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

Abstract: 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.