东昆仑造山带索拉沟地区三叠纪赞岐质闪长岩的成因机制及其对古特提斯造山作用的启示

具有特殊成因机制的赞岐岩是探究深部岩浆动力学过程与区域构造演化的重要岩石探针之一.以东昆仑索拉沟地区赞岐质闪长岩为研究对象,开展系统的岩石学、年代学、元素地球化学和Lu-Hf同位素分析,厘定其岩石成因,揭示其对古特提斯造山作用的启示.锆石U-Pb年代学分析表明,索拉沟闪长岩形成于中三叠世（~243 Ma）.岩石具有较低含量的SiO₂（50.26%~57.40%）,较高的全碱成分（Na₂O+K₂O=3.5%~6.3%）、MgO（6.0%~7.1%）和Mg#值（50.1~60.9）,属于高镁钙碱性准铝质岩石.索拉沟闪长岩具有较高的Sr（622×10-6~1 041×10-6）、Cr（30×10-6~161×10-6）和Ni（19×10-6~79×10-6）以及中等的Y（7.6×10-6~24.3×10-6）和Yb（0.62~1.87）含量,与典型的高镁安山质赞歧岩成分类似.岩石富集轻稀土和大离子亲石元素（如Rb、K和Pb）,亏损高场强元素（如Ta、Nb和Zr）,具有弧岩浆岩的化学属性.Lu-Hf同位素研究揭示,索拉沟闪长岩起源于富集岩石圈地幔（ε_Hf（t）=-2.4~-0.4,T_DM=0.89~0.99 Ga）.岩石成因分析表明,东昆仑索拉沟中三叠世赞岐质高镁闪长岩形成于陆缘弧背景,岩浆起源于富集岩石圈地幔,且岩浆经历了以角闪石和黑云母为主的分离结晶作用.中三叠世是东昆仑造山带古特提斯洋壳俯冲和地体碰撞的构造转换阶段,索拉沟赞岐质高镁闪长岩可能是俯冲作用结束时板片断离的岩浆响应.      

川西甘孜-理塘结合带洋岛型玄武岩地球化学、年代学特征及其地质意义

甘孜-理塘结合带作为三江特提斯造山带与松潘-甘孜造山带的重要组成部分,发育大量的洋岛-海山岩石组合,洋岛玄武岩的存在标志着洋盆已发育成熟的洋壳.木里地区的洋岛-海山岩石组合由"基性火山岩+碳酸盐岩"构成,对基性火山岩的地球化学特征分析表明：基性火山岩SiO₂含量为40.17%~49.19%,TiO₂含量1.77%~4.86%,Al₂O₃含量9.67%~15.39%,MgO含量3.85%~17.75%,全碱含量（K₂O+Na₂O）0.87%~6.33%,属碱性玄武岩系列.稀土总量∑REE值较高,为106.21×10^-6~378.83×10^-6,（La/Yb）_N比值为9.23~39.41,为轻稀土富集型,Eu、Ce无异常,配分模式为右倾型.微量元素表现出大离子亲石元素Rb、Ba、Th、K富集,而亏损高场强元素Nb、Zr.稀土元素及微量元素特征与标准洋岛型玄武岩相近,源区为软流圈地幔石榴石橄榄岩,且部分经历了岩浆结晶分异,形成于洋岛台地环境.玄武岩的锆石U-Pb定年结果为218.96~221.71 Ma,表明洋岛型玄武岩形成于晚三叠世中期.     

东昆仑五龙沟矿集区中三叠世辉绿岩脉的岩石成因:年代学、地球化学特征及其构造意义

本文对五龙沟矿集区4条辉绿岩脉中的锆石进行LA-ICP-MS U-Pb定年研究,发现其中大部分锆石为捕获锆石,而拥有最新年龄的锆石可代表岩浆结晶时的锆石,其加权平均年龄为（242.8±2.1）Ma,属于中三叠世。这些辉绿岩脉的w（SiO₂）为42.82%~52.94%,w（Na₂O）为1.98%~4.51%,w（K₂O）为0.70%~2.04%,属于钙碱性和高钙碱性系列;4条岩脉中V、Cr、Ni等质量分数与MgO质量分数均呈正相关,且质量分数较小,表明其岩浆演化过程中经历了以橄榄石、辉石为主的分离结晶作用;其微量元素具有相对富集大离子亲石元素（Rb、Ba、Eu）和轻稀土元素,亏损高场强元素的特征,Nb/La为0.39~0.50,Nb/Ta为12.16~16.69,Zr/Hf为45.82~47.06,明显和亏损软流圈地幔来源的岩浆不同。综合分析认为,五龙沟矿集区的中三叠世辉绿岩脉大地构造背景为俯冲环境,是受俯冲板片流体改造的岩石圈地幔部分熔融的产物,并经受了一定的地壳混染。     

