西秦岭夏河花岗岩的地球化学、年代学及地质意义

Geochemistry,chronology and geological significance of the granitoids in Xiahe,West Qinling

甘肃西秦岭夏河花岗岩主要由二长花岗岩及花岗闪长岩组成,岩体内发育较多暗色微粒包体。全岩地球化学分析结果表明,二长花岗岩SiO2和全碱含量(分别为66.07%~69.18%、7.18%~7.69%)高于花岗闪长岩的平均值(分别为60.18%~66.22%、4.73%~6.72%),而Al2O3和CaO含量(分别为15.21%~16.37%、2.23%~3.04%)则低于花岗闪长岩的平均值(分别为16.60%~17.39%、3.54%~5.86%),两类岩石的K2O/Na2O比值均大于1,Mg#偏高(平均为55),铝饱和指数A/CNK介于1.0~1.1,属于弱过铝质的高钾钙碱性I型花岗岩类。夏河花岗岩类富集轻稀土而亏损重稀土,(La/Yb)N比值介于8.97~33.48,无或弱负Eu异常,以富集Rb、Ba、Th、U、K并相对亏损Nb、Ta、P、Ti、Zr为特征。锆石LA-ICP-MS U-Pb测年结果确定夏河岩体的侵位年龄为244~248Ma,属印支早期。夏河花岗岩类具有较高的(87Sr/86Sr)i(0.70714~0.70743)、较低的εNd(t)(-6.21^-8.36)和高放射性成因Pb(206Pb/204Pb=17.850~18.003,207Pb/204Pb=15.489~15.510,208Pb/204Pb=37.815~37.931)同位素组成,表明该花岗岩主要来自中元古代下地壳变基性岩的部分熔融,且幔源岩浆的参与可能对该岩体的形成具有重要作用。综合该区已有研究成果,本文提出夏河岩体形成于大陆边缘弧构造环境,推测西秦岭地区阿尼玛卿-勉略洋盆的闭合时间晚于244Ma。

Xiahe granitoids consisting of monzogranite and granodiorite are located in West Qinling, Gansu Province, in which mafic microgranular enclaves are also well developed. The whole rock geochemical data shows that the content of SiO2 and total alkali of monzogranite （66. 07% - 69. 18%, 7. 18% - 7.69%, respectively ） are slightly higher than those of granodiorite （60. 18% 66. 22%, 4.73% -6. 72% , respectively）, while the content of Al2O3 and CaO of monzogranite （ 15.21% - 16. 37% , 2. 23% - 3.04%, respectively） are lower than those of granodiorite （ 16. 60% - 17.39%, 3.54% - 5.86%, respectively）. These granitoids characterized by high Mg# value （with an average of 55 ）, K2O/Na2O 〉 1 and A/CNK = 1.0 - 1.1 belong to the slightly-peraluminous high-K calc-alkaline rock series and show similar geochemical characteristics to I-type granite. The rocks are enriched in light rare earth elements （ Rb, Ba, Th, U and K） and depleted in Nb, Ta, P, Ti and heavy rare earth elements, having no or weak negative Eu abnormality and a wide variability of the （La/Yb） N values （ 8. 97 - 33.48）. The zircon U-Pb data show that the emplacement age of Xiahe granitoids is 244 -248Ma, belonging to the early stage of Indonian. The high （87Sr/86Sr）i（0. 70714 -0. 70743）, low eNd（t） （ -6. 21 - --8. 36 ） and high radiogenic Ph isotopic compositions （206 Pb/204 Pb = 17. 850 - 18. 003, 207 pb/204 pb = 15.489 - 15.510, 20s pb/204 Pb = 37. 815 - 37. 931 ） indicate that Xiahe granitoids were mainly originated from the partial mehing of ancient lower crustal materials and the mantle-derived magmas may play an important role in the formation of the host granitoids. Based on the existing research results, we suggest that Xiahe granitoids were formed in a continental marginal are setting which, in other words, implies that the Ahyemagen-Mianlue ocean basin closed after 244Ma in West Qinling.