Summary The first known occurrence of kornerupine in Australia is described. The mineral is a minor component of a phlogopite-rich garnet-corundum-spinel metasomatic zone, which cuts mafic and ultranlafic spinel-plagioclase-orthopyroxene-hornblende rocks. The kornerupine-bearing assemblage was formed during the second major metamorphic episode that affected the terrain, probably under low-grade granulite or upper amphibolite facies conditions. Kornerupine appears to have formed at significantly lower levels of P-T than those reached in the terrain during the peak of the second granulite metamorphism.
The rocks of the north-eastern Strangways Range, central Australia, display a broad range of chemical types and have been subjected to at least four periods of metamorphic activity.Of the six rock sequences delineated five are dominated by felsic rocks and include meta-tuffaceous and pre-metamorphic intrusive igneous rocks; the sixth sequence contains an extensive occurrence of interbedded calc-silicates and marbles. The mafic rocks, accompanied by rare, interlayered ultramafic types lacking plagioclase, are characterised by the ubiquitous presence of hornblende and by low SiO2, K2O, Na2O and a high MgO/FeO ratio. The presence of aluminous rocks, particularly in the Western Zone Granulites sequence, has facilitated the development of the rare minerals sapphirine, hogbomite, kornerupine and yoderite (?) and the unusual mineral assemblages sillimanite - orthopyroxene and kyanite - cordierite.Repeated metasomatic addition of K, Rb and Ba has extensively modified rock chemistry, particularly of the felsic rocks. Metamorphic depletion of Rb and high K/Rb ratios have not been observed. In all rock types anomalously low contents of Sr are attributed to low Sr levels in the mantle underlying the Strangways Range, at least prior to early Proterozoic times. Low Sr appears to be a feature typical of a number of other metamorphic terrains in the Australian Shield. Unusually low Sr in the marbles (80 ppm) is attributed to diagenetic and metamorphic depletion of initially low levels at sedimentation.Four metamorphic episodes, M1 - M2 - M3 - M4, have been petrologically categorised, geochronologically delineated and the stable mineral assemblages geochemically contrasted.The first recognisable event. M1, is a granulite metamorphism dated elsewhere in the Strangways Range as 1860 ± 60 m.y., and is characterised by the stability of hornblende and cordierite. High KD values for the distribution of Fe and Mg between orthopyroxene and clinopyroxene in mafic granulites, similar Alⅳ and Alⅵ levels in orthopyroxene from mafic granulites, Fe/Mg distribution between garnet and phlogopite, the Mg content of calcite and the stable mineral assemblages indicate pressure-temperature conditions of 8.5 to 9 kb and 700°-750°C.The second recorded event, M2, is another granulite metamorphism, dated as 1470 ± 70 m.y., in which three phases are recognised. The first phase, characterised by the instability of hornblende and its replacement by pyroxene, and the replacement of cordierite by sillimanite, orthopyroxene and quartz represents pressure-temperature conditions of ll.5 kb and 900°C. The second phase of M2 is reflected in the development of garnet in pyroxene assemblages and the stability of hornblende. Pressure-temperature conditions of approximately 9 kb and 850°C prevailed. The third phase is characterised by upper amphibolite facies conditions reflected in the development of amphibolite facies minerals in many of the M2-recrystallised felsic rocks.The M3 event, dated as 700-800 m.y., caused partial to complete argon degassing of many of the M1 and M2 micas and hornblendes but only very limited mineral reconstitution.The M4 event was a structurally-controlled retrogressive event dated as 327 m.y. to 352 m.y. Metamorphic conditions were of the order of 6 kb and 600°C.The study has illustrated the rewards of a unified approach to unravel the geological history of a polymetamorphic terrain.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3cpath fill='%23fff' d='m8.751-17.959 10.11 11.373L3.997-9.844l13.94-6.1-7.692 13.129z'/%3e%3c/svg%3e)