Sapphirine-bearing rocks with sedimentary and volcanogenic protoliths from the Arunta Block
AT least 15 separate localities for sapphirine-bearing rocks have been found in the Arunta Block, a Precambrian metamorphic complex in central Australia (Fig. 1), although only two have been documented in any detail1–3. Such rocks are the most spectacular examples of a suite of rocks with a distinctive, characteristic chemistry. This suite includes, inter alia, the better known cordierite–anthophyllite rocks, and is widely distributed both within the Arunta Block and world-wide. From field observations it is proposed here that two different geological processes, one sedimentary or diagenetic and the other volcanic, generated rocks that are now indistinguishable in their whole-rock chemistry and mineral assemblages. It had formerly been proposed that sapphirine-bearing rocks of volcanogenic and sedimentary origins were mineralogically and chemically distinct4.
The origin of cordierite—anthophllite rocks in the Land's End aureole
Summary The prevalence of low-variance assemblages, the frequency of the buffered association anthophyllite—cordierite—cummingtonite—plagioclase, and the ubiquity of aluminium-rich clots of diaspore incompatible with the ortho-amphibole of the rock suggest that the cordierite—anthophyllite rocks of the Land's End aureole are not the product of metasomatism, but resulted from the thermal metamorphism of pre-existing rock-compositions. The origin of many kyanite-, amphibole- and cordierite-rich bodies by pre-metamorphic alteration is of economic significance in regions where such curious silicate assemblages accompany sulphide deposits.
Natural boron-free kornerupine and its breakdown products in a sapphirine rock of the Limpopo Belt, Southern Africa
Large crystals of boron-free kornerupine occurring in MgAl-rich inclusions within meta-anorthosites are partially replaced by symplectitic pseudomorphs consisting essentially of the assemblage sapphirine-cordieritegedrite. The highly magnesian, hydrous kornerupines (F= 0.10–0.14) have compositions close to the oxide ratio (Mg, Fe) O· Al2O3· SiO2. Sapphirines (F=0.09) show decreasing Al-contents with continued grain growth and equilibration. Gedrite (F=0.15) contains sodium in amounts near the limit of solid solution, although the kornerupine starting material is free of this element and it is very rare in the enclosing rock. Cordierite (F=0.05) is also free from sodium.For conditions of PH2O = Ptot the appearance of boron-free kornerupine requires relatively high temperatures (> 700 °C) and a minimum pressure near 4 kb within this zone of the Limpopo Belt. The subsequent replacement reaction occurred nearly isochemically except for Na and probably H2O, which were introduced into the symplectite. Textural features suggest that the breakdown reaction of kornerupine is actually governed by the magnitude of sodium activity: Relatively low values would favor the appearance of boron-free kornerupine, whereas higher values lead to the more common assemblage sapphirine-cordierite-gedrite.