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1.
The Precambrian crystalline basement of Sinai represents a low-pressure metamorphic terrain intruded by large volumes of granitic rock. Based on detailed fieldwork, a general assessment of the metamorphic and tectonic history of the Wadi Kid area, southeastern Sinai, is presented. Three lithostratigraphic units can be traced over the whole area; the Umm Zariq Formation (arkoses, greywackes, pelites), the Tarr Formation (dolomitic-calcareous rocks) and, unconformably overlying the previous two units, the Heib Formation (flows, pyroclastics, conglomerates). D1 deformation of this 3.5 km thick sequence resulted in upright folds, with changing strike of the axial planes from NE to NW across the area. Low-grade conditions prevailed during this phase. D2 produced recumbent folds and a subhorizontal cleavage, leading to transposition of D1 structures in the higher grade parts of the area. Metamorphism reached its peak conditions around D2. Pressures are estimated at 2.5–3.5 kb, whereas temperatures vary from 450–660°C. In the central Wadi Kid area, garnet, staurolite, cordierite and andalusite occur in metapelitic rocks. Highest grade rocks are syn-D2 andalusite—K-feldspar gneiss diapirs. Metamorphic zones are shallow dippin and form a domed pattern. Most of the metavolcanics and the syntectonic and late tectonic plutonic rocks belong to the calc-alkaline suite.The Kid Group sediments and volcanics were deposited in a shallow basin and subaerially, respectively, probably on older sialic basement. This basement is at present not exposed because post-orogenic uplift directly after the Pan-African event was relatively small (3–6 km). Metamorphism and the D2 formation phase can both be related to a rising (mafic?) diapir. The Sinai Peninsula may have been a continental margin or a cratonized, mature island arc, in Late Proterozoic times.  相似文献   

2.
The Upper Proterozoic ophiolite complex of Bou Azzer, Morocco, includes ultramafic rocks, cumulate gabbros, sheeted dykes, pillow lavas and diorite-quartz diorite intrusions and an overlying volcano-sedimentary sequence. The gabbroic cumulates, basaltic flows and dykes have compositions similar to recent ocean-floor rocks (N- and/or T-type). Among other features, they have comparable light REE-depleted patterns and relations of Ti-Zr and La-Nb. Although fractional crystallization played an important role in the evolution of these rocks, the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and/or by a dynamic partial melting process. Diorites, quartz diorites and the volcanic rocks of the overlying sequence are calc-alkaline, genetically unrelated to the tholeiitic suite and indicative of an island arc setting. A possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc.  相似文献   

3.
Age-determinations on a complex array of mafic dykes across the eastern Kaapvaal craton (Olsson et al., 2010) are complemented herein by field evidence, dyke trend analysis, and petrological characterization of 58 dyke samples. ∼2.95 Ga SE-trending, a ∼2.65 Ga E- to SE-radiating, and ∼1.90 Ga NE-trending swarms can be distinguished. Prominent Archean (∼2.95 and ∼2.65 Ga) dyke ridges contain basement xenoliths, and have a more quartz-oversaturated, andesitic and calc-alkaline character. Proterozoic (∼1.90 Ga) dykes are, on the other hand, more aeromagnetically prominent tholeiitic basalts with higher modal (Fe, Ti)-oxide contents. Multi-elemental statistics indicate that the best geochemical discrimination is found between Archean high-Sr/V and La/Yb and Proterozoic low-Sr/V and La/Yb dyke swarms. The calc-alkaline character of Archean dyke swarms is augmented by LILE-enriched spider-element patterns with steeper REE-slopes as compared to the Proterozoic swarm. Geochemical similarities are roughly consistent with the ∼2.95 and ∼2.65 Ga dykes having fed coeval lavas within the Nsuze Group and Allanridge Formation (upper Ventersdorp Supergroup), respectively. ∼1.90 Ga dykes match coeval sills on both the Zimbabwe and Kaapvaal cratons, and are presumed feeders to Soutpansberg Group lavas. This new information provided by dated feeder dyke swarms leads to a re-evaluation of petrotectonic settings, ranging from continental back-arc to radiating swarms emanating from igneous (plume?) centers.  相似文献   

