准噶尔北缘中泥盆统北塔山组富辉橄玄岩的岩石成因及其对晚古生代洋脊俯冲的启示

准噶尔北缘中泥盆统北塔山组富辉橄玄岩是一种富含单斜辉石斑晶的岛弧玄武岩,被认为是洋脊俯冲作用的产物,但具体成因机制并不清楚。本文对老山口地区富辉橄玄岩进行了岩石地球化学研究,同时系统测定了其单斜辉石斑晶的成分及其环带变化规律。结果表明,富辉橄玄岩的相容元素和不相容元素的变化规律具有岩浆混合作用的特征,而透辉石斑晶的反环带则将岩浆混合作用限定于高Mg~#熔浆的上涌过程。富辉橄玄岩极高的相容元素含量将上涌的高Mg~#熔浆限定为苦橄质熔浆,而富辉橄玄岩中顽透辉石是被苦橄质熔体携带而来的斑晶物质。主导老山口富辉橄玄岩形成的岩浆过程是苦橄质熔浆的连续补给过程,而苦橄质熔浆被认为是板片窗处软流圈高程度部分熔融的产物。老山口富辉橄玄岩形成的岩浆过程与洋脊俯冲的板片窗模型吻合,因此老山口富辉橄玄岩可能是洋脊俯冲的产物。     

新疆巴楚瓦吉里塔格金伯利岩矿物中岩浆包裹体研究

笔者在代表金伯利质熔浆结晶演化产物的橄榄石巨斑、金云母斑晶和磷灰石粗晶中首次发现了岩浆包裹体。捕晶角闪石、单斜辉石及橄榄石中岩浆包裹体研究证实,其源于基性-超基性杂岩。依据充填物相态和成分,将本区岩浆包裹体划分为两类:A,硅酸盐熔融包裹体;B,流体熔融包裹体,前者又细分出一个亚类(A′),即晶体-玻璃包裹体。经岩浆包裹体均一化温度值的测定和压力估算,原始金伯利质熔浆晶出巨斑橄榄石的温度略高于1116℃,压力约4.5GPa。     

新疆巴楚瓦吉里塔格金伯利岩矿物中岩浆包裹体研究

笔者在代表金伯利质熔浆结晶演化产物的橄榄石巨斑、金云母斑晶和磷灰石精晶中首次发现了岩浆包裹体。捕晶角闪石、单斜辉石及橄榄石中岩浆包裹体研宄证实,其源于基性-超基性杂岩。依据充填物相态和成分,将本区岩浆包裹体划分为两类:A,硅酸盐熔融包裹体;B,流体熔融包裹体,前者又细分出一个亚炎(A′),即晶体-玻璃包裹体。经岩浆包裹体均一化温度值的测定和压力估算,原始金伯利质熔浆晶出巨斑橄榄石的温度略高于1116℃,压力约4.5GPa。     

新疆东天山土墩基性-超基性杂岩体橄榄石成分特征及其成因意义

橄榄石通常是玄武质岩浆最早结晶出的矿物之一,其化学成分可以很好地反演母岩浆成分、岩浆结晶分异、硫化物熔离等成岩及成矿信息。本文以土墩镁铁质-超镁铁质杂岩体为研究对象,采用电子探针对岩体中的橄榄石矿物颗粒进行化学成分测试。利用橄榄石的Fo值和其中Ni含量,计算得到土墩杂岩体母岩浆中Mg O含量约为12.95%,是一种富镁的玄武质岩浆。同时,定量模拟结果表明,土墩杂岩体母岩浆中硫化物熔离几乎与橄榄石结晶作用同时进行,早阶段由橄榄石结晶(分离结晶程度约2%)而导致硫化物的熔离程度为0.2%。随后,橄榄石分离结晶程度在6%~7%时,硫化物熔体的熔离程度仅为0.01%。这些表明土墩杂岩体发生过一定程度的硫化物熔离,但成矿前景不是很好。此外,部分数据显示出Ni-Fo的负相关性,表明少许富铁橄榄石和晶间硫化物熔浆发生了Fe-Ni物质交换反应,这对橄榄石的成分有重要影响。     

