Constraining the timing of veins, faults and fractures in crystalline rocks by in situ Rb-Sr geochronology

Sammanfattning: Precambrian cratons are continent cores archiving the oldest crustal historieson Earth. The crystalline basement of cratons is typically characterized bycomplex arrays of multiple fracture and fault generations hosting mineralsformed by fluids flowing through fracture networks. Disentangling absolutechronologies of the various fracturing, faulting and fluid flow events have todate been difficult given the micro-scale mineral intergrowths and zonations,inhibiting conventional dating techniques. In the general lack of ageconstraints, deformation and mineralization mechanisms cannot be attributedto specific tectonic regimes, hampering reconstruction of local and regionalevents of fluid flow and mineral precipitation, and ultimately of the geologicalevolution of cratons. This thesis presents diverse studies utilizing theradiogenic decay of fracture, fault and shear zone mineral assemblagessampled from the crystalline basement of the Fennoscandian Shield, aiming atdetecting episodic fracturing reactivation, mineralization and microbialprocesses throughout the craton history.The analytical procedures involve, foremost, Rb-Sr geochronology, alongwith U-Pb and U-Th/He geochronology, stable isotope and trace elementgeochemistry, fluid inclusion thermometry and biomarkers. The in situ agedeterminations enabled 1) linking of greisen and distal veins to magmatic andpost-magmatic fluid circulation, 2) slickenfibre growth to distinct faultingepisodes, and 3) mineral precipitation in fractures, veins and shear zones toregionally extending deformation events across the Fennoscandian Shield. Inaddition, dating of mineralization related to deep fracture-hosted microbial lifeconstrained the timing of such activity at several sites. The precipitationepisodes stretch from Paleoproterozoic to Jurassic times with overgrowthgenerations separated in time by up to one billion years in single veins andeven within individual crystals. The findings of the thesis demonstrate that themethodological protocol has potential to directly date a wide range of mineralassemblages in fractures, faults, veins and shear zones given that the isochronrequirements are fulfilled. Fulfillment is ensured through detailed petrologicaland structural characterization followed by geochronological analysis andthorough data reduction allowing validation of isotopic data down to submicrometerlevel. The outcomes have implications for tectonic reconstructionsat various scales, for the tracing of the deep ancient biosphere and forcomprehending hydrothermal ore deposition, with direct societal relevance inthe detection of ancient microbial activity and fracture reactivation at thecandidate site for a spent nuclear fuel repository in Sweden.