Serpentinization and chloritization of metamorphosed Iherzolites in Darreh-Deh (east of Nain Ophiolite, Central Iran): Calcium source for rodingitization and tremolitization

摘要

The petrography and mineral chemistry of the metamorphosed lherzolite in Darreh-Deh massif (east of Nain Ophiolite, Central Iran) is investigated in order to fmd the calcium source for rodingitization and tremolitization. In comparison with olivine and orthopyroxene, the clinopyroxene has lower modal content and is more alteration-resistant. The microprobe data and petrography of these lherzolites indicate that Ca2+ cations can be released during serpentinization of orthopyroxene (with similar to 18 vol% and CaO similar to 2.7wt%) and clinopyroxene (with similar to 6 vol% and CaO similar to>20 wt%). In contrast, pervasive serpentinization of mantle olivine with similar to 70 vol% and CaO similar to 0.02-0.07 wt% is another expected source for producing Ca2+ rather than metamorphic olivine with CaO similar to<0.02 wt%. The released Ca2+ cannot be completely accommodated in crystal lattice of produced serpentine (with CaO similar to 0.02-0.06 wt%), talc and chlorite (with CaO similar to 0.015 wt%), but it can participate in fonnation of Ca-bearing tremolite (CaO similar to 13 wt%), as a result of serpentinization of clinopyroxenes or subsequent metamorphism of peridotites at amphibolite facies and in formation of coarse-grained clinopyroxene blades and tremolite during rodingitization. Therefore, the calcium content in clinopyroxene, orthopyroxene and olivine of a plagioclase-free peridotite is a potential source of Ca2+, depending on the degree of serpentinization or chloritization.