特提斯喜马拉雅带中段东部三叠纪火山岩的地球化学和岩石成因

三叠纪火山岩分布于特提斯喜马拉雅带中段东部,古地理上位于大印度北东部.分别采用原子吸收分光度法、ICP-MS和全岩同位素稀释法对三叠纪火山岩的主量元素、微量元素和Sr-Nd同位素进行了分析,并用来研究其成分特征和成因.岩相学和地球化学指标显示三叠纪火山岩为玄武岩,MgO含量均很低（范围在4.27%～7.72%）,属于演化岩浆.早三叠世热马组和中三叠世桑康组具有较高的TiO2、TFeO和P2O5含量,稀土元素总量和轻重稀土元素的分馏程度均较高,Ti/V、Ti/Y、Zr/Y比值较高,富集高场强元素（如Nb、Zr等）,地球化学特征与峨眉山高Ti玄武岩相似,以更富集La、Ce、Zr、Hf等元素而有别于夏威夷碱性玄武岩,它们属大陆边缘裂谷背景的具有OIB型地球化学特征的碱性玄武岩;晚三叠世涅如组玄武岩以较低TiO2、TFeO和P2O5含量为特征,稀土元素总量和轻重稀土分馏程度均较低,高场强元素（如Nb、Zr等）丰度很低,为大陆边缘裂谷背景的具有E-MORB地球化学特征的拉斑玄武岩.三叠纪玄武岩Nd同位素组成随时间从轻度富集到亏损（εNd（t）=-1.2→-0.5→4.4）,指示岩浆源区具有从轻度富集到亏损的演化趋势.不相容元素（如La/Sm、La/Ta以及（Th/Ta）PM、（La/Nb）PM等）指示热马组和桑康组玄武岩遭受了一定程度下地壳混染,而涅如组没有遭受地壳混染.热马组和桑康组玄武岩还显示岩石圈地幔物质的印记,并与石榴石相二辉橄榄岩的低度部分熔融有关,而涅如组玄武岩除了具有E-MORB特征外,还与尖晶石相二辉橄榄岩的较高程度部分熔融有关.它们被解释为上涌的软流圈地幔物质与岩石圈地幔物质混合后在不同程度的拉张背景下发生减压熔融的产物.特提斯喜马拉雅带的三叠纪裂谷作用很可能具有主动裂谷作用的性质,较小规模的火山岩浆作用可用裂谷带之下上涌的地幔物质由于传导作用而变冷或正在孕育的地幔柱成因模式来解释.     

江南造山带形成过程中若干新元古代地质事件

报道了江南造山带( 中-) 西段在“俯冲造山阶段”形成的科马提质玄武岩、石英角斑岩、流纹质凝灰岩及“后造山伸展阶段”形成的两类基性岩的特征,研究表明,科马提质玄武岩是形成于岛弧环境的高MgO 玄武岩。高的Ni 和Cr含量说明它们为来自弧下地幔的原始岩浆,较高的MgO 显示这些高MgO 玄武岩浆是在相对较干的条件下从地幔源区分离的。“后造山伸展阶段”形成的两种基性岩分别来自软流圈和岩石圈地幔,具有不同的地球化学特征。基底地层中砂岩的碎屑锆石及夹层中基性—酸性火山岩的锆石U-Pb 定年表明,在扬子板块东南缘,新元古代岛弧岩浆活动的时间约为878~822 Ma,岛弧地区陆源碎屑的沉积作用发生在872~835 Ma 期间,表明存在同时代的（coeval）“岛弧岩浆作用”和“沉积作用”。     

