Zircon Trace Element and Hf Isotopic Composition of Granitoid Rocks in the eastern Selwyn Basin, Northwest Territories: a reconnaissance-scale studyThursday, November 21, 2019 - 2:40pm to 3:00pm Theatre One
Crustally derived intermediate to felsic plutons were emplaced across the eastern Selwyn Basin (SB) in the Northwest Territories within a post-accretionary tectonic setting from ~100-90 Ma. This magmatism is economically notable for the association between world-class W skarn mineralization and the Tungsten plutonic suite in the region. Recent work has divided these plutons into several suites and sub-suites on the basis of U-Pb age, whole rock geochemistry and radiogenic isotopic composition, and demonstrated that the magmas were likely derived from intermediate-composition infracrustal rocks that are more isotopically evolved in the northeast SB (older ± more felsic). The influence of magmas derived from supracrustal rocks and the mantle is also apparent; however, these melt sources are now interpreted to be less significant in the eastern SB than previously thought. Detailed compositional investigations of magmatic and inherited zircon can provide direct information on the age and composition of melt sources in the middle to lower crust underlying the eastern SB. A pilot study of trace element (TE) and Hf isotopic composition of zircon (magmatic rims and antecrystic cores) was conducted on samples from representative plutons in the eastern SB to test and better constrain earlier conclusions regarding magma petrogenesis. This study also included a limited examination of inherited zircon cores to further elucidate the age and composition of crustal melt sources.
Initial results generally support the earlier petrogenetic interpretations regarding the composition of magma source rocks, as well as the previously noted change in melt source composition from southeast to northeast. Zircon morphologies vary between plutonic suites, reflecting differing magma temperatures and alkalinity during precipitation. The range of TE compositions of magmatic zircon in the southeast is compositionally restricted and has a relative HREE-enrichment, whereas magmatic zircon from the northeast is more compositionally variable and LREE- and Th-enriched. TE contents and ratios (e.g., La, Ti, Yb/Gd, Lu/Hf) and εHf100 vary with plutonic suite and geographically from southeast to northeast. In the southeast SB, magmatic zircon (rims and antecrystic cores) and inherited zircon all have similar Hf isotopic compositions, which are consistent with a relatively ‘young’ infracrustal melt source (TDM = 1.1-1.5 Ga). In zircons from the northeast SB, Hf isotopic compositions of magmatic rims and antecrystic cores indicate the involvement of a slightly more evolved or older melt source (TDM = 1.4-1.8 Ga), whereas inherited cores are significantly more isotopically evolved and consistent with the incorporation of zircons derived from much older rocks (TDM = 2.0-2.75 Ga). Zircons from samples of the Tungsten plutonic suite in the southeast and northeast SB also display these regional variations. Although limited, these preliminary data complement previous interpretations but also provide new petrogenetic information on the plutonic suites and sub-suites that raises further questions and highlights the complexity of magmatism in the eastern SB. Additional work is needed to better define geographic and intra-/inter-suite variation in magmatic zircon composition, and systematic analysis of inherited zircon will better constrain the age and composition of the lower to mid-crustal melt sources.