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1.
Drillcores and waters from Wairakei and Broadlands geothermal areas New Zealand have been analyzed for Li, Rb, Cs, Na, K, Mg, Ca, Al, Ti, Mn, and Be. The drillcores were altered to various degrees at temperatures below 300°C in slightly alkaline chloride water, probably derived from rock-water interaction in untapped horizons at higher temperature. It changes its composition as it leaches Ca and Na from the rock and adds K, Rb, Cs and Li. Evaluation of these changes in relation to the dimensions of the altered zone under observation suggests that a high mass ratio of water to rock (e.g., 100) and a period of up to 1 million years are responsible for the present stage of alteration.Increase of K and Rb in the altered rocks is a result of the formation of abundant adularia in addition to illite. The KRb ratio of the rock decreases during alteration but remains higher than that of the fluid. Only clay materials and zeolites that preferentially absorb Rb give slightly lower KRb ratios than the fluid.The mineral phases responsible for the uptake of lithium during alteration are chlorite (300 ppm Li) and quartz (up to 430 ppm Li). Li uptake in quartz is considered to be the mechanism by which Al-rich quartz crystallises from alumino-silicates. LiAl atomic ratios of 0.3–0.57 and Al concentrations up to 3000 ppm have been observed.Relatively small concentrations of Cs are found in potassic minerals (e.g. 10 ppm Cs in adularia, 44 ppm Cs in illite). However, 240 ppm Cs are found in wairakite from Wairakei equilibrated at 235°C. Lower equilibration temperatures may lead to higher cesium concentrations. This effect, in conjunction with a more concentrated hydrothermal fluid, could explain a content of 4500 ppm Cs in wairakite extracted from a drillcore taken in the El-Tatio geothermal field in Chile.  相似文献   

2.
A. nal 《Geological Journal》2008,43(1):95-116
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.  相似文献   

3.
Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.  相似文献   

4.
In western Anatolia, a thick volcanic succession of andesitic to rhyolitic lavas and volcaniclastic rocks crops out extensively. On Foça Peninsula, the westernmost part of the region, a dominantly rhyolitic sequence is exposed where massive rhyolites occur as dome or domelike stubby lava flows. These rhyolite domes vertically and laterally pass into blanketing volcaniclastic sequences. The gradational boundary relations and the facies characteristics of the surrounding volcaniclastic sequences indicate that the silicic domes directly intruded a subaqueous environment and were shattered upon sudden contact with water to form hyaloclastic blankets.

In and around these rhyolite domes, we have defined six different volcanic and volcaniclastic facies, consisting of: (1) massive rhyolite; (2) massive perlite; (3) hyaloclastic breccias; (4) rhyolite pumice and lithic fragment-bearing volcaniclastic rocks; (5) subaqueous welded ignimbrites; and (6) brecciated perlite. The massive rhyolite facies have distinct structures from the centers to the peripheries of the domes and stubby lava flows. Massive lava facies gradually pass into hyaloclastic breccias and massive perlite facies, indicating water-magma interaction during the emplacement. Phreatomagmatic explosive activity and doming caused the subaqueous pyroclastic flows on the flanks of the volcanic center. Welding in the upper parts of these pyroclastic flow deposits indicates the high-temperature emplacement of the pyroclastic material and relatively slow cooling caused by the cushioning effect of the gas-vapor mixture and rapid deposition of younger pyroclastic units.  相似文献   

5.
Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the 18O16O ratios of the rocks.NH4 contents of biotites averaged 22 ppm in granitic rocks of non-metamorphic terranes, 67 ppm in granitic rocks in the Ryoke metamorphic belt, and 279 ppm in metamorphosed sedimentary rocks of the Ryoke belt. In granitic rocks, enrichment of NH4 in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH4 content in biotites is due to inheritance from original organic material in sedimentary rocks.Biotites from migmatites of the Ryoke belt contain more NH4 (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH4.  相似文献   

