南极乔治王岛第三纪火山岩中斜长石成分特征及成因意义

乔治王岛第三纪火山岩含大量斜长石斑晶。斜长石的主元素、稀土和微量元素分析表明,它们主要为中长石－钙长石。斜长石种类与寄主岩之间有较好对应关系,即玄武安山岩和安山岩中以中长石和拉长石为主,而高铝玄武岩中为倍长石和钙长石;它们的ΣＲＥＥ、Ｌａ／Ｙｂ和δＥｕ等之间也有明显对应关系。但不同种类的斜长石之过渡金属元素特征相近,反映了过渡金属元素在斜长石中的含量和分异趋势主要受制于斜长石的晶体结构。综合研究表,斜长石成分取决于其母岩浆成分,并反映了区域岩浆演化的趋势．     

Collision-related slab break-off volcanism in the Eastern Anatolia,Kepez volcanic complex (TURKEY)

The Neogene–Quaternary volcanic products, related to Arabian and Anatolian Plate collision along the Bitlis Suture Zone, cover wide areas on both plates. One of these volcanic exposures on the Arabian Plate is the Kepez volcanic complex (KVC). This study aims explain to petrogenesis of KVC. Although some examples display alkaline affinities, the majority of the volcanic rock is calc-alkaline and can be defined in three main groups. ⁴⁰Ar/³⁹Ar data obtained from dacite, basalt and andesite rock groups within the KVC yield ages of between 13.5 and 15.5 Ma. Geochemical and petrographical data show that the andesitic rocks are products of homogeneous mixing between basic end-member magmas and dacitic magmas which are the products of partial melting of lower crustal compositions. Basaltic products of KVC are asthenospheric mantle derived, while dacitic and andesitic volcanic rocks are crustal origin. High Sr and Nd isotope ratios may indicate that andesitic and dacitic rocks originated from continental crust. The lithospheric mantle, which is subducting underneath the Anatolian plate, must have experienced slab break-off processes 13–15 million years ago and sunk into the asthenosphere. KVC were produced with the collision between Arabian and Anatolian Plates and related uplift of the East Anatolia region.     

俯冲带流体作用的地球化学示踪

俯冲带流体包括俯冲带岩石脱水形成的水流体和熔融作用产生的熔体。俯冲带流体循环及伴随的化学作用是壳幔演化和物质交换的重要机制。岛弧火山岩地球化学研究可以示踪流体的性质、成分。造山带变质岩内高压脉是俯冲带流体活动的直接记录。通过高压脉及其主岩地球化学研究 ,可以示踪流体成分和来源。俯冲带水流体成分与流体释放深度有关 ,俯冲带深处 ( >5 0km)形成的水流体内溶质相当可观。水流体流动方式有大规模沟道式和小规模弥散式 ,与其相应的流体来源分别为外来的和内部的。俯冲带流体组分、流动方式、物质交换机制与俯冲带深度、热状态、物质结构的定量关系 ,以及矿物 /水流体间元素分配系数的高压实验可能将是俯冲带流体今后研究的重要课题。中国西天山高压变质带榴辉岩及蓝片岩中广泛发育高压脉 ,根据脉的矿物组合 ,高压脉可分成 4种类型。高压脉氧同位素研究表明 ,脉体与流体间氧同位素交换宏观与局部平衡 ,流体活动为大规模的 ;流体活动方式既有沟道式 ,又有渗透式 ;流体的δ18O值为 + 10 .8‰左右 ,流体来源于主岩经过海水低温蚀变的大洋玄武岩。西天山广泛发育的高压脉 (尤其是蓝片岩中的高压脉的发现目前在世界范围尚属首例 ) ,是研究古俯冲带流体作用良好的野外天然实验室。     

