Geophysical imaging of buried volcanic structures within a continental back-arc basin: The Central Volcanic Region, North Island, New Zealand

Hidden beneath the ~ 2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~ 20 × 20 km structures, at Tokoroa and Mangakino, form an adjacent gravity high and low, respectively. Interpretations from seismic refraction arrivals and gravity modelling indicate the − 65 mgal Mangakino residual gravity anomaly can be modelled, in part, by two low-density bodies that reach depths of ~ 6.5 km, whereas the Tokoroa gravity anomaly is due to a higher density rock coming, at most, to within ~ 650 m of the surface. The Mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1.2 Ma. An andesite volcano or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. The size of the putative volcanic structure is comparable to the presently active Tongariro Volcanic Complex in the centre of North Island.     

东昆仑祁漫塔格寒武纪岛弧:来自拉陵高里河地区玻安岩型高镁安山岩/闪长岩锆石U-Pb年代学、地球化学和Hf同位素证据

祁漫塔格构造带是东昆仑造山带的重要组成部分,在青藏高原北部早古生代汇聚板块边缘地质研究中具有重要意义.祁漫塔格构造带在早古生代通常被认为是弧或弧后盆地,但其早期形成阶段的地质记录尚未报道,初始俯冲的时代也有待进一步限定.本文选择了祁漫塔格东段拉陵高里河地区祁漫塔格群火山岩和闪长岩进行岩石学、地球化学、锆石U-Pb年龄学...     

Magma plumbing system beneath Ontake Volcano, central Japan

Ontake Volcano in central Japan was last active from ~ 100–35 Ka. The eruptions contained rhyodacite pumice and lavas in the first stage (stage O1, > 33 km³), followed by eruptions of andesite lavas and pyroclastics (stages O2 and O3, > 16 km³). Modeling of major and incompatible elements with Sr isotope ratios suggests that the primary magma was a high-alumina basalt. One andesite magma type appears to have evolved from the basalt in a closed system magma chamber, in part by fractional crystallization, and its generation included crustal assimilation. The other andesite magma type is considered to have evolved in an open system magma chamber in which repeated input of primary magma occurred together with wall-rock assimilation and fractional crystallization. The rhyodacite is inferred to have evolved in a closed system magma chamber by fractional crystallization of the second type of andesite. These genetic relationships require that the magma chamber functioned alternately as an open and a closed system. Geobarometry indicates that there may have been multiple magma chambers, located in the upper crust for the rhyodacite, near the upper–lower crust interface for the andesite and in the mid-lower crust for the basalt. These chambers were stacked to form the magma plumbing system of Ontake. Incompatible element compositions of the basalt are considered to have changed during the eruptions, suggesting that two different plumbing systems for stage O1 magma and for stages O2, O3 magmas existed during the 65 Ka of activity. Evolutionary history of the systems implies that the primary magma was introduced into the magma plumbing system each for ~ 17 500 years and that the life span of a magma plumbing system was shorter than 40 Ka.     

湘南宜章地区辉绿岩、花岗斑岩、安山岩的形成时代和成因——锆石U-Pb年龄和Hf同位素组成

湘南宜章县长城岭地区广泛出露以辉绿岩、花岗斑岩为主的燕山早期岩体,而宜章平和地区主要分布安山岩体,其年龄和成因有待进一步研究。分别用SHRIMP和LA-ICP-MS法测试了它们的年龄和Hf同位素组成,长城岭辉绿岩的SHRIMP锆石U-Pb年龄为153.7Ma±3.1Ma、227.0Ma±4.2Ma;长城岭花岗斑岩的LA-ICP-MS锆石U-Pb测年结果为153Ma±14Ma和231.58Ma±0.67Ma;平和安山岩的SHRIMP锆石U-Pb测年结果为159Ma±14Ma和229.3Ma±7.6Ma,LA-ICP-MS锆石U-Pb测年结果为161Ma±1Ma,显示为燕山早期岩浆侵位和印支期基性岩浆底侵。锆石Lu-Hf同位素原位分析结果表明,εHf（t）值在正值和负值范围内变化,指示岩浆为壳幔混合来源。结合长城岭花岗岩体中继承锆石的年龄信息,认为花岗质岩石可能来源于中元古代基底的重熔。岩体岩浆很可能是由元古宙火成岩石部分熔融形成的,并伴有年轻或新生幔源物质的加入,岩浆上升侵位的过程中发生了混合作用。     

Geochemical and isotopic characteristics and evolution of the Jurassic volcanic arc between Arica (18°30′S) and Tocopilla (22°S), North Chilean Coastal Cordillera

