PETROGRAPHY AND GEOCHEMISTRY OF SUBSURFACE GRANITOID PLUTONIC ROCKS FROM TOTTEN WELL T-3, SOUTH HAMILTON, MASSACHUSETTS

Forty-two thin-sections of granitoid igneous rocks were examined to determine their lithology, geochemical characteristics, and to compare with other granitoid rocks of similar age observed in the Avalon Terrane. Ten of these samples were also analyzed by (ICP)-Mass Spectrometry.

Granitoid samples were taken at depths ranging from 3,737-5,926 feet from a 6,000 ft-long core. Thirty-two samples are granodiorites, six are quartz monzodiorites, two are granites, and two are quartz diorites, according to QAP modal mineralogical analysis. All samples are phaneritic and porphyritic, with quartz (12.3-46.4%), orthoclase (3.6-23.3%), microcline (3.3-25.7%), perthite (12.1-43.1%), plagioclase (5.6-24.2%), and amphibole (trace-10.5%) as the major minerals. Minor minerals include biotite (trace-4.9%), while accessory minerals (<1%) include zircon, apatite, rutile, and Fe-Ti oxides. The samples plot as two distinct groups on the TAS, variation, AFM, Rb-Ba-Sr, and tectonomagmatic diagrams. Group I has silica concentrations from 72.82 wt.% to 76.64 wt.%, while Group II ranges from 64.78 wt.% to 66.69 wt.%. The rocks of this study appear to be a bimodal intrusion of two groups of granitoid magma. The samples are subalkaline on a TAS diagram and can be further categorized as calc-alkaline according to an AFM diagram. Group II is classified as M-type metaluminous granitoids, while Group I is classified as M- to I-type metaluminous to weakly peraluminous granitoids. Group I plot as volcanic arc granitoids, while Group II plot as within-plate granitoids based on the Rb-(Y +Nb) tectonomagmatic discrimination plot. All the samples show a prominent negative europium anomaly, indicating a plagioclase-rich restite. It was originally thought, due to the location of the T-3 well in an area mapped as Cape Ann Granite, that the rocks of this study would be Cape Ann Granite. However, the rocks of this study are M- to I- type, calc-alkaline, metaluminous, within-plate and volcanic arc granitoids unlike the granitoids of the Cape Ann area, which are A-type, commonly peralkaline and entirely within-plate granitoids. The samples of this study are more similar to the granitoids of similar age (429 + 4 Ma) from the Avalon pluton of Newfoundland, which are M- to I-type, calc-alkaline, metaluminous, within-plate and volcanic arc granitoids.