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 pyro xene, muscovite, epidote, allanite, and hastingsitic amphibole occur i n a few of the samples. The Na-Y garnet can be divided into almandine- spessartine-grossular [Al > 1.6 per formula unit (pfu) for O = 12, her eafter denoted as Al-garnet], and andradite-spessartine (Fe3+ > 1.3 pf u, hereafter denoted as Fe3+-garnet) groups. Average composition of th e Al-garnet in each sample ranges from Alm(35)Sps(13)Prp(1)Grs(49)Adr( 2) to Alm(21)Sps(67)Prp(0)Grs(5)Adr(7). The Fe3+-garnet contains a cal derite component (Cdr) and its composition is Alm(4-7)Sps(10-21)Prp(1- 2)Adr(59-81)Cdr(3-12). Na, Y, and Yb contents of the Al-rich garnet re ach 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 Na-[8]([8])(Y,Yb) (Ca,Mn)(-2) substitution, whereas in the Fe3+-garnet, Y and Yb are dom inantly incorporated by the ([8])(Y,Yb)([4])(Al,Fe3+)(Ca,Mn)(-1)Si--1 substitution with up to 3 mol% Y + Yb for Ca + Mn. Comparison of Na/(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 c onditions, Y and Yb probably substitute into the garnet structure by N a-[8]([8])(Y,Yb)(Ca,Mn)(-2), instead of the ([8])(Y,Yb)([4])(Al,Fe3+)( Ca,Mn)(-1)Si--1-type substitution effective under lower-pressure condi tions.