Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle

Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhön, Central European Province, Germany) yield new high-precision 40 Ar/ 39 Ar ages of 24.0–24.1 Ma. Major and trace element compositions demonstrate that these high-TiO 2 (4–7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from magmatic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks ( 87 Sr/ 86 Sr 24 = 0.7035–0.736, εNd 24 = +3.8–+4.0, 206 Pb/ 204 Pb 24 = 19.21–19.37, 207 Pb/ 204 Pb 24 = 15.58–15.62, 208 Pb/ 204 Pb 24 = 38.95–39.16), but two samples show contrasting isotopic compositions (εNd 24 = −4.0 and −2.9; 206 Pb/ 204 Pb 24 = 17.08 and 18.11; 207 Pb/ 204 Pb 24 = 15.51 and 15.58; 208 Pb/ 204 Pb 24 = 37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 ‰, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhön volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 °C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale.