Geochemistry and origin of the Neoproterozoic Natkusiak Flood Basalts and related Franklin Sills, Victoria Island, Arctic Canada

Abstract

ABSTRACTlacked correspondingly high Lu/Hf. We suggest that the source region for the southwestern Natkusiak basalts and related sills included isotopically matured oceanic crust, which was recycled through the asthenospheric mantle into a laterally heterogeneous plume. The distinct trace element signatures of the southern and northern sources became attenuated with the onset of voluminous melting (corresponding to emplacement of the Type 2 doleritic sills and sheet flow basalts) and may reflect contributions from hydrous eclogitic material emplaced into the lithospheric mantle during the ca. 1.9 Ga Wopmay Orogeny. As both the northern and southern volcanic rocks exhibit contrasting isotopic signatures throughout the preserved stratigraphy, the magma plumbing system must have experienced limited lateral mixing and homogenisation, which allowed for the expression of distinct mantle source signatures in the high-level sills and basaltic lavas.

Emplacement of the Franklin large igneous province and initiation of the Sturtian Snowball Earth

“…The best exposures of the intrusive and extrusive relationships of the Franklin LIP units are on Victoria Island where the bulk of the volcanostratigraphic relationships has been established, also providing a geochemical framework for the rest of the Franklin LIP (23,(25)(26)(27). On Victoria Island, the >4-km-thick Shaler Supergroup includes organic-rich shale and sulfate evaporites (28,29) and is succeeded by a >1-km-thick flood basalt sequence, the Natkusiak Formation (30).…”

“…The Natkusiak Formation consists of basal rubbly flows, volcaniclastic units, and thick sheet flows of basalt (26,32). Franklin sills intruded the basal Natkusiak Formation and have been linked stratigraphically with crosscutting dykes and geochemically with different extrusive flow types (11,26,27), providing coarse-grained equivalents to the extrusive units that can be more easily and reliably dated. Evidence for glaciation has not been documented below or within the Natkusiak Formation, and the sub-Cambrian unconformity (33) precludes further stratigraphic constraints on the relationship between the Franklin LIP and the Sturtian glaciation.…”

“…Most of the Franklin LIP has tholeiitic compositions of 45 to 51 wt % SiO 2 and 6 to 11 wt % MgO ( 24 , 27 ). Low-Ti [<1.2 wt % TiO 2 ; ( 36 )] type 1 intrusive and volcanic rocks are stratigraphically below or are crosscut by the younger, high-Ti type 2 igneous rocks and tend to have incorporated more crustal material, resulting in relatively lower Nb/La ratios, enriched LREEs and steeper REE slopes, and lower initial ε Nd (−6.1 to −0.8) and ε Hf (−4.3 to +4.6) values ( 27 , 36 ).…”

“…Most of the Franklin LIP has tholeiitic compositions of 45 to 51 wt % SiO 2 and 6 to 11 wt % MgO ( 24 , 27 ). Low-Ti [<1.2 wt % TiO 2 ; ( 36 )] type 1 intrusive and volcanic rocks are stratigraphically below or are crosscut by the younger, high-Ti type 2 igneous rocks and tend to have incorporated more crustal material, resulting in relatively lower Nb/La ratios, enriched LREEs and steeper REE slopes, and lower initial ε Nd (−6.1 to −0.8) and ε Hf (−4.3 to +4.6) values ( 27 , 36 ). Type 1 rocks also include olivine-rich cumulates and generally have higher wt % MgO ( 25 , 27 ).…”

“…Type 2 rocks are generally more differentiated; are dominated by clinopyroxene and plagioclase; and have lower Ce/Yb ratios, higher Nb/La ratios, and higher initial ε Nd values ( 27 ). Type 2 Franklin sills and upper Natkusiak sheet flow basalts have values of ε Nd = +1 to +8.8 and ε Hf = +4.7 to +15.8, which indicate lesser degrees of contamination by continental crust ( 27 ).…”