Boninite primary magmas: Evidence from the Cape Vogel Peninsula,PNG

Boninites from Cape Vogel, PNG, are dominantly pyroxene-glass rocks, but many contain olivine, sometimes as refractory as Fo₉₄. We derive a parental magma for this suite (in equilibrium with Fo₉₄) which contains 20 wt.% MgO and is quartz-normative. This liquid is hydrous, and from petrographie evidence and whole rock H₂O⁺ values, we estimate it to contain 2–3 wt.% H₂O. These data suggest olivine fractionation and primary magmatic water are important in boninite genesis, but both are often obscured by later alteration. The derived parental magma has probably formed at 1,250–1,300° C and low pressures (< ?10kB) and is similar to those which gave rise to olivine-clinoenstatite phyric boninites from New Caledonia and from Howqua, Australia, and possibly to a proposed parental magma for the Bushveld Complex.     

High potassium calc-alkaline rocks from Cape Nelson,eastern Papua

High-K calc-alkaline rocks from Cape Nelson, eastern Papua are dominated by andesites containing numerous basic inclusions. High-Al basalts and dacites are subordinate. The slight iron enrichment and a systematic variation in K₂O/SiO₂ correlation observed in these rocks suggests a relationship to nearby rocks of a shoshonite association. The chemical composition of the inclusions illustrates the trends of major and minor element evolution which contradict the idea of complimentariness of high-K calc-alkaline rocks and alpine ultramafic rocks. The chemical character of the lavas (high K, Rb and Ba as well as high Cr and Ni) can be explained by fractional melting involving a mica phase.     

Volcanic evolution in eastern Papua

Late Mesozoic and Cenozoic volcanic rocks in eastern Papua record a complex series of volcanotectonic events which reflect interaction between the Indo-Australian and Solomon sea plates. Basement formations of Upper Cretaceous and Eocene submarine basalt are comparable to those characteristic of sea floor spreading centers and are thought to have originated during volcanic activity associated with spreading in the Coral Sea basin. Arc-trench type andesitic volcanism was prominent during the late Cenozoic but shows no clear relationship to a subduction event. An alternative explanation links the development of thickened crust and consequent crust/mantle interaction with the generation of andesitic magmas. The tectonic environment of eastern Papua during the late Cenozoic was one of block faulting and uplift associated with crustal tension. The presence of Quaternary peralkaline rhyolites suggests that this environment is now being replaced by active rifting.     

青藏高原那日尼亚地区新近纪查保玛组火山岩岩石地球化学特征

那日尼亚地区铅锌矿主要位于新近纪火山岩中,成矿的层控型明显,查保玛组火山岩是最主要的含矿层位之一。对地球化学特征研究表明,该组火山岩为一套钙碱性-碱性系列的中酸性岩石组合,以富碱高钾含量为特征。稀土配分模式高度分异,轻重稀土元素分馏明显。δEu为0.73～0.97,无明显负Eu异常。微量元素的配分模式近似一致,相对于原始地幔,富集大离子亲石元素(K、Ba、Rb、Sr)和Th、U等活泼的不相容元素,而Zr、Ti、Ta、P等高场强元素则相对亏损。Rb/Sr为0.07～0.27,Rb/Sr值为0.07～0.27,显示壳幔混合特点。岩石可能源于EMⅡ型富集地幔含金云母的地幔橄榄岩的部分熔融,经过分离结晶的岩浆房富钾岩浆沿构造薄弱部位上侵,与地壳物质发生混合形成混合岩浆,在地壳浅部就位。微量元素中具有高的铅锌背景值,为成矿提供主要物质来源。在R1-R2构造判别图解上,样品主要落入碰撞后伸展和后碰撞环境。结合区域构造背景分析,该套火山岩形成于陆内伸展构造背景。     

甘肃西秦岭区新生代碱性火山岩中的钛金云母

本文对甘肃西秦岭区新生代火山岩及所携带的上地幔包体中钛金云母进行矿物学和矿物化学研究。本区火山岩中的金云母与金伯利岩、钾镁煌斑岩和一般云煌岩中的金云母不同，其化学成分显示了一种过渡类型特征。巨晶和粗晶金云母是在＞１３００℃，３０×１０￣８ｐａ，富ＣＯ＿２和Ｈ＿２Ｏ的条件下的早期结晶相。微晶金云母为近液相线快速冷却的产物。橄榄岩包体和球状体中金云母的成因似与流体交代作用有关，这种钛金云母的出现进一步证明新生代火山岩起源于被交代了的地幔，交代流体具有富Ｋ、Ｎａ、Ｔｉ等不相容元素和挥发分ＣＯ＿２、Ｈ＿２的特征。     