青海沱沱河地区开心岭一带中二叠世—早三叠世基性岩浆事件及其地质意义

青海省沱沱河地区开心岭一带中二叠世—早三叠世构造背景是影响该区成矿规律和找矿方向分析的重要因素。对开心岭一带中二叠世诺日巴尕日保组玄武岩地球化学及侵入其中的辉长岩进行年代学研究,以期对该区中二叠世—早三叠世成矿构造背景提供一定的约束。采用LA-ICP-MS技术测得的辉长岩中锆石206Pb/238U年龄加权平均值为247.0±1.7Ma,属于早三叠世,该期岩浆事件可能是龙木错—双湖缝合带和可可西里—金沙江缝合带在此阶段发生俯冲的产物。地球化学特征表明,中二叠世玄武岩具有高镁、富铁、贫碱的特征,属于拉斑系列玄武岩;富集LREE和部分HFSE,REE表现为轻、重稀土元素分馏明显的右倾型配分型式。具有相对高的Zr含量(平均含量大于70×10-6)和Zr/Y值(5.2~7.0),Nb、Ta、Zr、Hf和Ti相对亏损,Nb/La值(0.1~0.3)较低,总体具有大陆玄武岩的特征,但受到不同程度陆壳或岩石圈地幔的混染。综合多种判别图解分析认为,该套玄武岩形成于大陆裂谷环境。     

泰宁地区中志留世辉绿岩脉成因及对陆内造山作用的制约

华南早古生代造山作用以武夷-云开造山带最为典型.然而，该次造山作用过程中岩石圈地幔性质及构造背景一直存在争议，还需要更多的基性岩证据.对武夷造山带中段泰宁地区何宝山辉绿岩脉开展了年代学、地球化学及Sr-Nd-Hf同位素研究.LA-ICP-MS锆石U-Pb年龄（430±3 Ma）表明辉绿岩脉形成于中志留世.岩石具有高Al₂O₃（15.23%~18.49%）、低Na₂O（2.82%~4.21%）和K₂O（1.63%~2.42%）含量特征，属亚碱性系列.稀土元素标准化图呈右倾，无明显Eu异常，显示富集大离子亲石元素（Rb、Ba、K、La、Sr）、不同程度亏损高场强元素（Ta、U、Hf、Ti）.岩石具有高Nb含量（5.74×10-6~8.45×10-6），其（Nb/La）_N和Nb/U值与富Nb岛弧玄武岩相似.全岩（87Sr/86Sr）_i=0.709 5~0.710 9，ε_Nd（t）=-0.8~-0.1，锆石ε_Hf（t）=-9.2~-2.7，对应二阶段模式年龄T_2DM（Hf）=1.4~1.8 Ga.地球化学、Sr-Nd及Hf同位素揭示泰宁地区基性岩浆源自富集岩石圈地幔13.3%~21.8%的熔融，但在熔融前曾受到古俯冲物质（俯冲板片熔体+5%沉积物熔体）的源区混染.武夷地区于早古生代为陆内造山环境，脉岩形成于后造山垮塌阶段，可能与冈瓦纳东缘造山作用的应力向大陆传播引起华夏地块内部俯冲有关.      

东天山七角井-巴里坤地区辉绿岩脉年代学及岩石成因

新疆东天山七角井-巴里坤一带石炭系中广泛发育辉绿岩脉,具有相对较高的Al2O3和MgO,低TiO2,贫P2O5及K2O的地球化学特征;岩石轻稀土富集、重稀土相对亏损的右倾型稀土模式,Cs、K等大离子亲石元素富集而Nb、Ta、Zr、Hf等高场强元素明显亏损,微量元素配分曲线,均一的SiO2含量和大离子亲石元素丰度的不显著波动等地球化学特征以及接近于软流圈地幔与岩石圈地幔界面温度的岩浆形成温度,暗示岩浆来源于受流体/熔体交代的富集岩石圈地幔;较高的Mg#(0.64~0.67),较低的Ce/Y(1.55~1.79)和Dy/Yb(1.97~2.02)以及La/Sm-La图解等表明其原始岩浆可能为石榴子石橄榄岩较高程度部分熔融的产物,分异演化程度较低;Zr、Hf、Y等稳定元素及判别图解指示岩石形成于板内环境;对辉绿岩中锆石进行LA-ICP-MS U-Pb定年,获得(314.7±1.5)Ma的形成年龄,表明从晚石炭世开始,博格达造山带已转变为伸展环境。     

