Spinel lherzolite xenoliths from the Premier kimberlite (Kaapvaal craton, South Africa): Nature and evolution of the shallow upper mantle beneath the Bushveld complex

Coarse-grained, granular spinel lherzolites xenoliths from the Premier kimberlite show evidence of melt extraction and metasomatic enrichment, documenting a complex history for the shallow mantle beneath the Bushveld complex. Compositions of orthopyroxene, clinopyroxene and spinel indicate equilibration within the spinel–peridotite facies of the upper mantle, at depths from 80 to 100 km and temperatures from 720 to 850 °C. Bulk compositions have lower Mg-number [atomic 100 Mg/(Mg + Fe*)] than previously studied spinel peridotites from Premier, and have higher Mg/Si than low-temperature coarse grained garnet lherzolites, suggesting shallower melting conditions or metasomatic enrichment. Clinopyroxene in one sample is highly LREE-depleted indicating very minor modification of a residue of 20% melt extraction, whereas the calculated REE pattern for a melt in equilibrium with a mildly LREE-depleted sample is similar to MORB or late Archean basalt, possibly related to the Bushveld Complex. Bulk and mineral compositions suggest minimal refertilization by silicate melts in four out of six samples, but REE patterns indicate introduction of a LIL-enriched component that may be related to kimberlite.     

Morphotectonic zones along the coast of the Pacific continental margin, southern Mexico

Geomorphic analysis, employing topographic, morphologic, geologic, and bathymetric maps, and field studies show that the morphology of the southern coast of Mexico can be linked to lateral variations in the geometry and tectonism of the subduction zone. A reconnaissance study, based on the regional morphological characteristics and correlation with seismotectonic segments, regional tectonics and major bathymetric features, allows identification of several morphotectonic zones along the coast of southern Mexico: (1) Jalisco zone, (2) Colima zone, (3) Michoacan zone, (4) Guerrero zone, (5) Oaxaca zone, and (6) Tehuantepec zone. A range of geomorphological evidence, including marine terraces, river terraces, uplifted notches, and elevated wave-cut platforms, indicates local and regional uplift along the coast of the Jalisco, Michoacan, Guerrero and Oaxaca zones. Coasts of the Colima and Tehuantepec zones show morphological evidence of subsidence.     

Black Sea chemocline oscillations during the Holocene: molecular and isotopic studies of marginal sediments

We measured δ¹³C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked difference in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites.     

扬子地台西缘北段震旦系露头层序地层学研究

运用露头层序地层学的方法，对四川汶川县七盘沟、广元市羊木东溪河震旦系剖面进行了详细的研究；将该区上震旦统地层划分为1个Ⅱ级层序，4个Ⅲ级层序和3个等时界面；建立了该区上震旦统等时地层格架，编制了相对海平面变化曲线。     

龙门山均衡重力异常及其对青藏高原东缘山脉地壳隆升的约束

青藏高原东缘处于不均衡状态,自西而东可分为青藏高原弱负均衡重力异常区、龙门山正均衡重力异常区和四川盆地负均衡重力异常区,表明该区的不均衡状态并未导致Airy均衡运动的产生,即龙门山没有均衡下降,而处于不断的隆升状态,显示该地区反均衡运动的构造抬升是导致龙门山隆升的主因。本次采用似三度体重力异常计算方法对该区的正均衡重力异常进行模拟和反],研究了大尺度地貌分异与均衡重力异常分区之间的相互关系,结果表明,龙门山的下地壳顶面抬升了11.2～12.6km,造成了龙门山的正均衡异常,揭示了构造抬升和剥蚀作用在相似的时间尺度上和空间尺度上控制着龙门山地貌的形成,龙门山的表面隆升是构造隆升和剥蚀作用相叠加的产物。     

湖缘峡谷及其含油性

在陆相断陷湖盆陡坡带,控盆断裂的剧烈活动导致在相对较短时期内形成盆地的快速沉降,与湖盆边缘地形形成巨大高差。沿此边缘,经过早期阵发性洪水及河流的强烈下蚀,并不断向源侵蚀,在古断面上形成宽达数km,深达上km的深切谷,称之为湖缘峡谷。东营凹陷北部陡坡带的Y921峡谷即为湖缘峡谷,其现今形态为长7000m、宽3500m、深3500m、剖面呈“U”型的特征。古构造及古生物分析认为,在古新世至渐新世时期,陈南断层剧烈活动,使断层两侧形成较大的高差,在干旱、阵发性洪水侵蚀下逐渐形成Y921峡谷,并在其底部形成侵蚀面。孢粉分析认为,仅沙四上段沉积时期就有近1000m的高差。后期,随湖平面上升,峡谷逐渐充填了古近系沙河街组四段、三段和部分二段地层,主要为辫状河粗碎屑砂砾岩、扇三角洲相交错层理砂岩和滨浅湖相泥岩、粉砂岩。这种峡谷的特点是短、宽、深,其充填模式主要为辫状河-扇三角洲-湖泊相沉积体系。由于峡谷前缘为深湖相泥岩作为生油岩,陈南断层和峡谷内的冲刷面作为油气运移通道,峡谷内砂砾岩作为储层,因此在峡谷内形成了多种非构造油气藏类型。     

华北麻粒岩相岩石的主要特征及今后研究中值得注意的几个问题

华北前寒武纪麻粒岩与国外同类组合相比有特殊性,如(1)具面状分布的特点;(2)麻粒岩类型缺少斜长岩-淡色辉长岩组合以及下部堆晶相辉长岩组合,灰色片麻岩比例少,并普遍受到钾质交代;(3)多为不亏损型麻粒岩,未发现熔融残留型麻粒岩;(4)变质压力为低—中压过渡型,堇青石在变质沉积岩中普遍存在;(5)华北克拉通经历了多期麻粒岩相变质作用。这些特征对于深入研究华北麻粒岩的成因、下部地壳的性质以及克拉通的形成和演化都是必须加以考虑的。     

黑龙江新城沟地区花岗岩成矿地球化学特征分析及资源潜力预测

东宁新城沟地区的花岗岩体位于兴蒙造山带东缘的活动大陆边缘带.通过岩相学和岩石地球化学分析认为,研究区内岩体的里特曼指数δ介于1.41~2.11之间,为钙碱性岩系;w(K2O)/w(Na2O)比值大于1,为高碱相对富钾;铝饱和指数ACNK=1.02~1.20,主要为弱过铝质I型花岗岩,部分具有S型同碰撞花岗岩的特征.岩体中稀土总量较低(∑REE=72.35×10-6~217.64×10-6),轻、重稀土分异较强(LaN/YbN=2.74~11.37),Eu具有较为明显的负异常,δCe为0.96~1.11,Ce异常不明显;Nb、Ta、P、Ti等相对亏损,Rb、Ba、K等相对富集,w(Nb)/w(Ta)比值为(6.18~26.33,平均为15.18)小于16.2,指示岩浆来源于上地幔或下地壳,形成过程中同化混染了较多壳源物质.区内岩浆的形成受古生代末和中生代初西伯利亚板块与华北板块发生陆陆碰撞以及晚三叠世古太平洋板块向欧亚大陆东缘俯冲的双重作用影响,岩浆侵入存在由早期向晚期阶段演化的过程,致使源岩存在一定差异,具有多期次、多阶段、复合成因的特点.物化探异常、岩石地球化学及含矿性参数显示,研究区具有寻找浅成低温热液型Au、Ag、Cu矿的资源潜力.