De Leeuw, G.A.M., Beintema, K.A., Wijbrans, J. R., White, S.H., 2001. Chemistry of Archaean gabbros intruding the Tabba Tabba shear zone, Pilbara, WA, 4IAS. AGSO, Record 2001/37, Perth, WA, pp. 143-145:
Introduction
The Archaean Pilbara Craton consists of six tectono stratigraphic domains that are bounded by major shear zones [Krapez and Eisenlohr (1998)]. The domains are assumed to represent amalgamation of terrains [Barley (1993)]. The two main terrains, the East and West Pilbara Cratons, are separated by the Tabba Tabba shear zone. The shear presently forms the eastern margin of the Mallina basin. An intrusive (ultra-) mafic igneous complex associated with the Tabba Tabba shear zone suggests that the shear zone is deeply rooted in the subcontinental lithospheric mantle. By implication we propose a tectonic origin for the igneous complex. Major and trace element geochemistry of selected rocks from the igneous complex reveal a complex intrusion history with at least two distinct sources.
Results & Interpretation
The mafic igneous complex consists mainly of gabbros (5 - 11 wt.% MgO) with minor peridotitic komatiites (34 - 38 .MgO). The gabbros can further be subdivided into Fe-rich gabbros with a tholeiitic affinity and Fe-low gabbros with calc-alkaline affinity. Fe-low gabbros have higher SiO2, Al2O3, MgO and CaO concentrations. They show calc-alkaline rather than tholeiitic characteristics, are LILE and LREE enriched and have lower Zr, Mn, Fe, P, Ti, Nb and Ta concentrations when compared to the Fe-rich gabbros.
These new geochemical data suggest that partial melting occurred in at least two distinct magma sources. Fe-rich gabbros and peridotitic komatiites are interpreted to be melts derived from a primitive mantle source, while Fe-low gabbros are interpreted to be melts from a mantle source enriched in incompatible elements. CaO/TiO2 and Al2O3/TiO2 versus TiO2 show for Fe-rich gabbros constant CaO/TiO2 and Al2O3/TiO2 ratios and a high and variable amount of TiO2, which is comparable to MORB. Fe-low gabbros show variable and high CaO/TiO2 and Al2O3/TiO2 ratios and less variable and low amounts of TiO2 which is comparable to island arc magmatism [Nesbitt and Sun (1979)]. Low Zr-concentrations in combination with low Nb, Ta, Mn, Fe, P and Ti concentrations in the Fe-low gabbros suggest high degrees of partial melting in the source area.
The intrusive mafic igneous complex along the Tabba Tabba shear is interpreted to be associated with a hot upwelling of asthenosphere, which first triggered partial melting of the primitive mantle and subsequently the enriched reservoir. This relative timing is indicated by field relations; the Fe-rich gabbros were intruded early during activity of the shear (see also Beintema et. al, this volume), and they are cross cut but by the Fe-low gabbros. The magmas are interpreted to have been channelled through the crust in an intracontinental setting along the Tabba Tabba shear zone.
The chemistry of gabbros belonging to the mafic igneous complex along the Tabba Tabba Shear Zone indicates the presence of an enriched mantle reservoir. This is consistent with the interpretation drawn from the chemistry of more felsic sanukitoids [Smithies and Champion (1999)] in this area. Both studies assume the existence of an enriched mantle below the Mallina basin. The enrichment of the mantle, possibly by subduction, must have happened prior to 2950 Ma [Smithies and Champion (1999)].
Conclusions
The analysed rocks from the mafic-ultramafic complex in the Tabba Tabba Shear Zone can be subdivided into Fe-rich and Fe-low gabbros. Fe-rich gabbros are found along the total explored area of the shear zone, while the Fe-low gabbros are only found at the northern part of the shear zone. Field relations indicate that the Fe-rich gabbros are the oldest rocks.
Fractional crystallization of olivine and clinopyroxene are similar for both types of gabbros. However, distinct chemistries indicate that the gabbros originate from different mantle sources. The low concentration of HREE and enrichment in LREE in Fe-low gabbros compared to Fe-rich gabbros indicates the presence of garnet in the residue. It indicates onset of melting at depths greater than 80 km.
Fe-low gabbros show calc-alkaline characteristics and are LILE and LREE enriched, indicating partial melting from an enriched mantle source. Zr, Mn, Fe, P, Ti, Nb and Ta concentrations are lower than in Fe-rich gabbros. The high MgO and low Zr concentration suggests a high degree of partial melting.
LILE and LREE enrichment and Nb-Ta depletion in the Fe-low gabbros can be produced in a subduction zone environment where dehydration reactions have triggered partial melting, or by continental influence. High SiO2, Al2O3 and CaO concentrations may originate from the oceanic lithospheric plate with overlying sediments. The enrichment of the mantle, either by subduction or an alternative mechanism, must have happened prior to 2950 Ma.