拟计算结果表明,保山东缘高Si花岗岩为一系列复杂作用的结果：高硅花岗岩母岩浆起源于该区沉积岩部分熔融;熔体形成后经高度分异演化,在侵位过程中同化混染围岩;岩浆冷凝至固相线下部分矿物再平衡。保山东缘高Si花岗岩体与平河花岗岩体具相似年龄和地球化学特征,暗示它们之间可能存在类似的成因机制。     

黑河市五道沟地区早二叠世A型花岗岩成因

根据岩石学、岩石化学、地球化学及同位素等特征,对黑河市五道沟地区早二叠世碱长花岗岩进行了构造成因研究。该套岩石具高硅、富碱、低钙镁、偏铝质-过铝质特点,且富Rb、Th、K等大离子亲石元素,相对亏损Ba、Sr、P、Ti、Nb等元素,稀土元素具有典型的海鸥式分配样式,总体具有典型A型花岗岩特征;其形成于额尔古纳—兴安地块与松嫩—张广才岭地块挤压拼贴后的伸展环境。黑河市五道沟地区A型花岗岩的最新厘定对该地区晚古生代构造演化的研究及区域成矿规律的探讨具有重要意义。     

Overview of the geomorphological and hydrogeological characteristics of the Eastern Desert of Egypt

Although, the Eastern Desert of Egypt forms about 22% of the surface area of the country, the area is undeveloped due to the limited availability of water. The morphologic units of the Eastern Desert consist of a number of drainage basins covering about 147,820 km² (66.5% of the total surface area of the Eastern Desert). The basins drain the occasional rainwater, either towards the Nile Valley or to the Red Sea, causing flood hazards. The availability of water from the hydrologic systems of these basins could be improved by constructing runoff controlling systems in these areas (e.g. dykes and partially effective dams), which could save and make use of a considerable amount of water. The groundwater resources in the Eastern Desert can be divided into four main water-bearing units: the fractured crystalline Pre-Cambrian aquifer, the Nubian sandstone aquifer, the fractured limestone and sandstone aquifer and the Quaternary aquifer. The most productive aquifer is the Nubian sandstone while the fractured limestone and sandstone (Miocene) are only productive along the eastern part of the desert. The Quaternary aquifer occurs along the major dry washes (wadis) and is considered of limited potential as it is recharged mainly from the occasional rainfall. Detailed assessment of these aquifers should be carried out locally for further development of the area.

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Resumen Aunque el Desierto Oriental de Egipto constituye alrededor del 22% de la superficie del país, esta área no se encuentra desarrollada por causa de la escasez de agua. Las unidades morfológicas del Desierto Oriental consisten de un número de cuencas de drenaje que cubren alrededor de 147,820 km² (66.5% del área superficial total del Desierto Oriental). Estas cuencas drenan las aguas lluvias ocasionales, bien hacia el Valle del Nilo o hacia el Mar Rojo, causando amenazas de inundación. La disponibilidad de agua a partir de los sistemas hidrológicos de estas cuencas, puede ser mejorada al construir sistemas de control de escorrentía allí (Ej. Diques y presas de efectividad parcial), las cuales podrían almacenar y permitir el uso de una cantidad considerable de agua. Los recursos de agua subterránea en el Desierto Oriental, pueden dividirse en cuatro unidades principales portadoras de agua: El acuífero fracturado cristalino Pre – Cámbrico, el acuífero de la Arenisca de Nubia, el acuífero de arenisca y caliza fracturadas y el acuífero Cuaternario. El acuífero más productivo es la arenisca de Nubia, mientras que la arenisca y caliza fracturadas (Mioceno), son productivas únicamente a lo largo de la parte oriental del desierto. El acuífero Cuaternario se encuentra a lo largo de las corrientes intermitentes mayores (Wadis) y se le considera de potencial limitado por ser recargado principalmente a partir de lluvia ocasional. Una evaluación detallada de estos acuíferos debe llevarse a cabo localmente, para el desarrollo adicional de esta área.

