Origin of Cambrian Fibrous Kerites of the Volyn Region

Fine-fibrous kerites of the Volyn region represent mummified remains of cyanobacterial mats that functioned in the thermal geyser spring. Alienation of mat from the substrate and its burial in a dry cavity of the self-sealed biogenic-geyser siliceous structure, where mummification of microorganisms occurred owing to the hydrocarbon component under specific conditions close to those in dry caves, favored the high-degree preservation of primary features of the community biomass (its general life-time structure, microorganism morphology, ultra-fine cell structure, and biochemical marks). Organic matter was destroyed under low temperatures and pressures. Paleomicrobiological analysis of Paleozoic graphites from the Botogol Massif of nepheline syenites (eastern Sayan) indicates that mummified microorganisms from the thermal mat can preserve their morphology even when primary biomass is carbonized up to the most perfect carbon forms. The obtained data can be important both for the interpretation of the origin of most ancient carbonaceous structures of the Earth and for recognition of the mummified remains of microorganisms in meteorites.     

Composition and Petrogenesis of Ti-Oxides Associated with Diamonds

Among Ti-bearing oxides, rutile is common in eclogitic (E-type) diamonds and diamondiferous eclogites of varied bulk chemistry, but rarely occurs in association with ultramafic (or peridotitic) (U-type) diamonds, especially in lherzolitic assemblages. Roughly 100 rutile samples associated with diamonds as inclusions (71 samples), intergrowths with polycrystalline diamond aggregates (20 samples), and extracted from diamondiferous eclogites (11 samples) were studied by electron micro-probe techniques for Fe, Al, Mg, Ca, Mn, Cr, Nb, and Zr contents. Rutiles included in diamonds from Yakutia and Venezuela dominate the study. Compositions of rutiles associated with diamond and/or coesite from crustal ultrahigh-pressure (UHP) metamorphic rocks from the Kokchetav massif, northern Kazakhstan, and the Dora Maira massif in the Italian Alps are presented for comparison.

Titanates of the LIMA (lindsleyite-mathiasite) series and perovskite are rare accessories in U-type diamonds. Ilmenite of a wide range of compositions, containing up to 80% geikielite, is closely associated with rutile in bimineralic intergrowths in a number of diamonds and diamondiferous eclogites. Diamond-associated rutiles contain up to 0.8 wt% Al₂O₃, 2.5 wt% Fe₂O₃, 1.4 wt% Nb₂O₅, 0.45 wt% ZrO₂, and 2.3 wt% Cr₂O₃ (in the U type). The majority of rutiles, including diamond inclusions, are heterogeneous with widely varying Al, Fe, and Mg contents. These heterogeneities are mainly caused by the presence of sigmoidal oriented lamellae of ilmenite, picroilmenite (geikielite), and corundum; these are documented for the first time in diamond-associated rutiles and may represent exsolution from a UHP precursor. These features reflect the complex P-T and f(O₂) history of the rutiles. The presence of complex titanates of the LIMA series and perovskite in diamond-bearing assemblages reflects the metasomatizing conditions of diamond formation.     

滇东寒武系磷矿成因新见解——风暴成矿模式初探

近年来,随着风暴流和风暴岩的新理论、新概念的不断引入,在分析沉积环境、沉积成矿作用时,不断提出了许多新思路,也不断地冲击和修正以往的传统认识,作者于1986年随同成都地矿所寒武系磷矿专题组^①对滇东一带包括昆阳梅树村、海口、鸣矣河、晋宁王家湾、二街、八街、安宁白登、江川清水沟、宜良大滴水、寻甸先锋等矿区的磷矿进行了详细的研究,在下寒武统中谊村段及其相当的地层中发现了多层磷质风暴沉积物。本文拟概略介绍磷质风暴沉积物的标志、特征、剖面结构并初步探讨磷矿富集与风暴事件关系的沉积演化模式。     

Petrogenesis of the Chaihulanzi Gneiss and its Tectonic Implications for the North China Craton

