STRUCTURAL SETTING AND MINERAL COMPOSITIONAL DATA FROM THE DURHAMTOWN MAFIC COMPLEX, EAST CENTRAL GEORGIA

The rocks of the Russell Lake Allochthon (Allard and Whitney, 1995) are metamorphosed mafic and ultramafic rocks that retain igneous bulk compositions (e.g., McFarland, 1992) which range from gabbroic to ultramafic. Mapping of one of the bodies which forms part of the Russell Lake Allochthon has emphasized the almost horizontal nature of the contact between the mafic body (herein called the Durhamtown mafic complex- DMC) and the gneisses that underlie the mafic rocks. The DMC outcrops in an area extending approximately 600 m in length and 250 m in width. The flat-lying nature of the basal contact between the DMC and underlying gneisses is exposed along a paved road, Randolf Church Road, that runs through the southern part of the DMC outcrop area. The DMC is a metagabbro that contains zoned amphibole, zoned plagioclase, unzoned clinopyroxene, epidote, clinozoisite/zoisite, quartz, and opaques. The amphiboles have magnesiohornblende cores with a tremolitic/actinolitic rim. The plagioclase is composed of anorthite-rich cores (An40-58) rimmed by anorthite-poor plagioclase (An4-6). The core compositions of amphibole and plagioclase reflect amphibolite facies conditions whereas the rims are consistent with greenschist facies conditions. The unzoned clinopyroxene is more calcium-rich (~Wo50) then typical igneous clinopyroxenes and we infer a metamorphic origin for the clinopyroxenes. At this point, the bulk compositions and metamorphic assemblages of these rocks are consistent with an origin as dismembered, metamorphosed mafic and ultramafic portions of an ophiolite.