Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

Upton, B. G. J.

doi:10.34194/geusb.v29.4692

Cited by 45 publications

(53 citation statements)

References 117 publications

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“…Ma intrusions from southern Greenland (Piper, 1992;Upton, 2013), indicate that Laurentia was at high latitudes, and at a near standstill, prior to the Keweenawan Track (Fig. 1).…”

Section: Background and Paleomagnetismmentioning

confidence: 99%

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

et al. 2019

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The late Mesoproterozoic was a time of large-scale tectonic activity both in the interior and on the margins of Laurentia-most notably the development of the Midcontinent Rift and the Grenvillian orogeny. Volcanism within the North American Midcontinent Rift between ca. 1109 and 1083 Ma, as well as other contemporaneous volcanism within Laurentia, has provided an opportunity to develop extensive paleomagnetic data sets spanning this time period. These data result in an apparent polar wander path (APWP) for Laurentia that goes from a high latitude apex known as the Logan Loop into a swath known as the Keweenawan Track. A long-standing challenge of these data was the appearance of asymmetry between relatively steep reversed polarity directions from older rift rocks and relatively shallow normal polarity directions from younger rift rocks. This asymmetry was used to support an interpretation that there were large non-dipolar components to the geomagnetic field at the time. Recent data sets support the interpretation that this directional change was progressive and therefore a result of very rapid motion of Laurentia from high to low latitudes rather than a stepwise change across non-dipolar reversals. We present high precision U-Pb dates from Midcontinent Rift volcanics that result in an improved chronostratigraphic framework for rift volcanics and unconformities that improves correlations as well as constraints on rift development. We use these dates in volcanostratigraphic context to temporally constrain a new compilation of Midcontinent Rift paleomagnetic poles. 1 These paleomagnetic poles include new data from the North Shore Volcanic Group and the Osler Volcanic Group. The U-Pb dates constrain the rate of implied plate motion more precisely than has previously been possible. We apply a novel Bayesian approach to assess the rate of implied plate motion through inverting for paleomagnetic Euler poles. If the path is to be explained by a single Euler pole these inversions reveal that motion of the continent exceeded 27 cm/year. The path is particularly well-explained by a model wherein there is continuous true polar wander in addition to rapid plate motion that changes direction and slows at ca. 1096 Ma. Laurentia's movement from high to low latitudes resulted in collisional tectonics on its leading margin which could be associated with such a change in plate motion. We propose that upwelling of the Keweenawan mantle plume was associated with an avalanche of subducted slab material with downwelling that drove fast plate motion. This fast plate motion was followed by the Grenvillian orogeny from ca. 1090 to ca. 980 Ma. Prolonged collisional orogenesis could have been sustained due to this strong convective cell that therefore played an integral role in the assembly of the supercontinent Rodinia.

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“…Ma intrusions from southern Greenland (Piper, 1992;Upton, 2013), indicate that Laurentia was at high latitudes, and at a near standstill, prior to the Keweenawan Track (Fig. 1).…”

Section: Background and Paleomagnetismmentioning

confidence: 99%

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

et al. 2019

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“…The Ilímaussaq complex (1160 ± 5 Ma; Waight et al, 2002;Krumrei et al, 2006) is part of the Mesoproterozoic Gardar Province, South Greenland (Upton, 2013). The complex was emplaced during at least four intrusive pulses successively crystallizing augite syenites, alkali granites, quartz syenites and the volumetrically dominant agpaitic nepheline syenites (see reviews in Larsen and Sørensen, 1987;Sørensen, 2001;Sørensen et al, 2006;Marks and Markl, 2015).…”

Section: Geological Settingmentioning

confidence: 99%

Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

et al. 2016

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The layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.

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“…These are calc-alkaline granitoids and granitoid-gneisses with varying degrees of megacryst and fabric development alongside less frequent hornblende diorite, an association reminiscent of the 'appinite' suite of Scotland and Ireland (Allaart 1967). The Gardar Province post-dates the Ketilidian and represents the products of two periods of rift-related alkaline magmatism during the intervals ~1300-1270Ma and 1180-1140 Gardar respectively (Upton et al 2003, Upton 2013. The earliest Gardar rocks preserved are sequences of interbedded clastic sediments and volcanic rocks referred to as the 'Eriksfjord formation'.…”

Section: Geological Settingmentioning

confidence: 99%

From Mantle to Motzfeldt: A genetic model for syenite-hosted Ta,Nb-mineralisation

Finch

McCreath

Reekie

et al. 2019

Ore Geology Reviews

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A genetic model for the Motzfeldt Tantalum-Niobium-rich syenite in southwest Greenland, considered to be one of the world's largest Ta prospects, is presented. The Motzfeldt primary magma formed early in regional Gardar (1273±6 Ma) rifting. Isotope signatures indicate that the Hf had multiple sources involving juvenile Gardar Hf mixed with older (Palaeoproterozoic or Archaean) Hf. We infer that other High Field Strength Elements (HFSE) similarly had multiple sources. The magma differentiated in the crust and ascended before emplacement at the regional unconformity between Ketilidian basement and Eriksfjord supracrustals. The HFSE-rich magmas crystallised Ta-rich pyrochlore which formed pyrochlore-rich crystal mushes, and it is these pyrochlore-rich horizons, rich in Ta and Nb, that are the focus of exploration. The roof zone chilled and repeated sheeting at the roof provided a complex suite of cross-cutting syenite variants, including pyrochlore microsyenite, in a 'Hot Sheeted Roof' model. The area was subject to hydrothermal alteration which recrystallized alkali feldspar to coarse perthite and modified the mafic minerals to hematite, creating the friable and striking pink-nature of the Motzfeldt Sø Centre. Carbon and oxygen isotope investigation of carbonate constrains fluid evolution and shows that carbonate is primarily mantle-derived but late-stage hydrothermal alteration moved the oxygen isotopes towards more positive values (up to 21 ‰). The hydrothermal fluid was exceptionally fluorine-rich and mobilised many elements including U and Pb but did not transport HFSE such as Ta, Hf and Nb. Although the U and Pb content of the pyrochlore was enhanced by the fluid, the HFSE contents remained unchanged and therefore Hf isotopes were unaffected by fluid interaction. While the effect on hydrothermal alteration on the visual appearance of the rock is striking, magmatic processes concentrated HFSE including Ta and the hydrothermal phase has not altered the grade. Exploration for HFSE mineralisation commonly relies on airborne radiometric surveying which is particularly sensitive to the presence of U, Th. A crucial lesson from Motzfeldt is that the best target is unaltered pyrochlore which was identified less easily by radiometric survey. Careful petrological/mineral studies are necessary before airborne survey data can be fully interpreted.

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Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

Cited by 45 publications

References 117 publications

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

From Mantle to Motzfeldt: A genetic model for syenite-hosted Ta,Nb-mineralisation

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