Elsevier

Journal of African Earth Sciences

Volume 42, Issues 1–5, July–September 2005, Pages 183-199
Journal of African Earth Sciences

Monazite U–Pb dating and 40Ar–39Ar thermochronology of metamorphic events in the Central African Copperbelt during the Pan-African Lufilian Orogeny

https://doi.org/10.1016/j.jafrearsci.2005.08.007Get rights and content

Abstract

New SHRIMP U–Pb age data on metamorphic monazite, as well as step-heating 40Ar–39Ar ages on metamorphic biotite, muscovite and microcline, from Katangan metasedimentary rocks of the Central African Copperbelt are presented. These rocks were deformed and metamorphosed during the Pan-African Lufilian Orogeny. Three samples of metamorphic monazite from the Chambishi structural basin give ages of 592 ± 22 Ma, 531 ± 12 Ma and 512 ± 17 Ma, which correspond respectively to the ages of eclogite facies metamorphism, high-pressure talc-kyanite whiteschist metamorphism, and of a regional metamorphism/mineralisation pulse elsewhere within the Lufilian Orogen. A biotite population from Luanshya gives a 40Ar/39Ar plateau age of 586.1 ± 1.7 Ma, coinciding with the oldest monazite age. Several samples from the Chambishi basin and the Konkola area give 40Ar/39Ar biotite plateau ages in the range of 496.6 ± 0.6–485.2 ± 0.9 Ma, and a muscovite plateau age of 483.6 ± 1.1 Ma, which are a manifestation of regional uplift and cooling that affected the whole Katangan basin. The youngest apparent 40Ar/39Ar ages obtained are from microcline at Musoshi and range from 467.0 ± 2.7 Ma to 405.8 ± 3.8 Ma, reflecting continued slow cooling of the Lufilian Orogen.

Introduction

This paper is part of a wider geochronological study of the Central African Copperbelt and its basement. After constraining the nature and evolution of the basement of the Copperbelt (Rainaud et al., 2003, Rainaud et al., this issue), and the provenance of key units within the Katanga Supergroup and their ages (Master et al., this issue), in this paper we provide new data dealing with the several metamorphic episodes which occurred in the Central African Copperbelt during the Lufilian Orogeny, and discuss their implications for the evolution of the Katangan basin.

Section snippets

Regional setting

The Katanga Supergroup is the host of the major stratiform sediment-hosted Cu–Co deposits, as well as numerous other deposits of Cu, Pb, Zn, U, Au, Fe etc., which constitute the Central African Copperbelt in Zambia and the Democratic Republic of Congo. This succession is a Neoproterozoic metasedimentary sequence which consists of the Roan Group, the Nguba Group, the Fungurume and the Biano Groups (Wendorff, 2001a, Wendorff, 2001b, Wendorff, 2002a, Wendorff, 2002b, Wendorff, 2003a, Wendorff,

Analytical methods

40Ar–39Ar analyses were performed at the Research School of Earth Sciences (RSES), Australian National University, Canberra. Muscovites were separated at the the Hugh Allsopp Laboratory, University of the Witwatersrand, Johannesburg, South Africa. Samples were reduced in a jaw crusher and through a pulverizer into a coarse powder which was then sieved. Extracts were purified at the Australian National University, using conventional magnetic separation and heavy liquid techniques. The resulting

Sampling

Eleven samples were utilised for the purpose of this study (Fig. 1): seven were located in the Chambishi basin in Zambia (Fig. 2), one in the Konkola area (also in Zambia), one in the Muliashi South deposit (Zambia, near Luanshya), one in the Nchanga mine (Zambia) and finally, one in the Musoshi Mine in the Democratic Republic of Congo (Fig. 1). Two of these samples were dated by two methods. In one sample (sample RCB2/112), monazites were extracted and dated with the SHRIMP U–Pb technique

Results

Samples analysed and dated in this study yielded several distinct age ranges. Three samples give an age range between 631.8 ± 1.8 Ma and 586.1 ± 1.7 Ma, 6 samples give an age range between 496.6 ± 0.6 Ma and 467.0 ± 2.7 Ma while individual samples give ages of 531 ± 12 Ma and 512 ± 17 Ma.

Discussion

The deposition of the Katangan sequence started somewhere after 877 Ma (Armstrong et al., 1999) and finished sometime after 573 ± 5 Ma (Master et al., this issue). Following deposition, the Katangan sedimentary sequence underwent several episodes of metamorphism during the late Neoproterozoic Pan-African Lufilian Orogeny, which gave the Copperbelt its arcuate shape (Cahen et al., 1971, Cosi et al., 1992).

Acknowledgements

We thank Claus Schlegel (Avmin Zambia), Tumba Tshiauka (Musoshi Mine, DRC), and the staff of Mufulira, Nchanga and Muliashi South mines and the Kalulushi core yard for access to the samples. We are grateful to Dr. Steve Prevec for his insightful and timeous review of this paper.

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