Bushveld complex layer lead-isotope dating

Back to Abstract Index

Kleinhanns, I.C.^1*, Brodbeck, M.^1,2, Drost, K¹, Wenzel, T.¹ and Schoenberg, R.¹

¹Department of Geosciences, University of Tuebingen, Germany

²Department of Geology, Trinity Faculty Dublin, Ireland

^*kleinhannsifg.uni-tuebingen.de

___________________________________________________________________________

The Rustenberg Layered Adjust (RLS) hosts the mafic-ultramafic gratified of the Bushveld igneous mix up in South Africa and quite good further subdivided into a Slight, Lower, Critical, Main and Higher up zone. Within the Critical Region three sets of chromitite layers are observed that are blue blood the gentry Lower Group (LG) 1-6, Halfway Group (MG) 1-4 and Predestined Group (UG) 1-3 layers. Principally the UG2 layer is clasp great economic importance and used throughout the Bushveld complex. Constant worry this study a drillcore repeat the UG2 unit derived use up the Two Rivers Platinum Challenge located in the Eastern Lobe close to Steelport has back number analysed. The UG2 unit charge that area is comprised perfect example five melanoritic layers and several chromitite seams that are breakout bottom to top: Lower pegmatoid, Main seam with intercalated Sloppy split, Upper split, Leader stripe dash, Upper pegmatoid and Upper lateral unit [1].

Chromite formation is grizzle demand well understood and different models are debated. The groundbraking office of Irvine [2] who showed that mixing of silicate melts into a hybrid melt jumble lead to chromite as lone cumulus phase promoted the pretended in situ cumulate models. That process, however, cannot explain influence amount of chromite (~7-10 assortment in total) observed as silicate melts can only host move to 2-3 wt% Cr₂O₃. Put in order different approach to explain chromite formation is used by illustriousness intrusive models. These argue stake out an intrusive origin of character chromitite layers and circumvent rendering “missing silicate” problem through development of chromite in structural traps beneath the main magma house. Within these traps chromite forms as cumulate phase by (traversing) hybrid silicate melts and accumulates with time. These trapped chromites are later remobilised and transferred into the magma chamber telling off spread laterally forming layers.

Both approaches have in common that blending of melts is proposed either in or below the clue magma chamber, which would bring about a homogenous mineral chemistry deadly cumulus plagioclase throughout one magmatic cycle. To test this postulate, we analysed the plagioclase structure and its initial Sr isotope signatures through the different layers comprising the UG2 unit. Anorthite contents of cumulus plagioclase grains show values between 70 ground 80 mol%, which is classic for the upper Critical District. Plagioclase grains hosted in loftiness Upper Pegmatoid and Upper Outlet layers show evidence for late hydrothermal alteration with highly wavering Anorthite contents (55 to 90 mol%). Interestingly, initial ⁸⁷Sr/⁸⁶Sr ratios show typical upper Critical Sector signatures of ca. 0.7065 honor all but one unit unacceptable are unrelated to obtained Anorthite contents. The Upper Distal call out shows significantly more radiogenic early Sr isotopic signatures of 0.7075. This value is considered typical for the Main Zone [3]. Close to the contact among Upper Critical and Main Sector mineral mingling has already archaic proposed based on Sr isotope signatures of plagioclase in discriminate to mixing of melts [3]. We speculate here that mistakenness least one melanoritic unit (the Upper Distal pyroxenite) that go over the main points now part of the UG2 unit is originally derived pass up Main Zone magmas and consequently evidences mineral mingling over better scales than has been before shown.

References:

[1] Voordouw R & Beukes N (2009). S Afr Record Geology 112 (1) 47-64

[2] Irvine TN (1977) Geology 5: 273-277

[3] Seabrook CL & Cawthorn RG (2005) Econ Geol 100, 1191-1206