Evolution of continental crust in the Proterozoic : growth and reworking in orogenic systems

Detta är en avhandling från Department of Geology, Faculty of Science, Lund University

Sammanfattning: To understand the growth of continental crust, the balance between juvenile mantle derived extraction, infracrustal reworking and crustal recycling, needs to be estimated. Since the beginning of the century, the use of coupled in situ zircon U–Pb, Lu–Hf and O isotope analyses as a tool to address these questions have increased exponentially. Numerous compilations of ever growing datasets have been presented, leading to new, and sometimes contrasting models of continental growth. Many of theses models, however, suffer from a number of assumptions, including a mantle reservoir that has been homogeneously and linearly depleted since the Hadean. Further, the use of (mainly) detrital zircon, taken out of their geological context, and the application of their depleted model-ages clearly hamper the validity of these models. To accurately address the question regarding continental crustal growth using combined zircon U–Pb-Lu–Hf(-O) isotope data, one needs to have contextual control and minimise the uncertainties of the applied models. In papers included in this thesis such an approach has been used on three different Palaeo- to Meso-Proterozoic orogenic belts; in Fennoscandia, in North American Grenville and in the Birimian terrane of the West African craton. The eastern part of the Sveconorwegian Province, located in the southwestern part of the Fennoscandian Shield, is made up of granitiod rocks that were emplaced through sequential tapping of a reservoir that formed through mixing between a 2.1–1.9 Ga juvenile component and Archaean crust. Between 1.7 and 1.4 Ga the continental crust of the Eastern Segment was reworked with little or no generation of new crust. Further to the west, in the Idefjorden terrane of the Sveconorwegian Province, 1.65 to 1.33 Ga rocks have isotopic signatures that indicate reworking of older continental crust, including sediments. However, overall the isotopic signatures in the Idefjorden terrane indicate an increase in juvenile material with time, consistent with development of an extensional back-arc rift geotectonic setting, accommodating deposition of the local metasedimentary basin, Stora Le-Marstrand. Isotope data from rocks within the Grenville orogen in subsurface Ohio suggest a common c. 1.65 Ga juvenile source to a majority of the sampled bedrock. Emplacement of this juvenile crustal contribution was followed by sequential reworking of that reservoir with little or no additional contribution to the source. The c. 2.31–2.06 Ga Birimian terrane in Ghana, West African craton, is a commonly cited example of plume initiated crustal growth, that is known to have largely juvenile signatures. However, we can show that reworked Archaean crust contribute in a much larger extent than previously known, once again highlighting the importance of infracrustal reworking during emplacement of continental crust. Further, the emplacement of felsic rocks during the Eoeburnean pre-dates suggested plume related rocks, contradicting a suggested plume initiated crustal growth. Collectively, these studies highlight the importance of infracrustal reworking in Palaeo- to Meso-Proterozoic accretionary orogens. These studies also provide good examples of combined zircon U–Pb-Lu–Hf-(O) isotope analyses on rocks and rock suites with known affinity where the validity of chosen models can be justified.