Volatiles in submarine volcanic rocks from the Mariana Island arc and trough

High temperature mass spectrometric analyses of glasses from quenched pillow rims of andesites dredged from 1170 m water depth in the northern portion of the Mariana Island arc indicate substantially less H₂O (~ 1 wt.%) and more CO₂ (~ 0.24 wt.%) than previously reported for volcanic arc rocks. Glass-vapor inclusions within plagioclase phenocrysts from quenched rims have $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios of 1:1. These results are similar to analyses of basaltic samples from the Mariana Trough (a back-arc basin). Generally, F and Cl contents are higher and S lower in the arc rocks compared to the samples from the back-arc basin. These results favor models for the production of island arc magmas which involve melting of the subducted slab, rather than just melting of the overlying mantle wedge because of the high volatile content needed to produce island arc magmas from peridotite (10–15 wt.%). The trough samples, although similar in non-volatile composition to mid-ocean ridge rocks, have much higher H₂O. somewhat higher CO₂ and lower S contents. Either near surface addition of voiatiles has enriched the magmas or H₂O must be a more important component in the generation and evolution of back-arc basin lavas than in the genesis of mid-ocean ridge basalts.     

Petrology of volcanic rocks from Kaula Island,Hawaii

A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island. The Kaula phonolites are the only documented phonolites from the Hawaiian Ridge. Among the accidental blocks, only the phonolites and a plagioclase basanite were amenable to K-Ar age dating. They yielded ages of 4.0–4.2 Ma and 1.8±0.2 Ma, respectively. Crystal fractionation modeling of major and trace element data indicates that the phonolites could be derived from a plagioclase basanite by subtraction of 27% clinopyroxene, 21% plagioclase, 16% anorthoclase, 14% olivine, 4% titanomagnetite and 1% apatite, leaving a 16% derivative liquid. The nephelinites contain the same phenocryst, xenocryst and xenolith assemblages as the tuff. Thus, they are probably comagmatic. The strong chemical similarity of the Kaula nephelinites and basanites to those from the post-erosional stage Honolulu Group on Oahu, the presence of garnet-bearing pyroxenites in the Kaula nephelinites (which previously, had only been reported in the Honolulu volcanic rocks) and the similar age of the Kaula basanite to post-erosional lavas from nearby volcanoes are compelling evidence that the Kaula basanites and nephelinites were formed during a post-erosional stage of volcanism.Now at Occidental Petroleum, Bakersfield, CA, 93309, USA     

Petrology,geochemistry, and tectonic significance of Mesozoic shoshonitic volcanic rocks,Luzong volcanic basin,eastern China

We studied the geochemical characteristics of three types of Mesozoic igneous rocks from the Luzong volcanic basin: basaltic trachyandesite at Shuangmiao, pyroxene monzonite at Bajiatan, and quartz-syenite (A-type granite) at Huangmeijian. Based on analyses of whole-rock major elements, all investigated rocks are enriched in K, Na, Ti, Al, but depleted in Ca, representing a shoshonitic series. Trace element analyses show that these rocks are characterized by enrichments of large-ion lithophile elements and high field strength elements. Positive Nb and Ta anomalies in the chondrite-normalized spider diagram indicate that the shoshonitic volcanic rocks share similar features with Nb-enriched basalts, which are different from normal island-arc volcanical rocks (they are typically strongly depleted in Nb and Ta). Bulk-rock chemical compositions and Sr–Nd isotopes indicate that the three types of igneous rocks are geochemically comagmatic, suggesting that the melts were derived from an enriched mantle reservoir. We postulate an extensional tectonic setting for the formation of Luzong volcanic basin, possibly related to subduction of a palaeo-Pacific plate beneath the east Chinese continent during the Yanshanian period (Cretaceous). Therefore, the petrogenetic features of those volcanic rocks as well as A-type granites in the Luzong basin indicate that the regional large-scale Fe–Cu–Au mineralization was associated with oceanic slab melting, but not delamination or recycling of the ancient lower continental crust, as previously proposed.     

