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- Spencer, C. J., et al.
(författare)
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Crustal reworking and orogenic styles inferred from zircon Hf isotopes : Proterozoic examples from the North Atlantic region
- 2019
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 10:2, s. 417-424
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Tidskriftsartikel (refereegranskat)abstract
- Zircon Hf evolutionary patterns are powerful tools to investiage magma petrogenesis and crustal evolution. The Hf-176/Hf-177 isotopic signature of a rock is particularly informative and can be used to derive an estimation of the time when mantle extraction and diagnose closed system reworking where successive samples through time define an Hf evolution array dependant on the source Lu/Hf ratio. However, many magmatic events require new mantle addition as the thermal impetus for melting pre-existing crust. In this situation, rather than simply reflecting reworking, the isotopic signature indicates mixing with contributions from both reworked crust and new radiogenic input. Different geodynamic settings have different propensities for either reworking or addition of new mantle-derived magma. Hence, Hf-time trends carry within them a record, albeit cryptic, of the evolving geodynamic environment as different tectonic configurations recycle and add new crust at different rates, magnitudes, and from different sources. As an example of the difference in apparent Hf evolution slopes, we present Hf-time compilations from three geographically distinct Meso-to Neoproterozoic orogenic belts in the North Atlantic Region whose geodynamic configurations remain a subject of debate. We use the epsilon Hf/Ma trajectory to assist in understanding their evolution. The epsilon Hf/Ma trajectory of the Sveconorwegian Orogen corresponds to a Lu-176/Hf-177 ratio of 0.012, which implies a process driven primarily by reworking of preexisting crust that is balanced with input from the depleted mantle resulting in a relatively shallow epsilon Hf/Ma slope. The Valhalla Orogen reveals a similar comparatively shallow epsilon Hf/Ma path. In stark contrast to these patterns is the steep epsilon Hf/Ma trajectory of the Grenville Orogen that requires a mixing process involving a greater contribution of old crust of at least similar to 1.8 Ga age. The degree of reworking required to produce the epsilon Hf/Ma trend of the Grenville Orogen is consistent with a continentecontinent collisional orogeny whereas both Sveconorwegian and Valhalla orogens appear more consistent with accretionary margins.
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| 2. |
- Spencer, C.J., et al.
(författare)
-
Generation and preservation of continental crust in the Grenville Orogeny
- 2015
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 6, s. 357-372
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Tidskriftsartikel (refereegranskat)abstract
- Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of ‘peaks’ and ‘troughs’. The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events, or preferential preservation of continental crust during global collisional orogenesis. The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks. Here these two end-member models (episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis) are assessed using U-Pb, Hf, and O analysis of detrital zircons from sedimentary successions deposited during the ∼1.3–1.1 Ga accretionary, ∼1.1–0.9 Ga collisional, and < 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland. The pre-collisional, accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology. Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from ∼1500 to 1100 Ma. However, post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma, similar to that observed in modern North American river sediments.Zircons within the post-orogenic sedimentary successions have progressively lower ɛHf and higher δ18O values from ∼1800 to ∼1200 Ma whereupon they have higher ɛHf and δ18O within the dominant 1085–985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material. The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Grenville Orogeny, which is a typical collisional orogenic belt. The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision. This temporal change in provenance highlights that the source(s), from which detrital zircons within syn-orogenic strata were derived, was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle. This may reflect some combination of tectonic burial, erosion, or possibly recycling into the mantle by tectonic erosion of the source(s). During continental collision, the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins. This tectonic isolation combined with sedimentary recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sediments. These results imply that zircon age peaks, which developed in conjunction with supercontinents, are the product of selective crustal preservation resulting from collisional orogenesis.
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