Geologische Rundschau

, Volume 67, Issue 2, pp 743–773 | Cite as

Petrogenesis of the felsic rock units of the late-Precambrian Sinclair Group, South West Africa

  • Brian R. Watters
Aufsätze

Abstract

The Sinclair Group is a late-Precambrian succession of basic to intermediate and felsic lava flows and intrusives, volcaniclastic and clastic units constituting part of an extensive volcanic, plutonic and clastic arc — the Rehoboth Magmatic Arc — which is regarded as having developed on the western and north-western margins of the sialic Kalahari Plate as a result of the subduction of oceanic crust.

The major felsic units comprise high-level granites, rhyolitic lava flows, plugs, domes, dykes and various volcaniclastics including ash-flow tuffs. Average major and trace element analyses of these units are presented and on this basis they can be separated into high-Ca and low-Ca groupings each with distinctive Ca content ranges and trace element distribution patterns.

The genesis of the felsic units is discussed and it is considered that the magmas were produced by partial fusion of sialic crust; the heat source for fusion can be found in the repeated injection of basic magma during evolution of the Group. The high-Ca magmas resulted from melting at the deepest crustal levels mainly involving a quartzofelspathic host rock and hornblende whereas the low-Ca melts were derived from the partial fusion of a similar host rock and biotite at shallower levels in the crust. Water content of the magmas, which was probably in the region of 1–2 per cent and not more than 4 per cent, could have been provided by these hydrous minerals and partial melting probably took place between depths of 18 km and 36 km in the continental crust at temperatures of about 750° C to 950° C.

Ash-flow tuffs resulted from vesiculation of the felsic magma at depths between 0 and 4 km, at temperatures between 800° C and 950° C and with water contents restricted to a range of about 1,5 to 4 per cent.

Keywords

Lava Flow Volcaniclastic Partial Fusion Rhyolitic Lava Element Distribution Pattern 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Zusammenfassung

Die Sinclairgruppe ist eine spätpräkambrische Folge, bestehend aus intermediären und sauren Lavaergüssen und Intrusivgestein, sowie auch aus pyroklastischen und klastischen Einheiten, die einen Teil eines ausgedehnten vulkanisch, plutonisch und klastischen Bogen — den Rehoboth magmatischen Bogen — bilden. Dieser Bogen entwickelte sich an den westlichen und nordwestlichen Rändern der sialischen Kalahariplatte, wahrscheinlich infolge einer Subduktion der ozeanischen Kruste.

Die wichtigsten kieselsauren Einheiten bestehen aus subvulkanischen Graniten, rhyolitischen Ergußgesteinen, Kuppen, Gängen und verschiedenen vulkanischen Lockerstoffen. Es werden durchschnittliche Haupt- und Spurenelementanalysen vorgelegt, die in Ca-reiche und Ca-arme Gruppen gegliedert werden können, jede mit unterschiedlichen Ca-Wert-Bereichen und Spurenelementverteilungs-Muster.

Der Ursprung kieselsäurereicher Einheiten wird besprochen, und es wird erwägt, ob Magmen sich bei einer Teilaufschmelzung der sialischen Kruste bildeten; die Schmelzwärme entstand beim wiederholten Eindringen des basischen Magmas, während der Entwicklung der Sinclairgruppe. Die Ca-reichen Magmen folgten einer Aufschmelzung überwiegend quarz-feldspatischen Gesteins und Hornblende des tiefsten Unterbaus der Erdkruste, wogegen die Ca-armen Magmen bei einer Teilaufschmelzung eines ähnlichen Gesteins und Biotit in einer geringeren Tiefe entstanden. Der Wasserinhalt der Magmen, der sich zwischen 1 und 2 Prozent und nicht über 4 Prozent befand, war womöglich von den hydroxidhaltigen Mineralien versehen. Die Teilaufschmelzung fand wahrscheinlich in einer Tiefe von zwischen 11 km und 36 km in der sialischen Kruste statt, bei Temperaturen, die zwischen 750° C und 950° C lagen.

Tuffite entstanden durch Gasblasenbildung in den felsischen Magmen in Tiefen zwischen 0 und 4 km, bei Temperaturen zwischen 800° C und 950° C, mit einem Wasserinhalt, der sich auf einen Bereich von 1,5 Prozent bis 4 Prozent beschränkte.

Résumé

Le groupe Sinclair représente une succession d'âge precambrien superieur composé de laves et d'intrusions basiques à intermédiaires, et de sédiments volcano-détritiques et clastiques. Ces roches font partie d'un large arc volcanique, plutonique et détritique — l'arc magmatique de Rehoboth — qui est considéré comme s'étant développé à la limite ouest et nord-ouest de la plaque sialique du Kalahari. Il résulte de la subduction de la croûte océanique.

Les principales unités felsitiques comprennent des granites de faible profondeur, des coulées de laves rhyolitiques, culots, dômes, et divers dépôts volcano-détritiques comprenant des tufs. Des moyennes d'analyses des éléments majeurs et mineurs de ces unités sont présentées; sur cette base elles peuvent être séparées en groupes, riches en chaux d'une part, et pauvres en chaux d'autre part. Chaque groupe est caractérisé par un éventail de valeurs distinctives en chaux, de même que par la distribution des éléments en traces.

On discute la genèse des unités felsitiques et on considère que les magmas se sont formés par fusion partielle de la croûte sialique; la source thermique ayant provoqué la fusion peut être recherchée dans des injections répétées de magma basique pendant l'évolution de la série. Les magmas riches en chaux résultent de la fusion à grande profondeur de roches quartzo-feldspathiques à hornblende tandis que les mélanges fondus pauvres en chaux se sont produits à plus faible profondeur et dérivent aussi de roches quartzo-feldspathiques mais à biotite. La teneur en eau des magmas, prpobablement 1–2%, pas plus de 4%, doit avoir pour origine les minéraux mentionnés précédemment. La fusion partielle s'est probablement produite entre 18 km et 36 km de profondeur dans la croûte continentale à des températures d'environ 750° C à 950° C.

Les coulées de tufs résultent de la vésiculation de magmas felsitiques à des profondeurs comprises entre 0 et 4 km, à des températures de 800° C à 950°C et avec des teneurs en eau variant approximativement de 1,5 à 4%.

Краткое содержание

Серия Синклера являе тся поздне-докембрий ской свитой, составленной из средних и кислых лаво вых потоков и интрузи вных пород, а также из пирокласти ческих и кластических компо нентов, образующих ча сть обширной вулканической, плуто нической и кластичес кой дуги — магматичес кой дуги Rehoboth. Эта дуга образовала сь на западном и северо-западном крае сиалической глыбы Калагари, вероя тно, в результате субд укции океанической коры.

Важные кремнеземные компоненты представ лены субвулканическими гранитами, риолитовы ми изверженными поро дами, куполами, жилами и различными рыхлыми п ородами вулканическ ого происхождения. Усред ненные результаты анализа н а основные и рассеянн ые элементы разрешают подразделить породы на богатые и бедные ка льцием; причем каждая из этих групп отличаетс я и картиной распреде ления рассеянных элементо в.

Обсуждается происхо ждение богатых кремн езёмом единиц и высказывает ся предположение: не обр азуются-ли магмы в рез ультате частичного выплавле ния сиалической коры; при повторных внедрения х базических магм во вр емя развития серии Синкл ер температура могла подниматься до уровня, достаточно го для выплавления. Бога тые кальцием магмы об разовались благодаря выплавлению преимущ ественно кварц-полев ошпатовых пород и роговой обман ки глубинного фундамен та коры, а то время, как бедные кальцием магмы возникали при ч астичном выплавлени и сходных пород и биотитов на незначительной гл убине. Содержание вод ы в магме составляет в среднем 1–2 %, и не превышает 4 %; она п роисходит, возможно, и з минералов, содержащих гидрооки си. Частичное выплавл ение происходило в сиалич еской коре, вероятно, на глуб ине 11 и 36 км при температ уре от 750 до 980 °C.

Туффиты возникли в ре зультате образобани я пузырей газа в фельзических магмах на глубине от 0 д о 4 км при температуре о т 800 до 950 °C, причем содержание воды здес ь колебалось в предел ах 1,5 до 4 %.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1978

Authors and Affiliations

  • Brian R. Watters
    • 1
  1. 1.Department of Geological SciencesUniversity of ReginaReginaCanada

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