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 qNd=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 qNd 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 8C 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.