Geochemistry of Archaean spinifex-textured peridotites and magnesian and low-magnesian tholeiites

Nesbitt, Robert W. and Sun, Shen-Su (1976) Geochemistry of Archaean spinifex-textured peridotites and magnesian and low-magnesian tholeiites. Earth and Planetary Science Letters, 31 (3), 433-453. (doi:10.1016/0012-821X(76)90125-4).

Abstract

Major and trace element (Rb, Sr, Ba, Zr, Y, Nb, Ni, Co, V, Cr) data are presented for 11 spinifex-textured peridotites (STP) and a number of high-magnesian and low-magnesian tholeiitic basalts. The STP, representing high-magnesian liquids, come from the Yilgarn Block of Western Australia, Munro Township in the Abitibi Belt of Canada and one sample from the Barberton area of South Africa. All of the basaltic samples come from the Yilgarn Block.

The STP and high-magnesian rocks are considered to belong to the komatiite suite (1 and 2) despite their low CaO/Al2O3 ratios. It is argued that the high values (about 1.5) reported for this ratio from the Barberton area can be explained by a combination of factors, viz. garnet separation, Al loss or Ca addition during metamorphism. The processes can be evaluated using CaO/TiO2, Al2O3/TiO2 ratios, the REE group and trace elements (e.g. Y, Sc). It would appear that most STP from other Archaean belts do not have abnormal CaO/Al2O3 ratios.

The STP display close to chondritic ratios for Ti/Zr, Zr/Nb, Zr/Y, and TiO2/Al2O3 and are considered to represent liquids produced by large amounts of partial melting of the Archaean mantle. The data suggest that virtually all phases other than olivine were removed by melting during the production of STP liquids. In the STP, Ti/V, Ti/P ratios are non-chondritic, suggesting original depletion and/or incorporation into the core.

For lower levels of partial melting, including mid-ocean ridge basalts (MORB) non-chondritic ratios are exhibited by Zr/Y, TiO2/Al2O3, TiO2/CaO, suggesting controlling phases in the residue for Y, Ca, Al. It is apparent that for STP, Cr is not being controlled, indicating the absence of chromite in the residual. However, at about 15% MgO the data suggest that chromite becomes a residual phase.

The transition metals, with the exception of Mn, have higher abundances in Archaean basaltic rocks than in MORB. This is interpreted as being mainly due to more extensive partial melting of the mantle in the Archaean, as a result of higher temperatures.

It is suggested that the generation of STP liquids with about 32% MgO is due to upwelling mantle diapirs which probably originated at depths greater than 400 km and at temperatures in excess of 1900°C.

Modern equivalents to Archaean greenstone sequences are lacking. The closest tectonic analogue would be the development of oceanic crust within a rifted continental block.

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