The Chaihulanzi area in eastern Inner Mongolia is tectonically situated on the northern margin of the North China Craton (NCC). The main Precambrian lithologies of the area have been referred to the Archean Jianping Group meta-supracrustal sequences. Based on field observations, petrographic, whole-rock geochemical, and zircon U-Pb geochronological results, a magmatic origin for the units is proposed. Our results show that the Chaihulanzi gneisses are mainly of granitic, dioritic and granodioritic compositions, and show typical magmatic rock textures and mineral assemblages. The dioritic and granodioritic gneisses show Na-rich tonalite–trondhjemite–granodiorite (TTG)-like affinity with zircon U-Pb dates of ca. 2.57–2.59 Ga, representing a juvenile continental growth for the northeastern NCC. The granitic gneiss is indeed potassic granite and yielded a zircon U-Pb date of ~2.50 Ga, which is contemporaneous with the Jining–Jiaoliao microblock collision (2.53–2.49 Ga), implying another crustal growth event. The well-developed gneissosity in 2.57–2.59 Ga dioritic and granodioritic gneisses together with the 2.5 Ga potassic granite, which crosscuts the gneissosity implies a 2.57–2.50 Ga (mainly 2.53–2.51 Ga) collisional orogeny, probably related to the Jining–Jiaoliao microblock collision. Our new geology and chronological results provide new evidence for the early Precambrian tectonic evolution of the NCC.     

Origin, diversification, and relationships of Cambrian lobopods

As an extinct group, the origin, diversification and relationships of Cambrian lobopods have long been one of the most hotly-debated subjects. Lobopods used to be regarded as closely related to modern onychophorans and tardigrades, but the similarities are based on the few species of Cambrian lobopods and often on a fairly general level. With the discovery of new creatures and based on new observations on fossil lobopods, we consider that Cambrian lobopods show great diversity and reveal a close relationship between Cambrian lobopods and arthropods. In addition, comparison between the dorsal spines of Cambrian lobopods and small shelly fossils suggest that Cambrian lobopods might have originated from the Meishucun radiation and diversified in the Qiongzhusi radiation.     

Geochronology,Geochemistry and Petrogenesis of the Bangbule Quartz Porphyry: Implications for Metallogenesis

The Bangbule skarn lead-zinc(Pb-Zn) deposit(>1 Mt Zn + Pb) is located in the western Nyainqentanglha polymetallic metallogenetic belt, central Tibet. Lenticular orebodies are all hosted in skarn and developed in the contact zone between the quartz porphyry and carbonate strata of the mid Paleozoic Middle to Upper Chaguoluoma Formation as well as in carbonate and sandstone beds of the Upper Paleozoic Laga Formation. As a newly discovered skarn deposit, the geological background and metallogene...     

石英晶格中微量元素组成对成岩成矿作用的示踪意义

相对其它造岩矿物,石英因其成分和结构具有特殊的稳定性,在地质演化过程中“携带”有更多的成岩成矿信息。 由于不同地质环境下石英晶格中微量元素的地球化学行为具有明显差异,造成岩浆-热液演化不同阶段以及不同地质背景 下形成的石英结构及其晶格中微量元素种类、含量显著不同。因此,利用阴极发光和微区原位测试技术对石英的结构以及 晶格中微量元素分布特征的研究,总结影响微量元素通过类质同象形式进入石英晶格的因素,找出特定环境下灵敏度高的 元素或元素组合,可以对不同来源的石英进行源区示踪,并可获取成岩成矿中有用的地质-地球化学信息。此外,石英晶 格中的Al,Ti含量与石英形成温度之间的线性关系可作为地质温度计而广泛应用。     

