Fluorine-rich calcic amphiboles in ultrahigh-temperature mafic granulite from Tonagh Island in the Napier Complex, East Antarctica: Preliminary report

Tsunogae Toshiaki, Osanai Yasuhito, Toyoshima Tsuyoshi, Owada Masaaki, Hokada Tomokazu, Crowe Warwick A.
Fluorme-nch calc1c amph1boles occur m ultrahigh-temperature mafic granuhte from Tonagh Island m the Napier Complex, East Antarctica The amph1boles are subd1v1ded mto two types high-grade browmsh to pale browmsh amph1bole and retrograde greemsh amph1bole The browmsh am ph1bole 1s pargas1t1c occurnng as a subhedral gram m two-pyroxene mafic granuhte It contains up to 2 2 wt% fluonne, which corresponds to an F /(F + Cl+OH) ratio of O 54 (sample B98020801) In sample B98020802B, browmsh amph1bole 1s
more » ... nmmed by an orthopyroxene-plag1oclase-quartz corona, probably formed by a devolat1hzat10n react10n It has a lower fluorme content of O 21-0 52 wt% (F/(F+Cl+OH)=0 04-0 13) Pale browmsh amph1bole 1s a mmor phase m metapyroxemte It has the highest fluorme content (2 6 wt%, F / ( F +Cl+ OH)=O 60, sample B98012802F) Greemsh amph1bole is present as fine-gramed aggregates with quartz, nmmmg ortho-and clmopyroxenes Fluonne 1s almost absent m this amphibole Holloway and Ford (1975) expenmentally determmed that pargasite with a F / (F +Cl+OH) rat10 ofO 48 is stable up to l l00 °C at 5 kbar, and that F /(F +cl+ OH) mcreases with mcreasmg temperature The fluonne-nch calcic amphiboles m mafic granuhtes are therefore stable under UHT metamorphic condit10ns The format10n of an orthopyroxene-plag10clase-quartz corona m sample B98020802B can therefore be explamed by breakdown of the amph1bole due to its low-fluonne content The ongm of the fluorme is not known, but it is probably denved from basic magma at the stage of protohth formation
doi:10.15094/00003068 fatcat:h2gr2v3dojhullx7ntigbkllme