Abstract

The majority of mafic complexes within the Dadeville belt of Alabama's Inner Piedmont contain two plutonic suites: the Doss Mountain suite, consisting of metanorite (DMTN) and metaorthopyroxenite (DMTO); and the Slaughters suite, which is composed of meta-gabbro (SG). All plutonic rocks are pre-regional metamorphism in age but in some places are predominantly free of macroscopic metamorphic fabrics. In the Doss Mountain area, the DMTN and DMTO show progressive mineralogical and textural changes into a poorly foliated and nonlayered amphibolite and an actinolite schist, respectively; such changes are essentially isochemical. The DMTN-DMTO-amphibolite-actinolite schist complex is interpreted to be a layered norite-orthopyroxenite intrusion that has suffered variable degrees of metamorphism. In other areas, the DMTN and DMTO intrude well-lineated and foliated amphibolites of contrasting composition that belong to the Ropes Creek Formation. The SG has well-developed olivine-plagioclase reaction coronas as the result of metamorphism. In all areas examined, the SG appears to intrude the amphibolites.

The SG has high-alumina and incompatible-element (Ba, Sr, Ti, Nb, Zr, Y, Pb, Th, Hf, ΣREE) concentrations and high La/Yb ratios. In contrast, the DMTN (and its metamorphosed equivalent) has low alkalies and other incompatible-element contents, has REE patterns that parallel chondritic abundances, and has low La/Yb ratios. On the basis of several geochemical parameters, the DMTN is tholeiitic, whereas the SG is calc-alkaline with some alkalic tendencies. Discriminant diagrams, together with the extremely low Ti concentration in the DMTN and the calc-alkaline nature of the SG, strongly indicate emplacement in an arc environment. High Al2O3/TiO2 and CaO/TiO2 ratios, as well as low incompatible-element concentrations in the DMTN, suggest that its origin was by partial melting of a depleted source. The DMTO represents a cumulate of the norite. Slaughters Gabbro magmatism resulted from partial melting of a less-depleted source.

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