A mechanism for Na incorporation in garnet: An example from garnet in orthogneiss from the Su-Lu terrane, eastern China

Sodian and yttrian (Na-Y) garnet occurs in orthogneisses of the Su-Lu terrane of eastern China with major quartz, microcline, and albite and subordinate biotite, titanite, hematite, and zircon. Acmite-rich pyroxene, muscovite, epidote, allanite, and hastingsitic amphibole occur in a few of the samples. The Na- Y garnet can be divided into almandine-spessartinegrossular [Al > 1.6 per formula unit (pfu) for O = 12, hereafter denoted as Al-garnet], and andradite-spessartine (Fe3+> 1.3 pfu, hereafter denoted as Fe3+-garnet) groups. Average composition of the Al-garnet in each sample ranges from Alm35Sps13Prp1Grs49Adr2to Alm21Sps67Prp0Grs5Adr7. The Fe3+-garnet contains a calderite component (Cdr) and its composition is Alm4-7Sps10-21Prp1-2Adr59-81Cdr3-12. Na, Y, and Yb contents of the Al-rich garnet reach 0.37, 1.46, and 0.42 oxide wt%, respectively, and Na/(Y + Yb) = 0.99 ± 0.16. The Fe3+-garnet contains up to 0.11,2.1, and 0.48 wt% of Na2O, Y2O3, and Yb2O3, respectively, and its Na/(Y + Yb) value (0.17 ± 0.1) is lower than that of the Al-garnet. We conclude that Na, Y, and Yb are incorporated in the Al-rich garnet by the [8]Na[8](Y,Yb)(Ca,Mn)-2substitution, whereas in the Fe3+-garnet, Y and Yb are dominantly incorporated by the [8](Y,Yb)[4](Al,Fe3+)(Ca,Mn)-1Si-1substitution with up to 3 mol% Y + Yb for Ca + Mn. Comparison ofNa/(Y + Yb) of garnets in the orthogneisses and in some pegmatites implies that increasing pressure favors incorporation of Na with Y and Yb, and that under the albite-epidote amphibolite facies or higher-pressure conditions, Y and Yb probably substitute into the garnet structure by [8]Na[8](Y,Yb)(Ca,Mn)-2, instead of the [8](Y,Yb)[4](Al,Fe3+)(Ca,Mn)-1S-1-type substitution effective under lower-pressure conditions.