Origin And Tectonic Implications Of Silurian Gabbros And I-type Granite In Central Fujian

The early-Paleozoic orogen in South China is possibly one of the rare intraplate orogen in the world. It is charcterized by an angular unconformity between post-Silurian cover and pre-Silurian strata and by the intensive and extensive early Paleozoic granitic plutonism. However, synchronous mafic-ultramafic rocks have not been reported, which are crucial for understanding the nature and evolution of the orogen. This paper firstly report systematically pertrological and LA-ICP-MS zircon U-Pb dating, whole-rock geochemical and Sr-Nd-Hf isotopic research for a Silurian gabbroic pluton (Dakang) and a coeval granitic pluton (Guiyang), that have recently been identified in central Fujian, in the southeastern part of the orogen. We disscuss the origins of mafic rocks and granitic rocks and their relationship to provide more constraints on the evolution of the early Paleozoic orogen in South China.LA-ICP-MS zircon U-Pb dating shows that the Dakang and Guiyang plutons were emplaced at 441-438 Ma and 431 Ma, respectively. The Dakang pluton is comprised of gabbros with SiO2 contentsof 48.1-52.0 wt.% and minor intermediate rocks, which have SiO2 contents (53.2-58.7 wt.%). These rocks are all metaluminous and potassic, which shows low Ga/Al ratios (10000*Ga/Al<2.6) and then can be of I-type granites. They are riched in LILE and depleted in HFSE. and have initial 87Sr/86Sr of 0.7066-0.7098, ?Nd(T) of -7.0 to -3.3 and average ?Hf(T) (in-situ zircon) of -5.2 to -4.4. Detailed geochemical data suggest that Dakang gabbroic primary magmas were generated by partial melting of old subduction-modified lithospheric mantle in the spinel-garnet transition zone at a high temperature (?1300?). These primary magmas underwent fractionation crystallization of clinopyroxene and amphibole, forming the Dakang gabbros and intermediate rocks, respectively. Guiyang granites have SiO2 contents (68.2-72.6 wt.%). They are mostly weakly peralumious (1.02-1.16), and have similar Mg# to that of pure crustal melts. Detailed geochemical data show Guiyang granites of I-type. Isotopically, They have initial 87Sr/86Sr of 0.7095-0.7113, ?Nd(T) of -6.7 to -5.9 and ?Hf(T) (in-situ zircon) of -7.7 to -0.8. Integrated geochemical data show Guiyang granites were generated by partial melting of a mixed source of Paleoproterozoic metaigneous and metasedimentary rocks in the lower crust.Our study, together with previous reported data, confirm that lithospheric delamination occurred along the metamorphic core of the early Paleozoic orogen of South China during the Silurian (442-420 Ma), and that represents a post-orogenic tectonic regime in the orogen.