4.
The composite Guerrero Terrane of western Mexico records much of the magmatic evolution of southwestern North America during Late Mesozoic time. The Guerrero includes three distinctive subterranes characterized by unique stratigraphic records, structural evolutions, and geochemical and isotopic features that strongly suggest they evolved independently. The eastern Teloloapan Subterrane represents an evolved intra-oceanic island arc of Hauterivian to Cenomanian age, which includes a high-K calc-alkaline magmatic suite. The central Arcelia–Palmar Chico Subterrane represents a primitive island arc-marginal basin system of Albian to Cenomanian age, consisting of an oceanic suite and a tholeiitic arc suite. The western Zihuatanejo–Huetamo Subterrane comprises three components that represent an evolved island arc-marginal basin-subduction complex system of Late Jurassic (?) –Early Cretaceous age built on a previously deformed basement. The Zihuatanejo Sequence includes a thick high-K calc-alkaline magmatic suite. The Las Ollas Complex consists of tectonic slices containing exotic blocks of arc affinity affected by high-pressure/low-temperature metamorphism included in a sheared matrix. The Huetamo Sequence consists mainly of volcanic-arc derived sedimentary rocks, including large pebbles of tholeiitic, calc-alkaline, and shoshonitic lavas. These sequences are unconformably underlain by the Arteaga Complex, which represents the subvolcanic basement. On the basis of available geology, geochemistry, geochronology, and isotopic data, we suggest that Late Mesozoic volcanism along the western margin of southern North America developed in broadly contemporaneous but different intra-oceanic island arcs that constitute a complex fossil arc-trench system similar to the present-day western Pacific island arc system.  相似文献   

5.
根据主量元素、微量元素和Sr-Nd同位素地球化学研究结果,觉罗塔格构造带中企鹅山群火山岩企鹅山群火山岩分为拉斑玄武岩和钙碱性玄武岩,拉斑玄武岩和钙碱性玄武岩均具有高铝、低钛和相对富集轻稀土元素（LREE）、大离子亲石元素的特征,与典型的岛弧玄武岩相似。钙碱性玄武岩具有显著的Eu的负异常,其不相容元素含量显著高于拉斑玄武岩,在La/Sm-La图解中,钙碱性玄武岩为结晶分异作用的产物,拉斑玄武岩则为平衡部分熔融的产物。Sr-Nd同位素特征表明企鹅山群火山岩源自亏损地幔。在玄武岩构造环境判别图中样品均落入岛弧玄武岩范围内。通过综合该区岩石组合特征、地球化学特征、地球物理特征,认为企鹅山群为康古尔洋壳向北俯冲在大南湖岛弧南侧形成的石炭纪岛弧火山岩。  相似文献   

6.
Abstract The Bergen-Jotun kindred rocks of this study, the Storådalen Complex (SCX), Svartdalen Gneiss (SG) and Mjølkedøla Purple Gabbro (MPG), have been shown to be a co-magmatic series with calc-alkaline affinities. The analyses of Ba, Nb, Y, and Zr presented here show no variation in these elements between the three rock units and are consistent with the calc-alkaline character of the rocks. The lithophile elements Ba, K, and Sr are enriched relative to MORB and the high field strength elements Nb, Y, and Zr are depleted relative to MORB, Zr especially so.
The SCX contains rocks with low (>30) differentiation indices which are interpreted as plagioclase + pyroxene ± olivine ± amphibole cumulates. The remainder of the SCX, together with the MPG and SG, is regarded as the congealed liquid in equilibrium with these cumulates. The distribution of trace elements between these two components of the SCX can be adequately modelled using a Rayleigh fractionation process, measured 'liquid'compositions, and calculated bulk distribution coefficients. It is thus concluded that the trace element geochemistry of the rocks of this study is consistent with subduction-related, mantle-derived magmas that fractionate within a continental or mature island arc environment. Subsequent high-grade metamorphism and deformation of Sveconorwegian age have been essentially isochemical.  相似文献   

