Elsevier

Chemical Geology

Volume 507, 5 March 2019, Pages 9-22
Chemical Geology

Nature and origin of the Mozambique Ridge, SW Indian Ocean

https://doi.org/10.1016/j.chemgeo.2018.12.027Get rights and content

Highlights

  • The volcanic nature of the Mozambique Ridge is clearly established.

  • It is a Large Igneous Province.

  • It is formed by a mantle plume originating from the African LLSVP.

Abstract

The Mozambique Ridge (MOZR) is one of several bathymetric highs formed in the South African gateway shortly after the breakup of the supercontinent Gondwana. Two major models have been proposed for its formation - volcanic plateau and continental raft. In order to gain new insights into the genesis of the Mozambique Ridge, R/V SONNE cruise SO232 carried out bathymetric mapping, seismic reflection studies and comprehensive rock sampling of the igneous plateau basement. In this study, geochemical data are presented for 55 dredged samples, confirming the volcanic origin of at least the upper (exposed) part of the plateau. The samples have DUPAL-like geochemical compositions with high initial 87Sr/86Sr (0.7024–0.7050), low initial 143Nd/144Nd (0.5123–0.5128) and low initial 176Hf/177Hf (0.2827–0.2831), and elevated initial 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb (Δ7/4 = 2–16; Δ8/4 = 13–167). The geochemistry, however, is not consistent with exclusive derivation from an Indian MORB-type mantle source and requires a large contribution from at least two components. Ratios of fluid-immobile incompatible elements suggest the addition of an OIB-type mantle to the ambient upper mantle. The MOZR shares similar isotopic compositions similar to mixtures of sub-continental lithospheric mantle end members but also to long-lived, mantle-plume-related volcanic structures such as the Walvis Ridge, Discovery Seamounts and Shona hotspot track in the South Atlantic Ocean, which have been proposed to ascend from the African Large Low Shear Velocity Province (LLSVP), a possible source for DUPAL-type mantle located at the core-mantle boundary. Interestingly, the MOZR also overlaps compositionally with the nearby Karoo-Vestfjella Continental Flood Basalt province after filtering for the effect of interaction with the continental lithosphere. This geochemical similarity suggests that both volcanic provinces may be derived from a common deep source. Since a continuous hotspot track connecting the Karoo with the MOZR has not been found, there is some question about derivation of both provinces from the same plume. In conclusion, two possible models arise: (1) formation by a second mantle upwelling (blob or mantle plume), possibly reflecting a pulsating plume, or (2) melting of subcontinental lithospheric material transferred by channelized flow to the mid-ocean ridge shortly after continental break-up. Based on geological, geophysical and geochemical observations from this study and recent published literature, the mantle-plume model is favored.

Introduction

Continental breakup leads to the formation of new ocean basins floored by new ocean crust. Breakup forms two types of rifted margins - magma-poor and volcanic-rifted margins (Franke, 2013 and references therein). Magma-poor margins (e.g. the South China Sea) are characterized by low magmatic activity and extensional features such as detachment faults and rotated crustal blocks over wide domains (i.e., >1000 km in the South China Sea). The crust breaks up before the lithospheric mantle extends, which contrasts with the formation of volcanic rifted margins. In the second scenario, initial lithospheric extension preceding crustal breakup leads to extensive magmatism over a short period of time. These large outpourings of magma are generally referred to as Large Igneous Provinces (LIPs) (e.g., Coffin and Eldholm, 1994; Courtillot et al., 1999; Dalziel et al., 2000). As the continental fragments drift apart as a result of new seafloor spreading, the continental crust along the margins cools and subsides, such that the sub-aerially erupted flood basalts on the new continental margins dip seawards, and are known as seaward-dipping reflectors. The super continent Gondwana started to disperse in the Middle Jurassic when Africa, South America, Antarctica, India and Australia rifted apart at different stages. The presence of the 183 Ma-old Karoo (Jourdan et al., 2005) and the 132 Ma-old Paraná-Etendeka (Renne et al., 1996) Continental Flood Basalt (CFB) provinces at the edge of the newly formed continents suggests that magmatic events may have contributed to continental breakup.