内蒙古赤峰北部百岔河地区新第三纪火山地质及基性火山岩岩石地球化学特征

赤峰北部新第三纪火山活动的产物为玄武岩－粗面玄武岩组合。经 研究对比，认为该时期基性火山岩的形成可能与慢隆伸展作用有关，为板 内裂谷、地幔柱活动的产物。     

滇东南建水地区高镁火山岩包体的成因和构造背景

在滇东南建水地区发现产于峨眉山玄武岩中的高镁火山岩包体,这对于地幔柱的形成演化具有重要研究意义.对这些包体进行了锆石U-Pb年代学、地球化学和岩矿分析.高镁火山岩包体具斑状结构,致密块状构造,斑晶主要为贵橄榄石和透辉石.13颗锆石U-Pb LA-ICP-MS加权平均年龄为259±2Ma(MSWD=1.9),显示与寄主岩石同期形成.包体岩石具有高镁(Mg~#=68～75)、低硅(SiO_2=45.11%～45.93%)特征,轻稀土元素(LREE)、高场强元素(HFSE)富集而重稀土元素(HREE)亏损,属于亚碱性、拉斑玄武岩系列,具有板内玄武岩(IPB)特征.火山岩包体的原始岩浆起源于石榴子石二辉橄榄岩低程度部分熔融的产物,岩浆演化过程中发生了橄榄石和单斜辉石的分离结晶作用,在侵位上升过程中未受明显的地壳混染作用.该高镁火山岩的存在,显示地幔柱除了垂直上升运动外,在地球深部不同的边界还有多次侧向扩展移动,表明滇东南晚二叠世存在峨眉山地幔柱的一个分支-地幔枝活动.     

Clinoenstatite in volcanic rocks from the Bonin Islands

Multiply-twinned clinoenstatite has been found in andesitic rocks from Chichi-jima and Mukojima in the Bonin Islands.The clinoenstatite occurs as (1) reaction rims around olivine, (2) composite crystals with bronzite, and (3) anhedral phenocrysts rimmed or included by bronzite.The clinoenstatite is chemically characterized by low contents of Ca (less than 0.5 wt.% oxide), Fs (9.2–11.5) and Al (less than 0.6 wt.% oxide) relative to the coexisting bronzite. The partition coefficient, K _{D op} ^cp =(Mg/Fe²⁺)^cp/(Mg/Fe²⁺)^op between coexisting clinoenstatite and bronzite, ranges from 1.02 to 1.32 (average 1.17). The Fe²⁺/Mg+Fe²⁺ ratios of coexisting clinoenstatite and bronzite are different in different rock types, which suggests variable inversion temperatures of protoenstatite to orthopyroxene in boninite.Bronzite phenocrysts in a specimen, ranging from Fs15 to Fs30, coexist with augite phenocrysts, whereas those in the other specimens, ranging from Fs12 to Fs18, do not coexist with augite phenocrysts. These differences in petrographical nature and mineral chemistry among the specimens examined may be due to variations in their quenching stages.The boninite clinoenstatite and bronzite are relatively rich in Ca and Mg, compared with the Papuan and Mariana pyroxenes, which seems to depend upon the rock chemistry.     

青藏高原冈底斯带中部朱诺地区林子宗群火山岩锆石U-Pb年龄和地球化学特征

林子宗火山岩是发育在青藏高原南部冈底斯带的古近纪火山岩, 被认为代表了新特提斯洋俯冲消减结束过渡到印度—亚洲大陆碰撞过程的产物, 其确切的时代对于限制印度—亚洲大陆的碰撞时限具有重要意义.对冈底斯带中部朱诺地区的林子宗火山岩进行了系统的LA-ICP-MS锆石U-Pb年代学、元素和Sr-Nd同位素地球化学研究, 获得的林子宗火山岩的典中组、年波组和帕那组LA-ICP-MS锆石U-Pb年龄分别为64.8±1.6 Ma、59.7±1.8 Ma和48.9±0.8 Ma.元素和同位素地球化学结果表明, 朱诺地区各组特征与区域上特征相似, 其中典中组和年波组火山岩属于钙碱性和高钾钙碱性系列, 具有岛弧火山岩特点; 帕那组出现大量的钾玄岩, 属于同碰撞火山岩.证明前人提出的雅鲁藏布江地区洋盆闭合和印度—亚洲大陆碰撞开始时间为50 Ma左右的观点.      

