GEMOC ARC National Key Centre

W.L. Griffin^1,2, Suzanne Y. O'Reilly¹, N.I. Fisher1,^1,3, J. Friedman^1,4, C.G. Ryan^1,2 and E. van Achterbergh¹

¹GEMOC Macquarie
²CSIRO Exploration and Mining
³CSIRO Mathematical and Information Sciences
⁴Dept. of Statistics, Stanford University

A statistical analysis of >12,000 analyses of garnet xenocrysts has revealed significant differences related to the tectonothermal age of the crust penetrated by the host volcanic rock [1]. To explore these differences, we are developing new statistical approaches to define natural populations in the garnet database. Some preliminary results are presented here to show the power of the method. CARP (Cluster Analysis by Recursive Partitioning) compares the real data with a "null" Monte Carlo set containing only clusters based on concentrations in each variable, to define clusters in the real data. We have identified 12 populations, accounting for 75% of the data. Comparison with xenolith data suggests that these populations represent consistent combinations of features produced by both primary depletion and later metasomatic modification. The correlation of some populations with xenolith types is shown below, with examples of their distribution in SCLM sections. Each garnet grain has been assigned to a CARP population, and placed in depth context by reference of a nickel-based equilibration temperature ( T_Ni ) to the localgeotherm.

Archean SCLM sections tend to be strongly stratified, and the degree of depletion decreases downward. The Slave craton is an extreme example of this type, with a highly depleted upper part separated from a more lherzolitic lower part by a boundary zone <10 km thick; this structure has been mapped across >12,000 km2 [2]. The Kaapvaal craton SCLM is less depleted but still markedly layered; granular phlogopite-bearing lherzolite (H10A population), which is a minor rock type in the Dalydn and Slave sections, is abundant at shallow depth, while deformed and metasomatised lherzolites (H13 garnet population) increase with depth. Proterozoic sections such as the Yangtze craton show a smaller range of rock types; harzburgites are rare, and stratigraphic layering is less pronounced. Typical Phanerozoic SCLM (not shown) is dominated by fertile spinel lherzolites, most of which have undergone <10% melt extraction; 90% of the garnet peridotites in these sections (typically near the base, due to high geotherms) classify as H10B, and are even less depleted than the spinel lherzolites.