辽宁省建平县大梁前膨润土矿地质特征及成矿规律初步研究

本区膨润土矿赋存层位为中生界白垩系义县组,成矿时代为白垩纪,成矿母岩为中性～中酸性凝灰岩、流纹岩,岩相古地理属湖泊相沉积。膨润土矿床成因类型为火山～蚀变～沉积型。     

广西合浦县白沙良港膨润土矿地质特征及找矿标志

广西合浦县白沙矿区良港矿段膨润土矿赋存于晚白垩系火山喷出岩中,矿体产出受地层及火山岩的控制,矿床的形成与火山岩活动及其风化水解作用有密切关系。通过对区域背景、矿区地质及矿体特征等进行分析,介绍本矿床地质特征及成矿模式,指出该类型膨润土矿的找矿标志。     

我国与火山岩有关的大型、超大型银矿床

通过对我国西南“三江”地区嘎村与老厂两个大型海相火山岩型银矿床和我国东部及新疆天山地区陆相火山岩-次火山岩型银矿的地质地球化学研究,认为海相火山岩型银矿床主要产于岛弧、裂谷构造环境的次级火山-沉积盆地中,矿床具有明显的分带现象,下部为脉状-网脉状矿化,上部为产于热水沉积岩中的层状矿化(主矿体),从地表向深部成矿元素为As(Au)→Ag,Pb,Zn→Cu,嘎村矿床的层状矿体全岩Rb-Sr同位素等时线年龄为(204±14)Ma,主成矿期为晚三叠世。陆相火山-次火山岩(斑岩)型银矿床银矿形成时代多晚于围岩,矿石构造以浸染状、细脉浸染状、网脉状和团块状为主。并详细地研究了陆相火山岩-次火山岩型银矿床的矿床地球化学特征和银矿床或银与萤石、卤素元素、锰的关系。     

我国与火山岩有关的大型、超大型银矿床

通过对我国西南“三江”地区嘎村与老厂两个大型海相火山岩型银矿床和我国东部及新疆天山地区陆相火山岩次火山岩型银矿的地质地球化学研究,认为海相火山岩型银矿床主要产于岛弧、裂谷构造环境的次级火山-沉积盆地中,矿床具有明显的分带现象,下部为脉状-网脉状矿化,上部为产于热水沉积岩中的层状矿化(主矿体),从地表向深部成矿元素为As(Au)→Ag,Pb,Zn→Cu,嘎村矿床的层状矿体全岩Rb-Sr同位素等时线年龄为(204±14)Ma,主成矿期为晚三叠世.陆相火山-次火山岩(斑岩)型银矿床银矿形成时代多晚于围岩,矿石构造以浸染状、细脉浸染状、网脉状和团块状为主并详细地研究了陆相火山岩-次火山岩型银矿床的矿床地球化学特征和银矿床或银与萤石、卤素元素、锰的关系.     

长江中、下游铜矿床的成矿模式

长江中、下游铜矿床主要为斑岩型、夕卡岩型及块状硫化物型矿床.文中从成矿地质背景、矿床类型及成矿作用等三方面讨论铜矿床的成矿模式,指出成矿热液主要来自岩浆水;夕卡岩型矿床及块状硫化物型矿床的形成与斑岩型矿床的形成一致,主要与中酸性侵入岩有关.     

中国东南沿海大（中）型浅成中低温热液矿床——斑岩型矿床评价 …

本文总结中国东南沿海高钾钙碱性－双峰式火山岩带中已勘查大中型矿床成矿环境的共性；矿床所处区域构造的部位，成岩与成矿时代，矿床与岩浆成因类型，火山构造及其基底构造控矿性，矿床与爆发角砾岩，矿床与矿化类型叠加与共生，矿化与蚀变的分带性、矿床定位深度与剥蚀深度。作者认为这八点可作为找寻与评价大（中）型矿床的地质准则。通过火山地质与矿床地质统一的研究提出本区晚中生代以火山为中心地热体系的成矿模式。并就三个     

西羌塘第三纪钠质基性火山岩的地球化学特征及成因探讨

西羌塘地区火山岩在时、空及地球化学性质上存在规律性变化：即从第三纪早期的富钠熔岩(始新世) 向中期的中性钾质熔岩(晚中新世)到最晚期的酸性次火山岩(上新世以后), 同时有Sr, Pb同位素的递增和Nd同位素递减的趋势. 富钠质基性火山岩出现的重要性表示了高原隆起前岩石圈的深部变化和源区的差异, 反映了岩石圈深部地质过程的不同特征. 隆起前的富钠熔岩为陆下软流圈原始地幔部分熔融, 可能与板内地幔柱的活动有关.     

