The geochemistry of the south Portuguese zone, Spain and Portugal


Mullane, Eta (1998) The geochemistry of the south Portuguese zone, Spain and Portugal. University of Southampton, Faculty of Science, School of Ocean and Earth Science, Doctoral Thesis , 469pp.

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Description/Abstract

Tectonic Setting Overview

Within this work the sediments (both detrital and chemical ) and igneous rocks of the South Portuguese
Zone are considered to have been deposited within a continental rift zone. This rift zone was linked to northwards
(present day directions) subduction of the Palaeotethys oceanic plate under the continental Ossa Morena Zone
plate. Within this setting the South Portuguese Zone is located on the northern margin of the Gondwana Plate
and the Ossa Morena Zone on the southern margin of the Armorican plate. Crustal thinning and rifting developed
due to stresses within the trailing plate. This allowed upward doming of the mantle under the area of thinning
(passive rifting), and with time rifting became active and no longer required the force from subduction to exist. As
a result of this morphology, two discrete basins were present in the Middle Devonian to Lower Carboniferous
times, i.e. a continental rifted basin to the south and an oceanic basin to the north.

Puio Do Lobo Domain

Sediments of the Pulo do Lobo Domain were deposited as melanges within the northern oceanic basin.
Two main groups are recognised within the Domain: the Ferreira-Ficalho Group to the north of the basin and the.
Changa Group to the south. The Peramora Melange Formation, at the base of the Ferreira-Ficalho Group is
interpreted as a tectonic melange produced by off-scraping of material from the subducting oceanic slab and from
the overriding continental wedge. The remaining formations constituting the domain are classified as sedimentary
melanges. Geochemistry indicates that sediments of the Changa Group and the Ferreira-Ficalho Group are
broadly similar, with some minor differences. These differences coupled with the spatial arrangement of the
groups suggest that each group derived from a discrete provenance. It is proposed that Changa Group
sediments are products of erosion of the continental flank on the southern margin of the oceanic basin, i.e. the
Gondwana Plate. Ferreira-Ficalho Group sediments derive from erosion of the northern Armorican flank. The
variation in provenance signature along strike in the Ferreira-Ficalho Group suggests that the Armorican source
area was not chemically homogenous. Eventual closure of the basin produced the present day imbricated
arrangement of the Ferreira-Ficalho Group and Changa Group. In Spain, sedimentary formations superseding
the Pulo do Lobo Formation are partially derived from a mafic/basaltic source and this source is the Beja-
Acebuches Amphibolite. Average model ages for each Ferreira-Ficalho Group formation range from 1204Ma in
the Pulo do Lobo Formation to 885Ma in the Horta da Torre Formation. The effects of incorporation of younger
Beja-Acebuches Amphibolite material also decreases up succession. Thus, it is concluded that the Ferreira-
Ficalho Group derives from an overturned sedimentary succession. The model age of the Peramora Melange
Formation is 649Ma, which is slightly older than the average for the Beja-Acebuches Amphibolite (424Ma). This
supports the mixing of relatively young oceanic material with older crustal material during Peramora Melange
Formation generation.

Beja-Acebuches Amphibolite

The spatial arrangement of Beja-Acebuches Amphibolite lithologies coupled with the average model age
(424Ma) and the average sNd value (+9.43) preclude any other origin, other than that of an obducted portion of the
oceanic crust, which was subducting to the north during the closure of the Palaeotethys. Obduction occurred in a
northerly direction, between the deposition of the Pulo do Lobo Formation and the Ribeira de Limas Formation (of
the Ferreira-Ficalho Group). This places a Middle Devonian age on obduction (based on the palynological
evidence of previous workers). Geochemical data infer a depleted, tholeiitic (N-MORB-like) origin for the
amphibolite. The marked incompatible element depletion observed suggests that the parent melts of the
amphibolite were generated in a setting far removed from any sources of enrichment.

