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

Abstract

Early Carboniferous high-K, calc-alkaline I-type plutonic rocks from the northern Vosges and Schwarzwald were studied for their chemical and Sr–Nd isotopic 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. These older granitoids (OG) and their contact metamorphic country rocks are cut 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 SiO 2 between 46 and 61 wt.% and are characterized by relatively high Mg# of 62–38, low contents of Na 2O (2.3–4.0 wt.%), high abundances of incompatible elements (LILE, Nb, and P) and enriched Nd–Sr initial isotopic signatures [ ε Nd(I)=−1.7 to −2.8; 87 Sr / 86 Sr (I)=0.7046–0.7061]. Chondrite-normalized (cn) REE patterns are relatively flat [(La/Yb) cn=5.1–7.8; (Tb/Yb) cn=1.2–1.8] and show small negative Eu anomalies (Eu/Eu*=0.73–0.90). All these characteristics suggest an origin of the diorites from enriched lithospheric mantle sources. Compared with the diorites, the granodiorites (2) show higher ε Nd(I) (+0.5 to −0.4) but similar 87 Sr / 86 Sr (I) (0.7051–0.7053). High values of molar CaO/(MgO+FeO tot) combined with low Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O ratios suggest an origin by dehydration melting from a metabasaltic to metatonalitic source. Radiogenic isotopic signatures of the older granites (3) are similar to those of the diorites [ ε Nd(I)=−1.8 to −2.5; 87 Sr / 86 Sr (I)=0.7048–0.7058]. Compared with the granodiorites the older granites show similar values of CaO/(MgO+FeO tot), but significantly higher ratios of Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O pointing to a metagreywacke source. REE patterns of both 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 fractionating phase. Instead, the residues were probably dominated by amphibole and plagioclase, and possibly also pyroxene. The YG (4) have elevated abundances 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 [ ε Nd(I)=−1.5 to −3.4; 87 Sr / 86 Sr (I)=0.7046–0.7060] are similar to those of the older granites. Relative to all OG, their REE patterns are characterized 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 abundances of Y, these characteristics point to an increasing role of garnet in the residues of the partial melts. Relatively low values of molar Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O in combination with variable molar CaO/(MgO+FeO tot) ratios suggest that these magmas were derived from heterogeneous metasedimentary sources.