Fourteen samples of calc-alkaline lamprophyres (CAL) (12 minettes, 2 kersantites) from the Schirmacher Oasis were analyzed for their chemical and isotopic compositions. Biotites and amphiboles are the mafic xenocrysts, present in a matrix of feldspars. Apatite, titanite, quartz, ore minerals and zircon are accessory constituents.
The major elements and some trace elements (Ba, Cr, Cu, Ni, Nb, Co, Rb, Sr, V, Zr) have been analyzed by X-ray
fluorescence spectrometry. The rare earth element (REE) concentrations as well as those of Cs, Pb, Th, U, Y, and Hf were analyzed by ICP-MS. Isotopic data were determined at the Universities of Bonn, München and Gießen.
The samples are basic to intermediate potassium-rich rocks which contain high amounts of volatiles. They have high abundances of compatible elements (Cr, Ni) and high mg#-values ranging between 58.3 and 75.3. This in-dicates a mantle-derived component in the petrogenesis of the CAL. Additionally the lamprophyres exhibit strong enrichments in incompatible elements, particularly the large ion lithophile elements LILE (up to 1000 x of the primitive mantle composition). The mantle-normalized element patterns show troughs for Nb and Ti, features commonly observed in subduction related magmas. Chondrite-normalized REE distribution patterns present a distinct fractionation between light and heavy REE (LaN/YbN = 19.6 - 36.7).
Lamprophyres are believed to originate in the upper subcontinental mantle. Two possible modes of enrichments of the incompatible elements have to be considered, crustal contamination during magma ascend and contamination of the mantle source. It seems to be unlikely that interactions of the ascending lamprophyric magmas with the continental crust are re-sponsible for the enrichments observed in the CAL. This is indicated by Sr concentrations in minettes varying between 690-1990 ppm and Ba 3920-7050 ppm, while the upper continental crust is characterized by 350 ppm Sr and 550 ppm Ba (Taylor and McLennan, 1985). Rather the chemical composition of the CAL reflects partial melting of a mantle reservoir, which was enriched in LILE and light REE before or during the melting process. However, an involvement of crustal materials in the petrogenesis of CAL cannot be ruled out. This is suggested by elevated d18O whole rock values. Pb isotope ratios (whole rock) also give some indications to a crustal component in the origin of the lamprophyres. 87Sr/86Sr ratios vs e-Nd of some samples are plotted in the figure. The isotope data suggest that the LILE-rich component in the genesis of CAL may be recycled continental crust.
Taylor, S. R. & McLennan, S. M. (eds.), The Continental Crust: its Composition and Evolution. (Blackwell Scientific Publications, 1985).
Fig. 1.