Phyllite-Quartzite Group

During the Palaeotethys closure to the north, a continental rift zone was developed on the northern
margin of the southern trailing Gondwana Plate. Sediments of the Phyllite-Quartzite Group represent the first
sediments deposited within this newly formed rift basin. Due to the location of the rift, the source of the Phyllite-
Quartzite Group sediments is likely to be totally confined to the basement of the South Portuguese Zone, and
sedimentary fades relationships infer deposition on a shallow marine platform. Element geochemistry suggests
that there were two discrete sources active during sedimentation. However, isotope geochemistry illustrates that
both groups show old model ages and sNd values, typical of old, recycled crustal material, with group two being
slightly less prevalent and older. Spatially, these groups appear to be randomly scattered. The presence of two
discrete geochemical signatures is not necessarily indicative of two separate sources, but may arise from
geochemical heterogeneity of a single provenance. However, the weight of evidence favours tow separate
sediment sources. At the top of the Phyllite-Quartzite Group, sedimentation changes to turbulent processes, and
this may coincide with the segmentation of the originally broad continuous basin into a set of sub-basins, with
differential subsidence rates.

Volcanic-Siliceous Complex

The episodic nature of Volcanic-Siliceous Complex magmatism is likely to be triggered by episodic
mantle doming, and it is likely that the base of the Volcanic-Siliceous Complex represents the transition from
passive to active rifting conditions. Basic tholeiitic magmas are initially derived from a depleted N-MORB source
due to their LREE characteristics. Subsequent contamination has given rise to the presently observed La-PR
depleted REE profiles. Trace and REE mass balance calculations suggest approximately 50% crustal
contamination, but many other compelling lines of evidence favour a significantly smaller degree of
contamination. Acid volcanics may be derived from partial melting of underplated basaltic magmas at the base of
the continental crust. Periods of no igneous activity are represented within the complex by black shales, massive
and stockwork sulphides and manganiferous cherts. Purple shales are randomly developed throughout the
complex, but are almost ubiquitously present between the second and third volcanic events. Geochemistry does
not clearly delineate the provenance of either the black or purple shales, but derivation from the Phyllite-Quartzite
Group is favoured, with additional material from acid igneous rocks.

Massive Sulphide And Stockwork Sulphide

The large tonnages of sulphide generated within the Volcanic-Siliceous Complex requires high heat flow,
and long-lived hydrothermal circulation. Mantle upwelling will compress the geothermal gradients and the
resulting high heat flow will provide the heat necessary for establishment and continuation of the hydrothermal
cells. Thus, sulphides are generated at the cessation of a phase of igneous activity, while the geothermal
gradients remain elevated. Positive Eu anomalies characterise the massive sulphides and a s a result
comparisons can be drawn between processes operating in the Volcanic-Siliceous Complex and present day
oceanic vent sites. Lack of sulphide oxidation, absence of positive Ce anomaly and preservation of the positive
Eu anomaly within massive sulphides indicates that interaction with seawater was minimised. Thus, sulphide
precipitation is envisaged to have occurred under a a bottom-hugging blanket of highly saline, reducing brine.
The origin of this brine is compared with the origin of Red Sea brines. Mass balance calculations illustrate that
the incorporation of silicate within sulphide analysis has a variable impact on the REE profiles, depending upon
the degree of contamination and the REE characteristics of the silicate phases.

Manganiferous Chert Deposits

Manganiferous cherts are dominantly biogenic, produced from radiolarian tests, with only minor
participation of hydrothermal and hydrogenous sources. Reduced detrital input is necessary for a thriving
radiolarian population, and as the depositional basins were compartmentalised and separated by small ridges,
this suggests that the surface area of sediment source areas was diminished. Absence of radiolarian tests is
attributed to recrystallisation, as tests initially comprise amorphous silica. Chert REE and Nd-isotope
geochemistry is variable and this is attributed to the presence or absence of detrital and/or volcanic material,
which overprints the initial hydrothermal characteristics. This variation in signature is dominantly controlled by the
spatial location of the chert, within the depositional basin., Genetically, both the massive sulphides and chert
deposits are independent, the cherts being biogenic and sulphides being hydrothermal. Additionally, both may be
precipitated concurrently. It has been previously suggested that sulphides were preserved and oxidation
hindered, by the presence of siliceous gel cap. This is not necessary if bottom-hugging brines were present. This
factor explains why sulphides are preserved in the absence of a capping horizon.

Item Type: Thesis (Doctoral)
Additional Information: Digitized via the E-THOS exercise.
Subjects: Q Science > QE Geology
Q Science > QD Chemistry
Divisions: University Structure - Pre August 2011 > School of Ocean & Earth Science (SOC/SOES)
ePrint ID: 42142
Date Deposited: 22 Nov 2006
Last Modified: 27 Mar 2014 18:27
URI: http://eprints.soton.ac.uk/id/eprint/42142

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