GEOCHEMICAL ANALYSIS OF JURASSIC GABBROS AND SYENITES FROM THE WHITE MOUNTAINS, NEW HAMPSHIRE

The White Mountains are composed of a series of plutons and associated extrusive igneous rocks located in New Hampshire and the westernmost part of Maine. The White Mountains have experienced two periods of magmatism: the older period occurred approximately 200-165 Mya and the younger period approximately 130-100 Mya. The Jurassic pulse of magmatism may be associated with the rifting of Pangea 200 Mya and the subsequent opening of the North Atlantic Ocean, but the petrogenetic processes behind the Jurassic magmatism are not well understood. Eby et al. (1992) propose that magmas derived from a non-crustal source underwent two periods of fractional crystallization, an earlier period at the base of the crust and a later period in the shallow crust. The Cretaceous magmatism is thought to be associated with the Great Meteor Hotspot, a mantle plume that begins in the Monteregian Hills in Quebec and extends out into the New England Seamounts in the Atlantic Ocean.

Previous studies suggest that the Jurassic White Mountains magmatism is an example of A-type granitoid magmatism with an ocean island basalt source (Eby et al., 1992). However, the White Mountains have not been studied extensively, especially in recent years. Many of the units have yet to be well characterized, mapped, and/or dated. We seek to characterize a suite of samples related to the period of Jurassic magmatism, including samples mapped as Jurassic gabbros and syenites (Lyons et al., 1997). The gabbro stock from the Three Ponds area in New Hampshire is characterized as having a mineral assemblage of plagioclase + biotite + hornblende ± augite, with previous studies suggesting that the hornblende is secondary (Malinconico et al., 2012). The porphyritic syenite from the Passaconaway area is characterized as having a mineral assemblage of alkali feldspar + plagioclase + oxides + biotite ± pyroxene ± amphibole. Because the Three Ponds gabbros and Passaconaway syenites were not included in the work of Eby et al. (1992), their geochemistry has not been explored in detail. We present whole rock geochemical data and mineral chemistry for these samples with the goal of better understanding the petrogenetic processes associated with magmatism during the Jurassic and how that fits into the regional context of the tectonic history of New England.