GEOCHEMISTRY OF PYROXENITIC AND HORNBLENDITIC XENOLITHS IN ALKALINE LAMPROPHYRES FROM THE SPANISH CENTRAL SYSTEM

Abstract

The alkaline lamprophyres and diabases of the Spanish Central System carry a heterogeneous suite of xenoliths which includes scarce pyroxenitic and hornblenditic types that can be divided in two groups: (a) pyroxenite xenoliths, including spinel clinopyroxenites and spinel websterites with granoblastic textures, and (b) hornblende-bearing clinopyroxenites and hornblendites (here after called hornblenditic xenoliths) characterised by the presence of Ti-rich kaersutitic amphibole and magmatic textures. Both groups of xenoliths can be assigned to the Al-augite series of Wilshire and Shervais (1975) [Wilshire, H.G., Shervais, J.W., 1975. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Phys. Chem. Earth 9, 257-272] with Al-rich and Cr-poor mafic phases. Clinopyroxenes show a very similar trace element composition in all of the ultramafic xenoliths, characterised by convex-upward chondrite-normalised REE patterns and low contents of incompatible elements such as Rb, Ba, Th and Nb. Kaersutite in the amphibole-bearing xenoliths shows a similar convex-upward REE pattern as clinopyroxene. Whole-rock and mineral geochemistry support an origin as cumulates from alkaline to subalkaline melts for most of the pyroxenites and hornblendites that have been studied. The Sr-Nd isotope ratios of pyroxenite xenoliths display two extreme compositional poles: one clinopyroxenite plots in the OIB field towards depleted values (87Sr/86Sr = 0.7028 and εNd = 6.2), whereas the other pyroxenites plot in enriched lithospheric fields (0.705 to 0.706 and -2.8 to -3.4, respectively), which implies that different magmas have been involved in their genesis. The hornblenditic xenolith suite has a very homogeneous isotopic composition, close to the isotopically depleted values of high εNd and low 87Sr/86Sr ratios of one of the pyroxenite xenoliths. Some of these ultramafic xenoliths fall within the isotopic compositional range of their host alkaline dykes, which also define a bipolar compositional field, suggesting that most of them are cogenetic with the lamprophyres. P-T estimates yield temperatures in the range of 970-1080 °C and pressures mainly from 0.9 to 1.2 GPa for pyroxenites, whilst hornblenditic xenoliths give lower (and probably underestimated) pressures (0.7-0.9 GPa). This pressure range is in agreement with pyroxenites being formed by an underplating event at the upper mantle-lower crust boundary, whereas pressure estimates for hornblenditic xenoliths suggest equilibration within the lower crust. © 2005 Elsevier B.V. All rights reserved.