Rb-Sr and Sm-Nd ages and isotopic geochemistry of Archaean granodioritic gneisses from Eastern Finland

Twenty granodioritic rocks and one amphibolitic enclave of the “basement” of the Suomussalmi-Kuhmo Archaean (2.65 Ga) greenstone belts (central-eastern Finland), have been chosen together with one greenstone sample for Rb-Sr and Sm-Nd geochronological and isotopic studies.The granitoïd rocks are subdivided into three groups: two generations of grey gneisses and a post-belt augen gneiss. The Rb-Sr ages of the first and second generation of grey gneisses are 2.86 ± 0.09 and 2.62 ± 0.07 Ga, respectively. These results are corroborated by Sm-Nd data. The post-belt augen gneiss gives an age of 2.51 ± 0.11 Ga. The results show that the two generations of grey gneisses, the greenstone belts and the post-greenstone augen gneiss, were developed over a period > 350 Ma. The two generations of grey gneisses show identical I_Sr values (0.7023 ± 8 and 0.7024 ± 6) which contrast with that of the augen gneiss (0.7049 ± 8). The low I_Sr and the near-chondritic ?_T^CHUR values indicate that the grey gneisses cannot derived from much older continental materials. Trace element studies suggest that these grey gneisses have had a multi-stage development. The augen gneiss with a moderately high I_Sr is likely to be derived from a granodiorite originated by partial melting of older sialic crust. The more probable parent rock seems to be the first generation grey gneisses. The I_Sr and average Rb/Sr values preclude the greenstone belt and the second generation of grey gneisses as the protolith.     

Mineralogy and geochemistry of volcanic and plutonic rocks from the boundaries of the Caroline plate: Tectonic implications

Chemical and mineralogical data from rocks dredged at six sites along plate boundaries around the Caroline Sea define at least three separate igneous suites. Three sites were located in the Sorol Trough, along the Caroline-Pacific plate boundary, and three sites were located in the Ayu Trough along the Caroline-Philippine plate boundary. Geophysical data suggest that both features have originated through processes of crustal extension.Weathered basalts with mid-ocean ridge basalt (MORB) affinities were recovered from the base of the northern margin of the Sorol Trough. Fresh transitional-to-alkalic pillow basalts from a seamount in the central part of the trough have geochemical and Sr-isotopic characteristics similar to enriched MORB or volcanics associated with off-ridge volcanism. A suite of tholeiitic rocks consisting of high-Mg basalt, ferrobasalt and a variety of mafic to ultramafic rocks dredged from the western section of the Sorol Trough has chemical and mineralogical characteristics of other intraplate tholeiitic suites like those from Hawaii. The ferrobasalts could have been derived from a high-Mg basaltic liquid through fractional crystallization of olivine, clinopyroxene and plagioclase. The geochemical data indicate that both MORB and “hot-spot” type sources have been involved in the generation of magmas beneath the Sorol Trough. Furthermore, the highly metamorphosed and tectonized mafic to ultramafic rocks recovered with the tholeiites suggest that a significant component of shear accompanied crustal accretion along “leaky” transform faults within the Sorol Trough.The chemical and isotopic characteristics of fresh to weathered pillow-lava fragments from the axial rift and eastern margin of the Ayu Trough, suggest that a MORB-type source was tapped during crustal accretion along a central spreading centre. An assemblage of metabasalts, metagabbros and silicic plutonic rocks (plagiogranites), recovered from a scarp along the western margin of the trough, have petrologic and chemical features analogous to rocks from plutonic complexes in ophiolites and some oceanic ridges. The plagiogranites could be differentiates from a relatively hydrous MORB magma, but cannot be related to the slightly LIL-enriched basalts in the Ayu Trough by crystal fractionation. Two separate mantle sources are implicated. Metamorphic features in the plutonic rocks are inferred to be the result of hydrothermal and dynamic events associated with the inception of spreading and to subsequent tectonism which exposed them.The petrochemistry of rocks from the Ayu and Sorol troughs is consistent with the hypothesis that crustal accretion occurred along the northern and western boundaries of the Caroline plate and further suggests that a variety of sub-oceanic mantle sources have been involved in the genesis of the crust in each region.     

He–Ar and Nd–Sr isotopic compositions of late Pleistocene felsic plutonic back arc basin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesis of young plutonic rocks in a back arc basin

We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. ⁴⁰Ar/³⁹Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K₂O + Na₂O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (⁸⁷Sr/⁸⁶Sr = 0.70454–0.71264, ¹⁴³Nd/¹⁴⁴Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (⁸⁷Sr/⁸⁶Sr = 0.70466–0.70892, ¹⁴³Nd/¹⁴⁴Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial ⁸⁷Sr/⁸⁶Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (³He/⁴He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.     

