南岭西段加里东期越城岭岩体锆石SHRIMP U-Pb年龄、地质地球化学特征及其形成构造背景

The zircon SHRIMP U-Pb dating, geochemical characteristics and tectonic setting of Caledonian Yuechengling pluton in the western segment of the Nanling Mountains

越城岭岩体位于南岭西段,主体为南部的加里东期花岗岩,北部为印支期花岗岩。加里东期花岗岩自早至晚依次为中细粒斑状黑云母二长花岗岩、细中粒斑状黑（二）云母二长花岗岩、细粒斑状黑（二）云母二长花岗岩和细粒黑（二）云母二长花岗岩。岩体东部和西部花岗岩分别具块状构造和片麻状构造。岩体西缘尚叠加了燕山期左行走滑-伸展型韧性剪切带。对中细粒斑状黑云母二长花岗岩和（糜棱岩化）细中粒斑状黑云母二长花岗岩各进行了1个样品的锆石SHRIMP U-Pb年龄测试,分别得到（436.6±4.8） Ma、（430.5±4.3） Ma的年龄值,反映花岗岩形成于早志留世晚期。岩石高硅、富铝、高钾、中碱, SiO2含量68.35%～78.10%,平均73.29%；Al2O3含量11.95%～15.55%,平均14.18%； K2O 含量4.12%～5.62%,平均4.95%；全碱（Na2O＋K2O）含量为6.18%～8.30%,平均7.58%；K2O/Na2O值在1.36～2.82之间,平均1.94。ASI值1.04～1.66,平均1.23。总体属高钾钙碱性系列过铝质花岗岩类。大多数样品Ba、Nb、Sr、P、Ti表现为较强烈亏损, Rb、（Th＋U＋K）、（La＋Ce）、Nd、（Zr＋Hf＋Sm）、（Y＋Yb＋Lu）等则相对富集；∑REE含量为50.43～328.81μg/g,平均173.39μg/g；δEu值0.21～0.68,平均为0.40；（La/Yb）N值为0.54～14.04,平均7.93；ISr值为0.71912和0.72415,εSr（t）值为208和279,εNd（t）值为–11.76～–7.80, t2DM为1.80～2.12 Ga。A/MF-C/MF图解显示源岩为泥质岩和碎屑岩。上述地球化学特征表明花岗岩为S型花岗岩,是陆壳碎屑岩石部分熔融的产物。花岗岩氧化物构造环境判别图解指示岩体形成于后碰撞构造环境。基于岩石成因、构造环境判别以及区域构造演化过程,推断加里东期越城岭花岗岩的具体形成机制为：奥陶纪末—志留纪初的北流运动导致地壳增厚、升温,早志留世中晚期在挤压减弱、应力松弛的后碰撞-减压构造环境下,中、上地壳酸性岩石发生部分熔融并向上侵位。

Yuechengling pluton in the western segment of the Nanling Mountains is mainly composed of Caledonian and Indosinian granites in the southern and northern parts, respectively. Caledonian granites change with time from medium-fine-grained porphyritic biotite monzogranite, fine-medium-grained porphyritic two-mica monzogranite, fine-grained porphyritic two-mica monzogranite and fine-grained two-mica monzogranite. The Caledonian granites in the eastern part are characterized by massive and gneissic structures. Late Mesozoic extensional-type ductile shear zones occur in the western margin of this pluton. U-Pb zircon dating for a porphyritic biotite monzogranite sample and a mylonitic porphyritic biotite monzogranite sample yield weighted average ages of （436.6±4.8） Ma and （430.5±4.3） Ma, respectively, which indicate that the Yuechengling pluton was formed in late Early Silurian. The rocks are silicon-high （SiO2= 68.35%―78.10%, 73.29% on average）, aluminium-rich （Al2O3= 11.95%―15.55%, 14.18% on average）, potassium-high （K2O = 4.12%―5.62%, 4.95%on average） and alkali-moderate （Na2O＋K2O = 6.18%―8.30%, 7.58% on average） with K2O/Na2O value of 1.36―2.82 （1.94 on average） and ASI value of 1.04―1.66 （1.23 on average）, thus belong to high-K Calc-alkaline series perluminous granitoids. Most samples are depleted in Ba, Nb, Sr, P and Ti, and enriched in Rb, （Th＋U＋K）, （La＋Ce）, Nd, （Zr＋Hf＋Sm） and （Y＋Yb＋Lu）. The studied sampels have ∑REE values of 50.43―328.81 μg/g （173.39 μg/g on average）, δEu values of 0.21―0.68 （0.40 on average）, （La/Yb）N values of 0.54―14.04 （7.93 on average）, ISr values of 0.71912 and 0.72415, εSr（t） values of 207 and 279,εNd（t） values of–11.76―–7.80 and t2DM ages of 1.80―2.12 Ga. The A/MF-C/MF diagram indicates that the Caledonian Yuechengling granites were form by partial melting of mudstones and clastic rocks. All above geochemical characteristics indicate that the granites are S-type granitoid. Multiple oxide-diagrams for discrimination of structural environment show that the granites were formed in post-collisional tectonic setting. According to petrogenesis, discrimination of structural environment and regional tectonic evolution, the formation mechanism of Caledonian Yuechengling granites is inferred as follows：the thickening of crust in Beiliu Movement （in the late Ordovician to the early Silurian） raised the crust temperature, granites formed through parting melting in post-collisional tectonic setting in middle-late Early Silurian.