7.
大别山北大别杂岩的大地构造属性   总被引：12，自引：1，他引：11  
北大别杂岩主要由花岗质片麻岩及斜长角闪岩组成 ,含有不同类型、大小不等的麻粒岩岩块和变质超镁铁质岩块 ,侵入有大量白垩纪花岗岩和辉石 -辉长岩类。其中的花岗质片麻岩、斜长角闪岩具有岛弧环境的岩石地球化学特征 ,代表拼贴于扬子陆块北缘的新元古代古岛弧。北大别杂岩北可与庐镇关群相连 ,南俯于超高压变质岩之下 ,在三叠纪扬子陆块与华北陆块的碰撞过程中 ,曾与超高压变质岩一起俯冲到地幔深度并经受榴辉岩相变质作用 ,然后在折返过程中叠加了麻粒岩相及角闪岩相变质作用 ,是扬子陆块北缘陆壳俯冲基底的一部分  相似文献   

8.
In the Aeolian island arc two different magmatological associations, calc-alkaline andesite series and shoshonites, occur in close vicinity. Although both associations erupted simultaneously during the last glaciation, there is a general tendency for the calc-alkaline rocks to be older. Shoshonitic activity is still going on.Calc-alkaline lavas include high-Al basalts, andesites and dacites, with the general characteristics of the island-arc type andesite series. Large cation trace elements (Rb, Ba, Sr) however are distinctly enriched.Shoshonite series include trachybasalts and latites, with which potassium-rich rhyolites can be associated. Leucite tephrites and potassic trachytes form a different evolution trend of the shoshonitic association.Petrology relates both associations of the Aeolian Islands to the island arc dynamics which is presently characterized by deep-focus earthquakes in the depth range of 200–350 km. The present-day gap in seismic activity from 50–200 km coincides with the present-day lack of calc-alkaline volcanic activity and is explained by the model of a detached slab which continues to sink into the mantle.  相似文献   

9.
The Shortland Islands lie in a northeast-southwest line across the western end of Solomon Islands, immediately adjacent to Bougainville. Three major islands dominate the group.Fauro and surrounding islands, in the northeast, have an altered basement suite comprising tholeiite, icelandite and tholeiitic dacite. This is intruded by a high-level calc-alkaline assemblage of microdiorite, hornblende andesite and rhyodacite and overlain by volcanogenic sandstones derived from an andesitic to dacitic volcano. Pyroclastics comprising high-alumina basalt and pyroxene andesite overlie the volcanogenic sandstones. The tholeiitic basement lavas may be of Late Oligocene to Early Miocene age, and the calc-alkaline rocks are probably also pre-Pliocene in age.Alu, in the centre of the group, also has an altered tholeiitic lava basement, which is intruded by a quartz diorite body and overlain by hypersthene-augite basaltic andesite. Pliocene siltstone and Quaternary shallow marine carbonates cover these igneous rocks over much of the island.Mono, in the southwest, has a small basement exposure of altered pillowed hawaiite, overlain by Miocene pelagic limestone, Pliocene siltstone and Quaternary reef limestone. Isolated clasts of pyroxene andesite and ?benmoreite occur in streams and on beaches.The younger, calc-alkaline suites on all islands were formed in an island arc environment, possibly related to subduction from the southwest beneath the New Britain Trench. The basement lavas on Alu are probably early island arc tholeiites, and both these lavas and the calc-alkaline rocks of Alu share a common trend on variation diagrams. The two igneous suites of Fauro, however, have distinctly different trends. The basement lavas have some chemical similarities with oceanic tholeiites, but an early island arc origin for these lavas cannot be ruled out. The altered hawaiite and benmoreite on Mono probably originated in an oceanic island environment.  相似文献   

