Potassic magmas of the Vosges Mts. (NE France) delimit the areal extent and nature of long-gone Variscan orogenic mantle domains
Abstrakt
Highly magnesian and potassic (Mg-K) magmatism is a characteristic feature of the European Variscan Belt. These magmatic rocks have an odd dual geochemical character: they have both high mg# and transition metal contents indicative of being derived from the mantle (and not having lost that character via magmatic differentiation), while simultaneously they are highly enriched in LILE and other fluid-mobile mantle-incompatible elements. There is a growing consensus that these magmas are likely produced by partial melting of metasomatized domains in the subcontinental lithospheric mantle (SCLM) that formed by influx of fluids rich in incompatible elements derived from subducted crust. The Sr and Nd isotopic signature of these fluids determined the isotopic character of the resulting mafic potassic magmas, and can be variable, depending on whether the subducted crust was continental vs. oceanic. The similar to 340 Ma Mg-K plutonic suites in the Central and Southern Vosges Mts. of northeastern France preserve differences in Sr-87/Sr-86(340), and to a lesser degree, epsilon(340)(Nd) that are consistent with continental underplating producing an isotopically enriched signature near the suture in the Central Vosges (CVMg-K) in contrast to an isotopically depleted/CHUR-like oceanic subduction signature in the Southern Vosges (SVMg-K). The resulting geodynamic model involves (1) pervasive metasomatism of the SCLM wedge by fluids from the leading oceanic part of the subducting slab during Andean-type subduction, (2) underplating of only the near-suture portion of the SCLM by the trailing continental part of the slab during collision, (3) slab break-off and/or partial delamination of the lowermost SCLM accompanied by asthenospheric influx triggering partial melting of the mantle metasomes, (4) generation of primary potassic magmas that sampled the isotopic characteristics of either side of the mantle boundary, reflecting whether the given part of the SCLM was underplated by continental crust or not, and that (5) carried evidence of that boundary into the plutons of the upper plate. In general, whereas some geochemical tracers in these odd Mg-K magmas preserve evidence of long-lost mantle domains, others were rapidly overprinted and instead preserve the complementary record of interaction with the upper plate.