湖南衡阳燕山早期川口过铝花岗岩地球化学特征、成因与构造环境

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

关键词:  衡阳  燕山早期  过铝花岗岩  地球化学  湖南

Key words:  Hengyang  early Yanshanian  peraluminous granite  geochemistry  Hunan

摘要:

川口过铝花岗岩为二云母二长花岗岩,高硅、中碱,SiO₂和K₂O含量分别平均为75.71%和4.39%,Na₂ O+K₂O平均为7.57%,K₂ O Na₂O平均为1.43,Al₂O₃平均为13.28%,总体属过铝—强过铝质钙碱性花岗岩类。Ba、Nb、Sr、Eu、Ti等元素表现为较强烈的亏损,Rb、U、Ta、Nd、Hf、Sm、Y+Yb等则相对富集;∑REE平均仅67.18×10^-6,(La Yb)_N平均为2.21,δ Eu值平均为0.16。I_Sr为0.75093,ε_Sr(t)为659,ε_Nd(t)为-11.82,t_2DM为1.92Ga,与湘桂内陆带花岗岩的背景值(2.4 ~ 1.8Ga)和区域基底的时代(2.7~ 1.7Ga)相吻合。川口岩体岩浆来源为中地壳结晶基底,属典型S型花岗岩;岩浆存在源于泥质岩的“低温”和源于砂岩的“高温”两种不同类型,至少有两个岩浆来源;岩体形成于后造山构造环境。分析认为,深部岩石圈拆沉与壳幔相互作用的规模差异,是造成湘东南燕山早期花岗岩一般为准铝—弱过铝质并有幔源物质加入,而川口岩体为过铝—强过铝质壳源花岗岩的原因。

Abstract:

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.