甘肃北山黑山岩浆铜镍硫化物矿床橄榄石特征及成因意义

黑山铜镍硫化物矿床是近年在甘肃北山发现的大型岩浆铜镍硫化物矿床,含矿岩体主要由含矿橄榄岩相和南部边缘的角闪辉长岩相构成。研究发现含矿岩体中的橄榄石属贵橄榄石(Fo值为81.54~86.87),其w(Ni)介于(801.53~2 703.19)×10-6;利用橄榄辉长岩中最高Fo值和主量元素反演,表明原始岩浆属高镁玄武质岩浆,w(MgO)=11.65%,w(FeO)=10.12%;橄榄石分离结晶模拟计算结果表明,橄榄石结晶过程中伴随有0.12%~0.17%硫化物熔离,深部岩浆房中橄榄石分离结晶程度小于3%,橄榄石与硫化物最小质量比约14∶1;隙间硅酸盐熔浆和硫化物熔浆作用明显,是造成早期结晶橄榄石成分变化的重要原因。     

用斑晶-包体法代替斑晶-基质法测定矿物元素的分配系数

测定矿物元素分配系数的方法之一是斑晶-基质法。通过进一步的分析研究,发现该方法有悖于分配定律,虽说斑晶和基质在空间上有紧密的联系,但斑晶是岩浆早期阶段的产物,即在岩浆库内就已品出,而基质则是由晚期残余粒间熔浆经过冷凝形成的,残余熔浆经过了较长时间的演化(包括结晶分异、熔离和去气作用等),因此它与早期斑晶并不处在同一个平衡体系。在许多火山岩、次火山岩斑晶(石英、长石)甚至副矿物(锆石)中可以见到大量岩浆包裹体,它们代表斑晶结晶时捕获的早期初始岩浆。有时我们甚至可以在同一个石英斑晶中既见到岩浆包裹体又见到锆石晶体,因此我们有充足的理由认为,斑晶与其捕获的初始岩浆     

安徽省绩溪黑云母二长花岗岩中高铁铁橄榄石大斑晶的研究

高铁铁橄榄石大斑晶产在安徽省绩溪县黑云母二长花岗岩中。岩石呈细粒花岗结构,矿物成分为条纹长石、更长石、石英和黑云母。大斑晶最大粒径可达5cm,其中含有微小的磁铁矿自形晶,呈浸染状或断续脉状。在表生条件下,高铁铁橄榄石局部氧化成褐铁矿。大斑晶的外形、颜色和光泽等酷似黑钨矿。 大斑晶形成于上地幔高温高压条件下,先从花岗岩浆中结晶出铁橄榄石的高压变晶,岩浆沿深断裂向上部运移过程中,随着氧分压的增加而氧化成高铁铁橄榄石。     

硫化物熔离对岩浆硫化物含矿岩体中橄榄石Ni含量的影响——以金川岩体为例

为探讨硫化物熔离对含矿岩体中橄榄石Ni含量的影响,在前人橄榄石结晶硫化物熔离模型的基础上,定量模似计算了分离结晶过程中橄榄石Ni含量,并将其应用于金川橄榄石成园研究。研究表明,部分橄榄石落于无硫化物熔离橄榄石结晶趋势线下方,暗示其母岩浆为S饱和。根据模拟计算S饱和母岩浆橄榄石分离结晶趋势线,指出金川深部岩浆房中母岩浆橄榄石的分离结晶程度小于或等于3％,而由橄榄石结晶所导致熔离的硫化物熔体与橄榄石之间质量比约为40。     