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Résumé Bien que le Désert Est occupe 22% de la surface de lEgypte, la région est sousdeveloppée à cause de la ressource en eau limitée. Les unités morphologiques consistent en des bassins drainant qui couvrent une surface de 147,820 km² ,représentant 66.6% de la surface du Désert Est. Les bassins drainent les plues intermittentes vers la vallée du Nil ou vers la Mer Rouge en provocant des inondations. On peut augmenter la ressource en eau dans cette région en réalisant des systèmes de contrôle de ruissellements (des digues...) qui peuvent sauver et utiliser un volume considérable deau. La ressource en eau souterraine du Désert Est est cantonnée dans quatre grandes structures: l› aquifère fracturé cristallin Précambrien, l› aquifère gréseux Nubien, l› aquifère calcaire et gréseux et l› aquifère quaternaire. Le plus productif est l› aquifère gréseux Nubien tendis que les calcaire et les grèses dage miocène sont productives seulement dans la partie est du désert. L› aquifère quaternaire se trouve au long des oueds sèches et on considéré quil a un potentiel limité, étant rechargé seulement par les plues intermittentes. Pour le futur développement de la région il est nécessaire une évaluation détaillée, à léchelle locale de ces aquifères.

     

柴北缘元古宙鹰峰环斑花岗岩及其共生岩石的地球化学特征、成因及地质意义

中元古代鹰峰岩体的主体是环斑花岗岩,与其共生的岩石有石英闪长岩-奥长花岗岩和辉绿岩。环斑花岗岩高碱（Na2O＋K2O=8.49%～9.39%）、富钾（K2O/Na2O=1.12～1.43）,铝近饱和,高铁镁比值[（ΣFeO）/MgO=4.91～7.19];富Rb、Ba、Ga、Th、Zr、Nb、Ta,贫Cr、Ni、V;高ΣREE（392.24×10-6～594.76×10-6）,稀土元素强分异[（La/Lu）N=12.67～17.09],弱铕负异常（δEu=0.58～0.78）,显示碱性花岗岩的特征,与密云环斑花岗岩相似。石英闪长岩-奥长花岗岩具钙碱性系列岩石的特征;与环斑花岗岩相比,其Rb、Ba、Ga、Nb、Ta、Th、Hf、Zr低,而Ni、Cr、V高;ΣREE较低（ΣREE=77.04×10^-6～129.85×10^-6）,轻重稀土分异明显,但（La/Lu）N的比值较小（11.62～14.06）,铕异常更弱（δEu=0.69～0.93）。辉绿岩具低碱、高ΣFeO的特征,属拉斑玄武质,与洋中脊拉斑玄武岩相比,K2O等不相容元素高,具大陆拉斑玄武质的特征。辉绿岩的ISr（1776Ma）为0.7066,εNd（1776Ma）为＋3.6,环斑花岗岩的ISr（1776Ma）为0.7181,εNd（1776Ma）为-5.5,显示辉绿岩起源于年轻的地幔,花岗质岩浆主要源自古老的地壳。综合分析显示,这些侵入岩形成于伸展背景,是北半球中元古代非造山环斑花岗岩的成员之一,在加里东期卷入到柴北缘造山带的古老地壳中。这在世界上提供了一个古老克拉通及环斑花岗岩卷入古生代造山带的一个实例。     

U-Pb Geochronology and Geochemistry of Stratiform Garnet from the Aqishan Pb-Zn Deposit, Eastern Tianshan, Xinjiang, NW China: Constraints on Genesis of the Deposit