浦城地区火山岩型铀成矿远景预测

本文根据浦城地区的成矿地质条件,利用信息量法、逻辑矢量长度法和逻辑矢量长度加权法,在已有成矿地质模型的基础上,筛选出有利的地质变量。根据地质变量对成矿的贡献大小,建立评价模型,圈出有利成矿区。经地质评价,认为各有利成矿区成矿条件有利,有较大的找矿前景。     

新疆雪米斯坦火山岩型铀矿成矿条件及远景预测

笔者在收集、整理前人资料的基础上,对新疆准噶尔西北部雪米斯坦地区进行铀成矿条件分析,认为区内铀成矿与大地构造环境、地层及火山作用、控矿构造、铀源等密切相关。结合已知铀矿床的产出地质特征和近年来的野外地质工作成果,总结了该地区火山岩型铀矿的分布规律,预测并圈定了一级铀矿远景区6片,二、三级铀矿远景区11片。     

Boninite petrogenesis and alteration history: constraints from stable isotope compositions of boninites from Cape Vogel,New Caledonia and Cyprus

¹⁸O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more ¹⁸O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a ¹⁸O value of up to 7.0 to which must be added a small amount of H₂O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low _Nd values typical of boninites do not correlate with the ¹⁸O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces ¹⁸O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater.     

东天山伊吾地区早石炭世火山岩地球化学特征及其构造意义

通过对东天山伊吾马依当地区早石炭世火山岩分布、成因与构造环境关系的调查研究,查明了该时期火山岩相从次火山相-爆发相-喷溢相-火山沉积相的分布特点;火山岩组合主要为一套岛弧相的中基性-中酸性火山碎屑岩、火山熔岩及火山沉积岩.结合该区火山岩地球化学特征及沉积岩特征综合研究认为:研究区早石炭世早期处于大陆裂谷向大洋裂谷转变的...     

Geochemical Characteristics of Gases from the Wudalianchi Volcanic Area,Northeastern China

The origins of gases in springs, pools and wells from the Wudalianchi (WDLC) volcanic area are discussed based upon molecular and isotope compositions of the gases. Nine gas and water samples were collected from bubbles and water of the springs and pools in the WDLC volcanic area, Northeastern China, in August 1997. The molecular components were measured with a MAT-271 mass spectrometer (MS), helium isotope ratios with a VG-5400 MS, and (13C with a MAT-251 MS in the Lanzhou Institute of Geology. The gases are enriched in CO2 , and most of the CO2 concentrations are over 80% (V). The helium and methane concentrations have relatively wide ranges of 0.7 to 380×10-6 and 4 to 180×10-6, respectively. The 3He/4He ratios are between 1.05 Ra and 3.1 Ra (Ra = 1.4×10-6); the 4He/20Ne values are between 0.45 and 1011, larger than the atmospheric value (0.32). The (13C (PDB) values of carbon dioxide range from (9.6 to (4.2‰. These geochemical data demonstrate that the spring water is from aquifers at different depths, and that helium and carbon dioxide are derived from the mantle, and are contaminated by crust gases during deep fluid migration. Also, there are larger fluxes of deep-earth matter and energy in the WDLC volcanic area.     

Volcanic and structural evolution of Pico do Fogo,Cape Verde

In recent months the media have drawn attention to the Cape Verde archipelago, with particular focus on the island of Fogo, the only island presently active and with an eruption that began on 23 November 2014, finally ceasing on 7 February 2015. The Monte Amarelo conical shield forms most of the 476 km² almost circular island of Fogo. After attaining a critical elevation of about 3500 m, the Monte Amarelo shield volcano was decapitated by a giant landslide that formed a caldera‐like depression (Cha das Caldeiras), which was subsequently partially filled by basaltic nested volcanism. This younger eruptive activity culminated in the construction of the 2829 m‐high Pico do Fogo stratocone, apparently entirely made of layers of basaltic lapilli. Continued growth of the Pico do Fogo summit eruptions was interrupted in 1750, most likely after the stratocone reached a critical height. Since then, at least eight eruptions have taken place inside the landslide depression at the periphery of the Pico do Fogo cone, including the 2014–2015 eruptive event. Strong geological similarities with the Canary Islands, 1400 km to the north, have been frequently noted, probably as a consequence of a common process of origin and evolution associated with a mantle hot‐spot. These similarities are particularly evident when comparing Fogo with the Teide Volcanic Complex on Tenerife, where a lateral collapse of the Las Cañadas stratovolcano also formed a large depression (the Caldera de Las Cañadas), now partially filled with the 3718 m‐high Teide stratocone. However, important geological differences also exist and probably relate to the contrasting evolutionary stages of both islands. The Las Cañadas volcano on Tenerife formed at a late post‐erosional stage, with predominantly evolved (trachyte and phonolite) magmas, while at Fogo basaltic volcanism is still dominant.     

