A MULTI-PHASE, CUSPIDINE-BEARING INTERGROWTH RESULTING FROM ÅKERMANITE BREAKDOWN IN A CALC-SILICATE XENOLITH, KIGLAPAIT INTRUSION, LABRADOR

Cuspidine [Ca₄Si₂O₇(F,OH)₂] is a rare calc-silicate mineral typically confined to contact metamorphic/metasomatic environments.  In most cases, cuspidine is F-rich, reflecting interaction of rocks with F-bearing fluids.  We report an occurrence of cuspidine as part of a complex intergrowth texture formed from the retrograde breakdown of åkermanite (Ca₂MgSi₂O₇) in a calc-silicate xenolith in the Kiglapait Intrusion, Labrador.  The m-sized xenolith occurs near the southeastern margin within Lower Zone troctolite, and is surrounded by a cm-thick pyroxenite reaction zone.  Dominant minerals in the xenolith include åkermanite (X_Mg=0.97, Na-mel₁₂Gh₃), forsterite (Fo₉₃), monticellite (X_Mg=0.86), strongly zoned aluminian-ferrian diopside (≤ 10 wt% Al₂O₃), along with minor spinel.  Textures among these minerals suggest that åkermanite + forsterite represent the peak T assemblage, and that  intergrown monticellite and diopside formed during cooling (åk + fo = mtc + di).  At low pressure (~1 kb), this fluid absent reaction occurs at ~875°C, suggesting that the xenolith was heated to at least this T.  Åkermanite grains are in local optical continuity, but are separated by regions composed of fine-grained symplectic intergrowths.  Under hydrous (or fluid absent) conditions, åkermanite is known to break down to monticellite + wollastonite at ~700°C.  However, at least 5 phases have been identified in these intergrowths, including cuspidine (near ideal F-bearing composition), monticellite (X_Mg=0.93), diopside (≤ 6 wt% Al₂O₃), grandite garnet (Gr₇₀), and serpentine.  This complex intergrowth probably reflects at least a two-stage retrogression path.  The first represents the breakdown of åkermanite in the presence of a F-rich fluid:  4åk + 2HF = cusp + 2di + 2mtc + H₂O.  These product phases appear to be the dominant ones in the intergrowth, and the Al-diopside was perhaps derived from the gehlenite component in the åkermanite.  The production of water in this stage then led to further reaction(s), possibly:  3CaTs (Ca-Tschermak's pyx) + 4di + 2 mtc + 4H₂O = 3gr + 2srp.  Fluorine-rich cuspidine requires interaction with a F-bearing fluid, probably derived from the surrounding magma.  This interpretation is consistent with the known volatile content of the Kiglapait, which contained F far in excess of H₂O and Cl.