湘西南苗儿山地区早燕山期花岗岩地球化学特征及形成环境

Geochemistry and tectonic setting of the early Yanshanian granites in the Miao′ershan area,southwest Hunan Province

湘西南苗儿山地区早燕山期花岗岩形成于晚侏罗世早期,侵入于加里东期花岗岩(构成苗儿山岩体主体)和印支期花岗岩中。主要岩石类型为细粒—中粗粒斑状黑云母二长花岗岩,局部发育细粒二云母二长花岗岩。岩石SiO_2为76.02%~80.26%、Al_2O_3为10.94%~12.88%、K2O为3.42%~5.34%、Na_2O+K_2O为5.37%~8.22%、ASI为1.04~1.31(平均1.14),总体属铁质、钙碱性系列过铝质花岗岩类。微量元素中Ba、Sr、P、Ti表现为强烈亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等相对富集。稀土总量较低(122.9~175.4μg/g),轻稀土略富集((La/Yb)N=2.55~3.79),具明显的负Eu异常(δEu=0.07~0.22)。岩体ISr值为0.99007和1.15860,εNd(t)值为-9.20和-8.80,两阶段Nd模式年龄(t2DM)为~1.7Ga。C/MF-A/MF图解显示源岩主要为变质泥质岩和变质杂砂岩。强过铝花岗岩样品的Al_2O_3/TiO_2比值部分<100。上述地球化学特征表明花岗岩为S型花岗岩,源岩主要为中、上地壳酸性岩石,并有少量地幔物质加入。花岗岩主量和微量元素构造环境判别图解以及区域构造演化过程表明花岗岩形成于后造山构造环境,岩浆形成与先期(中侏罗世)陆壳增厚升温及软流圈地幔的热传递有关。

The early Yanshanian granites emplaced in early Late Jurassic in the Miao′ershan area,southwest Hunan Province,were mainly intruded in the Caledonian and Indosinian granites and were mainly composed of fine grain and medium to coarse grain porphyritic biotite monzogranites with minor fine grain dimicaceous monzogranites.The rocks are characterized by their content of SiO_2（76.02% ~80.26%）,Al_2O_3（10.94%~12.88%）,K_2O（3.42%~5.34%）,Na_2O＋K_2O（5.37%~8.22%）and ASI values（1.04~1.31,1.14 on average）,showing features of the ferric,calc-alkaline perluminous granitoids.On the PM-normalized incompatible elements spiderdiagram,significant Ba,Sr,P and Ti trough and Rb,（Th＋U＋K）,（La＋Ce）,Nd,（Zr＋Hf＋Sm）and（Y＋Yb＋Lu）humps are observed.The rocks have lower total REE content（ΣREE =122.9~175.4μg/g）,slightly enrichment of LREE（（La/Yb）N=2.55~3.79）and pronounced negative Eu anomaly（δEu = 0.07~0.22）.As for isotope,they have ISrvalues of0.99007 and 1.15860,εNd（t）values of-9.20and-8.80 and t2DMages of^1.7Ga.The C/MF-A/MF diagram indicates that the granites were derived from metamorphic mudstones and clastic rocks,Al_2O_3/TiO_2 values of some strong perluminous granite samples are less than 100.All above geochemical characteristics reveal their S-type signatures and suggest that the granites were derived from partial melting of acid rocks of middle-upper crust with participation of mantle materials.Multiple oxide-and trace element-diagrams for discrimination of structural environment and regional tectonic geology show that the granites were formed in a post-orogenic tectonic setting.The forming of magma was related with the former thickening of crust in Middle Jurassic and the up-welling and heat transference of asthenosphere mantle.