西伯利亚板块东南缘晚古生代伸展体制新证据：东乌旗角闪辉长岩年代学及地球化学研究

为研究西伯利亚板块东南缘晚古生代板内伸展开始时间及地幔属性。基于1∶5万野外宏微观调查,采用LA-MC-ICP-MS锆石U-Pb定年法对分布于东乌旗西部的角闪石辉长岩进行了年龄测定和岩石地球化学测试。结果表明:角闪辉长岩主要由斜长石、角闪石和单斜辉石组成,锆石U-Pb年龄为280.8±1.5Ma,属早二叠世。地球化学数据显示:岩石具较高SiO2(48.87%~53.70%)、TiO2(0.73%~2.27%)、Al2O3(15.05%~16.69%)的含量,中等至较高的MgO(4.84%~9.25%)、FeOT(6.91%~10.47%)和较低的CaO(5.80%~7.94%)。富Na2O(2.75%~3.90%),贫K2O(1.01%~1.90%),为铁质—富铁质碱性和拉斑玄武岩系列岩石组合。稀土总量较高(∑REE为97.50×10-6~251.16×10-6),轻重稀土分异程度中等[(La/Yb)N=4.23~7.12],Eu异常不明显(δEu=0.85~0.98)。岩石富集大离子亲石元素(LILE)Ba、Sr、K等,相对亏损高场强元素(HFSE)Nb、Ta、Th等,高的Zr(87×10-6~289×10-6)和Zr/Y(4.03~7.66)比值,与板内幔源岩浆作用的产物一致。综上,我们认为角闪辉长岩为板内伸展构造体制下富集型地幔橄榄岩部分熔融形成的玄武质岩浆岩浆经分离结晶作用后形成的,标志着~280 Ma西伯利亚板块东南缘晚古生代造山过程进入到板内非造山阶段。     

松潘-甘孜地体下仓界地区晚三叠世基性岩的发现及其地质意义

笔者在松潘-甘孜地体下仓界地区发现了晚三叠世基性岩,对其开展了岩相学、地球化学、斜锆石U-Pb年代学等研究。该基性岩由玄武岩和辉绿岩组成,均为钙碱性系列,蚀变较强,形成时代为208.0±7.0 Ma。岩石La/Nb值为1.13～1.95,La/Ta值为7.96～25.79,TiO₂含量为1.51%～2.22%,Zr/Nb值为11.35～16.33,指示岩浆源自岩石圈地幔与软流圈地幔的混合源区。根据(Yb/Sm)_P-(Tb/Yb)_P模型计算结果,推测下仓界基性岩可能源于软流圈物质诱发岩石圈地幔尖晶石-石榴子石过渡相(石榴子石约占0～25%)发生约5%的部分熔融。岩石中发现捕获的古老斜锆石,轻稀土元素富集,重稀土元素亏损,Nb、Ta和Ti元素弱亏损。结合区域地质资料,认为松潘-甘孜地体应该具有陆壳基底,其晚三叠世(<211 Ma)的构造环境为碰撞后的板内伸展环境。     

CHARACTERISTICS OF VOLCANIC ROCKS AND ITS TECTONIC SETTING IN WESTERN PART OF LAZHULONG-JINSAJIANG STRUCTURAL BELT

CHARACTERISTICS OF VOLCANIC ROCKS AND ITS TECTONIC SETTING IN WESTERN PART OF LAZHULONG-JINSAJIANG STRUCTURAL BELT     

甘肃省文县刘家坪蓟县系变基性火山岩地球化学特征及构造环境

详细的野外地质调查显示,甘肃省刘家坪蓟县系为一套变基性火山岩,岩石以变玄武岩为主。岩石地球化学特征表明:A/CNK平均值为059%,具有铝不饱和特征;Na2O/K2O值为4700～14400,表现为钠质;在TAS图解中,显示原岩为亚碱性系列;在AFM图解中,样品点均落入拉斑玄武岩系列;稀土元素含量∑REE为5822×10-6～9559×10-6,平均为6781×10-6;δEu=083～117,δCe=088～095,稀土配分模式为平坦型;微量元素中,高场强元素Zr、P、Y、Yb相对亏损,Th、Hf、Nb相对富集,低场强元素K亏损明显,Rb、Ba相对富集,且Zr/Y值为238～391,Ti/Zr值为9230～14723,Th/Ta值为177～221,大于160。Sm Nd同位素测试结果表明,变玄武岩等时线年龄为(1 387±32) Ma。根据地球化学构造环境判别图,结合区域地质背景,认为刘家坪蓟县系变基性火山岩主要形成于与岛弧相关的构造环境,这一认识对研究碧口地块的构造演化具有重要意义。     