10.
锡林浩特岩群是内蒙古中东部锡林郭勒微地块内原锡林郭勒杂岩的重要组成部分,由一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等变质岩组成,形成于中元古代［1］。岩群中片麻岩样品显示轻稀土元素富集的中等分异特征((La/Yb)N=4.71~7.97),斜长角闪岩具有平坦型((La/Yb)N=0.74~0.95)和轻稀土元素微弱富集型((La/Yb)N=1.06~3.63)两种稀土元素配分模式。在微量元素N MORB标准化图解中,斜长角闪岩显示出洋中脊玄武岩和岛弧玄武岩的过渡类型特征。同时结合岩相学和岩石地球化学图解分析,推断片麻岩原岩为富含泥质的亚杂砂岩,而斜长角闪岩原岩为一套具有拉斑质到钙碱性过渡型特征的玄武岩;变质沉积岩形成于活动大陆边缘大陆岛弧环境;变质基性火山岩形成于岛弧弧后盆地。二者指示锡林浩特岩群形成于活动大陆边缘体系。  相似文献   

11.
The Nain and Ashin ophiolites consist of Mesozoic melange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent(CEIM).They largely consist of serpentinized peridotites slices;nonetheless,minor tectonic slices of sheeted dykes and pillow lavas-locally stratigraphically associated with radiolarian cherts-can be found in these ophiolitic melanges.Based on their whole rock geochemistry and mineral chemistry,these rocks can be divided into two geochemical groups.The sheeted dykes and most of the pillow lavas show island arc tholeiitic(IAT)affinity,whereas a few pillow lavas from the Nain ophiolites show calc-alkaline(CA)affinity.Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting.Petrogenetic modeling shows that these components were represented by pure aqueous fluids,or sediment melts,or a combination of both,suggesting that the studied rocks were formed in an arc-forearc tectonic setting.Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous.Previous tectonic interpretations suggested that the Nain ophiolites formed,in a Late Cretaceous backarc basin located in the south of the CEIM(the so-called Nain-Baft basin).However,recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic,which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location.This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction,below the CEIM,of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia.As a major conclusion of this paper,a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.  相似文献   

12.
The Banda arc of eastern Indonesia manifests the collision of a continent and an intra-oceanic island arc. The presently active arc is located on what appears to be oceanic crust whereas the associated subduction trench is underlain by continental crust.Recent lavas from the Banda arc are predominantly andesitic and range from tholeiitic in the north through calc-alkaline to high-K calc-alkaline varieties in the southern islands. Defining this regular geochemical variation are significant increases in the abundances of K (2,600–21,000 ppm), Rb (10–90 ppm), Cs (0.5–7.0 ppm), and Ba (100–1,000 ppm) from tholeiitic to high-K calc-alkaline lavas. 87Sr/86Sr ratios in the tholeiites are relatively low, from 0.7045 to 0.7047. In the calc-alkaline lavas, 87Sr/86Sr ratios range from 0.7052 to 0.7095, and in the high-K calc-alkaline lavas from 0.7065 to 0.7080. There is no correlation between 87Sr/86Sr and major and trace element abundances, even among lavas from the same volcano. Late Cenozoic cordierite — bearing lavas from Ambon, north of the presently active arc, are highly enriched in K, Rb and Cs, which together with 87Sr/86Sr ratios of approximately 0.715 is consistent with their derivation from partial melting of pelitic material in the locally — thick crust.The high 87Sr/86Sr ratios in the Recent calc-alkaline lavas are interpreted to result from mixing of a sialic component with a mantle derived component. The most likely cause is subduction and subsequent melting of either sea-floor sediments or continental crust. However, it is probably unrealistic to model this type of deep contamination by simple two-component mixing. Such contamination implies that the volcanic rocks from the Banda arc are at least partly a manifestation of melting at or near the Benioff seismic zone. Temperatures of at least 750–800 ° C at the top of the subducted lithospheric slab at depths of approximately 150 km are also implied; temperatures very close to the solidus of hydrous basalt (eclogite) at such pressure. It is concluded that partial melting of the crustal component of the subducted lithospheric slab may play a significant role in island arc petrogenesis.This paper is the result of a cooperative project with the Geological Survey of Indonesia, Ministry of Mines and Energy  相似文献   

13.
Abstract

The east central part of the Kohistan magmatic arc is made up principally of the Jaglot Group. From bottom to top it consists of I) paragneisses and schists intercalated with amphibolites and calc-silicates (Gilgit Formation), II) Gashu-Confluence Volcanics (GCV) and III) the Thelichi Formation comprising a volcanic base (Majne volcanics) and turbidites, marble, volcanoclastic sediments and lava flows. Metamorphic grade varies up to the sillimanite zone. The GCV are correlated with the Chalt volcanics and the Thelichi Formation with the Yasin Group. Other lithologies include the Chilas Complex, the Kohistan Batholith and part of the Kamila Amphibolite. Metavolcanics show a broad range in chemical composition. Geochemical parameters used to specify the tecto-nomagmatic regime suggest affinities of both island arc and MORB-like back-arc basin basalts. Kohistan can be divided into three tectonic zones, I) the southern (Kamila) zone comprises amphibolitized basalts, and mafic and ultramafic rocks, II) the central Chilas Complex, and III) the northern (Gilgit) zone i.e., the Jaglot Group. Previous tectonic models considered the southern two zones as the crust of a Cretaceous island arc. This investigation concludes that only the southern zone represents a true island arc. The Jaglot Group derives from back-arc basin assemblages and the Chilas Complex is a magmatic diapir emplaced in the back-arc basin.  相似文献   

14.
Mumbai City, situated on the western Indian coast, is well known for exposures of late-stage Deccan pillow basalts and spilites, pyroclastic rocks, rhyolite lavas, and trachyte intrusions. These rock units, and a little-studied sequence of tholeiitic flows and dykes in the eastern part of Mumbai City, constitute the west-dipping limb of a regional tectonic structure called the Panvel flexure. Here we present field, petrographic, major and trace element and Sr–Nd isotopic data on these tholeiitic flows and dykes, best exposed in the Ghatkopar–Powai area. The flows closely resemble the Mahabaleshwar Formation of the thick Western Ghats sequence to the east, in Sr–Nd isotopic ratios and multielement patterns, but have other geochemical characteristics (e.g., incompatible trace element ratios) unlike the Mahabaleshwar or any other Formation. The flows may have originated from a nearby eruptive center, possibly offshore of Mumbai. Two dykes resemble the Ambenali Formation of the Western Ghats in all geochemical characteristics, though they may not represent feeders of the Ambenali Formation lavas. Most dykes are distinct from any of the Western Ghats stratigraphic units. Some show partial (e.g., Sr–Nd isotopic) similarities to the Mahabaleshwar Formation, and these include several dykes with unusual, concave-downward REE patterns suggesting residual amphibole and thus a lithospheric source. The flows and dykes are inferred to have undergone little or no contamination, by lower continental crust. Most dykes are almost vertical, suggesting emplacement after the formation of the Panvel flexure, and indicate considerable east–west lithospheric extension during this late but magmatically vigorous stage of Deccan volcanism.  相似文献   

15.
Since Mesozoic time, Java and Bali have formed part of an evolving system of island arcs comprising the Sunda arc of Indonesia. The present tectonic setting is relatively simple with subduction occurring at the Java Trench to the south. A north-dipping Benioff seismic zone delineates an underthrust lithospheric slab to depths of approximately 600 km beneath the Java Sea. Quaternary lavas of the normal island arc association range from tholeiites to high-K calc-alkaline lavas over Benioff zone depths from 120–250 km, respectively. More abundant calc-alkaline lavas lie between these extremes. High-K alkaline lavas are found over Benioff zone depths in excess of 300 km.Both within and between these groups of rocks there are consistent spatial variations in the observed geochemistry. For approximately 200 rocks, incompatible elements such as K, Rb, Cs, Sr, Ba, light REE, U and Th show an increase in abundance of almost an order of magnitude with increasing depth to the seismic zone. Abundances of compatible elements show little consistent variation and trace elements such as Ni, Co, Cr, and Sc are characteristically depleted except in some of the alkaline lavas. Major element abundances in rocks of the normal island arc association show little variation, except for K and P, which both increase in abundance across the arc and Al, which shows a relative decrease.The major and trace element data are inconsistent with the derivation of the analyzed rocks by partial melting of the crustal component of the subducted lithosphere. On the other hand, low Ni abundances (20 ppm) in the basalts suggest that most of the lavas are fractionated and few if any represent primary mantle-derived melts. The spatial variations in the geochemistry of erupted lavas across Java and Bali are best explained by a combination of two processes: melting of a geochemically zoned mantle source and smaller degrees of partial melting of that material at progressively greater depths. Primary tholeiitic magmas could be formed by 20–25% melting at depths of 30–40 km, primary high-K calc-alkaline magmas by 5–15% melting at 40–60 km depth, and primary alkaline magmas by 5% melting at depths of 80–90 km. The geochemical zoning in the mantle, which is also manifested by increasing 87Sr/86Sr ratios in lavas across the arc, is interpreted to result from the addition of a small melt fraction derived from the crustal component of the subducted lithosphere.  相似文献   

16.
钦-杭结合带斑岩型铜矿的基本地质特征及成因分析   总被引：15，自引：10，他引：5  
钦-杭结合带是重要的斑岩铜矿带.斑岩铜矿在北、中、南三段均有产出,主成岩成矿年龄主要集中燕山期.钦-杭结合带与成矿有关的斑岩主要为钙碱性岩浆系列的中酸性岩,岩石类型主要为花岗闪长斑岩、花岗斑岩和次英安斑岩等,具有与新元古代岛弧火成岩类似微量元素特征.它们来源于软流圈的岩浆与中下地壳物质混合后部分熔融的结果,与受到过早期俯冲作用影响的岩石圈地幔有关,存在壳幔混染现象.钦-杭结合带燕山期的斑岩铜矿成岩成矿的动力学背景与太平洋板块俯冲关系密切,但斑岩和铜矿体在地球化学特征上显示出的具有弧岩浆作用特点,这与华南地区中生代构造转折事件发生以前的地质演化有关.钦-杭结合带的中酸性斑岩体来自于元古代岛弧底部玄武质岩石(下地壳)在中生代时期的部分熔融,本质上该类矿床带有岛孤俯冲环境的特征遗传.  相似文献   

17.
Although volcanogenic massive sulfide (VMS) deposits can form within a wide variety of rift-related tectonic environments, most are preserved within suprasubduction affinity crust related to ocean closure. In stark contrast to the VMS-rich Appalachian sector of the Grampian-Taconic orogeny, VMS mineralization is rare in the peri-Laurentian British and Irish Caledonides. Economic peri-Gondwanan affinity deposits are limited to Avoca and Parys Mountain. The Tyrone Igneous Complex of Northern Ireland represents a ca. 484–464 Ma peri-Laurentian affinity arc–ophiolite complex and a possible broad correlative of the Buchans-Robert’s Arm belt of Newfoundland, host to some of the most metal-rich VMS deposits globally. Stratigraphic horizons prospective for VMS mineralization in the Tyrone Igneous Complex are associated with rift-related magmatism, hydrothermal alteration, synvolcanic faults, and high-level subvolcanic intrusions (gabbro, diorite, and/or tonalite). Locally intense hydrothermal alteration is characterized by Na-depletion, elevated SiO2, MgO, Ba/Sr, Bi, Sb, chlorite–carbonate–pyrite alteration index (CCPI) and Hashimoto alteration index (AI) values. Rift-related mafic lavas typically occur in the hanging wall sequences to base and precious metal mineralization, closely associated with ironstones and/or argillaceous sedimentary rocks representing low temperature hydrothermal venting and volcanic quiescence. In the ca. 475 Ma pre-collisional, calc-alkaline lower Tyrone Volcanic Group rift-related magmatism is characterized by abundant non-arc type Fe-Ti-rich eMORB, island-arc tholeiite, and low-Zr tholeiitic rhyolite breccias. These petrochemical characteristics are typical of units associated with VMS mineralization in bimodal mafic, primitive post-Archean arc terranes. Following arc-accretion at ca. 470 Ma, late rifting in the ensialic upper Tyrone Volcanic Group is dominated by OIB-like, subalkaline to alkali basalt and A-type, high-Zr rhyolites. These units are petrochemically favorable for Kuroko-type VMS mineralization in bimodal-felsic evolved arc terranes. The scarcity of discovered peri-Laurentian VMS mineralization in the British and Irish Caledonides is due to a combination of minimal exploration, poor-preservation of upper ophiolite sequences, and limited rifting in the Lough Nafooey arc of western Ireland. The geological and geochemical characteristics of the Tyrone Volcanic Group of Northern Ireland and peri-Gondwanan affinity arc/backarc sequences of Ireland and northwest Wales represent the most prospective sequences in the British and Irish Caledonides for VMS mineralization.  相似文献   

18.
The Alaska–Aleutian island arc is well known for eruptingboth tholeiitic and calc-alkaline magmas. To investigate therelative roles of chemical and temporal controls in generatingthese contrasting liquid lines of descent we have undertakena detailed study of tholeiitic lavas from Akutan volcano inthe oceanic Aleutian arc and calc-alkaline products from Aniakchakvolcano on the continental Alaskan Peninsula. The differencesdo not appear to be linked to parental magma composition. TheAkutan lavas can be explained by closed-system magmatic evolution,whereas curvilinear trace element trends and a large range in87Sr/86Sr isotope ratios in the Aniakchak data appear to requirethe combined effects of fractional crystallization, assimilationand magma mixing. Both magmatic suites preserve a similar rangein 226Ra–230Th disequilibria, which suggests that thetime scale of crustal residence of magmas beneath both thesevolcanoes was similar, and of the order of several thousandyears. This is consistent with numerical estimates of the timescales for crystallization caused by cooling in convecting crustalmagma chambers. During that time interval the tholeiitic Akutanmagmas underwent restricted, closed-system, compositional evolution.In contrast, the calc-alkaline magmas beneath Aniakchak volcanounderwent significant open-system compositional evolution. Combiningthese results with data from other studies we suggest that differentiationis faster in calc-alkaline and potassic magma series than intholeiitic series, owing to a combination of greater extentsof assimilation, magma mixing and cooling. KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales  相似文献   

19.
西藏冈底斯东段叶巴组火山岩的岩石地球化学特征研究表明:该火山岩为一套岩性连续分布的钙碱性火山岩,其中基性火山岩主要来源于岩石圈地幔的部分熔融,并受到流体的交代作用,中酸性火山岩则主要来源于地壳的部分熔融.叶巴组火山岩形成于岛弧或活动大陆边缘的构造环境,其动力来源应为新特提斯洋向北的俯冲消减.叶巴组发育的岛弧地区是形成具重要经济价值的VMS矿床的最佳地域.叶巴组火山岩的研究对早、中侏罗世的生物事件、气候变化以及海退或海侵事件也有重要的意义.  相似文献   

20.
辉长岩大多为地幔岩石部分熔融的产物,辉绿岩脉及碱性正长岩的形成通常与伸展构造有关,本文对海南岛万宁辉长岩及辉绿岩脉和分界洲正长岩进行了系统的年代学和岩石地球化学研究,并以此来讨论其构造意义。LA-ICP-MS锆石U-Pb定年结果表明,万宁辉长岩及辉绿岩脉形成约在240 Ma,分界洲正长岩形成约在231 Ma,主量元素特征表明万宁辉长岩和辉绿岩分别属碱性系列和亚碱性系列,分界洲正长岩属于典型的碱性岩浆岩。万宁辉长岩及辉绿岩脉的稀土元素具有轻稀土富集的特点(LREE/HREE=7.22～8.50和8.11～11.10),微量元素具有岛弧型火山岩的特征,富集大离子亲石元素K、Rb、Ba、Th,贫高场强元素Nb、Ta和Zr、Hf;分界洲正长岩的微量元素特征显示出与A型花岗岩类似的特征。海南岛三叠纪中基性岩如分界洲正长岩和万宁辉长岩及辉绿岩脉形成的构造背景为陆内伸展环境,指示海南岛在240～230 Ma处于印支造山运动的应力松弛阶段。  相似文献   

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