The Formation and Modification of Cratonic Lithospheric Roots

A Petrological and Geochemical Study of Xenoliths from the Kaapvaal Craton case history form the Catalan Coastal Ranges

ISBN:  -
Promotor:
Prof. Dr. P.A.M. Andriessen
Prof. Dr.  G.R. Davies
Copromotor:
Dr. D.G. Pearson
Funding: Netherlands foundation for Scientific Research (NWO) project # 809-31.001 (component A)
Netherlands Research School of Sedimentary Geology (NSG) publication no. 20040401

The physical properties of the lithosphere are largely dependent on its chemical composition and mineralogy. In order to interpret geodynamic processes it is therefore important to understand the geochemical evolution of lithosphere. Moreover, sub-cratonic lithospheric mantle (SCLM) forms a significant reservoir and its composition has to be known to evaluate the role of lithospheric mantle in magmatic processes. In general, the deep mantle is not directly accessible for geologic sampling. In southern Africa, however, fragments of the deep mantle (more than 200 km) were transported to the surface in kimberlite eruptions, and abundant samples became available through excavation of the kimberlite pipes for diamond mining. The southern African Kaapvaal craton is Archaean in age and underlain by an at least 200 km thick lithospheric keel. It therefore provides an excellent location for the study of formation and evolution of the SCLM. Due to its more than 3 billion years history, the study of mantle xenoliths might also give a hint to the role of plate tectonics in the early Earth. Previously, considerable attention has been paid to the nature and timing of processes that led to the formation of mantle xenoliths that are modally metasomatized, i.e. contain minerals like mica, amphibole, ilmenite etc., which do not belong to the typical peridotite mantle mineralogy (olivine, orthopyroxene, ±clinopyroxene, ±garnet, ±spinel). In contrast, this study focuses on the origin of "normal", cold, coarse garnet and spinel peridotites that are believed to be more representative of the entire southern African lithospheric mantle. The SCLM is distinct from oceanic mantle in that it is more depleted in magmaphile elements (Fe, Al, Ca, HREE), but at the same time strongly enriched in incompatible trace elements (LILE, LREE). The Kaapvaal SCLM also has a distinctly higher Si/Mg ratio and therefore higher modal orthopyroxene content (at a given Mg/Fe) than oceanic and most continental upper mantle. Particular attention is paid to the origin of the incompatible trace element enrichment and to the question whether peridotite minerals like garnet, clinopyroxene and orthopyroxene might also have been introduced by a fluid or melt subsequent to the original melt depletion, i.e. if they are of metasomatic origin. A detailed petrological and geochemical study was performed on low-temperature peridotite xenoliths for Kimberley and northern Lesotho in order to obtain information about the processes that led to the depletion and re-enrichment of the Kaapvaal SCLM. Samples have been characterized for Re-Os isotope systematics, major and trace element concentrations in whole rocks and minerals, and garnet and cpx Lu-Hf, Sm-Nd and Rb-Sr isotope systems. The combined results require a multistage history of the Kaapvaal mantle. They are most consistent with a model that involves ancient trace element enrichment of the garnets and orthopyroxenes by aqueous fluids, possibly derived from a subducting oceanic slab. This is consistent with the observed enrichment in Si (orthopyroxene) and the Re-Os isotope systematics. Infiltration of aqueous fluids would also increase the degree of mantle melting and could therefore explain the strong major element and HREE depletion of the SCLM. The Nd-Hf isotope characteristics of the garnets require the trace element enrichment to be ancient. I therefore suggest that melting and metasomatism of the Kaapvaal SCLM took place in subduction zone settings, probably during amalgamation of smaller pre-existing terranes in the Late Archaean. Trace element and Nd and Hf isotope disequilibrium between garnet and clinopyroxene is preserved in many samples and indicates that garnet and clinopyroxene are not co-genetic. Calculated equilibrium liquids and Nd-Hf isotope compositions for clinopyroxene suggest that most diopside in the xenoliths studied crystallized from an infiltrating kimberlite-like melt, shortly prior to eruption.