6.
浙江新昌早白垩世复合岩流中的岩浆混合作用   总被引：35，自引：7，他引：35  
周金城  俞云文 《岩石学报》1994,10(3):236-247
浙江新昌拔茅地区早白垩世复合岩流中各种火山岩（Ｒｂ－Ｓｒ等时线年龄为９６．３Ｍａ）属高钾钙碱性岩系，在其中发现了中生代火山活动中岩浆混合作用的确凿证据，岩相学及地球化学研究表明，这种复合岩流中的安山质岩浆是由同时代橄榄拉斑玄武岩浆和流纹岩浆相互混合而形成的。  相似文献   

7.
The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains 1% K2O at 60% SiO2. Trace element abundances of a typical ignimbrite with 73% SiO2 are Sr 225 ppm, Rb 130 ppm, Y 32 ppm, Th 12 ppm, Zr 200 ppm, and Nb 15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.  相似文献   

8.
西秦岭天水地区分布有新生代陆相酸性火山岩，主要由流纹岩、流纹质角砾凝灰熔岩、流纹质熔结(角砾)凝灰岩及少量角砾凝灰岩、火山角砾岩等组成，具有富硅、富碱、低铝、低钙特征，属于非造山偏碱性岩石，类似于大陆裂谷碱性流纹岩。研究表明，新生代陆相酸性火山岩为地壳岩石部分熔融作用的产物，形成于陆内拉张构造环境，与新生代早期渭河断裂带的左行走滑剪切构造作用有关。  相似文献   

9.
The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma.  相似文献   

10.
The Thalanga volcanic‐hosted massive sulfide deposit occurs in the Cambro‐Ordovician Mt Windsor Subprovince in northern Queensland. The orebody comprises steeply dipping, stratiform, sheet‐like, polymetallic massive sulfide lenses. Overall, the volcanic facies architecture at Thalanga is dominated by quartz‐ and/or feldspar‐phyric lavas and synvolcanic intrusions that comprise coherent facies and in situ and resedimented autoclastic facies. Systematic phenocryst logging (mineralogy, abundance, size) has been used to discriminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and dacite types can also be distinguished using immobile‐element geochemistry. Rhyolitic lavas and intrusions in the footwall are weakly to strongly altered. Apparent clastic textures resulting from hydrothermal alteration and metamorphism are widely developed in the coherent facies. Genuine clastic textures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow‐laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clasts and matrix, and normal grading. Mass‐flow‐emplaced, rhyolitic breccia units delineate palaeo‐sea‐floor positions in the footwall that are potentially prospective for exhalative massive sulfide mineralisation. A comparison of the distribution of clastic and coherent facies with the geometry of strongly altered zones in the footwall indicates that intense hydrothermal fluid flow was independent of the facies arrangement. The massive sulfide lenses conformably overly altered footwall rhyolite and occur in a distinctive facies association which includes coarse quartz‐phenocryst‐rich rhyolitic sills with peperitic contacts and crystal‐rich polymictic breccia. The hangingwall to the orebody consists of largely unaltered dacitic lavas and synvolcanic intrusions and minor dacitic pumice breccia, dacitic breccia and polymictic volcanic breccia. The facies architecture shows that the Thalanga massive sulfide deposit formed in a below‐storm‐wave‐base depositional environment on top of an elevated, lava‐dominated, rhyolitic volcanic centre. A modern analogue for the setting of the Thalanga massive sulfide is the PACMANUS hydrothermal field on the crest of the dacite lava‐dominated Pual Ridge in the eastern Manus backarc basin (Papua New Guinea).  相似文献   

11.
小岭旋回火山岩是辽东中生代火山岩的重要组成部分,主要岩性有安山岩、英安岩以及流纹质火山碎屑岩,属钾质钙碱性系列。其地球化学特征显示,火山岩总体具有高K、Na,低Al、Fe、Ca的特点。其中,火山熔岩具有富集Nb、Th、Hf、Zn,相对亏损Cs、Li、Sr的特点;火山碎屑岩普遍具贫Sr、V、Cu、Li,富含Hf的特点。火山岩总体为稀土富集型,安山岩及安山质火山碎屑岩,略显Eu正异常;流纹岩及流纹质火山碎屑岩具明显Eu负异常。火山岩成岩物质来源于地壳,成因以部分熔融为主,部分岩石显示具陆壳混染特点。火山岩形成动力来自于太平洋板块向古欧亚板块的俯冲,形成于早期拉张—晚期挤压的构造背景,具有板内活动带火山岩的特征。  相似文献   

12.
Geochemical and geochronological data for rocks from the Rajahmundry Traps, are evaluated for possible correlation with the main Deccan province. Lava flows are found on both banks of the Godavari River and contain an intertrappean sedimentary layer. Based on40Ar/39 Ar age data, rocks on the east bank are post K-T boundary, show normal magnetic polarity, and belong to chron 29N. Their chemistry is identical to lavas in the Mahabaleshwar Formation in the Western Ghats, ∼1000km away. It was suggested earlier that the genetic link between these geographically widely separated rocks resulted from lava flowing down freshly incised river canyons at ∼ 64 Ma. For the west bank rocks, recent paleomagnetic work indicates lava flows below and above the intertrappean (sedimentary) layer show reversed and normal magnetic polarity, respectively. The chemical composition of the west bank flow above the intertrappean layer is identical to rocks on the east bank. The west bank lava lying below the sedimentary layer, shows chemistry similar to Ambenali Formation lava flows in the western Deccan.40Ar/39 Ar dating and complete chemical characterization of this flow is required to elucidate its petrogenesis with respect to the main Deccan Province.  相似文献   

13.
Vein distributions in line samples from four epithermal Au–Ag deposits of the Hauraki Goldfield were logged and quantified by vein spacing, vein density, vein thickness and percentage of vein extension. One deposit is hosted in andesite lavas (Martha Hill), one in andesite lavas and dacite porphyry, dacitic tuffs and pyroclastic breccias (Golden Cross), and two in rhyolite lavas and rhyolitic tuffs with minor andesite lavas or andesite dikes (Ohui and Wharekirauponga). The vein systems in these deposits form fault-controlled arrays of extensional veins. Vein spacing distributions are non-fractal over two to three orders of magnitude (1 mm to 5 m), and therefore fractal dimension statistics are not applicable. The coefficient of variation (Cv) of vein spacing was used as a measure of the degree of vein clustering. Rock type has a marked influence on vein spacing distributions, with veining in rhyolite lava having lower average thickness and percentage extension, but a generally higher degree of vein clustering compared with veining in andesite lava in the same deposit. Vein spacing distributions in well-jointed lithologies, mainly andesite lava, have Cv values (0.8–1.2) that are indicative of anticlustered to weakly clustered patterns, particularly in the vein stockwork of the upper part of the Golden Cross deposit. These Cv values are consistent with field observations that joints are a major control on vein spacing. In the poorly jointed dacitic and rhyolitic rocks, the veins are weakly to strongly clustered as shown by higher Cv values (1.2–2.4), and are commonly associated with normal faults. Overall, andesite lava and dacite porphyry and pyroclastics host thicker and more persistent veins than rhyolite lava and tuff. These larger veins contain significant volumes of high-grade gold mineralisation. The higher chemical reactivity to hydrothermal fluids of andesite and dacite compared with rhyolite may have aided propagation and thickening of the veins in andesite-hosted deposits. Within an individual epithermal deposit, location close to thick veins, representing major fluid conduits, commonly overrides the effect of different lithologies. Sites that are deeper and located within or adjacent to major vein structures have higher average vein thickness, percentage extension and degree of vein clustering. Systematic collection and analysis of vein spacing, thickness and density data can be used to define trends that are useful in the exploration of gold-bearing epithermal vein deposits. Received: 25 August 1998 / Accepted: 23 December 1999  相似文献   

