Stratigraphy,distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics,South Australia

Three widespread felsic volcanic units, the Eucarro Rhyolite, Pondanna Dacite Member and Moonaree Dacite Member, have been distinguished in the Mesoproterozoic Gawler Range Volcanics. These three units are the largest in the Gawler Range Volcanics, each in excess of 500 km³. Each unit is ～300 m thick and includes a black, formerly glassy base, a granophyric columnar‐jointed interior, and an amygdaloidal outer margin. The units are very gently dipping and locally separated by thin (<20 m) lenses of either ignimbrite (Mt Double Ignimbrite), tuffaceous sandstone or faults. The youngest unit, the Moonaree Dacite Member, covers a central area with a diameter greater than 80 km. The southern two units have east‐west extents in the order of 180 km, but are much less extensive from south to north (5–60 km). All three units are dominated by euhedral phenocrysts and are relatively crystal rich. Both the Eucarro Rhyolite and Moonaree Dacite Member contain clasts of basement granitoid and other lithologies and are locally heterogeneous in texture and composition. Some granitoid clasts have disintegrated, liberating feldspar and quartz crystals into the surrounding host. These liberated crystals cause textural variations, but can be identified on the basis of shape (amoeboid or skeletal) and/or size (megacrysts). Textural and lithofacies characteristics are consistent with the interpretation that these units are lavas; the strongly elongate distribution and wide extent of the Eucarro Rhyolite and Pondanna Dacite Member could indicate that vents were aligned along an extensive east‐west‐trending fissure system. Stratigraphic nomenclature has been revised to better reflect the presence of the three emplacement units. The oldest unit, the Eucarro Rhyolite, is dominated by plagioclase‐phyric rhyolite that locally includes granitoid clasts and megacrysts. Along the northern margin, the rhyolite is amygdaloidal and has mingled with a quartz‐rich rhyolite (Paney Rhyolite Member). The Eucarro Rhyolite and Paney Rhyolite Member replace the formerly defined ‘Eucarro Dacite’, ‘Nonning Rhyodacite’, ‘Yannabie Rhyodacite’ and ‘Paney Rhyolite’. The two younger units, Pondanna Dacite Member and Moonaree Dacite Member, are compositionally and spatially distinct, newly defined members of the Yardea Dacite.     

辽阳河栏地区辽河群中变质基性熔岩的锆石U-Pb年龄与形成构造背景

在海城－草河口的北辽河群中分布大量变质基性火山岩和基性岩墙,其中辽阳河栏镇南部大石桥组中的基性火山岩保存有良好的枕状构造。这些变质枕状熔岩化学成分上低碱低钾富铁,具有岛弧拉斑玄武岩的特征,岩石稀土和微量元素特征指示变质枕状熔岩形成于与岛弧相关的构造背景,很可能是弧后盆地环境的产物,而非大陆裂谷。锆石年代学研究显示：变质枕状熔岩的锆石SHRIMP U-Pb年龄为（1 869±28） Ma;变辉绿岩墙的形成年龄为（1 828±13） Ma,并有一组~2 080 Ma的捕获锆石年龄;花岗伟晶岩锆石U-Pb年龄为（1 869.7±7.6） Ma。结合岩石的产出状态和前人研究资料:推测变质枕状熔岩形成年龄为~1 880 Ma,可以代表北大石桥组的形成时代;花岗伟晶岩的年龄为~1 860 Ma,可作为造山作用结束的标志;变基性岩墙为~1 830 Ma,可作为后造山伸展张裂的标志。     

Exploration of the extension of two lava tubes,faults and dikes using very low frequency-electromagnetic technique in NE Jordan

The very low frequency-electromagnetic (VLF-EM) technique was used to delineate two sub-parallel lava tubes, faults and dikes in Umm El-Quttein area, NE Jordan. The investigation of the lava tubes was conducted through 22 VLF-EM profiles across lava strike; the length of profiles ranged from 700 to 1700 m. The lava tubes outcrop at two sites: Azzam cave and Al-Howa tunnel, characterized by slightly weathered basalt, columnar joints and fissure zones; qualitative interpretation of Fraser and Karous-Hjelt maps differentiate those zones as linear, elongated and circular anomalous zones. The 2-D tipper inversion of VLF-EM data and resistivity imaging had the potential to screen out three anomalous zones of likely resistivity contrast: the lava tube body with resistivity over 2500 Θ·m, the fractured zones with resistivity less than 500 Θ·m, and the host vesicular basalt with resistivity of 1500 Θ·m. The strike of lava tubes varied from SW to NE direction with depth less than 20 m and width from 10 to 30 m.     

