中天山东段花岗岩类钕锶氧同位素及地壳形成年龄

A STUDY OF Nd, Sr, AND O ISOTOPES AND THE CRUSTAL GROWTH AGE OF GRANITES IN EASTERN SEGMENT OF MIDDLE EAST TIANSHAN BELT, CHINA

作者对中天山东段7个花岗岩类岩体作了Nd、Sr和O同位素研究,认为平顶山、选矿场后山和天湖东岩体属原地-半原地改造型;砂垄东侧岩体属于幔源型和同熔型的过渡类型;尾亚岩体属同熔型;白石头泉岩体为高度分异的改造型。上述改造型花岗岩类来源于早、中元古代的地壳物质,而同熔型花岗岩类是来源于较年轻的地幔派生物与上述古老地壳物质的混合源区。中天山最老的地壳形成时代约为2.0Ga,然而也不排除外来的少量的晚太古代地幔物质参与了中天山陆壳增长。1.7～2.0Ga是中天山陆壳增长的重要时期,它与元古代时期内的一次全球性陆壳增长的重大地质事件相同步。

Systematical research on Sm-Nd, Rb-Sr and O isotopes and the crustal growth age were made on seven granitic intrusions in the eastern segment of middle East Tianshan belt. The Rb-Sr isochron ages, average value of Nd, Sr and O isotopic compositions in seven granitoids are given below:Pingdingshan augen-gneissic monzonitic granite,913.8 Ma, ε_(Nd)(t)=- 3.9,ε_(Sr)(t)=+208,δ^(18)O= 10.5‰,, t_(Dm)^(Nd)= 2.05Ga; Back hill gneissic granodiorite, 724.0Ma, ε_(Nd) (t) = -4.7, ε_(Sr)(t)=+165, t_(DM)^(Nd)= 1.82 Ga; Tianhudong gneissic granodiorite, 696.6Ma,ε_(Nd)(t) = - 7.7, ε_(Sr)(t) = + 132, δ^(18)O= 14.4‰, t_(DM)^(Nd)= 1.83Ga; Shalongdong monzonitic granite, 470.0 Ma, ε_(Nd)(t)= +2.5, ε_(Sr)(t)=-2.3, δ^(18)O=8.3‰, t_(DM)^(Nd)=0.82Ga; Weiya manzonitic granite (internal zone), 379.1 Ma, ε_(Nd)(t)=-0.95,ε_(Sr)(t)=+10.1, δ^(18)O=7.5‰, t_(DM)^(Nd)=1.14Ga; Weiya monzonitic granite and granodiorite (intermediate zone), 379.7Ma, ε_(Nd)(t)=-1.3, ε_(Sr)(t)=+28.3, δ(18)O= 8.9‰, t_(DM)^(Nd)= 1.15 Ga; Baishitou Spring amazonite granite, 209.6Ma, ε_(Nd)(t)=+0.36, ε_(Sr)(t)= (an anomaleous value), δ^(18)O=9.3‰, t_(DM)^(Nd)= (an anomaleous value. ) In combination with the geological evidence and other geochemical data, it is believed that the granites in eastern segment of middle East Tianshan belt are composed of granitoids of different ages, different genesis, and different source materials.The Xuefengian and Chengjiangian granites belong to a transformation type or S-type and are mainly autoch-parautochthonous syntectonic granites characterized by gneissic structure. Middle Caledonian granites posibly belong to an intermediate type between mantle-derived type and syntectonic type or Ⅰ-type derived from a mixed mantle-crustal source (predominantely mantle). Upper Caledonian granites belong to syntectic type or Ⅰ-type derived from a mixed mantle-crustal source and possible formed by a differentiated mantle magma assimilated and contaminated with crustal materials. The only difference in the syntectic-type granites is that the Upper Caledonian granites have a higher crustal component to mantle component ratio than that of Middle Caledonian. Late Indosinian granites are highly evolved and distinct in their two-mica and amazonite components, and not belong to autochthonous transformation type or S-type, and possible formed by partial melting of predominantly sedimentary parent rocks (possibly also containing some deep-source materials). According to the trend of evolution of ε_(Nd)(0) in the granitoids, it was suggested that the age of the old basement is 1.0～2.0Ga. Available Sm-Nd isotopic data for late Proterozoic to early Paleozoic granitoids in the eastern segment of middle East Tianshan belt show Middle Proterozoic to early Pro terozoic model ages(t_(DM)^(Nd)) from 0.9 to 2.1Ga with a mean of 1.5Ga.The Sm-Nd model age arguments do not exclude the possibility of a mixed source for Upper Archaean rocks and younger juvenile crust.However, Lower Proterozoic model age about 1.7～2.0 Ga is believed to represent an important period for continental growth in the Middle Tianshan and the age of the oldest basement (or crustal formation age) in Middle Tinsahan is about 2.0 Ga.