New information on the origin of mantled gneiss domes comes from a study of the Oliverian Domes in western New Hampshire. Fieldwork indicates that the core of the Mascoma Dome (a representative Oliverian Dome) can be subdivided into two major units: (1) massive gneiss (stratified core-rock) of intermediate igneous composition lying stratigraphically beneath the Ordovician Ammonoosuc Volcanics, and (2) a sub-central pluton of granite and quartz monzonite (unstratified core-rock) which crosscuts the massive gneiss and probably the Ammonoosuc Volcanics, but which lies unconformably beneath the Late Lower Silurian Clough Formation. Within limits imposed by analytical uncertainty and the metamorphic disturbance of the rocks, an Rb-Sr age of 440 ± 40 million years (initial Sr87/Sr86 = 0.706 ± 0.002) is determined for whole-rock samples of the granitic sub-core of the Mascoma Dome, and for whole-rock samples of the Ammonoosuc Volcanics. Zircon separates from both the gneissic and granitic units within the core of the Mascoma Dome yield Pb207/Pb206 ages of 450 ± 25 million years. The data indicate that the dome formed in the following stages: (1) Ordovician volcanism followed by intrusion of granitic rocks, (2) uplift and local unroofing followed “by deposition of Lower Silurian through Lower Devonian strata, and (3) garnet- to staurolite-grade post-Lower Devonian metamorphism and deformation. The crosscutting relationships were established by Ordovician plutonic activity and not by post-Lower Devonian plutonic activity or anatexis. The core-rock of the dome appears to be the result of volcanic and intrusive activity towards the end of the Ordovician, and not the result of in-place remobilization or anatexis of Precambrian basement subsequent to deposition of the mantling strata. The other Oliverian Domes, particularly those in New Hampshire, resemble the Mascoma Dome, and probably originated in much the same manner.

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