Insights into garnet growth in S-type granite from Lu–Hf dating and trace element mapping

Abstract

Garnet is a common minor phase in S-type granites and pegmatites, but its petrogenesis remains poorly constrained. Garnet in these systems may have crystallized from the melt or it may represent inherited grains derived from the source or xenocrysts from the wall rocks. Although garnet has the potential to provide unique insights into the magmatic evolution and crystallization intervals of S-type granites and pegmatites, its geochemical and chronological record is underexplored. In this study, we investigate the Lu–Hf age and trace-element record of garnet in the Neoarchean Decelles Batholith (c. 2670–2620 Ma), southeastern Superior Craton, Canada. The batholith comprises one of the most voluminous granites in the region and was likely sourced from anatexis of the metasedimentary Pontiac Group at depth. Garnet Lu–Hf geochronology yielded ages between 2667.5 ± 3.9 and 2656.0 ± 7.1 Ma, overlapping with U–Pb monazite ages from the batholith and Lu–Hf ages from the host rocks. LA-ICP-MS trace-element mapping revealed well-preserved sharp oscillatory—locally sector—zoning in Li, P, Sc, Ti, Y, Zr, REE, Hf, Th, and U, contrasting with weak major element zoning. Garnet grains exhibit a core with concentric zoning and an overgrowth domain truncating core patterns, reflecting both trace-element uptake controlled by varying crystal growth rates, element supply and diffusion at the garnet-matrix interface in the presence of melt, and cation supply limitations due to co-crystallization of muscovite, monazite, apatite, and zircon. The data support a magmatic origin for garnet in the peraluminous granite and demonstrate that the oscillatory zoning can be diagnostic. Moreover, the new Lu–Hf garnet dates place new constraints on the timing of crystallization of the Decelles Batholith. This study provides new insights into the conditions of garnet crystallization in granitic systems and illustrates the versatility of garnet in constraining the onset and later evolution of peraluminous granitoid magmatism. Ultimately, our study underscores the necessity of case-by-case assessment of garnet origins in S-type granites, emphasizing trace-element mapping as a key tool for petrogenetic interpretation.