东天山二红洼岩体橄榄石成分对CuNi-VTiFe复合矿化的启示

对二红洼镁铁-超镁铁岩体Cu-Ni硫化物和Fe-Ti氧化物复合矿化的岩浆作用过程的研究有助于探讨CuNi-VTiFe复合成矿作用的机制.首先通过对两个侵入期次不同侵入相的橄榄石进行了系统的电子探针分析,其次,对岩浆作用过程的分异演化程度和硫化物熔离程度进行了模拟.研究表明,二红洼岩体的母岩浆在深部岩浆房过程中并未发生硫化物熔离作用,其中第一期岩浆作用演化程度较低,橄榄石Fo值为77.0~83.4,岩浆在运移或者就位过程中约占橄榄石1/140~1/25的硫化物熔离,造成该岩体Cu-Ni硫化物矿化;第二期岩浆作用是深部岩浆房高度分异演化的产物,橄榄石Fo值为61.1~66.7,高度的分异演化作用使Fe-Ti等成矿物质在残余岩浆中富集.两期不同演化程度的基性-超基性熔浆的先后侵位造成了二红洼岩体Cu-Ni硫化物和Fe-Ti氧化物的复合矿化.     

论金川硫化铜镍矿床成因

本文在以往调查、勘探和综合研究的基础上,通过我国金川硫化铜镍矿床大量地质、地球化学资料的分析,论证了该矿床的成因。认为:1)该矿床的形成与多相熔体多次侵入,复合有关,第一、二次侵入是携带硫化物液滴和橄榄石晶粒的硅酸盐岩浆;第三次侵入的是运载大量榄榄石晶粒的硫化物矿浆;第四次侵入的是杂质很少的硫化物矿浆。2)各次所侵入的多相熔浆流之间具有密切的亲缘关系,它们是同一股含矿岩浆在过渡岩浆房中熔离—结晶—重力分异的产物。3)成矿母体为源自大陆地幔的含矿拉斑玄武质苦橄岩浆。4)成矿构造环境属克拉通区断裂带。     

Kalsilite in the lavas of Mt. Nyiragongo (Belgian Congo)

A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nepheline. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K₃NaAl₄Si₄O₁₆. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the ‘giant’ leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nepheline phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution.     

Kalsilite in the lavas of Mt. Nyiragongo (Belgian Congo)

A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nephe-line. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K₃NaAl₄Si₄O₁₆. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the ‘giant’ leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nephe-line phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution.     

Mineral chemistry,crystallization conditions and magma mixing‐mingling at Orduzu volcano (Malatya), Eastern Anatolia,Turkey

The Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley & Sons, Ltd.     

Recognition,Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites,Dongargarh Supergroup,Central India

A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the extensive silicic lava flow has been overwhelming in the recent literature. However, a rethinking, after Walker (1983), has brought into light the concept of ‘grade’ referring to the degree and extent of welding between the pyroclasts. Various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been made to suggest extensive occurrence of high-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrysts and degree of stretching. The presence of vitroclastic texture, melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies — A, B, C and D from bottom to the top respectively, have been identified from field studies around Salekasa. Facies-A and B represent clast-supported/matrix-supported welded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia carapace is represented by facies D. A complete gradation of facies A/B to D through C exists. High to extremely high-grade nature of welding in these deposits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.     

相山碎斑熔岩岩石特征及其成因探讨

相山主体岩石碎斑熔岩在岩石成因、岩相归属上长期以来有多种不同的看法,原因在于碎斑熔岩边缘相具有熔结凝灰岩甚至是凝灰岩的特点,过渡相具有熔岩的特征,中间相具有潜火山岩的特征,并且各种相之间呈渐变过渡关系。笔者从碎斑熔岩的岩石矿物组成,稀土、微量元素及同位素特征,提出了碎斑熔岩是由基底变质岩重熔而形成的观点。从斑晶钾长石的低结构态和出溶机理,认为碎斑熔岩的形成环境接近于火山岩和潜火山岩,而与深成岩差别较大。碎斑熔岩中斑晶多呈碎裂状,与火山口内隐爆作用有关,因此将该岩石归属于火山侵出相。     

