Geophysical mapping of the mafic and ultramafic rocks of the Seiland Igneous Province from the kilometer to the micrometer scale
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The Seiland Igneous Province (SIP) is the largest complex of mafic and ultramafic intrusions in northern Norway, and one of the best preserved deep-seated magmatic plumbing systems in the world. The province, generated from a voluminous set of igneous melts intruding the deep crust between 580 – 560 Ma at a still disputed location, was later moved and uplifted, and finally emplaced onto the Baltican margin during the Caledonian Orogeny. This thesis presents the results of a multidisciplinary study on the SIP. It addresses the following key questions: Is it possible to add any independent structural evidence for the deep plumbing system nature of the SIP? What is the current volume of the SIP? What is the proportion of mafic versus ultramafic rocks? How can the physical and magnetic properties of the SIP rocks be used to investigate the syn- and post-emplacement history of the complex? How relevant is remanent magnetization in the interpretation of the magnetic signature of these rocks? Geophysical interpretation of potential field data is commonly used for mapping and modeling of the spatial distribution of rocks’ physical and magnetic properties, where there is a lack of direct access. 3D modeling of potential-field anomalies is applied here on three different scales, ranging from the kilometer to the micrometer scale. This modeling is supported by petrophysical analysis, rock magnetic methods, mineral chemistry and petrological data. Large, kilometer-scale studies investigated the deep structure of the entire SIP and the petrological variation within it. The first part focuses on gravity interpretation. The SIP expresses the most prominent gravity anomaly on land in northern Scandinavia and has a distinct magnetic signature. In the gravity model enclosing the SIP and extending outside the target area, the mafic and ultramafic rocks of the SIP are modeled with densities of 3300 kg/m3, the surrounding rocks by densities of 2700 and 2900 kg/m3 for upper and lower crust, respectively. Based on the gravity modeling results, most of the SIP has a thickness between 2 and 4 km and its minimum volume is estimated to 17000 km3. The province is modeled with multiple roots arranged in an annular pattern, two of which reach a depth of approximatively 9 km. Sensitivity tests on these roots lower the minimum estimate of the maximum depth to 7 km. However, due to the high percentage of exposed mafic rocks in the SIP, and many of the collected samples showing lower density than the one used in the most conservative model (3300 kg/m3), larger depths for the roots of the SIP are expected. The relatively undisturbed shape of the annular root pattern indicates that the SIP has not been subjected to strong tectonic reworking during the Caledonian orogeny. These geometrical elements are relevant for both the study of deep plumbing systems that the SIP represents, and for the geodynamic evolution of the province. The annular pattern and the funnel-shaped roots are indicative of a high energy system, with high driving pressure and high flux of magma compatible with a mantle plume hypothesis. The second part of the large-scale studies focuses on modeling of the magnetic anomalies, which helped to map the petrological variation within the SIP. According to these results, the dominant magnetic sources are restricted to a depth of 3 km. Most of the rocks of the SIP, and particularly those forming the deep roots are weakly magnetic. Furthermore, the occurrence of numerous metal deposits around one of the roots of the SIP suggests that the root could be a potential target area for mineral exploration. The next part of the thesis addresses the Reinfjord ultramafic complex (RUC), one of the major ultramafic complexes of the SIP, which has a lateral extension of approximately 10 km. The aims of this work were to reconstruct the subsurface model of the RUC, identify areas of alteration and mineralization, and evaluate the architecture of the RUC in the larger framework of the SIP. The RUC was modeled using land gravity data, acquired and processed in this project, and high-resolution helicopter-borne magnetic data. The modeling results indicate that the RUC extends down to a minimum depth of 1400 m. Its subsurface petrological variation originated from magmatic processes and later alteration, is derived from modeled densities, magnetic attributes and is constrained with surface geology. The ultramafic rocks in Reinfjord occupy at minimum a volume of 6.5 km3. Sensitivity tests suggest that the deeper part of the eastern side of the exposed complex has either near-vertical boundaries or is dipping toward east. The maximum depth of the complex of 1400 m results in a good match between the observed, and the calculated gravity and magnetic anomalies. However, due to the limited coverage of the gravity data and according to sensitivity tests larger depths cannot be excluded. Modeling of the RUC also allowed for estimating the depth extent of serpentinization in selected areas, which at most reaches a depth of 400 m; uncertainties relate to the remanent magnetization of the rocks at those areas. The last part of the thesis investigates magnetic anomalies at a submillimeter scale combining scanning magnetic microscopy, with other microscopy techniques, rock magnetic methods and magnetic modeling. Three thin section magnetic scans of a pristine dunite sample and of two serpentinized samples from the RUC are investigated. Scanning magnetic microscopy is used to measure, in near field-free conditions, the vertical component of the remanent magnetic fields of the samples. This emerging technique helps to discriminate different behaviors of rocks constituent phases, which are necessary for a complete understanding of the origin of bulk behavior measured in both the laboratory and in magnetic surveys. Furthermore, it can provide key evidence concerning primary and secondary geological processes (e.g. serpentinization) and their role in determining the magnetic response. Here, forward and inverse modeling of the magnetic anomalies in combination with chemical and magnetic properties analyses allowed the characterization of the main magnetic carriers in the thin sections. In serpentinized samples, the magnetic carrier is end-member magnetite occurring as large discrete grains and small grains in µm-scale veins. By contrast, the pristine dunite sample contains large Cr-spinel grains with very fine exsolved blebs ranging in composition from ferrichromite to end-member magnetite, as the magnetic phases. Modeling of the magnetic anomalies over isolated grains indicated heterogeneous sources of natural remanent magnetization (NRM) within the grains with intensities varying from 2 to 14 A/m and variable directions. These estimates are slightly higher for the serpentinized sample grains than the pristine dunite. Furthermore, the observation of numerous dipolar anomalies with variable directions and intensities over the larger grains in the magnetic scans of the serpentinites may suggest that the magnetization was acquired over a long time interval. This result together with the composition, the percentage of magnetic minerals and the fine-grain size of the magnetic material in the serpentinized sample explain its high bulk magnetic properties of susceptibility and NRM. Modeling of the magnetic anomalies in these studies provided information at different length, which contributed to the nature and understanding of the link between magnetic petrology, bulk magnetic properties and the mapped magnetic anomalies. The application of gravity modeling in two of the studies improved our understanding of the deep structure of the complex and provided a new geometry of the SIP with structural evidence for a plumbing system. Detailed studies on the rocks’ properties in combination with their magnetic and gravity responses helped to evaluate uncertainties, and answering or refining some of the questions on this world-class exposure of deep magmatic rocks.