Resolving crystallisation ages of Archean mafic-ultramafic rocks using theRe-Os isotope system

Rhenium-Osmium (Re-Os) isotope and elemental data are presented for mafic-u ltramafic rocks from the central region of the Lewisian Archean terrain in northwest Scotland. These results give a best estimate for the time of empl acement of the mafic-ultramafic bodies of 2686.7+/-14.7 Myr (2 sigma). The initial Os-187/Os-188 isotope ratio of 0.10940 +/- 0.00076 indicates that s uch material possessed a chondritic Os isotope composition, which suggests that these rocks were formed by direct melting of mantle material, consiste nt with major and trace element constraints on their formation. Nevertheles s, the Re-Os systematics of some of the mafic-ultramafic rocks in the Lewis ian have been significantly disturbed, such that the original age informati on has been lost. These rocks lie on a regression line that defines an age of similar to 3260 Myr, and a negative initial Os isotope composition, sugg esting perturbation of the Re-Os system, either through assimilation or pos t-emplacement elemental exchange. Such a process also appears to have affec ted the Sm-Nd systematics in the same samples. Crustal assimilation can acc ount for the observed Os and Nd isotope variations but only if the assimila ted material possessed Os-187/Os-188 values of ca. 25 at similar to 2687 My r. In contrast, the surrounding gneisses and metasediments preserve present -day measured Os-187/Os-188 values of between 3 and 16. Rather, the spatial variation of initial Os and Nd isotope compositions suggests that isotope perturbation was caused by local sub-solidus element exchange between diffe rent lithologies, consistent with major element data and petrographic obser vations. Taken together, these results highlight the utility of the Re-Os i sotope system for obtaining precise ages for Archean mafic-ultramafic rocks , and as a sensitive petrogenetic tracer capable of discriminating between assimilation or elemental exchange. (C) 2000 Elsevier Science B.V. All righ ts reserved.