Zircon Hf isotope perspective on the origin of granitic rocks from eastern Bavaria, SW Bohemian Massif

The petrogenetic potential of in situ laser ablation Hf isotope data from melt precipitated zircons was explored through the analyses of about 700 individual crystals derived from about 20 different granitic intrusions covering the Variscan basement segment of eastern Bavaria, SE Germany. In combination with geochemical features, four major suites of granitic rocks can be distinguished: (1) NE Bavarian redwitzites (52–57 wt% SiO₂, intrusion ages around 323 Ma) have chondritic εHf(t) values (+0.8 to –0.4). The redwitzites are hybrid rocks and the Hf data are permissive of mixing of a mantle progenitor and crustal melts. (2) Various intermediate rock types (dioritic dyke, granodiorite, palite, 59–63 wt% SiO₂, 334–320 Ma) from the Bavarian Forest yield negative εHf(t) values between –3.4 and –5.1. These values which apparently contradict a mantle contribution fingerprint an enriched (metasomatized) mantle component that was mixed with crustal material. (3) Voluminous, major crust forming granites sensu stricto (67–75 wt% SiO₂, 328–298 Ma) are characterized by a range in εHf(t) values from –0.5 to –5.6. Different crustal sources and/or modification of crustal melts by various input of juvenile material can explain this variation. (4) Post-plutonic (c. 299 Ma) porphyritic dykes of dacitic composition (64–67 wt% SiO₂) from the southern Bavarian Forest have chondritic εHf(t) values (+0.6 to –1.1) and display large intergrain Hf isotope variation. The dykes form a separate petrogenetic group and the Hf data suggest that the zircons crystallized when a pristine mantle-derived parental melt was modified by infiltration of crustal material. The zircon Hf data form a largely coherent positive array with the whole-rock Nd data and both systems yield similar two-stage depleted mantle model ages (1.1–1.7 Ga).