GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 19-5
Presentation Time: 9:20 AM


EBY, G. Nelson, Environmental, Earth & Atmospheric Sciences, University of Massachusetts, Lowell, MA 01854,

Triassic-Jurassic intraplate magmatism in northeastern US and southeastern Canada is represented by the Coastal New England (CNE) and Eastern North America (ENA)provinces, largely composed of mafic dikes and lava flows, and the Older White Mountain (OWM) province which is predominately felsic plutons. The ENA is associated with the tholeiitic Central Atlantic Magmatic Province (CAMP), which is comprised of dikes, sills and lava flows of tholeiitic composition that were emplaced within a few million years at ~200 Ma (very close to the Triassic-Jurassic boundary). CNE magmatic activity starts at 247 Ma and extends to 162 Ma, and is represented by both silica-undersaturated and silica-oversaturated mafic dikes and a few lava flows. OWM magmatic activity starts at 238 Ma and ends at 155 Ma. However, on a volumetric basis OWM magmatism does not become significant until after 200 Ma and most of the plutons are emplaced between 190 Ma and 170 Ma.

Based on trace element and limited isotopic data, the CNE and OWM provinces are geochemically distinct from the ENA. Petrogenetic modeling suggests that the mafic CNE magmas are derived from the subcontinental lithosphere at depths ranging from 60 to 150 km. Nepheline-bearing rocks are found in the OWM at Red Hill, Rattlesnake Mtn, and Litchfield. It is possible to derive magmas of appropriate composition to form these plutons by fractional crystallization of silica-undersaturated CNE magmas. The volumetrically much more abundant silica-saturated magmas were produced by interaction between CNE magmas and the deep continental crust. Further evolution of the magmas occurred through low-pressure fractional crystallization with limited interaction with the upper continental crust. This model is well-illustrated by the White Mountain batholith where individual plutonic units evolved by closed system fractional crystallization, but with decreasing age each unit shows increasing interaction with the continental crust (a MASH model). CAMP magmatism plays an important role in this process in that the CAMP magmas provide the heat for the partial melting of the continental lithosphere. However, elemental and isotopic data indicate that there was no transfer of material between these reservoirs, only heat energy was transferred.