Metamorphic charnockite in contact aureoles around intrusive enderbite from Natal, South Africa

In the Port Edward area of southern Kwa-Zulu Natal, South Africa, charnocki tic aureoles up to 10 m in width in the normally garnetiferous Nicholson's Point Granite, are developed adjacent to intrusive contacts with the Port E dward Enderbite and anhydrous pegmatitic veins. Mineralogical differences b etween the country rock and charnockitic aureole suggest that the dehydrati on reaction Bt + Qtz -> Opx + Kfs + H2O and the reaction of Grt + Qtz -> Op x + Pl were responsible for the charnockitization. The compositions of flui d inclusions show systematic variation with: (1) the Port Edward Enderbite being dominated by CO2 and N-2 fluid inclusions; (2) the non-charnockitized granite by saline aqueous inclusions with 18-23 EqWt% NaCl; (3) the charno ckitic aureoles by low-salinity and pure water inclusions (< 7 EqWt% NaCl); (4) the pegmatites by aqueous inclusions of various salinity with minor CO 2. As a result of the thermal event the homogenization temperatures of the inclusions in charnockite show a much larger range (up to 390 degrees C) co mpared to the fluid inclusions in granite (mostly < 250 degrees C). Contrar y to fluid-controlled charnockitization (brines, CO2) which may have taken place along shear zones away from the intrusive body, the present "proximal " charnockitized granite formed directly at the contact with enderbite. The inclusions indicate contact metamorphism induced by the intrusion of "dry" enderbitic magma into "wet" granite resulting in local dehydration. This w as confirmed by cathodoluminescence microscopy showing textures indicative for the local reduction of structural water in the charnockite quartz. Two- pyroxene thermometry on the Port Edward Enderbite suggests intrusion at tem peratures of similar to 1000-1050 degrees C into country rock with temperat ure of < 700 degrees C. The temperature of aureole formation must have been between similar to 700 degrees C (breakdown of pyrite to form pyrrhotite) and similar to 1000 degrees C. Charnockitization was probably controlled la rgely by heat related; to anhydrous intrusions causing dehydration reaction s:and resulting in the release and subsequent trapping of dehydration fluid s. The salinity of the metamorphic fluid in the contact zones is supposed t o have been higher at an early stage of contact metamorphism, but it has lo st its salt content by K-metasomatic reactions and/or the preferential migr ation of the saline fluids out of the contact zones towards the enderbite. The low water activity inhibited the localized melting of the granite. Mine ral thermobarometry suggests that after charnockite aureole genesis, an iso baric cooling path was followed during which reequilibration of most of the aqueous inclusions occurred.