湖南川口印支期花岗岩成因及与钨成矿关系

川口花岗岩体位于南岭成矿带北西侧,赋存有大型川口钨矿床.川口花岗岩主要由二长花岗岩和白云母花岗岩组成,具有高硅、富碱的特点,属高钾钙碱性系列,为分异的S型花岗岩.SHRIMP锆石U-Pb同位素测年与辉钼矿Re-Os同位素测年表明,川口花岗岩体中细粒含斑二云母二长花岗岩成岩年龄为223.1±2.6 Ma,成矿年龄为225.8±1.2 Ma,表明其成岩成矿时代为印支期.元素地球化学及Sr、Nd同位素特征表明:川口花岗岩来源于古元古代泥质岩的部分熔融,形成于印支期陆内伸展-减薄的构造环境.川口花岗岩与钨矿床在时空上高度吻合,钨矿与花岗岩的Y/Ho比值相近,且两者在Y-Ho图解上显示出良好的线性关系,结合钨矿石中Re同位素和流体包裹体氧同位素组成特征,认为川口花岗岩岩浆经历了高度分离结晶,产生了富挥发分的成矿流体而形成钨矿.作为华南地区印支期花岗岩成岩成矿事件的典型实例,川口花岗岩与川口钨矿指示了华南地区印支期花岗岩具备较大的钨矿成矿潜力.     

北阿尔金野马泉二长花岗岩成因及其构造意义

野马泉二长花岗岩为北阿尔金地区出露面积最大的花岗岩体,为探讨其成因、形成环境及其与北阿尔金区域构造演化的关系,对其进行了岩石学、地球化学、锆石U-Pb年代学及Hf同位素等方面的研究.研究结果表明,野马泉岩体为中-粗粒等粒或似斑状二长花岗岩,岩体侵位时代为450~453 Ma.岩石具较高的Na₂O/K₂O比值为1.72~2.29,铝饱和指数A/CNK为0.99~1.10,P₂O₅与SiO₂含量呈负相关,具Ⅰ型花岗岩特征.轻稀土富集而重稀土亏损,较弱的负Eu异常,微量元素特征显示富集Rb、Ba、Th、U、K等元素,相对亏损Nb、Ta、P、Ti等元素.锆石ε_Hf（t）值为5.52~10.75,二阶段模式年龄t_DM2为0.75~1.09 Ga,其源岩可能主要是0.75~1.09 Ga的新生地壳（基性岩）.结合区域构造背景,野马泉二长花岗岩体可能形成于同碰撞-后碰撞环境,为造山带根部基性岩石部分熔融形成.      

塔里木盆地西北缘寒武系中热水白云石团块特征及成因研究

在塔里木盆地西北缘寒武系中,发育有团块状的热水白云石,其具有明显的层控性:产出在中寒武统顶到上寒武统底的蒸发岩向白云岩过渡段,且仅赋存于粉晶白云岩中.热水白云石具有典型的鞍状白云石特征:弯曲的晶面,波状消光,边缘多发生去白云岩化.地球化学分析显示,其Ca/Mg值稍高于基质白云石,Sr、Fe、Mn和Ba含量也略高于基质白...     

Origin of the Lower Cambrian phosphatic bed at Vassbo, Sweden

An extensive phosphatic bed characterized by a black apatite-cemented sandstone is developed in the uppermost part of the Lower Cambrian formation over the entire Vassbo area, Sweden. The thickness of the layer is generally 0.1 m, increasing in places to 0.2 m. The lower boundary of the bed is transitional to a sandstone; the upper boundary displays a contact that is both sharp and irregular with a Middle Cambrian bituminous black shale.
The phosphatic bed was diagenetically formed by inorganic in situ precipitation of apatite in a sandstone at a depth of about 0.2 m below the sediment-water interface, where reducing conditions were developed, as a result of oxygen consumption during the degradation of organic matter. Sulphur isotope data from associated pyrite suggest that organic processes, such as bacterial sulphate reduction, were also involved in the formation of the phosphatic bed. The depositional environment of the bed was a marine shelf with a zero to very low sedimentation rate. The upper part of the apatite-cemented sandstone was partly reworked by bottom currents as demonstrated by a board containing numerous pebbles. Sedimentological as well as chemical data confirm a southern hemisphere position of the Baltic Shield in the early Cambrian in which an oceanic upwelling and high organic productivity was the probable source of the phosphorus along the palaeo-west-coast.     