唐古拉山东段莫云地区二叠纪玄武岩地球化学特征及源区性质

在青藏高原腹地,二叠纪火山岩分布于唐古拉山东段,古地理上位于北羌塘盆地东缘。对唐古拉山东段莫云地区中二叠世栖霞期尕笛考组火山岩的主量元素、微量元素和Sr-Nd同位素进行了分析。岩相学和地球化学指标显示二叠纪火山岩为玄武岩,该套岩石低SiO2、MgO、K2O和Mg#值(0.39～0.48),富Na2O、TFeO、TiO2、P2O5含量,富集LREE和高场强元素(Nb、Ta、Zr、Hf、P、Ti),(La/Yb)N=14.89～23.23,(Gd/Yb)N=2.30～3.58,Eu异常不明显(δEu=0.76～0.99),具有明显的Th正异常和K、Sr负异常,显示与板内碱性玄武岩相似的地球化学特征;同位素组成以低(87Sr/86Sr)i(0.7033～0.7039)、高εNd(t)(+4.2,t=271Ma)为特点,反映岩浆源区既有亏损地幔源(DMM)又有富集地幔源(EMⅠ)的双重属性。岩浆起源可能与地幔柱诱导的软流圈上涌导致含石榴子石橄榄岩的岩石圈地幔部分熔融有关。莫云玄武岩形成于板内伸展环境(初始裂谷),具有主动裂谷作用性质。     

扬子地块北缘新元古代望江山层状岩体矿物成分和铂族元素特征:对岩浆演化过程和构造环境的制约

望江山层状岩体位于扬子地块北缘新元古代汉南杂岩带中,岩体从底部到顶部由超镁铁质岩过渡为中性岩:底部主要由辉石岩和橄长岩组成;中部为辉长苏长岩和辉长岩;上部为辉长岩和闪长岩。研究以中部岩相带橄榄辉长苏长岩、辉长苏长岩和辉长岩为对象,通过主要矿物的主微量元素和全岩主微量元素的分析,查明望江山岩体来源于尖晶石二辉橄榄岩组成的大陆下岩石圈地幔,并且地幔源区受到了来自俯冲板片流体的交代,岩体中部带的母岩浆为拉斑玄武质岩浆。钛铁矿—磁铁矿矿物对成分计算表明,母岩浆在形成时具有较高氧逸度。通过单斜辉石压力计得到岩体的侵位深度约为12.9~18 km。对岩体母岩浆橄榄石分离结晶过程的模拟计算表明,中部带橄榄石为母岩浆经过~28%分离结晶的产物。此外,铂族元素（PGE）组成暗示岩体并未经历过大规模的硫化物熔离,可能与缺乏地壳物质混染有关。岩体中单斜辉石与岛弧环境堆晶岩中单斜辉石成分相似,不同于裂谷环境中堆晶单斜辉石的成分;同时,全岩Th/Yb和Nb/Yb比值也与岛弧玄武岩比值相似,因此矿物和全岩成分均说明望江山层状岩体应形成于岛弧环境。研究认为扬子北缘在新元古代长期的俯冲过程中,大洋板片断离导致软流圈上涌,提供热源使交代大陆下岩石圈地幔部分熔融形成具有岛弧特征的镁铁质岩浆,在局部伸展环境中上升侵位形成汉南杂岩带中镁铁—超镁铁质层状岩体。      

Chemical and Isotopic Characteristics of the Kuroko‐Forming Volcanism

The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma.     

羌塘盆地东部那益雄组玄武岩地球化学特征及构造意义

羌塘盆地东部那益雄组玄武岩作为裂谷演化最后阶段的喷发产物,其成岩年龄和地球化学特征为裂谷的关闭时间和二叠纪构造演化提供了重要约束.在剖面地质调查基础上,对那益雄组玄武岩进行了LA-ICP-MS锆石U-Pb测年及全岩分析测试,结果显示:那益雄组玄武岩锆石U-Pb年龄为257.2±2.9 Ma,形成于晚二叠世;该玄武岩属于大陆拉斑玄武岩系列,轻微富集Ta元素而轻微亏损Nb元素,是软流圈地幔物质上涌与岩石圈地幔相互作用的产物,形成于裂谷关闭碰撞后的伸展背景.羌塘地块东部二叠纪玄武岩的地球化学数据显示,早二叠世-晚二叠世玄武岩具有由OIB型玄武岩向火山弧型玄武岩过渡的演化趋势,表明羌塘地块东部板内裂谷在早二叠世打开,中二叠世进入裂谷演化阶段,于晚二叠世关闭.      

