Christian O. Böhm

U–Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array

This study reports U–Pb geochronological data for zircon obtained by laser ablation-multi-collector-ICP-MS using a new collector block design that includes three ion counters and twelve Faraday buckets. The collector configuration allows for simultaneous detection of ion signals from mass 238U to 203Tl, an important factor for the achievement of highly precise and reproducible Pb–Pb and Pb–U ratios. The main advantage of the multiple ion counting system is the capability to readily measure low Pb ion signals ( 50) of zircon grains.

Discovery of pre-3.5 Ga exotic crust at the northwestern Superior Province margin, Manitoba

New U-Pb geochronological and Sm-Nd isotopic results across the northwestern margin of the Superior Province in Manitoba, Canada, require a major reinterpretation of the nature, structure, and location of the Superior–Trans-Hudson boundary in the area. A significant finding of this study is that an area northwest of Assean Lake, previously interpreted to be dominantly Early Proterozoic, contains ancient crust. Detrital zircon U-Pb results from two metagraywacke samples in this ancient crustal segment reveal ≥ 3.75 Ga source material, consistent with crustal residence Nd model ages ( T CR ) of 3.85 and 3.76 Ga, and the U-Pb zircon age for one tonalite gneiss sample is ca. 3.54 Ga ( T CR of 3.63 Ga). This newly discovered pre-3.5 Ga crust provides the first evidence for the possible preservation of some very old rocks (pre-3.8 Ga) in this region, older than any known Superior Province crust.

Chapter 6.2 The Assean Lake Complex: Ancient Crust at the Northwestern Margin of the Superior Craton, Manitoba, Canada

Publisher Summary This chapter describes the Assean Lake Complex (ALC) at ancient crust at the Northwestern margin of the Superior Craton, Manitoba, and Canada. An initial tectonic model for the Assean Lake area indicated that a regionally extensive high-strain zone running through the lake marks the suture between Archean high-grade crustal terranes of the Superior Craton to the southeast and Paleoproterozoic rocks of the Trans-Hudson Orogen to the northwest. Detailed geologic remapping combined with isotopic and geochemical studies led to a re-interpretation of the crust immediately north of the Assean Lake high-strain zone as Mesoarchean. The study area straddles the boundary between the Archean Superior Craton and the ca.1.90–1.84 Ga arc and marginal basin rocks of the Trans-Hudson Orogen, which represent the remains of ca. 1.83–1.76 Ga ocean closure and orogeny. It is indicated that the gneisses of the Split Lake Block consist primarily of meta-igneous protoliths of gabbroic to granitic composition. Tonalite and granodiorite are the most volumetrically dominant, but an anorthosite dome is also present in the northeast. Mapping, isotopic, and age data combined with high-resolution aero-magnetic data indicate that the Mesoarchean ALC is a crustal slice up to 10 km wide, and has a strike length of at least 50 km.