The early Precambrian ultramafic lava - layered intrusions, distributed in northeastern Jiangxi, are composed of dunite, peridotite, enstenite-bearing peridotite, olivine-bearing pyroxenolite, pyroxenolite, sefstromite-bearing ultramafic rock and ultramafic komati-ite. The series is characterized with high magnesium, poor alkali, low calcium and aluminium, as well as inhomogeneous titanium. The contents of Cr, Ni and Co increase to the maximum during the MgO being 33 %-36% , then decrease with increasing of the MgO. The komatiites of the Ⅱ a-type (LREE enriched sub-type in the HREE depleted type) seems to be dominate, and the komatiite of Ⅲ-type (the HREE enriched type) might be existent. The average MgO content is 34. 66% , a little higher than that in South Africa and Southeastern Brazil. Because of alteration, the CaO content is lower. The Content of Cr, Ni, Co and Zr in the komatiite are similar with ultramafic komatiites all over the world, while Ti and Y relatively lower , so that the ratios of Ti/Zr and Y/Zr lower.
The ultramafic lava - layered intrusions should be relics of Late Archean greenstone belt. The 2741. 40 Ma of Nd iso-topic model age comprehends that there develops the Late Archean geological record in northeastern Jiangxi. The granite-greenstone belt terrains in the region and Sichuan, being a part of the Yangtze Craton, have unique geochemical features, such as low-Al type TTG suite, ** a-type komatiite, etc. , which are similar with that of the Swaziland Gneiss and ko'matiite in the Onverwacht Group in the South Africa respectively. Possibly, the Yangtze Craton had a relatively closer relation with the South African Craton during early Precambrian, even was once a part of the latter.
Geotectonica et Metallogenia
komatiite of Ⅱ a-type, Late Archean greenstone belt, northeastern Jiangxi