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The Lindeques Drift and Heidelberg Intrusions and the Roodekraal Complex,Vredefort, South Africa: comagmatic plutonic and volcanic products of a 2055 Ma ferrobasaltic magma

Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: sybrand{at}dewaals.com

I.T. Graham

Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. Geoscience, Australian Museum, 6 College St, Sydney, NSW 2010, Australia. e-mail: iang{at}austmus.gov.au

R.A. Armstrong

PRISE, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. e-mail: richard.armstrong{at}anu.edu.au

The Lindeques Drift and Heidelberg Intrusions and the volcanic Roodekraal Complex belong to a high-Ti mafic igneous suite (HITIS) that was emplaced in the Kaapvaal Craton as km-sized bodies, to the north and north-east of the Vredefort impact structure, prior to impact. New zircon SHRIMP ages for the Lindeques Drift Intrusion (2054.8 ± 5.7 Ma) and Roodekraal Complex (2053 ± 9.2 Ma) are indistinguishable from that of the Bushveld magmatic event (~2.06 to 2.05 Ga). The dominant rock types in the Lindeques Drift and Heidelberg Intrusions are even-grained and porphyritic spessartites which essentially are clinopyroxene-magnetite (-ilmenite) ± olivine cumulates with variable fractions (estimated 15 to 30%) of interstitially crystallized magma. The concentration of magmatic amphibole (edenite-magnesiohastingsite), responsible for the porphyritic texture of the spessartite, is directly proportional to the fraction of trapped magma, which also crystallized interstitial plagioclase, sphene, sulphide, apatite, microperthite and biotite. The Lindeques Drift spessartite is cut by fine-grained dykes and sills(?) of diorite and syenodiorite. Low-silica diorite in the Lindeques Drift Intrusion was produced by the reaction of the parental magma with the dolomitic country rock of the Chuniespoort Group through desilicificaton and de-alumination. The Roodekraal Complex is composed of multiple flows of predominantly mugearite lava with sub-flow intrusions of diorite. The diorite of Lindeques Drift and the lava and diorite of Roodekraal are chemically comparable in terms of both major and trace elements, suggesting a co-magmatic derivation. The magma parental to the three bodies is proposed to be a low-Al ferrobasalt with alkaline affinities (MgO = 4.8%, TiO₂ =1.8%, P₂O₅ = 0.52%, Zr/(P₂O₅· 10⁴ ) = 0.02, Nb/Y=1.47, K₂O/Yb =1.3, Ta/Yb = 1.1). The latter was probably derived by deep crustal fractionation of amphibole and plagioclase from an alkali basalt(?) precursor in an embryonic continental rift setting. The probability that a gabbro dyke, stratigraphically below and perpendicular to the Lindeques Drift Intrusion, is a feeder to the latter is regarded as small.

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