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 es...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 consistent with a continent-continent collisional orogeny whereas both Sveconorwegian and Valhalla orogens appear more consistent with accretionary margins.展开更多
The presence of eclogites within continental crust is a key indicator of collisional orogenesis.Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane(NHT)have been redated in order to p...The presence of eclogites within continental crust is a key indicator of collisional orogenesis.Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane(NHT)have been redated in order to provide more accurate constraints on the timing of collision within the northern arm of the Grenville Orogen.The eclogites yield dates of ca.1200 Ma which are interpreted to record the onset of continent–continent interaction,and the NHT as a whole is thought to represent the lower plate in successive 1200–1000 Ma collision events.The Eastern Glenelg basement inlier is viewed as a fragment of the leading edge of the NHT continental basement that was partially subducted along a suture and then exhumed back up the subduction channel.Differences in ages of igneous protoliths and intrusive histories,and metamorphic events(this paper)between the NHT basement and the Laurentian foreland,suggests that they were separate crustal blocks until after ca.1600 Ma.We therefore suggest that:(1)the NHT represents a fragment of Archean–Paleoproterozoic crust that was reworked within the ca.1.7–1.6 Ga Labradorian-Gothian belt,although whether it was derived from Laurentia or Baltica is uncertain,and(2)amalgamation of the NHT with the Laurentian foreland did not occur until the terminal stages of the Grenville collision at ca.1000 Ma.展开更多
基金funding from the Swedish Research Council (Grant 621-2014-4375)
文摘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 consistent with a continent-continent collisional orogeny whereas both Sveconorwegian and Valhalla orogens appear more consistent with accretionary margins.
文摘The presence of eclogites within continental crust is a key indicator of collisional orogenesis.Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane(NHT)have been redated in order to provide more accurate constraints on the timing of collision within the northern arm of the Grenville Orogen.The eclogites yield dates of ca.1200 Ma which are interpreted to record the onset of continent–continent interaction,and the NHT as a whole is thought to represent the lower plate in successive 1200–1000 Ma collision events.The Eastern Glenelg basement inlier is viewed as a fragment of the leading edge of the NHT continental basement that was partially subducted along a suture and then exhumed back up the subduction channel.Differences in ages of igneous protoliths and intrusive histories,and metamorphic events(this paper)between the NHT basement and the Laurentian foreland,suggests that they were separate crustal blocks until after ca.1600 Ma.We therefore suggest that:(1)the NHT represents a fragment of Archean–Paleoproterozoic crust that was reworked within the ca.1.7–1.6 Ga Labradorian-Gothian belt,although whether it was derived from Laurentia or Baltica is uncertain,and(2)amalgamation of the NHT with the Laurentian foreland did not occur until the terminal stages of the Grenville collision at ca.1000 Ma.
