THE UNDER-APPRECIATED ROLE OF ENRICHED MANTLE DURING GENESIS OF VARISCAN ULTRAPOTASSIC MAGMAS

The Variscan Orogen of western and central Europe contains voluminous exposures of (ultra)-potassic intrusive rocks, the most characteristic being durbachites (strongly porphyritic Amph-Bt qtz syenite-melagranite). Coevally intruded at ca. 345-335 Ma, these rocks can yield information on geodynamic processes operating at the height of collision. We focus on similar sets of plutons in the Bohemian Massif in the Czech Republic and the Vosges Mts. in NE France, which share a strange dual geochemical character: mantle-like high mg# and high transition element contents, coupled with highly enriched LILE and crust-like Sr and Nd isotopic signatures.

Petrogenesis likely involved subduction of mature continental crust, which polluted the originally-depleted lithospheric mantle (Janoušek & Holub, 2007; Schulmann et al. 2014). Subsequently, heating, either by asthenospheric influx following slab break-off (Davies & von Blankenburg, 1995) or in-situ radiogenic decay (Lexa et al. 2011), led to partial melting of these metasomatized mantle domains. In the Bohemian Massif, these primary melts had more evolved Sr and Nd isotopic signatures than the crust of the upper plate with which they interacted. For example, this resulted in odd negative correlations of MgO content and initial ε_Nd (increasing incorporation of crust led to less-crustal ⁸⁷Sr/⁸⁶Sr and ε_Nd). Along the Vosges, a NE-SW gradient in ⁸⁷Sr/⁸⁶Sr and ε_Nd in primitive magmas indicates variable degree of mantle contamination along dip of the subduction channel, possibly reflecting differences in age of the subducting crust (Tabaud et al. 2015).

Thus, gauging relative contributions of mantle and crust in potassic magmas of the Variscan Orogen (and possibly elsewhere) becomes highly dependent on which element or isotopic ratio is modeled (e.g. crustal recycling is underestimated if relying on mg# and mantle contribution is underestimated when considering Sr and Nd isotopes).