Adakitic lavas in the Central Luzon back‐arc region,Philippines: lower crust partial melting products?

Abstract Volcanism in the back-arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side-main volcanic arc igneous rocks. The back-arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite-like geochemical characteristics of these back-arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet-bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.     

新疆乌鲁木齐东部早二叠世枕状玄武岩火山岩系的发现及大地构造意义

新疆乌鲁木齐东部野生动物园附近的下二叠统下部发育了一套以枕状玄武岩为代表的火山岩系。该玄武岩在公路上发育有两层,上部一层较厚,可达7～8m,下部一层2～3m。岩枕近圆形、肾状、枕状、条带状或蠕虫状等,多呈顶突底凹。岩枕长轴平行排列,长轴基本顺层面分布;有的岩枕中含大量海百合茎、珊瑚及腕足等生物化石。在TAS图中大部分样品位于玄武粗安岩,而在K_2O-SiO_2图上主要落在低K拉斑玄武岩区间。MnOTiO_2-P_2O_5图解显示以岛弧为主,常量元素的特征总体上更接近岛弧玄武岩。在Hf/3-Th-Ta图解上也以岛弧为主;在Zr/4-2Nb-Y图解显示以火山弧为主,常量元素的特点总体上更接近岛弧玄武岩。微量元素原始地幔标准化蛛网图表明为同源岩浆产物,具高度相似的演化过程,多种微量元素判别图揭示岛弧-弧后盆地环境;稀土总量明显较低,在稀土元素球粒陨石标准化图解上,其稀土分布曲线一致性较好,呈轻稀土富集右倾型,Eu为负异常,揭示了乌东玄武岩岩浆有一定分异。稀土元素配分曲线与弧后盆地玄武岩(BABB)具有很好的一致性。U-Pb和谐年龄为283±8Ma,结合地层及古生物资料推测乌东玄武岩喷发的时间为早二叠世早期。乌东枕状玄武岩-灰岩之下有一套(磨拉石)底砾岩,初定为石炭-二叠系的界限。底砾岩之下为下石炭统的中厚层灰岩,具有明显的喀斯特化,为不整合接触,揭示在两者之间发生了造山作用。通过对新疆乌东早二叠世早期的枕状玄武岩的地质特征、地球化学特征、形成环境和时代的研究,表明乌东一带早二叠世主要为一个岛弧和弧后盆地环境,进一步揭示了北天山北缘石炭-二叠世碰撞闭合造山之后又发生了松弛扩张形成了弧后盆地,海水再度大规模入侵。最终的闭合碰撞造山的时间最早可能在晚二叠世。由于乌东枕状玄武岩在喷出地表后受到了生物化石和陆源碎屑的污染,再加上侵入过程中地壳的污染,使其具有非常复杂的地球化学特点和多解性。乌东玄武岩的研究对于了解博格达山、甚至天山中段在晚古生代的构造沉积演化及造山作用具有重要意义,同时对准噶尔盆地、吐哈盆地及三塘湖盆地油气资源的形成与分布具有重要意义。     

A North-South Transect across the Central Mexican Volcanic Belt at ~100{degrees}W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism

Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K₂O basaltic andesitesand andesites; (2) medium-K₂O basaltic andesites, andesitesand dacites; (3) high-TiO₂ basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K₂O and medium-K₂O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO₂ group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K₂Omelts that are relatively enriched in fluid-mobile elementsand have the highest ⁸⁷Sr/⁸⁶Sr ratios. Medium-K₂O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO₂, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K₂O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K₂O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic ⁸⁷Sr/⁸⁶Sr from 0·704245 to 0·704507,¹⁴³Nd/¹⁴⁴Nd values ranging from 0·512857 to 0·512927(_Nd = 4·27–5·63), and ²⁰⁸Pb/²⁰⁴Pb valuesfrom 38·248 to 38·442, ²⁰⁷Pb/²⁰⁴Pb values from15·563 to 15·585, and ²⁰⁶Pb/²⁰⁴Pb values from18·598 to 18·688. The medium-K₂O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of ⁸⁷Sr/⁸⁶Sr, but similar ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁸Pb/²⁰⁴Pbvalues to those of the high-K₂O series. In contrast, the high-TiO₂series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO₂ lavas are isotopically distinct in theirlower and narrower range of ¹⁴³Nd/¹⁴⁴Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas     

Stratigraphy and paleomagnetism of a 3-km-thick Miocene lava pile in the Mjoifjördur area,eastern Iceland

We have carried out stratigraphic mapping in the Upper Miocene basalt lava pile around the fjords Mjoifjördur and Seydisfjördur, eastern Iceland. The mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups. These units are also used to provide correlations with the Nordfjördur area south of Mjoifjördur. We present a 3-km composite stratigraphic column for the area between Mjoifjördur and Seydisfjördur. The geology of this area shows some differences from the classical model of Walker for the structure of eastern Iceland partly due to the fact that most of Mjoifjördur is not in the vicinity of central volcanoes. Detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles. The local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria. Over 20 geomagnetic reversals are recorded in the eastern Iceland lava pile in a period approximately 13-10 Ma ago. The geomagnetic field during this period averages to a central axial dipole field, and its overall statistical properties resemble those obtained in earlier surveys in Iceland.     

