CONTROL OF REDOX STATE AND SR ISOTOPIC COMPOSITION OF GRANITIC MAGMAS- A CRITICAL-EVALUATION OF THE ROLE OF SOURCE ROCKS

The current underlying assumption in most geochemical studies of grani tic rocks is that granitic magmas reflect their source regions. Howeve r, the mechanisms by which source rocks control the intensive and comp ositional parameters of the magmas remain poorly known. Recent experim ental data are used to evaluate the 'source rock model' and to discuss controls of (1) redox states and (2) the Sr isotopic compositions of granitic magmas. Experimental studies have been performed in parallel on biotite-muscovite and tourmaline-muscovite leucogranites from the H igh Himalayas. Results under reducing conditions (logf(o2)=FMQ-0.5) at 4 kbar and variable f(H2O) suggest that the tourmaline-muscovite gran ite evolved under progressively more oxidising conditions during cryst allisation, up to f(o2), values more than four log units above the FMQ buffer. Leucogranite magmas thus provide an example of the control of redox conditions by post-segregation rather than by partial melting p rocesses. Other experiments designed to test the mechanisms of isotopi c equilibration of Sr during partial melting of a model crustal assemb lage show that kinetic factors can dominate the isotopic signature in the case of source rocks not previously homogenised during an earlier metamorphic event. The possibility is therefore raised that partial me lts may not necessarily reflect the Sr isotopic composition of their s ources, weakening in a fundamental way the source rock model.