Neoproterozoic zirconium-depleted boninite and tholeiitic series rocks from Adola, southern Ethiopia

In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4–10 km wide, north-south-trending belts bounded by high-grade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite complex. Preliminary geochemical and geochronological data highlight that the high-grade rocks in southern Ethiopia and northern Kenya include a significant portion of juvenile rocks that were accreted at the same time as ophiolitic rocks at 885-765 Ma. This is also the time of widespread oceanic magmatism and closure in the Arabian-Nubian Shield to the north.The Adola mafic rocks were previously described as island arc tholeiites and mid-ocean ridge basalts (MORB). New chemical analyses on the Megado belt rocks reveal the presence of boninites and related dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relatively low Zr/Sm (≤32). Boninites with similarly low ratios have not been reported from elsewhere.The Adola tholeiites have high Ti/Zr (150–300). Mixing between tholeiite and boninite magmas may have resulted in elevated Ti/Zr (80–126) in some Adola boninites. Otherwise, Ti/Zr in the latter is low (20–40). Low Ti/Zr is characteristic of Tertiary boninites in the west Pacific. The fact that both Ti/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertiary boninites at constant Ti/Zr suggests that Ti might be an element that is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.     

辽西白塔地区侏罗纪火山岩岩石学、同位素年代学与地球化学特征

出露于辽西白塔地区的侏罗纪中酸性火山岩及火山碎屑岩组合,主要是由粗安岩、流纹质岩屑晶屑熔结凝灰岩、粗面英安岩和流纹岩组成。通过系统的LA-ICPMS锆石的U-Pb同位素年代学研究,研究区火山岩形成年代为165~149 Ma。岩石地球化学特征显示,酸性火山岩属于钙碱性火山岩系,中性火山岩分别属于碱性火山岩系和中-高钾钙碱性火山岩系,中性岩具有埃达克质岩的地球化学特征,SiO_2≥56.20%、Al_2O_3≥15.09%、MgO≤2.89%、Sr≥561×10~(-6)、Y≤13.15×10~(-6)、Yb≤1.47×10~(-6)和无明显Eu异常。研究区火山岩普遍具有富集大离子亲石元素(Rb、K、Ba、Sr、Pb)和轻稀土元素,亏损高场强元素(Ti、Nb、Ta、P)的特征。其原始岩浆来源于基性下地壳物质的部分熔融,形成于古太平洋板块俯冲影响下板内挤压造山的构造环境。     

东天山卡拉塔格钠质火山岩岩石学、地球化学及成因

东天山卡拉塔格早泥盆世卡拉塔格组火山岩主要岩性为海相富钠质的安山岩、流纹岩和英安岩，夹少量的钠质玄武安山岩和钠质玄武岩以及正常系列的玄武安山岩。钠质火山岩的特征是：Na2O=1．69～6．38％，Na2O／K，O〉1，多在2～100间：无论是斑晶还是基质，斜长石均为富钠长石；酸性岩石斑晶主要是钠长石和石英，中基性岩石斑晶主要是钠长石（少量更长石）：酸性岩以斑状结构、霏细结构、球粒结构为主，而中基性岩以交织结构、玻晶交织结构为主。各类岩石全岩化学分析计算结果均含标准矿物Q、Hy、Hm，一部分含Di或C，在TAS图上全都投于亚碱性区；∑REE=25、96-96．11ppm，LaN／YbN=1．68-5．32，Eu／E^＊=0．46-1．25，LaN／SmN=1．01-2．95；强不相容元素适度富集，Nb和Ta强烈亏损，部分样品Cs、Rb、K、Zr和Hf适度亏损。这些特征表明，该区钠质火山岩的原始岩浆可能来自富钠俯冲洋壳熔体与地幔楔岩石反应的产物部分熔融。这样的岩浆形成机制与前人有关卡拉塔格是早泥盆世岛弧的认识相一致。     

南秦岭富钠长石岩石的岩石学和地球化学研究

钠长石岩是秦岭造山带中的一类特色岩石,主要分布于南秦岭凤-太、山-柞和镇-旬盆地的泥盆纪和志留纪地层中,与秦岭众多大中型铅锌矿、金矿成矿关系密切。钠长石岩呈层状、似层状产于碎屑岩或碳酸盐岩岩系中,与区域地层整合产出,渐变过渡。具有条带状、块状、角砾状构造;XRF分析数据显示钠长石岩化学成分以富Na2O、Al2O3、SiO2等常量元素及Pb、Zn、Au、Ag、Cu等微量元素为特征;富含元素Ba和Fe-Mn-(Co+Cu+Ni)×10分布图投影表明钠长石岩具有热水沉积的特征。而TiO2与Al2O3关系图解又反映出钠长石岩中有陆源碎屑物混入的特点,其Ti、Al可能来自水成沉积物中的粘土;ICP-MS分析所得稀土元素数据显示,钠长石岩与地层稀土元素在稀土总量、轻重稀土量和配分模式均十分相似,其较高∑REE就是成岩过程中水成沉积物混入造成的。富钠长石岩石在形成时间上发生在沉积和早期成岩阶段,与正常沉积岩同时形成。兼具有正常沉积岩石和热水沉积岩石的特点,是一种有别于二者的混合型非典型热水沉积岩石。研究认为钠长石岩是由一种富钠和金属元素的热水通过对地层元素的萃取并与海底未固结沉积物混合而成的混合热液经沉积成岩作用形成的。     