辽河盆地沙三期火山-侵入岩地球化学与岩石成因

辽河盆地沙三期火山-侵入岩为一套偏碱性的双峰式岩系,其基性端元为碱性玄武岩,中偏碱性端元为粗面质熔岩和侵入岩。碱性玄武岩富集高场强元素(如Nb、Th、Zr、Hf、V等)和轻稀土、Sr、Ba等大离子亲石元素,而亏损Rb和K,具有与板内碱性玄武岩和洋岛玄武岩类似的特征。粗面质岩石显示与基性端元相似的地球化学特征,其不相容元素含量总体上高于碱性玄武岩,但具强烈的Sr和Eu亏损。矿物学、岩石学及地球化学证据表明,玄武质岩石是软流圈地幔低程度部分熔融的产物,并经历了橄榄石和辉石的分离结晶作用,其源区可能有金云母和石榴石残留。玄武质岩浆上升到较浅部位后进一步发生橄榄石、辉石、斜长石和磁铁矿等的分离结晶作用而形成粗面质岩浆,地壳物质混染作用不显著。     

Petrology and geochemistry of Tertiary volcanic rocks from the İkizce (Ordu) area, NE Turkey: Implications for the evolution of the eastern Pontide paleo-magmatic arc

Tertiary volcanism in the İkizce region at the western edge of the eastern Pontides paleo-magmatic arc is represented by basaltic and andesitic rocks associated with sediments deposited in a shallow basin environment. The basaltic rocks contain plagioclase (An_58–80), olivine (Fo_82–84), clinopyroxene (Wo_44–48En_35–42Fs_7–17), hornblende (Mg# = 0.68–0.76) phenocrysts, and magnetite microcrysts, whereas the andesitic rocks include plagioclase (An_25–61), clinopyroxene (Wo_46–49En_38–43Fs_11–13), hornblende (Mg# = 0.48–0.81), biotite (Mg# = 0.48–0.60) phenocrysts, titanomagnetite, apatite, and zircon microcrysts.Geochemical data indicate magmatic evolution from tholeiitic-alkaline transitional to calc-alkaline characteristics with medium-K contents. The geochemical variation in the rocks can be explained by fractionation of common mineral phases such as clinopyroxene, olivine, hornblende, plagioclase, magnetite, and apatite. The trace elements’ distributions of the volcanic rocks show similarities to those of E-Type MORB, have a shape that is typical of rocks from subduction-related tectonic setting with enrichment in LILE and to a lesser extent in LREE, but depletion in HFSE. The rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks, the latter formed through clinopyroxene ± olivine controlled fractionation in a high level magma chamber. The andesitic rocks developed through hornblende ± plagioclase controlled fractionation in shallow level magma chamber(s).     

Cones and craters on Mount Pavagadh,Deccan Traps: Rootless cones?

Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Myvatn in northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism.     

早侏罗世叶巴组与桑日群火山岩:特提斯洋俯冲过程中的陆缘弧与洋内弧?

雅鲁藏布特提斯洋的演化对研究青藏高原的形成具有重要的意义,一般认为广泛分布于拉萨地块南部叶巴组和桑日群火山-沉积岩系是该特提斯洋早期的俯冲岩浆产物。本研究选取了拉萨附近达孜地区叶巴组中的2个中基性火成岩样品进行了锆石LA-ICPMS U-Pb分析,结果显示其年龄分别为188±2Ma和175±2Ma,与已发表的叶巴组中酸性火成岩的形成时代(174~193Ma)一致,已发表的桑日群火山岩的年龄也在相同范围内,因此叶巴组和桑日群火山岩喷发时间主要为早侏罗世。叶巴组和桑日群基性及中酸性岩浆均类似岛弧型火山岩,但前者具有相对高的Nb、Zr含量,Th/Y比值及相对较低的La/Nb比值,呈现出大陆地壳组分增加的趋势,叶巴组火山岩表现为典型的大陆边缘弧特征而桑日群类似于洋内弧火山岩,桑日群火山岩分布于叶巴组南侧,并呈碎片似展布于拉萨地块南部,同时显示了与叶巴组不同的岩性组合,暗示同时代的叶巴组和桑日群火山-沉积岩可能分别代表特提斯洋俯冲过程中的形成的陆缘弧和洋内弧。     

Ichthyofauna (Chondrichthyes,Osteichthyes) from the Upper Cretaceous intertrappean beds of Piplanarayanwar,Chhindwara District,Madhya Pradesh,India