Geology of the Side Crater of the Erebus volcano,Antarctica

The summit cone of the Erebus volcano contains two craters. The Main crater is roughly circular (∼ 500 m diameter) and contains an active persistent phonolite lava lake ∼ 200 m below the summit rim. The Side Crater is adjacent to the southwestern rim of the Main Crater. It is a smaller spoon-shaped Crater (250–350 m diameter, 50–100 m deep) and is inactive. The floor of the Side Crater is covered by snow/ice, volcanic colluvium or weakly developed volcanic soil in geothermal areas (a.k.a. warm ground). But in several places the walls of the Side Crater provide extensive vertical exposure of rock which offers an insight into the recent eruptive history of Erebus. The deposits consist of lava flows with subordinate volcanoclastic lithologies. Four lithostratigraphic units are described: SC 1 is a compound lava with complex internal flow fabrics; SC 2 consists of interbedded vitric lavas, autoclastic and pyroclastic breccias; SC 3 is a thick sequence of thin lavas with minor autoclastic breccias; SC 4 is a pyroclastic fall deposit containing large scoriaceous lava bombs in a matrix composed primarily of juvenile lapilli-sized pyroclasts. Ash-sized pyroclasts from SC 4 consist of two morphologic types, spongy and blocky, indicating a mixed strombolian-phreatomagmatic origin. All of the deposits are phonolitic and contain anorthoclase feldspar.     

Lava-flow hazard on the SE flank of Mt. Etna (Southern Italy)

A method for mapping lava-flow hazard on the SE flank of Mt. Etna (Sicily, Southern Italy) by applying the Cellular Automata model SCIARA_-fv is described, together with employed techniques of calibration and validation through a parallel Genetic Algorithm. The study area is partly urbanised; it has repeatedly been affected by lava flows from flank eruptions in historical time, and shows evidence of a dominant SSE-trending fracture system. Moreover, a dormant deep-seated gravitational deformation, associated with a larger volcano-tectonic phenomenon, affects the whole south-eastern flank of the volcano.     

Morphology of rubbly pahoehoe (simple) flows from the Deccan Volcanic Province: Implications for style of emplacement

Lava flows with preserved bases and brecciated upper crusts constitute a morphological type that differs in character from typical pahoehoe and a'a: such flows have been reported from many provinces around the world. Previous studies had referred to these flows informally as ‘pahoehoe flows with rubbly tops’, ‘broken-top pahoehoe’ and ‘rubbly pahoehoe’. Recent studies have formally applied the latter term to describe parts of the well-studied Laki flow in Iceland as well as flows from the Columbia River Basalt province. Rubbly pahoehoe flows are abundant in the upper stratigraphic formations of the Deccan Volcanic Province (DVP), and are more commonly known as simple flows. This study presents detailed observations of such flows from various parts of the DVP and discusses their implications for understanding flow emplacement. These flows, which appear to be single units at the outcrop-scale, are generally much thicker and significantly more extensive than individual pahoehoe lobes that dominate the lower formations of the Deccan stratigraphy. They are characterised by preserved, gently undulating tachylitic bases but variably disrupted crustal zones that grade into flow-top breccias. The breccias are constituted of highly vesicular and oxidised fragments of varying sizes that appear to have been derived from previously formed pahoehoe crusts. Previous work has indicated that the morphology of these flows might be related to initial inflation, accompanied by rapid volatile exsolution and an increase in effusion rate and/or viscosity with time. This agrees reasonably well with the qualitative and quantitative models of emplacement developed for the Laki flow. The abundance of such flows in the upper formations of the Deccan stratigraphy clearly hints at a significant shift in the nature of the Deccan eruptions; this could be indicative of higher eruption rates during this period. This, in turn, raises the possibility of hazardous impact on the climate during the eruption of these flows, which is also discussed in the paper.     

