Evidence for crustal assimilation by turbulently convecting,mafic alkaline magmas: Geochemistry of mantle xenolith-bearing lavas from northern Sardinia |
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| Institution: | 1. Research School of Earth Sciences, Australian National University, Acton, ACT 2601, Australia;2. School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom;1. Institute of Geophysics, University of Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany;2. Institute of Geology, University of Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany;3. Institute of Oceanography, University of Hamburg, Bundesstrasse 53, D-20146 Hamburg, Germany;1. CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. CAS Key Laboratory of Earth''s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia;2. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 109017, Russia |
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| Abstract: | Alkali basalt, trachybasalt and basanite magmas, containing abundant xenoliths of upper mantle origin, were erupted during the Plio-Pleistocene (2.4-0.14 Ma) in northern Sardinia. The magmas are enriched in K, Rb, Th and Ba relative to mid-ocean ridge basalts (MORB) and most ocean island basalts (OIB), resulting in high K/Nb, Th/Nb, Ba/Nb and Rb/Nb ratios. The large number of spinel peridotite inclusions in these lavas suggests that these chemical features cannot be explained by combined assimilation and fractional crystallization within the continental crust. However, volcanic rock chemistry can be explained by the assimilation of sialic rocks by turbulently convecting, mafic magmas during their ascent to the surface. Fractionation of Ba and K from the light rare earth elements (LREE) is required to explain the positive correlation of K/La and Ba/La with 87Sr/86Sr(i). Consequently, bulk assimilation of crystalline basement rocks by rising, hot basaltic magmas cannot explain the observed chemical trends, and preferential melting of a low melting quartzo-feldspathic crustal component probably occurred, leaving the REE in residual phases such as apatite, zircon, sphene and amphibole. Alternatively, large ion lithophile element (LILE) enrichment may have been related to interaction of rising mafic lavas with metasomatized lithospheric mantle or enriched asthenosphere. |
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