The Composition of Sub-Continental Lithospheric Mantle: Garnet-Based Estimates

Abstract

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 Archaean mantle, and that the dominant lherzolites become progressively less depleted (in terms of major-element composition) from Archaean 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 crustmantle coupling over periods measured in aeons. In most xenolith suites, concentrations of majorand minor elements are well-correlated with A1203 contents, while in garnet peridotites there is a good correlation between the Cr203 content of the garnet and the A1203 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 aveages or medians of large xenolith suites of both Archaean 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 Archaean, Proterozoic and Phanerozoic age (Table 2). Proterozoic GntSCLM 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,