|Title||The Franklin Large Igneous Province on Victoria Island, linkages between the plumbing system and the lavas|
|Author||Bédard, J H; Dell'Oro, T; Weis, D; Scoates, J; Beard, C; Williamson, N; Cousens, B; Hryciuk, M; Wing, B; Hayes, B; Rainbird, R; Ootes, L|
|Source||40th Annual Yellowknife Geoscience Forum, abstracts of talks and posters; by Watson, D M (ed.); Northwest Territories Geoscience Office, Yellowknife Geoscience Forum Abstracts Volume 2012, 2012 p. 2-3
|Links||Online - En Ligne|
|Alt Series||Earth Sciences Sector, Contribution Series 20130432|
|Meeting||40th annual Yellowknife Geoscience Forum; Yellowknife; CA; November 13-15, 2012|
|Subjects||igneous and metamorphic petrology; igneous rocks; igneous petrology; lavas|
|Abstract||The Neoproterozoic Minto Inlier of Victoria Island comprises a 4-km thick succession of sedimentary rocks (Shaler Supergroup) capped by Natkusiak Fm. Flood basalts, and intruded by coeval diabasic sills
(ca720 Ma Franklin event). The Natkusiak basalts outcrop as 2 erosional remnants (NE & SW lobes). The lowermost lavas (<100 m) are valley-filling primitive basalts (?11 wt% MgO) erupted from multiple vents, with the earliest deposits interbedded with
underlying fluvial sandstones. The unit is characterized by LREE-LILE-enrichment, high L/HREE, high87/86Sri (?0.70791), intermediate ?Nd (4.0-8.1) and ?Hf (0.03-6.7), high 208/204Pb (?39.136), high 207/204Pb (?15.686), and high 206/204Pb (?18.978).
After a hiatus marked by reworking of lavas, at least 1 km of basalt erupted as two differentiation cycles (10-6% MgO). Only a truncated section of cycle 1 basalts are preserved in the SW, however. The flood lavas in the SW have higher ?Nd (7.7-9.6),
lower 87/86Sri (0.70251-0.70605), higher ?Hf (4.1-9.7), lower 208/204Pb (36.196-37.623), lower 206/204Pb (16.147-17. 787), and lower 207/204Pb (15.383-15.605) than basal basalts. Cycle 2 lavas from the NE are even more depleted, suggesting that the
source became more depleted with time. Although the volcano-stratigraphy can be correlated throughout the area, NE lobe lavas have systematically higher ?Nd values (by about 4-5 units) than SW lobe lavas, suggesting compartmentalization of the
plumbing system, with an eruptive facies change somewhere within the NE lobe. |
The plumbing system is dominated by sills with localized dykes. Younger diabasic sills have trace element signatures matching the sheet flow lavas, whilst older sills,
some with olivine-rich bases, match the basal lavas. Data so far implies that most of the sills in the W Minto Inlier flowed towards the SW, presumably from a feeder zone located east of the head of Minto Inlet. All of the sills examined, including
some that directly underlie the SW lobe lavas, have isotopic signatures that match the NE lobe lavas, not the overlying SW lobe lavas. The plumbing system from which the SW lobe lavas erupted has yet to be discovered.
Most sills have ?34S
signatures that are systematically higher (+2 to +4) than depleted mantle values (ca. +0), suggesting that most of the magmas have acquired crustal sulfur during passage through the crust, possibly by interacting with carbonate rocks. Many
calc-silicate skarns generated by reaction between basalt and carbonate hosts have enriched ?34S signatures between +8 and +10. Small-scale sulfide immiscibility has been observed in fault-hosted dyke tips, and S-isotopic signatures imply that some
gypsum-hosted sills (?34S +11 to +13) have assimilated sulfate-rich sediments (?34S ca +15). Mixing calculations indicate that assimilation of up to 0.7% host sulfate can reproduce the intra-sill ?34S signatures, but less than half the sulfur
required by the isotopic data is still present in these contaminated sills. This suggests that the sills lost S after mixing, either by extensive S-degassing, or by loss of an immiscible sulfide phase upstream of the point of sampling.