湘西南印支期五团岩体锆石SHRIMP U-Pb年龄、地球化学特征及形成背景

Zircon SHRIMP U-Pb age, geochemistry and tectonic setting of Indosinian Wutuan pluton in southwestern Hunan Province

五团岩体位于湘西南,主要由黑云母二长花岗岩和二云母二长花岗岩组成,具块状构造。用SHRIMP对黑云母二长花岗岩中的锆石进行了U-Pb同位素测试,其中9颗锆石给出的206Pb/238U年龄加权平均值为(220.5±4.4)Ma,该年龄被解释为岩体的形成年龄,表明岩体形成于晚三叠世中期。岩石高硅、富铝、高钾、中碱,Si O2含量70.64%~75.32%,平均72.33%;Al2O3含量13.24%~16.37%,平均14.47%;K2O含量4.27%~6.66%,平均5.10%;全碱(Na2O+K2O)含量为6.71%~8.46%,平均7.65%;K2O/Na2O比值为1.54~5.34,平均2.23。ASI值平均为1.24,总体属铁质、高钾钙碱性-钾玄岩系列过铝质花岗岩类。Ba、Nb、Sr、P、Ti表现为明显亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集。ΣREE含量为84.35~222.90μg/g,平均156.81μg/g;δEu值0.17~0.52,平均0.39;(La/Yb)N值为1.81~17.12,平均10.57。ISr值为0.71813~0.72110,εSr(t)值为193.5~235.6,εNd(t)值为-9.78^-9.70,t2DM为1.78~1.79Ga。C/MF-A/MF图解显示源岩为变质泥质岩和碎屑岩。强过铝花岗岩样品的Al2O3/Ti O2比值大部分小于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。花岗岩微量元素构造环境判别图解主要显示为后碰撞构造环境。基于岩石成因、构造环境判别以及区域构造演化过程,推断五团岩体的具体形成机制为:在中三叠世印支运动之后挤压应力相对松弛、深部压力降低的后碰撞构造环境下,因地壳增厚而升温的中、上地壳岩石减压熔融并向上侵位;此外,深部软流圈上涌和热量的向上传递对五团花岗质岩浆的形成可能起到一定作用。

The Wutuan granitic pluton in southwestem Hunan is mainly composed of biotite monzogranite and dimicaceous monzogranite. U - Pb zircon dating of a biotite monzogranite sample was carried out, and analyses of 9 zircon grains yielded an average age of （220.54-4.4）Ma, which indicates that the granites were formed in middle Late Triassic. The rocks are silicon-high （SiO2=70.64%-75.32%, 72.33% on average）, aluminum-rich （Al2O3=13.24%-16.37%, 14.47% on average）, potassium-high （K2O= 4.27%-6.66%, 5.10% on average） and alkali-moderate （Na2O＋K2O=6.71%-8.46%, 7.65% on average） ones, with K2O/Na2O ratios being 1.54-5.34 （2.23 on average） and ASI value being 1.24 on average, thus belonging to ferric, high-K calc-alkaline series and shoshonite series perluminous granitoids. Ba, Nb, Sr, P and Ti are pronouncedly depleted, while Rb, （Th＋U＋K）, （La＋Ce）, Nd, （Zr＋ Hf＋Sm） and （Y＋Yb＋Lu） are comparatively concentrated. The rocks have low XREE values of 84.35-222.90μg/g （156.81μg/g on average）, 3Eu values of 0.17-0.52 （0.39 on average）, （La/Yb）N values of 1.81-17.12 （10.57 on average）, Is, values of 0.71813- 0.72110, esr（O values of 193.5-235.6, εNd（t） values of-9.78-9.70 and tZDM ages of 1.78-1.79 Ga. The C/MF-A/MF diagram indicates that the granites were derived from mudstones and clastic rocks. Al2O3/TiO2 ratios of most strongly perluminous granite samples are less than 100. All these data point to S-type granitiods and suggest that the granites might have come mainly from acid rocks of the middle- upper crust. Trace element diagrams for discrimination of the structural environment show that the granites were formed in a post-collisional tectonic setting. According to the petrogenesis, the discrimination of structural environment and regional tectonic evolutional setting, the authors infer that the formation mechanism of the Wutuan pluton should be as follows： the thickening of the crust in Indosinian movement caused the rise of the temperature of the crust, and later the weakening of the stress in the post-collisional tectonic setting caused the melting of the middle-upper crust, with the magma emplaced in a relatively open environment. In addition, upwelling and heat transference of asthenosphere mantle might have played a certain role in the formation of granitic magma.