Zircon U-Pb geochronology, geochemical characteristics and tectonic implications of Caledonian granites from the Xuanhe area, Southwestern Fujian Province

The NE-trending Xuanhe granitic mass is the largest Yanshanian-Caledonian complex rock mass in southwestern Fujian, which is exposed in southwestern Fujian Province in the arc form. However, there are still different opinions in the formation age of rock mass and its petrogenetic environment, which restricts the discussion of the tectonic environment of southwestern Fujian. Based on detailed field geological survey, the authors investigated the Xuanhe syenogranites, mainly on the basis of the LA-ICP-MS zircon U-Pb geochronology, petrochemistry and Sr-Nd isotope analysis, and discussed the rock types, magma source and tectonic environment. The Xuanhe granites are mainly composed of syenogranites in the forms of speckle fine grain, oligophyric medium-fine grain, porphyritic fine grain, porphyritic fine coarse grain and porphyritic fine grain. The LA-ICP-MS zircon U-Pb age of four samples from the granites are (424.6±2.8)Ma, (426.7±2.6)Ma, (435.5±2.4)Ma and (447±3.6)Ma respectively, indicating that the formation age of Xuanhe granite is Caledonian instead of Indosinian as previously held. The rocks belong to high-K calc alkaline peraluminous rock series, characterized geochemically by high Si, high K (K₂O>Na₂O), low TiO₂, low FeO and low MgO, with average A/CNK value being 1.23. The rocks are S-type grantie, with the characteristics of high content of rare earth elements, obvious right steeply inclined REE fractionation curve, middle negative Eu anomaly, enrichment of large ion lithophile elements such as K, Rb, Pb and Nd and high field strength elements of Th, U and Ba, and relative depletion of Sr, Nb and Ce. The initial Sr isotope ratios are from 0.71291 to 0.71399, the values of ε_Nd(t) vary from -9.1 to -13.4, and two stage Nd model ages correspond to 1.91-2.27 Ga. It is inferred that the granites were formed by partial melting of crustal material of Mayuan Group metamorphic rocks, being probably a product of intraplate Caledonian orogeny in South China.