太古代武夷群煌斑岩中的钻石 Diamonds in Archean Shoshonitic Lamprophyres.pptVIP

太古代武夷群煌斑岩中的钻石 Diamonds in Archean Shoshonitic Lamprophyres - Conclusions The mixed peridotitic - eclogitic character of diamond inclusions; low-P inclusions (plagioclase) Shoshonitic character of the host rocks Old-source Metasomatism in sub-arc style low-Al websterites Orogenic Setting of host dikes and related plutonic and volcanic rocks Association with Lode Gold -- All implicate a shallow diamond source that requires a cold finger effect in the Archean Upper Mantle The P-T requirements can only be met Subduction Tectonics Diamond-Bearing Shoshonitic Lamprophyres In situ diamonds first found in 2.7 Ga lamprophyres near Wawa, then in an Abitibi occurrence. Ongoing analysis by the Ont Geol Survey continues to find more Abitibi occurrences Timing, setting, host rocks are all well-constrained Diamonds in Xenolithic Lamprophyre Dikes (2674 ± 8 Ma) and Breccia Diatremes (2679.2 ± 2.1 Ma ) In diamond host lamprophyres the matrix is inherently primitive (14 wt% Mg) ). A special magma? No, a spectrum of variably evolved 2674 Ma magmas exists but longer crustal residence times guarantees no diamond preservation in low-Mg examples. Abundant ultramafic xenoliths signifies rapid (± energetic) ascent. Metasomatised Low-Al Websterite Xenoliths: Higher Rare Earth Content = “Old depleted mantle” Nd isotope signature (previously melted) Geodynamic Constraints Any model must: Be consistent with lamprophyre geochemistry (and that of related shoshonitic syenites, trachytes, etc) that requires a subduction-style modified mantle source Meet diamond P-T requirements via a shallow mantle magma Fit the upper crustal orogenic context associated with the time of lamprophyre emplacement Be consistent with the emplacement of Orogenic (vein) Gold deposits Accommodate arc-basement style (cumulate) websterites overprinted by water-rich fluids that have “seen” old rocks Subduction Diamonds: Griffin et al. Model for NSW Another distinctive feature of the southern Superior is