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摘要
The amphibolite facies Eoarchaean Isua supracrustal belt (northern part of the Nuuk region, southern West Greenland) is dominated by strongly deformed metabasalts, with chert, banded iron formation, felsic volcanic and volcano-sedimentary rocks and minor gabbro and sedimentary carbonates. It comprises a suture zone between a northern terrane formed at ca. 3700 Ma and a southern one formed at ca. 3800 Ma. At the junction between these two terranes is a strongly tectonised, thin unit of metachert, BIF and carbonate-bearing rocks with minor detrital components, named the dividing sedimentary unit. Away from the belt, the northern terrane is dominated by ca. 3700 Ma tonalites and the southern one by ca. 3800 Ma tonalites. The Isua supracrustal belt chemical metasedimentary rocks give low yields of small (commonly <= 50 mu m long), prismatic, oscillatory-zoned zircons with less common, somewhat rounded zircons of volcanic and/or detrital origin. The most siliceous of the felsic schists give a larger yield of volcanic or detrital zircons. The ages of these zircons are distinct in different parts of the belt. In the northern terrane, most have ages of 3710-3690 Ma, with a few slightly older <3750 Ma grains. In the southern 3800 Ma terrane, a fuchsite-quartzite has 3900-3810 Ma zircons. Three samples of the dividing sedimentary unit along the whole length of the belt yielded both 3920-3880 Ma (in two samples) and ca. 3750 Ma detrital zircons (in all three samples). The part of the ca. 3700 Ma northern terrane in the Isua supracrustal belt is interpreted as lithologically diverse and imbricated assemblage of ca. 3700 Ma arc-related volcanic rocks (boninitic, island arc tholeiite-picrite and andesite-dacite). In the associated metasedimentary rocks, the lack of >3750 Ma detrital zircons suggests that these volcanic assemblages evolved remote from 3800 Ma crust. In contrast, the up to 3940 Ma detrital zircons from the dividing sedimentary unit and the southern fuchsite quartzite shows that the southern ca. 3800 Ma terrane, although dominated by arc-related rocks, might contain older (pre-3800 Ma) crust. We propose that the dividing sedimentary unit was deposited on top of already deformed southern terrane rocks, and then acted as a decollement when the northern terrane ca. 3700 Ma arc complex collided with the southern terrane by ca. 3660 Ma (from previous age constraints). Thus the detrital zircon age data is compatible with proposals from other evidence for juxtaposition of ca. 3700 and 3800 Ma terranes in the Isua area. Some thin siliceous metasedimentary rocks along post-assembly mylonite zones in the Isua supracrustal belt yield some <3660Ma zircons. A tentative interpretation of these siliceous rocks (requiring confirmation) is that they could be younger, post-collisional sedimentary rocks, which were incorporated as tectonic lenses along some of the post-terrane-assembly <3660Ma mylonites which partition the collisional assemblage.
- 出版日期2009-8
- 单位中国地质科学院