松辽盆地林深3 井火山岩的锆石U——Pb 年龄与Hf 同位素组成

根据松辽盆地北部林深3 井火山岩的岩相、全岩主微量元素、锆石U--Pb 同位素和锆石Hf 同位素特征的分析结果,探讨了该火山岩的形成时代和岩浆源区。锆石LA--MC--ICPMS U--Pb 研究结果表明: 林深3 井火山岩形成时代约为120 Ma,处于早白垩世中晚期,属于营城组火山岩。锆石的LA-- MC--ICPMS Hf 同位素研究显示,本区火山岩的锆石εHf ( t) 全部为较高的正值,二阶段Hf 模式年龄为789 ～ 1 494 Ma,表明其主要起源于中--新元古代增生的地壳物质,暗示中--新元古代时期兴蒙造山带东段存在地壳增生事件。     

Neoproterozoic subducted materials in the generation of Mesozoic Luzong volcanic rocks: Evidence from apatite geochemistry and Hf-Nd isotopic decoupling

A combined study including apatite geochemistry, zircon U-Pb, Lu-Hf isotopes and whole-rock geochemistry including Nd isotopes was carried out for the late Mesozoic volcanic rocks from the Luzong Basin, in the lower Yangtze River region, South China. Whole-rock geochemistry indicates the enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) as well as depletions of Nb, Ta and Ti. The extremely low Cl contents in apatites strongly contrast with the rather high-K contents in whole rocks. Potential loss of Cl during syn- and post-magmatic processes having been ruled out, Cl-K decoupling is attributed to be a feature inherited from the primary magma, which indicates the involvement of highly dehydrated sediments and altered oceanic crust in the mantle source. A calculation based on apatite and whole-rock geochemistry further illustrates that the source was composed of four end-members in the perspective of Cl/K, Cl/Nb and F/K ratios. The Hf-Nd isotopes are decoupled for the basaltic trachytes from the lower volcanic sequences in the Luzong Basin, with rather low ε_Hf(t) values (mean = − 10.3) and inconsistent Hf-Nd model ages (Hf ~ 1.8 Ga, Nd ~ 1.3 Ga), which indicate the “zircon effect” that in turn requires the incorporation of continental detritus in the source via subduction. However, Hf and Nd isotopes are nearly coupled for the rocks from the upper volcanic sequences in the Luzong Basin. Late-Mesoproterozoic two-stage Hf and Nd model ages (ca. 1.2 Ga) of rocks from the upper volcanic sequences in the Luzong Basin are similar to those of the Neoproterozoic igneous rocks from the Jiangnan orogen, suggesting their relationship with the same subduction event. Based on the combined apatite geochemistry and Hf-Nd isotopes, this work suggests that the source of Luzong volcanic rocks might incorporate Neoproterozoic subducted slab fragments and detrital sediments that had been blocked in the deep lithospheric mantle below the Luzong area since the Neoproterozoic assembly between the Yangtze and Cathaysia blocks. The partial melting may be triggered by the back-arc lithospheric extension related to the subduction of Paleo-Pacific plate in the late Mesozoic.     

Strontium isotopic geochemistry of the volcanic rocks and associated megacrysts and inclusions from Ross Island and vicinity,Antarctica

Twelve whole-rock samples of volcanic rocks and a composite of 11 basanitoid samples from Ross Island and vicinity, Antarctica show a narrow range of ⁸⁷Sr/⁸⁶Sr ratios from 0.7030₅ to 0.7033₉. This range is consistent with a model of differentiation from a single parent magma, but the data allow a 30% variation in the ⁸⁷Rb/⁸⁶Sr ratio in the source region if the average ratio is less than 0.057 and if the source region has existed as a closed system for 1.5 b.y. Megacrysts of titaniferous augite, kaersutite, and anorthoclase are isotopically indistinguishable from the host volcanic rocks and therefore are probably cogenetic with the volcanic sequence. A single trachyte sample is isotopically distinct from the rest of the volcanic rocks and probably was contaminated with crustal strontium.Ultramafic and mafic nodules found in association with basanitoids and trachybasalts have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7027₅ to 0.7057₅. Several of these nodules exhibit evidence of reaction with the melt and are isotopically indistinguishable from their hosts, but data for seven granulite-facies nodules show an apparent isochronal relationship. Although this isochron may be fortuitous, the resulting age of 158±22 m.y. is similar to ages reported for the voluminous Ferrar Dolerites, and suggests isotopic re-equilibration within the lower crust and upper mantle. These nodules are not genetically related to the Ferrar Dolerites, as evidenced by their lower initial ⁸⁷Sr/⁸⁶Sr ratios.Three ultramafic nodules are texturally and isotopically distinct from the rest of the analyzed nodules. These are friable, have larger ⁸⁷Sr/⁸⁶Sr ratios, and may represent a deeper sampling of mantle rock than the granulite-facies nodules. They were, however, derived at a shallower depth than the alkalic magma. Thus they are not genetically related to either the magma or the granulite-facies nodules.     