Probing the magma plumbing of Erebus volcano,Antarctica, by open-path FTIR spectroscopy of gas emissions

Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H₂O, CO₂, CO, SO₂, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO₂ and H₂O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H₂O/CO₂ proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO₂ flux determinations and measured gas ratios. In the case of CO₂ and water, ∼ 1 and ∼ 0.4 m³ s^− 1, respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.     

遥感技术在长白山天池火山熔岩流表壳类型结构研究的可行性分析

溢流式熔岩喷发是火山喷发的一种常见形式,容易造成火山灾害.从火口或裂隙呈液态喷发出的溶岩,沿地表向前流动,熔岩温度逐渐降低,黏度随之增大,流动速度逐渐减慢,直至停滞、凝结.炙热熔岩流在流动过程中,会对沿途及周边植被、建筑物和人员等造成破坏和伤亡.Andrew和Rowland(2015)对aa和pahoehoe的力学原理...     

膏盐层在矽卡岩型铁矿成矿中的作用

矽卡岩型铁矿是我国富铁矿的重要类型,约占全国富铁矿总储量的60%。虽然膏盐层与矽卡岩型铁矿的关系已引起部分矿床学家的关注,但膏盐层的控矿机制尚不清楚。本文以我国最重要的大冶式和邯邢式矽卡岩型铁矿为例,探讨了膏盐层在矽卡岩型铁矿成矿中的作用。膏盐层富含碳酸盐、石膏和石盐等,不仅可以为成矿提供大量Na⁺、Cl^-、CO3²⁺等矿化剂,使围岩发生钠长石化、方柱石化（氯化）和矽卡岩化等蚀变,使Fe²⁺以NaFe-Cl等络合物形式搬运,膏盐层还是地壳深处最重要的氧化障,能够将硅酸盐熔体和成矿溶液中的Fe²⁺氧化成Fe³⁺,富集形成铁矿床,是矽卡岩型铁矿成矿的关键因素。大冶地区的膏盐层属于中三叠统下部的嘉陵江组,邯邢地区的膏盐层属于中奥陶统马家沟组—峰峰组。大冶和邯邢式矽卡岩型铁矿中硫化物的δ³⁴S_V-CDT值异常高,计算结果表明矿床中约80%的硫来自膏盐层硫酸盐的还原,还原温度多在500℃以上,但硫化物的沉淀温度相对较低,就位时间稍晚;硫酸盐的δ³⁴S_V-CDT值和还原温度越高,硫化物的δ³⁴S_V-CDT值越高,原始岩浆硫所占比例越高,硫化物的δ³⁴S_V-CDT值越低。当炽热的岩浆与膏盐层（CaSO₄）发生同化混染时,SO₄^2-将硅酸盐熔体中的Fe²⁺氧化成Fe³⁺,Fe³⁺无法进入硅酸盐矿物晶格,而形成Fe₃O₄/Fe₂O₃进入熔体,铁氧化物在磷、氯化钠、水等挥发分和盐类物质的作用下在岩浆房中与硅酸盐熔体发生不混熔,形成铁矿浆,沿构造有利部位贯入,形成矿浆型铁矿床。在矽卡岩型铁矿床中,矿浆充填型和热液交代型矿体同时存在,二者在空间上具有一定的分带性,有时渐变过渡,矿浆充填型铁矿体通常位于深部靠近成矿岩体的部位,而热液交代型铁矿体位于成矿流体运移的前方。在SO₄^2-氧化Fe²⁺的同时自身被还原为S^2-,与Fe²⁺结合形成硫铁矿,分布在铁矿的上部或边部。     

内蒙古正镶白旗碎斑熔岩长石特征及其岩石学意义

正镶白旗碎斑熔岩是一种特殊成因的火山岩,是岩浆沿火山通道侵出地表的产物。碎斑熔岩中钾长石主要为低透长石、高正长石、中正长石和中正微长石;斜长石则以更长石为主,其有序度低,指示了岩石高温火山成因的特征。从岩体边缘相到中心相至根部相,长石类型、化学成分和有序度具有一定的变化规律,反映了岩体不同岩相带成岩条件的差异性。