西天山石炭纪火山岩岩石学及Sr-Nd同位素地球化学研究

西天山石炭纪火山岩连续从玄武岩、粗面质玄武安山岩、粗面安山岩、粗面岩一直演化到流纹岩。不同地区火山岩的同位素组成差别很大。出露在拉尔敦达坂一带的晚石炭世粗面安山岩-粗面岩具有较高的εNd（t）值（＋2、68～＋4．29）和较高的初始^87Sr／^86Sr值（0、7015～0．7051）。新源城南的早石炭世粗面安山岩．流纹岩具有相对较低的εNd（t）值（-0．22～＋0．87）和变化较大的初始^87Sr／^86Sr值（0．7045～0．7068）。早石炭世玄武岩的初始^87Sr／^86Sr值较低且变化范围较小（0．7045～0．7058）,εNd（t）值高且变化范围大（＋0．89～＋3．04）。本文的同位素地球化学资料以及前期的年代学研究表明,西天山晚泥盆-早石炭世岛弧自西向东逐渐消亡,取而代之的是晚石炭世碰撞后富钾岩浆的喷发。     

西藏狮泉河地区高钾-钾玄质火山岩的岩石学、地球化学及锆石U-Pb年龄

报道的高钾-钾玄质火山岩位于狮泉河镇南东方向约20km处,向东延伸。高钾-钾玄质火山岩Si O2变化于60.35%~68.68%之间,属中酸性岩范畴;具有高的K2O+Na2O含量(8.8%~10.66%),K2O/Na2O值在1.92~2.49之间,Mg O含量较低,介于0.88%~3.47%之间,Al2O3含量为14.02%~14.91%,属于高钾-钾玄质系列。岩石强烈富集大离子亲石元素(LILE)Rb、Ba、Th、U和轻稀土元素(LREE),高场强元素(HFSE)Nb、Ta、Ti具有明显负异常,Cr、Ni、Co相容元素含量低于或接近地壳的平均含量,结合Th/Yb-Ta/Yb、(Th×100)/Zr-(Nb×100)/Zr判别图及La-La/Yb图解,暗示岩浆源区可能为下地壳。在左左乡南东约2km处和狮泉河水泥厂北东约1km处各采集1个高钾-钾玄质火山岩样品,对其中的锆石进行LA-ICP-MS U-Pb同位素测定,得到的206Pb/238U年龄加权平均值分别为22.04±0.42Ma和22.29±0.31Ma,此年龄被解释为狮泉河一带高钾-钾玄质火山岩的喷发时代,即中新世阿启塔期。由此表明,该火山岩是印度板片向北俯冲时在狮泉河一带俯冲板片断离,岩浆发生部分熔融的产物。     

Trace element geochemistry of Archean volcanic rocks

K, Rb, Sr, Ba and rare earth elements of some Archean volcanic rocks from the Vermilion greenstone belt, northeast Minnesota, were determined by the isotopic dilution method. The characteristics of trace element abundances, supported by the field occurrences and major element chemistry, suggest that these volcanic rocks were formed in an ancient island arc system. A felsic rock is suggested to be derived by partial melting of a basaltic source, presumably in an ancient subduction zone.It is well known that the distribution coefficients (liquid/source) for the above trace elements are almost invariably greater than one. Continuous extraction of volcanic liquids from the upper mantle through geologic time would result in depletion of these elements in the upper mantle. However, all trace element abundances in many Archean volcanic rocks are almost identical to their modern equivalents. This gross constancy of trace element concentration in rocks of different geologic age raises some important questions as to the evolution of the upper mantle. It is proposed that the trace elements have been repeatedly and fully recycled in a restrictive and closed system of crust and upper mantle during the last three billion years (recycled mantle), or the trace elements have been replenished from the lower part of the mantle by some undefined process (replenished mantle). It is believed that interplay of both recycling and replenishment have been responsible for crust-mantle evolution in geological history.     

