太行山南段西安里早白垩世角闪辉长岩的成因:锆石U-Pb年龄、Hf同位素和岩石地球化学证据

Petrogenesis of the Early Cretaceous Xi'anli Hornblende-Gabbros from the Southern Taihang Mountains: Evidence from Zircon U-Pb Geochronology,Hf Isotope and Whole-Rock Geochemistry

介绍了太行山南段西安里岩体角闪辉长岩的锆石U-Pb年代学、锆石Hf同位素以及岩石地球化学资料,以便对该类岩石的成因给予制约.西安里岩体主要由一套含橄榄石角闪辉长岩-角闪闪长岩-闪长岩构成.角闪辉长岩中的锆石自形程度较高,内部结构均匀,呈条痕状吸收,Th/U值介于1.44~2.85之间,表现出岩浆成因特征,其206Pb/238U年龄介于135~127 Ma之间,15个点的加权平均年龄为131±1 Ma,即角闪辉长岩形成于早白垩世;锆石的εHf(t)值介于-23.5^-19.2之间.角闪辉长岩中SiO2=50.27%~50.89%,Mg#=0.76~0.78,Na2O/K2O=2.18~2.37,富含过渡族元素Sc(19.2~20.3)×10-6、Cr(939~1 050)×10-6、Co(54.6~58.2)×10-6、Ni(645~718)×10-6.同时角闪辉长岩相对富含大离子亲石元素(LILEs,如Cs、Ba、Sr)和轻稀土元素(LREEs)、强烈亏损高场强元素(HFSEs,如Nb、Ta、Zr、Hf)和重稀土元素(HREEs).结合角闪辉长岩中纯橄岩包体和橄榄石捕虏晶的矿物化学和地球化学研究,认为西安里角闪辉长岩是拆沉的陆壳物质熔融的熔体与地幔橄榄岩反应的产物.

This paper reports zircon U-Pb geochronology,Hf isotope,and whole-rock geochemical data,aimed at constraining the petrogenesis of the hornblende（Hb）-gabbros from the Xi’anli intrusion in the southern Taihang Mountains（Mts.）.The intrusion is composed mainly of olivine-bearing Hb-gabbro,Hb-diorite,and diorite.Zircons from the Hb-gabbros display euhedral shape and zonal homogeneous absorption on CL images,and have high Th/U ratio（1.44-2.85）,suggesting a magmatic origin.Their 206Pb/238U ages range from 135 to 127 Ma,yielding a mean age of 131±1 Ma（MSWD=6.5,n=15）,which suggests that the Xi’anli Hb-gabbros were formed in the Early Cretaceous.The εHf（t） values of selected zircons range from-23.5 to-19.2.The Xi’anli Hb-gabbros have SiO2 = 50.27%-50.89%,Mg# =0.76-0.78,and Na2O/K2O= 2.18-2.37,and are enriched in transitional elements such as Sc（19.2-20.3）×10-6,Cr（939-1 050）×10-6,Co（54.6-58.2）×10-6,and Ni（645-718）×10-6.Moreover,these gabbros are characterized by relative enrichment in large ion lithophile elements（LILEs,such as Cs,Ba,Sr） and light rare earth elements（LREEs）,and intensive depletion in high field strength elements（HFSEs,such as Nb,Ta,Zr,Hf） and heavy rare earth elements（HREEs）.Combined with the mineralogical and geochemical study of the dunite xenolith and the olivine xenocrysts hosted in the Hb-gabbros,we propose that the Xi’anli Hb-gabbros may have been produced by the reaction of the delaminated lower continental crust-derived melt and mantle peridotite.