Peraluminous granites frequently with mantle-like isotope compositions: the continental-type Murzinka and Dzhabyk batholiths of the eastern Urals

Abstract

Murzinka and Dzhabyk are continental-type batholiths of the middle and southern East Uralian domain. They comprise mainly undeformed peraluminous K-rich granites whose elemental composition is similar to some late-Variscan granites of western Europe, but with much more primitive Sr and Nd isotope ratios. Murzinka (2545 Ma) is composed of silica-rich granites forming two different rock series with a 87 Sr/ 86 Sr init of 0.709 and 0.704, respectively. Both series have enormous variations in eNd 255 (±11.9 to ±0.1 and ±8.9 to +4.1) that reveal derivation from heterogeneous sources. Dzhabyk (2914 Ma) also comprises two coeval magmas which yielded voluminous granites and quartz-monzonites, respectively, with smaller differences in 87 Sr/ 86 Sr init and eNd 290 (~0.7043, +0.8 to +1.6 and~0.7049, 0.0 to +0.8). Despite their isotope compositions both batholiths lack evidence of genetic involvement of a mantle-derived parental magma. Moreover, we suggest that Dzhabyk granitoids were generated by polybaric partial melting of Paleozoic island-arc material, whereas Murzinka granitoids derived from an extremely heterogeneous source consisting mainly of Paleozoic and Proterozoic metagreywackes. This implies a relative fast reworking of juvenile arc crust and burial of the protoliths during the orogenic evolution of the Urals. Since there is neither evidence of significant extension, nor a direct link with subduction, we suggest that the main cause for late-orogenic anatexis was elevated heat production and fertility in the protolith, perhaps combined with some additional heat from unexposed mafic intrusions.

Making continental crust: origin of Devonian orthogneisses from SE Mongolian Altai

“…The relatively low A/CNK values, Rb/Sr (mostly below unity) and Rb/Ba (< 0.45) as well as high CaO/Na 2 O ratios (0.15-0.84) are typical of magmas generated by partial melting of feldspar-rich psammitic or intermediate meta-igneous sources (Sylvester 1998;Jung and Pfänder 2007). Such a notion is also supported by the major-element plots of Gerdes et al (2002) and Jung and Pfänder (2007) shown in Fig. 14.…”

Protolith and deformation age of the Gneiss-Plate of Kartali in the southern East Uralian Zone

“…5). This data shows a good consensus with Gerdes et al (2002), who published isotope data from the Dzhabyk pluton.…”

“…However, this model is inconsistent with geochemical and geophysical data. Nd-isotope data from Gerdes et al (2002) document a crustal residence time that is too brief to support the microcontinental concept. The crustal density below the East Uralian Zone is significantly higher than in a typical continental crust (Döring & Götze 1999).…”

“…The crustal thickening can be explained by two mechanisms: the stacking of the Magnitogork island arc or the westward thrusting of the Mag- A geodynamic model describing the Uralian orogenesis has to support one of these two mechanisms. Therefore, it is desired to characterize the magma source using the data from Gerdes et al (2002) and Popov et al (2001) in addition to our own data.…”

Deformation-driven differentiation of granitic magma: the Stepninsk pluton of the Uralides, Russia

“…Despite their modest size ( Fig. 1) almost every pluton comprises a great variety of rocks, a continuum from gabbros or diorites to leucogranites (Fershtater, 1984;Fershtater et al, 1994;Bea et al, 2002;Gerdes et al, 2002). Though this notable geochemical diversity might have resulted from different factors, compelling evidence firmly points to crystal fractionation as the main cause of differentiation (Fershtater, 1987).…”