GEOCHEMICAL AND SR-ND-PB ISOTOPIC COMPOSITIONS OF MTS PIENINY DYKES AND SILLS (WEST CARPATHIANS): EVIDENCE FOR MELTING IN THE LITHOSPHERIC MANTLE

Abstract

Mineralogical, geochemical and Sr-Nd-Pb isotopic data are presented for the Middle Miocene (13.5-10.8 Ma) Mts Pieniny magmatic rocks which occur in a portion of the outermost post-collisional volcanic arc in the Carpathian orogen. The analysed rocks cover a compositional range from basaltic andesites to andesites of the medium-K calc-alkaline series. Despite their arc-like trace element signature, Pieniny rocks show unusual isotopic compositions different from the Neogene-Quaternary calc-alkaline volcanic rocks erupted in the nearby Carpathian-Pannonian region. Their 87Sr / 86Sr (0.7053-0.7070) and 143Nd / 144Nd (0.5121-0.5124) ratios suggest that these magmatic rocks have originated from lithospheric sources enriched in Rb with low Sm / Nd ratios. Pb isotopic compositions (206Pb / 204Pb = 18.62-18.86; 207Pb / 204Pb = 15.66-15.75; 208Pb / 204Pb = 38.74-39.84) are also consistent with a lithospheric origin of Pieniny rocks. Sr and Nd depleted mantle model ages suggest that parental magmas of Pieniny rocks derived from a metasomatized subcontinental lithospheric mantle recording ancient (> 1.1 Ga) subduction-related enrichments. Geochemical features of Pieniny basic rocks allow an estimate of the mineralogy of their lithospheric mantle source region. The small range (from 11 up to 15) of La / Yb and the lack of significant Eu anomalies preclude the presence of residual garnet and plagioclase, respectively; the low Rb / Sr and Rb / Ba ratios as well as the range observed in Ba / Th, K / Ba and La / Yb ratios require involvement of an heterogeneous source in terms of amphibole abundance. The delamination process of the European plate that occurred in this sector of the Carpathian orogen during Middle Miocene should have favoured partial melting of metasomatized amphibole-bearing veins within the lithospheric mantle of the overriding Alcapa micro-plate, giving rise to magma genesis in the Mts Pieniny post-collisional arc segment. © 2006 Elsevier B.V. All rights reserved.