U–Pb geochronology and trace element composition of zircon from the Horseshoe Lake greenstone belt, Superior Province, Canada: implications for the tectonic and metamorphic history

Abstract

Depth profile techniques for U–Pb geochronology and rare earth element (REE) geochemistry were conducted on unpolished, Archean zircon with metasomatic rims from metasedimentary rocks within the Horseshoe Lake greenstone belt, western Superior Province, Canada. These zircon crystals are shown to have isotopically distinct rims (typically <5 μm thick) compared with the interiors of the crystal. Secondary ion mass spectrometry (SIMS) U–Pb depth profile analyses of the rims define two different 207Pb/206Pb age populations at ca. 2920 and 2869 Ma, which are >100 million years younger than the cores. The 207Pb/206Pb rim ages can be temporally correlated with regional magmatism and with a later, potentially Au-bearing, hydrothermal event synchronous with greenschist-facies metamorphism and regional deformation. Notably, the zircons do not record evidence of local ca. 2741–2715 Ma magmatism manifested by the emplacement of a quartz–feldspar porphyry dike swarm. Laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) REE depth profile analyses on the same unpolished zircon show that the rims are commonly characterized by low Th/U ratios, elevated Hf, and variable REE concentrations in comparison with the interior of the grains. The variations in rim and core chemistry suggest that these elements, along with common Pb, were mobilized by fluids and interacted with zircon in the metasediments, resulting in the production of the metasomatic rims. In summary, this paper shows that depth profiling techniques applied to unpolished zircon can be useful to elucidate the tectonic, and potentially metallogenic, history of a complex Archean terrane.

Zircon Macrocrysts from the Drybones Bay Kimberlite Pipe (Northwest Territories, Canada): A High-Resolution Trace Element and Geochronological Study

“…In the present work, we did not observe any evidence of zircon re-equilibration with invading melts or fluids, such as overgrowths or fracture fillings differing from the bulk of the macrocrysts in their trace element budget (e.g., [33]). In fact, the measured REE distributions are remarkably consistent in the Drybones Bay samples irrespective of their CL response ( Figure 5).…”

“…These compositional characteristics are consistent with kimberlitic zircon [32] and rule out crustal rocks or the Defeat granitoids as a potential source of these macrocrysts (cf. [33]). Another notable characteristic of the studied material is its enrichment in Sc (570-930 ppm), which is far in excess of what has been reported for felsic rocks [34].…”

“…Neither Eu nor Y anomalies are observed, and the average Y/Ho ratio is essentially chondritic (25 ± 2). The REE characteristics of zircon (i.e., the shape of chondrite-normalized profiles, Ce, Eu, and Y anomalies) are sensitive to its interaction with fluids and melts [33,35,36]. Because the Y/Ho ratio shows the least variation across the dataset, we used it as a "common denominator" to gauge the extent of Pb loss (or gain).…”

A zircon LA-ICPMS reverse depth profiling analysis method and its geological application

“…Previous studies have applied depth proling to the U-Pb dating and trace element analysis of metamorphic zircon, detrital zircon, monazite, and apatite with complex interior structures. 22,[31][32][33][34] The depth proling technique allows determining the age and trace elements of zircons and other accessory minerals with complex interior structures. However, there are still some problems in the geological application: (1) without polishing the zircon before ablation, the internal structure of the grain is unknown, and the results cannot be predicted before/during laser ablation.…”

“…Previous studies have applied depth profiling to the U–Pb dating and trace element analysis of metamorphic zircon, detrital zircon, monazite, and apatite with complex interior structures. 22,31–34…”