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia,North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.     

Complementary trace-element fractionation in volcanic and plutonic rocks: imperfect examples from ocean-floor basalts and gabbros

A simple trace-element model that is generalized from one devised by Albarede (1976) can be used to treat plutonic rocks as mixtures of cumulate phases and trapped liquid. The model is based on the Rayleigh fractionation law, mass balance, and modal data and, with certain assumptions, can give estimates of the mass fraction of the liquid component (X ^l) in a cumulate rock and of some of the characteristics of that liquid, including trace-element concentrations and the mass fraction of the original liquid (F) that remained in the magma chamber when some of it was trapped. Application of the model to Cr and Zr data for ten dredged gabbros from the Oceanographer Fracture Zone (OFZ) in the North Atlantic yields results in the range X ^l=0.014–0.17 and F=0.04–0.22. A more complex model extends the equations developed by Greenland (1970) and combines them with constraints from 1-atm experiments on ocean-floor basalts to effectively map phase-saturation boundaries onto trace-element variation diagrams. On a Zr-Sr plot, the model is capable of mimicking mixing chords and differences in liquid lines of descent (or parental compositions) inferred from major-element chemistry for several OFZ basalts. Qualitatively, the associated gabbros can also be rationalized as cumulate/melt mixtures on this diagram, although the allowable range of liquid compositions is large. In addition, there is some suggestion that the Zr-Sr liquid line of descent fitted to the OFZ basalts may have more general relevance to other ocean-floor rocks (e.g., Galapagos and FAMOUS glasses).     

Stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the Nagu-Korpo area,SW Finland

The volcanic rocks of the Nagu-Korpo area in SW Finland form a thin (< 1000 m) formation that is folded into narrow synforms separated by migmatite-filled antiforms. In Nagu the volcanic formation comprises three units: a lower unit of subvolcanic banded gabbro sills, a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer. The volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses. Geochemically the metavolcanic rocks are tholeiitic and plots based on Ti, Y, Zr and other trace elements show that they resemble recent within-plate lavas. MORB- and mantle-normalised trace element plots show patterns strongly differing from those of recent arc lavas, indicating a tectonic environment of (plate boundary related?) initial rifting. The upper ultramafic lavas are enriched in LREE relative to the lavas of the much thicker amphibolitic middle unit below. A model for the initial formation of the volcanic rocks of the Svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc-like environment.     

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。     

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。     

西藏新嘎果地区典中组火山岩年代学、Hf同位素及地球化学特征

新嘎果地区位于西藏南冈底斯中段北部,洛巴堆-米拉山断裂南侧,区内发育的林子宗群典中组火山岩主要为流纹质岩屑晶屑凝灰岩,出露面积小。该区典中组火山岩具有富硅(SiO_2=75.09%~78.10%)、贫碱(K_2O+Na_2O=4.84~6.52%)、贫镁(MgO=0.24~0.37%)、过铝质(A/CNK=1.02~1.75)特征,富集轻稀土元素,轻重稀土元素分馏明显,(La/Yb)N=9.49~11.30,Eu异常从弱负异常到弱正异常(δEu=0.76~1.15),相对富集大离子亲石元素(Rb、Th、U、K),亏损Nb、Ta、P、Ti等高场强元素,并且明显亏损Ba和Sr,属于钙碱性过铝质系列,具有陆源弧火山岩的特点。锆石U-Pb年龄为70.89±0.31 Ma,是目前典中组火山岩最老的年龄;锆石176Hf/177Hf值比较均一,分布于0.282 820~0.282 939之间,εHf(t)值为+3.2~+7.4,tDM模式年龄变化于662~930 Ma之间。地球化学研究表明,新嘎果地区典中组火山岩形成于俯冲构造环境,岩浆源区主要来源于地幔组分,并在岩浆演化过程中混染古老地壳。     

