Rb-Sr geochronology in favour of polymetamorphism in the Pan African Damara belt of Namibia (South West Africa)

New Rb-Sr whole rock age data are reported from two metasedimentary sequences of the Damara Supergroup in the central Pan African Damara belt of Namibia (South West Africa).Calc-granofels rocks of the Karibib Formation (Swakop Group) near Usakos are dated at 665±34 Ma (⁸⁷Rb=1.39×10^–11 a^–1) which is interpreted as reflecting a high-grade metamorphic event predating widespread granite intrusion.Pyroxene-bearing feldspathic gneisses of the Khan Formation (Nosib Group) from the Khan-Swakop River area east of Swakopmund show incomplete homogenization at 474±16 Ma. In view of similar ages obtained on the nearby Rössing alaskite granite and on biotites from a variety of rock assemblages this age is interpreted as reflecting a second Damaran metamorphic event rather than a specific stage in a long cooling history as previously thought.It is probable that the new ages characterize two distinct Pan-African tectono-metamorphic events previously named Katangan and Damaran episodes respectively (Clifford, 1967). The younger of these has affected large areas of south western Africa both within orogenic zones and on the neighbouring Kalahari Craton and may reflect crustal processes of sub-continental proportion during the closing stages of the Pan-African tectogenesis.

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Zusammenfassung Neue Rb-Sr-Gesamtgesteins-Isochronenalter von zwei sedimentären Abfolgen der spätpräkambrischen Damara-Supergruppe in der Zentralzone des Pan-Afrikanischen Damara-Orogens von Namibia (Südwest-Afrika) deuten auf eine mehrphasige metamorphe Geschichte hin.Kalk-Granofels-Gesteine der Karibib-Formation (Swakop-Gruppe) südöstlich von Usakos ergaben ein Alter von 665±34 M. J. (⁸⁷Rb=1,39×10^–11 a^–1) und wir interpretieren dieses Alter als das Resultat isotoper Homogenisierung während einer intensiven Regionalmetamorphose, die vor dem Eindringen weitverbreiteter Granite stattfand.Pyroxenführende und feldspathaltige Gneise der Khan-Formation (Nosib-Gruppe) aus dem Gebiet der Khan- und Swakop-Flüsse östlich von Swakopmund zeigen unvollständige Homogenisierung ihrer Sr-Isotope um 474±16 M. J. In Anbetracht ähnlicher Alter für den benachbarten Rössing-Alaskitgranit und für Biotite verschiedener Gesteinstypen aus dem gesamten Zentralbereich des Orogens interpretieren wir obiges Alter eher als den Höhepunkt einer zweiten Regionalmetamorphose als ein bestimmtes Stadium in der post-orogenen Abkühlungsgeschichte.Es ist wahrscheinlich, daß unsere Altersbestimmungen zwei deutlich voneinander getrennte pan-afrikanische tektono-metamorphe Phasen charakterisieren, vor allem wenn man sie in Zusammenhang mit publizierten Daten betrachtet, und wir nehmen an, daß diese den vonClifford (1967) postulierten Katanga- und Damara-Episoden entsprechen. Die jüngere dieser Episoden hat nicht nur die pan-afrikanischen orogenen Bereiche im südwestlichen Afrika, sondern auch weite Teile des Kalahari-Kratons erfaßt und mag auf tektogenetische Prozesse von nahezu kontinentalen Ausmaßen in und unter der Kruste im frühen Paläozoikum zurückzuführen sein.

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Résumé Deux séquences métasédimentaires du supergroupe du Damara ont été datées par la méthode Rb/Sr.Des gneiss calciques («granofels») de la formation de Karibib (groupe de Swakop), récoltés près d'Usakos ont été datés à 665±34 Ma (avec⁸⁷Rb=1.39× 10^–11 a^–1). Cet âge est interprété comme le résultat d'un événement métamorphique de degré é levé antérieur à la mise en place des grandes masses de granite.Des gneiss feldspathiques à pryoxène de la Formation de Khan (Groupe de Nosib), échantillonnés dans la région de la Khan River — Swakop à l'Est de Swakopmund, montrent une homogénéisation isotopique incomplète à 474±16 Ma. Des âges voisins ayant été obtenus pour l'Alaskite de Rössing qui affleure dans la même zone et pour les biotites de différentes roches, cet âge est interprété comme le résultat d'un second métamorphisme damarien plutôt que comme un stade de la longue histoire de refroidissement comme cela avait été imaginé auparavant.Ces nouveaux âges caractériseraient les deux épisodes tectonométamorphiques distincts de l'orogénie Pan-Africaine, qui avaient été appelés respectivement épisode katangien et épisode demarien parClifford (1967). Le plus récent de ces événements a affecté une grande partie de l'Afrique du S.W. aussi bien dans les zones orogéniques que dans le craton voisin du Kalahari; il peut refléter une évolution crustale d'échelle subcontinentale se produisant pendant les derniers stades du tectogène Pan-Africain.

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Rb/Sr - , - ; , .- , , 665±34 (⁸⁷Rb=1,39 10^–1a^–1); , . , , , , 474 ± 16 . - ; , , . , - - , , . , 1967 , . - - , ; , , .

