Geochemical and isotopic (Sr, Nd and O) constraints on sources of Variscan granites in the Western Carpathians - Implications for crustal structure and tectonics

Abstract

A Sr, Nd, and O isotopic study of Variscan granitoid rocks from the Western Carpathians reveals the dominance of heterogeneous crustal sources for the most of the granitic rocks. Their neodymium crustal index (NCI) is 0.4 to 1.0 (mainly 0.6-0.8). Initial (87Sr/ 86Sr)350 of 0.7053 to 0.7078 and ε Nd(350) of -0.6 to -6.9 preclude a simple mantle and/or crustal origin for most of granitoids and suggest more complex sources, such as vertically zoned lower crust consisting of old metaigneous, amphibolitic and metasedimentary rocks. Apparent crustal residence ages, indicated by two-stage depleted-mantle Nd model ages ranging from 1.6 to 1.1 Ga, are comparable with other segments of the European Variscan belt. The whole rock δ 18O(SMOW) values of the granites are heterogeneous and range from 8.4 ‰ in tonalites to 11.3 ‰ in leucogranites, reflecting source compositions ranging from mafic to silicic. Petrographically, these granites are representative of common crustal anatectic rocks with magmatic muscovite; however, their isotopic signature reflects petrogenesis related to subduction processes at active continental margins. Recent metamorphic, sedimentary and/or structural studies of the Variscan basement of the Western Carpathians suggest a continental collisional rather than a volcanic arc setting. The Western Carpathian granitic rocks were most likely generated by partial melting of mainly recycled Proterozoic crustal material during subduction-collisional processes of the Variscan orogeny, with possible input of mafic magmas from the mantle. These mafic magmas may have served as a heat source for melting of the lower crust.