Precambrian Research 105 2001 57 – 71
Marine evaporites from an oceanic island in the Neoproterozoic Adamastor ocean
H.E. Frimmel
a,
, S.-Y. Jiang
b
a
Department of Geological Sciences, Uni6ersity of Cape Town, Pri6ate Bag, Rondebosch
7701
, South Africa
b
State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing Uni6ersity, Nanjing
210093
, People
’
s Republic of China Received 28 October 1999; accepted 17 July 2000
Abstract
We report a hitherto unknown occurrence of ancient Neoproterozoic evaporite deposits from an allochthonous terrane in the Pan-African Gariep belt in Namibia. Low contents of Rb, Cs, Ba, Zr, Hf, Th, and U, flat
chondrite-normalised rare earth element REE patterns,
87
Sr
86
Sr ratios as low as 0.7075, Na – Cl – Br systematics of fluid inclusion leachates, and high d
11
B values for stratiform tourmalinites, together with geologic evidence, such as association with oceanic basalt, gabbro, and stromatolitic dolomite, point to a marine evaporitic origin. An atoll
environment on an oceanic island is envisaged as a likely depositional setting. In the associated mafic sequence we found a diamictite with metre-sized ice rafted detritus, suggesting the presence of sea ice cover at relatively low
latitude around the time of evaporite deposition. Based on chemostratigraphic
87
Sr
86
Sr, d
13
C comparison with passive continental margin sediments in the para-autochthonous external part of the Gariep belt, a correlation of the
mafic diamicitite with the global Varangian 590 – 560 Ma glaciation is proposed. © 2001 Elsevier Science B.V. All rights reserved.
Keywords
:
Gariep belt; Marmora terrane; Neoproterozoic; Evaporite; Boron isotopes; Varangian glaciation www.elsevier.comlocateprecamres
1. Introduction
The record of Precambrian evaporite deposits is sparse which is largely due to the fact that pri-
mary evaporite minerals do not survive even low- grade metamorphism easily and post-depositional
metasomatism in these rocks often obliterates pri- mary geochemical signatures. Yet, knowledge of
the distribution of evaporite deposits is pivotal for the reconstruction of stratigraphic correlation, pa-
leogeography and paleoclimate. The presence of Archean evaporite deposits in the Barberton
greenstone belt, South Africa, has been inferred from d
11
B values for tourmaline Byerly and Palmer, 1991. Only few Proterozoic examples of
former evaporite occurrences exist. Most of them, such as the 2.1 Ga borate deposits in the Liaohe
group of eastern Liaoning, China Jiang et al.,
Corresponding author. Tel.: + 24-21-6502901; fax: + 24- 21-6503783.
E-mail address
:
hefgeology.uct.ac.za H.E. Frimmel. 0301-926801 - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 1 0 4 - 2
1996 and the ] 1.7 Ga Thackaringa group Willyama supergroup in New South Wales, Aus-
tralia Stevens et al., 1988; Slack et al., 1989, or the Neoproterozoic Duruchaus formation Nosib
group in the Damara belt, Namibia Behr et al., 1983, are believed to have been deposited in
non-marine, playa lake environments in rift grabens.
Within the Gariep belt, which forms a coast- parallel branch of the larger network of Pan-
African orogenic belts in southwestern Africa Fig. 1, we found a dolomite-dominated metased-
imentary succession that bears many similarities to the inferred former playa deposits in the Dam-
ara belt. The succession occurs, however, in a very different geologic setting compared with other
Proterozoic former evaporite deposits — it is not associated with rift sediments but with mafic and
ultramafic rocks which have been previously de- scribed as representing either an oceanic island or
an aseismic ridge Frimmel et al., 1996a.
One of the enigmas in the reconstruction of Neoproterozoic paleogeography in southwestern
Africa is the width, or existence at all, of oceanic basins between the various crustal fragments that
amalgamated during the Pan-African orogenies. The Marmora terrane in the Gariep belt Fig. 1
provides one of the very few examples of true oceanic crust in the Pan-African belts of south-
western Africa. It remains unclear whether this oceanic crust formed in a wide, open ocean basin
or in a narrow sea. Thus, the question arises whether inferred former evaporite deposits there
are of marine or non-marine origin. To address this question we studied the whole rock geochem-
istry and Sr, O and C isotopic compositions of various dolomite beds, determined the mineral
chemistry and boron isotopic composition of tourmaline from stratiform tourmalinite, and
analysed the chemistry of fluid inclusions in tour- maline, early quartz and calcite veins.
2. Geologic setting and lithology
