Geochemistry, petrogenesis and tectonic setting of the early Yanshanian peraluminous granites in the Chuankou region, Hengyang, Hunan

The Chuankou peraluminous granites in Hengyang, Hunan consist dominantly of two-mica monzonitic granites which have high Si and moderate alkali contents, average SiO₂ and K₂O contents ranging from 75.71% and 4.39%, average Na₂O +K₂O content of 7.57%, average K₂O Na₂O ratio of 1.43, and average Al₂O₃ content of 13.28%, indicating that these granites should be assigned to the peraluminous and strongly peraluminous calc-alkaline granites. The elements Ba, Nb, Sr, Eu and Ti are strongly depleted, while the elements Rb, U, Ta, Nd, Hf, Sm and Y +Yb are relatively enriched. These granites are also marked by average ∑REE content of only 67.18×10^-6the average (La Yb)_N value of 2.21, the average δ Eu value of 0.16, the average I_Sr value of 0.75093, the ε_Sr(t) value of 659,ε_Nd(t) value of -11.82, t_2DM of 1.92 Ga coinciding with the ages for the granites from Hunan and Guangxi (2.4 -1.8 Ga) and for the basement of the study area (2.7 -1.7 Ga). The magmas of the Chuankou granites are derived from the middle crustal crystalline basement, typical of the S-type granites. The magmas may be divided into two types:"low-temperature" type from mudstones and "high-temperature" type from sandstones. The Chuankou granites were formed from the post-orogenic tectonic setting. The detachment of the deep-seated lithosphere and crust-mantle interaction suggest the evidences that the early Yanshanian granites in southeastern Hunan are metaluminous and weakly peraluminous whereas the Chuankou granites belong to the crust-derived peraluminous and strongly peraluminous granites.