Origin of CFB Magmatism: Multi-tiered Intracrustal Picrite–Rhyolite Magmatic Plumbing at Spitzkoppe, Western Namibia, during Early Cretaceous Etendeka Magmatism

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 6 PAGES 1119^1154 2007 doi:10.1093/petrology/egm012 Origin of CFB Magmatism: Multi-tiered Intracrustal Picrite^Rhyolite Magmatic Plumbing at Spitzkoppe, Western Namibia, during Early Cretaceous Etendeka Magmatism R. N. THOMPSON1*, A. J. V. RICHES1y, P. M. ANTOSHECHKINA2, D. G. PEARSON1, G. M. NOWELL1, C. J. OTTLEY1, A. P. DICKIN3, V. L. HARDS4, A.-K. NGUNO5 AND V. NIKU-PAAVOLA5z 1 DEPARTMENT OF GEOLOGICAL SCIENCES, DURHAM UNIVERSITY, SOUTH ROAD, DURHAM DH1 3LE, UK 2 DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY, MC 170-25, PASADENA, CA 91125, USA 3 DEPARTMENT OF GEOLOGY, MCMASTER UNIVERSITY, 1280 MAIN STREET WEST, HAMILTON, ONTARIO, CANADA L8S 4M1 4 BRITISH GEOLOGICAL SURVEY, KEYWORTH, NOTTINGHAM NG12 5GG, UK 5 GEOLOGICAL SURVEY OF NAMIBIA, 1 AVIATION ROAD, PRIVATE BAG 13297, WINDHOEK, NAMIBIA RECEIVED FEBRUARY 22, 2006; ACCEPTED FEBRUARY 27, 2007 ADVANCE ACCESS PUBLICATION APRIL 28, 2007 Early Cretaceous tholeiitic picrite-to-rhyolite dykes around Spitzkoppe, western Namibia, are part of the extensive Henties Bay^Outjo swarm, penecontemporaneous with
132 Ma Etendeka lavas
100 km to the NW. Although only intermediate to rhyolitic dykes contain clinopyroxene phenocrysts, the behaviour of Ca, Al and Sc in the dyke suite shows that liquidus clinopyroxeneçtogether with olivineçwas a fractionating phase when MgO fell to
9 wt %. Both a plot of CIPW normative di^hy^ol^ne^Q and modelling using (p)MELTS show that a mid-crustal pressure of
06 GPa is consistent with this early clinopyroxene saturation. Sr, Nd, Hf and Pb isotope variations all show trends consistent with AFC contamination (assimilation linked to fractional crystallization), involving Pan-African Damara belt continental crust. The geochemical variation, including isenthalpic AFC modelling using (p)MELTS, suggests that the picrites (olivine-rich cumulate suspensions) were interacting with granulite-facies metamorphic lower crust, the intermediate compositions with amphibolite-facies middle crust, and the rhyolitic dykes (and a few of the basalts) with the Pan-African granites of the upper crust. The calculated densities of the magmas fall systematically from picrite to rhyolite and suggest a magmatic system resembling a stack of sills throughout *Corresponding author. E-mail: yPresent address: Department of Earth Sciences,The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. zPresent address: Directorate of Diamond Affairs, Ministry of Mines and Energy,1 Aviation Road,Windhoek, Namibia. the crust beneath Spitzkoppe, with the storage and fractionation depth of each magma fraction controlled by its density. Elemental and isotopic features of the
20 wt % MgO picrites (including Os isotopes) suggest that their parental melts probably originated by fusion of mid-ocean ridge basalt (MORB) source convecting mantle, followed by limited reaction with sub-continental lithospheric mantle metasomatized just prior to the formation of the parental magmas. Many of the distinctive features of large-volume picritic^ basaltic magmas may not be derived from their ultimate mantle sources, but may instead be the results of complex polybaric fractional crystallization and multi-component crustal contamination. KEY WORDS: flood basalts; Spitzkoppe; picrite; trace elements; hafnium isotopes; Etendeka I N T RO D U C T I O N Attempts to understand the genesis and evolution of magmas can be subdivided into: (1) determining the physical and chemical conditions of magma genesis; (2)
The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 48 understanding the processes that have affected the magmas between their genesis and final solidification. Researchers interested in the first process are inclined to see the second one as little more than a nuisance, standing between them and their objectives. For this reason it is common for such students of magmas to focus on samples, techniques or datasets that appear to them to evade the ‘problem’ of post-genesis processes. The flood basalts of large igneous provinces (LIPs) in general, and the early Cretaceous Parana¤^Etendeka province in particular, have been and remain favourite battlefields for differing views about the relative importance of the two processes. The many publications that attempt to discern the ultimate sources of these magmas are split between favouring either the sub-continental lithospheric mantle (SCLM) or the underlying convecting mantle, with or without a contribution from either delaminated SCLM or older subducted material. Recent examples of such studies are those by Peate (1997), Ewart et al. (1998a, 2004a), Gibson et al. (2000, 2005), Marsh et al. (2001), Thompson et al. (2001), Trumbull et al. (2004a) and Tuff et al. (2005). All these studies acknowledge the probability that hot upwelling magmas will react with and absorb relatively fusible continental crust. Some studies attempt to make allowances for such crustal contamination, usually by comparing the predicted chemical effects of a likely crustal contaminant, such as the local granite basement, with geochemical trends observed in the magmatic suite. Most published studies assume a priori that any such crustal contamination takes place by a process of assimilation linked to the fractional crystallization of the melt (AFC), such that magmas become progressively more contaminated with falling liquidus temperature. Hence, they routinely reverse this reasoning and propose that the best shortcut to identifying magmas that have undergone little or no reaction with continental crust is to focus only on those with relatively high MgO contents, and hence high liquidus temperatures. This is difficult to do in the Parana¤^Etendeka province. For example, the compendium of data by Peate (1997) shows almost no Parana¤ lava analyses with 47 wt % MgO. Likewise, Marsh et al. (2001) and Ewart et al. (2004a) showed that relatively few Etendeka lavasç excluding the basal ferropicrites (Gibson et al., 2000)ç have 49 wt % MgO. Even this higher MgO content falls far short of the lowest value calculated for melts to be in equilibrium with a peridotite mantle. Primary magmas do not fall below 13^14 wt % (Thompson et al., 2005) and rise to values above 20 wt % MgO. Therefore all arguments that the elemental and isotopic variations in ‘more Mg-rich’ Parana¤^Etendeka lavas are inherited from their mantle sources and unaffected by subsequent polybaric fractional crystallization and concomitant crustal NUMBER 6 JUNE 2007 contamination (e.g. Erlank et al., 1984, and many subsequently) are, to some extent, acts of faith. This study is concerned with a picrite^rhyolite suite of dykes exposed within the Etendeka igneous province, south of the lavas. They are approximately contemporaneous with the Etendeka extrusive ro (...truncated)