南阿尔金陆块科克萨依新元古代花岗岩成因及地质意义

南阿尔金陆块是阿尔金造山带的重要组成部分.大量新元古代花岗岩出露于南阿尔金亚干布阳-帕夏拉依裆-科克萨依一带.这些花岗岩记录了与Rodinia超大陆汇聚有关的动力学信息,因此对其进行研究有利于对阿尔金造山带演化历史的认识和理解.选取了科克萨依花岗岩岩体进行了岩相学、地球化学、锆石U-Pb年代学和Hf同位素组成的研究.研究结果表明:（1）科克萨依二长花岗岩的主要矿物有:石英、钾长石、斜长石、黑云母和白云母;花岗岩的锆石U-Pb年龄为947~945 Ma.（2）地球化学特征显示,岩石具有高SiO₂（71.54%~74.69%）、高Na₂O+K₂O（6.33%~7.40%）,低CaO（1.59%~2.00%）,低MgO（0.43%~0.61%）和TiO₂（0.25%~0.37%）的特征,相对富钾,K₂O/Na₂O比值为1.02~1.71,A/CNK在1.10~1.14之间,属高钾钙碱性系列的过铝质花岗岩.富集Rb、Th、K、La等元素,亏损Nb、Ta、P、Ti等元素;轻稀土富集而重稀土亏损,具有明显的负Eu异常.（3）锆石ε_Hf（t）为-4.09~+3.87之间,二阶段模式年龄t_DM2为1.6~2.0 Ga.这些特征表明科克萨依二长花岗岩是古老地壳富长石贫黏土的（变）杂砂岩部分熔融形成的S型花岗岩.结合相邻地区新元古代花岗岩类的地球化学、同位素特征及阿尔金区域构造资料,认为科克萨依二长花岗岩形成于新元古代时期,是碰撞造山环境下的产物,是Rodinia超大陆汇聚碰撞过程的响应.      

阿尔泰山南缘中-酸性斑岩的地球化学特征及其岩石成因探讨

阿尔泰山南缘中—酸性斑岩侵入于中泥盆统北塔山组火山岩地层中,斑岩体规模较小,主要岩石类型为闪长斑岩、石英闪长斑岩、花岗闪长斑岩、石英二长斑岩等。对哈腊苏、卡拉先格尔、喀腊萨依、托斯巴斯陶和乔夏哈拉等5个与铜矿有关矿区的斑岩研究表明,其地球化学特征既有相似性,又有不同的特点,其中SiO2含量为534%~662%,A/CNK为098~127(大多数小于1),稀土元素表现为轻稀土相对富集、不具Eu异常的特点。微量元素表现为明显的Nb和Ta的负异常。但是Harker图解以及稀土元素配分曲线表明,不同地区的斑岩不存在演化关系。斑岩的Sr和Nd同位素组成基本上与与其共生的玄武岩相同,因此根据实验岩石学以及稀土模拟结果推测,它们可能是岛弧阶段底侵的基性岩石部分熔融形成的,源区相当于下地壳无水的角闪岩相,但含有少量的石榴子石,其中石榴子石和角闪石比例的差别可能是导致不同地区斑岩地球化学特征差别的主要原因。     

Melting of Refractory Mantle at 1{middle dot}5, 2 and 2{middle dot}5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting

Boninites are an important ‘end-member’ supra-subductionzone magmatic suite as they have the highest H₂O contents andrequire the most refractory of mantle wedge sources. The pressure–temperatureconditions of boninite origins in the mantle wedge are importantto understanding subduction zone initiation and subsequent evolution.Reaction experiments at 1·5 GPa (1350–1530°C),2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C)between a model primary high-Ca boninite magma composition anda refractory harzburgite under anhydrous and H₂O-undersaturatedconditions (2–3 wt % H₂O in the melt) have been completed.The boninite composition was modelled on melt inclusions occurringin the most magnesian olivine phenocrysts in high-Ca boninitesfrom the Northern Tongan forearc and the Upper Pillow Lavasof the Troodos ophiolite. Direct melting experiments on a modelrefractory lherzolite and a harzburgite composition at 1·5GPa under anhydrous conditions (1400–1600°C) havealso been completed. Experiments establish a P, T ‘meltinggrid’ for refractory harzburgite at 1·5, 2 and2·5 GPa and in the presence of 2–3 wt % H₂O. Theeffect of 2–3 wt % dissolved H₂O produces a liquidus depressionin primary boninite of     