北天山东段石英滩地区早二叠世火山岩岩石组合与岩浆生成动力学机制

北天山东段早石炭世雅满苏岛弧带位于中天山北缘的阿其克库都克断裂和康古尔断裂之间。哈尔加乌组为二叠纪的火山岩地层。在区域上,哈尔加乌组火山岩与下伏地层上石炭统吐古吐布拉克组呈角度不整合接触。上覆地层为中-上二叠统阿其克布拉克组,二者呈微角度不整合接触。哈尔加乌组火山岩属于典型双峰式火山岩组合,流纹岩、英安岩占主体地位,而玄武安山岩、玄武岩较少。该组火山熔岩锆石U-Pb SHRIMP年龄为（280.6±1.8）Ma,鉴于区域内在该时段内属于后碰撞伸展环境。因此,认为该组火山岩形成于后碰撞伸展环境。岩石地球化学特征表明,该组玄武岩属碱性玄武岩系列,其他样品均属钙碱性火山岩系列。玄武安山岩具有轻稀土含量较高,重稀土含量较低、低Ti高Al的典型岛弧岩浆地化特征,这表明该组火山岩主体部分的形成是先期岛弧环境的物质在后碰撞伸展环境下再熔融所致。玄武安山岩及玄武岩的4个样品全部落在尖晶石稳定域,为软流圈物质上涌到达岩石圈的深度部分熔融的产物,玄武安山岩源自于被消减板片交代改造的岩石圈地幔;英安岩、流纹岩是不同深度的地壳物质重熔的产物。     

青海省同仁地区早白垩世大陆裂谷环境火山岩的确定及意义

西秦岭西端在同仁地区沿近南北向断裂分布一套典型的灰紫色陆相火山岩,岩性组合为灰紫色块层状-杏仁状橄榄玄武岩、安山玄武岩、火山角砾岩夹灰紫色厚层状复成分砾岩、灰紫色—青灰色中厚层状含砾粗砂岩和灰紫色薄层状泥岩,火山岩以碱性系列为主,其形成时代为早白垩世。其地球化学显示轻稀土富集,Eu具轻度正异常,不相容元素富集,岩石具有较高的87Sr/86Sr、143Nd/144Nd、εNd和206Pb/204Pb特点。岩石的εNd均值为7.69,εSr均值为-10.7,其物源与典型的EM或HIMU型洋岛玄武岩明显不同,物源受部分大陆壳的混染,岩浆源成分属于EM与HIMU幔源间混合成因。综合分析其为大陆板块内部拉张环境下的岩浆活动产物,是形成于伸展构造背景下类似于大陆裂谷环境的火山岩。     

庐山地区新元古代海相火山活动及岩石特征

庐山南麓发育一套变细碧－石英角斑岩系火山岩,岩性组合从早到晚反映了由基性到酸性的岩浆演化规律,火山活动具有喷发相（喷溢亚相）－喷发－沉积相的特点,构成了具多韵性的火山喷发特征,经同位素年龄研究属晋宁期火山喷发产物,时代归属为新元古代早期。     

新疆哈巴河县白哈巴村一带火山岩地质特征及构造意义

该区火山岩分为南北两带,二者既有一定的成因联系,也有一定的个体差异.据岩石化学成果估算出南带火山岩岩浆房顶部的近似深度为3.97km,而北带火山岩岩浆房顶部的近似深度为1.65km.表明岩浆房是向南倾斜的.该区基底属过渡壳海盆基底性质,火山活动具有类双峰特征,但又不是陆内裂谷.岩石建造及微量、稀土、同位素等地球化学特征反映具有活动陆缘特征.流纹岩Sr初始比值87Sr0/86Sr0=0.711130±0.00083,据"不同成因类型花岗岩类的(87Sr/86Sr)年龄和岩浆源区的关系图解",投入下部壳源型区,表明岩浆可能主要来自下部地壳.     

江苏仪征捺山火山机构及玄武岩石柱林

江苏仪征捺山为一个典型的古火山口,由玄武质岩石组成的火山溢流相、爆发相、通道相等岩相在空间上呈环状分布,且均向内倾斜,构成一个上大下小、向北东倾斜的喇叭口。火山喷发时间在距今约1000万年前,属中新世晚期产物。尤其是玄武岩石柱林规模宏大,气势雄伟,千恣百态,其火山石柱林价值已不再沉寂于地学标尺上,而以一种奇致的新姿态将成为本地域旅游业的新亮点。