霞石岩的成因及深部碳循环  

Petrogenesis of nephelinites and the deep carbon cycling

作　　者：石苡宁 张招崇[1,2] 程志国[1] SHI YiNing;ZHANG ZhaoChong;CHENG ZhiGuo(State Key Laboratory of Geological Process and Mineral Resources,China University of Geosciences(Beijing),Beijing 100083,China;Frontiers Science Center for Deep-time Digital Earth,China University of Geosciences(Beijing),Beijing 100083,China)

机构地区：[1]中国地质大学(北京),地质过程与矿产资源国家重点实验室,北京100083 [2]中国地质大学(北京)深时数字地球前沿科学中心,北京100083

出　　处：《岩石学报》2024年第7期2238-2254,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42030302);深时数字地球前沿科学中心“深时数字地球”中央高校科技领军人才团队项目(2652023001)联合资助。

摘　　要：霞石岩是以发育霞石和单斜辉石斑晶为特征的过碱性火山岩,其单体出露体积较小,常以碳酸岩-碱性杂岩体或小体积熔岩流的形式广泛分布在板内环境中,产出的时代跨度大(从太古宙至今),产出量总体上随地球的演化而增多。通常认为霞石岩在喷发过程中上升速度快且较少受到地壳物质混染,可以很好地反映地幔源区特征。然而,目前对其源区性质仍存在争议,其争论的焦点在于源区是否存在碳酸盐,即碳酸盐化源区是否是霞石岩形成的必要条件。本文基于全球各地霞石岩的地质-地球化学数据分析,将其分为两组:第一组霞石岩的SiO_(2)和碱含量(Na _(2)O+K_(2)O)变化范围较大,出现Rb、K、Pb、Zr、Hf、Ti、Y元素的负异常,放射性同位素组成普遍较为亏损;第二组相较第一组具有更高的SiO_(2)、碱含量,更低的MgO、TiO_(2)、P_(2)O_(5)含量和Na _(2)O/K_(2)O比值,并出现U、Nb、Pb、Zr元素的正异常以及Ti元素的负异常,Nb-Ta及Zr-Hf发生分馏,放射性同位素组成较第一组更富集。结合实验岩石学结果,研究认为第一组霞石岩的原始熔体为碳酸盐化榴辉岩部分熔融熔体与地幔橄榄岩反应形成的富碳硅酸盐熔体,其直接喷发或只经历低程度的分离结晶作用;第二组霞石岩与第一组霞石岩具有相似的源区特征,但熔体经历了强烈的分离结晶及不混溶作用。因此,碳酸盐化源区是霞石岩形成的必要条件。对霞石岩露头的时间分布进行统计发现,霞石岩最早形成于太古宙,但形成量在约550Ma和80Ma突增,碳酸盐俯冲通量的增加可能是导致霞石岩形成量突增的原因,而形成于太古宙的霞石岩说明太古宙地幔中存在局部的“碳”富集或输入。Nephelinite is a peralkaline(mostly sodic with high Na _(2)O/K_(2)O ratios)volcanic rock.It generally occurs as nephelinite-carbonatite complexes or small-volume lava flows in intraplate,both continental and oceanic environments,and is characterized by nephelite and clinopyroxene phenocrysts.Although rare,it spans the entire geological history from Archean to present.Overall,the output increases with the evolution of the Earth.It is generally believed that due to high ascent rate,nephelinitic magmas are less contaminated by crustal materials,and thus they can well indicate the nature of the mantle sources.However,the petrogenesis of nephelinite still remains controversial,and the debate focuses whether the mantle sources contain carbonate or whether carbonated mantle is prerequisite for the formation of nephelinite.In this paper,we compile the geological information and geochemical data of nephelinites from various locations worldwide and divided them into two groups.Group 1 nephelinites display a wide range of SiO_(2)and total alkali(Na _(2)O+K_(2)O)content,with negative Rb,K,Pb,Zr,Hf,Ti and Y anomalies,coupled with depleted Sr-Nd isotopic compositions.Group 2 nephelinites have higher SiO_(2)and total alkali(Na _(2)O+K_(2)O),lower MgO,TiO_(2),P _(2)O _(5) content and Na _(2)O/K_(2)O ratio than Group 1,and exhibit positive U,Nb,Zr,Pb anomalies and negative Ti anomalies,as well as Nb-Ta and Zr-Hf fractionation.These distinct geochemical characteristics imply their different petrogenesis.The primary magmas of Group 1 nephelinites can be carbonate-rich silicate melt through interaction between carbonated eclogite-derived melt and peridotite.These melts may erupt directly,or experience limited fractional crystallization.The two groups of nephelinites may originate from a similar mantle source,but primary Group 2 nephelinite magmas have undergone significantly fractional crystallization and silicate-carbonatite liquid immiscibility.Thus,it can be inferred that carbonated mantle source is a prerequisite for nephelinite

关 键 词：霞石岩 岩石成因 深部碳循环 碳酸盐化地幔 不混溶作用 

分 类 号：P581[天文地球—岩石学] P588.15[天文地球—地质学]