Primary carbonatite melt inclusions in apatite and in K-feldspar of clinopyroxene-rich mantle xenoliths hosted in lamprophyre dikes (Hungary)

Abstract

We studied clinopyroxene–apatite–K-feldspar–phlogopite xenoliths (CAKP), collected from the Late Cretaceous lamprophyre dikes of the Alcsutdoboz-2 (Ad-2) borehole, Transdanubian Central Range, Hungary. Apatite and K-feldspar contain a large number of primary, negative crystal shaped carbonatite melt inclusions. Chemically, the melt inclusions are phosphorous dolomitic in apatite and dolomite-bearing alkaline-aluminosiliceous in K-feldspar. As these melts in apatite and K-feldspar cannot be the differentiation product or residuum of each other, and appear to have been present in the host rock at the same time, they are likely to have formed by liquid immiscibility. Clinopyroxene and phlogopite are interpreted as products of metasomatic reactions between ultramafic mantle and carbonatite melt initially infiltrating the mantle rock. During this process the composition of the metasomatizing carbonatite melt changed, resulting in the separation of two immiscible melts. One melt was phosphate carbonatite in composition, and the other was a carbonate-bearing alkaline aluminosiliceous melt. Both melts were over-saturated with respect to apatite and K-feldspar. This process initiated the crystallization of the host minerals from a continuously percolating melt and, consequently, led to the entrapment of large populations of melt inclusions. The initial metasomatizing melt had a phosphorous Mg-calcitic carbonatite composition (with low Mg#), and is likely to have been formed in a subducted lithospheric slab.