粤西北大桂山岩体LA-ICP-MS锆石U-Pb测年、岩石成因及构造环境分析

LA-ICP-MS Zircon U-Pb Dating,Petrogenesis and Tectonic Setting of the Daguishan Pluton in Northwestern Guangdong

为了加深对粤西北地区加里东期岩浆岩演化的认识,对粤西北地区大桂山岩体进行了岩石学、岩相学、同位素年代学、岩石地球化学及Sm-Nd同位素地球化学等研究。研究显示:大桂山岩体位于广东佛冈复式岩体的北西缘,岩性为中粗粒含斑黑云母正长花岗岩;利用LA-ICP-MS锆石U-Pb法测得其年龄为(445.9±3.6)Ma,属晚奥陶世侵入岩;岩石地球化学具有高硅、富钾、高分异指数和低钙、低镁、贫铁的特征;稀土元素总量中等-偏低,轻重稀土元素分馏程度较低,具明显的负Eu异常;微量元素表现为大离子亲石元素Rb、Th、U、K、La、Ce、Nd等的富集,亏损P、Sr、Ba、Ti等高场强元素;(^(87)Sr/^(86)Sr)_(i)为0.0253~0.1002,ε_(Nd)(t)值较高,为-3.15~-5.26,T_(DM2)值较低。以上特征表明大桂山岩体为高分异或低熔融程度的铝质A型花岗岩,其源区为成熟度较低的地壳物质或者是一定比例壳幔物质的混合,因造山带垮塌、软流圈上涌引起地壳伸展,由于地幔上隆,幔源基性岩浆岩和钾玄质岩浆岩上侵或底侵而形成。

To deepen our understanding on Caledonian magmatic rock evolution in northwest Guangdong,the petrology,petrography,isotope chronology,petrogeochemistry and Sm-Nd isotope geochemistry of the Daguishan pluton in northwest Guangdong is carried out.The result shows the Daguishan pluton is located in the northwest margin of the Fogang complex pluton in Guangdong Province,and the lithology is medium-/coarse-grained biotite syenogranite.LA-ICP-MS zircon U-Pb dating shows that its age to be(445.9±3.6)Ma,belonging to the Late Ordovician intrusive rock.The Daguishan pluton is rich in Si,K,high differentiation index and low Ca,Mg and Fe.The rocks show the total amount of REE is medium to low,the fractionation degree of LREE and HREE is low with negative Eu anomaly.The monzogranite is also enriched in LILEs(e.g.,Rb,Th,U,K,La,Ce and Nd)and depleted in P,Sr,Ba and Ti.(^(87)Sr/^(86)Sr)_(i) value is 0.025,3-0.100,2,andε_(Nd)(t)value is-3.15--5.26 with lower T_(DM2) value.Based on the geochronological and geochemical characteristics of the Daguishan pluton,it is inferred that the source area is the crust material with low maturity or the mixing of a certain proportion of crust mantle material.The crust gets extended due to the collapse of orogenic belt and the upwelling of asthenosphere.Due to mantle uplift,mantle-derived mafic magmatites and potassic-basalt magmatites were formed by upwelling or underplating.