Facies architecture of a silicic intrusion-dominated volcanic centre at Highway–Reward, Queensland, Australia

The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings.The lithofacies associations at Highway–Reward collectively define a submarine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0.5 km³. Single porphyritic units vary from <10 to 350 m in thickness and some are less than 200 m in diameter. Ten of the porphyritic units studied in the immediate host sequence to the Highway–Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lavas and intrusions. Direct evidence for eruption at the seafloor is limited to a single partly extrusive cryptodome. However, distinctive units of resedimented autoclastic breccia indicate the presence nearby of additional lavas and domes.The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaqueous emplacement environment. Due to rapid quenching and mixing with unconsolidated clastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units. Magma preferentially invaded the sediment, avoiding the older units or conforming to their margins. Large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma.     

Geochemical trends of sequential lava flows from Meseta volcano, Guatemala

The Meseta and Fuego volcanoes closely overlap and collectively are known as the Fuego Volcanic Complex. Historic activity occurs exclusively at Fuego, the southern center, and consists of high-Al basalts. Meseta, the inactive northern center, is predominantly composed of basaltic andesites with minor basalt and andesite. A thick sequence of lava flows and dikes is exposed by a steep collapse escarpment on the east flank of Meseta. The upper 75% of the sequence was sampled from three interfingering stratigraphic sections consisting of 27, 10 and 4 lavas, respectively. Temporal geochemical trends of each section indicates a complex evolutionary history. A major trend toward more evolved compositions upward in the section is consistent with crystal fractionation. This trend is sharply interrupted by the youngest lavas which become distinctly more mafic in composition. Magma mixing is apparently the dominant magmatic evolution process that generated these lavas. The two trends have distinct Sr signatures that suggest a change in parental magma compositions. This abrupt change in composition is interpreted to signal high input rates of mafic magma into the subvolcanic magma chamber. These changes eventually led to sector collapse of Meseta volcano and deposition of the Escuintla debris avalanche. Eruptive activity then migrated to the Fuego volcano where historic activity is similar to that of Meseta immediately prior to its collapse.     

Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 2: Eastern part of the Dzhavakheti Highland. Regional geological correlation

The results of isotopic-geochronological study of the Pliocene volcanic rocks in reference sections and volcanic edifices of eastern part of the Dzhavakheti Highland (the northwestern Lesser Caucasus) are considered. The isotopic-geochronological data obtained here are correlated with data on western part of the Dzhavakheti Highland, which have been considered in previous part of this work. Based on correlation, time spans of principal volcanic events of the Pliocene in the study region as a whole are determined, and general trends of the young magmatism evolution within the region are established. In sum, the isotopic-geochronological dates evidence that the Pliocene magmatism of the Dzhavakheti Highland developed practically without essential breaks during the period of about 2 Ma long, from 3.75 to 1.75–1.55 Ma ago. The areal basic volcanism that was most widespread at that time is divisible into five discrete phases according to the isotopic dates obtained. Comparatively short pauses, which separated these phases of magmatic activity, were a few hundreds thousand years long, not more. Chemical composition of moderately acidic to silicic volcanics, which are of a limited distribution in the Dzhavakheti Highland, and their age relations with basic lavas of the region suggest that they are most likely the differentiation products of parental basic mantle-derived magmas. The analyzed distribution of volcanic centers, which erupted basic lavas of the Dzhavakheti Highland, evidence that first two phases of basic magmatism were connected here with volcanic activity in southwestern part of the region (northern termination of the Egnakhag Ridge), whereas activity of volcanoes situated on the east, predominantly in water-shed part and on slopes of the submeridional Dzhavakheti Ridge, controlled development of the third and fourth phases. Consequently, magmatic activity of the Pliocene stage in history of the Neogene-Quaternary magmatism of the Dzhavakheti Highland laterally migrated from the west to the east, being controlled by development of regional submeridional extension zones. Volcanic ridges marking the latter are formed by volcanic edifices, which are amalgamated at their bases and have erupted lavas of close age and composition. The migration of volcanic activity can be described in terms of the “domino effect,” when cessation of volcanism in one zone led to formation of the other submeridional zone of extension and magmatic activity displaced from the west eastward in sublatitudinal direction. In general, evolution of the Pliocene magmatism of the Dzhavakheti Highland, was similar, despite the essential regional peculiarities, to the generalized trend of magmatism evolution in the continental rifts and intraplate zones of the “hot-spot” type.     

拉萨地块南部得明顶地区叶巴组火山岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其成因

作为拉萨地块南部形成时代最早的火山岩,叶巴组火山岩对于确定新特提斯洋的俯冲过程及印度欧亚大陆碰撞前的演化等具有重要意义。前人的研究结果显示,叶巴组火山岩是一套岩性由基性到酸性连续的、具有岛弧地球化学特征的火山岩。目前在叶巴组火山岩中只有酸性岩获得了精确的锆石U-Pb年龄,显示其形成时代为早侏罗世。然而对叶巴组中基性火山岩目前尚无精确的年代学报道。另外,有关叶巴组火山岩形成的动力学背景,目前还存在争议。对拉萨地块南部得明顶地区的叶巴组火山岩进行了锆石U-Pb年龄和地球化学研究,获得其锆石U-Pb年龄为188.8±1.8Ma,表明与酸性火山岩形成时代一致。得明顶地区叶巴组火山岩地球化学数据显示其具有类似于岛弧火山岩的特征,富集大离子亲石元素、亏损高场强元素、轻重稀土元素分异明显,部分样品具有Sr及Eu(δEu=0.75~0.83)的略微异常。结合前人的研究成果认为,叶巴组火山岩很可能形成于大陆边缘弧,其形成与新特提斯洋的北向俯冲有紧密的联系。     

Geochemistry of basaltic lavas from the southern Lau Basin: input of compositionally variable subduction components

We present new major element, trace element, and Sr–Nd–Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e.g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr–Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher ⁸⁷Sr/⁸⁶Sr for a given ¹⁴³Nd/¹⁴⁴Nd ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.