14.
在松辽盆地东南隆起区营城组标准剖面营三段古火山口附近识别出岩浆期后热液活动的地质记录。岩石学特征表现为隐爆角砾岩,即,原有的近火山口相岩石(原岩)被高压流体炸碎形成原地角砾、之后又被灌入的富含矿物质"岩汁"胶结形成的原地角砾岩。采集隐爆角砾岩及其上覆和下伏三个层位共10个样品进行地球化学和年代学对比研究,包括两套4个对应的原岩和岩汁、下伏4个玄武粗安岩和上覆2个流纹岩。原岩高精度~(39)Ar/~(40)Ar坪年龄113.4±0.7Ma;岩汁为含铁酸性流体的隐晶质析出物,其~(39)Ar/~(40)Ar坪年龄112.9±19.6Ma;二者的年龄差反映岩浆主期与期后热液活动的时代间隔(1Ma之内)。10个样品的共性是:(1)稀土总量中-高(∑REE=81×10~(-6)～202×10~(-6))且轻重稀土分异明显((La/Yb)_N=4.91～11.45);(2)MORB标准化蛛网图上P和部分不相容元素(Cs、Th、La和Zr)正异常,而另一部分不相容元素(Sr和Pb)负异常。整体表现为双峰式裂谷火山岩特点。主要差别在于:(1)铕和钡表现为两种情况,下伏玄武粗安岩和隐爆角砾岩中的粗面岩具正异常,而隐爆角砾岩中的流纹岩和岩汁及其上覆的流纹岩具负异常;(2)只有2个岩汁样品显示K负异常,其它8个岩石样品均为K正异常;(3)下伏玄武粗安岩(4个样)显弱的Ti正异常且Rb/Sr比低(0.04～0.05),而其它6个样品为Ti负异常且Rb/Sr比高(0.62～2.83),其中的2个岩汁样品Ti负异常最强。岩汁与其下伏粗面质原岩差别显著,而与其上覆流纹岩(SHRIMP年龄110.6Ma)的地化特征相似(见正文)。该火山期后热液活动是深源热流体萃取壳源物并沿古火山通道(构造薄弱带)运移到近地表的,可能是后续流纹质火山活动的先驱。这种高压的岩浆期后热液导致围岩炸裂、发生角砾岩化、形成大量角砾间孔和裂缝。这是造成火山口-近火山口相带成为优质储层的重要因素。该类火山岩储层改善作用早于烃类运移,可构成有利于成藏的时空配置。与该期热液活动相伴生的深源天然气早于上覆圈闭的形成,因此对成藏没有贡献。  相似文献   

15.
碎斑熔岩在黑矿型矿床成矿作用中的地位   总被引：2，自引：0，他引：2  
白银矿田火山碎屑岩及赋矿岩石的深入研究表明,该地区存在一种长英质侵出相火山岩——碎斑熔岩。其主要特征是：①产于矿田酸性火山岩穹的火山活动中心或古火山喷口位置,代表中酸性火山作用晚期岩浆活动的产物,其典型者如折腰山石英角斑质碎斑熔岩体,规模为2×0．5km2,有次火山相石英钠长斑岩和各种火山碎屑岩如晶屑凝灰岩、集块岩及熔岩等分布在其四周;②碎斑熔岩中碎斑为长石和石英,具破而不碎、碎而不离或离而不远的特征,③基质具霏细结构,但多数已发生绢云母化、绿泥石化和黄铁矿化等;④岩石化学上同该地区喷出相石英角斑岩相当,SiO2均大于70ωB％,具有较高的Al2O3含量和Na2O／K2O值,分别为11．16～14．90和1．78～7．43,具I型与S型的混合型特征,⑤稀土元素配分型式具平坦型和富集型两种,分别与白银矿田东西两段酸性火山岩成因类型相对应,微量元素地球化学研究表明碎斑熔岩与区内石英角斑岩属同源岩浆的衍生物．海相火山岩中碎斑熔岩的确立,不仅增加了海相火山作用形式,即便出作用,而且因其占据火山活动中心或岩浆通道位置,在成岩或火山作用期后便成为海底热液对流循环成矿作用中心即热液喷流口．它既标志着黑矿型块状硫化物矿床成矿的相对时期,是在火山作用晚期侵出相碎斑熔  相似文献   

