PROGRESS IN NATURAL SCIENCE V ol .15 , No .6 , June 2005 Petrological implication of wagnerite-Ma5bc in the quartzofeldspathic gneiss, Larsemann Hills, East Antarctica＊ REN Liudong 1＊＊ , Edw ard S .GREW2 , XIONG M ing3 and WANG Yanbin1 (1 .Institute of Geology , C hinese Academy of Geological S ciences, Beijing 100037 , China;2.Departmen t of Earth Sciences, University of M aine, 5790 Edw ard T .Bryand Research Center , Orono , Maine 04469-5790 , USA;3 .China University of Geosciences, Beijing 100083 , China) Received September 23 , 2004 ;revised October 21 , 2004 Abstract Wagnerite-Ma5bc polytype is found in a gneiss associated with cordierite-prismatine-bearing gneiss, northern Stornes Peninsula, Larsemann Hills, East Antarctica.Wagnerite coexists with apatite and both of them are presen t as rock-forming minerals.Ca depletion is not essential to the formation of w agnerite;sufficient Mg , P and F components, and a relatively low Ca/ Mg ratio is critical, otherwise only apatite crystallizes.S trong anatexis of the quartzofeldspathic gneiss of the area results in differentiation and separate en rich- ment of leuco- and m elanocratic components.P and F tend to accumulate in the mafic-rich melanocratic componen t, and w hen their abun- dance reaches certain concentration , w agnerite forms.It is perhaps the chemistry of the setting that con trols the appearance of wagnerite , w hereas the PT condition is responsible for the variety of w agnerite polytype.A disordered polytype crystallizes at relatively high tempera- ture, then cools dow n slow ly and transforms into one of the ordered polytypes, in this case, the Ma5 bc polytype. Keywords: wagnerite-Ma5 bc polytype, anatexis, formation condition, volatile components, Larsemann Hills. M g-dominant minerals of w ag nerite-triplite assemblage in the study area . g roup, i .e .(Mg , Fe , M n)2 (PO 4)(F , OH), are relatively rare , but have been found in a wide variety 1 Regional geology of geologic environments, including metamorphic rocks of all g rades[ 1] such as quartz-carbonate veins of The Larsemann Hills were once situated at the hydrothermal origin , pegmatites , kyanite-quartzite , East Antarctic craton of the Gondwana superconti- mica schist , sillimanite gneiss , and magnetite ores . nent , w here the c .550 M a Pan-African tectonother- mal event w as intensively developed[ 9] .A suite of Up to now , w agnerite is present mainly as the high amphibolite-g ranulite facies rocks is present in Ma2bc poly type , an accessory mineral in calcium- this region , and among them the sillimanite-garnet poo r rocks[ 2 , 5] in w hich apatite , if present , often oc- gneisses , and the quartzofeldspathic gneisses are the curs as retrog rade rim around wagnerite .Henriques[ 6] main types , w hich are intercalated w ith pyroxene-bi- reported a small amount of w ag nerite and apatite in otite-plagioclase g neiss , biotite-plagioclase gneiss, mi- kyanite-quartzite , but the relation between the two nor granitic g neiss , hornblende-tw o pyroxene g ran- minerals w as not mentioned .The wagnerite-bearing ulite , and calc-silicates .In addition , some g ranites rocks generally have o ther phosphate minerals like and pegm atites are also present . monazite and xeno time . The gneiss containing w agnerite-Ma5bc occurs The poly type with a 5-fold b-dimension repeat in the core of w hat appears to be a fold in banded w as first discovered on Sto rnes Peninsula in the cordierite-prismatine gneiss (Fig .1).Prismatine and Larsemann Hills , East Antarctica[ 7, 8] .The Larse- cordierite also form coarse-g rained segregations and mann Hills rock is distinctive in that wagnerite coex- nodules associated w ith quartzofeldspathic seg rega- ists with apatite and bo th are present as major rock- tions . Given the presence of Al-rich minerals forming constituents , w hich has not been reported cordierite and prismatine , the gneisses illustrated in befo re .This paper focuses on the petrolog ical implica- Fig .1 most likely have sedimentary precursors that tions and geological setting of the w agnerite-apatite were subsequently modified by migm atization . ＊S upported by the National Natural Science Foundation of China (Grant No.40172065 ), the Chinese Geological Survey (Grant No . 