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Abstract
AbstractA novel way to investigate the petrogenesis of ancient polymetamorphosed terranes is to study zircon‐hosted mineral inclusions, which are sensitive to melt evolution such as apatite. Recent contributions on such inclusions in unmetamorphosed granitoids can provide valuable petrogenetic information and, in turn, represent a way to circumvent effects of metamorphism. Yet the impact of metamorphism on apatite inclusion has never been studied in detail. To address the issue of chemical and isotopic preservation of primary signals in apatite crystals both in the matrix and armored within zircons, we have studied apatite crystals from four 3.6–4.0 Ga TTG granitoids from the Acasta Gneiss Complex (Canada). Our results demonstrate that U‐Th‐Pb isotope systematics in matrix apatite crystals were reset at 1.8–1.7 Ga (Wopmay orogen) whereas primary REE signatures were preserved in many crystals. In contrast, zircon‐hosted apatite inclusions all preserved primary REE signatures despite variable ages between 1.7 and 4.0 Ga. We interpret reset ages to be a consequence of metamorphism that managed to affect U‐Th‐Pb systematics because of advanced radiation damage accumulation in host‐zircon lattices. Only the most pristine zircon crystal has an apatite inclusion with a concordant age consistent with the magmatic age of the zircon (4.0 Ga). In addition, our results show that apatite crystals from TTG have distinct REE composition from post‐Archean granitoids apatites, that is preserved even in some apatites with reset ages. This capacity to retain primary information and discriminate granitoid types makes apatite a very valuable tool for reconstructing the nature and evolution of ancient crustal rocks through the use of detrital minerals.
Highlights
Rare-Earth bearing minerals such as apatite (Ca5(PO4)3(OH, Cl, F)), have proven to be extremely valuable in retrieving the nature of their host magmas and the crystallization history of granitoids (Chu et al, 2009; Jennings et al, 2011; Bruand et al, 2016)
Chondrite-normalized REE patterns of apatites from the matrix define two groups: group 1 shows depleted LREE compared to HREE and a negative Eu anomaly (Eu/Eu*= EuN/√[(SmN).(Gd)N]< 0.5; Fig. 3A, Table 2) whereas group 2 patterns are sub-parallel to group 1 but with lower REE content and no Eu anomaly
We were able to demonstrate that primary REE composition of apatite in the matrix can be preserved for large crystals, whereas their U-Th-Pb isotope systematics were fully reset by HT metamorphism around 1.8-1.7 Ga
Summary
Rare-Earth bearing minerals such as apatite (Ca5(PO4)3(OH, Cl, F)), have proven to be extremely valuable in retrieving the nature of their host magmas and the crystallization history of granitoids (Chu et al, 2009; Jennings et al, 2011; Bruand et al, 2016). They can be very useful complements to zircon in crustal evolution studies (e.g., Iizuka and Hirata, 2005; Belousova et al., 2010; Dhuime et al, 2012). They have shown that Sr/Sm ratio in apatite can discriminate felsic magmas from mafic ones and, as previously suggested by Belousova et al (2001), that Sr content in apatite inclusions can be used as a proxy for the degree of differentiation of their parental magma (evaluation of the SiO2 and Sr contents)
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