新疆博格达山东段早石炭世火山岩地球化学特征及其构造意义

Geochemical characteristic and tectonic significance of the early carboniferous volcanic rocks in eastern Bogda mountains of Xinjiang region

博格达山东段北缘分布有早石炭世玄武岩和流纹质火山碎屑岩及少量英安岩夹层,在时空上构成双峰式火山岩组合.玄武岩Rb-Sr等时线年龄为322±3 Ma(1σ-),w(SiO_2)为47.74%～49.34%,Ti的质量分数(w(TiO_2)=1.51%～1.75%)略高于N-MORB的,高Al(w(Al_2O_3)=16.82%～17.54%),富钠贫钾(w(K_2O)/w(Na_2O)=0.21～0.28),低Mg(w(MgO)=3.80%;7.33%,w(Mg~#)=42～47).表明玄武岩浆发生过橄榄石、辉石分离结晶作用.玄武岩具较高的w(Zr)/w(Y)(4.1～5.5),w(Zr)/w(Nb)(27～33),相对富集LIL(Sr,Rb,Ba,K),亏损Nb,Ta,Th,U等高场强元素,以及(w(^(87)Sr)/w(^(86)Sr))_I为0.703188～0.703 295(<0.704 5),正εNd(t)值(+7.10).岩石LREE适度富集,(w(La)/w(Yb))N=2.56～3.66,Eu无异常或轻微正异常(δEu=1.08～1.15).地球化学特征表明:研究区玄武岩源于亏损地幔,且在岩浆作用过程中可能发生过下陆壳物质的混染作用,但斜长石分离结晶作用不明显,形成于陆内裂谷环境.

Early carboniferous basalt, acidic volcanic classic rocks and intercalated dacite developing in the north edge of eastern Bogda orogenic belt, Xinjiang, northwest China, are bimodal volcanic, rocks. The basalt with Rb-Sr isochron age of 322±3 Ma（1σ） has w（SiO2） content of 47.74%-49.34%, relatively high Ti （w（TiO2）=1.51%-1.75%）, Al （w（Al2O3）=16.82%- 17.54%）, and low Mg contents （w（MgO）=3.80%-7.33%, w（Mg^#）= 42-47）, and rich in Na but depleted in K contents （w（K20）/w（Na2O）=0.21-0.28）, showing obvious fractional crystallization of olive and pyroxene during basaltic magmatism; the rocks exhibit relatively high w（Zr）/w（Y）（4.1-5.5） and w（Zr）/w（Nb） （27-33）, rich in LIL （Sr, Rb, Ba and K） and moderately enriched in LREE, （w（La）/w（Yb））N=2.56-3.66, slightly positive Eu anomalies （SEu=1.08-1.15）, and depleted in HFSE （Nb, Ta, Th and U）, and have （w（^87Sr）/w（^86Sr））i=0.703188-0.703295 （〈0.7045） and positive εNd （t） （＋7.10）. All the above geochemical features suggest that the basalts originated from the partial melting of depleted mantle, without the fractional crystallization of plagioelase but with contamination of lower continental crust during the basaltic magmatism, and formed in the continental rift.