Petrology and U–Pb zircon age of the Variscan porphyroclastic Rand Granite at the southeastern margin of the Central Schwarzwald Gneiss Complex (Germany)

Results of stratigraphic, UPb geochronological, and geochemical study are reported for rocks in a 2800 km2 area along the southeastern margin of the Archean Rae Province on north-central Baffin Island. Archean rocks include a gneiss complex, two greenstone belts of the Mary River Group, and various younger plutonic rocks. The 30002800 Ma gneiss complex contains intrusions of orthogneiss, dated at 27802770 Ma. Intermediate-felsic volcanism in overlying greenstone belts occurred at 27402725 Ma and was accompanied and outlasted by calc-alkaline plutonism (27302715 Ma). Peraluminous plutonism at ca. 2700 Ma, possibly associated with low- to medium-pressure metamorphism, represents the culmination of the Archean tectonic cycle. Dating of metamorphic zircon and titanite in Archean gneissic rocks indicates that overprinting, high-grade metamorphism in the northwest part of the area (footwall of the Isortoq fault zone) is Paleoproterozoic (ca. 1820 Ma). A weaker, somewhat older thermal disturbance (ca. 18501840 Ma with large errors) is recorded in the hanging wall of this zone. Additional tectonothermal events at ca. 15001400 Ma and ca. 700 Ma may, respectively, correlate with Mesoproterozoic faulting and emplacement of the Franklin dyke swarm. Unlike their age-correlative counterparts in the Mary River area and on the mainland to the southwest, the greenstone belts at Eqe Bay lack abundant orthoquartzite and komatiitic volcanic rocks: calc-alkaline volcanic rocks predominate, suggesting a fundamentally different tectonic environment. Striking similarities, both in lithology and age, to greenstone belts of the Minto block of the Superior Province raises the question of RaeSuperior correlation.

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Persistence of melt-bearing Archean lower crust for >200 m.y.—An example from the Lewisian Complex, northwest Scotland

Geology ◽

10.1130/g46834.1 ◽

2019 ◽

Vol 48 (3) ◽

pp. 221-225 ◽

Cited By ~ 7

Author(s):

Richard J.M. Taylor ◽

Tim E. Johnson ◽

Chris Clark ◽

Richard J. Harrison

Keyword(s):

Trace Element ◽

Lower Crust ◽

Mafic Magma ◽

High Grade ◽

Trace Element Data ◽

Metamorphic Zircon ◽

Geochronological Data ◽

Zircon Age ◽

Gneiss Complex ◽

High Grade Metamorphism

Abstract Geochronological data from zircon in Archean tonalite–trondhjemite–granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean.

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Persistence of melt-bearing Archean lower crust for >200 m.y.— An example from the Lewisian Complex, northwest Scotland

10.5194/egusphere-egu2020-12837 ◽

2020 ◽

Author(s):

Tim Johnson ◽

Rich Taylor ◽

Chris Clark

Keyword(s):

Trace Element ◽

Lower Crust ◽

Mafic Magma ◽

High Grade ◽

Trace Element Data ◽

Metamorphic Zircon ◽

Geochronological Data ◽

Zircon Age ◽

Gneiss Complex ◽

High Grade Metamorphism

<p><strong>Geochronological data in zircon from Archaean tonalite&#8211;trondhjemite&#8211;tonalite (TTG) gneisses is commonly difficult to interpret. A notable example are TTG gneisses from the Lewisian Gneiss Complex (LGC), northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchaean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at c. 2.7 Ga and c. 2.5 Ga, with intermediate ages reflecting variable Pb-loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchaean ages, they exhibit subtle but key step changes in trace element compositions that are difficult to ascribe to diffusive resetting, but which are consistent with emplacement of regionally-extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 Myr or more during the Neoarchaean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace element compositions suggesting derivation from a mafic source, and define a well-constrained U&#8211;Pb zircon age of c. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence for hundreds of millions of years of melt-bearing lower crust was probably the norm during the Archaean.</strong></p>

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A zircon age from gabbro-anorthosite inclusions in the gneisses of the Angmagssalik area, South-East Greenland

Rapport Grønlands Geologiske Undersøgelse ◽

10.34194/rapggu.v66.7399 ◽

1974 ◽

Vol 66 ◽

pp. 21-31

Author(s):

P.D Nunes ◽

R.H Steiger ◽

D Bridgwater

Keyword(s):

Major Part ◽

Coarse Grained ◽

Zircon Age ◽

East Greenland ◽

The North ◽

Gneiss Complex ◽

Age Determinations

Age determinations were made on 3 zircon fractions extracted from gabbroanorthosite inclusions which occur as tectonically dislocated pods in quartzofeldspathic gneisses on the eastern end of Kitak island, South-Bast Greeniand (fig. 5). The anorthosites were selected for study because they are derived from a lithologically distinctive suite of rocks found over a large part of the Archaean craton of the North AtIantic area (Bridgwater et al., 1973b). While there is no evidence that all the anorthosites of this type are contemporaneous their formation appears to require special conditions in the ernst and they are thus likely to be a useful stratigraphic tool. Furthermore the coarse-grained refractory nature of these rocks means that they retain their lithological identity during later tectonic, anatactic and metasomatic events to a far greater degree than the quartzofeldspathic, semipelitic and amphibolitic units making up the major part of the gneiss complex in which the anorthosites occur.

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