Synkinematic intrusion of granitoid sheets, with implications for molybdenite deposits in the Knaben Zone - Sirdal Magmatic Belt, SW Norway
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The Knaben Zone represents a N-S-elongated zone with abundant xenolith screens andmolybdenite deposits within the late Mesoproterozoic Sirdal Magmatic Belt (SMB) batholithin the Sveconorwegian orogen, in SW Norway. The SMB represents a significant componentof the western vicinity of the Sveconorwegian orogen, and has recently received attention dueto its lately defined sheer size and weak deformation. Through determining the significance of xenolith-rich zones the understanding of the evolution of the SMB can be greatly improved. The main objective in this MSc thesis is to determine the lithological relationships andmagmatic and tectonic processes for the Knaben Zone. An evaluation of the evolution andpotential of the molybdenite deposits is also put forward, together with a regional comparison. Detailed fieldwork and microscopic analysis of the Knaben Zone provide the basis for thisproject, supported by whole-rock geochemistry and zircon geochronology.Xenolith screens representing pre-Sveconorwegian bedrock (1252.8±8.7 Ma), and granitoidsheets (1043.3±5.6 Ma) related to the SMB, dominate the geometry of the Knaben Zone, witha pervasive moderate east-dipping parallelism. These are presented in a detailed geologicalmap of the area. The SMB granitoids are generally undeformed to weakly deformed, withstrain concentrated along xenolith screen contacts and local mylonite zones. Compressionaltop-to-the-W kinematics are consistent at all scales along the deformation zones, and meltpresent deformation is widely documented. The deformation is largely expressed as ductileduplex ramps, ramping up towards the west. Whole-rock geochemistry and crosscutting fieldrelations suggest a continuously evolving melt, from the least evolved K-feldspar porphyriticbiotite granite to highly evolved molybdenite bearing grey granite and leucocratic granites.The whole-rock geochemistry displays a decoupling signature and distinct Nb, Pb, P and Tianomalies, a typical active margin signature. Disseminated molybdenite of magmatic originwithin the grey granite is the most economically viable molybdenite deposit, while smallerdeposits are largely confined to thrust-related, subhorizontal pegmatites. Results from fieldwork, microscopic, geochemical and geochronological analysis aresynthesized to a synkinematic intrusion model for the granitoid sheets in the Knaben zone,relative to the E-W-compression. The xenolith screens are essential for the molybdenitedeposits, geochemically by providing precipitating agents and structurally, by concentratingfertile melts within the shear zones and anisotropies. The emplacement model, deduced fromkey findings within the Knaben Zone, is likely to be scale-invariant, and provides astructurally and petrological consistent emplacement model for the SMB at whole. Timingand kinematics correlate with thrusting along major shear zones in the Sveconorwegianorogen. The vast melt volume, and the geochemical signature related to the SMB, is bestexplained by a tectonic setting in the vicinity of an active margin.