西准噶尔早泥盆世马拉苏组火山岩岩石成因研究

新疆西准噶尔北部地区的早泥盆世马拉苏组出露有少量富钠低钾的拉斑质中基性熔岩,这些分布于谢米斯台断裂北侧的玄武安山岩和玄武岩多呈夹层状断续产出于火山碎屑岩之中。马拉苏中基性熔岩的Mg#与主、微量元素协变关系及Th-Th/Nd图反映了其并非同源岩浆演化的结果。马拉苏火山岩中的玄武安山岩富集LILE、亏损HFSE,具有较高的Th含量及较低的Hf/Th和(Nb/Th)PM比值,显示出弧火山岩的地球化学特征。其中的玄武岩则具有略为平坦的稀土元素分配样式,较低的Th含量及较高的Hf/Th和(Nb/Th)PM比值,此同MORB地球化学特征极为相似;虽然其也显示有轻微的LILE富集、HFSE亏损,但是较高的La/Nb比值则暗示这同地壳或俯冲物质组分的卷入有关,且一系列构造环境判别图解也进一步印证了马拉苏组内的玄武岩应属似MORB基性熔岩。此外,两类岩石的高场强元素比值Zr/Nb、Hf/Ta同全球平均大洋中脊玄武岩的相应比值极为接近,反映了马拉苏组中基性火山岩的物质源区主体均为MORB地幔物质源区。La/Yb-Gd/Yb原始地幔标准化比值的模拟计算进一步显示了马拉苏组玄武安山岩与受改造(俯冲沉积物或地壳物质的混染)的石榴子石或尖晶石-石榴子石地幔橄榄岩物质源区的部分熔融作用有关,而似MORB型玄武岩则源自尖晶石地幔橄榄岩源区的部分熔融。结合区内同期的蛇绿岩、火山岩和碱性花岗岩的地球化学研究,我们可以进一步推断此类兼具有似MORB和弧火山岩地球化学特征的早泥盆世马拉苏火山岩应当是西准噶尔地块北部在早古生代受后期俯冲作用影响下经历弧后扩张形成的火山-岩浆地质记录。

Petrogenesis of volcanic rocks from Early Devonian Marasu Formation, West Junggar

Volcanic rocks from Marasu Formation of Early Devonian, occurred in northern West Junngar, are sodium-rich and Low-K tholeiitic series, these basaltic andesites and basalts on the north side of Xiemistai fault are generally distributed in the outcrops of pyroclastic rocks intermittently. The Hark diagrams of Mg^# versus major and trace elements and plots of Th-Th/Nd suggest these basic- and intermediate-lavas were not formed only by the comagmatic evolution. The basaltic andesites in this study are characterized by enrichments in large ion lithophile elements (LILE) and depleted in high field-strength elements (HFSE), low Th contents and high Hf/Th and (Nb/Th)_PM ratios, all of which show the typical geochemical features of subduction zones. The basalts have nearly flat REE patterns, high Th contents and low Hf/Th and (Nb/Th)_PM ratios, similar to the geochemical features of MORB. Although they also show various enrichments in LILE and depleted in HFSE, the high La/Nb ratios suggest that they are related to the added crust or subducting materials. Besides, some geotectonic discrimination diagrams also can reflect they are MORB-like basalts. HFSE/HFSE ratios (e.g. Zr/Nb, Hf/Ta) of both are similar to that of mid-oceanic ridge basalts, which should be the partial melting products of MORB sources. The primitive mantle normalized ratios of La/Yb and Gd/Yb suggest the basaltic andesites were originated from garnet-bearing or spinel-garnet mantle peridotites with subducting sediments or crustal materials added, and MORB-like basalts were formed by the partial melting of the spinel-bearing mantle peridotites. Several lines of evidence suggest that the Marasu volcanic rocks in northern West Junggar, with both geochemical characteristics of island arc and MORB-like volcanics, may be the magmatic records of back-arc spreading in tectonic collage.