河南新县花岗岩岩基的岩石成因、来源及对西大别构造演化的启示

Petrogenesis and Origin of the Xinxian Granitic batholith in Henan Province and Its Implication for the Tectonic Evolution of the Western Dabie Area

本文对新县花岗岩岩基进行了系统的锆石U-Pb定年,全岩元素地球化学、Sr-Nd-Pb-Hf同位素测试,研究探讨其成岩年龄、岩石成因、物质来源以及地球化学动力学背景。花岗岩的锆石U-Pb年龄为125.5±1.5Ma,属于早白垩世产物。全岩元素地球化学成分表现为高硅、富碱,贫镁、铁和钙,富集轻稀土元素、大离子亲石元素(Rb、K、Th、U)和Pb,亏损重稀土元素、高场强元素(Nb、Ta、Ti)和Sr、Ba。全岩同位素具有中等略偏高的Sr初始比值[(87Sr/86Sr)i=0.706949~0.707086],低的放射性Pb同位素组成[(206 Pb/204 Pb)i=16.611~17.152,(207Pb/204Pb)i=15.304~15.432,(208Pb/204 Pb)i=37.316~37.702],低的Nd初始比值[εNd(t)=-13.65^-13.51]和老的Nd模式年龄(tDM2=2.12~2.13Ga)。锆石原位Hf同位素具有较低的初始比值[εNd(t)=-22.92^-19.40]和老的Hf二阶段模式年龄(tDM2=2.39~2.61Ga)。综合以上元素地球化学特征、锆石U-Pb定年结果及Sr-Nd-Pb-Hf同位素组成,新县花岗岩岩基应属于分异的高钾钙碱性I型花岗岩,是化学成分类似于扬子板块北缘新元古代TTG型岩浆岩的扬子下地壳在非加厚下地壳(深度小于35km)环境下部分熔融的产物。

In this study, we carried out a comprehensive research using zircon U Pb dating, petrogeochemistry and Sr-Nd-Pb-Hf isotope analysis on the Xinxian granitic batholith to investigate its age, petrogensis, rock source and geochemical dynamic background. Analysis shows that the Xinxian granitic batholith has a zircon 2：58 U-206 Pb age of 125.5 4-1.5 Ma. Whole-rock chemical compositions are characterized by high silica and alkali contents, lower values in Mg, Fe and Ca. The granites are enriched in light REE and large ion lithophile elements （Rb, K, Th, U） and Pb, but depleted in heavy REE, high field strength elements （Nb, Ta, and Ti ）, Sr, and Ba. Isotopic analyses of the Xinxian granitic batholith yieldedinitial Sr ratios （sT Sr/S6Sr）i ranging from 0. 706949 to 0. 707086, low initial Pb isotopic ratios E（2G Pb/TM Pb）,- 16. 611 17. 152, （2Tph/2CPb）=15. 30415. 432,（zSPh/24pb）--37. 31638. 7021, as well as low eNd（t） values （--13. 65 --13. 51） and old Nd model ages （trM2=2. 122. 13 Ga）. Zircon has low Cnr（t） values （-- 22. 92 --19. 40） and old Hf model ages （tDMe =2. 392. 61 Ga）. Based on the results of the whole-rock geochemistry, geochronology and isotopic compositions, it can be concluded that the Xinxian granitic batholith belongs to a high-K calc-alkaline fractionated I type granite, which resulted from partial melting of the lower crust of Yangtze Block which probably consists of Neoproterozoic TTG-like magmatic rocks at the northern margin of the Yangtze Block under a non-thickened lower crust setting （35 km）.