Geochemical and isotopic (Sr, Nd and O) constraints on sources for 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 (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 18 O (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 subductioncollisional 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.

U-Pb zircon age of the youngest magmatic activity in the High Tatra granites (Central Western Carpathians)

“…All applied geothermometers gave similar, relatively high temperature range for magma crystallization: 720-890°C, supporting the suggested mafic magma influx Nabelek, 2008 andGawęda, 2009).…”

“…The High Tatra granite originated most likely by partial melting of recycled Proterozoic crustal rocks, with the simultaneous input of mafic magmas from the mantle. The mafic magma portions are thought to be the heat source for melting of the continental crust Uher, 2001 andNabelek, 2008). The age of that granite body is still controversial.…”

Tracking potential source areas of Central European loess:examples from Tokaj (HU), Nussloch (D) and Grub (AT)

“…Compared to loess, many potential source rocks have a di erent isotopic signature with much higher ϵ ND values (Figure 7, Figure 9, Figure 10), including basalts from Hungary and Austria (Carpathian Basin (CB); [56]), igneous rocks from the Central Alps [57], igneous rocks from Hungary and Romania (Eastern Carpathians (EC); [58]) and Bohemian Massif granites (BM; [59]). Our loess samples plot close to, or within, the rectangles of East Carpathian ysch [60], granites from the Tisia terrane in Southwest Hungary (Carpathian Basin (CB); [61]), Bohemian Massif sedimentary rocks [62], Western Carpathian granites (WC; [63]), and at some distance to Slovakian gneiss (WC; [64]) and Czech metamorphic rocks (BM; [65]). A more detailed analysis of single data points (Figure 10) reveals that the loess samples overlap with several rock samples, most notably with East Carpathian ysch [60].…”

Titanite composition and SHRIMP U–Pb dating as indicators of post-magmatic tectono-thermal activity: Variscan I-type tonalites to granodiorites, the Western Carpathians

“…The Paleozoic basement rocks of the Inner Western Carpathians occur in three Alpine tectonic units: Tatric, Veporic, and Gemeric. The Variscan (Devonian to Carboniferous) calc-alkaline granitic plutons of I-and S-type affinity (e.g., Petrík & Kohút 1997;Kohút et al 1999;Broska & Uher 2001;Kohút & Nabelek 2008;Broska et al 2013) occur in the Tatric and Veporic units, they intruded high-to medium-grade Paleozoic metamorphic rocks (mainly metapelites to metapsammites) of the Variscan nappes, which show a pre-Alpine, generally south vergency (e.g., Putiš 1992;Bezák et al 1997;Bielik et al 2004). Moreover, small bodies of Permian post-orogenic to anorogenic S-and A-type granitic rocks are also present in various tectonic units of the Inner Western Carpathians, mainly in the Gemeric Unit (e.g., Uher & Broska 1996;Broska & Uher 2001).…”