Formation of corona structures from the troctolitic gabbros of Chainigund, Kargil, Ladakh, NW Himalayas, India: Petrological implications

Well-developed corona structures are observed and described in detail in the cumulate troctolites from Chainigund village, Kargil. The gabbro-troctolite unit is situated 5 km NW of Kargil city and consists of gabbros, troctolites, and anorthosites with doleritic dykes cross-cutting the unit at places. The host gabbros are fresh and display both fine and coarse-grained varieties. Troctolites occur as pods and veins within the gabbro and are composed of plagioclase (77-80 vol%), olivine (10-16 vol%), pleonaste spinel (6-8 vol%), amphiboles (2 -3 vol%) and opaques (0.5-2vol %). Both olivines and plagioclases are unzoned with spectacular coronas around the olivines (Fo 74.9-76.7) at the contact with plagioclase feldspar (An90.6-95.2). From center outwards, the discontinuous reaction series consists of the following members: Olivine, enstatitic orthopyroxene, magnesio-hornblende (Amph1) enclosed by a symplectitic rim of pargasite (Amph2) and pleonaste spinel and concludes at the plagioclase interface i.e. Ol-Opx-Amph1-Amph2-Spl-Plg. The mineral textures of the corona structure indicate formation in the presence of an interstitial fluid trapped between cumulus olivine and plagioclase. The reaction of this fluid with the olivine resulted in a rim of peritectic orthopyroxene around olivine which was subsequently replaced to form Amph 1 between the orthopyroxene and plagioclase. This is evident by the horse-shoe shaped outline and intermingling boundary shared by orthopyroxene and Amph 1. The formation of outer Amph 2 and spinel symplectite layers could be attributed to the replacement of precursor clinopyroxene and plagioclase at high temperatures (1050-1150° C ± 40° C). The Amph-Spl symplectites, presence of oxidizing conditions (magnetite and ilmenite), discontinuous reactions and local or short-range diffusion phenomena thus indicate that the corona structures are a result of metasomatic interaction of cooling magma with the previously formed minerals.

Keywords: Corona structures; troctolite gabbro; olivine- plagioclase contact; Kargil; Ladakh; India.