Petrography, petrology and geochemistry of crustal xenoliths in kimberlites and carbonatites of the Gibeon Kimberlite Field, southern Namibia

Abstract

Abstract by author:

Gneiss xenoliths are mainly enclosed in carbonatites around the Gross Brukkaros. Rb-Sr isotope data of two gneiss xenoliths give an isochron age of 895 ± 7 Ma (whole rock + 3 mica; biotite gneiss) and an errorchron age of 1002 ±11 Ma (plagioclase + 3 micas; hornblende bearing biotite gneiss), which imply that these rocks represent relative young crust formed during two different metamorphic events in the Mid Proterozoic. The metamorphism is probably related to the Namaqua event in the Mid to Late Proterozoic at 1100-1000 Ma, where significant crustal reworking of pre-existing Early to Mid Proterozoic crust took place and which also influenced parts of the Kheis-Magondi Belt west to the Gibeon Kimberlite Field

Metasomatic alteration of wall rocks, called fenitization, is a general feature of carbonatite complexes. At the Gross Brukkaros Carbonatite Complex, fenitization is documented by low-grade and high-grade fenitized gneiss xenoliths. The fenites were formed by the influence of hydrothermal fluid derived from a crustal magma reservoir situated at a maximum depth of about 20 km

The metasomatic alteration of the gneissic wall rocks caused changes of the initial gneissic mineral assemblage (plagioclase + alkali feldspar + quartz ± muscovite ± biotite) with proceeding fenitization. In the low-grade fenites, which are often characterized by a layered texture, quartz is partly replaced by aegirine-augite, which forms rims around single quartz grains. Furthermore, biotite decomposes to form alkali feldspar and aegirine-augite rims around biotite aggregates. Quartz is absent in the high-grade fenites and plagioclase is often totally replaced by carbonate. High-grade fenitized rocks are composed of aegirine-augite, alkali feldspar, plagioclase, calcite, apatite, ilmenite and magnetite

Quantitative mass balance calculations of these high-grade fenites showed that fenitization of the granitic gneisses caused a volume decrease between 1 and 7 percent with a depletion of SiO2, A12O3 and LREE and an enrichment of Fe2O3, FeO, MgO, CaO, Na2O, P2O5 and CO2 during fenitization. The degree of depletion or enrichment increased with proceeding fenitization grade (F. I.) of the xenoliths, which is in agreement with petrographical observation. In addition, two chemical fenitization trends can be observed in the investigated fenite xenoliths. The less fenitized xenolith, characterized by a fenitization index (F. I.) of 12. 5 shows a potassic trend, whereas the mostly fenitized sample (F. I. = 33. 3) is characterized by a sodic trend, which can be explained in several ways. (1) The chemical change may be the result of temporal evolution from an early sodic fenitization trend to a later potassic fenitization trend. (2) A further possible explanation is that the sodic high-grade fenitized sample formed close to the contact in the presence of a supercritical CO2-poor aqueous fluid at temperatures above 500°C, whereas the less fenitized potassic sample may have formed at lower temperatures (T = 400-450°C) in greater distance to the carbonatitic magma chamber. (3) The sodic and potassic fenites may be the result of fenitization at different crustal depth, whereas potassic fenitization took place at the upper region of the carbonatite complex and sodic fenitization occurred in deeper crustal levels

The model crustal profile for the Gibeon Kimberlite Field indicates a crustal thickness of about 60 km. With respect to an explosion seismic profile at the northern margin of the study area, which shows that the crust-mantle boundary is situated at an approximate depth of about 55 km, the model profile seems to be realistic. In addition, a thickened crust underneath the Gibeon Kimberlite Field is also indicated geochemically by the pyriclasite and garnet pyroxenite xenoliths, which show strong subduction related chemical characteristics

Two types of pyriclasites can be distinguished. The non kyanite-bearing pyriclasites show a textural equilibrium formed by plagioclase, clinopyroxene and orthopyroxene at temperatures between 660 and 715°C (P = 10 kbar). In contrast, the kyanite-bearing pyriclasites with the initial mineral assemblage plagioclase + clinopyroxene I + orthopyroxene ± garnet ± scapolite are characterized by reaction textures around primary orthopyroxene. These rims are composed of fine-grained clinopyroxene II and kyanite needles which were produced by the reaction: orthopyroxene + plagioclase = clinopyroxene II + kyanite at 910°C and 13 kbar. The calculated peak temperatures and pressures for the initial mineral assemblage fall in the range between 620 and 710°C at about 11 kbar. The calculated P-wave velocities combined with pressure estimates clearly show that the non kyanite-bearing pyriclasites (Vp = 7. 0-7. 2 km/sec) occur at higher crustal levels (20-36 km) than the kyanite-bearing pyriclasites (Vp = 7. 5-7. 8 km/sec), which probably derive from depths between 36 and 43 km. Geochemically, the non kyanite-bearing pyriclasites are characterized by higher AI2O3- but lower MgO- and Cr-contents compared to the kyanite-bearing pyriclasites. Discrimination diagrams, MORB-normalized element patterns and chondrite-normafized REE patterns strongly imply a subduction related tectonic environment (island arc, back arc) for their protoliths. Sm-Nd whole rock model ages for one non kyanite-bearing pyriclasite indicate that the protolitbic material of this lower crustal xenolith has not resided in the crust for longer than approximately 1800 Ma (DM model age). The model ages can be correlated with an Early Proterozoic major phase of crustal growth during the Orange River event (Nd model ages: 1500-2300 Ma) in the adjacent Namaqua-Natal Belt south to the Gibeon Kimberlite Field, but also plots well in the range of rocks from the Rehoboth Basement Inlier (Nd model ages: 1663-2372 Ma) within the Ghanzi-Chobe Belt north to the Gibeon Kimberlite Field

Garnet pyribolites and pyribolites (plagioclase + amphibole ± clinopyroxene ± orthopyroxene ± garnet ± biotite ± scapolite ± aluminosilicate: kyanite?/sillimanite? ± apatite ± ilmenite ± magnetite) are characterized by Vp between 6. 8 and 7. 2 km/sec, which is consistent with an origin from the base of the middle crust or from the lower crust. Pressure estimates indicate that the garnet pyribolites originate from depths between 36 and 46 km (11-14 kbar), whereas the garnet-free pyribolites derive from higher crustal levels at about 26 km (8 kbar). Temperature estimates yield 750°C and 860°C for two garnet-free pyribolites and 710-800°C for garnet-bearing samples

A broad spectrum of amphibolite facies xenoliths associated with amphibole felse and clinopyroxene-amphibole felse xenoliths derived from mid-crustal levels chiefly occurs in kimberlites and carbonatites near the Gross Brukkaros Volcanic Complex. Granites, granitic gneisses, garnet biotite gneisses, and fenites are mainly enclosed in carbonatites and represent fragments from the middle and upper crust

The amphibolites contain amphibole or feldspar porphyroblasts as relicts of a former magmatic texture. The association of amphibolite, amphibole felse and clinopyroxene-amphibole felse xenoliths within the same kimberlite or carbonatite as well as petrographical features indicate the presence of an inhomogenous ampbibontic body beneath the Gross Brukkaros Volcanic Complex. P-wave velocities of about 6. 8 km/sec in the amphibolite xenoliths showed that amphibolites are located at the base of the middle crust at depths of about 20 km (6 kbar). They equilibrated at temperatures between 739 and 820°C