Reequilibration of chromite within Kilauea Iki lava lake,Hawaii

Chromite mainly occurs as tiny inclusions within or at the edges of olivine phenocrysts in the 1959 Kilauea Iki lava lake. Liquilus chromite compositions are only preserved in scoria that was rapidly quenched from eruption temperatures. Analyses of drill core taken from the lava lake in 1960, 1961, 1975, 1979, and 1981 show that chromite becomes richer in Fe⁺², Fe⁺³, Ti and poorer in Mg, Al, Cr than the liquidus chromite. The amount of compositional change depends on the time elapsed since eruption, the cooling history of the sample, the extent of differentiation of the interstitial melt, and the position of the chromite inclusion within the olivine phenocryst. Compositional changes of the chromite inclusions are thought to be a result of reequilibration with the residual melt by cationic diffusion (Mg, Al, Cr outwards and Fe⁺², Fe⁺³, Ti inwards) through olivine. The changing chemical potential gradients produced as the residual melt cools, crystallizes and differentiates drives the reequilibration process. Major and minor element zoning profiles in olivine phenocrysts suggest that volume diffusion through olivine may have been the major mechanism of cationic transport through olivine. The dramatic compositional changes observed in chromite over the 22 years between eruption and 1981 has major implications for othe molten bodies.     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 2: Geology and genesis of the orebodies

The Black Swan Ultramafic Succession hosts a number of magmatic Fe–Ni–Cu–PGE sulfide ore shoots, ranging from high grade massive ore to low grade disseminated sulfides. Of these, the most economically significant is the Silver Swan massive sulfide orebody, associated with the basal contact of the succession. The deposit varies in thickness between 5 and 20 m, reaches a N–S strike length of 75 m, extends for at least 1.2 km of vertical plunge and is open at depth. Overlying matrix (net-textured) ore is rare. Inclusions of dacite are abundant within the lower 5 m of the massive sulfide. They range from angular fragments through smooth sinuous and plumose morphologies to fine lace-like intergrowths with the sulfide matrix, and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts. Dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures, some well above their liquidus temperature. The composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30% komatiite. The consistent thickness of the inclusion-bearing basal layer within the massive sulphide is interpreted as the extent of 3-D physical connectivity between the inclusions and a partially molten underlying hybrid layer. Primary contacts between the Silver Swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse-grained chevron-textured chromites with skeletal textures. Compositions of these chromites match those from Kambalda, Perseverance and other localities, and are inconsistent with a metamorphic origin. They are interpreted as markers of primary magmatic contacts. The combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates. Taken together with observations on the internal fractionation of platinum group elements within the massive ores, these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube. The floor of the tube had been previously heated by passage of large volumes of lava, such that it had reached its melting range. The felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma. This constitutes strong evidence for the operation of thermo-mechanical erosion during ore emplacement. The disseminated Cygnet and Black Swan orebodies show a number of distinctive features. Cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip-up, transport and redeposition of sulfides from a pre-existing massive sulfide orebody, of which Black Duck may be a remnant. The Black Swan orebody, by contrast, does not show xenolithic features, but is characterised by an association of sulfide blebs with segregation vesicles, and by unusually coarse-grained olivine. The Black Swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals.Editorial handling: Peter Lightfoot     

Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS

The El Laco magnetite deposits consist of more than 98 % magnetite but show field textures remarkably similar to mafic lava flows. Therefore, it has long been suggested that they represent a rare example of an effusive Fe oxide liquid. Field and petrographic evidence, however, suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs. We determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from various settings at El Laco and compared them with magnetite from both igneous and hydrothermal environments. This new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite “lava flows” formed. The trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite, including primary magnetite phenocrysts from the unaltered andesite host rocks at El Laco. Instead, the El Laco magnetite is most similar in composition to hydrothermal magnetite from high-temperature environments (>500 °C), such as iron oxide-copper-gold (IOCG) and porphyry-Cu deposits. The magnetite trace elements from massive magnetite are characterised by (1) depletion in elements considered relatively immobile in hydrothermal fluids (e.g. Ti, Al, Cr, Zr, Hf and Sc); (2) enrichment in elements that are highly incompatible with magmatic magnetite (rare earth elements (REE), Si, Ca, Na and P) and normally present in very low abundance in magmatic magnetite; (3) high Ni/Cr ratios which are typical of magnetite from hydrothermal environments; and (4) oscillatory zoning of Si, Ca, Mg, REE and most high field strength elements, and zoning truncations indicating dissolution, similar to that formed in hydrothermal Fe skarn deposits. In addition, secondary magnetite in altered, brecciated host rock, forming disseminations and veins, has the same composition as magnetite from the massive lenses. Euhedral magnetite lining both open-spaced veins in the brecciated host rock and along the walls of large, hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and/or physio-chemical conditions of the fluid. Thus, the chemical fingerprint of magnetite from the supposed El Laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows, by dissolution and precipitation of magnetite from high-temperature fluids, rather than a magmatic origin from an effusive Fe oxide liquid.     