A new fossiliferous intertrappean section is found 92 km southwest of the extensively studied intertrappean beds of Mohgaonkalan and Jhilmili in Chhindwara District, Central India. Application of the bulk screen‐washing method led to the recovery of a rich microvertebrate fauna represented by fish, amphibians, crocodiles, snakes and dinosaurs. In this paper, the ichthyofauna recovered from the new intertrappean section consisting of Igdabatis indicus Prasad and Cappetta, 1993, Lepisosteus indicus Woodward, 1908, Osteoglossidae gen. et sp. indet., Pycnodontidae gen. et sp. indet. and Siluriformes indet. is described. The ichthyofauna dominantly represented by the teeth of Igdabatis indicus is suggestive of a nearshore, deltaic or estuarine palaeoenvironment and Late Cretaceous (Maastrichtian) age for the intertrappean beds. The remarkable similarity of the new intertrappean fauna to that of Asifabad and the infratrappean beds of Marepalli also is suggestive of coastal‐plain conditions all along the Godavari lineament in the Late Cretaceous. Although the presence of planktonic foraminifera in the intertrappean beds of Jhilmili in the Mandla Lobe of Deccan volcanic province has been inferred in terms of a short term marine incursion from the west coast along the Narmada lineament in the Early Palaeocene, currently there is no definitive evidence for the prevalence of marine or coastal‐plain conditions along the Narmada lineament at least in the latest Cretaceous.     

皖南新元古代井潭组火山岩锆石U-Pb定年和同位素地球化学研究

对皖南江南造山带新元古代井潭组火山岩样品进行了锆石微区U-Pb定年、全岩主微量元素分析和Sr-Nd同位素分析以及矿物氧同位素分析。结果表明,井潭组火山岩存在二期火山岩,形成时代分别为820±16 Ma和776±10 Ma。所有样品都具有较高的基性组分(ΣTiO2+ Fe2O3 T+MgO) 含量(4.7%~16.36%),LREE富集,中等Eu负异常,大离子亲石元素K,Rb,Ba,Th,U等富集,Nb,Ta,Sr,Ti等相对亏损。εNd (t )值为-2.79~-1.71,指示岩浆源区含有显著的新生地壳组分。这些岩石具有高A/CNK(1.37~1.61)和高δ18O值(锆石6.52‰~8.98‰),显示为壳源S型岩浆岩的特点。石英、斜长石和钾长石等矿物氧同位素具有很大的变化范围,表明井潭组火山岩经历了不同程度的亚固相中温热液蚀变。根据锆石U-Pb定年和元素以及同位素分析结果,776±10 Ma井潭组火山岩与石耳山花岗岩为同期岩浆岩,井潭组火山岩直接来源于早新元古代新生地壳重熔,而石耳山花岗岩源岩为约825 Ma岩浆岩,间接来源于早新元古代新生地壳的再造。这些新生地壳是江南弧–陆碰撞造山带的组成部分,在Rodinia超大陆裂解时发生拉张跨塌熔融,既不是地幔柱岩浆活动产物,也不是岛弧岩浆作用的直接产物,而是板块–裂谷岩浆活动产物。     

Hydrocarbon prospects across Narmada-Tapti rift in Deccan trap, central India: Inferences from integrated interpretation of magnetotelluric and geochemical prospecting studies

As the Mesozoic sediments contribute most of the oil and gas reserves of the world, we present an integrated interpretation approach using magnetotellurics (MT) and surface geochemical prospecting studies to demarcate hydrocarbon prospective Gondwana (Mesozoic) formations underneath the Deccan flood basalts of Late Cretaceous age across Narmada-Tapti rift (between Bhusawal and Barwah) in Central India. The MT interpretation shows deep (∼5 km) basement structure between southern and central part of the MT profile however, it gradually becomes shallower to either ends of the profile with a predominant basement depth reduction in the northern end compared to the southern end. The geophysical results suggest thick (2-3.5 km) Mesozoic sediments in the area characterized by deep basement structure. The geochemical analysis of the near surface soil samples indicate higher concentrations of light gaseous hydrocarbons constituents over the area marked with thick sub-basalt Mesozoic formations. Analyses of the geochemical data imply that these hydrocarbons are genetically related, generated from a thermogenic source and these samples fall in the oil-producing zone. The temperature-depth estimations in the region supports favorable temperature conditions (80-120 °C) for oil generation at basement depths.