The opening of the Southern Ocean results from the fragmentation of Eastern Gondwana (i.e. Africa, Antarctica and Australia; 184–171 Ma, Nguyen et al., 2016). The first oceanic crust between Africa and Antarctica formed at ca. 155 Ma (Jokat et al., 2003). Numerous structures such as the Mozambique Ridge (MOZR), Agulhas Plateau (AGP), Maud Rise, and Madagascar Rise (Fig. 1a) were then emplaced between Southern Africa and Antarctica on the newly formed oceanic crust, but little is known about possible links to the Gondwana breakup. The continental margin of the Dronning Maud Land (Antarctica) and its African counterpart have been classified as volcanic-rifted (Jokat et al., 2004; Eagles and König, 2008; Mueller and Jokat, 2017). The nature and origin of these bathymetric highs are enigmatic due to the lack of detailed marine-based investigations, which has led to opposing models (volcanic plateau versus thinned continental crust) for their formation (e.g., Tucholke et al., 1981; Ben Avraham et al., 1995; König and Jokat, 2010; Gohl et al., 2011). In order to better understand the nature, origin and spatial and temporal evolution of the MOZR and its relationship to Gondwana breakup, the Research Vessel SONNE (expedition SO232 in April–May 2014) carried out a comprehensive bathymetric and seismic reflection survey on the MOZR accompanied by a detailed sampling of the plateau basement, and preliminary sampling of the NW part of the AGP. In this paper we present a comprehensive new geochemical data set for samples from the MOZR and AGP, including major and trace element and Sr-Nd-Hf-Pb (double spike) isotope data.

Section snippets

Geological setting

The MOZR is an elongated plateau striking roughly parallel to the SE coastline of South Africa between 25° and 35°S (Fig. 1). It rises ca. 4000 m above the abyssal plain located at ca. 5000 m below sea level. It is composed of three sub-plateaus, divided by E-W and NW-SE trending valleys: 1) southwestern, 2) central and 3) northern plateaus (Fig. 1b). A fourth sub-plateau, in the southeast, is less distinct.

The following section reviews the different models that have been proposed since the

Petrography of the samples

Fifty-one successful dredges were carried out during SO232 covering the entire MOZR except the northernmost part (Fig. 1b). Massive lavas, mostly sheet and pillow lavas, were recovered in 35 dredges and volcaniclastic rocks from a further 16 dredges. The samples are variably altered with the groundmass ranging from brownish to grayish in colour. Mn crusts, present on most samples, and alteration halos were cut off with a rock saw onboard to prepare the least altered inner core of individual

Effects of alteration

Because the samples are of submarine origin and have been exposed to seawater for up to ~130 Ma, it is crucial to evaluate the degree of alteration and possible effects of alteration on whole rock chemistry. As stated earlier the thin sections reveal that some of the samples are heavily altered, as highlighted by reddish matrix and altered pyroxenes.

An important criteria for evaluating the freshness of whole rock chemistry is the loss on ignition (LOI), which ranges from 0.5 to 13 wt%, but 32

Nature of the MOZR: evidence for excess volcanism

The exact nature of the MOZR is controversial, and contradicting models (i.e., volcanic versus continental) have been proposed. DSDP Leg 25 (Site 249) on the MOZR northern plateau (Fig. 1b) was the first sampling attempt of the plateau basement and Cretaceous tholeiitic rocks were recovered (Thompson et al., 1982). In contrast, several cruises in the late 1980s dredged continental crustal rocks (Fig. 1b). Mougenot et al. (1991) dredged gneiss, metagabbros and anorthosites samples, with similar

Conclusions

This study presents the first comprehensive geochemical data set (major and trace elements, and Sr-Nd-Pb-Hf isotopes) from an extensive sampling campaign (SO232) of the Mozambique Ridge. We conclude the following:

  • (1)

    The evidence for a volcanic nature for the MOZR is clearly supported by material recovered through dredging during the SO232 cruise. With the exception of exclusively continental material from a suspect continental block immediately north of the MOZR, volcanic rocks were recovered

Acknowledgments

We thank Captain Detlef Korte and his officers and crew of R/V SONNE for their professional and enthusiastic service during the SO232 cruise. We also thank S. Hauff and K. Junge from GEOMAR and U. Westernströer from Kiel University for their support with the isotope and trace element analytical processes, respectively. We are grateful to two anonymous reviewers and the editor for their help in increasing the quality of this paper. The cruise SO232 and the “SLIP” project were primarily funded by

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