南祁连党河南山地区拐杖山岩群玄武岩年代学、地球化学及其构造环境

计波; 李向民; 黄博涛; 王磊; 王国强

doi:10.12029/gc20200619003

南祁连党河南山地区拐杖山岩群玄武岩年代学、地球化学及其构造环境

Zircon U−Pb dating and geochemistry of basalt in Guaizhangshan Group from the Southern Danghe Mountain in South Qilian and its tectonic setting

摘要

摘要:
研究目的 前人在南祁连志留纪巴龙贡噶尔组中解体出新元古代拐杖山岩群，对该群中玄武岩的年龄、成因与成岩构造环境的研究是厘清南祁连地区构造演化的重要手段。
研究方法 本文基于 LA−ICP−MS 锆石 U−Pb 年龄和全岩地球化学分析等方法研究拐杖山岩群火山岩的形成年代、地球化学特征及地质意义。
研究结果 锆石U−Pb定年结果显示，玄武岩喷发年龄为（786.6±5.8）Ma，表明拐杖山岩群形成于新元古代早期。岩石地球化学分析结果表明，玄武岩SiO₂含量在48.09%~50.97%，高TiO₂（1.34%~2.55%），低MgO（5.78%~7.11%），显示亚碱性拉斑玄武岩特征。岩石轻、重稀土分异明显（(La/Yb)_N=3.76~4.51），无明显Eu异常（δEu=0.80~1.05），大离子亲石元素Ba、Th、U相对富集，Pb强富集，亏损Nb、Ta、Ti等高场强元素。
结论岩石地球化学特征暗示拐杖山岩群玄武岩是遭受陆壳物质混染的地幔源区玄武岩，该火山岩形成于大陆裂谷环境，响应了全球新元古代Rodinia超大陆裂解事件。

Abstract:
This paper is the result of geological survey engineering.
Objective The Neoproterozoic Guaizhangshan Group was disintegrated from the Silurian Balonggonggaer Formation. The study of the petrogenesis and tectonic setting of the basalt in Guaizhangshan Group is crucial to clarify the tectonic evolution of South Qilian.
Methods Based on LA−ICP−MS zircon U−Pb datingand whole−rock geochemistry analysis, the formation age, geochemical characteristics and geological significance of the basalt in Guaizhangshan Group are constrained.
Results The dating results indicate the eruption age of the basalt is (786.6±5.8) Ma, suggesting that the Guaizhangshan Group formed during Early Neoproterozoic. Geochemical analyses of the basalt show that: SiO₂=48.09%–50.97%, TiO₂=1.34%–2.55%, MgO=5.78%–7.11%, displaying characteristics of subalkaline tholeiite; Significant differentiation between light rare earth element (LREE)and heavy rare earth elements (HREE) ((La/Yb)_N=3.76–4.51), and inconspicuous Eu anomalies (δEu=0.80–1.05); Enrichment of Ba, Th, U, Pb, and depletionin Nb, Ta, Ti.
Conclusions These geochemical characteristics indicate that the basalts were derived from mantle and contaminated by crustal materials. The basalt was formed in continental rift setting, which may be relevant to the break−up of the Rodinia supercontinent.

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