Intergrowths between anthophyllite, gedrite, calcic amphibole, cummingtonite, talc and chlorite in a metamorphosed ultramafic rock of the KTB pilot hole, Bavaria

Abstract

A metamorphosed ultramafic rock, penetrated by the KTB pilot hole at a depth of 1382.36 m, contains the assemblage Ca-amphibole — anthophyllite — chlorite — talc, formed at about 630°C/10 kbar under conditions of the high-P amphibolite facies. EMP analyses of the Ca-amphibole yielded magnesiohastingsite (to pargasite) compositions with (Al+Fe 3+ +Cr+Ti) [6] of 1.0-1.2 p.f.u. and (Fe 2+ +Mn)/(Mg+Fe 2+ +Mn) ratios of 0.10-0.16, while coexisting anthophyllite has (Al+Fe 3+ +Cr+Ti) [6] ≤ 0.16 and (Fe 2+ +Mn)/(Mg+Fe 2+ +Mn) ratios of 0.21-0.29. Chlorite is clinochlore with Al [4] /(Al [4] +Si) of 0.27-0.30 and Fe 2+ /(Fe 2+ +Mg) of 0.12-0.14. TEM investigations revealed, for the first time, a complex lamellar intergrowth in the sequence talc → anthophyllite → anthophyllite/cummingtonite intercalations → anthophyllite → Ca-amphibole. This lamellar intergrowth is about 15 μm in width, but individual cummingtonite lamellae intercalated with dominant anthophyllite are only 1 μn wide. All the amphiboles share the directions b * and a * ; anthophyllite and the monoclinic amphiboles are intergrown along (100); anthophyllite and talc are intergrown with (100) Ath //(001) Tlc . In other areas anthophyllite and chlorite are intergrown with a * Ath // c * Chl and b * Ath // b * Chl and Ca-amphibole exsolves lamellae of cummingtonite //(100). With one possible exception, cummingtonite has the space group C 2/ m . Anthophyllite displays chain multiplicity faults //(010), anthophyllite and cummingtonite chain arrangement faults //(100). Microstructures suggest that anthophyllite was formed at the expense of cummingtonite that is interpreted as a high-T precursor phase testifying to a possible earlier, granulite-facies metamorphic stage. During the retrograde P-T path, on cooling presumably below 500-550°C, anthophyllite exsolves platelets or lamellae of gedrite with {hk0} composition planes. Orientations close to {230} and {110} have been recorded, so far not described in the literature. Alteration products of biotite are composed of clinochlore and a sheet silicate intermediate in composition between chlorite and biotite, i.e. , with partly high K contents, but with chlorite metric.