Rutile Ages and Thermometry Along a Grenville Anorthosite Pathway

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 degrees 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.

Automated summary

The Grenville Orogen in North America contains Proterozoic anorthosite massifs that were emplaced episodically. These magmas were formed through fractional crystallization of plagioclase from mafic magma. U-Pb geochronology and Zr-in-rutile analyses of the Lac Malbaie complex in Quebec provide information on the tectonothermal history of anorthosite emplacement. The rutile ages obtained suggest variable modification of cooling ages during later intrusion and fluid mobilization events.