Geological mapping and the geochemical and petrological characterisation of the ‘Woman – in – White’ amphibolite and associated felsic rocks

Abstract

Sampling and analysis of mafic and felsic rocks from the Woman-in-White Mine and Cathedral rock areas (Olary Domain SA) revealed four intrusive rock types including three distinctively similar amphibolites and one A-type granitoid. The ‘Woman-in-White’ amphibolite (WIWA) sill can be subdivided into three broad units, mafic (mesocratic), intermediate and felsic (leucocratic). The mafic unit is characterised by low SiO2, K2O, Na2O and high Fe2O3 and MgO. The intermediate unit is characterised by high Al2O3, Fe2O3 and low K2O with the felsic unit high in SiO2, Na2O and generally lower in Fe2O3 and Al2O3. The ‘Drew Hill’ amphibolite (DHA) is enriched in Fe2O3, TiO2 and low in Al2O3 and similar in CaO, Na2O and K2O to the mafic and intermediate units. The ‘Drew Hill’ granite (DHG) is characterised by high SiO2 and low Na2O and K2O. Rb and Ba concentrations for the DHG are extremely low along with negative Ti, P and Sr anomalies. The trace element distributions in the three amphibolites is distinctly similar with enrichment of LREE and HFSE. Nd values revealed a depleted mantle origin for the WIWA, DHA and DHG, with the DHA and mafic unit less differentiated from the depleted mantle. The DHG, intermediate and felsic components show significant crustal contamination by either pre-existing Palaeoproterozoic or Archaean crust indicating AFC. Similar Nd values and depleted mantle model ages for other Olary mafics and felsic suggests magmatism within the OD was derived from the same or similar source. The geochemical characteristic, including sub alkaline to basaltic compositions, A-type affinity of the DHG and crustal contamination indicate magmatism occurred within a continental rift zone. Emplacement relatively low in the stratigraphic sequence suggesting passive rifting was dominant over active rifting. S3 foliations defined by hornblende generally overprint a less intense S2 hornblende fabric within the WIWA and DHA. F3 folding with an axial planar S2 fabric within the DHG, indicate at least two deformational events. The two deformational events within the region and the passive rift environment suggest pre OD1 emplacement for the WIWA, DHG, and DHA. The lack of a S1 fabric is attributed to metamorphic overprinting.