Zircon and titanite geochronology of the Furstenstein granite massif, Bavarian Forest, NW Bohemian Massif: Pulses of the late Variscan magmatic activity

摘要

The Furstenstein granite massif is the largest late Variscan granitoid body exposed in the Moldanubian basement of the Bavarian Forest. The granite massif comprises different rock types, ranging from diorite to granite in composition. This study provides precise zircon and titanite ages, obtained by the U-Pb isotopic dilution and Pb-Pb evaporation methods, and Nd-Sr isotopic data on this massif. Zircon dating constrains an intrusive sequence as following: Dioritic rocks crystallised about 334 to 332 Ma ago, earlier than the high-temperature peak stage of the Variscan metamorphism of the Moldanubian basement (325 to 320 Ma) in the Bavarian Forest. The medium- to coarse-grained Tittling granite was emplaced about 10 Ma later, between 324 and 321 Ma, while the medium-rained Eberhardsreuth granite crystallised between 316 and 312 Ma, contemporaneously with the coarse-grained porphyritic Saldenburg granite. Titanites from the dioritic rocks give U-Pb ages of about 321 4 Ma, indicating thermal resetting or crystallisation due to the intrusion of the Tittling granite. The diodtes have initial epsilon(Nd) values of -1.3 to -2.9 and relatively low initial Sr-87/Sr-86 ratios of 0.7056 to 0.7067, which suggest mantle contribution to melts or melting of young mafic lower crust. The granites have similar initial 6,, values of -3.6 to -4.1 but show some variation in their Sr isotopic composition (initial Sr-87/Sr-86 ratios of 0.7063 to 0.7074), probably indicating the involvement of crustal material in their genesis.