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    The complex age of orthogneiss protoliths exemplified by the Eoarchaean Itsaq Gneiss Complex (Greenland) SHRIMP and old rocks [查看] KenjiHorie、AllenP.Nutmanc、ClarkR.L.Friend、HiroshiHidaka
    Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840Ma zircons vary from very rare single-phase metatonalites to morecommoncomplexly banded tonalitic migmatites. The former metatonalites have simple oscillatoryzoned ≥3840Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840Ma and 3650–3600Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships.With only 100–160ppm Zr in the tonalites and likely melt generation temperatures of >1000 ◦C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the≥3840Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur.With an established igneous age of ≥3840Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800 ◦C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations,whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations.Gneisses with ≥3840Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65km east Akilia) and on the north of Ivisaartoq (∼150km northeast of Akilia). These include from Itilleq a 3891±6Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ~3800, 3750 and 3700Ma tonalite generations.Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800Ma igneous zircon are mutually exclusive from migmatite samples with ~3700Ma igneous zircon.This suggests that prior to an amalgamation event followed by 3660–3600Mahigh-grade metamorphism,≥3840Ma tonalites might have resided in a terrane discrete from ∼3700Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ~3800Matonalites with a minor associated≥3840Macomponent and a terrane with ~3700Matonalites were tectonically juxtaposed at ~3660 Ma.
    Adjacent terranes with ca. 2715 and 2650 Ma high-pressure metamorphic assemblages in the Nuuk region of the North Atlantic Craton, southern West Greenland Complexities of Neoarchaean collisional orogeny [查看] AllenP.Nutman、ClarkR.L.Friend
    In the gneiss complex of the Nuuk region of the North Atlantic Craton in southern West Greenland, terrane juxtaposition was followed by Neoarchaean folding under amphibolite facies conditions, with widespread low-pressure recrystallisation (5 kbar and 550–700 ◦C). The complex metamorphic overprinting requires that the P–T history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the U/Pb dating, petrology and geochemistry of metamorphic zircons.In the south of the region, the Færingehavn terrane (Eoarchaean orthogneisses) is tectonically overlain by a supracrustal package of amphibolites and paragneisses (ca. 2840 Ma felsic volcano-sedimentary protoliths). This package is juxtaposed against a higher tectonic level represented by the Tre Brødre terrane (2825 Ma orthogneisses) and the Tasiusarsuaq terrane(2920–2810 Ma orthogneisses). The terranes were assembled by 2710–2720 Ma, as shown by dating of granitic sheets intruded along the terrane boundary mylonites. In the Færingehavn terrane and in the overlying 2840 Ma supracrustal package, relict early high-pressure assemblages (12–8 kbar, 700–750 ◦C) are clinopyroxene + garnet + plagioclase + quartz±hornblende in mafic rocks and garnet + kyanite + rutile bearing assemblages in paragneisses. These are commonly replaced by lower pressure assemblages (7–5 kbar) such as cordierite±sillimanite±garnet in paragneisses and hornblende + plagioclase + quartz±garnet in mafic rocks.In situ partial melting took place during both low- and high-pressure regimes. Metamorphic zircon in the high- and low-pressure assemblages yields dates of ca. 2715 Ma, mostly with errors of <±5 Ma, thereby demonstrating rapid decompression at high temperatures.Zircons in the overlying Tre Brødre and Tasiusarsuaq terranes show little response to the ca. 2715 Ma event supporting structural interpretations that they were at a higher structural level at ca. 2715 Ma. High temperature recrystallisation continued after ca. 2715 Ma, as demonstrated by intergrowth of sillimanite with 2680 Ma metamorphic zircon.The Kapisilik terrane (3050–2960 Ma orthogneisses) and another supracrustal assemblage of amphibolites and ca. 2800 Ma quartzo-feldspathic metasedimentary rocks are exposed north of the Færingehavn terrane and within fold cores along its western margin, are bounded by folded Neoarchaean mylonites. The Kapisilik terrane and this supracrustal assemblage include high-pressure metamorphic remnants in amphibolites and metasediments (metamorphic segregations with garnet + clinopyroxene and kyanite,respectively) that formed at ca. 2650 Ma. These remnants are overprinted by high temperature, but lower pressure metamorphic events at ca. 2630, 2610 and 2580 Ma. Thus remnants of early metamorphism
    2635 Ma amphibolite facies gold mineralisation near a terrane boundary (suture) on Storø, Nuuk region,southern West Greenland [查看] AllenP.Nutmana、OleChristiansen、ClarkR.L.Friend
    Gold on Storø (in the Nuuk region of southern West Greenland) occurs in a slice of strongly deformed, amphibolite facies,Neoarchaean quartzo-feldspathic metasedimentary rocks and amphibolites in tectonic contact with the Eoarchaean Færingehavn terrane and the Meso- to Neoarchaean Akia terrane. The gold is associated with either l¨ollingite (FeAs) + arsenopyrite or pyrrhotite, which belong with the amphibolite facies silicate mineral assemblages. Zircons were U/Pb dated by SHRIMP to constrain the timing of mineralisation. The 2700–2830 Ma oscillatory-zoned, high Th/U volcano-sedimentary zircon as whole grains and cores are interpreted to give the maximum depositional age of the rocks hosting the gold. A granite sheet barren of gold that cuts gold mineralised rocks gives an age of ca. 2550 Ma, which is the absolute minimum age of mineralisation. Metamorphic zircons associated with auriferous arsenopyrite (particularly close to secondary l¨ollingite), pyrrhotite and in the coexisting amphibolite facies silicate minerals were dated in situ, in polished thin sections. These have lower average Th/U and yielded dates of ca. 2635 Ma, which is interpreted as the age of the gold-bearing mineral assemblages and hence probably the mineralisation.Supracrustal packages containing abundant quartzo-feldspathic sedimentary rocks with depositional ages of ca. 2800 Ma and metamorphosed at 2650–2600 Ma also occur as folded tectonic intercalations along the western edge of the Færingehavn terrane south of Storø. Moreover, in the 3080–2960 Ma Kapisilik terrane to the north and east of Storø, high-grade metamorphic events also occurred between 2650 and 2600 Ma. However, the Eoarchaean Færingehavn terrane to the south and east of Storø shows widespread zircon growth and recrystallisation during 2720–2700 Ma regional metamorphism, but not at 2650–2600 Ma. The Storø gold prospect is thus located near an important tectonic terrane boundary (cryptic suture?) between domains with different Neoarchaean metamorphic history. Storø mineralisation at ca. 2635 Ma probably occurred during deformation of this boundary shortly after it was created, and at Storø maybe was focussed in an area of low strain in a major antiform. The general tectonic frameworks for ca. 2635 Ma gold mineralisation on Storø and in the Yilgarn Craton are similar. These gold provinces are compared and contrasted, and the likely reason why so much less gold occurs in the Nuuk region is discussed.
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