NEW APPROACHES TO CRUSTAL EVOLUTION STUDIES AND THE ORIGIN OF GRANITIC-ROCKS - WHAT CAN THE LU-HF AND RE-OS ISOTOPE SYSTEMS TELL US

The Lu-Hf and Re-Os isotope systems have been applied sparsely to eluc idate the origin of granites, intracrustal processes and the evolution of the continental crust. The presence or absence of garnet as a resi dual phase during partial melting will strongly influence Lu/Hf partit ioning, making the Lu-Hf isotope system exceptionally sensitive to eva luating the role of garnet during intracrustal differentiation process es. Mid-Proterozoic (1.1-1.5 Ga) 'anorogenic' granites from the wester n U.S.A. appear to have anomalously high epsilon(Hf) values, relative to their epsilon(Nd) values, compared with Precambrian orogenic granit es from several continents. The Hf-Nd isotope variations for Precambri an orogenic granites are well explained by melting processes that are ultimately tied to garnet-bearing sources in the mantle or crust. Resi dual, garnet-bearing lower and middle crust will evolve to anomalously high epsilon(Hf) values over time and may be the most likely source f or later 'anorogenic' magmas. When crustal and mantle rocks are viewed together in terms of Hf and Nd isotope compositions, a remarkable mas s balance is apparent for at least the outer silicate earth where Prec ambrian orogenic continental crust is the balance to the high-epsilon( Hf) depleted mantle, and enriched lithospheric mantle is the balance t o the low-epsilon(Hf) depleted mantle. Although the continental crust has been envisioned to have exceptionally high Re/Os ratios and very r adiogenic Os isotope compositions, new data obtained on magnetite mine ral separates suggest that some parts of the Precambrian continental c rust are relatively Os-rich and non-radiogenic. It remains unclear how continental crust may obtain non-radiogenic Os isotope ratios, and th ese results have important implications for Re-Os isotope evolution mo dels. In contrast, Phanerozoic batholiths and volcanic arcs that are b uilt on young mafic lower crust may have exceptionally radiogenic Os i sotope ratios. These results highlight the unique ability of Os isotop es to identify young mafic crustal components in orogenic magmas that are essentially undetectable using other isotope systems such as O, Sr , Nd and Pb.