The present-day North Chilean Coastal Cordillera between 18°30′S and 22°S records an important part of the magmatic evolution of the Central Andes during the Jurassic. Calc-alkaline to subordinate tholeiitic members from four rock groups with biostratigraphically constrained age display incompatible element pattern characteristic of convergent plate-margin volcanism, whereas alkaline basalts of one group occurring in the Precordillera show OIB-type trace element signatures. The correlation of biostratigraphic ages, regional distribution, and composition of the volcanic rocks provides a basis for the discussion on geochemical evolution and isotope ratios.Major and trace element distributions of the volcanic rocks indicate their derivation from mantle-derived melts. LILE and LREE enrichments in calc-alkaline basaltic andesites to dacites and some of the tholeiites hint at the involvement of hydrous fluids during melting and mobile element transport processes. A part of the Early Bajocian to ?Lower Jurassic and Oxfordian andesites and dacites are adakite-like rocks with a substantial participation of slab melt and are characterized by high Sr/Y ratios and low HREE contents. The Middle Jurassic tholeiitic and calc-alkaline basalts and basaltic andesites have been transported and partly stored within a system of deep-seated feeder fissures and crustal strike-slip faults before eruption.The isotopic composition of Sr (⁸⁷Sr/⁸⁶Sr_i=0.7032-0.7056) and Nd (εNd_i=2.2-7.1) of the Jurassic volcanic rocks mostly fall in the range characteristic for mantle melts although some crustal components may have been involved. A few samples show slightly more radiogenic Sr isotopic composition, which is probably due to interaction with ancient sea-water. The Pb isotopic composition of the arc rocks is uncoupled from the isotopic composition of Sr and Nd and is dominated by the crustal component. Since the Cretaceous and Modern arc volcanic rocks show Pb isotopic compositions that can be largely explained by in situ Pb isotope growth of Jurassic arc volcanic rocks, we argue that the various Andean arc systems between 18°30′S and 22°S formed on the same type of basement.Most of the investigated samples have high Ba, Zr, and Th concentrations compared to island arc mafic volcanic rocks. About 20% of the Jurassic arc volcanics comprise of dacitic to rhyolitic rocks. These characteristics combined with the Pb isotopic composition that shows the influence of a Palaeozoic (or partly older) basement point to a continental margin setting for the North Chilean Jurassic arc. The distribution of the magmatic rocks throughout time, their textures, and the character of intercalated sedimentary rocks reflect westward movement of the magma sources and of the arc/back-arc boundary relative to the current coast line during the Early Bajocian on a broad front between 19°30′ and 21°S.     

鲁西中生代高Mg闪长岩的成因:年代学与岩石地球化学证据

鲁西高Mg闪长岩分布较为广泛, 莱芜铁铜沟岩体和淄博金岭岩体是其典型代表.铁铜沟岩体早期苏长辉长岩和晚期辉石闪长岩、金岭黑云母闪长岩的LA-ICPMS U-Pb谐和加权平均年龄分别为(131.4±4.9)Ma(n=15)、(134.5±2.3) Ma(n=13)和(132.8±4.2)Ma(n=12), 它们代表了岩体的侵位结晶年龄——早白垩世.铁铜沟岩体晚期辉石闪长岩中浑圆状锆石的LA-ICPMS U-Pb谐和年龄为(2 513±54)Ma(n=8), 代表了华北克拉通基底的存在.该类岩石以高Mg# (> 60)、富钠、富含轻稀土元素、贫重稀土元素以及高场强元素(Nb、Ta、Zr、Hf)的明显亏损为特征, 显示埃达克岩(adakite) 的特征.该类岩石中地幔橄榄岩包体的广泛存在以及高Mg的特征表明原始岩浆的地幔成因, 而Sr-Nd同位素组成(I_sr为0.704 75-0.707 72和ε_Nd(t)值为-3.95--13.30)和高场强元素(Nb、Ta、Zr、Hf)的亏损以及太古代继承锆石的存在均暗示岩浆源区应有陆壳物质的涉入.铁铜沟岩体和金岭岩体闪长岩成分的差异应归因于岩浆源区性质的差异和部分熔融程度的不同.基于该类岩石的地球化学和所含包体, 同时结合华北克拉通东部中生代早期岩石圈演化历史, 可以认为鲁西中生代早白垩世高Mg闪长岩的形成应为拆沉的岩石圈(地幔+下地壳)与软流圈混熔的产物.      