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Contribution No. 27 of the South African Geodynamics Project, presented at the 9th Colloquium on African Geology, Göttingen, April 1977.     

Geochemistry and geotectonic setting of Late Proterozoic Katangan basic rocks from Kibambale in central Shaba (Zaire)

The Late Proterozoic Katangan basic rocks (dolerites and basalts) outcropping at Kibambale in central Shaba (Zaire) are classified as extension-related tholeiites. The concentrations of immobile major and trace elements resemble those encountered in Phanerozoic enriched oceanic ridge basalts (E-MORB) emplaced in incipient oceanic rifts such as Red Sea and Tadjura Gulf troughs. This conclusion is also sustained by: elemental ratios which are MORB-like; multielement chondrite- and MORB-normalized patterns; sedimentological data, especially the shallow water conditions which prevailed during the deposition of lower Kundelungu tillite in which these basic rocks are interstratified; and the geological and structural framework, notably the emplacement of these igneous rocks in a basin not distant from the Kibaran continental land mass.In this area the geological units containing the igneous rocks are not affected by significant tangential tectonism and so the present structural relationships are relatively undisturbed compared with those which prevailed during the emplacement of the igneous rocks.     

冀东金矿带中生代岩浆作用及其成矿意义——来自锆石矿物学证据

冀东金厂峪金矿被认为与青山口花岗岩体侵入有关,而同样距离矿区较近的腰岭子花岗闪长岩体则研究较少,由于两岩体在岩性、构造背景等方面有很大的相似性,因此有必要探讨两成矿条件和成矿潜力差异的可能原因。本文对腰岭子和青山口岩体中的锆石进行透射光和阴极发光（CL）图像分析、晶体形态学以及统计学分析,结果显示两岩体的形成温度相差不大,分别为650～800℃和650～850℃。较高的岩浆热水含量和富碱环境可能是青山口岩体成矿的必要条件。应用LA- ICP- MS方法对腰岭子花岗闪长岩中的锆石进行U- Pb年代学测试,其结晶年龄为161.5±0.9 Ma,与峪耳崖金矿区内的闪长岩脉年龄相近,且两者在含矿岩体特征方面有很大的相似性,因此认为腰岭子岩体的成金潜力较大。两岩体稀土元素地球化学特征显示轻稀土相对亏损而重稀土相对富集。同时,有明显的Ce正异常和微弱的Eu负异常,结合野外地质观察,认为两岩体可能经历了壳幔混合作用。结合前人对青山口岩体年龄的测试结果及大地构造背景认为两岩体可能为来自同一岩浆房不同阶段侵位的产物。     

冀东金矿带中生代岩浆作用及其成矿意义——来自锆石矿物学证据

冀东金厂峪金矿被认为与青山口花岗岩体侵入有关,而同样距离矿区较近的腰岭子花岗闪长岩体则研究较少,由于两岩体在岩性、构造背景等方面有很大的相似性,因此有必要探讨两成矿条件和成矿潜力差异的可能原因。本文对腰岭子和青山口岩体中的锆石进行透射光和阴极发光（CL）图像分析、晶体形态学以及统计学分析,结果显示两岩体的形成温度相差不大,分别为650～800℃和650～850℃。较高的岩浆热水含量和富碱环境可能是青山口岩体成矿的必要条件。应用LA- ICP- MS方法对腰岭子花岗闪长岩中的锆石进行U- Pb年代学测试,其结晶年龄为161.5±0.9 Ma,与峪耳崖金矿区内的闪长岩脉年龄相近,且两者在含矿岩体特征方面有很大的相似性,因此认为腰岭子岩体的成金潜力较大。两岩体稀土元素地球化学特征显示轻稀土相对亏损而重稀土相对富集。同时,有明显的Ce正异常和微弱的Eu负异常,结合野外地质观察,认为两岩体可能经历了壳幔混合作用。结合前人对青山口岩体年龄的测试结果及大地构造背景认为两岩体可能为来自同一岩浆房不同阶段侵位的产物。     

中非刚果（金）加丹加铜钴矿带主要矿化类型及特征

文章在总结前人研究成果的基础上,结合野外勘查实践与认识,简要概括了加丹加铜钴矿带的区域地质背景、岩石地层划分、构造演化及沉积环境的变迁、铜钴矿床的空间分布及层控特征.在同生沉积成因理论的基础上,依据铜钴矿床在卢菲利弧内的空间分布规律、加丹加造山运动对矿床的改造程度,将加丹加铜钴矿带的层状铜钴矿床归纳为受D1变形作用改造的同生沉积铜钴矿床以及受D2-D3变形作用改造的同生沉积-热液改造铜钴矿床2种类型.前一类型矿床主要分布在外部褶皱推覆带的北侧,赋矿地层为罗安群的矿山亚群,褶皱构造控制着矿体的形态与空间展布,矿床受热液改造的程度低,氧化程度亦较低,氧化带深度一般为20～50 m,带内矿石矿物以孔雀石和辉铜矿为主,铜主要以结合氧化铜形式存在,矿床的钴含量相对较低,铜钴比值较大.后一类型矿床主要分布在外部褶皱推覆带的南侧,赋矿地层为罗安群的迪佩特亚群和木瓦夏亚群,线型构造如韧性剪切带控制着矿体的形态及空间展布,矿体呈层状、似层状,受热液改造程度高,矿床氧化程度较高,氧化带最深可达300 m,带内矿石矿物以孔雀石和水钴矿为主,钴含量高,铜钴比较值小,铜和钴主要以自由氧化铜及自由氧化钴形式存在.文章解剖了典型矿床的特征,并指出了进一步找矿的方向.     

