Fumarolic activity of Avachinsky and Koryaksky volcanoes, Kamchatka, from 1993 to 1994

 Volcanic gas and condensate samples were collected in 1993–1994 from fumaroles of Koryaksky and Avachinsky, basaltic andesite volcanoes on the Kamchatka Peninsula near Petropavlovsk–Kamchatsky. The highest-temperature fumarolic discharges, 220  °C at Koryaksky and 473  °C at Avachinsky, are water-rich (940–985 mmol/mol of H₂O) and have chemical and isotopic characteristics typical of Kamchatka–Kurile, high- and medium-temperature volcanic gases. The temperature and chemical and water isotopic compositions of the Koryaksky gases have not changed during the past 11 years. They represent an approximate 2 : 1 mixture of magmatic and meteoric end members. Low-temperature, near-boiling-point discharges of Avachinsky Volcano are water poor (≈880 mmol/mol); Their compositions have not changed since the 1991 eruption, and are suggested to be derived from partially condensed magmatic gases at shallow depth. Based on a simple model involving mixing and single-step steam separation, low water and high CO₂ contents, as well as the observed Cl concentration and water isotopic composition in low-temperature discharges, are the result of near-surface boiling of a brine composed of the almost pure condensed magmatic gas. High methane content in low-temperature Avachinsky gases and the 220  °C Koryaksky fumarole, low C isotopic ratio in CO₂ at Koryaksky (–11.8‰), and water isotope data suggest that the "meteoric" end member contains considerable amounts of the regional methane-rich thermal water discovered in the vicinity of both volcanoes. Received: 2 May 1996 / Accepted: 5 November 1996     

Fissure swarms and fracture systems within the Western Volcanic Zone,Iceland – Effects of spreading rates

The Western Volcanic Zone (WVZ) in Iceland is ∼120 km long and 40 km wide. It offers an opportunity to study rift zones in a local ultra-slow spreading area close to a hotspot. Fractures were mapped from aerial photographs and digital elevation models. Most surface fractures are located in the southern part of the WVZ. The majority of the fractures have a north-northeasterly orientation, some deviations occur from this, especially in the north part of the WVZ. Fracture orientations are therefore quite uniform in the southern, faster spreading part of the WVZ, but more irregular in the slower-spreading northern part. This suggests different stress fields in the north part, which could be due to the influence of the Hreppar microplate and possibly also due to stress fields induced by crustal deformation because of changes in glacial load in the area. Such glacially-induced stress fields may have similar or even more influence than crustal spreading in the slower spreading northern part of the WVZ. Lower fracture density towards the north within the WVZ suggests lower frequency of rifting events in the north part, in accordance with less spreading in the north as measured by GPS geodetic measurements.     

粤东北仁差盆地早侏罗世花岗岩基底LA-ICP-MS锆石U-Pb年龄及其地质意义

钻探勘查揭露表明,粤东北仁差盆地基底主要由古生代变质岩和文象花岗岩组成。为进一步查明其基底岩体形成的时代,提高对该地区地质演化的认识,对盆地内2个勘查钻孔内的文象花岗岩岩心取样,进行LA-ICP-MS锆石U-Pb同位素定年,测得谐和年龄为179±1Ma和186Ma。此结果表明,仁差盆地基底文象花岗岩的形成时代为早侏罗世晚期,即其为燕山期第一幕岩浆活动的产物。此年龄不仅加深了对仁差盆地形成演化的认识,也为粤东北地区早侏罗世时期缺乏岩浆岩活动的报道提供了新的年代学资料。     

新疆甜水海地块种羊场石炭纪火山岩年代学和地球化学:岩石成因和地质意义

甜水海地块西段的种羊场地区发育一套互层状产出的玄武岩-玄武安山岩-流纹岩,本文对其进行了岩石学、同位素年代学和地球化学研究。结果表明,流纹岩LA-ICP-MS锆石U-Pb定年获得三组年龄:343.5±4.1Ma表明火山岩的形成时代为早石炭纪,2439±26Ma和1988±36Ma说明甜水海地块存在前寒武纪结晶基底。其中玄武质岩石岩性从拉斑系列、钙碱性系列向碱性系列过渡,呈现出E-MORB(OIB)、大陆板内拉张和岛弧的混合特征,与典型弧后盆地Okinawa玄武岩有一定的差异,表明其可能是异常陆缘弧后盆地拉张裂解的产物。玄武质岩石和流纹岩的主量元素、稀土元素和微量元素比值对的差异表明它们不是同源岩浆演化的产物,玄武质岩石的源区为类似E-MORB(OIB)的岩石圈地幔,且发生了部分熔融,原始岩浆上升过程中经历了矿物分离结晶和地壳混染作用。流纹岩属于高硅高碱的钙碱性火山岩,是上地壳部分熔融的产物。种羊场早石炭纪火山岩可能代表了古特提洋西端早期扩张的记录,为西昆仑-喀喇昆仑地区晚古生代多岛洋格局提供了新的证据。     

琼州海峡发现海底火山锥

琼州海峡位于雷州半岛和海南岛之间,其规模仅次于渤海海峡和台湾海峡,为我国第三大海峡。平均宽度约30km,最窄处仅19.5km,东西长约80km。海底普遍发育陡坎、深穴,地形非常复杂。特别是在海底地形图上,常见有一些略呈卵形的孤丘,引人注目。这种孤丘地貌,有的学者曾经怀疑是海底火山锥,甚至在     

