Mineral chemistry, petrology and geochemistry of the Sebago granite-pegmatite system, southern Maine, USA

Abstract

The Permian (293 ± 2 Ma) Sebago Pluton is a homogeneous, two-mica granite situated in the Oxford pegmatite field, southwestern Maine. Surrounding the pluton is an area designated as the Sebago Migmatite Domain (SMD) dominated by metapelitic migmatites and diatexites with subordinate intrusions of heterogeneous, two-mica and biotite granites, pegmatitic leucogranites and granitic pegmatites. The Sebago Pluton plus the SMD formerly defined the extent of the Sebago Batholith. Most of the granitic pegmatites and bodies of pegmatitic leucogranites occur either within, or barely outside, the margins of the SMD. The pegmatitic leucogranite facies displays units typical of fertile granites (granites that produce granitic pegmatites) and include: megacrystic graphic K-feldspar, sodic aplite and potassic pegmatite pods hosting rare tourmaline, beryl and columbite-tantalite. Over 100 granitic pegmatite bodies (Sebago Pegmatite Group -SPG) intrude the outer portions of the SMD and neighboring granitoids and metasedimentary rocks. The pegmatite population includes mineralogically simple to complexly zoned pegmatites that are characterized by a LCT-type (Li, Cs and Ta) geochemical signature, extensive replacement of primary zones and gem-bearing miarolitic cavities. Evidence that supports the pegmatitic leucogranites as the likely parent to the SPG includes the close spatial distribution of the pegmatites to the leucogranite bodies, texturally and mineralogically similar units observed within the leucogranite and the neighboring pegmatites plus gradual, yet overlapping, rare-element fractionation from the leucogranites to the associated pegmatites. A few pegmatites (e.g., the Lord Hill pegmatite and amethyst-bearing pegmatites) show NYF tendencies unlike any other pegmatites of the SPG. Differences in fractionation degree, evolution and/or relation to another fertile granite-pegmatite system may be responsible for this apparently anomalous group of pegmatite dikes.

The Mata Azul pegmatitic field, Tocantins/Goiás, central Brazil: geology, genesis and mineralization

“…Much evidence corroborates the proposition that the Mata Azul Suite is the source of the studied pegmatites; its complex textural variation presented at Levantina quarry (coarse leucogranite, aplite and crosscutting pegmatites) and the geochemical signature previously described are very similar to many descriptions of granite sources of pegmatites around the world (Černý 1982, 1991, Martin and De Vitto 2005, Beurlen et al 2009, Wise and Brown 2010. a pegmatite body with ordinary mineralogy: quartz, microcline, biotite, muscovite and almandine.…”

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part I: accessory Sn, Nb, Ta and Ti minerals in leucogranites, aplites and pegmatites

“…Thus, they have a Mn/(Mn + Fe) ratio varying between 0.401 and 0.603 and a Ta/(Ta + Nb) ratio ranging from 0.564 to 0.739. In general, similar evolutionary trends are represented by the columbite-group minerals of other LCT pegmatites, such as the northern group of the Cross Lake pegmatite field (Černý and Ercit 1985) and Greer Lake (Černý et al 1986), both in Manitoba; the beryl-columbite phosphate pegmatites of the Sebago granite-pegmatite system, Maine, USA (Wise and Brown 2010), Los Chilenitos, Argentina (Sosa et al 2002), and Cap de Creus, Spain (Alfonso et al 1995).…”

Detrital zircons from modern sands in New England and the timing of Neoproterozoic to Mesozoic Magmatism

“…It was targeted for sampling because lithium-bearing pegmatites occur within the drainage (Wise and Brown, 2010). We were unable to find an undisturbed sand deposit at the river mouth near Portland, and instead collected sand from low bluffs above the river in Shaw Park in Gorham about 15 km upstream from tidewater, but well within the area that was inundated by the sea after glaciation (Belknap and others, 1987).…”