摘要
白石头泉含黄玉的天河石花岗岩体Rb-Sr等时线年龄209.6±9.6Ma,从下至上可分为5个连续过渡的岩相带,即淡色花岗岩(a带),含天河石花岗岩(b带),天河石花岗岩(c带),含黄玉天河石花岗岩(d带)以及黄玉钠长花岗岩(e带)。岩体的岩石地球化学特征是高F(>2%)、高Rb(500×10-6～1087×10-6),低P2O5(≤0.06%),Na2O>K2O,弱过铝(A/NKC=1.00～1.11)、翼型稀土元素配分曲线(ΣREE=28.6×10-6～231.9×10-6)、低(La/Lu)N值(0.11～0.68)、强烈Eu负异常(Eu/Eu*=0.0005～0.0110)、Nd同位素富集(εNd(t)=-4.4～-4.9)。该岩体的岩浆是中地壳云母片麻岩部分熔融的产物。从a带到e带的地球化学变化是:(1)F,A2O3和Na2O含量逐渐增加,而SiO2,(Fe2O3+FeO+MgO+MnO)、FeO和K2O含量逐渐减少,在标准矿物的Qz-Ab-Or图上总体向Ab角顶移动;(2)总体而言,Cr,Ni,Co,V,W,Nb,Zr,U,Th和Y含量逐渐减少,而F,Li,Rb,Hf,Ta,Sn,Sc,Ga和Zn含量逐渐增加,但d带到e带间存在Li,Rb,Sn,Sc和Zn含量的突降;(3)K/Rb,Al/Ga,Nb/Ta和Zr/Hf值下降,但K/Cs,Th/U,(La/Lu)N值上升;(4)全岩的δ18O值从a带的9.25‰～9.75‰降低到e带的7.32‰,d带与e带间存在2.1‰的δ18O值突降。岩浆从a带到e带的垂向分带是分离结晶和流体输运的共同结果。岩体的d带与e带存在明显的成分间断。在矿物成分上表现为黄玉、钠长石和白云母的剧增,钾长石和天河石的剧减。在主量元素上表现为Na2O和CaO含量的剧增,SiO2和K2O含量的剧减。在微量元素上表现为F,Ga,Sr和Ba含量的剧增,Li,Rb,Sc,Zn和Sn含量的剧减。在稀土元素上,Eu/Eu*和(La/Lu)N值增加,而ΣREE值降低。在氧同位素特征上,δ18O值显著降低。这种间断不仅受分离结晶和流体输运的制约,也与天水加入、围岩混染和亚固相线淋滤有关。
The Mesozoic Baishitouquan （BST） topaz-bearing amazonite granite pluton has been dated at 209.6±9.6 Ma by the Rb-Sr isochron method. This pluton exhibits five lithological zones, which, gradational from the lowest level upwards, are leucogranite （zone a）, amazonite-bearing granite （zone b）, amazonite granite （zone c）, topaz-bearing amazonite granite （zone d） and topaz albite granite （zone e）. Geochemically, The rocks are characterised by higher F （〉2%） and Rb （500-1087×10^-6）, lower P2O5 （≤0.06 %）, Na2O〉K2O, A/NKC=1.00-1.11, ∑14 REE=28.6-231.9×10^-6 with gull wing-shaped distribution patterns （LaN/LuN=0.11-0.68） and strong Eu depletion （Eu/Eu^＊ = 0.0005-0.0110）. δ^18O=9.75-7.32 ‰, εNd（t）= -4.4～-4.9. The magma for this pluton was derived by partial melting of a mica gneiss in the middle crust. The pluton exhibits the following geochemical transition from zone a to zone e： 1） Increasing F, Al2O3 and Na2O, and decreasing SiO2, （Fe2O3＋FeO＋MgO＋MnO） and K2O. Plots of normative compositions on the Qz-Ab-Or diagram move gradually towards the Ab apex. 2） Overall, contents of Cr, Ni, Co, V, W, Nb, Zr, U, Th and Y decrease, while contents of F, Li, Rb, Hf, Ta, Sn, Sc, Ga and Zn increase. 3 ） K/Rb, Al/Ga, Nb/Ta and Zr/Hf decrease, and K/Cs, Th/U, and （La/Lu）N increase; 4） Whole-rock δ^18O decreases from 9.25-9.75‰ in zone a to 7.32‰ in zone e. It is interpreted that crystallisation of the magma started from zone a and proceeded upwards to zone e, and the vertical zoning was produced by fractional crystallisation accompanied by fluid fractionation. There is a compositional gap on the transition trend between zone d and e. This gap is manifested in mineralogy by sharp increase in topaz, albite and muscovite and decrease in K-feldspar and amazonite; in major elements by sharp increase in Na2O and CaO and decrease in SiO2 and K2O; in trace elements by sharp increase in F, Ga, Sr and Ba, and sudden decrease in Li, Rb, Sc, Zn and Sn; in
出处
《高校地质学报》
CAS
CSCD
北大核心
2007年第2期207-223,共17页
Geological Journal of China Universities
基金
国家自然科学基金(40603008
40472042
40672040)
国家重点基础研究发展规划(No.2001CB409802)项目资助
关键词
淡色花岗岩
萤石
黄玉
天河石
分带
分离结晶
东疆
leucogranite
fluorine
topaz
amazonite
zoning
fractional crystallization
eastern Xinjiang