榴辉岩相高压-超高压变质岩的He同位素地球化学研究现状与问题

He同位素是区分地壳、地幔物质,研究壳-幔相互作用最灵敏的示踪剂之一,但其在高压-超高压变质作用过程中的地球化学行为目前仍不清楚,因而制约其在榴辉岩研究中的应用。中国作为高压-超高压榴辉岩带分布的重要地区,榴辉岩产出得天独厚,大洋、大陆两种俯冲成因榴辉岩均有分布。本文在归纳榴辉岩相高压-超高压变质岩研究进展的基础上,分析了He同位素示踪在榴辉岩研究中的应用现状。     

Petrology and geochemistry of boninite series volcanic rocks from the Mariana trench

Boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the Mariana Trench. These hauls included olivine boninites, boninites, boninitic andesites and boninitic dacites, as well as island arc tholeiitic basalts and andesites. The boninite series volcanics range from 52 to 68% SiO₂, and are characterized by very low abundances of high-field-strength cations and heavy-rare-earth elements. Boninites and olivine boninites have phenocrysts of olivine and orthopyroxene, the andesites phenocrysts of orthopyroxene and clinopyroxene, and the dacites orthopyroxene, clinopyroxene, and plagioclase. Most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine, orthopyroxene, clinopyroxene, and plagioclase in the proportions 2.5412, leaving 47% residual liquid. The fractionation must be in part open-system: reverse zoned phenocrysts, resorbed olivine and plagioclase xenocrysts, and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation. Two boninitic magma stems can be identified, with similar high-field-strength element abundances, but different amounts of Ca, Na, Al and light-rare-earth elements. There is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites. The compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid. The Mariana forearc boninite series lacks a strong iron enrichment, but produces andesites with lower Ti, Al and Y/Zr, and higher Mg, Ni and Cr than typical calcalkaline arc andesites and dacites. Boninites in the Mariana system were erupted only in the earliest phases of subduction zone activity.     

K-Ar ages and Pb,Sr isotopic characteristics of Cenozoic volcanic rocks in Shandong Province,China

28 samples of Cenozoic volcanic rocks collected from Shandong Province have been dated by K-Ar method. They are mainly Neogene with an age range of 4–19 m. y. The basalts from Linqu and Yishui in west Shandong Province are Miocene and those from Penglai and Qixia in east Shandong Province are Miocene and Pliocene in age. The basalts from Wudi in north Shandong Province are Middle-Early Pleistocene in age. In each area the duration of volcanic eruption was estimated at about 2–3 m. y. Pb and Sr isotopic compositions and U, Th, Pb, Rb, Sr, and major elements in most of the samples were determined. The isotopic compositions are:²⁰⁶Pb/²⁰⁴Pb—16.92-18.48,²⁰⁷Pb/²⁰⁴Pb—15.30-15.59,²⁰⁸Pb/²⁰⁴Pb—37.83-38.54, and (⁸⁷Sr/⁸⁶Sr)_i—0.70327-0.70632. There are some positive or negative linear correlations between²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb, Pb isotopes and Pb content, Pb isotopes and Sr isotopes, and Sr isotopes and other elements. The basaltic rocks from east and west Shandong Province have somewhat differences in isotopic composition and element content. The basalts probably are products of multi-stage evolution of the mantle. They have preserved the primary features of the source, although they were influenced, to some extent, by the contamination of crustal materials.     

Geochemistry and petrogenesis of late proterozoic volcanic rocks from north-western Africa

The Upper Proterozoic volcanism of northwestern Africa is characterized by the predominance of calc-alkaline rocks. Volcanics with tholeiitic affinities and alkali basalts are rare. The geochemistry and the relative proportions of calc-alkaline rocktypes in the Silet zone (Algeria) and the Ouarzazate formation (Morocco) are similar to those of recent island arc suites where basalts are most abundant while in the Tassendjanet and Gara Akofo zones (Algeria) they resemble contintal margin volcanic suites with a predominance of andesites. The volcanic rocks have undergone low-grade metamorphism which strongly affected alkali and alkali-earth elements and also to a smaller degree, the less mobile elements such as REE, Zr, Hf, Nb, and P. The geochemistry of the calc-alkaline rocks point to a complex origin involving low-pressure fractional crystallization, crustal contamination and derivation from a source already enriched in LILE.     

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors.     

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.