NEW GEOCHEMICAL AND ISOTOPIC DATA FROM THE THREE PONDS GABBRO, NH: INSIGHTS INTO POST-ACADIAN MAGMATISM

An abundant magmatic history in New Hampshire is associated with various tectonic events. The New Hampshire Plutonic Suite (NHPS) formed during the Acadian Orogeny and consists of four components, including the syntectonic Bethlehem Granodiorite (410 ± Ma), Kinsman Granodiorite (411 ± 19 Ma), and Spaulding Tonalite (393 ± 5 Ma), and the post-tectonic Concord Granite (~365 Ma) (Dorais, 2003). The former three were intruded and deformed during the Acadian orogeny (Malinconico et al., 2012). The White Mountain Magma Series (WMMS), a Mesozoic felsic igneous province, is related to the opening of the Atlantic Ocean. The oldest pulse of magmatism in its Batholith occurred from 198.5 Ma to 180 Ma. (Kinney et al., 2022). The youngest magmatic rocks in New Hampshire are associated with the New England-Quebec igneous province (NEQ) and have dates between 140 to 100 Ma with a variety of explanations for their petrogenesis (Boemmel et al., 2021 and Kinney et al., 2022). With an interest in connecting various magmatic pulses to tectonic events, we focus on a gabbro mapped as Jurassic on the NH State Geologic map, which has been termed both the Three Ponds Gabbro and the Black Hill Gabbro, whose overall magmatic history is enigmatic. This post-paleozoic body makes contact with pelitic country rocks (Malinconico et al, 2012). The Three Pond Gabbro has a zircon date from Kinney et al. (2022) at 388.91 ± 4.98. This project is meant to identify and understand the petrogenesis and tectonic framework of the Three Pond Gabbro.

Major trace element analysis of samples collected showed a consistent SiO₂ 45-48 wt%, which is consistent with a low-silica gabbro. Basaltic dykes that crosscut the gabbro are characterized by OIB-like trace element patterns, while the gabbro has a composition similar to subduction zone magmatism. Radiogenic isotopic values for the gabbro and dykes plot in the direction of Enriched Mantle. Petrographic analysis of the samples shows a varying degree of hydrothermal alteration across the gabbro. We use all this information to evaluate the connection of the Three Ponds Gabbro within a regional context.

References:

Boemmels et al., 2021 https://doi.org/10.2475/03.2021.03

Dorais, 2003 https://doi.org/10.2475/ajs.303.5.447

Kinney et al., 2022 https://doi.org/10.1130/g50181.1

Malinconico et al., 2012 104th meeting of NEIGC