湘中印支期关帝庙岩体地球化学特征及成因

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

关键词:  地球化学特征  S型花岗岩  中三叠世  晚三叠世  印支运动  关帝庙岩体  湘中

Key words:  geochemistry  S-type granite  Middle Triassic  Late Triassic  Indosinian orogeny  Guandimiao granitic pluton  central Hunan

摘要:

关帝庙岩体位于湘中盆地,据年龄资料可分为中三叠世末和晚三叠世中期两个形成时期。中三叠世花岗岩中发育闪长质暗色微粒包体,自早至晚依次为细中粒角闪石黑云母花岗闪长岩、细中粒斑状角闪石黑云母二长花岗岩、细-细中粒(斑状)黑云母二长花岗岩。晚三叠世花岗岩自早至晚依次为细中粒-粗中粒斑状二云母二长花岗岩、细粒二云母二长花岗岩。岩石高硅、富铝、高钾,(Na₂O+K₂O)含量为6.80%~8.87%,平均7.74%;K₂O/Na₂O比值在1.35~2.66之间,平均为1.58;ASI值为0.99~1.40。总体属镁质、高钾钙碱性系列过铝质花岗岩类。中三叠世花岗岩Ba、Nb、Sr、P、Ti表现为明显亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集。中、晚三叠世花岗岩ΣREE含量为121.60~197.56μg/g,平均为158.70μg/g;δEu值0.28~0.68,平均为0.53;(La/Yb)_N值为5.94~17.53,平均13.80。中三叠世花岗岩I_Sr值为0.71302~0.71758,εSr(t)值为121~186,εNd(t)值为-9.95~-8.74,t_2DM为1.72~1.82Ga。C/MF-A/MF图解显示源岩主要为碎屑岩、少量基性岩和泥质岩。地质地球化学特征表明,印支期关帝庙花岗岩属S型花岗岩,形成于碰撞-后碰撞构造环境。

Abstract:

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.