中条山上玉坡地区黑云母片岩成岩时代及成因

中条山上玉坡背斜核部主要有北峪花岗岩、变富钾流纹岩及(角闪石/方柱石)黑云母片岩为主的变镁铁质杂岩出露.上玉坡的黑云母片岩类的归属及成因争论已久.空间上, 上玉坡黑云母片岩类岩石及地球化学特征形成了一个以背斜核部为中心的环形的递变带, 岩石的主要造岩矿物特征及K、Ca、Na、Rr、Sr、Ba受退变质韧性剪切带高盐度流体改造均发生系统变化, Ta及LREE也显示有明显变化.上玉坡变富钾流纹岩LA-MC-ICPMS测年获得207Pb/206Pb加权平均年龄为2 164±8.9 Ma, 与绛县群竖井沟组富钾流纹岩(207Pb/206Pb加权平均年龄2 161.3±1.5 Ma)同时代产出.上玉坡产出的变富钾流纹岩-黑云母片岩与铜矿峪分布的绛县群双峰式火山建造岩性组合类似, 推断上玉坡背斜核部产出的黑云母片岩产出于2.20~2.15 Ga.使用在流体中保持相对稳定的高场强元素对图解, 判断中条山黑云母片岩的原岩更可能为亚碱性镁铁质火成岩, 并非前人认识的"超钾基性火山岩", 综合中条山古元古代地质特征, 该地区古元古代可能处于与俯冲活动有关的张性构造环境.      

东昆仑哈拉森地区花岗岩类岩石成因及地质意义

哈拉森地区位于东昆仑东段,分布着大量花岗岩,对其研究不仅有助于认识东昆仑造山带在晚古生代-早中生代的构造-岩浆演化历史,而且可以为东昆仑古特提斯洋俯冲时限及洋盆闭合时限提供约束.对区内花岗岩进行了岩石学、年代学以及岩石地球化学分析,结果表明哈拉森地区的钾长花岗岩和细粒二长花岗岩锆石LA-ICP-MS U-Pb年龄分别为239.2±1.7 Ma（MSWD=0.19）和232.4±1.2 Ma（MSWD=0.76）,属中三叠世花岗质岩浆作用的产物.岩石主微量元素分析显示该地区花岗岩具有高硅铝、富碱和低钛特征,属于高钾钙碱性到钾玄岩系列的过铝质花岗岩,富集轻稀土元素（LREE）及K、Th、Rb等大离子亲石元素（LILE）,明显亏损Nb、Ti、P、Ta等高场强元素（HFSE）,具有非常明显的Eu负异常（δEu为0.27~0.65）.哈拉森地区花岗岩具有高分异Ⅰ型花岗岩的特征,是同碰撞背景下幔源岩浆与其诱发地壳物质熔融产生的长英质岩浆在地壳深部混合,随后这一混合岩浆又经过高程度的分异演化形成的,进一步证明东昆仑古特提斯洋的俯冲作用一直持续到早三叠世,至中三叠世才逐渐转入陆内碰撞造山阶段.      