新疆特克斯东南大哈拉军山组玄武安山岩地球化学特征:岩石成因和构造背景探讨

特克斯东南大哈拉军山组玄武安山岩具有钙碱性火山岩的地球化学特征,Nb、Ta等高场强元素强烈亏损,强不相容元素比值明显不同于原始地幔、亏损地幔和富集地幔而倾向于地壳,具有岛弧火山岩的特点,与新源县南及其东部大哈拉军山组火山岩一起指示了西天山在早石炭世-晚石炭世处于汇聚而不是伸展的大地构造背景。与岛弧作用有关的俯冲沉积物的交代作用是其源区受改造的最主要方式,岩浆演化以包括橄榄石、单斜辉石在内的分离结晶作用为主。     

埃达克岩的原义、特征与成因

论述了埃达克岩的原义与综合特征 ,并针对其与太古宙TTG之间的区别和联系及今后研究埃达克岩的建议提出了自己的见解。埃达克岩 (adakite)的原义是指一类具有镁铁质斑晶的隐晶质火山岩 ,属于岛弧型岩浆钙碱性岩系 ,一般形成于年轻的 (<2 5Ma)、地热高的岛弧环境 ,是俯冲板块和上覆地幔相互作用产生的杂化熔液通过结晶分异形成的。综合总结埃达克岩 (原义 )的地球化学特征如下 :(1)原生标志 ,高Mg# 、低FeO /MgO、高Cr及Ni;(2 )微量元素标志 ,高LILE、高LREE、低HREE、低HFSE以及高分异的REE型式等。对实验岩石学研究资料的总结可知杂化 (hybridized)熔液是由小数量的板块熔液与地幔楔反应经交代作用、同化作用形成的 ,可分异直至出现酸性岩浆 ,这一过程称为“地幔同化及分离结晶作用 (mantle AFC)”。在橄榄岩的同化作用中 ,原有熔液Mg# 迅速上升 ,并在熔液成分加多后 ,向高Mg# 区迅速发展。在近代一些埃达克岩及相关岩石研究中 ,部分学者认为太古宙TTG与新生代板块 (榴辉岩 )重熔的TTD岩系类似。同时 ,亦有学者认为太古宙“绿岩带”中与TTG有关的深成岩系是一类Mg质花岗闪长岩Mg质二长闪长岩 ,成因与Sanukite相似 (太古宙sanuk itoid岩系 ) ,相当于富集橄榄岩重熔形成的岩系。这些研究重新引发了太古宙     

新疆三塘湖盆地早二叠世火山岩地球化学特征与构造环境分析

新疆三塘湖盆地早二叠世火山岩地球化学特征与构造环境分析林克湘闫春德龚文平（江汉石油学院，湖北荆沙４３４１０２）关键词安山岩活动陆缘盆地板块构造早二叠世三塘湖盆地位于新疆东北部，与蒙古接壤，盆地呈北西—南东向展布，长约５００ｋｍ，宽４０～５０ｋｍ，面积...     

西天山玉希莫勒盖达坂石英闪长岩的微量元素地球化学及同位素年代学研究

玉希莫勒盖达坂石英闪长岩的主量元素组成相对稳定,SiO2、TiO2、Al2O3、Fe2O3T、MgO、CaO、Na2O和K2O的含量分别为57.08%～59.95%、0.70%～0.85%、16.34%～16.81%、6.73%～8.51%、2.82%～3.55%、5.72%～6.91%、3.22%～3.90%和1.44%～1.96%。该类岩的CA/TH指数在1.2左右变化,应属钙碱性系列岩石。主量元素和微量元素地球化学特征显示,玉希莫勒盖达坂石英闪长岩与区域内的大哈拉军山组钙碱性玄武安山岩为同源岩浆岩,它们是富集的尖晶石二辉橄榄岩部分熔融的产物,原始地幔楔的富集受俯冲板块脱水形成流体控制,这种源区富集交代过程导致玉希莫勒盖石英闪长岩具有明显的陆壳混染特征。玉希莫勒盖石英闪长岩明显亏损Nb-Ta、Zr-Hf、Ti等高场强元素,同时富集Pb,显示出与板块俯冲作用有关造山带岩浆岩的特征,形成于活动陆缘岛弧。与大哈拉军山组高钾系列火山熔岩形成于板片撕裂诱发的伸展构造环境不同,石英闪长岩形成于挤压的构造环境。单颗粒锆石U-Pb定年结果显示,石英闪长岩的形成年龄为309.9±2.7Ma,暗示阿吾拉勒东段大哈拉军山组火山岩的形成时代为晚石炭世。