| 扬子地台北缘白勉峡组和三湾组火山岩形成构造环境及岩石成因的地球化学约束 |
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| 引用本文： | 徐学义,陈隽璐,李向民,马中平,王洪亮,李平,李婷.扬子地台北缘白勉峡组和三湾组火山岩形成构造环境及岩石成因的地球化学约束[J].地质学报,2009,83(11):1703-1718. |
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| 作者姓名： | 徐学义 陈隽璐 李向民 马中平 王洪亮 李平 李婷 |
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| 作者单位： | 1. 中国地质调查局西安地质调查中心(西安地质矿产研究所),西安,710054
2. 长安大学资源学院,西安,710069 |
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| 基金项目： | "十一五"国家科技支撑课题，国土资源大调查项目(编号 1212010611804;1212010610319)资助的成果 |
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| 摘 要： | 依据中基性火山岩主量和微量元素地球化学特征的差异,白勉峡组可分两部分,一部分火山岩TiO_2大于1%,变质程度较高,主要分布在下段;另一部分火山岩TiO_2小于1%,变质程度较浅,主要分布在上段.下段火山岩属拉斑玄武岩系列,上段主体属钙碱系列,稀土总量高(∑REE=83.4～180.8μg/g),轻重稀土分异较低(LREE/HREE=2.17～5.85),有弱的Eu负异常(δEu=0.79～1.01),微量元素原始地幔蛛网图显示有弱的Nb、Ta亏损,具有板内火山岩的地球化学特点,形成于板内裂谷环境.上段火山岩稀土总量低(∑REE=40.3～82.4μg/g),轻重稀土分异较高(LREE/HREE=2.3～7.6),无Eu负异常(δEu=0.90～1.11),微量元素原始地幔蛛网图发育明显的Nb-Ta槽和Zr-Hf槽,Ti、Sr发育较强的低谷,具有典型岛弧玄武岩的地球化学特点,形成于岛弧或大陆边缘弧环境.三湾组玄武岩和安山岩稀土元素分配型式呈LREE亏损的左倾型或呈近平坦型,类似于N-MORB,明显不同于白勉峡组,岩石组合和地球化学特点类似于弧后盆地火山岩.火山岩及相关侵入岩LA-ICPMS锆石U-Pb定年及元素及Sr-Nd同位素地球化学研究揭示,白勉峡组下段火山岩形成时代可能为1144Ma,其源区为与洋岛玄武岩类似的软流圈地幔源区,部分熔融发生在石榴子石二辉橄榄岩稳定区,岩浆在演化过程中经历了一定分离结晶作用(分离结晶矿物为斜长石+单斜辉石)和地壳混染作用.白勉峡组上段火山岩形成时代可能为437Ma,有可能跨到晚古生代,其源区为受俯冲作用改造的富集地幔区,部分熔融亦发生于石榴子石二辉橄榄岩稳定区.三湾组中基性火山岩源于N-MORB近似的亏损地幔源区.白勉峡组下段代表中元古代末板内拉张事件的地质记录,白勉峡组上段和三湾组目前的火山岩样品可能代表了古生代同一洋陆转化的地质记录.
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| 关 键 词： | 白勉峡组和三湾组 构造环境及岩石成因 LA-ICPMS锆石U-Pb定年 岩石地球化学 扬子地台北缘 |
| 收稿时间： | 2009/2/15 0:00:00 |
| 修稿时间： | 2009/9/11 0:00:00 |
Geochemical Constrains on the Petrogenesis and Tectonic Setting Discrimination of Volcanic Rocks from the Baimianxia and the Sanwan Formations |
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| XU Xueyi,CHENG Junlu,LI Xiangmin,MA Zhongping,WANG Hongliang,LI Ping,and LI Ting.Geochemical Constrains on the Petrogenesis and Tectonic Setting Discrimination of Volcanic Rocks from the Baimianxia and the Sanwan Formations[J].Acta Geologica Sinica,2009,83(11):1703-1718. |
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| Authors: | XU Xueyi CHENG Junlu LI Xiangmin MA Zhongping WANG Hongliang LI Ping and LI Ting |
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| Abstract: | On the basis of petrogeochemical data, the volcanic lavas of Baimianxia Formation can be classified into two major parts. These are a high TiO_2 (>1%) type situated in lower part with high-grade metamorphism; and low TiO_2(<1%) type situated in upper part with low-grade metamorphism. The volcanic rocks in lower part has dominantly tholeiitic lavas with chondrite normalized REE patterns exhibiting high REE contents (∑REE=83.4～180.8μg/g), high LREE/HREE ratios (LREE/HREE=2.17～5.85)and weak negative Eu anomaly(δEu=0.79～1.01), also the trace element patterns display weak Nb-Ta anomalies with respect to Th, K, La, Ce, elemental and isotopic data is similar to the signatures of within plate lavas and suggest the lavas in lower part originated from within plate setting. In contrast, the volcanic rocks in upper part exhibit low REE contents, low LREE/HREE ratios, and pronounced Nb-Ta anomalies with respect to Th, K, La, Ce, these are typical arc or continental arc signature. While the basalts and andesites in Sanwan Formation show flat or LREE depletion patterns, similar to N-MORB, together with the rock associations, we suggest these lavas formed in back-arc basin setting. LA-ICPMS zircon U-Pb dating for basalts in upper part of Baimianxia Formation and elemental and Sr-Nd isotopic data suggest the age of lavas in lower part of Baimianxian Formation is 1144Ma or so, and which originated form and asthenospheric oceanic-island-basalt-like mantle source within the garnet stability field with fractional crystallization and crustal contamination and continental lithospheric mantle also contributing significantly to the formation the the lavas. Instead, the lavas in upper part of Baimianxia Formation formed about 437Ma and maybe continued to late Paleozoic, they may originate from arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in garnet stability field. The samples of basalts and andesites show they are derived from depleted mantle source similar to N-MORB. Finally, we can conclude that the lavas in upper part of Baimianxia Formation represent the geological records of rift-related volcanism in middle Proterozoic. Whereas, the lavas in upper part of Baimianxia Formation and Sanwan Fromation may be generated by the oceanic and continental conversion occurred in Paleozoic. |
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| Keywords: | Baimianxia and Sanwan formations petrogenesis and tectonic setting LA-ICPMS zircon dating petrogeochemistry northern margin of Yangtze Plate |
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