MINERALOGY OF THE DELHI SYENITE, GEORGIA

The Delhi syenite (DS) in northeast Georgia is part of a series of gabbro ± syenite complexes in the Piedmont of Georgia and the Carolinas. No evidence of gabbro is found near the DS. Do the minerals of the DS show any relations to a gabbroic magma? This question was investigated by the students and faculty of the Earth Materials class at UGA during the fall of 2009.

Students mapped the syenite with careful attention to age relations between the syenite and other rocks. Outcrop area of the DS was smaller than previously recognized. Dikes of DS intrude fine-grained granite at several locations.

The DS is medium to coarse-grained and composed mostly of coarsely perthitic alkali feldspar with distinct clumps of mafic minerals. Individual perthite grains contain about 40% albite (Ab 93-99 ) in a host of K-feldspar (Or 93-100). Fine-grained, dark-brown amphibole (wt % SiO₂ = 37-39; mg# 5-10) forms aggregates that resemble pseudomorphs of some larger more elongate mineral. Recalculation of the microprobe analyses reveals the amphibole is hastingsite. Less abundant, finer-grained green to brown ferroactinolite (wt % SiO₂ = 49-50; mg# 6-9)occurs as coronas on rare clinopyroxene grains. A few grains of green to brown ferrohornblende (wt % SiO₂ = 40-45; mg# 8-11) fill the compositional gap between the other amphiboles. Rare grains of green clinopyroxene contain elevated contents of nonquadrilateral components (Na/Na+Al = 0.8) and is aegirine-augite. Dark brown annite (mg # 7-10) is associated with the hastingsite. Magnetite with coarse ilmenite exsolution lamella, pyrite, and, less common, chalcopyrite and sphalerite grains occur with the amphibole. Accessory minerals include apatite and titianite.

Amphibole and biotite are extremely rich in Fe indicating an oxidizing crystallization history. The form of amphibole-biotite clumps suggests replacement of some preexisting, elongate phase; perhaps another amphibole or clinopyroxene. Extensive exsolution in the alkali feldspar and FeTi oxides also indicate recrystallization. The high Fe content of the DS minerals is unique in the region and suggests an affinity for a mafic magma.