Multiple Metamorphic Events at Broken Hill, Australia. Evidence from Chloritoid-bearing Parageneses in the Nine-Mile Mine Region

Abstract

Pressure (P) and temperature (T) conditions for two distinct metamorphic events are quantified in the Nine-Mile Mine region of the central Broken Hill Block, New South Wales, Australia. Parageneses of the earlier event formed contemporaneously with D2 which produced a pervasive foliation. The event occurred at conditions around 650°C and 4–5 kbar, but may have been preceded by higher peak temperatures, possibly during an early M1 event. Thus the event described here is called M2. This is supported by geochronological studies that report at least two high-grade partial melting events in the region. A later paragenesis, here called M3, overprinted statically the M2–D2 and D3 fabrics. PT conditions during M3 reached 480°C at 5 kbar. Possible PT paths between M2 and M3 are inferred from the interpretation of chloritoid-bearing parageneses using (1) a new calculated petrogenetic grid, (2) calculated compatibility diagrams and (3) thermodynamic pseudosections. In particular, two possible PT paths are discussed: (a) a direct path involving negligible pressure change during cooling and (b) cooling with minor decompression from M2 and much later compression and heating to M3. PT path (b) is interpreted from pseudosections. It is also considered more likely because: (1) the orientation and position of the regional isograds for M2 and M3 do not coincide across the Broken Hill Block and (2) D3 is interpreted as a substantial shortening event. Thus, a much larger P difference would be expected between M2 and M3 if the two events occurred on the same PT path. The interpretation of independent PT paths for M2 and M3 allows the possibility that the M3 event is much younger than the mid-Proterozoic high-grade events.