The Temagami Greenstone Belt (TGB) is a small 2.7 billion year old greenstone belt in the Temagami region of Northeastern Ontario, Canada. It represents a feature of the Superior craton, an ancient and stable part of the Earth's lithosphere that forms the core of the North American continent and Canadian Shield. The belt is composed of metamorphosed volcanic rocks that range in composition from basalt to rhyolite. These form the east-northeast trend of the belt and are overlain by metamorphosed sedimentary rocks. They were created during several volcanic episodes involving a variety of eruptive styles ranging from passive lava eruptions to viscous explosive eruptions.
Part of the Canadian Shield, the TGB contains some of the oldest known rocks on Earth. The belt is made up of a number of geologic features such as batholiths, stocks, dikes, volcanic complexes, layered intrusions and deformation zones. These are situated in several geographical townships in the municipality of Temagami, including Chambers, Strathy, Strathcona, Briggs and possibly Best.
Geology Geologists assume that greenstone belts were formed by many geological processes, such as tectonism, magmatism, metamorphism and sedimentation. They are important economically for large metal deposits, and for the insight they provide into crustal evolution and the tectonics of the early Earth. The TGB is wide and long. It contains the southernmost remnants of Archean intrusive and supracrustal rocks in Eastern Ontario, as well as some of the most ancient felsic magmatic events in this section of the Superior craton. Uranium-lead dating has established that the Iceland Lake Pluton, as well as an adjacent rhyolitic lava flow, is about 2,736 million years old. Therefore, at least some intrusions were likely formed during the first volcanic phases in the belt and may have been conduits for volcanic eruptions.
The variety of volcanic deposits and intrusions in the TGB indicates that magmatic activity played a significant part in its formation. Pillow lava is found throughout the belt, indicating lava erupted underwater. Its pyroclastic deposits are remnants of explosive volcanism. The oldest exposed rocks within the belt are fine to medium-grained basalts and andesites. Lava flow units range in thickness from to . Mafic agglomerate and breccia are relatively abundant, being either massive and undeformed, or sheared. Dacitic lava flows or tuffs overlie these metamorphosed volcanic rocks along with intermediate volcanic breccias, and are overlain by rhyolite lava flows and tuffs. Acidic lava flow units range in thickness from to and are common in the Vermilion Lake and Link Lake areas. The felsic tuffs are normally altered and sheared. The most recent intrusive activity in the TGB was the formation of a rhyolite porphyry dike million years ago. This age correlates well with the 2675–2700 million year old intrusions throughout the Abitibi Subprovince, but the 2736 million year old magmatic events in the TGB are older than the closest exposed portion of the Abitibi Subprovince, about north of Kirkland Lake.
Along with nearby granitic intrusions, the TGB is bounded by layers of rock comprising the Huronian Supergroup. Strathy Township is dominated by metamorphosed volcanic rocks of the northeastern portion of the belt. It is approximately north of the Grenville Front Tectonic Zone. The volcanic rocks possibly total as much as thick. However, portions of the sequence might have been repeatedly sheared by one or several local fault zones. Every large volcanic event is capped by metamorphosed sedimentary rocks and/or iron formations. The metamorphosed sedimentary units range in thickness from to and consist of laminated slate and greywacke with or without volcanogenic tuffs. The iron formations are composed of alternate layers of magnetite, white quartzite, jasper, grey cherty quartz, and/or tremolite-chlorite tuff. They are intruded by sills composed of medium-grained, white-weathering, quartz diorite that range in thickness from to . These rocks are similar to the coarse thicker parts of lava flows, but are interpreted to be partly intrusive, likely conduits that produced mafic volcanism.
Intrusions A layered intrusion composed of diorite, pyroxenite, gabbro and anorthositic gabbro has been found in northwestern Strathy Township. Pyrrhotite is common in associated pyroclastic rock. Several northeast-trending shear zones less than wide intersect the edifice, extending along the Net Lake-Vermilion Lake Deformation Zone. This layered intrusion might be similar in age to the Kanichee layered intrusive complex, and may represent a magma chamber that was the source of tholeiitic volcanic activity.
The Kanichee layered intrusive complex, also known as the Kanichee Intrusion and Ajax Intrusion, is the most voluminous mafic-ultramafic body in metamorphosed felsic and mafic volcanic rocks of the northern TGB. It is an oval-shaped layered intrusion that was formed during five phases of magmatic activity. A series of south-southeast dipping cyclic magmatic layers make up the intrusion, similar to those of the surrounding metamorphosed volcanic rocks, indicating that the rocks of the intrusion were formed horizontally and likely close to the surface. Numerous magmatic events may have breached the surface to produce volcanic eruptions. The overall structure of the intrusion indicates it is cylindrical in shape and has a long axis plunging to the southeast at a somewhat steep angle. Its fairly steep angled axis was formed by at least one period of deformation that similarly folded and deformed the surrounding volcanic rocks.
