| 浙北湖(州)-安(吉)火山岩盆地：锆石U-Pb年代学、地球化学与岩浆成因 |
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| 引用本文： | 马芳,薛怀民.浙北湖(州)-安(吉)火山岩盆地：锆石U-Pb年代学、地球化学与岩浆成因[J].地质学报,2017,91(2):334-361. |
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| 作者姓名： | 马芳 薛怀民 |
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| 作者单位： | 北京大学,中国地质科学院地质研究所 |
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| 摘 要： | 处于浙-赣火山岩带东北缘的湖(州)-安(吉)盆地内的火山岩/潜火山岩从中性到酸性,中间没有明显的成分间断,以中酸性—酸性组分占绝对优势,中性组分相对较少,缺少基性组分,代表一套连续的中性—酸性岩浆系列。岩石化学总体表现为富碱和高钾的特征,中性岩属橄榄玄粗岩系列,中酸性—酸性岩类属高钾钙碱性系列。盆地内的火山岩在地球化学上均表现为富集大离子亲石元素和轻稀土元素,而高场强元素Nb、Ta、Ti等则有一定程度的亏损。火山岩中主量和微量元素的变异规律揭示分离结晶作用是盆地内岩浆演化的主要机理,但岩浆演化的不同阶段分离的矿物相有所差异,斜长石自始至终都是分离结晶的重要矿物相,在中性—中酸性岩浆演化阶段,角闪石可能也是重要的分离矿物,而中酸性—酸性岩中钾长石和黑云母的分离结晶也起着重要影响。本文所作的精确定年结果表明,盆地内三期火山活动产物的LA-ICPMS锆石U-Pb年龄在误差范围内几乎一致,介于128~130 Ma之间,指示盆地内主要火山活动持续的时间很短。另外测得粗安岩集块的SHRIMP锆石U-Pb年龄为136±1 Ma,可能意味着该盆地初期有少量偏基性的火山活动。推测湖安火山岩盆地发育于活动大陆边缘的后造山环境,岩浆的形成可能受控于岩石圈的拆沉或俯冲板片的断落,母岩浆主要是由镁铁质下地壳物质部分熔融形成的,几乎没有地幔物质的参与,所形成的安山质-英安质岩浆在浅部岩浆房中发生过强烈的分异演化。
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| 关 键 词： | 锆石U-Pb定年 高钾钙碱性系列 橄榄玄粗岩系列 后造山环境 岩石圈拆沉 莫干山 |
| 收稿时间： | 2015/5/4 0:00:00 |
| 修稿时间： | 2015/10/25 0:00:00 |
Huzhou-Anji Volcanic Basin of Northern Zejiang Province: Zircon U-Pb Dating, Geochemistry and Magma Genesis |
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| Ma Fang and Xue Huaimin.Huzhou-Anji Volcanic Basin of Northern Zejiang Province: Zircon U-Pb Dating, Geochemistry and Magma Genesis[J].Acta Geologica Sinica,2017,91(2):334-361. |
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| Authors: | Ma Fang and Xue Huaimin |
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| Institution: | Peking University,Institute of Geology, Chinese Academy of Geological Sciences |
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| Abstract: | Located in the Huzhou-Anji volcanic basin in the northeastern margin of Zhejiang-Ganzhou volcanic rock belt, the volcanic/subvolcanic rocks consist dominantly of intermediate-felsic to felsic rocks with minor intermediate component and lack of mafic component, and represent a intermediate-felsic magmatic rocks series. The rocks are lithochemically characterized by high-K and rich alkali, suggesting that intermediate rocks belong to shoshonite series and the intermediate-felsic and felsic rocks belong to high potassium calc-alkaline series. In geochemistry, the volcanic rocks in the basin are enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high field strong elements (HFSE) such as Nb, Ta and Ti. Major and trace elements variation regulation of the volcanic rocks reveals that fractional crystallization is the main mechanism of the magma evolution in the basin. Different mineral facies is produced at different stage of magma evolution, with plagioclase being the key mineral of fractionation and crystallization. Hornblende may be an important fractionation mineral during the intermediate to intermediate-feslic magma evolution, and K-feldspar and biotite minerals also play an important role in the fractionation of intermediate-felsic to felsic rocks. Our dating results show that the LA-ICP-MS zircon U-Pb ages of three episodes of volcanic activities in the basin are around 128~130 Ma, which fall within the error range, indicating that the main volcanic activities lasted for very short time. The SHRIMP zircon U-Pb dating yields an age of 136±1 Ma for the trachyandesite agglomerates, probably indicating small-scale weak mafic volcanic activities at the initial stage of the basin. It can be speculated that the Huzhou-Anji volcanic basin developed in a post-orogenic environment in the active continental margin, the magma formation was probably controlled by lithosphere delamination or subduction slab breakoff, with parent magma formed by partial melting of mafic lower crust material and with barely mantle material involvement, and the resulting andesitic-dacitic magma experienced strong fractional crystallization at the shallow magma chamber. |
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| Keywords: | zircon U-Pb dating high-K calc- alkaline series shoshonite series post-orogenic environment lithosphere delamination Mogan mountain |
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