GEOCHEMICAL CHARACTERISTICS OF XENOLITHS IN ARCHEAN DIAMONDIFEROUS LAMPROPHYRES: IMPLICATIONS FOR THE ARCHEAN MANTLE

Diamonds occur in Archean shoshonitic lamprophyres across the southern margin of the Superior Province. This study reports data for xenoliths in diamond-hosting lamprophyres of the 2.97 - 2.7 Ga eastern Wawa subprovince. Four groups of xenoliths have been recovered from the ~ 2.674 Ga lamprophyres, including low-Al websterites, eclogitic or mafic lithologies, and pyroxenites that fall into both the “mantle” and “arc” Mg#-Al₂O₃ fields. The consistent HFSE characteristics of Group I samples, combined with a trend to higher εNd_T at greater LREE contents, indicates that they originally had arc tholeiite-like characteristics (LREE depletion, minor negative to positive Nb and negative Ti anomalies) prior to a late metasomatic event. Other xenolith groups are mainly related to the late tectonic shoshonite magmatism responsible for syenites and the host lamprophyres. Trace elements distinguish two styles of metasomatism: i) LILE enrichment caused by hydrous fluids and ii) combined LILE and Zr-Hf ± Nb-Ta enrichment due to small degree melts. εNd_T values for the xenoliths extend over a wide range from -3.3 to 29.7 and indicate that both processes sporadically involved long-term depleted reservoirs, most plausibly in the shallow sub-arc lithospheric mantle. Consistent with the recognition that so-called kimberlitic indicator minerals in nearby sediments are actually derived from the lamprophyres, the xenoliths provide no evidence of the deep (~ 150 km) keel now observed beneath the southern Superior Craton. Given that such keels are characteristic of Archean cratons worldwide, the apparent dilemma can be explained by a geodynamic history involving mantle plume – subducted slab interaction similar to that defined by recent 3D laboratory and computer-based models. The results have significant implications for the unusual abundance of large and giant orogenic gold deposits along the southern Superior Province margin.