漠河盆地东南部中生代火山岩年代学与地球化学研究

漠河盆地东南部中生代火山岩地层主要由光华组和甘河组组成,锆石U--Pb定年结果确定两组火山岩分别形成于128~125 Ma和126~120 Ma。光华组流纹岩高硅富碱、贫钙铁镁;轻重稀土分馏较明显[w(La/Yb)N=13.01~17.99],铕异常中等(δEu=0.49~0.67);富集大离子亲石元素Rb、K、Th、U,亏损高场强元素Nb、Ta、P、Ti;锆石εHf(t)值为-2.5~3.0,两阶段Hf模式年龄为989~1 130 Ma;岩浆源区为中元古代末期的地壳物质。甘河组粗安岩和玄武粗安岩富碱高钾,镁指数较大(Mg#≈48);稀土元素配分曲线为轻稀土富集的右倾型[w(La/Yb)N=12.67~23.21],铕异常不明显(δEu=0.77~0.91);富集Rb、Ba、K、Th、U等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素;锆石εHf(t)值为-1.2~4.1;岩浆源区为俯冲流体交代过的岩石圈地幔,岩浆经历了一定程度分离结晶作用。     

西藏狮泉河地区高钾-钾玄质火山岩的岩石学、地球化学及锆石U-Pb年龄

报道的高钾-钾玄质火山岩位于狮泉河镇南东方向约20km处,向东延伸。高钾-钾玄质火山岩SiO2变化于60.35%~68.68%之间,属中酸性岩范畴;具有高的K2O+Na2O含量（8.8%~10.66%）,K2O/Na2O值在1.92~2.49之间,MgO含量较低,介于0.88%~3.47%之间,Al2O3含量为14.02%~14.91%,属于高钾-钾玄质系列。岩石强烈富集大离子亲石元素（LILE）Rb、Ba、Th、U和轻稀土元素（LREE）,高场强元素（HFSE）Nb、Ta、Ti具有明显负异常,Cr、Ni、Co相容元素含量低于或接近地壳的平均含量,结合Th/Yb-Ta/Yb、（Th×100）/Zr-（Nb×100）/Zr判别图及La-La/Yb图解,暗示岩浆源区可能为下地壳。在左左乡南东约2km处和狮泉河水泥厂北东约1km处各采集1个高钾-钾玄质火山岩样品,对其中的锆石进行LA-ICP-MS U-Pb同位素测定,得到的206Pb/238U年龄加权平均值分别为22.04±0.42Ma和22.29±0.31Ma,此年龄被解释为狮泉河一带高钾-钾玄质火山岩的喷发时代,即中新世阿启塔期。由此表明,该火山岩是印度板片向北俯冲时在狮泉河一带俯冲板片断离,岩浆发生部分熔融的产物。     

松辽盆地火山岩的同位素年代、地球化学特征及意义

沉积盆地内部的火山岩是研究盆地形成的构造环境以及探讨盆地发展演化的动力学特征的重要线索。松辽盆地具有二元结构特征，断陷构造层发育大量的火山岩。但是，前人尚未对比进行深入系统的研究。根据采自钻井岩芯的火山岩样品，通过同位素测年（K／Ar)和地球化学分析等结果，讨论了松辽盆地晚中生代火山岩的岩石学、岩石化学和地球化学特征，探讨了盆地发育早期的构造环境及其地球动力学背景。指出松辽盆地在其形成早期（即断陷期）是一个发育陆缘岩浆弧构造背景上的走滑拉张盆地，火山岩岩浆源区具有多成分、复合性的演化特点。     

Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus

Here we present the first radiometric age data and a comprehensive geochemical data set (including major and trace element and Sr-Nd-Pb-Hf isotope ratios) for samples from the Hikurangi Plateau basement and seamounts on and adjacent to the plateau obtained during the R/V Sonne 168 cruise, in addition to age and geochemical data from DSDP Site 317 on the Manihiki Plateau. The ⁴⁰Ar/³⁹Ar age and geochemical data show that the Hikurangi basement lavas (118-96 Ma) have surprisingly similar major and trace element and isotopic characteristics to the Ontong Java Plateau lavas (ca. 120 and 90 Ma), primarily the Kwaimbaita-type composition, whereas the Manihiki DSDP Site 317 lavas (117 Ma) have similar compositions to the Singgalo lavas on the Ontong Java Plateau. Alkalic, incompatible-element-enriched seamount lavas (99-87 Ma and 67 Ma) on the Hikurangi Plateau and adjacent to it (Kiore Seamount), however, were derived from a distinct high time-integrated U/Pb (HIMU)-type mantle source. The seamount lavas are similar in composition to similar-aged alkalic volcanism on New Zealand, indicating a second wide-spread event from a distinct source beginning ca. 20 Ma after the plateau-forming event. Tholeiitic lavas from two Osbourn seamounts on the abyssal plain adjacent to the northeast Hikurangi Plateau margin have extremely depleted incompatible element compositions, but incompatible element characteristics similar to the Hikurangi and Ontong Java Plateau lavas and enriched isotopic compositions intermediate between normal mid-ocean-ridge basalt (N-MORB) and the plateau basement. These younger (∼52 Ma) seamounts may have formed through remelting of mafic cumulate rocks associated with the plateau formation. The similarity in age and geochemistry of the Hikurangi, Ontong Java and Manihiki Plateaus suggest derivation from a common mantle source. We propose that the Greater Ontong Java Event, during which ∼1% of the Earth’s surface was covered with volcanism, resulted from a thermo-chemical superplume/dome that stalled at the transition zone, similar to but larger than the structure imaged presently beneath the South Pacific superswell. The later alkalic volcanism on the Hikurangi Plateau and the Zealandia micro-continent may have been part of a second large-scale volcanic event that may have also triggered the final breakup stage of Gondwana, which resulted in the separation of Zealandia fragments from West Antarctica.     

