THE NORTHEAST KINGDOM BATHOLITH, VERMONT - MAGMATIC EVOLUTION AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF ACADIAN GRANITIC-ROCKS

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasing SiO2 (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K2O/Na2O ( < 1), K/Rb (60-350), Zr/Hf (30-50), Nb/Ta (2-22), Hf/Ta (up to 10), and Rb/Zr ( < 2) in the range of plutonic rocks found in continental magmatic arcs. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5-22), Th/Yb (0-27), Ba/La (0-80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Yb(cn) < 10) and negative Eu anomalies are small or absent. The metaluminous to peraluminous intermediate plutons are relatively enriched in the light REE (La/Yb(cn) > 40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80-164), Th/Ta ( < 9), Rb/Cs (7-40), K/Cs (0. 1 -0.5), Ce/Pb (0.5-4), high values of Rb/Sr (1-18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilation scheme that accounts for all trace element variations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks ( < 50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.