Chronology, Geochemistry And Geodynamic Significance Of The Mafic-ultramafic Rocks In Fujian Province Since Late Mesozoic

The SE China is located at the southern section of eastern Asia continental margin, and lies among the Yangtze Plate, Indian Plate and Pacific Plate. It is an active region due to the interaction between Pacific Plate and Eurasian Plate in Mesozoic, so for a long time it attracts geologists' attention worldwide. In Late Mesozoic, the subduction of paleo-Pacific Plate to Eurasian Plate and large scale calc-alkaline magmatism are important tectono-magmatic events in SE China. The tectonic transformation time, the continental dynamic evolution processes, the lithospheric extension stages and the deep geological processes all become the key topics of current researches. This paper focuses on petrology, mineralogy, element geochemistry, isotope geochemistry and K-Ar ages of the mafic dikes and alkaline mafic-ultramafic rocks in Fujian province. By systematical geologic-geochemical studies on the western mafic-ultramafic rocks, northern bojite stocks, western and southeastern mafic dikes in Fujian province, we analyzed in detail of the petrogenesis, source region characteristics and intrusion ages, preliminarily discussed the geodynamic setting of SE China since Mesozoic, and retrieved the crustal extension stages, mantle source and evolution characteristics underneath Fujian province in Mesozoic. The following conclusions have been obtained:1. The Liucheng limburgites of western Fujian province formed in Cenozoic are intrusive pipes. The petrochemical study indicates that they belong to Si-unsaturated agpaitic alkaline ultramafic rocks and systematical trace element geochemical studies show that crustal assimilation was infrequent during the ascend and emplacement processes of magma. The limburgites had successive trace element geochemical characteristics of HIMU mantle source region, and the HIMU mantle was resulted from the subduction of dehydrated oceanic slab and the recycle of sediments. The Liucheng limburgites originated by means of 2.9% partial melting of mantle. The whole rock K-Ar age of limburgites is 12.6±1.3Ma (Miocene), and the geotectonic setting for these rocks is continental rift.2. Systematical geologic-geochemical studies indicate that the Chimen bojite stocks in northern Fujian belong to low-K(tholeiite) rock series which characterized by high Al₂O₃ and Na₂O>K₂O. These stocks are enriched in LILE and LREE, and deficited in HFSE. The source of stocks is enriched mantle with the features of EM â…¡. Compared to the coastal gabbro, the Chimen bojite stocks can be used as an indicator for the transference from the crustal extension into continental rift. The whole rock K-Ar age is 67.6±5.4Ma, being in Late Cretaceous, and the geodynamic setting for these rocks is extension belt - primary rift.3. The wastern Fujian mafic dikes mainly distributed in NW direction. Studies on petrology, mineralogy and geologic-geochemistry show that they belong to subalkaline rock series. They can be divided into two types in geochemical characteristics of REE, incompatible elements and Sr-Nd isotopes, which indicate that there are two type mantle source regions in western Fujian, namely sub-depleted mantle source region and enriched mantle source region. The mafic dikes originated from the sub-depleted mantle are characterized by LILE sub-enrichment, no Nb, Ta,Ti negative anomalies, Nd isotopic sub-deficiency and Sr isotopic sub-enrichment; while the mafic dikes originated from the enriched mantle are characterized by varying degree enrichment of LILE and LREE, Nb, Ta, Ti negative anomalies and Nd, Sr isotopic enrichment, the mixing of the crust and the mantle...