Part I. Geology of the Manicouagan-Mushalagan Lakes Structure. Part II. Geochronology of the Manicouagan-Mushalagan Lakes Structure

Citation

Wolfe, Stephen Howard (1972) Part I. Geology of the Manicouagan-Mushalagan Lakes Structure. Part II. Geochronology of the Manicouagan-Mushalagan Lakes Structure. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YB0X-BZ53. https://resolver.caltech.edu/CaltechETD:etd-02112004-095253

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Manicouagan-Mushalagan Lakes structure is an area of anomalous geological features in the Grenville Province terrain of northeastern Canada. Three features of this structure have been revealed by geological studies: (1) The Grenville gneisses and anorthosites which outcrop in the area between the arcuate lakes have been shock metamorphosed. (2) The shocked Grenville gneisses are covered by a thin, discontinuous layer of shock breccia and thin outcrops of Ordovician limestone. (3) An annular-shaped body of Permo-Triassic, igneous, andesitic rock, which shows no signs of shock metamorphism, overlies the shocked gneisses, the shock breccia and the Ordovician limestone. This rock (monzonite) is 800 feet thick at a maximum. The shocked basement rocks in the structure were most probably caused by the impact of an extraterrestrial body, but the origin of the igneous rock unit is not completely understood. This latter unit has been hypothesized to be either (1) a shock-melted breccia or (2) a later, secondary igneous event occurring at the crater site due to disruptions of the earth's thermal regime around the impact site. A geochronological study of this structure was made to determine which, if either, of the above modes of formation for the igneous material is likely. [...] data for the igneous material and rock units related to it yield a well-defined crystallization age of [...]. [...] data for this unit indicate that it has experienced no subsequent metamorphism. Argon isotopic ages obtained for a series of shock-sequence anorthosites do not indicate a well-defined time of shock metamorphism; the ages could characterize a system shocked at anytime between 320 and 210 m.y. ago which then suffered a secondary gas loss. It is shown that anorthosites yielding ages between 280 and 320 m.y. probably underwent complete or nearly complete outgassing at the time of shock. These latter ages should therefore yield a good impact age. The time interval within which the impact took place is thought to be 280-320 m.y. ago. The [...] data show unexpected evidence for the occurrence of a secondary event at about 100-110 m.y. which affected the anorthosites in the crater but not the igneous monzonite. Since it is not possible to say with certainty when the meteoritic impact took place, the difference between the monzonite formation time and the impact time is unclear. The suggested impact time is, however, significantly earlier than the crystallization time of the igneous material. This suggests that a secondary formation mode for this material is more likely than an impact melt formation mode. A secondary mode is also compatible with evidence that the monzonite formed in a subsurface environment with [...].

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