200413000023)and Programme of Excellent Young Scientists of the Ministry of Land and Resources ＊＊ To w hom correspondence should be addressed .E-mail:ldren @cags.net .cn 524 w ww .tandf.co .uk/journals Prog ress in Natural Science Vol .15 No .6 2005 The g neiss lacks obvious foliation .Majo r con- cracks and g rain boundaries of w ag nerite-Ma5bc stituents are plagioclase , w ag nerite-Ma5bc , apatite (Fig .1).Sy mplectitic intergrow ths of cordierite and m agnetite ; subordinate are biotite , quartz , w ith K-feldspar were developed (Fig .2), w hich may potassium feldspar , cordierite , minerals of the mon- correspond to the sapphirine-orthopy roxene symplec- azite and xenotime g roups , co rundum , lamellar tite after w ag nerite in Eastern Ghats Land , India[ 14] . hem atite-ilmenite interg row ths , hercynite , and sul- In sum mary , tex tures are consistent w ith a primary fide . assemblage :wagnerite-Ma5bc + plagioclase + ap- atite(1) + mag netite + ilmenite-hem atite . The conditions for the metamorphic peak gran- ulite facies in the Larsem ann Hills are 750 to -860 ℃ at 0 .6 —0 .7 GPa[ 10 —13] . Fig .2 . BSE image of Kfs-Crd symplectite after wagnerite (the background is Kfs). Although interlayered w ith cordierite-prismatine gneiss, the w ag-bearing gneiss has no t the contact Fig.1 . BS E image of Wag-Ap and late Kfs and Ap2. tex ture betw een w ag and prs .By combining the rela- tively earlier crystallization of wag than monazite- 2 Petrography and texture feature xenotime in the g neiss w hile in the prs g neiss mnz- x nt formed earlier , the possible mineral cry stallization Generally situated between mineral g rains such sequence relative to wag can be summarized as follow s [ 28] as plagioclase , wagnerite has a straight contact with (mineral abbreviations are after Kretz ): apatite (Fig .1), w hich suggests the essentially co- Bt , Pl※Mt -Ap1 +Wag +Mnz +Xnt +Ilm existing feature of the tw o minerals .Wag nerite local- -Hem +Crn ※Prs※Kfs , ly occupying the position of apatite implies the earlier Crd -Kfs ;Ap2 ※pinite . beginning of apatite and late finishing of w ag nerite . Predating prism atine and the crd-kfs sy mplectite , the Following apatite and w agnerite are monazite and fo rmation of w agnerite is related to the biotite dehy- xenotime in small amount , and betw een them are ba- dration reactio n w ith plag ioclase involved in anatexis, sically co-existing relationships .Meanw hile , opaque and should be the stage of g arnet in quartzofeldspathic minerals like m agnetite , ilmenite and hematite are gneisses , that is, wagnerite formed at the peak meta- formed , and their volume in the rock can reach 15 %. morphism . Wagnerite-Ma5bc forms anhedral to euhedral Semi-quantitative energy spectrum com position g rains mostly 0 .5 —2 mm across (the maximum is scanning demonstrates that from Ap1 to Ap2 , Mg and about 2 .5 mm).Some g rains have a tabular habit and P tend to increase , w hile Na , Ca , Fe , and S tend to the thickness can be less than 0 .1 mm ;a few of them decrease . show two terminal faces .Wagnerite-Ma5bc is com- 3 Crystal structure of wagnerite monly dusty with fine inclusions o r discolored by in- cipient alteration .Textures suggest two generations By using a Bruker Smart Apex CCD sy stem and of apatite :relatively coarse grains with alig ned acicu- g raphite-monochrom ated Mo Kαradiation , the crystal lar inclusions ; rare margins fring ing w agnerite- structure of w agnerite is measured and the unit-cell Ma5bc (Fig .1)or overgrow ths separating w ag ner- parameters w ere obtained by least-squares refine- ite-Ma5bc from oxide and sulfide inclusions .Micro- ment , a =9 .645(2) , b =31 .659(6) , c = scopic seams of apatite have also developed along 11 .914(2) , β =108 .26 (3)°, V =3455(1) 3 , Prog ress in Natural Science Vol.15 No .6 2005 w ww .tandf.co .uk/journals 525 and Z =40 .The wagnerite-Ma5bc[ 7 , 8] , space group 4 Mineral chemistry Ia , distinct to that of the type w ag nerite , 2b poly- type (space g roup P 2/ a), is a new poly type .The Microprobe analyses of w ag nerite are given in prim ary difference betw een the w ag nerite-Ma5bc and Table 1 .Assuming Fe , Ti , Ca , and M n substitute w agnerite-Ma2bc is ordering of the (F , OH)posi- fo r M g , all the spots(except spots 1-1 and 3-1) tions.F can occupy one of the two positions , w hich give the average composition as follows :(Mg19.050 can result in tw o distinct configurations along the a Fe1.006Ti0.192Ca0 .030 Mn0.023)20.301(P9.982Si0.018)10.000 direction .Other polyty pes have been summarized by O40 F6 .485 (OH )3 .545 ;the presence of hydroxy l was Chopin et al .[ 15] . confirmed by Raman spectroscopy . Table 1 . Microprobe analyses of w agneritea) Oxides 1-1 1-2 2-2 2-3 2-5 2-6 2-7 3-1 3-2 3-3 4-1 8-4 8-2 MgO 43.59 44 .13 44 .99 45 .15 44 .29 45 .05 45 .07 42 .59 44 .45 43 .85 44 .80 44 .85 45 .03 SiO 2 0.064 0 .056 0 .061 0 .077 0 .053 0 .027 0 .080 0 .051 0 .057 0 .053 0 .084 0 .072 0 .075 CaO 0.088 0 .105 0 .081 0 .084 0 .071 0 .075 0 .080 2 .374 0 .089 0 .097 0 .084 0 .129 0 .158 TiO 2 0.974 0 .954 0 .851 0 .826 0 .821 0 .745 0 .722 0 .975 1 .028 0 .949 0 .974 1 .007 0 .924 MnO 0.084 0 .096 0 .088 0 .058 0 .095 0 .078 0 .106 0 .089 0 .132 0 .121 0 .065 0 .088 0 .12 FeO 4.153 4 .809 4 .236 4 .100 4 .610 2 .563 4