INFERENCES FROM SR-ISOTOPE SYSTEMATICS FOR MAGMA EVOLUTION OF NW BAVARIAN GRANITIC INTRUSION SEQUENCES

Many granites occur as sequences of intrusions which show certain rela tions between its members. In this work the systematics of isochron sl opes (i.e. isochron ages) and initial Sr-87/Sr-86 ratios are derived m athematically for two different types of granite intrusion sequences: fractional crystallization and magma generation by partial melting. Th e equations derived in this paper are applicable to any radioactive mo ther-radiogenic daughter system as Rb/Sr, Sm/Nd, U/Pb, etc. If granite s are produced by magma fractionation, it can be shown that in an Rb-S r isochron diagram the average isotope ratios y(i) = (Sr-87/Sr-86) and x(i) = (Rb-87/Sr-86) of a member G(i) of a granitic intrusion sequenc e plot at the intersection of the individual isochrons of the two subs equent granites Gi and Gill which have intrusion ages of ti and t(i-1) , In a Compston-Jeffery diagram, where the initial ratios b(i) are plo tted vs the ages t(i) (or isochron slopes a(i)), hi increases monotoni cally with decreasing age t(i). Two subsequent granites G(i) and G(i+1 ) with the coordinates x(i), y(i) and x(i+1), y(i+1) are connected by a line with a minimum slope of x(i) and a maximum slope of x(i+1). If granites G(i) are produced by progressive partial melting of a source rock, the present-day average values (x(i), Y-i) of all granites G(i) plot on an isochron which yields the age and the initial ratio of the source rock, The Compston-Jeffery diagram, contrary to the fractional crystallization model, does not necessarily yield a monotonic curve. T he isotopic criteria derived in this payer, together with geochemical criteria, are applied to a granitic intrusion sequence in the Fichtelg ebirge where the different granites are probably produced by fractiona l crystallization. A granitic intrusion sequence which covers a large area ill the Oberpfalz only a few kilometres east of the KTB and appro ximately 30 km south of the Fichtelgebirge. however, probably is deriv ed from magma generated by partial melting.