新疆西准噶尔巴尔努克早石炭世富Nb岛弧玄武岩的发现及其地质意义
Discovery and significance of Early Carboniferous Nb-enriched basalts in Barnuke, West Junggar, Xinjiang
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摘要: 黑山头组玄武岩-玄武安山岩组合是西准噶尔早石炭世海相火山-沉积建造的重要组成部分,其与高Mg安山岩及O型adakite共生。此次分析的所有样品均为玄武质岩石,SiO2含量为49.62%~55.68%,平均为52.70%;明显高TiO2(1.16%~1.99%),平均为1.56%;显著富Na2O而贫K2O,Na2O为3.17%~6.35%,平均为4.90%,Na2O/K2O为1.19~26.08,绝大多数样品该比值>4(平均8.93)。样品明显富集Nb、Sr、Zr等,Nb含量均>7×10-6(7.29×10-6~12.32×10-6,平均为9.48×10-6);全部样品Sr均>400×10-6(618×10-6~1107×10-6,平均为825×10-6);Zr>158×10-6(159×10-6~217×10-6,平均为182.6×10-6);Zr/Y比值>4(6.63~11.09,平均为8.62)。显著高于一般岛弧玄武质岩类。轻稀土明显富集而重稀土亏损显著,且有基本一致的弱正Eu异常。(La/Nb)PM比值>0.7(1.23~2.26,平均1.94),(87Sr/86Sr)i=0.7035~0.7039;143Nd/144Nd=0.5128~0.5129;εNd(t)=5.49~7.13。这些地球化学特征与Sajona et al.(1993,1994,1996)确立的富Nb岛弧玄武岩基本一致,而与一般岛弧玄武岩有显著区别。这一发现,为确认西准噶尔早石炭世存在富Nb岛弧玄武岩提供了地球化学佐证,丰富了新疆乃至我国富Nb岛弧玄武岩的产地与层位,也为确认本区早石炭世岛弧构造环境提供了重要依据。
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关键词:
- 富Nb玄武岩-玄武安山岩 /
- 岛弧构造环境 /
- 早石炭世 /
- 巴尔努克 /
- 西准噶尔
Abstract: The Nb-enriched basalts-basaltic andesites in Heishantou Formation, which are associated with high magnesium andesite and adakite, is main composition of the Early Carboniferous marine volcano-sedimentary in West Junggar. All analyzed samples are basaltic in composition with SiO2 contents ranging from 49.62% to 55.68%, and the average value is 52.70%; Moreover, the samples have high TiO2 (1.16%~1.99%) with average 1.56%; Also Na2O enriched but K2O depleted, with Na2O from 3.17% to 6.35%, the average is 4.90%. The ratios of Na2O/K2O in samples ranging from 1.19 to 26.08, and most ratios >4 (average is 8.93). The Nb-enriched basalts are characterized by obvious Nb, Sr and Zr enrichment, with the contents Nb>7×10-6 (7.29×10-6~12.32×10-6, average is 9.48×10-6), the contents Sr>400×10-6 (618×10-6~1107×10-6, average is 825×10-6), the contents Zr>158×10-6 (159×10-6~217×10-6, average is 182.58×10-6), the ratios of Zr/Y>4 (6.62~11.09, average is 8.62). This all features distinguished differ from island arc basaltic rocks. Moreover, the samples show light rare earth elements enrichment and heavy rare earth elements depletion [(La/Nb)PM>0.7 (1.23~2.26), average is 1.94], very weak Eu positive anomalies. Isotopically, the studied basalts all display consistently Sr-Nd isotopic compositions [(87Sr/86Sr)i=0.7035~0.7039, 143Nd/144Nd=0.5128~0.5129, εNd(t)=5.49~7.13]. This geochemical characteristics are consistent with the Nb-enriched basalts defined by Sajona et al. (1993, 1994, 1996), but obviously different from island arc basalts. The discovery is geochemical evidence for the Nb-enriched basalts occurring in Early Carboniferous, West Junggar. This is also abundant in location and horizon of the Nb-enriched basalts in Xinjiang and even China. Furthermore, this is more important proof for confirming island arc settings in study area at Early Carboniferous. -
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