scholarly journals Early (3700 Ma) Archaean rocks of the Isua supracrustal belt and adjacent gneisses

1983 ◽  
Vol 112 ◽  
pp. 5-22
Author(s):  
A.P Nutman ◽  
D Bridgwater ◽  
E Dimroth ◽  
R.C.O Gill ◽  
M Rosing
Keyword(s):  
Amphibolite Facies ◽  
Iron Formation ◽  
Ultramafic Rocks ◽  
Banded Iron Formation ◽  
Sedimentary Structures ◽  
The North ◽  
Gneiss Complex ◽  
Supracrustal Belt ◽  
Silicate Rocks

A coherent stratigraphy is recognised in the highly deformed, amphibolite facies early Archaean Isua supracrustal belt. The supracrustal belt consists of layered rocks (in which sedimentary structures are locally preserved), ultramafic rocks and units of garbenschiefer (a massive Mg-Al rich, leucoamphibolite). The layered supracrustal rocks form two sequences, which are separated from each other tectonically. When folding is taken into account, these sequences are now less than 200 m thick. Sequence A forms most of the belt. In it there is a transition upwards from predominantly layered amphibolites with banded iron formation horizons to calc-silicate rocks, carbonates and layered felsic metasediments. Sequence B is restricted to the western edge of the eastern part of the supracrustal belt. It changes upwards from predominantly layered felsic metasediments to ferromagnesian mica schists. The supracrustal belt is regarded as a thin fragment from a thicker, more extensive volcanosedimentary pile. The early Archaean gneisses adjacent to the supracrustal belt consist of early multiphase tonalites which were first intruded by mafic dioritic dykes and then by granitic sheets. The granitic sheets were originaIly horizontal to gently inciined and form up to 40 per cent of the gneiss complex. Interdigitation of supracrustal rocks and gneisses in the Isukasia area is due to both the style of intrusion ofthe gneisses and to tectonic intercalation. Archaean basic dykes that cut the supracrustal belt and adjacent gneisses are ofseveral generations. Within and south of the supracrustal belt they are generally strongly deformed and have been recrystallised under amphibolite facies conditions; but in the north of the area they are generally better preserved. The dykes cut across several generations of structures in the supracrustal belt and the adjacent gneisses.

scholarly journals The early Archaean to Proterozoic history of the Isukasia area, southern West Greenland

1986 ◽  
Vol 154 ◽  
pp. 1-80
Author(s):  
A.P Nutman
Keyword(s):  
Volcanic Rocks ◽  
Amphibolite Facies ◽  
Iron Formation ◽  
Banded Iron Formation ◽  
Stratigraphic Sequence ◽  
Supracrustal Belt ◽  
History Of ◽  
High Grade Metamorphism ◽  
Polyphase Metamorphism ◽  
Chemical Sediments

The c. 3800 Ma Isua supracrustal belt and associated smaller bodies of supracrustal rocks are intruded by >3600 Ma orthogneisses. A coherent stratigraphic sequence is recognised consisting of interlayered metabasic rocks, metasediments derived from volcanic rocks, chemical sediments, and metabasic and ultramafic intrusions. Despite repeated deformation and high-grade metamorphism sedimentary structures are locally preserved. The depositional environment was probably an immersed volcanic region remote from areas of significantly older crust. Conglomeratic structures in a metachert and banded iron formation unit suggest shoaling and shallow water conditions. Felsic sediments locally preserve evidence of deposition from turbidite flows. The Isua supracrustal rocks are regarded as thin fragments of a thicker, more extensive sequence. The orthogneisses that intrude the supracrustal rocks consist of 3750-3700 Ma multiphase tonalites (the grey gneisses) which were first intruded by the basic Inaluk dykes, then by abundant shallow-dipping swarms of c. 3600 Ma granite sheets (the white gneisses) and finally by c. 3400 Ma pegmatitic gneiss sheets. These early Archaean rocks were metamorphosed under amphibolite facies conditions and repeatedly deformed prior to intrusion of the Tarssartôq basic dykes in the mid Archaean. In the late Archaean (3100-2500 Ma) there was polyphase metamorphism up to amphibolite facies grade and two or more stages of deformation and local intrusion of granitic gneiss sheets and pegmatites. However, despite general strong deformation there is a large augen of low deformation preserved within the arc of the Isua supracrustal belt. During the Proterozoic there was intrusion of basic dykes, major faulting with associated recrystallisation under uppermost greenschist to lowermost amphibolite facies conditions, followed by heating and intrusion of acid dykes at c. 1600 Ma. No profitable mineralisations have been located.

