Paleoproterozoic andesite porphyrites, Kursk block, Eastern Sarmatia: geochemistry and sources of melts

Abstract

The subvolcanic andesitic porphyrites of the Glazunovskaya suite within the Kursk block of Eastern Sarmatia are high potassium and show the strong fractionation of REE distribution ((La/Yb) n = 33–49) without the Eu negative anomaly (Eu/Eu * = 0.89–0.99). The LILE enrichment and Nb, Ti, and HREE depletion are established. Andesitic porphyrites are characterized by the adakite geochemical characteristics, marked by distinctly high La/Yb and Sr/Y ratios. Volcanic rocks demonstrate the negative εNd(2067) values (from –3.8 to –4.7). We have two contrasting groups of the primary isotopic composition of hafnium. The first group has εHf(2067) values from + 3.3 to + 6.8, the second group has εHf(2067) values from -12.7 to -13.3. Adakite signatures of porphyrites are supposed to have appeared as a result of a two-stage process: 1) assimilation of crustal rocks at the base of the delaminated crust and 2) crystallization differentiation of basalt magmas in the igneous chambers at the upper crust levels. The source of parental magmas was water enriched garnet peridotite most likely, the melting of which occurred at P > 1.5 GPa and T > 1000 °C. The most likely intrusive comagmates of andesitic porphyrites are the gabbro-diorite-granodiorite rocks associations of the Stoilo-Nikolaevsky complex.