MULTIPLE METAMORPHIC EVENTS AT BROKEN-HILL, AUSTRALIA - EVIDENCE FROMCHLORITOID-BEARING PARAGENESES IN THE 9-MILE MINE REGION

Pressure (P) and temperature (T) conditions for two distinct metamorph ic events are quantified in the Nine-Mile Mine region of the central B roken Hill Block, New South Wales, Australia. Parageneses of the earli er event formed contemporaneously with D-2 which produced a pervasive foliation. The event occurred at conditions around 650 degrees C and 4 -5 kbar, but may have been preceded by higher peak temperatures, possi bly during an early M-1 event. Thus, the event described here is calle d M-2. This is supported by geochronological studies that report at le ast two high-grade partial melting events in the region. A later parag enesis, here called M-3, overprinted statically the M-2-D-2 and D-3 fa brics. PT conditions during M-3 reached 480 degrees C at 5 kbar. Possi ble PT paths between M-2 and M-3 are inferred from the interpretation of chloritoid-bearing parageneses using (1) a new calculated petrogene tic 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 an d (b) cooling with minor decompression from M-2 and much later compres sion and heating to M-3. PT path (b) is interpreted from pseudosection s. It is also considered more likely because: (1) the orientation and position of the regional isograds for M-2 and M-3 do not coincide acro ss the Broken Hill Block and (2) D-3 is interpreted as a substantial s hortening event. Thus, a much larger P difference would be expected be tween M-2 and M-3 if the two events occurred on the same PT path. The interpretation of independent PT paths for M-2 and M-3 allows the poss ibility that the M-3 event is much younger than the mid-Proterozoic hi gh-grade events.