METAMORPHIC EVENTS IN THE EASTERN ARUNTA-INLIER .2. ND-SR-AR ISOTOPICCONSTRAINTS

In the Harts Ranges, the Central Province of the eastern Arunta Inlier is characterised by upper amphibolite facies ortho- and paragneisses. Previously published U-Pb dating of zircons, as well as new Sm-Nd and Rb-Sr data indicate that the intrusive ages of both mafic and felsic orthogneisses are in the range 1780-1740 Ma (Strangways Orogeny). Thes e rocks had initial epsilon(Nd) values of 0 to -3 and depleted mantle model ages of 2.5-2.1 Ga and they intruded supracrustal rocks which ar e now preserved as paragneisses with initial epsilon(Nd) of about -15. Sm-Nd and Rb-Sr whole-rock isotope data indicate that the hornblende- rich amphibolitic gneisses and mylonites in the Harts Range Detachment Zone (Ding and James, 1989) were equilibrated at similar to 1473 Ma ( +/-133) when the rocks experienced hydration, isotopic homogenisation and thrust faulting. This is the Anmatjira Event described elsewhere i n the Arunta Inlier and is a predominantly structural influence with t hrust faults and hydrous fluid flux. Accordingly the record of this ev ent by radiogenic geochronometers has been diffuse, a problem also exa cerbated by the probable influence on these rocks, of least one more s ubsequent lower-temperature amphibolite facies metamorphic event. In s pite of the indications of similar to 1470 Ma isotopic homogenisation of amphibolites in the Harts Ranges, Rb-Sr isotopic analyses of all mi nerals and Ar-40-Ar-39 from hornblendes yield ages in the range 450-30 0 Ma. Using the Sm-Nd system, a few garnet-hornblende pairs give 1400- 1250 Ma ages, but several of these pairs also yield early Palaeozoic a ges. These data suggest that the Central Province of the eastern Arunt a Inlier also experienced an early Palaeozoic thermal event, having co oled from temperatures > 550 degrees C during and immediately before t he Carboniferous Alice Springs Orogeny. In the Southern Province, sout h of the Illogwa Shear Zone, micas record Mesoproterozoic ages (1500-1 250 Ma) for the Rb-Sr, K-Ar and Ar-Ar systems, equivalent to the whole -rock ages obtained from the Central Province, to the north of the Ill ogwa Shear Zone. Clearly the exposed southern basement cooled to < 350 degrees C, presumably as a result of uplift and exhumation during the Anmatjira Event, when basement now exposed in the Central Province re mained at depth, undergoing one or more additional amphibolite facies events. The spectacular contrast in cooling histories across the Illog wa Shear Zone emphasises that this is an important crustal scale tecto nic structure.