High-potassium, calc-alkaline I-type plutonism in the European Variscides:northern Vosges (France) and northern Schwarzwald (Germany)

Early Carboniferous high-K, calc-alkaline I-type plutonic rocks from the no rthern Vosges and Schwarzwald were studied for their chemical and Sr-Nd iso topic compositions. Intrusion relationships and mineralogical and chemical characteristics allow to distinguish four suites. The oldest intrusions are diorites (1), followed by a granodioritic (2) and a granitic (3) suite. Th ese older granitoids (OG) and their contact metamorphic country rocks are c ut by younger high-K to shoshonitic granitic plutons (YG) (4). Still later, peraluminous S-type granitic magmas intruded (not included in this study). Diorites (1) have SiO2 between 46 and 61 wt,% and are characterized by rel atively high Mg# of 62-38, low contents of Na2O (2.3-4.0 wt.%), high abunda nces of incompatible elements (LILE, Nb, and P) and enriched Nd-Sr initial isotopic signatures [epsilon(Nd)(I) = -1.7 to -2.8; Sr-87/Sr-86(I) = 0.7046 -0.7061]. Chondrite-normalized (cn) REE patterns are relatively flat [(La/Y b)(cn) = 5.1-7.8; (Tb/Yb)(cn) = 1.2-1.8] and show small negative Eu anomali es (Eu/Eu* = 0.73-0.90). All these characteristics suggest an origin of the diorites from enriched lithospheric mantle sources. Compared with the dior ites, the granodiorites (2) show higher epsilon(Nd)(I) (+0 5 to -0 4) but s imilar Sr-87/Sr-86(I) (0.7051-0.7053). High values of molar CaO/(MgO + FeOt ot) combined with low Al2O3/(MgO + FeOtot) and K2O/Na2O ratios suggest an o rigin by dehydration melting from a metabasaltic to metatonalitic source. R adiogenic isotopic signatures of the older granites (3) are similar to thos e of the diorites [epsilon(Nd)(I) = -1.8 to -2.5; Sr-87/Sr-86(I) = 0.7048-0 .7058]. Compared with the granodiorites the older granites show similar val ues of CaO/(MgO + FeOtot), but significantly higher ratios of Al2O3/(MgO FeOtot) and K2O/Na2O pointing to a metagreywacke source. REE patterns of bo th the granodiorites and the granites are characterized by relatively low ( Tb/Yb)(cn) ratios (1.2-1.7) excluding substantial amounts of garnet as a fr actionating phase. Instead, the residues were probably dominated by amphibo le and plagioclase, and possibly also pyroxene. The YG (4) have elevated ab undances of large ion lithophile elements (K, Rb, Th, U, Ba, and Sr) and of some high field strength elements (Nb and P). Their isotopic signatures [e psilon(Nd)(I)= -1.5 to -3.4; Sr-87/Sr-86(I) = 0.7046-0.7060] are similar to those of the older granites. Relative to all OG, their REE patterns are ch aracterized by higher ratios of (La/Yb)(cn) (11.8-38.9) and (Tb/Yb)(cn) (1. 3-2.6) but lower values of Eu/Eu*. Combined with higher Mg# and lower abund ances of Y, these characteristics point to an increasing role of garnet in the residues of the partial melts. Relatively low values of molar Al2O3/(Mg O + FeOtot) and K2O/Na2O in combination with variable molar CaO/(MgO + FeOt ot) ratios suggest that these magmas were derived from heterogeneous metase dimentary sources. (C) 2000 Elsevier Science B.V. All rights reserved.