湘东南志留纪彭公庙花岗岩体的地质地球化学特征及其构造环境

Geology and Geochemistry of the Silurian Penggongmiao Granitic Pluton in the Southeastern Hunan Province and Its Implication for Tectonic Setting

志留纪彭公庙岩体地处湘东南,岩石类型主要为黑云母花岗闪长岩、黑云母二长花岗岩、二云母二长花岗岩。各岩石单元从早至晚,w(S iO2)总体由低变高,变化在65.30%~73.42%之间;w(K2O)平均为4.34%,w(Na2O+K2O)为6.46%~8.32%,平均为7.09%;w(K2O)/w(Na2O)平均为1.58;岩石富铝,w(A l2O3)平均为14.13%。岩体总体属铁质、强过铝质高钾钙碱性花岗岩。Ba、Nb、Sr、P、Ti相对于原始地幔为负异常,Rb、(Th+U)、(La+Ce)、Nd、(Zr+H f+Sm)、(Y+Yb)等则相对富集。w(ΣREE)平均为221.85×10-6;轻稀土富集,w(La)N/w(Yb)N值平均为8.66;Eu弱亏损,δ(Eu)平均为0.53。ISr值为0.712 30~0.718 31,εSr(t)为110.7~196.1,εNd(t)为-8.0^-8.7,t2DM为1.81~1.87 Ga。上述地球化学特征表明彭公庙岩体为S型花岗岩。在C/MF-A/MF图解中,样品点部分落入基性岩区,部分落入变质碎屑岩区;在岩体氧化物比值的Harker图解中,各样点构成良好的线性关系;在w(La)/w(Sm)-w(La)图解中样品点分布散乱;酸性程度最高的晚期恩垄岩石单元的轻稀土相对富集更为明显;岩石中发育镁铁质岩浆包体等,表明彭公庙岩体具岩浆混合成因。地球化学构造环境判别图解、岩体侵位地质特征以及区域构造演化背景等表明彭公庙岩体形成于后造山构造环境。岩体各岩石单元侵位的先后次序可能与岩浆的粘度密切相关,基性程度相对较高的岩浆向上运移速度更快,因而侵位更早。

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 K20 content is 4. 34% , and their Na20 ＋ K20 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 AI 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 8Eu value of 0. 53. Their lsr values range from 0. 712 30 to 0. 718 31, 110.7 to 196. 1 for εSr（t） , and -8.0 to -8. 7 for εNd （t） with t2DM 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-clasticrocks. A good linear relationship can be observed on the diagrams of SiO2 ratios are no correlated with La. The late stage of the Enlong Unit with th vs. major oxides. In contrast, La/Sm e 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 di agrams, 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.