A new volcanic province: evidence from glacial erratics in western North Greenland

2000 ◽  
pp. 35-41 ◽  
Author(s):  
Peter R. Dawes ◽  
Bjørn Thomassen ◽  
T.I. Hauge Andersson
Keyword(s):  
Volcanic Rocks ◽  
Iron Formation ◽  
Banded Iron Formation ◽  
Common Component ◽  
Volcanic Province ◽  
North West ◽  
North East ◽  
The North ◽  
Surficial Deposits ◽  
North Greenland

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Dawes, P. R., Thomassen, B., & Andersson, T. H. (2000). A new volcanic province: evidence from glacial erratics in western North Greenland. Geology of Greenland Survey Bulletin, 186, 35-41. https://doi.org/10.34194/ggub.v186.5213 _______________ Mapping and regional geological studies in northern Greenland were carried out during the project Kane Basin 1999 (see Dawes et al. 2000, this volume). During ore geological studies in Washington Land by one of us (B.T.), finds of erratics of banded iron formation (BIF) directed special attention to the till, glaciofluvial and fluvial sediments. This led to the discovery that in certain parts of Daugaard-Jensen Land and Washington Land volcanic rocks form a common component of the surficial deposits, with particularly colourful, red porphyries catching the eye. The presence of BIF is interesting but not altogether unexpected since BIF erratics have been reported from southern Hall Land just to the north-east (Kelly & Bennike 1992) and such rocks crop out in the Precambrian shield of North-West Greenland to the south (Fig. 1; Dawes 1991). On the other hand, the presence of volcanic erratics was unexpected and stimulated the work reported on here.

SHRIMP zircon age of a Proterozoic rapakivi granite batholith in the Gyeonggi massif (South Korea) and its geological implications

2005 ◽  
Vol 142 (1) ◽  
pp. 23-30 ◽  
Author(s):  
MINGGUO ZHAI ◽  
ZHIYAO NI ◽  
CHANG WHAN OH ◽  
JINGHUI GUO ◽  
SEON GYU CHOI
Keyword(s):  
South Korea ◽  
North China ◽  
Granulite Facies ◽  
Iron Formation ◽  
Rapakivi Granite ◽  
Banded Iron Formation ◽  
The North ◽  
Gyeonggi Massif ◽  
Granite Batholith ◽  
New Evidence

A large rapakivi granite batholith in the Neo-Archaean/Palaeoproterozoic Odesan complex, northeastern Gyeonggi massif, South Korea, has been dated at 1839±10 Ma using SHRIMP U–Pb analysis of zircons. The age, petrological and geochemical characteristics of this batholith are similar to those of the rapakivi granite batholiths exposed in the Rangnim massif of North Korea and in the Miyun–Chengde complex of North China. The country rocks of these rapakivi granite batholiths are also comparable; all are composed of granitic gneisses and banded iron formation (BIF)-bearing supracrustal rocks metamorphosed to amphibolite- to granulite-facies. This study provides new evidence for the suggestion that the Gyeonggi and Rangnim massifs may share an affinity with the Precambrian basement of the North China craton. The study provides new insight into the possible eastward extension of the Sulu orogenic belt in the Korean peninsula and further provides evidence to correlate the Korea basement to a possible global 2.1–1.8 Ga supercontinent.

Depositional and Environmental Constraints on the Late Neoarchean Dagushan Deposit (Anshan-Benxi Area, North China Craton): An Algoma-Type Banded Iron Formation

2021 ◽  
Author(s):  
Xiaoxue Tong ◽  
Changle Wang ◽  
Zidong Peng ◽  
Yuhao Li ◽  
Weiduo Hao ◽  
...  
Keyword(s):  
Water Column ◽  
North China Craton ◽  
Ferrous Iron ◽  
Iron Reduction ◽  
North China ◽  
Hydrothermal Fluids ◽  
Iron Formation ◽  
Carbonate Facies ◽  
Banded Iron Formation ◽  
The North

Abstract The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ56Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ13C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

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