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Response to the Comment by Reimold et al. on ⁴⁰Ar/³⁹Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex (South African Journal of Geology, 109, 393–410), by Alexandre et al. (2006)

SUERC, Rankine Avenue, Technology Park, East Kilbride, G75 0QF, United Kingdom

Marco A.G. Andreoli

South African Nuclear Energy Corporation, P.O. Box 582, Pretoria 0001, South Africa, School of Geosciences, University of the Witwatersrand, Private Bag 3, 2050 Wits, South Africa

Antony Jamison and Roger L. Gibson

School of Geosciences, University of the Witwatersrand, Private Bag 3, 2050 Wits, South Africa

The first 20% of the full text of this article appears below.

	Introduction

 
Reimold et al.’s observations on Alexandre et al.(2006) concern the ⁴⁰Ar/³⁹Ar analytical data, their statistical treatment, and the suggested age interpretations, without commenting on the extensive and detailed structural data presented. Their criticisms can be divided into three categories: (a) not taking into account the recoil effects on the age calculation leads to erroneous ages; (b) the ⁴⁰Ar/³⁹Ar ages we suggest are in fact lower than the real crystallization ages, due to a suggested "bias" between ⁴⁰Ar/³⁹Ar and U/Pb ages; and (c) the metamorphism dated is not the result of the Transvaalide orogeny (Andreoli, 1988a), but of the Bushveld "metamorphic" episode. Reimold et al. also criticize our use of the top-of-the-bell ages and what they consider to be a non-rigorous approach in age calculation. In conclusion, Reimold et al. assert that our data are neither robust nor reliable and cannot be related to a discrete metamorphic event (the proposed Transvaalide orogeny: Andreoli, 1988a) that occurred at the time we suggest (2042.1 ± 2.9 Ma: Alexandre et al., 2006) Hereafter, we will answer their comments point by point.

	Recoil effect on the calculated ages

 
Firstly, we would like to state that we have been very careful not to misrepresent our analytical data: we have thoroughly described the results obtained and commented about the validity of the calculated ages. The recoil effect observed in our samples TTW12 and P2 is very limited (Alexandre et al., 2006: Figure 6); it is more pronounced in sample PEL-PIK where it is combined with superficial ⁴⁰Ar* loss, while sample TT3 exhibits a significant ⁴⁰Ar* loss. However, we are not certain - and have never suggested - that the age spectra of samples PEL-37 and PEL-34B indicate unambiguously (or at all) ³⁹Ar_K recoil; indeed, there is some disagreement about the . . . [Full Text of this Article]