GEOLOGY OF NICHOLSONS-POINT GRANITE, NATAL METAMORPHIC PROVINCE, SOUTH-AFRICA - THE CHEMISTRY OF CHARNOCKITIC ALTERATION AND ORIGIN OF THE GRANITE

In the Port Edward area of southern Kwa-Zulu Natal, South Africa, char nockitic aureoles up to similar to 4 m in width are developed adjacent to contacts with Port Edward enderbite and pegmatites intruded into t he normally garnetiferous Nicholson's Point granite. Other mineralogic al differences between the aureoles and the granite include increased myrmekite and significantly less biotite in the former and the replace ment of pyrite by pyrrhotite in the charnockitic rocks. No significant differences in major element chemistry between the garnet-biotite Nic holson's Point granite and charnockitic Nicholson's Point granite are seen, except possibly for higher CaO and TiO2 in the charnockite. High er Rb, Th, Nb and Y contents in the garnet-biotite granite suggest tha t these elements have been locally depleted from garnet-biotite granit e during charnockitization. This depletion is considered to be related to the reduction in biotite. Strontium and Pa contents are significan tly higher in the charnockite. Generally higher S contents in the char nockite suggest S metasomatism, with S possibly being added from the e nderbite. No differences in delta(18)O isotope data are seen between t he garnetiferous and hypersthene bearing granite. In the charnockite t he LREEs are weakly depleted whereas the HREEs show greater depletion compared to the garnetiferous granite. The depletions in REEs are thou ght to be related to the breakdown of garnet. Europium is marginally e nriched or unchanged in the charnockite relative to the garnetiferous granite. Two-pyroxene thermometry on the Port Edward enderbite suggest s that it was intruded at temperatures of similar to 1000-1100 degrees C. The replacement of pyrite by pyrrhotite is also consistent with a thermal auroele. Consequently the charnockitic zones developed around the intrusions of Port Edward enderbite may result from the thermally driven dehydration of biotite. The aureoles developed adjacent to pegm atites are not considered to have resulted from heat but probably by d estabilisation of biotite by a low aH(2)O fluid phase, possibly hypers aline brines. The Nicholson's Point granite has geochemical characteri stics typical of within-plate granites, A-type granites and rapakivi g ranites, however the stable and radiogenic isotope characteristics sug gest a significant crustal component in the source. Copyright (C) 1997 Elsevier Science Ltd.