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。ISr为0.75093,εSr(t)为659,εNd(t)为-11.82,t2DM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。
关键词:
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衡阳
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燕山早期
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过铝花岗岩
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地球化学
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湖南
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 ISr value of 0.75093, the εSr(t) value of 659,εNd(t) value of -11.82, t2DM 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.
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