Abstract:

The Silurian Penggongmiao granitic pluton in the southeastern Hunan Province is composed mainly of biotite granodiorite, biotite monzogranite and twomica monzogranite. The SiO₂ contents increase from early to late granitic units, ranging from 65.73% to 73.42%. Their mean K₂O content is 4.34%, and their Na₂O+K₂O contents vary from 6.46% to 8.32%, with an average of 7.09% while their mean K₂O/Na₂O ratio is 1.58. The rocks are enriched in Al with an average Al₂O₃ content of 14.13%. Thus, the rocks belong to ferric, strong peraluminous high-K calc-alkaline series. Their primitive mantle-normalized trace element patterns display slightly negative Ba, Nb, Sr, P and Ti anomalies and positive Rb, Th, U, La, Ce, Nd, Zr, Hf, Sm, Y and Y anomalies. Their average total rare earth element (REE) content is 221.85×10^-6.They are characterized by enrichment in light REE with average (La/Yb)n ratio of 8.66, and slightly negative Eu anomalies with mean δEu value of 0.53. Their ISr values range from 0.712 30 to 0.718 31, 110.7 to 196.1 for ε_Sr(t), and -8.0 to -8.7 for ε_Nd(t) with t_2DM value of 1.81 to 1.87 Ga. All above geochemical characteristics indicate S-type granitoid. On the diagram of C/MF vs. A/MF, they plot in the fields of basic rocks and meta-clastic rocks. A good linear relationship can be observed on the diagrams of SiO₂ vs. major oxides. In contrast, La/Sm ratios are no correlated with La. The late stage of the Enlong Unit with the most felsic has the most pronounced enrichment in LREE. The above geochemical characteristics as well as mafic microgranular enclaves suggest that the Penggongmiao pluton was related to magmatic mixing processes. The geochemical-tectonic discrimination diagrams, emplaced characteristics and regional tectonic evolution suggest that the Penggongmiao pluton was formed in the post-collisional setting. The emplaced sequences of the various units were likely related to magma viscosities: the more basic magma ascended more quickly, and therefore emplaced earlier

Key words:  Penggongmiao, granite, geochemistry, magma mixing, post-orogenic, southeastern Hunan Province