Petrochemistry of Coexistent Mafic Silicates from the Mecklenburg Gabbro-Metagabbro Complex, North Carolina

Abstract

The Mecklenburg Complex, which crops out in the Carolina Piedmont, consists of older metagabbro and felsic metaplutonic rocks of upper amphibolite rank, and a younger post-metamorphic gabbro. Electron microprobe analyses have been made of 9 olivines, 18 orthopyroxenes, 21 clinopyroxenes, 19 hornblendes, and 14 biotites from the mafic rocks. Gabbroic rocks contain the 5 mafic phases listed above, and display hypidiomorphic textures characterized by numerous reaction relationships among olivine, orthopyroxene, clinopyroxene, and hornblende. In contrast, metagabbro lacks olivine and exhibits textures that range from relict igneous varieties (Type 1) to recrystallized granoblastic types (Type 2) that appear to represent equilibrium metamorphic assemblages. In general, mafic silicates from gabbro are higher in Mg, Al, Ti, K, and Na, and lower in Fe and Mn relative to respective phases in metagabbro. Compositions of unzoned minerals from Type 1 rocks fall intermediate between those homogeneous phases from gabbro and Type 2 metagabbro, and imply that: (1) minerals of the Type 1 metagabbro have not been completely equilibrated during the metamorphic episode, and (2) the recrystallization process evidently involved continuous rehomogenization of the mafic phases. Partitioning of Mg, Fe, Mn, and Ti between coexistent phases is generally systematic. Coexistent pairs from gabbro commonly exhibit less pronounced fractionation compared to metagabbroic pairs, and may be explained by higher temperatures of equilibration or by compositional variables not considered in the exchange reaction.