湘西南苗儿山岩体北段新元古代晋宁期花岗岩岩石学、地球化学特征及其形成构造背景

Petrological,Geochemical Characteristics and Formation Tectonic Setting of Neoproterozoic Jinning Granite in the Northern Section of Miaoershan Pluton in Southwestern Hunan

苗儿山岩体位于湘西南与桂北交界处,其北段处于湖南省境内,南段主要位于广西省境内,岩体总出露面积约1 600km2。该岩体大地构造位置上处于扬子地块东南缘江南造山带与华夏地块的结合部位,为晋宁期、加里东期、印支期和燕山期花岗岩组成的复式岩体。其中晋宁期花岗岩可划分为第一、第二和第三侵入期次,分别对应猫儿界岩体群、谭家坳岩体群和报木坪岩体群,其成岩年龄分别为811.3,(807±11),(806±9)Ma,不同侵入期次间隔时间短,岩浆活动连续,并在空间上具有从北东往南西依次侵入的特点。晋宁期花岗岩经加里东早期动力变质作用后,已经全部变为糜棱岩化花岗岩或花岗质糜棱岩,按化学成分主要属花岗闪长岩。晋宁期花岗岩属铝过饱和的钙碱性酸性花岗岩类,其Al_2O_3含量高,铝饱和度(A/CNK)和铝饱和指数(ASI)均大于1.1,富集Rb、K、Th、U和稀土元素,强烈亏损Ba、Nb、Sr、P、Ti元素,Nb/Ta比值(9.36)较小,显示出地壳物质熔融形成的强过铝质S型花岗岩的特征,但其SiO_2质量分数(69.38%)较低,K/Na比值(0.85)小于1,且铁镁质成分TFeO和MgO质量分数(4.04%和2.34%)偏高,与典型的华南壳源花岗岩略有不同,暗示可能有少量幔源物质混入。Rb/Sr-Rb/Ba图解表明晋宁期花岗岩的源岩主要是难熔的低成熟度的砂屑岩。多组微量元素构造环境判别图解表明晋宁期花岗岩为与洋壳俯冲有关的岛弧型花岗岩,暗示华南洋俯冲板片的折断和拆沉引发深部地幔上涌致使基底地壳的部分熔融而形成花岗质岩浆活动,同时有少量幔源成分的加入。晋宁期花岗闪长岩糜棱岩化形成于郁南运动和北流(崇余)运动之中,反映在加里东早、中期的造山运动中,除了强烈的碰撞挤压作用外,还兼具水平方向的韧性剪切作用。

Miaoershan pluton, exposed about 1 600 kmz in area, is located at the junction of southwestern Hunan and northeastern Guangxi. The pluton, a complex pluton composed of Jinning, Caledonian, In- dosinian and Yanshan granites, is at the binding site of Jiangnan orogenic belt of southeastern margin of the Yangtze block with Cathaysia Block. The Jinning granites can be divided into the first, second and third time intrusions, respectively corresponding to the Maoerjie intrusion group, Tanjiaao intrusion group and Baomuping intrusion group, whose diagenetic ages are respectively 811.3 Ma, （807±11） Ma and （806 ±9）Ma. The interval time of intrusions intruding indifferent times is short, and the magmatic activity is continuous, and thus has the characteristics of invading successively from the northeast to southwest in space. After going through early Caledonian dynamic metamorphism, The Jinning granites have all turned into mylonitized granite or granitic mylonite. Accordingto the chemical composition, the Jinning granites are mainly granodiorite. The Jinning granites are peraluminous calc-alkaline acidic granitoids, which have high content of Al2O3, and their alumina saturation （A/CNK） and alumina saturation index （ASI） are both greater than 1.1, and they are enriched in Rb, K, Th, LT and rare earth elements, and strongly de- pleted in Ba, Nb, Sr, P, Ti elements, and their Nb/ Ta ratio （9.36） is relatively small. The above char- acteristics show that the Jinning granites are probably strongly peraluminous S type granite formed by melting of crustal material, but their SiO2 content （69.38%） is low, and their ratio of K/Na （0.85） is smaller than 1, and their femic components content （TFeO and MgO （4. 04% and 2. 34%） ） is high, which are slightly different from the typical South China crust-derived granite, suggesting that there may be a small amount of mantle material mixed with the Jinning granites. The Rb/Sr--Rb/Ba diagram shows that the source rocks of the Jinning granite are mainly refractory low-mature psammite. Multiple trace ele- ment tectonic discrimination diagrams show that the Jinning granites are island-arc granites related to the subduction of oceanic crust, indicating that the break and delamination of the subducting slab of Huanan- yang caused deep mantle upwelling, which resulted in partial melting of basement crust, and formed gra- nitic magmatism, mixed with a small amount of mantle source composition. The mylonization of Jinning granodiorite formed in the Yunan movement and the Beiliu （Chongyu） movement, reflects that in the early and middle Caledonian orogeny, except strong collision effect, there was ductile shearing in the horizontal direction.