Intrusive rocks and tectono-metamorphic evolution of the Mako Paleoproterozoic belt (Eastern Senegal,West Africa)

The Kedougou Kenieba Inlier (KKI) (Paleoproterozoic of Eastern Senegal) is a portion of the West African Craton (WAC) containing a granite-greenstone terrain that experienced three distinct periods of magmatic activity, peaking at 2200, 2160–2130 and 2100–2070 Ma. In the Inlier, Paleoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Our structural data and deformation sequence indicate that the Mako Greenstone Belt and the adjacent granitoid complexes have undergone a multi-phase evolutionary history that is spread over a prolonged period. The available geochronological data and field studies allowed classification of the granitoids of the KKI into four generations GI, GII, GIII and GIV.The current data suggest that the oldest rocks in the KKI, the Badon granites (2198 ± 2 Ma) and the tonalitic gneisses from Tonkouto (2200–2198 Ma) (GI), could be correlated with an early Birimian magmatic event. The gneisses, crystallized at depth, record the earliest deformation and in contrast to other tonalites, do not appear to have intruded volcanic rocks. The second manifestation of magmatism was intrusion of mafic diorite – the Gabbro Sandikounda Layered Igneous Complex type (GII) and development of the Laminia Kaourou Plutonic Complex (LKPC) (2160–2130 Ma). These bodies pre-date or are sometimes synchronous with a major deformational episode, and may, therefore, have formed very early in convergent Birimian orogenesis.The third major peak of magmatic activity occurred after the above major episode with the development of the oval shaped Diombalou and Bouroumbourou plutons (GIII). The orientation of these plutons parallel to the regional strike of the schistosity indicates structural control on granite emplacement. Eburnean magmatism was terminated in the Mako Belt following compressional Eburnean deformation, with the emplacement of the Tinkoto, Mamakono plutons (GIV) in the east of the complex and continued in the Dialé–Daléma supergroup with the syntectonic emplacement of the Saraya batholith. Garnitiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2080 Ma.Field observations suggest the early plutons of the complex granitic (Kakadian) batholith intruded during convergent deformation whereas later igneous activity accompanied regional orogen-parallel extension, followed by exhumation. In the Mako Belt, thickening of the crust was proposed to have caused heating and the ‘apparent diapiric rise’ of the Diombalou and Bouroumbourou plutons.     

Pan African plate tectonics and its repercussions on the crust of northeast Africa

Pan African belts of the African mainland and the Arabian-Nubian Shield exhibit evolutionary features which are either compatible with intracontinental ensialic development or with plate margin and Wilson cycle tectonics during the time period 1100–500 Ma ago.It is suggested that both the ensialic and plate margin developments were caused by the same fundamental subcrustal forces that are reponsible for present-day lithospheric motion, and the Pan African event is therefore representative of a transition from Precambrian ensialic plate tectonics to Phanerozoic Wilson cycle tectonics.The North African craton between the central Hoggar and eastern Egypt/Sudan is characterized by widespread late Pan African thermal activity, documented by voluminous calc-alkaline granitoid intrusions and alkaline to peralkaline ring complexes, as well as a general resetting of mineral isotopic systems in older basement rocks (Schürmann, 1974;Vachette, 1974). Nagy et al. (1976) interpreted these features as evidence for progressive eastward growth of the West African craton but I suggest that pre-Pan African continental crust including Archaean elements (Ouweinat) was already in existence in northeast Africa. This crust was strongly affected by intraplate stresses resulting from closing of the Pharusian ocean in the western Hoggar and from collisional tectonics in the Arabian shield.Much of the late Pan African deformation in northeast Africa appears to have occurred by strike slip motion and there is a striking analogy with the Cenozoic deformation pattern behind the Himalayan belt in northeast Asia.Following the model ofMolnar andTapponier (1975) the deformation patterns of Asia and northeast Africa are compared, and it is possible to relate the Pan African structures and magmatism between the Hoggar and Egypt/Sudan to continental collision and identation of a semi-infinite hot crustal segment by a cold rigid plate (the West African craton).

