TIMESCALES FOR CONSTRUCTION OF THE EXETER GRANODIORITE, A DEVONIAN SHEET-LIKE PLUTON IN SOUTHERN NEW HAMPSHIRE

Large intermediate composition plutons allow insight into intrusive timescales and magmatic flux rates in magmatic arcs. Southern New Hampshire and northeastern Massachusetts contain a number of these large plutons that are compositionally diverse and well exposed. The Exeter pluton is representative of these plutons, is exposed on the campus of the University of New Hampshire and is a favorite of Wally Bothner. Starting over 30 years ago, Wally, his colleagues, and students, mapped the pluton, determined its gravity and magnetic signatures, and dated it using Rb-Sr as well as multi- and single-grain U-Pb geochronology. Since the senior author was greatly influenced by Wally and this pluton, this symposium is the ideal place to return to his batholithic roots.

We report new high-precision U-Pb geochronology for the Exeter Granodiorite, a large composite pluton exposed in southern New Hampshire that intrudes deformed Silurian rocks of the Merrimack trough. The Exeter ranges in composition from gabbro to pyroxene-bearing diorite, quartz-diorite and minor granite. High-precision CA-TIMS zircon analyses from multiple phases of the pluton indicate that most of it was emplaced in less than 400 Ky with a volume of 350-450 cubic kilometers and that the gabbroic phase post-dated the voluminous dioritic intrusion.

Gravity data are consistent with a tabular intrusive body with exposed dimensions of 32 km by 7 km and model thicknesses ranging from 1 to 3 km. Magnetic fabric data obtained from oriented core samples of the diorite show strongly developed magnetic foliation at the margins that may be related to magmatic flow or post-emplacement strain or both. The interior of the pluton lacks a magnetic fabric consistent with rapid emplacement of a large pulse of magma.