GEMOC ARC National Key Centre

W.L. Griffin^1,2, Suzanne Y. O'Reilly¹ and C.G.Ryan²

1 GEMOC Macquarie, 2 CSIRO Exploration and Mining

Data from xenoliths, garnet concentrates and peridotite massifs demonstrate secular evolution in the composition of subcontinental lithospheric mantle (SCLM), related to the last major tectonothermal event in the overlying crust. The garnet data show that subcalcic (cpx-free) harzburgites are restricted to Archean mantle, and that the dominant lherzolites become progressively less depleted (in terms of major-element composition) from Archean through Proterozoic to Phanerozoic time. This 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 measured in aeons.

In most xenolith suites, concentrations of major- and minor elements are well-correlated with Al2O3 contents, while in garnet peridotites there is a good correlation between the Cr2O3 content of the garnet and the Al2O3 content of the host rock. Algorithms relating garnet composition to bulk-rock composition allow calculation of mean SCLM compositions from garnet concentrates; this procedure gives good agreement with averages or medians of large xenolith suites of both Archean and Phanerozoic age (Table 1).

Application of this approach to garnet concentrates (>13,000 analyses) from 28 regions of different crustal age yields estimates of mean composition ("Gnt-SCLM") for SCLM of Archean, Proterozoic and Phanerozoic age (Table 2). Proterozoic Gnt-SCLM is similar to averages of orogenic peridotite massifs and xenolith suites of known Proterozoic age. Phanerozoic Gnt-SCLM and garnet peridotite xenoliths are similar to Zabargad Island peridotites, but less depleted than the average of spinel peridotite xenolith suites from extensional regions with Phanerozoic crust; these suites may include relict older SCLM. Even if the spinel peridotite data are used as an estimate of mean Phanerozoic SCLM, these data demonstrate the secular evolution of SCLM composition toward lower degrees of depletion, as measured by Al, Ca, Na, mg#, cr#, Mg.Si and Fe/Al, from Archean through Proterozoic time to the present (Table 2).

Depletion in Cr and a strong Cr-Al correlation in Archean xenolith suites indicate that Cr behaved incompatibly during generation of Archean mantle. Most Archean SCLM probably was derived by high-degree melting at depths 3150 km, with no Cr-Al phase present on the liquidus. Observed variations in olivine/orthopyroxene ratios may reflect both sorting of olivine and high-T opx, and variable degrees of melt interaction leading to more olivine-rich rocks. Comparison of SCLM xenolith suites with peridotites from convergent-margin settings and ocean basins suggests that accretion of subducted oceanic or sub-arc mantle is not a major process in the production of Proterozoic or Phanerozoic SCLM. We propose that most Proterozoic and Phanerozoic SCLM has been generated in extensional environments; typical Phanerozoic SCLM has experienced 210% melt extraction.
 

Table 1. Comparison of mean mantle compositions calculated from garnets, with median compositions of xenolith suites (after Griffin et al., 1998)
 

Table 2. Calculated mean compositions for Archean, Proterozoic and Phanerozoic SCLM (after Griffin et al., 1998)
 

References

Griffin, W.L., O'Reilly, S.Y. and Ryan, C.G. 1998. In: Y.Fei (ed.) Mantle Petrology: Field observations and high-pressure experimentation (in press).