准噶尔盆地陆东-五彩湾地区石炭系火山岩地球化学特征及其构造背景

Geochemical characteristics and tectonic settings of Carboniferous volcanic rocks from Ludong-Wucaiwan area,Junggar basin.

准噶尔盆地陆东—五彩湾地区石炭系火山岩位于上石炭统巴塔玛依内山组地层中,主要为玄武岩、玄武质安山岩和安山岩,基本属于钙碱性系列。火山岩的Mg~#介于38～57之间,全碱含量较高(K_2O+Na_2O为4.75%～6.16%),TiO_2的含量(0.96%～2.04%)和Na_2O/K_2O比值(均大于2)较高,轻稀土元素(LREE)相对重稀土元素(HREE)富集((La/Yb)_N=2.4～4.7),同时显示有轻微的重稀土分馏((Gd/Yb)_N=1.5～1.8),无明显的Ce异常,大部分样品具有轻微的Eu负异常(δEu= 0.89～1.05),大离子亲石元素(LILE)相对高场强元素(HFSE)富集,Nb、Ta相对于LREE和LILE明显亏损,La/Nb比值为2.0～2.8,Th/La比值(≈0.1)较低,Zr/Y比值(3.95～7.03)较高。火山岩(^(87)Sr/^(86)Sr);=0.70278～0.70365,ε_(Nd)(t)=+5.69～+8.24,t_(DM)为0.47～0.67Ga。这些特征连同相关判别图解一起表明,这套火山岩不是形成于典型的板内或岛弧环境,而是形成于后碰撞期伸展背景下,是软流圈物质上涌发生部分熔融、产生的岩浆在上升和侵位的过程中受到了晚石炭世之前弧组分混染的产物。其所携带的弧岩浆特征继承自混染的碰撞前的弧组分。

Carboniferous volcanic rocks from Ludong-Wucaiwan area of Junggar basin mainly composed of basalts, basaltic andesites and andesites occur within Upper Carboniferous Batamayineishan Formation, and are mainly Calc-alkaline series. These rocks have moderate Mg-#(38 similar to 57) values, relatively high alkali K2O + Na2O)=4.75%similar to 6.16%), TiO2 (0. 96%similar to 2.04%) contents and Na2O/K2O ratios(>2). They are enriched in LREE ((La/Yb)(N)=2.4 similar to 4.7), and with slight fractionation of HREE ((Gd/Yb)(N)= 1.5 similar to 1.8), and possess weak Eu anornalies(delta Eu=0.89 similar to 1.05), without significant Ce anomalies. These samples are enriched in LILE but relatively depleted in HFSE, with strong negative Nb-Ta anomalies. They have high La/Nb (2.0 similar to 2.8), Zr/Y (3.95 similar to 03), but low Th/La(approximate to 0. 1) ratios. Furthermore, They have relatively low I-M (0.70278 similar to 0.70365, iota(DM) (0.47 similar to 0.67Ga), and high (iota) (+5.69 similar to+8.24). These characteristics, along with the discrimination diagrams, indicate that these, volcanic rocks are from extensional setting of post-collisional period, rather than typical intraplate or island are environments, and they were produced by the ascending and melting of asthenospheric material, and the magmas were. contaminated by the are component formed before I,by Carboniferous in the processes of uplift and emplacement. The are charUCteristics carried by the volcanic rocks were inherited from the pre-collisional are component which had contaminated the source region.