The Geochronology, Geochemical And Petrogenesis Of Zixi-Guangze Complexes In Northern Wuyishan Region: Constraint On Tectonic Setting Of The Indosinian Magmatism In South China Interior

The Triassic granites are widespread in Southern China Interior, especially in Wuyishan to Xuefengshan regions. In this thesis, the geochronology, petrography and whole-rock geochemical and Sr-Nd isotopic compositions of the Zixi-Guangze granite-gabbro complexes in North Wuyishan region were studied. The high precise SHRIMP and La-ICP-MS U-Pb zircon geochronological results show that the Zixi granites were formed during 231.7-238.0 Ma, Guangze gabbros were formed during 220.1-231.7Ma. Geochemical feature of the Zixi granites show that(1) they have higher SiO2 and K2 O contents and ACNK values of 1.00-1.11, belonging to weakly peraluminous granites of the high-K calc-alkaline series.(2) these rocks are depleted in Ba, Sr, Nb, Ta, Ce and Ti, and enriched in Th, U, Nd, La, Pb and obvious enriched light rare earth elements(LREE) related to heavy rare earth elements(HREE) with(La/Yb)N= 28.8 and(Sm/Yb)N= 4.4, as well as evident negative Eu anomalies(δEu= 0.35-0.47);(3) the whole-rock εNd(t) values of the granites are in the range of-11.94 to-7.66 and initial 87Sr/86 Sr ratios from 0.7068 to 0.7113. These geochemical characteristics suggest that Zixi granites may derived from partial melting of a crustal material with minor mantle material contamination. Geochemical feature of the Guangze gabbros show that(1) they are lower in SiO2(51-59%) and K2 O, Na2 O and Al2O3, higher in MgO, MnO, Ti O2, Fe2O3 and P2O5, belonging to high-K calc-alkaline series to shoshonite series rocks;(2) the gabbros are depleted in Ba, Sr, Nb, Ta, Ce and Ti, enriched in Th, U, Nb, La and Pb, resembling to Zixi granites in trace elements characteristic;(3) whole-bulk rock εNd(t) values in the range of-10.08 to-9.16 and initial 87Sr/86 Sr ratios from 0.71096 to 0.71169, which higher than those of the Zixi granites. These geochemical characteristics, as well as arc-signature, i.e. depleted in Nb-Ta, enriched in Th and light rare earth elements(LREE) suggest that Guangze gabbros were derived from an enriched lithospheric mantle modified by subduction fluid. Furthermore, combined with our new ages and compilation of age data of magmatic rocks of the Indosinian granites from the South China Interior, the Indosinian magmatism in the South China Interior shows a younging-trend of NW-ward toward the cratonic interior and a broad horizontal linear distribution from Southwest to northeast along the Wuyishan region. A number of ca. 240 Ma granitic rocks renewed product in the Wuyishan region at a distance of approximately 400~500 km away from the trench, suggesting that a renewed continental magmatic arc was probably initiated since ca. 240 Ma along the Wuyishan region after a magmatic gap due to flat-slab subduction of the paleo-Pacific, resembling to flat-slab subduction of the Laramide orogeny in northern America and Mexican Volcanic Belt in Central Mexico. Our analysis indicates that the renewed arc magmatism in the Wuyishan region is relative to dehydration of serpentinization mantle wedge at front of the flat-slab, the water released from the mantle upwell into thicken and warmed orogenic crust to result in partial melting of the middle-lower crust and formed large numbers of Indosinian granites in the Wuyishan region.