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

2007; E. Schweizerbart; Volume: 183; Issue: 2 Linguagem: Inglês

10.1127/0077-7757/2007/0064

2363-7161

Hans-Jürgen Förster, B. Gottesmann, G. Tischendorf, Wolfgang Siebel, Dieter Rhede, Reimar Seltmann, J. Wasternack,

Geochemistry and Geologic Mapping

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-Variscan, post-collisional subvolcanic magmatism. Most rhyolites experienced variable degrees of highand 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 % Na2O) 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 eNd(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 Eibenstock granite pluton display features in common with aluminous A-type granites and may be subvolcanic equivalents of shallowly 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.