A Study On The NiangNiangshan FormationVolcanic Rocks In Tongjin, Jiangsu Province

The volcanic rocks of the Niangniangshan (NNS) Formation formed by the latest eruption of the Mesozoic volcanic activities in the Ningwu Basin are evidently different in volcanic eruption type and magmatic character from those of the Longwangshan (LWS), Dawangshang (DWS), and Gushan (GS) Formations formed in the early 3 volcanic eruptions. A detailed petrographical, mineralogical and petrochemical study has been carried out and contributes to the following achievements.1. The main rock types of the NNS Formation are psedoleucite phonolite formed in the 1st subcycle of eruption and hauyne phonolite in the 2nd subcycle, as well as minor phonolitic ignimbrite, rhyakkumulate, and trachyte;2. The psedoleucite phonolite is rich in potassium. In contrast, the hauyne phonolite is rich in sodium. However, both are significantly rich in LREE and LILE, relatively poor in HREE and HFSE, and greatly similar in chondrite-normalized REE patterns and TE spiderdiagrams, which may imply their same magma source region;3. The mineral chemical and thermodynamic data combined with BSE image and related diagrams support the occurrence of a large amount of irregular nosean and sodalite scattered in hauyne phenocrysts and the existence of pyrite and pyrrhotite in hauyne phenocrysts as inclusions rather than schlierens;4. Hauyne phonolite is dated at 120±9 Ma using the whole-rock plus single minerals isochron method and represents product of the early Cretaceous volcanic eruption;5. Hauyne phonolite crystallization P-T conditions have been determined using the mineral-liquid thermobarometers and are consistent with the corphase diagrams;6. The REE simulation and comprehensive geochemical analysis results indicate that the NNS Formation volcanic rocks cannot be formed by eruption of magma with composition of the LWS and DWS volcanic rocks through fractionation of the predominant phenocrysts includeing plagioclase, amphibole and pyroxene in them, or by partial melting of the "normal" Yangtze crust, but more probably by partial melting of pre-enriched mantle source followed by crystal fractionation with slight crustal contamination.