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Large-scale hydrothermal dolomitisation in the southwestern Cantabrian Zone (NW Spain) : causes and controls of the process and origin of the dolomitising fluids

Gasparrini, Marta

German Title: Großräumige hydrothermale Dolomitisierung in der südwestlichen Kantabrischen Zone (NW Spanien) : Ursachen und Steuerung des Prozesses und Herkunft der dolomitiierenden Fluide

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Translation of abstract (English)

The Cantabrian Zone in NW Spain represents the foreland belt of the Variscan Iberian Massif and is made up of a Precambrian basement covered by Palaeozoic sediments. This succession was thrusted and folded in Late Carboniferous time during the Variscan Orogeny, resulting in several thin-skinned thrust units. The rocks underwent deformation, diagenetic to epizonal thermal events, and several episodes of fluid-flow. A spectacular product of fluid circulation in this area is a large-scale dolomitisation, mostly affecting the carbonates of the Late Carboniferous succession. The aim of this work is to reconstruct the main episodes of late diagenesis in the Carboniferous carbonates of the Bodón thrust unit and to define the causes of the dolomitisation as well as the origin of the dolomitising fluids. Employed methods are transmitted light and cathodoluminescence microscopy, X-ray diffraction and ICP-ES analyses, O, C and Sr isotope geochemistry and fluid inclusion study, comprising microthermometry, Raman spectroscopy and crush-leach analyses. The precursor carbonates underwent burial and deformation prior to dolomitisation, resulting in the development of bedding parallel stylolites and calcite veins. The dolomitisation post-dated the main Variscan compressinal event in the study area (Westphalian B - Stephanian B) and pre-dated the precipitation of ore minerals (~280-260 Ma). The dolomite bodies display irregular morphology and are often associated with rock discontinuities, such as fault, bedding, lamination and stylolite planes. The dolomitisation consisted in two major events: the first replacive and the second void-filling. “Zebra-structures” are common. These are given by the repetition of mm-scale replacive and void-filling dolomite sheets and bear cavities locally filled by later calcite cements. The dolomites display features typical of burial dolomitisation, such as scarce preservation of precursor fabrics, coarse crystallinity, non-planar texture and abundant dolomite of the saddle type. The replacive and void-filling dolomites have a very similar geochemical signature. They both have a dull red and unzoned CL and are nearly stoichiometric and well ordered. They display depletion in Sr, and enrichment in Fe and Mn relative to the precursor carbonates. They have wide-ranging d18O values (-3 to -12 ‰ PDB), distinctly lower than those of the precursors. On the contrary, the d13C values (1,7-5,4 ‰ PDB) were strongly buffered by the precursor carbonates. Both replacive and void-filling dolomites formed from hot (100-170 °C) and saline (19,0-22,6 eq. wt % MgCl2) fluids, slightly radiogenic and strongly enriched in 18O relative to Late Palaeozoic seawater. A continuous dolomitisation process which evolved from a replacive stage towards a void-filling stage in a nearly isochemical system is postulated. The dolomitising fluids were hydrothermal and hypersaline brines, with isotopic signature of chemically modified Late Palaeozoic seawater. The dolomitisation possibly occurred during the Early Permian, when predominantly extensional tectonics followed the main Variscan compression. During this period the Cantabrian Zone underwent crustal thinning, possibly accompanied by increased heat flow from the basement. Shallow water sedimentation in semiarid climatic conditions occurred in isolated basins controlled by normal faults. Regional Variscan fractures (e.g. León Fault) were reactivated and controlled the occurrence of volcanic and magmatic phenomena. It is proposed that Early Permian seawater, concentrated by evaporation, circulated downwards through fractures and mixed with formation waters. The latter carried abundant 18O, radiogenic 87Sr and high salinity, because of fluid-rock interaction with ambient rocks at high temperatures. Thermal convection, induced by the increased heat flow, homogenised the surface seawater and the subsurface formation waters and made them circulate through the permeable precursors. The concentrated seawater furnished continuous supply of Mg to the convecting system. Main pathways for the dolomitising fluids were Variscan thrust and fault planes. The León Fault possibly played a major role for fluid circulation as suggested by the almost complete dolomitisation in the region of the Bodón Unit closer to this fault. The dolomitisation was followed by the precipitation of burial calcite from fluids of the same hydraulic system than the dolomites, but having higher salinities and slightly lower temperatures. Much later in the diagenetic history of the studied carbonates a low temperature calcite precipitated from fluids having a meteoric signature, suggesting the exposure of the study area.

Item Type: Dissertation
Supervisor: Bechstädt, Prof. Dr. Thilo
Date of thesis defense: 20. June 2003
Date Deposited: 07. Jul 2003 11:16
Date: 2003
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institut für Geowissenschaften
Subjects: 550 Earth sciences
Controlled Keywords: Kantabrische Zone, Versenkungsdiagenese, hydrothermale Dolomitisierung, hypersaline Lösungen, Thermalkonvektion
Uncontrolled Keywords: Cantabrian Zone , burial diagenesis , hydrothermal dolomitisation , hypersaline brines , thermal convection
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