A Field and Geochronologic Study of Mantled Gneiss Domes in Central New England

Citation

Naylor, Richard Stevens (1967) A Field and Geochronologic Study of Mantled Gneiss Domes in Central New England. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7TWJ-DJ70. https://resolver.caltech.edu/CaltechTHESIS:05032017-084934764

Abstract

New information on the origin of mantled gneiss domes comes from a study of these structures in Central New England. The domes have cores of massive granite and gneiss encircled by concordant mantles of well-stratified metamorphic rocks, and appear to originate through intense metamorphism of rock sequences in which massive, chiefly quartzo-feldspathic rocks are overlain by less competent strata. Contrary to previous hypotheses, the new work indicates that neither unconformable separation of the core and mantle nor re-mobilization or anatexis of the core rocks are essential elements in the formation of mantled gneiss domes. Two contrasting types of gneiss domes have been identified in central New England.

Examples of the first type are domes of the Chester Dome group in southeastern Vermont. Formation of these domes involved kyanite-staurolite grade metamorphism of Precambrian gneissic basement overlain unconformably by Paleozoic strata. The angular relationships at the unconformity have been obscured by differential movement of the core-rocks relative to the mantling strata. The Precambrian rocks in the cores of the Chester Dome and the nearby Green Mountain Anticlinorium have been badly disturbed by Paleozoic metamorphism, but generally yield Precambrian zircon and Rb-Sr whole-rock ages.

The second type of gneiss dome is exemplified by the Mascoma and Lebanon (Oliverian) Domes exposed about thirty miles to the east in central New Hampshire. No Precambrian rocks have been identified in the cores of these domes. Fieldwork indicates that the core of the Mascoma Dome can be subdivided into two major units: (1) massive gneiss of intermediate igneous composition lying stratigraphically beneath the Ordovician Ammonoosuc Volcanics, and (2) a sub-central pluton of granite and quartz monzonite 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, a common age of 440 ± 40 million years (initial Sr⁸⁷/Sr⁸⁶ = 0.706 ± 0.002) is determined for whole-rock samples of the granitic sub-cores of the Lebanon and Mascoma Domes, 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 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 Lower Silurian through Lower Devonian strata, and (3) garnet- to staurolite- grade post-Lower Devonian metamorphism and deformation. Most of the crosscutting relationships were established by Ordovician plutonic activity and not by post-Lower Devonian plutonic activity or anatexis. The core-rocks of these domes appear 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 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 the same manner.

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