Features And Origin Of The Basic Rock In The Tongling Area, Anhui Province, China

The Tongling area in Anhui Province is an important part of the Middle-lower Yangtze River polymetallic metallogenic belt. There occur Mesozoic intensive magmatic activity and mineralization in the area. The previous researches indicate input of mantle material into upper crust during magmatism and ore-forming process. However, shortage of fresh outcrops of Mesozoic basic rocks in this area places restrictions on our understanding of mantle characteristics as well as Mesozoic magmatism, ore-forming process and crust-mantle interaction. A detailed petrographical, mineralogical and petrochemical study has been carried out on the basic rocks in the Tongling area. The following are major achievements obtained in this thesis.1. The major lithology of the basic rocks in the Tongling area is gabbro. Its mafic mineral is dominated by augite and biotite, sialic mineral by andesine and oligoclase, and accessory mineral by magnetite and apatite.2. The petrochemical analyses indicate that the basic rocks belong to quasi-aluminous and alkalic series. Their smaller Mg~# values support the formation of them by crystallization of evolved magma. They are high in REE and have similar normalized REE patterns, with high ratios of LREE to HREE, indicating that they are products of the same source magma evolution. The basic rocks are rich in LILE such as Rb, Ba, Th, U, K and Sr, but poor in HFSE such as Nb, Ta and Ti, with obvious Nb, Ta, and Ti negative anomaly, Pb positive anomaly, and slight Sr positive anomaly.3. The trace element and isotopic data are consistent with crystallization fractionation and less contamination of the initial magma with crustal materials during its ascending and emplacement.4. It is inferred that the mantle source place of the basic rocks in Tongling has an isotopic character similar to that of the EMⅡend member, showing little effect of the asthenosphere matter on it.5. The basic rocks are dated at 123.9±8.3Ma and 122.9±2.8Ma using the whole-rock plus single minerals isochron method and represents product of the early Cretaceous volcanic eruption.