摘要
南兴安岭地区的晚中生代中酸性火山岩的元素-同位素地球化学分析结果显示,区域上存在同期的两类中酸性火山岩组合。第一类火山岩以满克头鄂博组的安山-英安岩和流纹岩组合为代表,其SiO2为60．0-74．9％,MgO变化在0．36- 2．04％之间,具有LREE略富集的REE配分模式(Ce／YbCN=2．6-3．0)和富集Rb、Ba、K和不同程度的Nb-Ta、Ti-P-Sr亏损的微量元素特征;在Sr-Nd同位素组成上类似于同期的玄武安山岩(87Sr／86Sr(i)=0．70522-0．70709;εNd(t)=-1．5- -0．4),而且在空间上与玄武岩共生,为玄武质岩浆高度分异或同化混染作用(AFC)的产物。第二类以玛尼吐组英安岩为代表,以富Al2O3(15．87-16．36％)、Na2O(4．63-4．79％,K2O／Na2O<0．4)、高Sr(709-832μg/g)、低Y(6．9-10．4μg／g) 和低HREE(如Yb=0．37-0．51μg／g),具有与现代俯冲带adakite岩石相似的特征;在Sr-Nd同位素组成上,它们较同期或近期玄武安山岩更低Sr和高Nd同位素比值(87Sr／86Sr(i)=0．70409-0．70425;εNd(t)=+0．9-+2．1),为造山带下地壳镁铁质岩石的部分熔融产物,其低Y和HREE以及高Sr的特点说明残留源区中有石榴子石存在,而斜长石为主要熔融相进入熔体。在熔融动力学方面,同期的玄武岩浆底侵提供大量的热源导致下地壳镁铁质岩石的部分熔融作用。南兴安岭地区晚中生代不同成因类型中酸性火山岩为造山带伸展背景下岩石圈地幔起源的玄武岩浆底侵作用和深部岩浆房分异的结果。
Major, trace element and Sr-Nd isotope data of two types late Mesozoic intermediate-felsic volcanic rocks from the southern Da Hinggan Mountain, NE China are reported in this paper. One type is represented by the andesitedacites and rhyolites occurring in the Manketou＇ebo Formation （Fm）. These rocks span a SiO2 range of 60 - 74.9% and MgO range of 0.36 - 2.04% ,characterized by moderate LREE/HREE fractionation （ Ce/YbCN = 2.6 - 3.0） , strong LILE enrichment and variable Nb-Ta and Ti-P-Sr depletion. They have more radiogenic Sr and less radiogenic Nd isotopic compositions （^87Sr/^86Sr（i） = 0. 70522 -0. 70709;εNd （t） = -1. 5 - - 0.4） than the contemporaneous basaltic lavas. In combination with the spatial coexistence between them and the basaltic lavas, we interpret these melts as differentiates of the primary basaltic magmas via variable degrees of fractional crystallization or assimilation and fractional crystallization （AFC）. The other type of intermediate-felsic lavas occurs in the Manitu Fm. They are dacites rich in Al2O3 （ 15.87 - 16.36% ） and Na2O （4.63 -4.79%, K2O/Na2O 〈0.4）, high Sr （709 - 832μg/g） and low Y （6.9 - 10.4 μg/g） and HREE （e. g. , Yb = 0.37 - 0. 51μg/g） , signatures of modern adakites from the subduction zones.Compared with the basaltic lavas, they have less radiogenic Sr but more radiogenic Nd isotopic compositions （^87Sr/^86Sr（i） = 0. 70409-0. 70425; εNd （t） = ＋0.9 - ＋ 2. 1 ）, arguing against an origin of differentiates of the contemporaneous basahs. These rocks are considered as melts of mafic protoliths from the lower crust beneath the orogen under the heating of the underplating basaltic magmas.The adakitic trace element features suggest the presence of garnet and amphibole during melting with decomposition of plagioclase. The petrogenesis of these two types late Mesozoic intermediate-felsic rocks favors a lithospheric extension model in post-orogenic stage,during which mantle-derived basahs underwent extensive different
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2005年第3期737-748,共12页
Acta Petrologica Sinica
基金
本文得到国家自然科学基金项目(No.40734343)中国科学院创新项目(KZCX2-104)的共同资助
关键词
地球化学
岩石成因
中酸性火山岩
晚中生代
南兴安岭
Geochemistry, Petrogenesis, Intermediate-felsic volcanic rocks, Late Mesozoic, Southern Da Hinggan Mountain
