辽西台里地区斑状花岗质片麻岩特征及其地质意义

Characteristics of Porphyritic Granitic Gneiss in Taili Area, Western Liaoning Province and Their Geological Significance

本文对辽西台里地区的斑状花岗质片麻岩进行了系统的岩石学、地球化学和锆石U—Ph年代学研究，这为了解该地区斑状花岗质片麻岩的特征和形成演化，以及探讨华北克拉通地质演化提供了新的岩石学证据。研究结果表明，台里地区花岗质岩石主要由花岗质片麻岩、斑状花岗质片麻岩和黑云母二长花岗岩组成。斑状花岗质片麻岩形成晚于花岗质片麻岩，与其呈侵入接触关系。其具斑状结构，含较多碱性长石和石英斑晶，结构构造在不同地段表现各异。远离变形带产出的斑状花岗质片麻岩斑晶较大，无构造变形现象，而在变形带内，其变形强烈，发生糜棱岩化，具“眼球状”构造。斑状花岗质片麻岩的Si02和K：O含量分别为67．54％～75．64％和2．89％。4．67％。其富集轻稀土和Ba，Sr，亏损重稀土和Ti，Nb，Ta，P等元素，存在弱的负Eu异常，推测当时源区残留相中含有较多石榴子石。斑状花岗质片麻岩中锆石为岩浆成因锆石，其发育振荡生长环带，并具较高的Th／U比（0．28～1．20）。两个样品的U—Ph定年结果（加权平均年龄）分别为（224．6±2．5）Ma和（216．4±0．5）Ma。斑状花岗质片麻岩的原岩为I型花岗岩，其源岩是下地壳的中基性火成岩。斑状花岗质片麻岩的原岩形成于晚三叠世，其为华北克拉通中生代岩石圈减薄的岩石学标志。

The characteristics, formation, and evolution of the porphyritic granitic gneiss in Taili area of western Liaoning Province was studied in detail based on petrology, geochemistry and zircon U-Pb dating. This study provides new petrological evidence for investigating the geological evolution of the North China Craton. The granitoid rocks are mainly composed of granitic gneiss, porphyritic granitic gneiss, and biotite adamellite in the study area. The porphyritic granitic gneiss intruded into and was formed later than granitic gneiss. The porphyritic texture is well-developed with different amount of alkali feldspar and quartz depending on the distance relative to the deformation zone. Phenocrysts are much bigger in the areas far away from the deformation zone, while the porphyritic granitic gneiss with eye-like structures has been highly deformed and mylonitized in the deformaltion zone. The SiO2 and K20 contents of the porphyritic granitic gneiss range from 67.54% to 75.64% and from 2.89% to 4.67%, respectively. It is enriched in light rare earth elements as well as in Ba and Sr; while depleted in heavy rare earth elements Ti, Nb, Ta and P. Chondrite-normalized REE distribution patterns of granitic rocks show weak negative Eu anomalies. Therefore we speculate that the residual phase of the source has more garnets. Geochronological studies have shown that zircons, which are characterized by growth bands and high Th/U ratios （0.28-1.20）, are attributed to magmatic origin. The LA-ICP-MS zircon U-Pb weighted average ages of two samples are （224.6 ± 2.5） Ma and （216.44 ± 0.50） Ma, respectively. In conclusion, we find that the protolith of porphyritic granitic gneiss belongs to I-type granite with source rocks of meso-basic volcanic rocks from the lower crust, and was formed in the late Triassic as evidenced by lithospheric thinning of the North China Craton in the Mesozoic.