Temagami Greenstone Belt - Geology

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 Temagami greenstone belt is 25 km (16 mi) wide and 32 km (20 mi) 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 Temagami greenstone belt 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 90 m (300 ft) to 1,500 m (4,900 ft). 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 90 m (300 ft) to 900 m (3,000 ft) 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 Temagami greenstone belt was the formation of a rhyolite porphyry dike 2687 ± 2 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 Temagami greenstone belt are older than the closest exposed portion of the Abitibi Subprovince, about 120 km (75 mi) north of Kirkland Lake.

Along with nearby granitic intrusions, the Temagami greenstone belt 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 24 km (15 mi) north of the Grenville Front Tectonic Zone. The volcanic rocks possibly total as much as 6,000 m (20,000 ft) 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 60 m (200 ft) to 300 m (980 ft) 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 100 m (330 ft) to 210 m (690 ft). 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.