GEMOC ARC National Key CentreW.L. GRIFFIN1,2, S.Y. O'Reilly1 and C.G. Ryan2
C.G. Ryan
1. GEMOC Macquarie
2. CSIRO Exploration and Mining
Chemical and petrographic observations on mantle-derived xenoliths and xenocrysts define a fundamental distinction between Archean cratonic mantle and Phanerozoic mantle. Archean xenoliths are more depleted on average, and have higher Si/Mg; subcalcic harzburgites are well-represented in Archean xenolith and xenocryst suites, but rare in younger ones. Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks. In xenoliths, the Cr content of garnet correlates well with the Al content of the rock, and Al is closely related to the contents of other major and minor elements. These correlations allow calculation of the mean composition of a mantle section, given the Cr content of garnet xenocrysts from that section. The calculated mean composition of SCLM beneath Archean, Proterozoic and Phanerozoic terrains shows a secular evolution in all measures of depletion, such as Al, Ca, mg#, and Fe/Al; Proterozoic SCLM is intermediate in composition between Archean and Phanerozoic SCLM. Cenozoic SCLM, exemplified by garnet peridotites from young extensional areas (China, Siberia, Australia) is only mildly depleted (210% melt extraction) relative to Primitive Mantle. SCLM beneath some Phanerozoic terrains, especially in Europe, is more depleted and may represent reworked Proterozoic SCLM. Most Archean SCLM probably was derived by high-degree melting at depths 3150 km. Comparison of xenolith suites with ophiolitic and abyssal peridotites suggests that subducted oceanic mantle is not a major component of Phanerozoic SCLM. Most existing Proterozoic and Phanerozoic SCLM probably has been generated in extensional post-orogenic environments. The broad correlation of SCLM composition with crustal age implies quasi-contemporaneous formation of crustal volumes and their underlying SCLM, and crust-mantle coupling over periods of aeons; it also requires an evolution in fundamental processes involved in the formation of continents and their roots.
