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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.
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   

5.
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   

6.
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   

7.
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   

8.
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   

9.
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   

10.
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   

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