Facies in an Upper Permian volcanic succession,Emmaville Volcanics,Deepwater, northeastern New South Wales

The Upper Permian Emmaville Volcanics at Deepwater, northeastern New South Wales, consist of a diverse succession of calc‐alkaline silicic‐intermediate ignimbrites, volcaniclastics and minor lavas. This 2.5 km‐thick sequence underlies and outcrops extensively along the northern margin of the Dundee Rhyodacite Outlier at Dundee. Detailed mapping and facies analysis have revealed eight locally mappable units namely; Magistrate Volcanic Member (rhyolitic ignimbrites), Wollundi Mudstone Member, Dellwood Ignimbrite Member, Marrawarra Rhyolite Member, Top‐Crossing Sandstone Member, Arranmor Ignimbrite Member, Yarramundi Andesite Member (lava, breccia) and Welcome Volcanic Member (rhyolitic ignimbrites). All volcanic units are contained in two fault‐bounded blocks of different lithology and structure. The volcanic succession ranges in composition from andesite to high‐silica rhyolite (58.6–78% SiO₂). Chemical characteristics include enrichment in K₂O (>3.5%), Al₂O₃ and large‐ion lithophile elements (LILE: Rb, K and light rare‐earth elements (LREE)), and depletion in high field strength elements (HFSE: Ti, Nb and Zr). These geochemical attributes reflect a continental subduction‐related signature. The facies architecture indicates that the principal volcanic features of the Late Permian palaeogeography in northeastern New South Wales was a topographically subdued depression flanked by low‐angle ignimbrite sheets with rhyolitic‐intermediate volcanic centres rising gently from the sloping terrain. The succession demonstrates that during the Late Permian andesitic volcanism was present, although localised. A modern analogue for the setting of the Emmaville Volcanics is the Quaternary Taupo Volcanic Zone (New Zealand).     

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO₂ contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO₂ contents and high Mg# (Mg# = 100 Mg/(Mg + Fe²⁺)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (⁸⁷Sr/⁸⁶Sr)_i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (⁸⁷Sr/⁸⁶Sr)_i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO₂ (77.63–82.52 wt.%) and low total Fe₂O₃, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.     

琼北地区北西方向长流-仙沟断裂带晚第四纪活动及与火山活动关系的讨论

琼北地区的火山活动以裂隙喷溢为主,晚更新世道堂期的射气岩浆喷发形成了众多的低平火山口,全新世雷虎岭期火山口主要分布于石山、永兴一带,沿NW向长流-仙沟断裂带分布。近2年在石山一带的射气岩浆喷发物中揭露出多条大规模的断裂,这些断裂带的单个断面虽然类似于地震活断层,但它们缺少断错地貌和断层方向的稳定性,一些断层组合成弧形。尽管这些断裂断面清晰,断距达4m,仍被认为是伴随火山喷发活动后期塌陷而形成的次级断层。此外,位于非火山岩分布区跨长流-仙沟断裂带的钻孔联合剖面探测表明,该断裂带在晚更新世晚期以来不活动。长流-仙沟断裂带晚更新世晚期以来的活动主要表现在作为深部岩浆的上涌通道。     

The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

A review of global stratospheric aerosol: Measurements, importance, life cycle, and local stratospheric aerosol

Stratospheric aerosol, noted after large volcanic eruptions since at least the late 1800s, were first measured in the late 1950s, with the modern continuous record beginning in the 1970s. Stratospheric aerosol, both volcanic and non-volcanic are sulfuric acid droplets with radii (concentrations) on the order of 0.1–0.5 µm (0.5–0.005 cm^− 3), increasing by factors of 2–4 (10–10³) after large volcanic eruptions. The source of the sulfur for the aerosol is either through direct injection from sulfur-rich volcanic eruptions, or from tropical injection of tropospheric air containing OCS, SO₂, and sulfate particles. The life cycle of non-volcanic stratospheric aerosol, consisting of photo-dissociation and oxidation of sulfur source gases, nucleation/condensation in the tropics, transport pole-ward and downward in the global planetary wave driven tropical pump, leads to a quasi steady state relative maximum in particle number concentration at around 20 km in the mid latitudes. Stratospheric aerosol have significant impacts on the Earth's radiation balance for several years following volcanic eruptions. Away from large eruptions, the direct radiation impact is small and well characterized; however, these particles also may play a role in the nucleation of near tropopause cirrus, and thus indirectly affect radiation. Stratospheric aerosol play a larger role in the chemical, particularly ozone, balance of the stratosphere. In the mid latitudes they interact with both nitrous oxides and chlorine reservoirs, thus indirectly affecting ozone. In the polar regions they provide condensation sites for polar stratospheric clouds which then provide the surfaces necessary to convert inactive to active chlorine leading to polar ozone loss. Until the mid 1990s the modern record has been dominated by three large sulfur-rich eruptions: Fuego (1974), El Chichón (1982) and Pinatubo (1991), thus definitive conclusions concerning the trend of non-volcanic stratospheric aerosol could only recently be made. Although anthropogenic emissions of SO₂ have changed somewhat over the past 30 years, the measurements during volcanically quiescent periods indicate no long term trend in non-volcanic stratospheric aerosol.