【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。更多还原

【Abstract】 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, Σ14 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). δ18O=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 δ18O 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 REE behaviour by sudden increase in Eu/Eu* and (La/Lu)N, and decrease in ΣREE; and in isotope composition by sharp decrease inδ18O. Such a gap might have been constrained not only by fractional crystallization and fluid fractionation, but also by involvement of meteoric water, contamination of wallrock-derived components, and leaching and dispersion due to subsolidus metasomatism.更多还原