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Zusammenfassung Die panafrikanischen Faltengürtel des afrikanischen Kontinentes und des Arabisch-Nubischen Schildes zeigen während des Zeitraumes von ca. 1100 bis 500 M. J. Entwicklungstendenzen, die teilweise im Sinne einer ensialischen Orogenese und teilweise als Folge einer Plattenrand-Evolution (Wilson Zyklus) gedeutet werden können.Trotz ihrer verschiedenartigen Anlage sind sowohl die ensialischen als auch die Plattenrand-Orogene wahrscheinlich auf die gleichen subkrustalen Kräfte zurückzuführen, die auch heute für die Bewegung von Lithosphärenplatten verantwortlich sind. Es ist daher anzunehmen, daß die panafrikanische Episode geotektonisch eine Übergangsphase von vorwiegend ensialischer Orogenentwicklung des Präkambriums zur phanerozoischen Plattentektonik darstellt.Im Bereich des nordafrikanischen Kratons zwischen dem zentralen Hoggar Gebirge und Ost-Ägypten/Sudan wirkte sich die panafrikanische Episode vorwiegend als Thermalphase aus, wobei weitverbreitet voluminöse Granitplutone und alkalische Ringkomplexe in die alte Kruste eindrangen und von einer regionalen isotopischen Rejuvenation der älteren Gesteine begleitet wurden (Schürmann, 1974;Vachette, 1974). Einige Autoren deuten die panafrikanischen Granitintrusionen in Nordostafrika als Resultat einer allmählichen Krustenakkretion, es gibt jedoch genügend Anhaltspunkte für die Annahme einer kontinuierlichen prä-panafrikanischen Kruste vom Zentralhoggar bis nach Ägypten, zu der auch die archaischen Elemente des Ouweinat Massives gehören. Dieser stabile Bereich wurde als Folge der spätpräkambrischen Kollisionstektonik im westlichen Hoggar und im Arabisch-Nubischen Schild starken Spannungen unterworfen und reagierte vor allem durch Bruchtektonik, wobei besonders Horizontalverschiebungen dominierten.Es besteht eine auffallende Ähnlichkeit zwischen dieser Verformung und dem Deformationsschema in Nordost-Asien hinter dem jungen Faltengebirge des Himalaya. Beide Bereiche werden verglichen und es scheint möglich zu sein, die panafrikanische Bruchtektonik und den zugehörigen Magmatismus im Sinne vonMolnar &Tapponier (1975) als Resultat von Plattenkollision zu deuten. Nach diesem Modell kollidierte im Westen der westafrikanische Kraton unter Schließung des pharusischen Mini-Ozeans mit Nordostafrika während im Osten im Bereich des heutigen Arabischen Schildes kontinuierliche Subduktionsvorgänge zur Bildung eines Kontinentalrandes vom Anden-Typus mit voluminösen kalkalischen Vulkaniten führten.

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Résumé Les zones mobiles pan africaines du continent africain et du bouclier arabo-nubien montrent des tendances évolutionnaires qui sont compatibles en partie avec le développement intracontinental et ensialique en partie avec les tectoniques de bord de plaques et du cycle de Wilson pendant la période: 1100–500 Ma.Il y a lieu d'admettre que le développement orogénique-ensialique et celui du bord des plaques sont provoqués part les mêmes activités fondamentales et sous-crustales qui sont responsables pour des mouvements lithosphériques d'aujourd'hui. Pour cette raison l'événement pan-africain peut être considéré comme une transition de la tectonique du plaqueensialique du Précambrien à la tectonique cyclique de Wilson du Phanérozoique.Le craton d'Afrique du Nord, entre le Hoggar central et l'Egypte de l'Est/Soudan, est caractérisé par une activité thermale tardive pendant l'épisode pan-africain. Cette activité est démontrée par des intrusions volumineuses de granite calcoalcalin et des complexes cirenlaires alcalins à peralcalins aussi bien que par une remise générale en place de systèmes isotopiques des minéraux dans les roches plus vieilles du socle. (Schürmann, 1974;Vachette, 1974). Nagy et al. (1976) ont interprêté ces caractères comme une preuve de la croissance progressive en direction de l'Est du craton ouest-africain; mais nous supposons que la croûte continentale pré-pan africaine y compris les éléments archéens (Ouweinat), a déjà existé en Afrique du Nord-Est. Cette partie de la croûte terrestre a été influencée fortement par des pressions internes qui résultent de la fermeture de l'océan pharusien dans l'ouest du Hoggar et de la tectonique de collision dans le bouclier arabe.Beaucoup de déformations pan-africaines tardives en Afrique du Nord-Est semblent être apparues àla suite d'un mouvement de décrochement. Il y a une analogie frappante avec le type de la déformation cénozoique à l'arrière de la chaine himalayenne de l'Asie nord-est. Les types de déformations de l'Asie et de l'Afrique du Nord-Est ont été comparés. Il semble possible de mettre en rélation les structures pan-africaines et le magmatisme de l'Hoggar et de l'Egypte/Soudan avec la collision continentale et l'indentation dune partie chaude de la croûte avec une plaque rigide et froide (le craton ouest-africain) après le modèle deMolnar &Tapponier (1975).

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- - 1100 500 , , , , ( ). , . , - . - - - , , , (Schürmann, 1974; Vachette, 1974). - , , - Hoggar , Ouweinat. Hoggar - , , . - . , , - , Molnar Tapponier (1975) , . , - - , , , , - .

     

The cooling history of a Pan African belt in the Adrar des Iforas, (Republic of Mali). Comparison with other Pan African belts

A study of biotite cooling ages in the Adrar des Iforas Pan African belt has been made using Rb/Sr techniques. The results indicate that uplift of this segment of the belt probably took place soon after intrusion of the last granites in each particular area. Comparison of the cooling ages in all the Pan African belts suggest there is a bi-partite age division and thus a diachronism in the Pan African.

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Zusammenfassung Abkühlungsalter von Biotiten aus dem Adrar des Iforas im Panafrikanischen Gürtel wurden mit der Rb/Sr-Methode untersucht. Die Ergebnisse zeigen, da\ der Aufstieg dieses Abschnittes des Gürtels wahrscheinlich kurze Zeit nach der Intrusion der letzten Granite in jedem einzelnen Gebiet erfolgte. Der Vergleich von Abkühlungsaltern in allen Panafrikanischen Gürteln legt nahe, da\ es eine zweiteilige Altersgruppierung gibt und somit einen Diachronismus im Panafrikanischen Ereignis.

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Résumé On a fait l'étude des âges de refroidissement des biotites dans la chaÎne panafricaine dans l'Adrar des Iforas suivant la technique Rb/Sr. Les résultats indiquent que le soulèvement de cette partie de la chaÎne eut lieu bientÔt après l'intrusion des derniers granites. La comparaison des âges de refroidissement dans toutes les chaÎnes panafricaines suggère qu'il y a une différence d'âge entre deux domaines et donc un diachronisme dans le Panafricain.

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Geochronological scale and evolution of late Cenozoic magmatism within the Caucasian segment of the alpine belt

Results of the isotope-geochronological studies of the Late Cenozoic magmatism of Caucasus have been considered. The Neogene-Quaternary volcanic activity is found to have evolved during the last 15 m. y. being most intensive in the Middle-Late Pliocene. Within separate neovolcanic areas of the Caucasus region, magmatism was of a clearly discrete character when intense eruption periods interchanged with prolonged (up to several million years) times of quiet conditions. Four stages of young magmatism of the Caucasus are recognized: the Middle Miocene (15–13 Ma), the Late Miocene (9–5 Ma), the Pliocene (4.5–1.6 Ma), and the Quaternary (less than 1.5 Ma). However, for certain areas the time limits of these stages were shifted relative to each other and overlap the whole age range from the mid-Miocene to the end of the Quaternary period. Therefore, within the collision zone, the Neogene-Quaternary magmatism evolved almost continuously during almost the last 9 m. y., but in the time interval of 13–9 Ma in the Caucasian segment, volcanic activity was possibly low. No evidence of directed lateral migration of volcanic activity within the entire Caucasus region was found. At the same time, in the Lesser Caucasus the young magmatism commenced earlier (∼15 Ma), compared to the Greater Caucasus (∼8 Ma).     

西昆仑西段晚古生代—中生代花岗质岩浆作用及构造演化过程

本文通过对西昆仑西段地区晚古生代—中生代花岗岩的岩石类型、形成时代和岩石地球化学资料的综合分析,探讨花岗质岩浆活动期次、岩石成因,结合区域资料,探讨构造-岩浆演化特征和碰撞造山过程。将该地区晚古生代—中生代构造-岩浆演化分为7个阶段:(1)388~324 Ma(特提斯Ⅰ、Ⅱ支洋向北俯冲消减阶段),具富钠贫钾特征的低温TTG岩石组合,形成于陆缘弧环境;(2)339~291 Ma(奥依塔格弧后盆地演化阶段),由于南部特提斯Ⅰ支洋持续往北俯冲,导致西昆仑北缘发生弧后扩展而形成弧后盆地,形成拉斑质具强烈富钠贫钾特征的低温大洋花岗岩;(3)258~241 Ma(特提斯Ⅰ支洋闭合、碰撞造山阶段),岩石中发育石榴子石和白云母,普遍具片麻状构造,属于S型花岗岩,陆壳部分熔融的产物;(4)234~210 Ma(特提斯Ⅰ后碰撞伸展阶段):岩体规模较大,为I型→A型花岗岩,伴随着地幔岩浆底侵和强烈的壳幔岩浆混合作用;(5)198~150 Ma(特提斯Ⅱ支洋向南俯冲消减阶段):类似TTG的岩石组合,形成于与洋壳俯冲有关的岩浆弧环境;(6)148~118 Ma(特提斯Ⅱ支洋闭合、碰撞造山阶段):弱片麻状二云二长花岗岩,属C型埃达克岩,为陆-陆碰撞过程中陆壳加厚发生部分熔融的产物;(7)111~75 Ma(特提斯Ⅱ后碰撞伸展阶段):发育规模较大,钾玄质系列,是古老地壳部分熔融的产物。根据各阶段花岗质岩浆活动特征和构造演化过程,初步提出了西昆仑西段晚古生代—中生代大地构造演化模式图。     

Post-collisional magmatism in the northern Arabian-Nubian Shield: The geotectonic evolution of the alkaline suite at Gebel Tarbush area, south Sinai, Egypt

Post-collisional alkaline magmatism (∼610–580 Ma) is widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield (ANS), i.e. the northern part of the Egyptian Eastern Desert and Sinai. Alkaline rocks of G. Tarbush constitute the western limb of the Katharina ring complex (∼593 ± 16 Ma) in southern Sinai. This suite commenced with the extrusion of peralkaline volcanics and quartz syenite subvolcanics intruded by syenogranite and alkali feldspar granite. The mineralogy and geochemistry of these rocks indicate an alkaline/peralkaline within-plate affinity. Quartz syenite is relatively enriched in TiO₂, Fe₂O₃, MgO, CaO, Sr, Ba and depleted in SiO₂, Nb, Y, and Rb. The G. Tarbush alkaline suite most likely evolved via fractionation of mainly feldspar and minor mafic phases (hornblende, aegirine) from a common quartz syenite parental magma, which formed via partial melting of middle crustal rocks of ANS juvenile crust. Mantle melts could have provided the heat required for the middle crustal melting. The upper mantle melting was likely promoted by erosional decompression subsequent to lithospheric delamination and crustal uplift during the late-collisional stage of the ANS. Such an explanation could explain the absence or scarce occurrence of mafic and intermediate lithologies in the abundant late- to post-collisional calc-alkaline and alkaline suites in the northern ANS. Moreover, erosion related to crustal uplift during the late-collision stage could account for the lack or infrequent occurrence of older lithologies, i.e. island arc metavolcanics and marginal basin ophiolites, from the northern part of the ANS.     

Geochemistry and petrogenesis of Biabanak-Bafq mafic magmatism: Implication for the evolution of central Iranian terrane

Precambrian magmatism in the Biabanak-Bafq district represents an extensive sequence of mafic magmatic rocks. Major, trace and rare earth elements reveal that the low-Ti basement mafic rocks are magnesium tholeiite and low-Ti cover a mafic rock belongs to Fe-tholeiite, whereas, the high-Ti alkaline mafic rocks, as well as dolerites, show much more Fe–Ti enrichment. Primitive mantle normalized trace element patterns show a relative enrichment of LREE and LILE and depletion of HFSE, but have an equally distinct continental signature reflected by marked negative Nb, Sr, P, and Ti anomalies. The composition of the intrusive rocks is consistent with fractional crystallization of olivine ± clinopyroxene ± plagioclase, whereas variations in the Sr and Nd isotope compositions suggest heterogeneous sources and crustal contamination. Low-Ti group samples contain a crustal signature in the form of high La/Yb, Zr/Nb, and negative \(\varepsilon \hbox {Nd}\) values. In contrast, high-Ti mafic magmatic rocks display an increase in La/Yb with a decrease in Proterozoic alkaline rocks recognized across the central Iran. The presence of diverse mafic magmatic rocks probably reflects heterogeneous nature of sub-continental lithospheric mantle (SCLM) source. The mafic magmatism largely represents magmatic arc or rift tectonic setting. It is suggested that the SCLM sources were enriched by subduction processes and asthenospheric upwelling.     

中非新元古代铜钴成矿带的地质背景和控矿因素

卢菲利安弧形构造带由泛非运动引起,该带由南往北可进一步划分为加丹加高地、复向斜带、穹隆区、外部褶皱逆冲带和前陆带等5个单元。泛非运动期间,卢菲利安弧经历了3个不同的构造变形阶段。文章对中非铜钴成矿带的地层层序进行了合理划分,将中非铜钴成矿带中的矿床类型划分为6种,最著名的为矿山亚群中的层状铜钴矿床,次为恩古巴群中以碳酸盐岩为容矿岩石的锌铅铜银矿床;总结了成矿带中矿床的时空分布规律,认为矿床受地层、岩性、构造和表生氧化作用的联合控制。     

Areal extent and first U-Pb age of the Pre-Damaran Abbabis complex in the central Damara belt of South West Africa (Namibia)

The Proterozoic Abbabis Complex emerges from beneath the metasediments of the Late Precambrian Damara Supergroup within mantled gneiss domes and anticlinoria over a wide area in the central granite zone of the Pan African Damara belt. U-Pb dating of composite-, size-, and non-magnetic fractions of metamict zircons from two samples of Abbabis granite-gneiss from the farm Abbabis yielded a concordia intercept age of 1925 _–280 ⁺³³⁰ Ma. It is speculated that the Abbabis basement rocks are continuous under the Damaran metasedimentary cover and may link up with the Franzfontein basement inlier of similar age. This conclusion supports the ensialic character of the northeastern branch of the Damara orogenic belt.

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Zusammenfassung Der proterozoische Abbabis-Komplex tritt nicht nur als Grundgebirgsaufwölbung unter den Metasedimenten der Damara-Supergruppe im bisher bekannten Gebiet südlich von Karibib auf, sondern vergleichbare Gesteine wurden auch in Domstrukturen und Antiklinorien nahezu im gesamten Bereich der Zentralzone des Pan-Afrikanischen Damara-Orogens von Südwest-Afrika gefunden. Dieses vorwiegend aus Granitgneis bestehende Grundgebirge wurde während der Damara-Orogenese unterschiedlich stark überprägt, durch Granite intrudiert und zum Teil remobilisiert, so daß sein ursprünglicher Charakter stellenweise nicht mehr erkennbar ist und daher eine Unterscheidung von hochmetamorphen und teilweise anatektischen Damara-Metasedimenten nahezu unmöglich wirdU-Pb-Altersbestimmungen an gemischten Konzentraten metamikter Zirkone von zwei Granitgneis-Proben des Abbabis-Komplexes sowie an verschiedenen Korngrößen- und nicht-magnetischen Fraktionen ergaben ein Concordia-Schnittalter von 1925 _–280 ⁺³³⁰ M. J. Die relativ große Ungenauigkeit dieses Alters ist wahrscheinlich auf die heterogene Natur der analysierten Zirkone und damit auf eine bisher nicht näher bekannte komplexe prä-Damara-Geschichte des Abbabis-Komplexes zurückzuführen. Eine Überprägung während der Damara-Orogenese ist aus den Meßdaten nicht ersichtlich.Ähnlichkeiten der Abbabis-Geochronologie mit der des Franzfontein-Grundgebirgskomplexes im Norden des Damara-Orogens lassen auf eine kontinuierliche sialische Kruste unter den Damara-Deckschichten vom Zentralbereich des Orogens bis zu seinem nördlichen Vorland schließen. Eine direkte Verbindung des Abbabis-Komplexes mit dem Grundgebirge im südlichen Vorland des Orogens wird jedoch von den vorhandenen Altersdaten bisher nicht gestützt, so daß die von einigen Autoren postulierte ensialische Entwicklung des Nordost-Astes der Damara-Geosynklinale, obwohl wahrscheinlich, noch nicht endgültig geklärt ist.

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Résumé Le complexe d'Abbabis (d'âge Protérozoique) affleure en boutonnières au sein des métasédiments du supergroupe du Damara (Précambrien terminal); il forme des anticlinoria et des dômes (mantled gneiss domes) bien développés dans la zone granitique centrale de la chaîne pan-africaine du Damara. Des mesures par la méthode U/Pb effectués sur des fractions composites, granulométriques et non magnétiques de zircons métamictes, extraites de deux échantillons de granito-gneiss d'Abbabis (provenant de la ferme Abbabis) ont fourni un âge de 1925 _–280 ⁺³³⁰ Ma (intersection de la courbe Concordia). La continuité de ce socle d'Abbabis sous les méta-sédiments damariens et sa corrélation avec la boutonnière de socle du même âge de Franzfontein sont proposés. Le caractère ensialique du rameau NE de la chaîne damarienne est ainsi mis en évidence.

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Contribution Nr. 29 of the South African Geodynamics Project, presented at the 9th Colloquium on African Geology, Göttingen, April 1977.     

西秦岭东段印支期花岗质岩浆作用及构造演化：地球化学、锆石U-Pb年代学及Hf同位素约束

西秦岭造山带东段发育大量印支期花岗岩,由北向南贯通整个西秦岭造山带。本文选取西秦岭东段的柴家庄、太白、周家山和迷坝4个花岗质岩体进行岩石学、地球化学、锆石U-Pb定年和Lu-Hf同位素研究。这些花岗质岩体的岩性主体为石英闪长岩-花岗闪长岩-二长花岗岩,属高钾钙碱性系列,形成时代在237～219Ma之间,并显示出两期岩浆活动的特点,早期柴家庄岩体形成于236.6±2.9Ma,晚期的太白、周家山和迷坝岩体形成于220.12～218.9Ma。早期柴家庄二长花岗岩显示出类似埃达克岩的一些地球化学特点,如高含量的Sr(578 × 10^-6～661 × 10^-6)和高的(La/Yb)_N比值(37.44～41.73),低含量的Y(3.48 × 10^-6-5.50 × 10^-6)和Yb(0.3 × 10^-6～0.5 × 10^-6),以及弱的负Eu异常(δEu=0.85～1);在晚期花岗岩中,迷坝和周家山岩体同样表现高Sr含量(Sr>400 × ...     

First evidence for Cambrian rift-related magmatism in the West African Craton margin: The Derraman Peralkaline Felsic Complex

West of the southern, Archean, part of the Reguibat Rise of the West African Craton the Oulad Dlim Massif consists of metamorphic nappes stacked during the Mauritanides (Variscan) orogeny. In the Derraman region, about 12 km west of the nappes, we have found strongly deformed hypersolvus aegirine-riebeckite A1-type granites with SHRIMP zircon U–Pb ages of ca. 525 ± 3 Ma, ε(Nd)_525Ma (− 5.2 to − 6.8.) and Nd model ages T_CR ≈ 1.85 Ga. These granites define two km-sized bodies and a few smaller satellites. One body is emplaced within a 3.12 Ga leucocratic gneiss. The other body and its satellites are emplaced within an Archean low-grade metasedimentary sequence with detrital zircons that have ages that peak at 2.84 Ga, 2.91 Ga, and 3.15 Ga. These Archean gneisses and metapelite rocks define a tectonic unit, hereafter called the Derraman-Bulautad-Leglat (DBL) unit, which was formed from the Reguibat basement at the very margin of the WAC. The ~ 525 Ma Derraman granites are the oldest post-Archean rocks in this unit and were generated in an intraplate rifting environment from melting of crustal fenites during the ubiquitous Cambrian rifting event that affected this part of northern Gondwana. At the present level of knowledge, however, we cannot decide whether the “old” Nd isotope signature of Derraman granites resulted from melting of an old (Paleoproterozoic) fenite source or reflects the signature of the mantle-derived metasomatising fluids. The just-discovered Derraman granites are strikingly similar to other rift-related Cambrian–Ordovician hypersolvus aegirine–riebeckite granites widespread in North Gondwana. Understanding the potential connections between them would help to understand the Cambrian–Ordovician breakdown of northern Gondwana.     

Chemical and Sr-Nd compositions and 40Ar/39Ar ages of NW-trending dolerite dikes of Burkina Faso: Evidence for a Mesoproterozoic magmatism in the West African Craton

The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as ⁴⁰Ar/³⁹Ar ages of these dikes with the following strikes N98°–N112°, and N114°–N124° in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti (TiO₂ ≤ 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly (0.86–0.99) and slightly fractionated REE patterns ((La/Yb)_N = 2.5–9.1; Yb_N = 9.5–19.9). The ratios of Th/Ta (1.3–11.4) and Ce/Pb (5.2–58.5) suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 °C (calculated from olivine compositions) and pressures of 6.9 kbar (calculated for pyroxene minerals). Calculated initial ⁸⁷Sr/⁸⁶Sr (0.70040–0.70260) and ɛ_Nd(t) = +2.1 to −3.5 at 1575 Ma, also point to a crustal contamination with the most primitive samples showing T_DM values of 1946 Ma and 2154 Ma. The low values of La/Ba (<0.2) and Nb/La (<1.0), contrast with the low Th/Nb (<0.9), and suggest a lithospheric mantle or subduction-modified mantle as possible source for the dikes. Sr-Nd data, Mg# and Nb-Ta-Zr-Y-Th-Tb-Yb compositions further suggest that the most primitive samples were emplaced in a none orogenic setting and their magmas were subjected to variable crustal contaminations. Literature and the present whole rock ⁴⁰Ar/³⁹Ar age determinations show that the dikes were emplaced during a widespread Mesoproterozoic magmatism between 1.6 Ga and 1.2 Ga, and were affected by a thermal event causing the argon systematics resetting, best constrained by the date of sample KK1 (1236 ± 20 Ma, ⁴⁰Ar/³⁶Ar = 294 ± 13, MSWD = 2.2). Contemporaneous 1590–1570 Ma extensive magmatism is reported in other crustal blocks in Baltica (Sveconorw-Goth, svecofennian) NW Laurentia (Slave craton, Yukon), and Australia (Gawler craton), and together with the 1575 studied dikes, are related to the breakup of the supercontinent Nuna.     

The genesis of felsic magmatism during the closure of the Northeastern Paleo-Tethys Ocean: Evidence from the Heri batholith in West Qinling,China

To better understand the origin of voluminous silicic rocks in a convergent continental margin, we conducted an integrated study in which we have obtained geochronological, mineralogical, and isotopic (including whole-rock Sr–Nd–Pb, in-situ zircon Hf) data of the Heri batholith in West Qinling on the NE Tibetan Plateau. The batholith is composed of metaluminous to weakly peraluminous granodiorites (235–233 Ma) and porphyritic granodiorites (230–223 Ma) with an I-type affinity. Both lithologies share similar major, trace elemental and SrNd isotopic compositions. Detailed elemental data demonstrate that these granodioritic rocks underwent fractional crystallization of hornblende and apatite, with plagioclase (i.e. sieve-textured plagioclase cores) accumulation to some extent. Except for porphyritic granodiorites, the Pb isotopes for other analyzed samples are characterized by high radiogenicity and uniformity ((²⁰⁶Pb/²⁰⁴Pb)_t: 17.263–18.472, (²⁰⁶Pb/²⁰⁴Pb)_t: 15.571–15.591, and (²⁰⁶Pb/²⁰⁴Pb)_t: 38.032–38.304), together with limited variations in initial Sr ((⁸⁷Sr/⁸⁶Sr)_t: 0.707251–0.708103) and Nd (ε_Nd(t) = −7.1 to −6.3) isotopes with two-stage model ages (T_DM2) of 1.58–1.52 Ga. These factors collectively point to a derivation from the Mesoproterozoic basement rocks at the lower crustal level, or a comprehensive mixing of different-age components that generated an average crustal residence age. The SrNd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of granodiorites. Compared with the experimental melt compositions of amphibolites, the Heri granitoids are probably derived from an amphibolitic source under fluid-absent conditions due to the incongruent breakdown of amphibole and biotite. Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution, our interpretation is that the Heri batholith was formed during the initial collision between the North China Craton (NCC) and the South China Craton (SCC), which was accompanied by the closure of the Paleotethyan Ocean. Considering both previously published data and our new data, we propose that the Heri granitoids were mainly generated by the partial melting of lower crustal amphibolites, with minor mantle-derived melts.