Possible petrogenetic relationships between the Pee Dee gabbro and Lilesville granite, Anson and Richmond counties, North Carolina

The Pee Dee gabbro occurs as a stock within the Lilesville granite batholith located in the Carolina slate belt in Anson and Richmond Counties, North Carolina. Gravity data indicate the gabbro is a slab that thins to the north and extends to a depth of approximately 4.3 km. Rb-Sr isotopic data indicating contemporaneous crystallization ages for the gabbro and granite, intertonguing contact structures, and the presence of modal quartz in the gabbro suggest that magma mixing or assimilation may have occurred.

The gabbro contains plagioclase, clinopyroxene, orthopyroxene, hornblende, biotite, magnetite, ilmenite, apatite and trace amounts of interstitial quartz. Based on Streckeisen's classification, three rock types constitute the pluton: gabbronorite, gabbro, and pyroxene-hornblende gabbronorite.

Major element mixing calculations do not support the gabbro-granite mixing hypothesis. Computer-modeled assimilation-fractional crystallization demonstrates that assimilation of 4% granite is plausible; however, due to thermal restraints assimilation was more likely restricted to isolated hybrid pods of tonalite at the contact.

Petrogenetic modeling of major element trends indicates the gabbro could be produced by 77% fractional crystallization of a tholeiitic basalt parental magma in the proportions 60% plagioclase (An₆₁), 22% olivine (Fo₆₁), and 14% clinopyroxene, 2% magnetite, and 2% ilmenite. Fractionation may have taken place at depth prior to the magma's emplacement into the Lilesville batholith.