Mantle processes in an Archean orogen:Evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths

摘要

The world's oldest diamond deposits occur in approx 2.67 Ga dikes and heterolithic breccias emplaced into greenstone belts of the Wawa and Abitibi Subprovinces,southern Superior Province,Canada.Thousands of white to yellow microdiamonds and macrodiamonds to 5 mm in width have been recovered by non-contaminating fusion techniques.The host rocks exhibit petrographic and compositional features that are characteristic of post-Archean minettes and spessartites of the calc-alkaline or shoshonitic lamprophyre clan.Based on chemical trends and petrographic evidence,host rocks that contain more than approx16 wt.% MgO represent lamprophyre magmas that entrained cumulate olivine,probably at the base of the crust.Breccia bodies that are tens of metres wide at the two localities are somewhat atypical of late Archean lamprophyre occurrences in the Superior Province and owe their size to optimum conditions for magma ascent that were required to preserve the diamonds.Abundant altered ultramafic xenoliths occur in the host rocks.The majority of xenoliths studied(10 of 14)display uniform major element compositions similar to websterite cumulate suites derived from crystal fractionation processes at the base of post-Archean volcanic arcs.The xenoliths display highly variable trace element abundances that are characteristic of cryptic metasomatism associated with the flux of an oxidised fluid above a subduction zone.The tectonic setting of the deposits and the nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths(100 to 160 km)rather than the deep keels of Archean cratons associated with traditional diamond deposit types.Models of low-temperature Phanerozoic diamond formation in active subduction zones,or rapid uplift and emplacement of peridotite massif occurrences,can be adapted to the Archean deposits.The stability field of diamonds in most Phanerozoic subduction scenarios,however,may be too deep to be accessed by the lamprophyric magmas.In contrast,shallow subduction,as invoked for the distinctive occurrence of adakitic(slab-melt)type rocks in the southern Superior Province,could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas.The multiple requirements imposed on Archean tectonic models by occurrences of diamonds in hydrous shoshonitic rock types(spessartite and minette lamprophyres),along with distinctively metasomatised xenoliths,strongly favour plate tectonic subduction models of orogeny.Evidence of slightly earlier mantle plumes,such as 2.7 Ga komatiites,only strengthens the need for a subduction-driven low-temperature thermal anomaly in the Archean mantle prior to lamprophyric magmatism.