16.
银山地区火山岩－次火山岩由流纹岩、流纹英安斑岩、英安质熔岩、英安斑岩和粗面安山斑岩组成,喷发和侵入存在着反序特征．通过各种物理化学方法确定了岩浆房中存在的化学成分和温度、密度、粘度梯度．建立了以英安质岩浆为主,顶部有一层富ＳｉＯ２、富Ｋ的流纹质岩浆,底部为少量粗面安山质岩浆的带状岩浆房．岩浆演化及带状岩浆房的形成是分离结晶、同化混染、岩浆对流和边界层机制共同作用的结果．  相似文献   

17.
比利亚谷铅锌银矿位于海拉尔—根河中生代火山盆地北西缘地区。该矿床赋存于上侏罗统满克头鄂博组酸性火山岩中,受北西向断裂构造控制,主矿体呈脉状产出。赋矿围岩流纹质火山角砾熔岩LA-ICP-MS锆石U-Pb测年结果表明,其成岩时代为(159.2±1.8)Ma,并依据赋矿围岩与成矿的关系推测成矿时代为晚侏罗世。矿床赋矿围岩流纹质火山角砾熔岩和流纹岩的主量元素具有富硅(68.31%~77.18%)、富钾(3.39%~4.28%)、低钠(0.14%~0.18%)、高铝(11.74%~14.93%)的特征,属高钾钙碱性系列;岩石REE总量较低(93.65×10~(-6)98.94×10~(-6)),以轻稀土相对富集、轻稀土和重稀土分异明显、弱的Ce负异常和Eu负异常为特征;结合岩石具有亏损Sr、高度富集轻稀土等特征认为火山岩产于板内,与其有关的岩浆可能来源于上地幔,并混染了壳源物质。综合本次研究和区域构造演化特征,初步认为矿床形成于鄂霍次克板块剪刀式闭合时期的拉张伸展环境。  相似文献   

18.
This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.  相似文献   

19.
杨喆  钟大康  张硕  郭强  路昭 《地球科学》2018,43(10):3733-3748
二连盆地白音查干凹陷下白垩统暗色沉积物中发育由白色颗粒构成的同沉积纹层、条带,其矿物组合中包含大量成因不明的钠沸石和方沸石.在薄片鉴定和电子探针（EMPA）分析的基础上,通过场发射扫描电镜（FE-SEM）和电感耦合等离子体质谱仪（ICP-MS）法对两种沸石矿物的形貌、微结构、共生矿物组合和微量元素成分进行分析.实验结果表明,钠沸石和方沸石自形程度较高,其中钠沸石为半自形-自形的棱柱状,长轴方向长度最大可达600 μm,集合体为纤维状或块状;方沸石呈自形的四角三八面体,粒径变化较大,介于10~300 μm.钠沸石和方沸石既能独立生长,也能互相交代,与重晶石、含铁白云石、菱镁矿和菱铁矿以及立方体黄铁矿等矿物紧密共生.与地壳及同区和同层位湖相泥岩微量元素配分曲线比较,含沸石岩石的全岩微量元素富集Pb、Sb、Tl、W、Mo、Bi,亏损V、Cr、Co、Ni,暗示了中高温含深源物质的碱性热液流体的加入.在综合分析沸石矿物形态结构、共生矿物组合及微量元素特征的基础上,认为在异常热流驱动下,中高温热液流体沿着断裂等运移通道喷出或溢流至湖底沉积物表面,与冷的湖水混合直接结晶沉淀形成方沸石、钠沸石和其他共生矿物,属于热水沉积成因.   相似文献   

20.
At Medicine Lake Volcano, California, the compositional gap between andesite (57–62 wt.% SiO2) and rhyolite (73–74 wt.% SiO2) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO2 andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO2 andesite.Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H2O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions.  相似文献   

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