Geochemistry and petrogenesis of the Indosinian Guandimiao granitic pluton in central Hunan

The Indosinian Guandimiao granitic pluton in central Hunan is composed of two generations:the latest Middle Triassic and middle Late Triassic granites. The Middle Triassic granites comprise meladioritic micrinites, and consist, in a progressively younger order, of fine-to medium-grained hornblende-biotite granodiorite, fine-to medium-grained porphyritic hornblende-biotite adamellite, and fine-to finely medium-grained (porphyritic) biotite adamellite. The Late Triassic granites consist, in a progressively younger order, of finely medium-grained to coarsely medium-grained porphyritic two-mica adamellite and fine-grained two-mica adamellite. Geochemically, these granites are characterized by high Si, enriched Al, high K, moderate alkali contents, SiO₂ contents ranging from 69. 50% to 74. 53% with an average of 71. 68%. The Late Triassic granites have Al₂O₃ contents from 13. 39% to 16. 31% with an average of 14. 62%; K₂O contents from 4. 15% to 6. 45% with an average of 4. 71%; total alkali (Na₂O + K₂O) contents from 6. 80% to 8. 87% with an average of 7. 74%; K₂O/Na₂O ratios from 1. 35 to 2. 66 with an average of 1. 58; ASI values from 0. 99 to 1. 40, indicating the magnesian high-K calc-alkaline peraluminous granites. The Middle Triassic granites are remarkedly depleted in Ba, Nb, Sr, P and Ti, and relatively enriched in Rb, Th + U + K, La + Ce, Nd, Zr + Hf + Sm and Y + Yb + Lu. The ΣREE contents for the Middle and Late Triassic granites vary from 121. 60 to 197. 56μg/g with an average of 158. 70μg/g; δEu values from 0. 28 to 0. 68 with an average of 0. 53, and (La/Yb)_N values from 5. 94 to 17. 53 with an average of 13. 80. The above-mentioned geological and geochemical signatures show that the Indosinian Guandimiao granites belong to the S-type granites generated in the collision-postcollision tectonic settings.