Petrology,volume and age relations of alkaline and saturated peralkaline volcanics from Terceira,Azores

Four volcanoes form Terceira, one of the islands of the Azores group; three contain both basaltic and peralkaline and one only peralkaline rocks. A recently active basaltic fissure zone trends NW-SE across the island.The rocks fall into the alkaline olivine basalt suite although some young basalts are of transitional affinity. The geochemistry shows two general basaltic series: 1) undersaturated, found in lavas of the oldest volcano and in some recent fissure zone basalts and hawaiites; 2) saturated, found in the younger basaltic lavas.Since the emergence of Terceira there has been a contemporaneity of basalt and salic peralkaline lavas. The younger rocks show a bimodal composition distribution, the most voluminous compositions being alkali olivine basalt and comendite with negligible volume in the benmoreite-trachyte range. Two processes appear viable for the derivation of voluminous oversaturated peralkaline rocks: 1) partial melting of upper mantle material giving small magma batches of contrasting composition or 2) fractionation from a transitional basaltic parental magma.Now at Department of Geology, Victoria University of Wellington, New Zealand.     

http://dx.doi.org/10.1016/j.gsf.2015.11.004

The alkali-basalt and basaltic trachy-andesites volcanic rocks of south Marzanabad were erupted during Cretaceous in central Alborz, which is regarded as the northern part of the Alpine-Himalayan orogenic belt. Based on petrography and geochemistry, en route fractional crystallization of ascending magma was an important process in the evolution of the volcanic rocks. Geochemical characteristics imply that the south Marzanabad alkaline basaltic magma was originated from the asthenospheric mantle source,whereas the high ratios of(La/Yb)Nand(Dy/Yb)Nare related to the low degree of partial melting from the garnet bearing mantle source. Enrichment pattern of Nb and depletion of Rb, K and Y, are similar to the OIB pattern and intraplate alkaline magmatic rocks. The K/Nb and Zr/Nb ratios of volcanic rocks range from 62 to 588 and from 4.27 to 9 respectively, that are some higher in more evolved samples which may reflect minor crustal contamination. The isotopic ratios of Sr and Nd respectively vary from 0.70370 to0.704387 and from 0.51266 to 0.51281 that suggest the depleted mantle as a magma source. The development of south Marzanabad volcanic rocks could be related to the presence of extensional phase,upwelling and decompressional melting of asthenospheric mantle in the rift basin which made the alkaline magmatism in Cretaceous, in northern central Alborz of Iran.     

Trace element geochemistry of high alumina basalt - Andesite - Dacite - Rhyodacite lavas of the Main Volcanic Series of Santorini Volcano,Greece

Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.     

Complex geometry of the Cenozoic magma plumbing system in the central Sahara,NW Africa

Topographic uplifts in the central Sahara occur in the Hoggar-Aïr and Tibesti-Gharyan swells that consist of Precambrian rocks overlain by Cenozoic volcanic rocks. The swells and associated Cenozoic volcanism have been related either to mantle plumes or to asthenospheric upwelling and to partial melting due to rift-related delamination along pre-existing Pan-African mega-shears during the collision between Africa and Europe. The Cenozoic volcanic rocks in the Hoggar generally range from Oligocene tholeiitic/transitional plateau basalts, which occur in the centre of the dome, to Neogene alkali basalts characterized by a decrease in their degree of silica undersaturation and an increase in their La/Yb ratios. The alkali basaltic rocks occur mainly along the margins of the dome and typically have less radiogenic Nd and Sr isotopic ratios than the tholeiitic/transitional basalts. The geochemistry of the most primitive basaltic rocks resembles oceanic island basalt (OIB) tholeiitic – in particular high-U/Pb mantle (HIMU)-type – and is also similar to those of the Circum-Mediterranean Anorogenic Cenozoic Igneous (CiMACI) province. These characteristics are consistent with, but do not require, a mantle plume origin. Geophysical data suggest a combination of the two mechanisms resulting in a complex plumbing system consisting of (a) at depths of 250–200 km, an upper mantle plume (presently under the Aïr massif); (b) between 200 and 150 km, approximately 700 km northeastward deflection of plume-derived magma by drag at the base of the African Plate and by mantle convection; (c) at approximately 150 km, the magma continues upwards to the surface in the Tibesti swell; (d) at approximately 100 km depth, part of the magma is diverted into a low S-wave velocity corridor under the Sahara Basin; and (e) at approximately 80 km depth, the corridor is tapped by Cenozoic volcanism in the Hoggar and Aïr massifs that flowed southwards along reactivated Precambrian faults.