The Aqishan lead-zinc deposit,located in the Jueluotag metallogenic belt of eastern Tianshan,Xinjiang,Northwest China,has a stratiform occurrence in the marine volcanic tuff of the Yamansu Formation.The ore body has a typical double-layer structure,having a stratified,stratoid,lenticular upper part and a veined,stockwork-like lower part.The occurrence of the upper orebody is consistent with that of the volcanic tuff wall rock.The ore minerals are mainly chalcopyrite,pyrite,sphalerite,galena and magnetite,the altered minerals mainly being silicified,such as sericite,chlorite,epidote,garnet.The garnetized skarn,being stratiform and stratoid,is closely related to the upper part of the orebody.Geological observations show that the limestone in the ore-bearing Yamansu Formation is not marbleized and skarnized.Spatially,it is associated with the ferromanganese deposits in the marine volcanic rocks of the Yamansu Formation.These geological features reflect the likelihood that the Aqishan lead-zinc deposit is a hydrothermal exhalation sedimentary deposit.The results from the EPMA show that the garnet is mainly composed of grossular-andradite series,contents being in a range of 34.791-37.8%SiO₂,32.493-34.274%CaO,8.454-27.275%FeO,0.012-15.293%Al₂O₃,0.351-1.413%MnO,and lower values of 0.013-1.057%TiO₂.The content of SiO₂ vs.CaO and FeO vs.Al₂O₃ has a significant positive correlation.The results of ICP-MS analysis for the garnet show that the REE pattern is oblique to right in general.The total amount of rare earth elements is relatively low,ΣREE=71.045-826.52 ppm,which is relatively enriched for LREE and depleted for HREE.LREE/HREE=8.66-4157.75,La_N/Yb_N=23.51-984.34,with obvious positive Eu and Ce anomalies(δEu=2.27-76.15,δCe=0.94-1.85).This result is similar to the REE characteristics of ore-bearing rhyolite volcanic rocks,showing that the garnet was formed in an oxidizing environment and affected by clear hydrothermal activity.The U-Pb isotopic dating of garnet by fs-LA-HR-ICP-MS gives an age of 316.3±4.4 Ma(MSWD=1.4),which is consistent with the formation time of the Yamansu Formation.According to the study of deposit characteristics and geochemical characteristics,this study concludes that the Aqishan lead-zinc deposit is a hydrothermal exhalation sedimentary deposit,the garnet being caused by hydrothermal exhalative sedimentation.     

华北克拉通北缘隆化地区蓝旗镇古元古代石榴石花岗岩的成因及地质意义

隆化地区蓝旗镇石榴石花岗岩位于华北克拉通北缘中部,与古元古代红旗营子杂岩密切共生。岩石主要由碱性长石、斜长石、石英、石榴石以及少量黑云母等矿物组成。锆石U Pb定年结果表明,石榴石花岗岩的形成时代为1894~1878Ma,属于古元古代中晚期。岩石的SiO2和K2O含量分别介于6409%~7080 %和364%~573 %之间,属于高钾钙碱性到钾玄岩过渡系列;岩石具有较高的Al2O3（1450%~1613%）和较低的CaO（125%~327%）和Na2O（180%~257%）含量,铝饱和指数（ACNK）介于107~134之间,表明该岩体属于S型花岗岩。岩石具有较高的CaO/Na2O比值（>03）,以及较低的Al2O3/Ti2O、Rb/Sr和Rb/Ba比值,指示岩浆源区为贫粘土、富斜长石的砂质岩。锆石Hf同位素分析进一步说明其物源可能是单塔子和红旗营子杂岩经风化作用的产物。蓝旗镇石榴石花岗岩具有与华北克拉通孔兹岩带内S型花岗岩相似的年代学和地球化学特征,结合区域变质岩和岩浆岩资料,本文认为孔兹岩带在空间上可能从内蒙古集宁地区向东延伸到了冀北地区。     

滇西镇康木厂A型花岗岩岩石学及地球化学特征

木厂A型花岗岩是由钠闪英碱正长岩、钠闪花岗岩,霓石花岗岩组成的复式岩 体。其形成与裂谷作用有关。岩体分异程度高,以高碱、高铁,贫镁、钙,稀土元素丰度高,轻稀土强烈分馏和明显的负铕异常为特征。 木厂A型花岗岩与上二叠统火山岩系具同源性,乃玄武岩浆与陆壳局部重熔岩浆的混合物。     

Age and tectonic setting of granitoid gneisses in the Eastern Desert of Egypt and south-west Sinai

Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (900 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called Older Granites of Egypt.²⁰⁷Pb/²⁰⁶Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 12 Ma and document granitoid activity over a period of about 23 Ma. A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 ± 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit. Single zircons from a dioritic gneiss from Wadi Feiran in south-west Sinai suggest emplacement of the protolith at 796 ± 6 Ma and this is comparable with ages for granitoids in north-east Sinai and southern Israel. None of the above gneisses is derived from remelting of older continental crust, but they are interpreted as reflecting subduction-related calc-alkaline magmatism during early Pan-african magmatic arc formation.