Abstract

External and internal morphologies are well shown by a newly discovered diatreme that is exceptionally well exposed in a cirque within the north face of Mount Dingley. The diatreme contains abundant brecciated host rocks mixed with highly altered, fine-grained, light-green igneous fragments (minerals include muscovite, chlorite, quartz, carbonate, and some remnant K-feldspar). The diatreme cuts Lower Ordovician rocks of the McKay Group. Olistostromes and other volcaniclastic rocks that are directly associated with the diatreme are bevelled beneath a regional unconformity below the Upper Ordovician Beaverfoot Formation. Lower Ordovician gastropods are present just below the volcaniclastic rocks and within what appears to be a lens of sediment within one of the olistostrome beds. These occurrences indicate a mid-Early Ordovician time of intrusion, but there is the possibility that the pipe was emplaced later within the interval mid-Early to early Late Ordovician. In the Western Ranges, three other episodes of emplacement of diatremes have been documented previously as within the intervals early Middle to early Late Ordovician, latest Early Silurian to early Middle Devonian, and Late Permian. Many of the diatremes are broadly contemporaneous with widespread, but volumetrically small, Ordovician and Lower Paleozoic volcanic and intrusive rocks found throughout the Canadian Cordillera. These volcanic and intrusive rocks have been interpreted as evidence of continued Lower Paleozoic extensional tectonism and some are associated with large base-metal deposits.

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