Evolution of magmatic AFM mineral assemblages in granitoid rocks; the hornblende + melt = biotite reaction in the Liberty Hill Pluton, South Carolina

The Liberty Hill pluton comprises a central, coarse-grained edenite + biotite granitoid facies grading to a biotite, pargasite + biotite, or muscovite + biotite granitoid marginal facies. The replacement of amphibole by biotite, mineral compositions, and “liquid” variation diagrams for the Liberty Hill help determine the AFM mineral-crystallization reactions in the Liberty Hill pluton. These reactions evolved from liquid = amphibole to liquid = biotite with an intervening liquid + amphibole = biotite reaction resulting from either (1) movement along the amphibole-biotite liquidus boundary from the even, liquid = amphibole + biotite, to the odd, liquid + amphibole = biotite, crystallization reaction with decreasing temperatures or (2) movement of the liquid composition from the amphibole field into the biotite field. The liquid + amphibole = biotite reaction involved additional phases; the full reaction may have taken the form liquid 1 + amphibole + ilmenite = biotite + quartz + anorthite + K-feldspar + titanite + magnetite + liquid 2, where the liquid is the interstitial melt. The extended crystallization history to lower temperatures and higher water activities at the margins of the pluton allowed the reaction to exhaust the amphibole before the liquid; the subsequent liquid = biotite crystallization reaction produced the biotite granitoid facies. Locally, muscovite crystallized according to the reaction liquid = muscovite + biotite, producing the muscovite + biotite facies. The pluton is intruded by a finer-grained biotite granitoid believed to be derived from interstitial melt of the coarser-grained granitoids. This derivation explains why biotite is the only AFM mineral in the finer-grained granitoids and why biotites from all Liberty Hill granitoids are generally similar in composition.

The average K_D value $[(XMgBt/XFeBt)/(XMgHb1/XFeHb1)]$ is 1.04 for coexisting Hbl and Bt. This K_D is higher than the 0.69-0.93 normally observed in igneous rocks, but equal to values for specific instances of biotite replacing amphibole in the Peruvian Coastal batholith. Combined with the results obtained from the contact aureole, the central granitoids preserve conditions of 725°C, P_total ≈ 4.5 kbar, P_fluid ≈ 0.5 P_total, and f_o2 ≈ 10^-15, slightly more oxidizing than the NNO solid buifer. Marginal granitoids yield conditions of 647°C, P_fluid ≈ P_total, and f_o2 ≈ 10^-16, suggesting a more prolonged reaction history.