南秦岭西坝花岗质岩体矿物学特征及成岩意义

西坝花岗质岩体位于南秦岭构造-岩浆带中部,为南秦岭印支期花岗岩带五龙岩体群的重要组成部分,与双王金矿有密切的空间关系。该岩体主要由石英二长闪长岩和花岗闪长岩组成,岩体中发育较多的镁铁质包体。通过对岩体及镁铁质包体的系统的矿物学特征研究发现,本区花岗质岩体属于I型花岗岩类,成岩物质主要来源于地壳,并且有地幔物质的加入。岩体及镁铁质包体显示有多种壳-幔岩浆混合的岩相学证据:如包体的冷凝边、斜长石环带成分显示核部和边缘偏基性,而幔部偏酸性,包体中见大量的长针状磷灰石及角闪石,包体中有两种产状的角闪石等。根据矿物温度压力计计算,西坝岩体的结晶温度为646.72~703.84℃,压力为1.67×108~3.66×108Pa,平均为3.12×108Pa。岩体形成于相对高氧逸度(lgfO2为-16.16×105~-17.06×105Pa)、中等水逸度(fH2O为46.82×105~136.85×105Pa)和氢逸度(fH2为0.32×105~0.91×105Pa)环境。岩体的这种高氧逸度条件有利于金进入熔体相或流体相,是金矿形成的有利条件。     

Petrogenesis of Neoproterozoic Granitoids and Related Rocks from the Seychelles: the Case for an Andean-type Arc Origin

The Seychelles islands consist of undeformed and unmetamorphosed,metaluminous monzogranites and granodiorites of Neoproterozoicage (     

鄂东南地区鄂城岩体的时代、成因及其对成矿作用的指示

鄂城岩体位于鄂东南地区的最北部,是鄂东南地区的六大岩体之一.在该岩体的南缘接触带上产出了长江中下游地区最大的矽卡岩型铁矿床——程潮铁矿床.众多研究表明,程潮铁矿化与鄂城杂岩体的岩浆演化密切相关,然而目前对于成矿作用究竟是与花岗质岩还是闪长质岩有关仍存在争议.通过对鄂城杂岩体开展系统的锆石U-Pb年代学、元素地球化学和Sr-Nd-Hf同位素研究,结果表明该岩体主要由花岗岩、石英二长岩、花岗斑岩以及小面积的闪长岩组成,最早侵位于140±1 Ma（中粒闪长岩）,之后依次侵位形成了细粒闪长岩（132±2 Ma）、花岗斑岩（130±2 Ma）、花岗岩（中细粒花岗岩129±2 Ma,中粒花岗岩129±1 Ma）和石英二长岩（129±1 Ma）.根据全岩地球化学特征,鄂城杂岩体的岩石组成大致可以分为两组:（1）花岗岩类,包括花岗岩、花岗斑岩和角闪石英二长岩,钾质,具有高SiO₂,低TiO₂、FeO_t、MnO、MgO含量等特征;（2）闪长岩类,包括中、细粒闪长岩,钠质,具有低SiO₂,高TiO₂、FeO_t、MnO、MgO含量等特征.这些岩石均富集轻稀土元素（LREE）和大离子亲石元素（LILE,如Rb、Th等）,亏损高场强元素（HFSE,如Nb、P、Ti）等,且花岗岩类具明显的负Eu异常,而闪长岩类则无此特征.在同位素组成方面,鄂城花岗岩类具有较负的全岩ε_Nd（t）值（-11.7~-10.1）和锆石ε_Hf（t）值（-22.91~-9.83）,闪长岩类则具有稍高的全岩ε_Nd（t）值（-7.6）和锆石ε_Hf（t）值（-12.04~-4.69）.元素和同位素地球化学特征共同表明,鄂城花岗岩类属于高分异Ⅰ型花岗岩,且主要来源于古元古代基底物质的部分熔融作用,源区可能有少量幔源物质的加入;闪长岩类主要来源于富集岩石圈地幔,且经历了一定的分离结晶作用.年代学结果显示,鄂城花岗岩类和细粒闪长岩的侵位时间均与程潮铁矿床的主成矿期吻合.结合野外接触关系以及前人的研究,程潮铁矿化可能与上述两类岩石均密切相关.从整个鄂东南地区的成矿作用来看,随着岩浆源区壳源物质贡献的增大以及岩浆分异程度的增加,岩浆作用与铁矿化的关系也更加密切.      

Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km². It has an ⁴⁰Ar/³⁹Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO₂ (2·2wt %) and total iron (14 wt % as Fe₂O₃), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar