Geologie und Bau des Sithonia-Ophioliths (Chalkidiki,NE-Griechenland): Anmerkungen zur Bildung ozeanischer Krusten

The Upper Jurassic Sithonia ophiolite of Chalkidiki (NE Greece) provides the opportunity to study the processes of the formation of fossil oceanic crusts in detail. This ophiolite consists from top to base of:

1. an over 900 m thick succession of shallow-water sediments frequently interlayered with basic volcanics;
2. an about 700 m thick basaltic layer which predominantly consists of hyaloclastics;
3. an excellently developed sheeted dyke complex which is at least 1.2 km thick. It is the main scope of this study.

The sheeted dyke complex consists entirely of dykes. They have intruded each other preferably along their margins. Two groups of dykes may be distinguished. The first group comprises those dykes which were obviously formed within the active spreading zone. These dykes make up the structure of the sheeted complex. They are in average 3.9 m thick and do not display a clear polarity of their chilled margins. The dykes of the second group were formed later, i.e. off-ridge. They are up to 0.8 m thick and almost always symmetrically chilled. The general strike of the dykes is N 54° E. This direction is almost perpendicular to the major tectonic units of Chalkidiki. The geometrical relations between the dykes can be best explained by formation at an oscillatory spreading axis. Available geological and geochemical data suggest a low to very low spreading rate. A periodic process of dyke injection is recognized. The dyke injection periods are of variable duration. This together with the restricted and variable magmatic differentiation within the different periods points towards a nonsteady state magma chamber beneath the spreading axis. The off-ridge dykes impart valuable information as regards the spreading direction and the relative position of the paleoridge. Some ideas about the spreading rate may also be obtained from these dykes. The shallow-water conditions during the sedimentation may be explained as due to the tectonic uplift of the Upper Jurassic oceanic crust possibly along a transcurrent fault. This interpretation is supported by other geological observations.     

Constraints on the formation of geochemically variable plagiogranite intrusions in the Troodos Ophiolite,Cyprus

The geochemistry and petrology of tonalitic to trondhjemitic samples (n = 85) from eight different plagiogranite intrusions at the gabbro/sheeted dyke transition of the Troodos Ophiolite were studied in order to determine their petrogenetic relationship to the mafic plutonic section and the lava pile. The plagiogranitic rocks have higher SiO₂ contents than the majority of the glasses of the Troodos lava pile, but lie on a continuation of the chemical trends defined by the extrusive rocks, indicating that the shallow intrusions generally represent crystallised magmas. We define three different groups of plagiogranites in the Troodos Ophiolite based on different incompatible element contents and ratios. The first and most common plagiogranite group has geochemical similarities to the tholeiitic lavas forming the lavas and sheeted dyke complex in the Troodos crust, implying that these magmas formed at a spreading axis. The second plagiogranite group occurs in one intrusion that is chemically related to late-stage and off-axis boninitic lavas and dykes. One intrusion next to the Arakapas fault zone consists of incompatible element-enriched plagiogranites which are unrelated to any known mafic crustal rocks. The similarities of incompatible element ratios between plagiogranites, lavas and mafic plutonic rocks, the continuous chemical trends defined by plagiogranites and mafic rocks, as well as incompatible element modelling results, all suggest that shallow fractional crystallisation is the dominant process responsible for formation of the felsic magmas.     

Sheeted dykes in Kandra ophiolite complex,Nellore schist belt,Andhra Pradesh — Vestiges of Precambrian oceanic crust

Extensive field and petrological studies carried out in Kandra ophiolite complex (KOC) in the southern part of Nellore schist belt (NSB) revealed the existence of sheeted dykes in Kandra- Kondakuru-Gollapalli section. The sheeted dykes occur in the northern part of the complex and compositionally vary from medium-grained dolerites showing typical sub-ophitic texture to diabase dykes that are extremely fine grained aphanitic to fine-grained cryptocrystalline parallel basic dykes (varying in width from 4 cm to 1.25 m). These dykes show chilled margins on either side with the host dykes into which these are intruded. Veins of oceanic plagiogranite (0.5 × 5 m) are noticed in a major sheeted dyke. Occurrence of oceanic plagiogranite as ovoid / semicircular patches of 6 cm diameter resembling “immiscible droplets” within basic dykes is also noticed. The southern part of the complex is made of a major NW-SE trending cumulus gabbro that grades into olivine gabbronorite in its eastern part. The southern gabbros represent the plutonic component and form the deeper part of the complex. In this paper an attempt has been made to present the field, petrographic and mineral chemistry account of various lithounits that constitute the Kandra ophiolite complex in a succinct manner.     

北祁连山中段早中奥陶世蛇绿岩中席状岩墙杂岩的发现及其地质意义

作为蛇绿岩套重要组成部分之一的席状岩墙杂岩(Sheeted dyke complex)近来在北祁连山中段肃南县大岔大坂北坡的早中奥陶世蛇绿岩中被发现。这一发现对于祁连山早古生代蛇绿岩来说尚属首次,而且对于研究蛇绿岩的发展演化及探讨奥陶纪时洋底扩张都有重要意义。席状岩墙杂岩由一系列具单向冷凝边的辉绿岩墙组成,以一墙挨一墙的形式产出,岩墙间无任何填充物。席状岩墙杂岩在矿物组合上,常量元素、稀土元素及痕量元素地球化学特征和配分模式,甚至金属硫化物矿化作用方面都有类似之处。这些证据表明席状岩墙杂岩是连通其下岩浆房与其上枕状熔岩的通道。席状岩墙的单向冷凝边为岩浆上升方式和扩张洋脊的存在提供了令人信服的证据。根据Zr/Y—Zr关系图式,得出该区洋脊的扩张速率大约为2cm/a。     

A sheeted dyke complex within the Coolac Ophiolite,southeastern New South Wales

A unit composed of sheeted dykes and an associated unit of sodic felsic rocks have been found within the Coolac Ophiolite, about 5 km east of Coolac township, southeastern N.S.W. The dyke complex consists of small multiple dykes intruding gabbro screens. About half the dykes are basaltic in composition and half are ande‐sitic. Felsic differentiates occur as minor intrusions within the dyke complex, and also constitute the sodic felsic unit. It is suggested that the dyke complex and associated felsic rocks be included in the Honeysuckle Beds, and that the absence of a mappable dyke unit further south within the Honeysuckle Beds is caused by tectonic dismembering of the ophiolite. The dyke complex bears considerable resemblance to the sheeted dyke members of other ophiolite sequences. However, the scale of development is roughly 10 times smaller than at Troodos or Newfoundland, and one‐way chilling is not well developed. The mechanism of intrusion in the Coolac dyke complex therefore is probably not symmetrical spreading about a well defined axis, but a diffuse injection of small multiple dykes within a relatively broad extensional zone. The environment in which such a mechanism might develop, and which is suggested by geochemical evidence, including the high proportion of intermediate and felsic rocks, is that of a small basin adjacent to or within an island‐arc system.     

Extensional deformation along the late Precambrian-Cambrian Baltoscandian passive margin: the Sarektjåkkå Nappe,Swedish Caledonides

The structural evolution of a part of the late Precambrian Baltoscandian passive margin just before the inception of seafloor spreading is described, recording the change from deformation by faulting to dominantly magmatic extension of the crust. The allochthon of the Scandinavian Caledonides contains the imbricated passive margin of continental Baltica overlain by various exotic terranes. The Sarektjåkkå Nappe in the Seve Nappe Complex, which contains the outer parts of Baltica's passive margin, consists of sedimentary rocks, occurring as screens between Vendian (573±74 Ma) diabase dykes. These dykes constitute 70–80% of the nappe and locally form sheeted dyke complexes. The Sarektjåkkå Nappe largely escaped penetrative Caledonian deformation and preserves igneous, metamorphic and structural elements that are linked to the evolution of a pre-Caledonian rift to a passive continental margin. Extensional deformation before dyke emplacement is recorded by normal faults, pull-apart structures and folds. Unconformities, dykes affected by brittle deformation, and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments, extensional deformation and dyke emplacement. The Sarektjåkkå Nappe is compared with other parts of the Baltica's passive margin and its tectonic evolution is discussed.     

The difference between oceanic and continental tholeiites: a fluid dynamic explanation

Continental tholeiites have higher SiO₂, K₂O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.     

Geochemistry and petrogenesis of the Proterozoic dykes in Tamil nadu, southern India: another example of the Archaean lithospheric mantle source

Approximately 1650-Ma-old NW/SE and NE/SW-trending dolerite dykes in the Tiruvannamalai (TNM) area and approximately 1800-Ma-old NW/SE-trending dolerite dykes in the Dharmapuri (DP) area constitute major Proterozoic dyke swarms in the high-grade granulite region of Tamil nadu, southern India. The NW- and NE-trending TNM dykes are compositionally very similar and can be regarded as having been formed during a single magmatic episode. The DP dykes may relate to an earlier similar magmatic episode. The dolerites are Fe-rich tholeiites and most of the elemental variations can be explained in terms of fractional crystallisation. Clinopyroxene and olivine are the inferred ferromagnesian fractionation phases followed by plagioclase during the late fractionation stages. All the studied dykes have, similar to many continental flood basalts (CFB), large-ion lithophile element (LILE) and light rare-earth element (LREE) enrichment and Nb and Ta depletion. The incompatible element abundance patterns are comparable to the patterns of many other Proterozoic dykes in India and Antarctica, to the late Archaean (～2.72 Ga) Dominion volcanics in South Africa and to the early Proterozoic (～2.0 Ga) Scourie dykes of Scotland. The geochemical characteristics of the TNM and DP dykes cannot be explained by crustal contamination alone. Instead, they are consistent with derivation from an enriched lithospheric mantle source which appears to have been developed much earlier than the dyke intrusions during a major crustal building event in the Archaean. The dyke magmas may have been formed by dehydration melting induced by decompression and lithospheric attenuation or plume impingement at the base of the lithosphere. These magmas, compared with CFB, appear to be the minor partial melts from plume heads of smaller diameter and of shallow origin (650 km). Therefore, the Proterozoic thermal events could induce crustal attenuation and dyke intrusions in contrast to the extensive CFB volcanism and continental rifting generally associated with the Phanerozoic plumes of larger head diameter (>1000 km) and of deeper origin (at crust mantle boundary).     

Fault-related carbonate breccia dykes in the La Chilca area,Eastern Precordillera,San Juan,Argentina

Carbonate fault breccia dykes in the Cerro La Chilca area, Eastern Precordillera, west-central Argentina, provide clues on the probable mechanism of both fault movement and dyke injection.Breccia dykes intrude Upper Carboniferous sedimentary rocks and Triassic La Flecha Trachyte Formation. The timing of breccia dyke emplacement is constrained by cross cutting relationships with the uppermost Triassic unit and conformable contacts with the Early Miocene sedimentary rocks. This study supports a tectonic-hydrothermal origin for these breccia dykes; fragmentation and subsequent hydraulic injection of fluidized breccia are the more important processes in the breccia dyke development.Brecciation can be triggered by seismic activity which acts as a catalyst. The escape of fluidized material can be attributed to hydrostatic pressure and the direction of movement of the material establishes the direction of least pressure.Previous studies have shown that cross-strike structures have had an important role in the evolution of this Andean segment since at least Triassic times. These structures represent pre-existing crustal fabrics that could have controlled the emplacement of the dykes. The dykes, which are composed mostly of carbonate fault breccia, were injected upward along WNW fractures.     

侵入岩脉风化壳对中林村残积土滑坡渗流场和稳定性的影响

岩脉侵入是一种普遍的地质现象,侵入岩脉及其风化壳对斜坡的渗流场和稳定性具有重要影响,但目前国内外在该方面的研究鲜见报道。我国东南沿海约50×104 km2的白垩系火山岩地层中广泛分布侵入岩脉及其风化壳,改变了该地区残积土斜坡的失稳机制。以浙江省文成县中林滑坡为对象,研究花岗岩脉风化壳对其渗流场和稳定性的影响。中林滑坡主要由凝灰岩及其残积土组成,后部有侵入的花岗岩脉及其残积土,利用室内土柱试验获取降雨作用下两种残积土渗透性质的差异,利用不同含水率下的直剪试验获取非饱和抗剪强度参数;利用Geo-studio软件中的Seep/W模块反演土柱降雨试验,获取两种土体的非饱和渗透参数;接着将上述参数应用于中林滑坡的渗流场模拟,获取降雨入渗下坡体的渗流场;将渗流场耦合到坡体稳定性计算中,获取了坡体的稳定性变化曲线及最危险滑面。结果表明,降雨通过侵入岩脉风化壳快速入渗,并顺着基覆面向凝灰岩残积土区侧向渗流,导致侵入区与原始斜坡的交界处以及基覆面附近的地下水位快速升高,孔隙水压力迅速增大,斜坡在该位置最易变形破坏,且失稳深度远大于普通的浅层残积土斜坡,可达6～10 m。该研究成果可为东南沿海地区有岩脉侵入的残积土斜坡的稳定性评价与防治设计提供理论依据。     

义县组主期中酸性岩墙群与熔岩地球化学特征及其地质意义

对义县组主期中酸性岩墙与熔岩的地球化学、成因岩石学对比分析显示：中酸性岩墙Mg^#平均36.68%,Na₂O/K₂O平均为1.07;微量元素标准化配分图表现出富集Rb、Ba、K大离子亲石元素,亏损Sr及高场强元素Nb、Ta、P、Ti;稀土元素标准化配分图上表现出负Eu异常,HREE强烈分异,Y/Yb平均10.10,（Ho/Yb）_N平均1.13,LREE配分曲线与熔岩LREE配分曲线重合.中性、中基性熔岩Mg^#平均大于55%,除无Sr负异常外,微量元素标准化配分图与岩墙相似;中性、中基性熔岩稀土元素标准化配分曲线相互平行,并且配分高低与SiO₂含量呈反相关关系,说明二者是部分熔融的非同源岩浆发生混合作用的产物;中性、中基性熔岩Y/Yb分别平均为11.27、11.98;（Ho/Yb）_N分别平均为1.25、1.32,Sr大于400×10^-6,Sr/Y均大于40,显示出了典型埃达克岩地球化学特征.地球化学特征表明中酸性岩墙与火山熔岩来自不同源区,前者来源于斜长石稳定的加厚角闪石麻粒岩地壳部分熔融,而后者来源于受幔源岩浆底侵并且混染过的加厚石榴石麻粒岩相下地壳部分熔融,并且岩墙母岩浆、熔岩岩浆与幔源底侵岩浆在形成过程中可能发生过不同比例的混合作用.结合义县组最底部高Mg^#幔源玄武质岩浆成因机制,主期中酸性岩墙与熔岩岩浆形成机制揭示了早白垩世期间华北板块壳-幔之间岩浆动力学过程.     

Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides

The Sarek Dyke Swarm (SDS) crops out in the Sarektjåkkå Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components: (1) a 4–5 km thick succession of rift-related sedimentary rocks, which is intruded by (2) a suite of tholeiitic dykes (the SDS) constituting 70–80% of the nappe. The nappe was deformed during Caledonian thrusting, but dykes and sedimentary rocks in the interior of the eastern parts of the SN are preserved in a pristine state. The tholeiitic dykes of the SDS commonly occur in sheeted dyke complexes, and up to 11 successive generations can be identified from crosscutting relations. The SN represents the fossil continent–ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading. Bubble-shaped pods and veinlets of diorite are present in the SDS sheeted dyke complexes. The pods are absent in the oldest dykes, but the younger a dyke, the more frequent the pods. The diorite pods are the equivalent of gabbro pegmatites, and both cogenetic and coeval with the dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally discordant, indicating a crystallization age of 608±1 Ma (²⁰⁷Pb/²⁰⁶Pb=607.9±0.7 Ma, MSWD=0.33). This age is inferred to date the onset of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin.     

The geometry of a dyke swarm as a result of dyke interaction with each other and with external stresses

In the framework of plane elastic problem we use a numerical approach to study the forms of opening and paths of growth of three parallel magma fractures as a model of dyke swarm formation and development. As expected, the internal dynamic mechanism of dyke interaction distorts their shapes in comparison with a single dyke shape, and curves their paths combining them into a divergent or a convergent system. The external dynamic mechanism of regional stress tends to align the growing dyke paths in parallel to the axis of the maximum compressive stress. The external and internal mechanisms compete with each other. The impact of the internal mechanism is stronger when the ratio of the distance between dykes to their length is less, the initial parallel dyke shift in relation to each other is larger, and the differential regional stress is less. Under the opposite conditions, the external mechanism prevails.     

Geometry and tectonic significance of Albian sedimentary dykes in the Sisteron area,SE France

Sedimentary dykes in the Sisteron area have penetrated pre-existing fracture sets in late Aptian and early Albian strata. Dynamic characteristics of such fractures are revealed by the bifurcation and the branching of dykes. Three-dimensional statistical analysis of the initial orientation of two vertical conjugate sets of dykes indicates their mechanics of formation, as well as the principal compressional stress direction during the development of the sill-dyke regime in the Sisteron area. Another set of sandstone dykes present in the area suggests syn-sedimentary and extensional faulting tectonic activity before the area became involved in compressional tectonics. Several significant dyke-forms clearly show the sense of shear along fractures filled with sand, and enable the stress pattern during dyke formation in the area to be recognized.     

The effects of layering and local stresses in composite volcanoes on dyke emplacement and volcanic hazards

Since most volcanic eruptions are fed by dykes, any assessment of volcanic hazards in an area must include an evaluation of the probability of injected dykes either reaching the surface or becoming arrested. Composite volcanoes are normally composed of alternating stiff (high Young's modulus) and soft (low Young's modulus) layers. Numerical models indicate that during unrest periods with magma-chamber inflation, the local stresses in composite volcanoes commonly prevent dyke-fed eruptions: while the stresses in the stiff layers may favour dyke propagation and seismogenic faulting, the local stresses in the soft layer remain seismically quiet and favour dyke arrest. Geodetic and field studies also indicate that most dykes never reach the surface, and that only a small fraction of the magma volume injected from a chamber erupts at the surface. I propose that for a dyke-fed eruption to occur, all the layers along the potential pathway of the dyke must have local stresses that favour magma-driven extension-fracture propagation. Thus, the stress field along the pathway must be homogenised. To cite this article: A. Gudmundsson, C. R. Geoscience 337 (2005).     

Rb–Sr and <Superscript>147</Superscript>Sm–<Superscript>143</Superscript>Nd systematics of gabbro and dolerites from fragments of ophiolite complexes of the Middle Urals

The Rb–Sr and ¹⁴⁷Sm–¹⁴³Nd age data obtained for sheeted dolerite dykes and rocks of the Platinum Belt of the Urals within the Tagil segment of the paleoceanic spreading structure (Middle Urals) are discussed. The study of the Rb–Sr isotope systematics of gabbro allowed us to reveal errochronous dependencies, which yielded ages of 415 and 345 Ma at (⁸⁷Sr/⁸⁶Sr)₀ = 0.70385 ± 0.00068 and 0.7029 ± 0.0010, correspondingly. The ¹⁴⁷Sm–¹⁴³Nd isotope age data demonstrate a specific coincidence of the chronometric ages of the sheeted dolerite dyke complex (426 ± 54, 426 ± 34, and 424 ± 19 Ma) and gabbro from the Revda gabbro–ultramafic massif (431 ± 27 Ma) and from screens between dolerite dykes in the sheeted dyke complex (427 ± 32 Ma, 429 ± 26 Ma). The proximity of the ¹⁴⁷Sm–¹⁴³Nd ages of gabbro and dolerite can be explained by the thermal effect of the basaltic melt, which is the protolith for the dyke complex, on the hosting gabbro.     

The Scourie dyke suite: Petrogenesis and geochemical nature of the Proterozoic sub-continental mantle

The petrogenesis of bronzite-picrite, olivine-gabbro, norite and quartz-tholeiite dykes, which make up the 2.39 b.y. Scourie dyke swarm cutting the Archaean Lewisian gneisses of N.W. Scotland, is interpreted on the basis of their major and trace element geochemistry. Most of the dykes bear primary amphibole and/or phlogopite and, with one exception, are all hypersthene- or quartz-normative. Apart from one tholeiite dyke which shows relative light rare-earth element depletion, all the dykes show enrichment in light rare-earths and large-ion lithophile elements. They do not however show an equivalent enrichment in other incompatible high field strength ions such as Nb and Ta, and in this respect resemble island arc and calc-alkaline basalts. The different dyke types have distinctive rare-earth patterns and other trace element ratios which are maintained over a range of major element compositions.Petrogenetic modelling of the major and trace element compositions of the various dykes demonstrates that very few can be related by fractional crystallisation. Indeed, even with partial melting mechanisms at least two different mantle sources, with different major and trace element compositons, are required to explain the compositional differences between the major dyke types. The high degrees of mantle melting implied for the generation of the magnesium-rich bronzite-picrites suggests that their rare earth and other trace element patterns closely reflect those of their mantle source. Similar arguments, though less well constrained, can be advanced for the other dyke types. The results suggest that the sub-continental mantle source feeding the dykes was heterogeneous with respect to both major and trace elements, and that their mantle sources must have been enriched in lithophile elements. Enrichment at the time the Lewisian gneisses were generated (i.e. 2.92 b.y. ago) would be compatible with the initial ⁸⁷Sr/⁸⁶Sr ratios of the dykes and the inferred Rb/Sr ratios of their mantle sources. The sub-continental mantle sources have thus retained the geochemical signature of the crustgenerating processes some 500 m.y. earlier.     

丁青蛇绿岩新知

1982年7—8月间,我们在青藏高原东部横断山地区综合科学考察工作中,对丁青一带的蛇绿岩进行了初步调查,发现了席状岩墙群和一个比较连续的蛇绿岩剖面。 丁青蛇绿岩位于班公湖—东巧—怒江蛇绿岩带的中段。近几年来,不少人对这个带进行了研究。由于构造破坏,蛇绿岩强烈肢解,迄今为止,还未报道过有如丁青那样连续的蛇绿岩剖面。丁青蛇绿岩的新发现,对于青藏高原第二条缝合线内蛇绿岩的性质以及古特提斯海演化历史的研究提供了新的资料。 席状岩墙群出露于西藏丁青县沙贡区海拔5120米的日隆山的北坡。岩墙走向NE—NNE,倾角很陡,倾向sE或NW。岩墙宽约4米至20厘米,有的仅数厘米。主要由辉绿岩组成,中粒至细粒,有的为隐晶质的。     

Geochemistry and petrogenesis of the post-orogenic bimodal dyke swarms in NW Sinai, Egypt: constraints on the magmatic–tectonic processes during the late Precambrian

The study area in the northwest Sinai represents one of the most significant regions in the Egyptian basement intensely invaded by post-orogenic calc-alkaline dyke swarms. Two post-orogenic dyke swarms have been recognized in NW Sinai namely: (1) mafic dykes of basalt, basaltic andesite and andesite composition and (2) felsic dykes of dacite, rhyodacite and rhyolite composition. These basaltic to rhyolitic dykes intruded contemporaneously and shortly after the intrusion of the post-orogenic leucogranite. The mafic and felsic dykes are enriched in incompatible elements, especially in the large ion lithophile elements (e.g. K, Rb, Ba) and depleted in high field strength elements with negative P, Ti and Nb anomalies. Major and trace element geochemistry indicates that investigated mafic and felsic magma types are not related via fractional crystallization. The protoliths of the mafic and felsic dykes appear to have evolved from different parental magmas. The incompatible trace element patterns favour a derivation of the mafic dykes from melting of a garnet peridotite source followed by fractional crystallization of olivine, clinopyroxene amphibole and zircon. The felsic dykes, on the other hand, could be generated by melting of garnet-free source modified subsequently by fractional crystallization of plagioclase, apatite and titanomagnetite. This implies variable source characteristics at the end of the Pan-African in the NW Sinai.The mafic and felsic dykes can be related to an intracontinental setting and that this was accompanied by a chemical evolution of the subcontinental lithosphere. Magma generation and ascent in the area was favoured by extensional movements, which is already known from other areas in NE Africa.     

Age and origin of early Proterozoic dolerite dykes in South-West Greenland

Samples of three dolerite dykes from South-West Greenland give a pooled weighted mean Rb-Sr whole-rock isochron age of 2,130±65 Ma. Initial ⁸⁷Sr/⁸⁶Sr ratios are 0.70155±0.00018 for one of the dykes (Naujat) and 0.70277±0.00012 for the other two (Torssut and Akinaq). A fourth dyke (Qaqarssuaq) did not yield an isochron. Torssut samples have significantly lower initial ²⁰⁷Pb/²⁰⁴Pb ratios than samples from the Naujat dyke, indicating that the Torssut dyke has been affected by crustal contamination. Samples of the dykes vary widely in chemical composition because of an uneven distribution of different primocryst phases throughout the dykes, and because of variable degrees of fractionation of the magma. For two of the dykes some constraints on the composition of the parent magma have been inferred from trace element data. Higher concentrations of K, Rb, Ba, Sr, Ce and perhaps Nb (at comparable levels of magmatic fractionation) in the dykes with higher Sr_i are probably related to crustal contamination of their parent magmas. Two of the dykes probably intruded from zoned magma chambers. The marginal zones of these dykes crystallised from a strongly fractionated magma, whereas the centres of the dykes consist of more primitive material with large proportions of olivine and plagioclase primocrysts. The central part of Torssut is less strongly contaminated with crustal Pb than the border zones. A more detailed analysis of the Pb-isotopic composition in the Torssut dyke shows that contaminant lead must have been derived from rocks in the deeper crust considerably older than the late Archaean gneisses which occur at the surface.