THE CHAIN LAKES MASSIF: LAURENTIAN SOURCE AND ANATECTIC HISTORY

Using recently-collected geochronological, structural, and petrologic information, we present a new interpretation of the history of the Chain Lakes massif and its role in Appalachian orogenesis. The Chain Lakes massif, located along the Maine-Quebec border, is a quartzofelsdpathic diatexite whose protolith was most likely a sedimentary-volcanic sequence deposited in an arc-marginal basin. Our chronological work sought to identify the provenance of the massif, the time of protolith formation, and the time of anatexis. SHRIMP-RG analyses of detrital zircon indicate that the sedimentary portion of the protolith contains dominant age populations of ca. 900-1200 Ma, 1400-1800 Ma, and 2600-2800 Ma. Such populations are consistent with a Laurentian source. Three lines of evidence support but do not require an early Ordovician age for the protolith of the massif. (1) The overall detrital age spectrum is similar to populations seen in Ordovician sediments in Newfoundland. (2) Igneous zircon from a felsic lithic fragment of probable volcanic origin crystallized at 479±6 Ma. The age of the fragment represents the age of active deposition. (3) Zircon separated from amphibolite portions of the massif exhibit xenocrystic cores of widely varying age with rims grown in the early Ordovician. We determined the time of anatexis by dating metamorphic monazite. Monazite grew at 469±4 Ma, an age that concurs with previously-determined results. The available data suggest that the Chain Lakes massif formed and was metamorphosed adjacent to the contemporaneous Notre Dame arc. The sediment source for the massif and the nucleus for the arc was probably a Laurentian-derived microcontinent. Such a microcontinent may have extended from Newfoundland through central New England.