17:15 〜 18:30
[BCG04-P01] カナダ北東部、原大古代ヌブアギツック表成岩帯の地質学的研究:苦鉄質岩や珪質岩の原岩の再検討
キーワード：原太古代、初期地球、地球進化、珪質堆積岩、生命進化
The early Earth’s evolution is the most important issue to decode the history of the earth, but it is very difficult directly to obain its information because only few Hadean and Eoarchean terrains are preserved. The Nuvvuagittuq supracrustal belt (NSB) is one of the oldest supracrustal rocks in the world; thus geological and geochemical study allows us to constrain the evolution of surface environment and solid earth. In addition, previous studies proposed the NSB was formed in the Hadean (~4.2 Ga). The geochronology led to a hot debate, but petrological, geochemical and geobiological studies are still insufficient because the NSB rocks underwent high-grade metamorphism and may have lost the original information. We made detailed geological investigation of the NSB to constrain their formation ages and precursors for the future comprehensive studies.
The NSB in the Hudson Bay Terrane of the Superior Province consists of ultramafic rocks, coarse-graind and fine-grained mafic rocks, banded iron formation (BIF), siliceous rocks, carbonate rocks, and conglomerate. The fine-grained mafic rocks are classifed into two groups: one group, named as Ujaraaluk unit, mainly consists of cummingtonite, biotite, and quartz with subordinate amounts of plagioclase, garnet, cordierite, and anthophyllite, whereas another group comprises greenschist with chlorite, actinolite, quartz, plagioclase, and carbonate. The coarse-grained mafic rocks, named gabbroic intrusions, mainly comprise hornblend, plagioclase, and quartz. The NSB was intruded by trondhjemitic intrusions (~3.82–3.75 Ga) within the belt and on the edge of the southeastern part, and tonalitic intrusions (~3.66 Ga) on the edge of the western part, which constrains the minimum age. In addition, the NSB was inturded by many gabbroic intrusions, pegmatitic intrusions and quartz veins. Previous studies showed an isochron age of ca.4.3 Ga in a 147Sm-143Nd system for the gabbroic intrusions, and a pseudo-isochron age of 4.2 Ga in a 147Sm-142Nd system for the Ujaraaluk unit.
We made a quite detailed geological map using a drone, which enables easily to trace even tens centimeter layers and intrusions. Some modification from previous studies are decribed below. The ultramafic rocks were interpreted as intrusions (sills), but we considered that they originated from flows because no chilled margins were recognizable, and our detailed mapping showed their discontinous occurrence was due to intrusion of the gabbroic dikes and later deformation. The precursor of the siliceous rocks is enigmatic: cherts, pegmatitic intrusion or quartz veins. We concluded that they originated from siliceous chemical sediments deposited in the sea based on their petrography, chemical composition and geological occurrence such as layering and cross-cutting relationship.
The Ujaraaluk unit was considered as hydrothermally-altered basaltic rocks. But they are highly depleted in Ca and enriched in K relative to basaltic rocks. In addition, the low-Ti group has a negative Y anomaly and a negative correlation between the Y anomaly and FeO contents. They resemble modern clay deposited in pelagic environments. The BIF occurs in three areas, and has different occurrence and petrography. The BIF in the northwestern area is accompanied by the ultramafic rocks, and comprises interlayering of magnetite and amphibole/serpentine/talc bands, indicating that they are secondary BIF formed during the metamorphism. The BIF in the southwestern area occurs together with fine-grained greenstone, and mainly contains hematite and quartz as well as putative biogenic fabric formed by iron-oxiding bacteria. We found that they occur as enclaves within a basaltic intrusion. The BIF in the southeastern part is interlayered with the Ujaraaluk unit, and mainly comprises magnetite and quartz, indicating that they originated from chemical sediments.
Many thin intrusions with intermediate composition are present in the Ujaraaluk unit, whereas no intrusions are apparently found in the gabbroic intrusions. In addition, we found many gabbroic intrusions in the trondhjemitic and tonalitic gneiss, indicating the gabbroic intrusions postdated the tonalitic gneiss (~3.66 Ga).
Geological reappraisal indicates that the isochron age of the gabbroic intrusions should be reconsidered and suggests mafic and siliceous rocks originated from pelagic sedimentary rocks.
The NSB in the Hudson Bay Terrane of the Superior Province consists of ultramafic rocks, coarse-graind and fine-grained mafic rocks, banded iron formation (BIF), siliceous rocks, carbonate rocks, and conglomerate. The fine-grained mafic rocks are classifed into two groups: one group, named as Ujaraaluk unit, mainly consists of cummingtonite, biotite, and quartz with subordinate amounts of plagioclase, garnet, cordierite, and anthophyllite, whereas another group comprises greenschist with chlorite, actinolite, quartz, plagioclase, and carbonate. The coarse-grained mafic rocks, named gabbroic intrusions, mainly comprise hornblend, plagioclase, and quartz. The NSB was intruded by trondhjemitic intrusions (~3.82–3.75 Ga) within the belt and on the edge of the southeastern part, and tonalitic intrusions (~3.66 Ga) on the edge of the western part, which constrains the minimum age. In addition, the NSB was inturded by many gabbroic intrusions, pegmatitic intrusions and quartz veins. Previous studies showed an isochron age of ca.4.3 Ga in a 147Sm-143Nd system for the gabbroic intrusions, and a pseudo-isochron age of 4.2 Ga in a 147Sm-142Nd system for the Ujaraaluk unit.
We made a quite detailed geological map using a drone, which enables easily to trace even tens centimeter layers and intrusions. Some modification from previous studies are decribed below. The ultramafic rocks were interpreted as intrusions (sills), but we considered that they originated from flows because no chilled margins were recognizable, and our detailed mapping showed their discontinous occurrence was due to intrusion of the gabbroic dikes and later deformation. The precursor of the siliceous rocks is enigmatic: cherts, pegmatitic intrusion or quartz veins. We concluded that they originated from siliceous chemical sediments deposited in the sea based on their petrography, chemical composition and geological occurrence such as layering and cross-cutting relationship.
The Ujaraaluk unit was considered as hydrothermally-altered basaltic rocks. But they are highly depleted in Ca and enriched in K relative to basaltic rocks. In addition, the low-Ti group has a negative Y anomaly and a negative correlation between the Y anomaly and FeO contents. They resemble modern clay deposited in pelagic environments. The BIF occurs in three areas, and has different occurrence and petrography. The BIF in the northwestern area is accompanied by the ultramafic rocks, and comprises interlayering of magnetite and amphibole/serpentine/talc bands, indicating that they are secondary BIF formed during the metamorphism. The BIF in the southwestern area occurs together with fine-grained greenstone, and mainly contains hematite and quartz as well as putative biogenic fabric formed by iron-oxiding bacteria. We found that they occur as enclaves within a basaltic intrusion. The BIF in the southeastern part is interlayered with the Ujaraaluk unit, and mainly comprises magnetite and quartz, indicating that they originated from chemical sediments.
Many thin intrusions with intermediate composition are present in the Ujaraaluk unit, whereas no intrusions are apparently found in the gabbroic intrusions. In addition, we found many gabbroic intrusions in the trondhjemitic and tonalitic gneiss, indicating the gabbroic intrusions postdated the tonalitic gneiss (~3.66 Ga).
Geological reappraisal indicates that the isochron age of the gabbroic intrusions should be reconsidered and suggests mafic and siliceous rocks originated from pelagic sedimentary rocks.