EMPLACEMENT OF THE MAIDSTONE AND BLACK HILLS PLUTONS, COMPOSITE INTRUSIONS ON THE MARGINS OF THE NORTHEAST KINGDOM BATHOLITH, VERMONT

The Maidstone and Black Hills plutons are located respectively on the eastern and western margins of the Northeast Kingdom batholith, a Devonian-aged complex in northeastern Vermont that intruded the regionally metamorphosed and deformed rocks of the Connecticut Valley – Gaspé Trough. Recent geologic, petrologic, and geochemical analyses of these two plutons show that they have overall similarities with respect to their geochemical variability and emplacement histories.

Diorite, tonalitic granodiorite, monzogranite, and leucocratic low-Ti monzogranite occur as discrete zones within the Maidstone pluton; similarly, discrete zones of quartz diorite, low-Ti quartz monzodiorite, and quartz monzogranite to granodiorite make up the Black Hills pluton. In each case, the pluton is composite, with contrasting mineralogy and geochemistry between individual intrusions that were produced by discrete magma pulses. Additionally, the region surrounding the Black Hills body includes discrete intrusions of the same lithologies, along with granodioritic dikes, that make up the main pluton.

The composite character of each pluton is particularly well supported by overlapping ranges of silica content accompanied by distinctive abundances of other elements, producing sharp graphical separation on discrimination diagrams. These relationships suggest development of composite plutons in the Northeast Kingdom batholith by sequential emplacement of evolving magma sources, and raise the question as to whether or not this process may be the rule rather than the exception.