Original paper

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

Müller, Wolfgang Friedrich; Schmädicke, Esther; Okrusch, Martin; Schüssler, Ulrich

Abstract

A metamorphosed ultramafic rock, penetrated by the KTB pilot hole at a depth of 1382.36m, contains the assemblage Caamphibole - 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+Fe3++Cr+Ti)[6] of 1.0-1.2 p.f.u. and (Fe2++Mn)/(Mg+Fe2++Mn) ratios of 0.10-0.16, while coexisting anthophyllite has (Al+Fe3++Cr+Ti)[6] 0.16 and (Fe2++Mn)/(Mg+Fe2++Mn) ratios of 0.21-0.29. Chlorite is clinochlore with Al[4]/(Al[4]+Si) of 0.27-0.30 and Fe2+/(Fe2++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 µm 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 C2/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.

Keywords

amphibole-chlorite-talc intergrowthsanthophyllitegedrite exsolutionscummingtoniteca-amphiboleorientationrelationshipschain multiplicity faultschain arrangement faultstem