准噶尔盆地陆梁地区基底火山岩的岩石地球化学及其构造环境

Geochemical characteristics and geological environment of basement volcanic rocks in Lulliang, central region in Junggar basin.

准噶尔盆地腹地陆梁隆起基底火山岩岩性为富钠玄武岩及流纹岩,总体显示出板内双峰火山岩特点。玄武岩的特征是:岩石的斑晶和基质中普遍出现橄榄石;辉石为普通辉石;斑晶和基质中的长石为偏酸性的斜长石(平均牌号为30～50);全岩化学成分CIPW计算结果表明,绝大部分含有Ne(2.8％～4.6％),均含有O1(19.3％～10.1％)和Di(10.2％～24.6％),标准矿物分子组合为Ne+O1+Di+An;在全碱-SiO2图上玄武岩投影于碱性区;Mgu<65;REE总量为110.29～158.06μg/g。(La/Y)N变化范围为3.10～4.51。δEu变化于0.93～1.04;玄武岩的微量元素标准化图解为LILE相对于LREE适度富集,Nb,Ta相对于LREE和LILE亏损。Ni,Gr含量略低于原始岩浆的参考值;以上特征表明,玄武岩总体上属于碱性橄榄玄武岩;玄武岩具有较高正的εNd(t)和低的87Sr/86Sr,而流纹岩则具有较低的εNd(t)和较高的87Sr/86Sr,反映它们的同源性和遭受陆壳物质同化混染程度的不同。同位素Rb-Sr等时线年龄和单颗粒铬石蒸发年龄集中在323～395Ma。以上特点表明,陆梁玄武岩来自于亏损的地幔源区,并经历了一定程度的分异作用和陆壳物质的混染作用.其形成于板内环境,与泥盆纪-石炭纪区域伸展作用有关,因此,陆梁隆起带基底很可能是一个大陆裂谷带。

Basement volcanic rocks in Luliang uplift in central region of Junggar basin are dominated by soda-rich basalts and rhyolites and shows bimodal, intraplate suite. The basalts are characterised by occurrence of olivine in phenocryst and groundmass. Pyroxens in samples are augite. Feldspar are acid plagioclases (An: 30similar to50). The whole-rock CIPW results show that most samples contain Ne(2.8%similar to4.6%), O1(19.3%similar to10.1%), and Di(10.2%similar to24.6%). Nomative mineral-assemblage is Ne+O1+Di+An. Basalts plotted in the alkalic area on the diagram of Si-Alkaline. Mg-# numbers are smaller than 65. The total REE ranges from 110.29similar to158.06 mug/g, (La/Y)(N) varies from 3.10similar to4.51. deltaEu varies from 0.93similar to1.04. Trace elements of basalts are characterized by moderate enrichment in LILE ralative to LREE, moderate depletion in Nb, Ta relative to LREE and LILE. Ni and Gr are lower than that of reference value of primitive magma. These characteristics indicate that basalts in Luliang belong to alkali-olivine basalt overall. Basalt has higher positive initial epsilon(Nd) (7.20similar to8.38) and lower Sr-87/Sr-86, while rhyolite has lower initial positive epsilon(Nd) (5.64similar to6.60) and higher Sr-87/Sr-86. These differences are attributable to the absence or presence respectively of a significant crustal component, but basalt and rhyolite were derived from the same depleted mantle source. Rb-Sr isochron age and zircon crystal ages vary from 323Masimilar to395Ma. The conclusions can be made that basalts were originated from a depleted mantle and then differentiated and contaminated by crust, and the basement volcanics were formed in the continental rifting environment which was related to limited extension during D-C. Therefore, the Luliang uplifting in basin can be explained to be a continental rift.