The geochemistry of the south Portuguese zone, Spain and Portugal

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. Asa 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. Thevariation 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 MelangeFormation 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 as 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.

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