Evolution of the Eoarchean orthogneisse/metaplutonic complex north of the Isua Supracrustal Belt, Itsaq Gneiss Complex, SW-Greenland

The Isua Supracrustal Belt (ISB) encloses circularly an orthogneiss/metaplutonic complex comprising mainly ~3.7 Ga old metatonalite/tonalitic orthogneiss and slightly younger ~3.65 Ga old granite (Baadsgaard et al., 1986; Nutman et al., 1996). The central part of this metaplutonic complex has experienced weaker deformation during its evolution and based on field observations and cross cutting relationships 4 different metatonalite generations were identified. From oldest to youngest these are: (T1) rare occurrences of mesocratic fine grained tonalite boudins in (T2) biotite-rich coarse grained tonalitic augengneiss. (T3) Well foliated felsic tonalitc gneiss with medium sized plagioclase crysts. (T4) Weakly foliated felsic and fine grained metatonalite. The metatonalite commonly encloses centimeter to several meter sized boudins and bands of amphibolite/metagabbro. The slightly younger granites (G) occur as dikes and larger irregular shaped bodies/patches intruding the metatonalite (T2,T3,T4). Towards the contact with the ISB the metaplutonic complex develops a mylonitic to ultramylonitic fabric obscuring the above mentioned relationships. The youngest magmatic event is seen in ultramafic dikes (Ameralik dikes) aligning in 3 different strike directions. Some of them enclose several centimeter sized granitic xenoliths.

Detailed petrographic observations and fluid inclusions (FIs) studies revealed a later postmagmatic overprint in the metatonalites, probably related to one of the metamorphic episodes recorded in the ISB. Magmatic allanite crystals show frequently rims of newly grown epidote while the destabilized allanite forms partly numerous tiny U-Th rich phases.

The investigated metatonalites for our FIs study contain recrystallized equigranular quartz with lobate grain boundaries. FIs have been investigated using Raman spectroscopy combined with fluid inclusion microthermometry. Texturally and chemically, FIs can be distinguished into two major types: (1) polyphase aqueous high-saline FIs (46-49 mass% NaCl) arranged within central domains of quartz grains. (2) One phase FIs arranged as intragranular fluid inclusion planes (FIPs) within quartz grains or as intergranular FIPs along quartz grain boundaries. Type 1 inclusions are interpreted as primary inclusions characterizing an early stage of tonalite crystallization. Their textural occurrence is always restricted to clusters within the core domains of quartz aggregates. Beside halite, solid inclusions like ferropyrosmalite and calcite occur frequently. Type 2 inclusions are arranged along planes that occur inside quartz grains (internal), along grain boundaries (intergranular) or crossing grain boundaries (transphase) and are likely interpreted as resulting from type (1) during recrystallization/metamorphic overprint.

Additional petrographic, geochronologic, mineral chemical and geochemical analyses of the orthogneisse/metaplutonic complex north of the ISB are required to further elucidate their emplacement and metamorphic history for a better comprehension of tectonic processes during the Archean time.

 

References

Baadsgaard, H., Nutman, A.P., Bridgwater, D. (1986). Geochronology and isotope geochemistry of the early Archaean Amîtsoq gneisses of the Isukasia area, southern West Greenland. Geochimica et Cosmochimica Acta 50, 2173-2183.

Nutman, A. P., McGregor, V. R., Friend, C. R., Bennett, V. C., & Kinny, P. D. (1996). The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma). Precambrian Research, 78, 1-39.