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Crustal reworking and orogenic styles inferred from zircon Hf isotopes:Proterozoic examples from the North Atlantic region

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Geoscience Frontiers 2019年第2期10卷 417-424页

作者： C.J.Spencer C.L.Kirkland a.r.prave r.A.Strachan V.Pease Earth Dynamics research Group The Institute of Geoscience Research (TIGeR)Department of Applied GeologyCurtin University Centre for Exploration Targeting d Curtin Node Department of Applied GeologyCurtin University Department of Earth and Environmental Sciences University of St Andrews School of Earth and Environmental Sciences University of Portsmouth Department of Geological Sciences Stockholm University

Zircon Hf evolutionary patterns are powerful tools to investiage magma petrogenesis and crustal evolution. The ^(176)Hf/^(177)Hf 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 εHf/Ma trajectory to assist in understanding their evolution. The εHf/Ma trajectory of the Sveconorwegian Orogen corresponds to a ^(176)Lu/^(177) Hf 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 shallowεHf/Ma slope. The Valhalla Orogen reveals a similar comparatively shallow εHf/Ma path. In stark contrast to these patterns is the steep εHf/Ma trajectory of the Grenville Orogen that requires a mixing process involving a greater contribution of old crust of at least ～ 1.8 Ga age. The degree of reworking required to produce the εHf/Ma trend of the Grenville Orogen is co

关键词： Zircon Hf Crustal evolution Grenville Sveconorwegian Valhalla

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Generation and preservation of continental crust in the Grenville Orogeny

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Geoscience Frontiers 2015年第3期6卷 357-372页

作者： Christopher J.Spencer Peter A.Cawood Chris J.Hawkesworth Anthony r.prave Nick M.W.roberts Matthew S.A.Horstwood Martin J.Whitehouse EIMF Department of Earth and Environmental Sciences University of St Andrews NErC Isotope Geosciences Laboratory British Geological Survey Swedish Museum of Natural History and Nordic Center for Earth Evolution(NordCEE) Edinburgh Ion Microprobe Facility University of Edinburgh

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 0 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 EHf and higher -lSO values from - 1800 to - 1200 Ma whereupon they have higher EHf and -3180 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

关键词： Zircon Continental crust Grenville Orogeny Crustal preservation

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