粤北下庄矿田小水铀矿床地球化学特征分析

粤北小水矿床是下庄矿田典型的交点型铀矿床。通过对矿床内发育的花岗岩、辉绿岩及铀矿石采样分析．发现小水矿床矿石具有轻稀土富集的稀土元素特征和富集大离子亲石元素Rb、Th的微量元素特征。与辉绿岩的相应特征十分吻合,与花岗岩的相应特征差异很大,推测其成矿物质来源于深部地幔流体（富含U、F、CO2）,且交点型矿石的形成很有可能伴有幔源成矿流体对与花岗岩有关的早期红化矿石的叠加改造作用。     

青海省查查香卡地区晚三叠世火山岩岩石学及其构造环境

通过对查查香卡地区晚三叠世火山岩岩石学特征、空间分布、形态、火山机构、火山岩与构造关系的研究,确定该套火山岩时代为晚三叠世。岩石地层单位为鄂拉山组,为一套陆相喷发火山岩,呈NW向展布于大海滩-都库隆瓦地区。以中-高钾、高钙、低钛为特征,属钙碱性系列。火山喷发活动由强到弱,岩性由中性向中酸性渐变,岩浆活动由喷发型向侵入型递进。表明岩石构造环境为陆内消减带火山岩中的造山区。是来自地壳下部的火山岩浆经分异结晶并在上涌过程中混入有上地壳物质而喷发形成。喷出时的大地构造环境为陆内造山环境,该火山岩最初可能形成于大陆边缘环境,由于A型俯冲构造活动,测区乃至鄂拉山地区产生一系列右旋走滑断裂带,受NW向右旋走滑断裂的影响,岩石孔隙加大,并出现强烈的热流活动,导致岩浆沿这些断裂带喷出地表。     

470铀矿床地球化学特征及成矿机理探讨

470火山岩型钿矿床主要有两种铀矿化类型，即铁－铀型和铀－铝型。含矿主岩主要为粗面岩，具有独特的“麻子构造”；通过对矿床岩石化学、共生及伴生元素、稀土元素、稳定同素、成矿温度、成矿年龄等地球化学特征研究，分析了矿床的成矿机理，建立了铀成矿地球化学模式。     

Genesis of the Nyiragongo lavas

Mt. Nyiragongo is one of the eight major volcanoes of the large Virunga volcanic field in the Lake Kivu area in the Eastern Congo. The lavas of Nyiragongo are rather unique. Starting from the top of the mountain, the rocks are nephelinites with some leucite and melilite. The molten material of the present-day lava lake belongs to this type of lava. Under the nephelinites, there is a thin series of leucite-rich lava beds. The main part of the volcano consists of bergalitic melitite lavas alternating with pyroclastics of similar composition. The nephelinitic material is considered to constitute the main portion of the pre-volcanic magma under the future volcano. It is pointed out that the Nyiragongo represents just the type of volcano with which the African volcanic carbonatites are associated. It is concluded that the Nyiragongo nephelinite must be interpreted in a way accepted for the Central African volcanic nephelinites in general. The bergalitic melilitite material is interpreted as a product of carbonation of the nephelinitic magma.     

Acid hyaloclastites

In exceptional cases also acid hyaloclastites can be originated by submarine extrusion of acid lavas. Acid hyaloclastites — which hitherto were not known — could be found for the first time on the western Ponza-Islands/Tyrrhenian Sea, Italy. The formation of acid hyaloclastites is closely connected with the process of auto-brecciation proceeding during the extrusion of acid lavas; both processes work together and cause the disintegration of parts of extruding lavabodies (domes etc.) into a mass of volcano-clastic materials. Hyaloclastization that is both the chilling of fluid lava in contact with water, ice or water bearing country rocks and the shattering of the congealed glassy skin by the pressure of the inflowing lava. Auto-brecciation that is destruction of parts of the extruding mass, not only by movements proceeding during the slow cruption but also by explosive phenomena depending on the release of the vapors. Hyaloclastization and auto-brecciation of acid lavas are an example of convergency: through both processes more or less similar deposits are formed. Thus brecciated acid hyaloclastites are not to be distinguished from auto-breccias. Two occurrences of brecciated lavas in California are interpreted as acid and intermediate hyaloclastites. Both are originated by the intrusion of rhyolitic respectively of andesitic lavas into water-bearing country rocks. In particularity for Iceland the possibility is discussed, that the formation of acid hyaloclastites also may occur in case of subglacial eruptions of acid lavas.     

