New information on the origin of mantled gneiss domes comesfrom a study of these structures in Central New England. The domeshave cores of massive granite and gneiss encircled by concordantmantles of well-stratified metamorphic rocks, and appear to originatethrough intense metamorphism of rock sequences in which massive,chiefly quartzo-feldspathic rocks are overlain by less competentstrata. Contrary to previous hypotheses, the new work indicates thatneither unconformable separation of the core and mantle nor re-mobilizationor anatexis of the core rocks are essential elements inthe formation of mantled gneiss domes. Two contrasting types ofgneiss domes have been identified in central New England.
Examples of the first type are domes of the Chester Dome groupin southeastern Vermont. Formation of these domes involved kyanite-staurolitegrade metamorphism of Precambrian gneissic basement overlainunconformably by Paleozoic strata. The angular relationships atthe unconformity have been obscured by differential movement of thecore-rocks relative to the mantling strata. The Precambrian rocks inthe cores of the Chester Dome and the nearby Green Mountain Anticlinoriumhave been badly disturbed by Paleozoic metamorphism, butgenerally yield Precambrian zircon and Rb-Sr whole-rock ages.
The second type of gneiss dome is exemplified by the Mascomaand Lebanon (Oliverian) Domes exposed about thirty miles to the eastin central New Hampshire. No Precambrian rocks have been identifiedin the cores of these domes. Fieldwork indicates that the core ofthe Mascoma Dome can be subdivided into two major units: (1) massivegneiss of intermediate igneous composition lying stratigraphicallybeneath the Ordovician Ammonoosuc Volcanics, and (2) a sub-centralpluton of granite and quartz monzonite which crosscuts the massivegneiss and probably the Ammonoosuc Volcanics, but which liesunconformably beneath the Late Lower Silurian Clough Formation.Within limits imposed by analytical uncertainty and the metamorphicdisturbance of the rocks, a common age of 440 ± 40 million years(initial Sr87/Sr86 = 0.706 ± 0.002) is determined for whole-rocksamples of the granitic sub-cores of the Lebanon and Mascoma Domes,and for whole-rock samples of the Ammonoosuc Volcanics. Zirconseparates from both the gneissic and granitic units within the coreof the Mascoma Dome yield Pb207/Pb206 ages of 450 ± 25 million years.The data indicate that these domes formed in the following stages:(1) Ordovician volcanism followed by intrusion of granitic rocks,(2) uplift and local unroofing followed by deposition of LowerSilurian through Lower Devonian strata, and (3) garnet- to staurolite-grade post-Lower Devonian metamorphism and deformation. Most ofthe crosscutting relationships were established by Ordovicianplutonic activity and not by post-Lower Devonian plutonic activity oranatexis. The core-rocks of these domes appear to be the result ofvolcanic and intrusive activity towards the end of the Ordovician,and not the result of in-place remobilization or anatexis of Pre-cambrian 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 thesame manner.
