Geochemistry of pyroxenitic and hornblenditic xenoliths in alkalineudlamprophyres from the Spanish Central System

摘要

The alkaline lamprophyres and diabases of the Spanish Central System carry a heterogeneous suite of xenoliths whichudincludes scarce pyroxenitic and hornblenditic types that can be divided in two groups: (a) pyroxenite xenoliths, including spineludclinopyroxenites and spinel websterites with granoblastic textures, and (b) hornblende-bearing clinopyroxenites and hornblenditesud(here after called hornblenditic xenoliths) characterised by the presence of Ti-rich kaersutitic amphibole and magmaticudtextures. Both groups of xenoliths can be assigned to the Al-augite series of Wilshire and Shervais (1975) [Wilshire, H.G.,udShervais, J.W., 1975. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Phys. Chem.udEarth 9, 257–272] with Al-rich and Cr-poor mafic phases. Clinopyroxenes show a very similar trace element composition in alludof the ultramafic xenoliths, characterised by convex-upward chondrite-normalised REE patterns and low contents of incompatibleudelements such as Rb, Ba, Th and Nb. Kaersutite in the amphibole-bearing xenoliths shows a similar convex-upwardudREE pattern as clinopyroxene. Whole-rock and mineral geochemistry support an origin as cumulates from alkaline toudsubalkaline melts for most of the pyroxenites and hornblendites that have been studied. The Sr–Nd isotope ratios of pyroxeniteudxenoliths display two extreme compositional poles: one clinopyroxenite plots in the OIB field towards depleted valuesud(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 hornblenditicudxenolith suite has a very homogeneous isotopic composition, close to the isotopically depleted values of high qNd and lowud87Sr/86Sr ratios of one of the pyroxenite xenoliths. Some of these ultramafic xenoliths fall within the isotopic compositionaludrange of their host alkaline dykes, which also define a bipolar compositional field, suggesting that most of them are cogeneticudwith the lamprophyres. P–T estimates yield temperatures in the range of 970–1080 8C and pressures mainly from 0.9 to 1.2 GPaudfor pyroxenites, whilst hornblenditic xenoliths give lower (and probably underestimated) pressures (0.7–0.9 GPa). This pressureudrange is in agreement with pyroxenites being formed by an underplating event at the upper mantle–lower crust boundary,udwhereas pressure estimates for hornblenditic xenoliths suggest equilibration within the lower crust.