The mineralization age of the Makeng Fe deposit, South China: implications from U–Pb and Sm–Nd geochronology

The Makeng Fe deposit is located in the southwestern Fujian district, South China. The Sm–Nd isochron ages of seven samples of pure garnet and five of pure magnetite separates from the Makeng ores yielded an isochron age of 157 ± 15 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the nearby exposed the Dayang–Juzhou (DJ) porphyritic biotite granite and fine-grained syenogranite yielded 206Pb/238U ages of 140.2 ± 1.1 and 140.1 ± 1.0 Ma, respectively. These results suggest that the intrusion of the DJ granite and the Makeng skarn alterations and Fe mineralization are contemporaneous. The DJ granite exhibits geochemical characteristics of A-type granites, including high values of Na2O + K2O (8.13–8.92 wt%), FeOt/MgO (3.4–21.5), and Ga/Al (2.64–3.45 × 10−4), and low Al2O3 (10.71–13.29 wt%) value. Chondrite-normalized rare earth element patterns are characterized by obviously negative Eu anomalies (δEu = 0.02–0.28) and primitive-mantle normalized spidergrams show the enrichment in high field strength element and depleting in Sr, Ti, Ba, and Eu. The geochemical characteristics of DJ granite suggest that the granite was derived from partial melting of the Paleoproterozoic metasedimentary rocks of the Cathaysia basement. And some underplating of mafic magma in the lower tholeiitic crust and/or depleted mantle might be involved and provide the heat source for the partial melting. The DJ granite also fits the spatiotemporal distribution of the Jurassic–Cretaceous coastward migration of both extensional and arc-related magmatism and fills the A-type granites gap in the early stage of the early Cretaceous (145–125 Ma). Therefore, it is suggested that the late Jurassic and early Cretaceous magmatism in southwestern Fujian district were generated in an extensional environment responding to the slab rollback and concomitant retreating arc system of the paleo-Pacific plate within the South China Block. And the Fe metallogeny in southwestern Fujian district is genetically linked with the magmatism during this period.