An intrusion of light-coloured diorite lies at the northern end of the Tetapaga Syncline, along the Milne-Sherman Road. Its colouration is from weathering of plagioclase, which comprises more than 50% of the intrusive rock. Several east-trending high-strain zones related to the Link Lake Deformation Zone, less than wide, are found in the intrusion, indicating that it formed at least before the last increment of strain along the Link Lake Deformation Zone. The dioritic intrusion might be the remains of a magma chamber that was the product of calc-alkaline feldspar-phyric felsic volcanism, the erupted products of which are mostly located in the Sherman Mine area.
At least three large granitoid intrusions penetrate the TGB. The Spawning Lake Stock in Briggs and Chambers townships contains megacrysts that reach to in diameter. It shears through the nearby fold axis of the Tetapaga Syncline, but remains undeformed by the western expression of the Net Lake-Vermilion Lake Deformation Zone. To the north and northeast, the massive Chambers-Strathy Batholith was formed during one magmatic event. It consists of intrusive rocks ranging from pink to grey quartz monzonite to granodiorite and intrudes metamorphosed volcanic rocks of the TGB. The Iceland Lake Pluton, situated in Strathcona and Briggs townships, consists of a complex series of chlorite trondhjemite, hornblende quartz diorite and hornblende trondhjemite magmas that were emplaced during more than one magmatic event. The trend of metamorphosed volcanic rocks in Briggs and Strathcona townships is parallel to the intrusion contact. A planar fabric, attributed to the earliest deformation of the TGB, exists in the outer margin of the intrusion.
Intrusive dikes composed of quartz and feldspar-quartz porphyry are widespread in Strathy Township. All dike types were placed in the Earth's crust during the formation of the TGB. At least some of these dikes may have been subvolcanic feeders that produced known calc-alkalic felsic volcanic events in the belt. Many pyroxenite dikes less than wide intrude every geologic formation in the township. A pyroxenite dike east of Highway 11 extends roughly to the north-northwest while two others, north of Arsenic Lake, extend to the east. This includes a dike exposed along the Kanichee Mine Road in Temagami North, where it intrudes the large granitic Chambers-Strathy Batholith. A series of northwest-trending dikes composed of diabase may represent extensions of the 1,250 million year old Sudbury dike swarm. This dike swarm is older than the Lake Wanapitei impact crater but younger than the meteorite impact that created the large Sudbury Basin crater about 1,850 million years ago. Therefore, the TGB predates these two impact craters, the Sudbury crater of which is the seconed largest known impact crater on Earth.
Felsic volcanic vents Volcanic vents composed of felsic rocks are thought to have been located at the iron-bearing Sherman Mine, the former Temagami garbage dump and adjacent to the Milne Townsite. Remnants of a large volcanic vent are present west of Sherman Mine, including the existence of two felsic lava flows that outcrop between Link Lake and Turtle Lake. The aspect of the most extensive felsic lava flow indicates that the volcanic vent they erupted from was adjacent to the western portion of Link Lake. Additionally, differentiation of facies and directions in which the lava traveled exist in course grained, resedimented conglomerate. This suggests that a more prominent structure, perhaps evidence of a prehistoric volcano, existed west of Link Lake. Also, coarsest felsic volcanic fragments occur in feldspar-phyric pyroclastic deposits exposed on the Sherman Mine property, suggesting the approximate location of a volcanic vent.
Just north of the Milne Townsite lies a minor felsic volcanic vent exposed along the Milne-Sherman Road. A quartz porphyry has intruded mafic and rhyolitic lava flows and a dike of diorite. In the middle of the intrusive body, the grain size of quartz phenocrysts averages roughly , but become smaller towards the edge of the intrusion. Broken fragmental rocks, interpreted to be carapace breccia, are exposed along the western margin of the intrusion. Exposed near a minor felsic lava dome is a fine grained, quartz-phyric felsic rock that may represent a rhyolite lava flow. An igneous body, interpreted to be a subvolcanic intrusion, is exposed approximately to the northwest and might have formed during the same magmatic event as the felsic dome. Exposed along the Sherman Mine railroad west of the former Milne sawmill lumber yard is a porphyritic body composed of quartz-feldspar. It is unknown if this igneous rock is a rhyolitic lava flow or an intrusion.
Several north-trending felsic dikes, composed mainly of feldspar and quartz-feldspar, are located north of the former Temagami garbage dump. These dikes are not known to extend across the Link Lake Deformation Zone, suggesting that they might represent the feeders of a minor volcanic vent, manifest now by felsic lava flows. Because certain lithologies can be correlated through the Link Lake Deformation Zone, it is unlikely that the lack of felsic dikes south of the former town dump area is an expression of displacement along the deformation zone.
Volcanic complexes