%0 Journal Article
%A Orejana García, David
%A Villaseca González, Carlos
%A Paterson, Bruce A.
%T Geochemistry of pyroxenitic and hornblenditic xenoliths in alkalinelamprophyres from the Spanish Central System
%D 2006
%@ 0024-4937
%U https://hdl.handle.net/20.500.14352/49590
%X The alkaline lamprophyres and diabases of the Spanish Central System carry a heterogeneous suite of xenoliths whichincludes scarce pyroxenitic and hornblenditic types that can be divided in two groups: (a) pyroxenite xenoliths, including spinelclinopyroxenites 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 magmatictextures. 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 allof the ultramafic xenoliths, characterised by convex-upward chondrite-normalised REE patterns and low contents of incompatibleelements such as Rb, Ba, Th and Nb. Kaersutite in the amphibole-bearing xenoliths shows a similar convex-upwardREE pattern as clinopyroxene. Whole-rock and mineral geochemistry support an origin as cumulates from alkaline tosubalkaline melts for most of the pyroxenites and hornblendites that have been studied. The Sr–Nd isotope ratios of pyroxenitexenoliths 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 hornblenditicxenolith suite has a very homogeneous isotopic composition, close to the isotopically depleted values of high qNd and low87Sr/86Sr ratios of one of the pyroxenite xenoliths. Some of these ultramafic xenoliths fall within the isotopic compositionalrange of their host alkaline dykes, which also define a bipolar compositional field, suggesting that most of them are cogeneticwith the lamprophyres. P–T estimates yield temperatures in the range of 970–1080 8C and pressures mainly from 0.9 to 1.2 GPafor pyroxenites, whilst hornblenditic xenoliths give lower (and probably underestimated) pressures (0.7–0.9 GPa). This pressurerange 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.
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