Ar-Ar geochronology: provenance and thermal history from laser ablation analysis of muscovite

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Many of the rock units in Atlantic Canada (and elsewhere) now juxtaposed began their lives in distant parts of the world typically across distant seas. It is important to identify the environment in which these rocks formed, and also to establish when they were accreted to the rest of North America. When rocks erode, 'detrital' minerals are deposited in sediments that may be preserved long after the source rocks have disappeared, or otherwise have been left behind as the source and sedimentary units or 'terranes' drifted apart. Zircon and muscovite are two important detrital minerals because their respective radiometric ages are together fairly definitive 'fingerprints' of the source rocks. Projects described in the current application involve the measurement of 40-Ar/39-Ar (essentially K-Ar) ages of muscovites separated from selected sedimentary units. Because the various muscovite grains in a sedimentary rock may have come from different sources, it is necessary to date each grain separately. In fact, recent data suggest that one must obtain intra-grain ages; that is, observe what age variation exists within each muscovite grain. The reason for this is that some parts of the grain may have been disturbed (e.g. radiometric 'clock' reset to zero) by later thermal events. Intra-grain age distribution patterns are most reliably made by measuring a large number of 'spot' ages using the UV laser ablation technique, the method of choice for the present research. When plate tectonic processes cause two terranes to collide together and become one, new muscovites are formed in certain geologic environments, and the ages of pre-existing muscovites may be completely or partially reset by the resulting thermal event. Here too, the UV laser ablation method can be used to reveal intra-grain age patterns that will reveal the timing of terrane amalgamation. In the current proposal, we target Late Precambrian to Cambrian (ca. 650-500 million year old) rocks in the Meguma terrane in SW Nova Scotia and in the Avalonia terrane, specifically where the latter is exposed in southern New Brunswick and in eastern Cape Breton Island. We seek clues to the origin of these terranes and to the times of their accretion to each other and to the rest of North America.