P‐T‐X (CO₂) conditions in mafic and calc‐silicate hornfelses from Oberon, New South Wales, Australia

Abstract

The Rockley Volcanics from near .Oberon, New South Wales occur within the aureole of the Carboniferous Bathurst Batholith and have been contact metamorphosed at P z 100 f 50 MPa ( I & 0.5 kbar) and a maximum T 'v 565°C in the presence of a C-0-H fluid. Prior to contact metamorphism the volcanics were regionally metamorphosed and altered with the extensive development of actinolite, chlorite, plagioclase, quartz and calcite. The contact metamorphosed equivalents of these rocks have been subdivided into: Ca-poor (cordierite + gedrite), Mg-rich (amphibole + olivine + spinel), mafic (amphibole + plagioclase) and Ca-rich (amphibole +garnet + diopside; diopside + plagioclase; garnet + diopside + wollastonite) rocks.The chemistry of the minerals in the hornfelses was controlled by the bulk rock chemistry and fluid composition. Pargasites and hastingsites as well as an unusual phlogopite with blue green pleochroism, are found in Ca-rich hornfelses. A comparison of the assemblages with experimentally derived equilibria suggests that the fluid phase associated with the Ca-rich hornfelses was water-rich (Xco, = 0.1 to 0.3) while that associated with the Mg-rich hornfelses was enriched in CO, (Xco, > 0.7). The different hornfels types have reacted to contact metamorphism independently in both their solid and fluid chemistries.

Noncarbonate, skarnlike Au-Bi-Te mineralization, Lucky Draw, New South Wales, Australia

“…= below detection limit lent to many cordierite-anthophyllite rocks in contact aureoles or medium-grade metamorphic terranes. For example, cordierite-anthophyllite rocks south of Yetholme (Vallance, 1969) and Diamond Hill (Andrew, 1984), 50 and 35 kin, respectively, north-northeast of Lucky Draw (Fig. 1A), are developed only within the contact aureole of the Bathurst Granite.…”

“…1A), are developed only within the contact aureole of the Bathurst Granite. Cordieritegedrite and gedrite-spinel-chlorite rocks at Diamond Hill are developed in the Rocldey Volcanics and contain rock and crystal fragments implying a tuffaceous protolith (Andrew, 1984). Outside the contact aureole of the Bathurst Granite, rock types laterally and compositionally equivalent rock types to the cordierite-anthophyllite rocks at Yetholme and Diamond Hill are hydrothermally altered mafic and ultramafic volcanics.…”

“…1A), was dated at 310 ___ 7 Ma (late Carboniferous) by Facer (1978) using K-Ar and Rb-Sr techniques. In the Burraga-Rockley district, andalusite and cordierite are restricted to contact metamorphic aureoles (Andrew, 1984). …”