湘西南印支期瓦屋塘岩体年代学、成因与构造环境

摘要

瓦屋塘岩体位于湘西南,主要由黑云母二长花岗岩和二云母二长花岗岩组成,少量黑云母花岗闪长岩。2个花岗岩样品的锆石SHRIMP U-Pb年龄分别为216.4±2.4 Ma、215.3±3.2 Ma,属晚三叠世。岩石具有富硅(SiO2=68.39%~77.77%)、富铝(Al2O3=12.39%~16.43%)、高钾(K2O=4.27%~6.02%)、中碱(Na2O+K2O=7.08%~8.57%)、高 ASI(平均1.19)的特点,总体属高钾钙碱性系列强过铝质花岗岩类。微量元素中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)等相对富集,稀土总量较低(ΣREE=81.72~216.23μg/g),轻稀土富集((La/Yb)N=1.91~12.18),具明显的Eu负异常(δEu=0.09~0.78)。岩体具有较高的ISr值(0.71061~0.71786)和较低的εNd(t)值(–8.63~–4.82),两阶段Nd模式年龄(tDM2)为1.38~1.69 Ga。C/MF-A/MF图解显示源岩为泥质岩和碎屑岩。多数样品Al2O3/TiO2<100,少量>100。上述地球化学特征,表明花岗岩源岩主要为中上地壳酸性岩石,并可能有少量地幔物质加入。岩石氧化物和微量元素构造环境判别图解主要显示为后碰撞构造环境。基于上述岩石成因、构造环境判别,并结合区域构造演化过程,推断瓦屋塘岩体的形成机制为：中三叠世印支运动导致地壳增厚、升温,晚三叠世中期进入挤压应力相对松弛、深部压力降低的后碰撞构造环境,中上地壳岩石减压熔融并向上侵位；此外,软流圈上涌和热量的向上传递可能对瓦屋塘花岗质岩浆形成也起到一定作用。%TheWawutang granitic batholith in southwestern Hunan consists mainly of biotite monzogranite and dimicaceous monzogranite, and minor biotite granodiorite. All the rocks have massive structure. Zircon U-Pb dating indicates that the batholith was formed in the Late Triassic with zircon U-Pb ages of 216.4±2.4 Ma to 215.3±3.2 Ma. Geochemically, it is a high-K calc-alkaline strongly peraluminous granitic batholith and characterized by high SiO2 (68.39%‒77.77%), Al2O3(12.39%‒16.43%) and K2O (4.27%‒6.02%) contents, moderate alkali (Na2O+K2O=7.08%‒ 8.57%), as well as high ASI (1.19 on average). The granites are depleted in Ba, Nb, Sr, P and Ti, but enriched in Rb, (Th, U, K), (La, Ce), Nd, (Zr, Hf, Sm) and (Y, Yb, Lu). The granites have rather low REE contents (ΣREE = 81.72‒216.23μg/g), relatively enriched in LREE ((La/Yb)N=1.91‒12.18) with negative Eu anomalies (δEu=0.09‒0.78). The highISr (0.71061‒0.71786) and lowεNd(t) (–8.63‒–4.82) values of the granites, as well as the C/MF-A/MF diagram demonstrate a crustal origin, and very likely derived from mudstones and clastic rocks. Actually, Al2O3/TiO2 values of most samples are less than 100, while only a few are greater than 100. All above geochemical characteristics and the occurrence of mafic microgranular enclaves suggest that the granites should come mainly from acid rocks of the middle crust with participation of mantle material. Multiple oxide- and trace element-diagrams for discrimination of structural environment show that the granites were formed in post-collisional tectonic setting. Based on petrogenesis and discrimination of structural environment, and combined with regional tectonic evolutional setting, the formation of the Wawutang granites might due to the partial melting of the thickened middle-upper crust in the Late Triassic post-collisional setting, during which heat from the upwelling of asthenospheric mantle might have played a role.