Effects of melt percolation on platinum group elements and Re-Os systematics of peridotites from the Tan-Lu fault zone, eastern North China Craton

摘要

Concentrations of platinum group elements (PGE) and Re-Os isotopic compositions were determined for a suite of peridotite xenoliths from Cenozoic Beiyan volcanoes within the Tan-Lu fault zone, eastern North China Craton (NCC). Previous petrographic and geochemical data indicate the existence of considerable modification by melt-peridotite interaction for these xenoliths. Thus, this suite of xenoliths provides an unusual opportunity to examine the effects of melt percolation on PGE abundances and Re-Os isotopes. Our data show that most of the Beiyan peridotites have flat chondrite-normalized PGE patterns whereas a considerable variation in total PGE abundances has been observed in lherzolites to highly fertile cpx-rich lherzolites and wehrlites. Their total PGE contents range between 0.97 and 61.4 ppb and their PGE relative abundances from 0.1 to 0.0001 3 CI-chondrites, respectively. In these rocks, the total PGE budget was reduced by more than 90% from lherzolite to cpx-rich lherzolite and to wehrlite, probably because intergranular sulphides were completely removed by silicate melt. However, Beiyan peridotites have low and variable Re (0.0002-0.5118 ppb) and Os (0.194-10.4 ppb) abundances and high ~(187)Os/~(188)Os (0.12167-0.14978) ratios. The ~(187)Os/~(188)Os ratios in some wehrlites are much higher than the value for the primitive mantle. The high 187Os/188Os ratios have also been observed in peridotites found in other localities within the Tan-Lu fault zone such as Shanwang and Nüshan. This could be the result of the addition of radiogenic Os during the melt percolation. Extremely low Os abundances in Beiyan peridotites suggest that Os may behave as an incompatible element during melt percolation and could mobilize with the dissolution of sulphides. The suprachondritic ~(187)Os/~(188)Os ratios in peridotites within the Tan-Lu fault zone relative to those away from it indicate that the Tan-Lu fault zone played an important role as a melt infiltrating channel in the radiogenic Os enrichment induced by melt percolation. This could be the reason for the Os isotopic heterogeneity observed in the eastern NCC. This study also confirms that Os isotopes and PGE appear to be mobile during massive melt percolation.