EVOLUTION OF THE COMPOSITE SHIRLEY-BLANCHARD PLUTON, CENTRAL MAINE: FIELD, PETROGRAPHIC AND GEOCHEMICAL EVIDENCE

The Shirley-Blanchard pluton (SBP), located S of Greenville in north-central Maine, lies within the Piscataquis Igneous Belt, a suite of ~ 40 compositionally diverse plutons. The pluton was mapped as a composite intrusion by Espenshade and Boudette (1964), with the Shirley phase exposed in the NE of the pluton and the Blanchard phase in the SW. The relative age of these phases is unclear, although zircon ages (Bradley et al., 2000) suggest the Shirley phase (404 ± 4.4Ma) is younger than the Blanchard phase (407 ± 1.5Ma). This study details the field relations and petrography of the SBP and, along with new whole-rock geochemistry, examines its evolution and assembly.

The Shirley phase is a leucocratic, biotite granite. It is fine- to medium-grained, texturally variable ranging from equigranular to porphyritic with a CI < 15 to 25. The Blanchard phase has a higher CI (~ 45). It varies from a melanocratic, pyx + hb bearing, coarse-grained, equigranular quartz diorite to a plagioclase-rich diorite to quartz diorite, sometimes within the same outcrop. Meter-sized, angular blocks of both these textural variants are observed within the Shirley phase at the mapped contact and leucocratic veins of the Shirley granite intrude the diorites confirming it is younger. Geochemical data show that the two phases are compositionally distinct. The Shirley biotite granite is more evolved (SiO₂ 66 – 73%), whereas the Blanchard phase is intermediate in composition (SiO₂ 52 – 57%) and has higher Al₂O₃, CaO, Fe₂O₃, MgO, and TiO₂. Harker diagrams display linear but separate trends for both phases with a definite compositional hiatus. However, if the trends displayed by the Blanchard phase are extrapolated to higher silica values they coincide closely with the data for the Shirley phase. The trace element data define more continuous variations which may indicate a fractionating comagmatic series.

The field evidence confirms that the Shirley granite is younger than the Blanchard diorite and initial examination of the geochemical data suggests they are separate intrusions. Alternatively, the two phases may be comagmatic with the Shirley granite produced by continuing fractionation of the Blanchard magma at depth and emplaced later as a magmatic pulse. Further geochemical modeling will test these hypotheses.