Coupled dichotomies of apatite and fluid composition during contact metamorphism of siliceous carbonate rocks

Abstract

The X CO 2 recorded by mineral-fl uid equilibria in contact metamorphosed siliceous carbonates commonly defi nes two groups of rocks in the same aureole. One group records relatively low X CO 2 that results from infi ltration of chemically reactive H 2 O-rich fl uid. The other records relatively high X CO 2 , up to 0.99, that results from decarbonation reactions with little or no infi ltration. A complementary dichotomy in apatite compositions exists in fi ve contact aureoles in Italy, Scotland, and the U.S.A. Apatite in the low-X CO 2 group is close to an F-OH solid solution. Apatite in the high-X CO 2 group is a relatively Cl-rich Cl-F-OH solution. The halogen content of fl uid coexisting with analyzed apatite was characterized in two aureoles to determine the origin and signifi cance of the dichotomy in apatite composition. Calculated a HF /a H 2 O , a HF /a HCl , a HF , and m F T (the total F molality of fl uid) are systematically higher in fl uid coexisting with the low-X CO 2 group. In contrast, a HCl /a H 2 O in the high-X CO 2 group may be higher than or overlap with a HCl /a H 2 O in the low-X CO 2 group. Calculated a HCl and m Cl T in the high-X CO 2 group are lower than or overlap with a HCl and m Cl T in the low-X CO 2 group. The Cl-rich apatites in the high-X CO 2 group are explained by crystallization at relatively low a H 2 O , a HF , and m F T rather than at high a HCl or m Cl T . In comparison, the F-OH apatites in the low-X CO 2 group formed by infi ltration of rock by and equilibration with relatively H 2 O-rich, high m F T /m Cl T fl uid, refl ecting the same metasomatic process responsible for F-rich humite-group minerals and skarns in many contact aureoles. Calculated halogen contents indicate that the non-CO 2 fraction of fl uid in equilibrium with both groups had modest, seawater-like salinity, and that the reactive H 2 O-and F-rich fl uid that infi ltrated the low-X CO 2 group had a plutonic source. Rock groupLow-X CO 2 High-X CO 2 X CO 2 0.13-0.28 0.96-0.99 Poorly constrained Notes: Retrograde minerals listed in italics. Minerals in bold formed during both prograde and retrograde metamorphism. Retrograde brucite formed after peak periclase. Mineral abbreviations: Act = actinolite, Ap = apatite, Bad baddeleyite, Brc = brucite, Cal = calcite, Cam = calcic amphibole, Chl = chlorite, Chn = chondrodite, Chu = clinohumite, Czo = clinozoisite, Di = diopside, Dol = dolomite, Fo = forsterite, Gk = geikielite, Gr = grossular, Hu = humite, Kfs = K-feldspar, Mag = magnetite, Ms = muscovite, Nrb = norbergite, Phl = phlogopite, Pl = plagioclase, Po = pyrrhotite, Prg = pargasite, Py = pyrite, Qnd = qandalite, Qtz = quartz, Rt = rutile, Scp = scapolite, Spl = spinel, Srp = serpentine, Tr = tremolite, Ttn = titanite, Urn = uraninite; Wo = wollastonite, Zrn = zircon.