Abstract

The Chain Lakes massif has long been an enigmatic component of the Appalachian orogen, but new structural, microstructural, and geochronological information provides the basis for the following new interpretation of the massif and its history. In the early Paleozoic, sediments and volcanic rocks from Laurentia or a Laurentian-derived microcontinent were deposited in a fore-arc basin on the western margin of the Iapetus ocean. Following intrusion of arc-related magmas, the sedimentary–volcanic sequence was heated sufficiently to melt in place, resulting in stratigraphic disaggregation and diatexite formation. We dated monazite growth from this metamorphic event at 469 ± 4 Ma. Although some melt may have left the system, much remained, including water dissolved in the melt. Upon crystallization, this water drove thorough retrogression of the massif, causing pervasive pseudomorphism of porphyroblasts. With cooling and crystallization, the Chain Lakes massif became sufficiently rigid that it was not significantly deformed during the Middle Ordovician through Devonian stages of Appalachian orogenesis involving the arrival of several peri-Gondwanan microcontinents.

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