The Zamu dolerite: A lower Proterozoic preorogenic continental tholeiitic suite from the Northern Territory, Australia∗

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Abstract
The term ‘Zamu Dolerite’ is proposed here to replace ‘Zamu Complex’, the name originally applied to the predominantly mafic intrusives of the South Alligator River area, and to include the other mafic supracrustal intrusive rocks that, with minor felsic differentiates, were emplaced into the strata of the Pine Creek Geosyncline before an 1800 m.y. phase of regional metamorphism. This suite of lower Proterozoic rocks generally forms conformable intrusive tabular bodies which are folded and, in most places, metamorphosed with the enclosing strata. The intrusions are mostly now amphibolite, but in the type area of Zamu Creek, folded but unaltered dolerite rocks are found. The Zamu Dolerite occurs in two broad zones of regional meta‐morphic grade: medium and/or high grades in the northeastern part of the region, and low grade elsewhere. Late‐orogenic Carpentarian granite diapirs have, however, locally superimposed a contact metamorphism on the low‐grade regional metamor‐phic event. Low‐temperature retrograde metamorphism is locally present throughout the Pine Creek Geosyncline, being particularly prevalent within areas of uranium mineralization. Compared with the dolerites, the amphibolites have the following chemical characteristics: total Fe and Ti enrichment; Mg, K, and Sr depletion; similar values for Zr, Y, Nb, and P2O5; lower K:Rb and higher K:Ba ratios. The dolerites and amphibolites are orthopyroxene normative and their major‐ and trace‐element chemistry demonstrates that they closely parallel the trends of continental tholeiitic suites. Compositions of the mafic suite of the Zamu Dolerite recast into the 1‐atm phase diagrams for simplified anhydrous tholeiitic magmas suggest that the more evolved members of this suite were produced by olivine and, possibly, plagioclase fractionation.