藏北羌塘地块新生代火山岩中麻粒岩捕虏体的岩石学和地球化学研究:对青藏高原新生代火山岩成因及下地壳性质的约束

青藏高原北部羌塘地块的新生代高钾钙碱性火山岩中含有很多下地壳捕虏体,这些捕虏体的主要岩石类型为二辉石麻粒岩和单斜辉石麻粒岩.本文对乌兰乌拉湖南侧枕头崖地区新生代高钾钙碱性火山岩中的9个麻粒岩样(6个基性麻粒岩和3个酸性麻粒岩)进行了系统的岩石学,矿物化学,地球化学和Sr-Nd-Pb同位素地球化学研究.其中,紫苏辉石具有高Mgo FeO,低Al2O3的特征,单斜辉石具有低Al2O3和TiO2的特征,黑云母具有高TiO2的特征,这些特征都表明这些矿物为变质成因.矿物温压计算表明二辉石麻粒岩形成的平衡温度为783℃～818℃.单斜辉石麻粒岩形成压力在0.845～0.858GPa之间,来源深度约28km.表明它们可能是来自青藏高原加厚陆壳中部的岩石样品.基性麻粒岩的SiO2=48.76%～58.61%,Al2O3=18.34%～24.50%,Na2O=3.16%～5.41%,K2O=1.58%～3.01%,低Mg#(30～67),富集轻稀土(LREE)和大离子亲石元素(LILE),其具有较高的Rb/Sr(0.09～0.21)和(La/Yb)N(17.32～49.35),具有较低的Nb/Ta(9.76～14.92),其Eu异常变化于0.19～0.89之间.基性麻粒岩的Sr-Nd-Pb同位素地球化学表现为87Sr/86Sr=0.710812～0.713241,εSr=169.13～203.88,143Nd/144 Nd=0.512113～0.512397;εNd=-4.70～-10.05,206Ph/204Pb=18.7000～-18.9565,207Pb/204Pb=15.7135～-15.7662,208pb/204Pb=39.1090～39.4733.和基性麻粒岩类似,酸性麻粒岩也表现出富集轻稀土(LREE)和大离子亲石元素(LILE)的特征,它们的87Sr/86Sr=0.712041～0.729088,εNd=180.71～430.59,143Nd/144Nd=0.512230～-0.512388;εNd=-4.74～-7.96,206Ph/204Pb=18.9250～-19.1717,207Pb/204Pb:15.7662～-15.7720,208Pb/204Pb=39.2109～39.6467.上述地球化学特征表明这些基性麻粒岩的源岩是下地壳岩石,而非地幔岩或玄武质堆晶岩,而酸性麻粒岩的源岩极有可能是准铝质的花岗质岩石.这就表明青藏高原新生代下地壳的地温梯度很高,并含有部分沉积岩,而非典型的辉长质下地壳.而且,详细的研究表明,这种特殊的下地壳可能对青藏高原新生代高钾钙碱性和橄榄粗安质岩浆的起源有重要作用.因此目前所认为的超钾质-橄榄粗-安质岩浆源于富集岩石圈地幔在对流减薄作用下发生部分熔融的观点值得重新考虑.     

Petrology and geochemistry of the mafic dyke rocks from precambrian almora crystallines of Kumaun Lesser Himalaya

Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85?38.58×10^?3SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo₆₁-Fo₃₃), clinopyroxene (Wo₄₆-En₄₂-Fs₂₂ to Wo₄₀-En₃₆-Fs₁₅) and plagioclase (An₅₈-An₁₂) have crystallized in the temperature range of ca1400–980°C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Na_pfu=0.033?0.025), (Mg^#=0.64–0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. Mg?Fe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs Mg^# variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene -plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions.