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Evidences for Polyphased Oceanic Alteration of the Extrusive Sequence of the Semail Ophiolite from the Salahi Block (Northern Oman)

  • C. Pflumio
Part of the Petrology and Structural Geology book series (PESG, volume 5)

Abstract

The extrusive sequence of the Salahi block (northern Oman) consists of a well-developed dyke complex and thick volcanic member. The latter is composed of three events (V1, V2 and V3). Lenses of pelagic, often metalliferous sediments are present within the lava flows. The largest exposures of these sediments are observed at the interface of the different volcanic units. The V1—V2 contact is also the locus of the fault-controlled, Zuha sulphide prospect.

A strong metamorphic zonation overprints the extrusive sequence of the Salahi block (greenschist-facies assemblage in the dyke complex, prehnitepumpellyite-facies assemblage in the lower part of the volcanic sequence, zeolite-facies and low-temperature assemblages in high stratigraphic level flows). Although the steep thermal gradients and static recrystallization suggest that the observed zonation had developed in response to seawater circulation, the dykes and lava flows alteration differs from that described in oceanic layer 2 in three respects: 1) by the pervasiveness of the recrystallization, 2) by the occurrence of a prehnite-pumpellyite-facies assemblage at the top of the first accretion-related volcanic event (V1), 3) by the relative scarcity of low-temperature minerals and the widespread development of phases uncommon in the modern oceanic crust (i.e. iron-rich pumpellyite) in the high stratigraphic level lavas (V2, V3).

Field observations and mineralogical study indicate that the Salahi block extrusive sequence has been subjected to three stages of hydrothermal circulation and to low-temperature oceanic alteration that were contemporaneous with the three magmatic events of the Semail complex. The superposition of alteration phases accounts for the peculiarities of the metamorphic zonation in the volcanic member of this ophiolite.

The origin of the Zuha mineralized zone is attributed to the hydrothermal phase activated by the second, off-axis, magmatic event. The Zuha prospect displays the characteristics of a hydrothermal discharge zone: sulphide-bearing lavas with a typical stockwork paragenesis (quartz, Fe-chlorite, rectorite, titanite), appear in the uppermost Vl sequence below the gossans. This assemblage results from an interaction between Vl lavas and hot (250–300°C), relatively low pH, metal-loaded, upwelling fluids. After interaction with the volcanics and formation of the sulphide mineralization, the hydrothermal fluids were responsible for the formation of large, Mn-rich, sedimentary lenses at the top of the Vl unit in the vicinity of the mineralized zone.

Keywords

Hydrothermal Alteration Magmatic Event Volcanic Event Volcanic Sequence Metalliferous Sediment 
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.

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© Springer Science+Business Media Dordrecht 1991

Authors and Affiliations

  • C. Pflumio
    • 1
  1. 1.Ecole Nationale supérieure des Mines de ParisC.G.G.M.Paris Cedex 06France

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