Permo-Carboniferous subvolcanic rhyolitic dikes in the western Erzgebirge/Vogtland, Germany: a record of source heterogeneity of post-collisional felsic magmatism

摘要

In the western Erzgebirge/Vogtland of Germany, a suite of dikes of compositionally and texturally diverse, often K-rich high-Si rhyolites/porphyritic microgranites are evidence of igneous activities some 10 Ma after the major period of granite magmatism (325-318 Ma) in the early/middle Carboniferous (Namurian). Available geochronological data (305-295 Ma, from occurrences at Gottesberg and Jungfernsprung) suggest a middle/late Carboniferous (Stefanian) age for this episode of late-Varis-can, post-collisional subvolcanic magmatism. Most rhyolites experienced variable degrees of high- and low-T alteration, such as greisenization, hematitization, sericitization, and argillitization, which disturbed and obscured the primary abundances of many mobile trace elements (F, Li, Rb, Cs, Ba, Sr, Tl, Pb, U, Sn etc.)- Several rhyolites seem to be intrinsically rich in Sn, W, and other ore elements, others received their enrichment in rare metals by superimposed greisenization. The Na(+ -Ca)-poor nature of superficially exposed rhyolites (<0.1 wt percent Na_2O) is mainly ascribed to plagioclase alteration as a result of weathering. High-T, pervasive K-phyllic alteration, which caused enrichment in K and removal of Na, Ca, and Mg, may have operated in some rhyolites prior and additional to weathering. Irrespective of postmagmatic and exogenic overprinting, less-mobile elements conserving primary magmatic patterns and the Nd-isotope signatures indicate that generation of the rhyolites involved melting of a minimum of two geochemically and isotopically different (lower crustal) sources. According to Nd isotopes, the rhyolites form two distinct groups, with xi Nd(300) of -5.5 to -6.4 and -1.8 to -3.8, respectively, indicating the rocks are predominantly S-type and transitional I-S-type with respect to source lithology. Some rare-element rich rhyolites (Morgenrothe, Hahnewald) crossing the Ei-benstock granite pluton display features in common with aluminous A-type granites and may be subvolcanic equivalents of shal-lowly intruded Li-F granites of the P-poor class. Contrasting Nd-isotope patterns imply that the rhyolites were derived from sources other than those of the granite plutons with which they are spatially associated.