Volcanology of the 2350 B.P. Eruption of Mount Meager Volcanic Complex, British Columbia, Canada: implications for Hazards from Eruptions in Topographically Complex Terrain

 The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total volume of 10⁹ m³ and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated to the 2350 B.P. eruption, although the post-eruption avalanche may have its origins in the over-steepened slopes created by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results from deposition in a narrow, steep-sided mountain valley containing a major river. Received: 20 January 1998 / Accepted: 29 September 1998     

Paleozoic submarine volcanoes in the high-P/T metamorphosed Chichibu system of Eastern Shikoku, Japan

The Sawadani greenstone in the Chichibu Paleozoic System is an ancient submarine volcanic complex consisting of pillow lavas and hyaloclastites. The volcanism is divided into two periods. Alkali basalt was erupted in the first period and two shield-shaped cones were formed. After a period of dormancy the volcanism of the second period took place and a cone was formed by eruptions of lavas ranging in composition from mildly alkaline to tholeiitic basalt. The top of the volcano nearly reached the sea surface and was finally about 3.7 km above the base. A limestone cap and volcanic conglomerate were deposited on the summit. The base rests conformably on upper Carboniferous sandstone and subordinate mudstone derived from a continent or mature island arc. Many feeding channels of lava cut the volcanic body and underlying sedimentary formation. Sedimentation proceeded concurrently on the surrounding sea floor, so that volcanic and sedimentary material is interlayered.The Sawadani greenstone, although it occurs in the high-P/T metamorphic belt, is not believed to be a fragment of oceanic crust (ophiolite complex) formed by oceanic ridge volcanism and later carried into a convergent zone. It is a seamount formed on and within a sedimentary sequence near a continent or island arc. The magma changed from alkaline to tholeittic as the volcano grew.It cannot be assumed that all metavolcanic rocks formed in high-pressure metamorphic terranes are fragments of oceanic crust.     

西昆仑阿什火山机构及岩石学、矿物学特征

阿什库勒盆地位于NE向阿尔金断裂与NW向康西瓦断裂的"弧形"交会处,构造活动十分活跃,盆地内发育10余座火山,其中阿什火山为该火山群中最新活动的火山。文中从火山地质、熔岩和斑晶成分、显微结构特征及地质温压计4个方面对阿什火山进行了详细研究。结果表明,阿什火山由火山锥和熔岩流组成,锥体由早期的渣锥和晚期的溅落锥组成,熔岩流分布面积约33km2,可划分为4个流动单元。熔岩属于钾玄岩系列,岩性为粗安岩,显微镜下呈斑状结构。斑晶以长石(主要为中长石)和辉石(包括普通辉石、古铜辉石和紫苏辉石)为主;基质为玻璃质、隐晶质、微晶质,部分含有大量的长石和辉石。斑晶与岩浆的平衡温度为1 104~1 194℃,压力为570~980MPa,对应的岩浆房深度为18.92~32.29km。     

Basanite glaciovolcanism at Llangorse mountain,northern British Columbia,Canada

The Llangorse volcanic field is located in northwest British Columbia, Canada, and comprises erosional remnants of Miocene to Holocene volcanic edifices, lava flows or dykes. The focus of this study is a single overthickened, 100-m-thick-valley-filling lava flow that is Middle-Pleistocene in age and located immediately south of Llangorse Mountain. The lava flow is basanitic in composition and contains mantle-derived peridotite xenoliths. The lava directly overlies a sequence of poorly sorted, crudely bedded volcaniclastic debris-flow sediments. The debris flow deposits contain a diverse suite of clast types, including angular clasts of basanite lava, blocks of peridotite coated by basanite, and rounded boulders of granodiorite. Many of the basanite clasts have been palagonitized. The presence and abundance of clasts of vesicular to scoriaceous, palagonitized basanite and peridotite suggest that the debris flows are syngenetic to the overlying lava flow and sampled the same volcanic vent during the early stages of eruption. They may represent lahars or outburst floods related to melting of a snow pack or ice cap during the eruption. The debris flows were water-saturated when deposited. The rapid subsequent emplacement of a thick basanite flow over the sediments heated pore fluids to at least 80–100°C causing in-situ palagonitization of glassy basanite clasts within the sediments. The over-thickened nature of the Llangorse Mountain lavas suggests ponding of the lava against a down-stream barrier. The distribution of similar-aged glaciovolcanic features in the cordillera suggests the possibility that the barrier was a lower-elevation, valley-wide ice-sheet.     

Elemental fractionation in lavas during post-eruptive degassing: Evidence from trachytic lavas,Rishiri Volcano,Japan

Trachytic lavas of Rishiri Volcano, northern Japan, show a peculiar geochemical variation across lava flow units. Samples collected systematically in a vertical cross section from a lava flow unit with a thickness of about 20 m are nearly homogeneous in major element compositions. However, some trace elements, including Li, B and Cs, are considerably depleted in samples collected from the main part of the flow unit, compared to those obtained from the surface of the lava flow (clinker layer). In particular, Cs content of the main flow unit is as low as ∼30% of the clinker layer. ¹¹B / ¹⁰B ratios of samples from the main flow unit are also slightly lower than those of the clinker samples, and the isotope compositions positively correlate with boron concentrations. These geochemical variations cannot be explained by magmatic processes in magma chambers, post-eruptive weathering, or alteration process. Rather, we infer these systematics resulted from escape of these elements from the lava flow unit during post-eruptive degassing. Vapor phases in which Li, B and Cs dissolved are suggested to have been transported through veins formed in the main flow unit as fractures due to slight shearing along the flow planes after lava emplacement. In the Tanetomi lava, only rocks of the clinker layer preserve original composition of magmas, although they are porous and brownish due to extensive oxidization. On the other hand, rocks of the main flow unit do not retain original magma compositions, although they are dense and grayish, and seem to be much fresher compared to the clinkers. A similar geochemical modification of lavas can occur in other volcanic systems, especially for lavas consisting of relatively thick flow units.     

Distinguishing strongly rheomorphic tuffs from extensive silicic lavas

High-temperature silicic volcanic rocks, including strongly rheomorphic tuffs and extensive silicic lavas, have recently been recognized to be abundant in the geologic record. However, their mechanisms of eruption and emplacement are still controversial, and traditional criteria used to distinguish conventional ash-flow tuffs from silicic lavas largely fail to distinguish the high-temperature versions. We suggest the following criteria, ordered in decreasing ease of identification, to distinguish strongly rheomorphic tuffs from extensive silicic lavas: (1) the character of basal deposits; (2) the nature of distal parts of flows; (3) the relationship of units to pre-existing topography; and (4) the type of source. As a result of quenching against the ground, basal deposits best preserve primary features, can be observed in single outcrops, and do not require knowing the full extent of a unit. Lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix; such deposits are unique to lavas. Because the chilled base of an ashflow tuff generally does not participate in secondary flow, primary pyroclastic features are best preserved there. Massive, flow-banded bases are more consistent with a lava than a pyroclastic origin. Lavas are thick to their margins and have steep, abrupt flow fronts. Ashflow tuffs thin to no more than a few meters at their distal ends, where they generally do not show any secondary flow features. Lavas are stopped by topographic barriers unless the flow is much thicker than the barrier. Ash-flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit. Lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents. Tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits. These criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava-like ignimbrite. The latter have characteristics of lavas except for great areal extents, up to many tens of kilometers. These rocks have been interpreted as ash-flow tuffs that formed from low, boiling-over eruption columns, based almost entirely on their great extents and the belief that silicic lavas could not flow such distances. However, we interpret the best known examples of lava-like ignimbrites to be lavas. This interpretation should be tested through additional documentation of their characteristics and research on the boiling-over eruption mechanism and the kinds of deposits it can produce. Flow bands, flow folds, ramps, elongate vesicles, and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic. Pumice and shards also occur in both tuffs and lavas, although they occur throughout ash-flow tuffs and generally only in marginal breccias of lavas. Dense welding, secondary flow, and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick, rheomorphic tuffs and thick lavas. High-temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts. Extensive silicic lavas could be appropriately termed flood rhyolites.     

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km³ in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km³, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.