陆上与水下喷发火山岩的区别及其对储层的影响——以松辽盆地营城组为例

陆上与水下喷发火山岩在岩性、结构构造、蚀变特征、产状、与下伏地层接触关系、孔隙和裂缝发育特点等6方面有显著区别。陆上喷发火山岩包括各种熔岩、碎屑熔岩、火山碎屑岩和沉火山碎屑岩;熔岩流纹构造发育,火山碎屑岩除发育常见层理外,还可见反丘构造;同生蚀变弱;与下伏地层多呈角度不整合接触,古风化壳常见,常含有陆相植物;主要储集空间为原生孔隙和冷凝收缩节理缝、次生溶蚀孔、矿物解理缝和构造裂缝。水下喷发火山岩多为具玻璃质结构的熔岩和含晶屑玻屑的层/沉凝灰岩、膨润土/伊利石岩/蒙脱石岩/沸石岩;常具枕状、球状构造,水平层理、粒序层理、变形层理;蚀变强烈;水下熔岩呈穹隆状、透镜状,凝灰岩为层状,近火山口的膨润土/伊利石岩/蒙脱石岩/沸石岩呈松散团窝状并夹有火山弹;与下伏地层呈整合、假整合或侵蚀接触;原生气孔、杏仁体内溶蚀孔和炸裂纹,岩球岩枕间孔和粒间孔,基质和斑晶蚀变孔缝,后期构造缝是主要储集空间。松辽盆地营城组陆上、水下喷发火山岩均有发育。其陆上喷发火山岩的典型标志为流纹构造、柱状节理,含炭化木/硅化木,与下伏地层呈角度不整合接触。水下喷发火山岩典型标志为珍珠岩、玻璃质结构、枕状构造、纹层状凝灰岩和膨润土。松辽盆地营城组的储层火山岩以陆上喷发火山岩为主;水下喷发火山岩中侵出相内带亚相珍珠岩为优质储层。     

Petrologic, stratigraphic and tectonic significance of Mesozoic volcanic rocks in the Río Wampú area, Eastern Honduras

A thick sequence of dark-red lava flows and clastic rocks with abundant volcanic debris crops out along the Río Wampú of eastern Honduras. Lithologic characteristics of these rocks, field interpretations of the Río Wampú stratigraphy, and limited K---Ar age determinations on volcanic samples indicate that these redbeds and associated volcanic units are correlative with the Upper Cretaceous Valle de Angeles Group of central Honduras. These strata uncomformably overlie dark clastic units which are tentatively correlated with the Jurassic Agua Fría Formation of the Honduras Group. The lava samples are predominantly microporphyritic andesites and basaltic andesites with abundant fine-grained phenocrysts of plagioclase, clinopyroxene and Fe---Ti oxides. Immobile-element whole-rock and mineral concentrations indicate a high-K tholeiitic composition. Elevated TiO₂ whole-rock and clinopyroxene concentrations found in the basaltic andesites apparently reflect a primary magmatic characteristic. Based on geochemical and modal data, it appears that these rocks were erupted from partial mantle melts which were modified by substantial fractionation of modal phases. The presence of widespread Mesozoic redbeds and associated volcanics in Honduras was previously cited as evidence of regional extensional tectonics. However, structural, geochemical and stratigraphic data collected during this study indicate that the Río Wampú volcanic suite was formed during a compressional (subduction-related) event. Similar high-TiO₂ basaltic andesites and andesites are reported along several active continental margins.