Rutile Ages and Thermometry Along a Grenville Anorthosite Pathway. (18th February 2023) - Liebmann, Janne
Abstract: The Grenville Orogen, in North America, contains a belt of Proterozoic anorthosite massifs with an episodic emplacement history. These distinctive magmas reflect extensive fractional crystallization of plagioclase from a mafic magma and contain layers dominated by Ti‐bearing minerals including ilmenite and rutile. U‐Pb geochronology and Zr‐in‐rutile analyses from the Lac Malbaie complex in the Saint Urbain area of Quebec help to constrain the tectonothermal history of repeated anorthosite emplacement into the crust. Geologically significant dates were obtained from low‐U (<2 ppm) rutile in anorthosite drill cores and a boulder. Rutile ages of c. 1158, 1069–1056, and 936–910 Ma are interpreted to reflect variable modification of cooling ages during later intrusion and fluid mobilization events. Zr‐in‐rutile temperatures track higher apparent temperatures of >800°C in earlier events. Apparently younger rutile grains, and those influenced by fast pathway diffusion, record lower apparent temperatures. The Grenville Orogen comprises major trans‐lithospheric structures that facilitate the repeated bottom‐up transit of mantle‐derived magmas. At depth this led to the emplacement of anorthositic magmas whose thermal pathway is tracked by rutile passing through their Pb blocking temperature during crustal unroofing. This same broad crustal architecture, in the comparatively shallow crust, provided a favorable pathway for top‐down hydrothermal fluids to interact with younger rutile as successive anorthosites were emplaced. Rutile demonstrates that the current crustal section represents the juxtaposition of minerals preserving information from different thermal regimes, retaining vestiges of a protracted history due to incomplete re‐equilibration with subsequent chemical or thermal conditions. Plain Language Summary: Anorthosite massifs reflect the differentiation of mafic magmas through voluminous plagioclase crystallization and constitute a large portion of the Grenville Orogen in North America. This study uses U‐Pb and Zr rutile (an oxide mineral composed of TiO2 ) data to constrain the emplacement and cooling histories of anorthosites in the Grenville Province, Quebec. These results reveal multiple pulses of anorthosite emplacement, supporting the presence of large‐scale structures in the Grenville Orogen that cross‐cut the continental crust and facilitate the transit of magmas from the bottom‐up and hydrothermal fluids from the top‐down. Key Points: We report new U‐Pb rutile dates and Zr‐in‐rutile temperatures from the hitherto undated Lac Malbaie anorthosite complex, Grenville Province Rutile U‐Pb and Zr track cooling and hydrothermal processes, elucidating the tectonothermal history of the composite Lac Malbaie complex Our results imply that the Grenville Orogen established a lithospheric pathway that facilitated the repeated transit of fluids