GRANITOID MAGMATISM OF THE NW BOHEMIAN MASSIF REVEALED - GRAVITY-DATA, COMPOSITION, AGE RELATIONS AND PHASE CONCEPT

The late Variscan granitoids of the NW Bohemian massif (northeast Bava ria, west Bohemia) constitute four partly contiguous granitoid complex es: Fichtelgebirge, northern Oberpfalz, Waidhaus-Rozvadov and Bor, inc orporating more than 20 intrusive units. Based on gravity data, the gr anites can be modeled as steeply inclined slab- and wedge-like bodies with thicknesses between 2 and 8 km. A rough estimate of the total vol ume of the granites is approximately 18 000 km(3). Within the four are as named above, composition ranges from less evolved dioritic rocks, k nown as the redwitzite suite, to highly evolved granites. The redwitzi tes comprise metaluminous rocks with dominant I type features. These r ocks yield aberrantly old Rb-Sr ages (545-415 Ma), low initial Sr rati os (0.706-0.708) and high and variable epsilon(Nd(T)) values (1 to -4) . Sr-Nd isotopes of the redwitzites show contamination trends towards the granites suggesting mixing between mantle magma and crustal granit ic melts. An older plutonic association (granites of Bor, Leuchtenberg , Weissenstadt-Rilarktleuthen, Zainhammer) is mildly peraluminous, dis playing features of both I and S type granitoids. These granites are c haracterized by Lower Carboniferous ages (Rb-Sr, K-Ar, U-Pb), low to i ntermediate initial Sr ratios (0.707-0.708) and high epsilon(Nd(T)) va lues (-2 to -4) which overlap with those of paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV) and from the western part of the Tepl a Barrandian. It is postulated that the older granites were formed eit her by partial melting of ZEV or Tepla Barrandian crust, or alternativ ely, of preexisting mature crust contaminated by mantle material. The younger granites are strongly peraluminous and of S type. They yield U pper Carboniferous Rb-Sr and K-Ar ages and exhibit a range towards hig h initial Sr ratios (0.710-0.720) and low epsilon(Nd(T)) values (-4 to -8). Similar Values are found in Moldanubian paragneisses and in Saxo thuringian metasediments, both of which provide potential source-rock lithologies for these granites. The age and isotope data discussed her ein suggest episodic rather than continuous magmatic activity. From a combination of field and analytical data, a three-stage cycle of grani toid intrusion is proposed: (a) a first phase (similar to 350-325 Ma) of two contrasting magma types coexisting in a close spatial context, the redwitzites (phase Ia) and the older granites (phase Ib), (b) a se cond phase with emplacement ages of 315-310 Ma comprising all younger granites of the northern Oberpfalz and the Waidhaus-Rozvadov complex a nd (c) a third phase with emplacement ages of 305-295 Ma restricted to the Fichtelgebirge.