【作者】 丘志力；

【导师】 杨树锋；

【作者基本信息】 浙江大学 ， 地质学， 2009， 博士

【摘要】 锆石是目前最重要的矿物地球化学探针之一,关于锆石的研究已经成为新的一门学问-锆石学。中国东部华南、华北和东北等不同构造单元新生代玄武岩和地幔包体中发现一套特殊的锆石巨晶组合,对这种巨晶成因及其与深部壳-幔作用过程的研究对于进一步深入理解中国东部发生的岩石圈减薄作用及其机理具有重要作用。本文在前人工作的基础上,运用红外及激光拉曼探针技术,电子探针分析（EPMA）,激光剥蚀电感耦合等离子质谱技术（LA-ICPMS）和配有激光器的多接收等离子体质谱仪（MC-ICPMS）对中国东部多个产地锆石巨晶的结构、包裹体、主微量元素、U-Pb年龄以及Hf同位素进行了详细分析,结合前人对中国东部玄武岩、地幔岩岩石地球化学及同位素的研究成果,探讨了锆石巨晶的成因及其与岩石圈减薄过程的关系,取得了如下的认识:1、中国东部和锆石巨晶有关岩石类型主要为碱性玄武岩,具有贫硅、富碱和高Fe/Mn比值、低Nb/Ta比值的特征,南（东南沿海）北（华北和东北）区域的特征基本一致;赋存巨晶的碱性玄武岩包含多种类型地幔岩包体橄榄石Mg/Mg+Fe平均88.63,易熔组分FeO、TiO₂、Al₂O₃、CaO和K₂O+Na₂O相对富集,经历过显性地幔交代作用;相关的锆石巨晶最常见柱面{110}单形,结晶度高,变生程度低,具有高的ZrO₂/HfO₂比值,阴极发光多数具有震荡环带,红外及显微红外光谱在3500～3700 cm^-1之间含有明显的吸收带,显示结构水的存在。2、研究的四个产地锆石巨晶存在共同的锆石-磁铁矿-CO₂矿物及流体-熔体包裹体组合。除此之外,巨晶还含有黑云母,斜锆石、磷灰石、镁铁矿、方解石、正长石、赤铁矿等矿物包裹体,以及富含稀土元素La、Ce的矿物包裹体（怀疑为独居石,绿帘石）:含有富含SiO₂（＞60%）和碱性组分以及富Al、Zn、Zr和Ca的硅酸质熔体包裹体和大量不混熔的流体-熔体相包裹体,显示出锆石巨晶形成于富含SiO₂、Fe、Zr、碱性组分、水和CO₂的流体-熔体环境,巨晶的平均结晶温度范围为657-692℃之间（Ti地质温度计）。巨晶包裹体特征和中国东部地幔岩包体中发现的富硅、碱地幔交代流体具有相似性显示出两者存在明显的成因联系。3、LA-ICPMS分析显示,锆石巨晶的稀土总量大部分数据集中在15～300ppm范围,巨晶具重稀土（HREE）明显富集,Ce正异常明显,没有或只有微弱的Eu负异常,不同产地锆石巨晶稀土配分模式呈现一致的从左向右倾斜的变形“N”字模式和一致的微量元素组成模式的特征;锆石巨晶从中心到边缘环带不同位置稀土元素和微量元素含量及比值有变化,但不显示一致的由高到低或者相反的变化规律,暗示中国东部玄武岩的锆石巨晶并非形成于简单的岩浆降温体系。但Th/U比值多数大于0.4,主体显示出岩浆锆石的特点。锆石巨晶Ta明显富集,具有很低的Nb/Ta比值,显示出锆石巨晶可能结晶于强烈演化的、具有低Nb/Ta和Nb/U比值的岩浆源区（壳源物质加入）或者结晶过程中源区Nb、Ta发生过明显的分异,显示巨晶成因可以用再循环地壳残余含金红石榴辉岩和地幔流体或熔体的交代作用进行解释。4、海南蓬莱锆石巨晶的U-Pb同位素平均加权年龄为4.37±0.24Ma,福建明溪锆石巨晶两组平均加权年龄分别为1.27±0.36Ma和3.75±0.63Ma;黑龙江穆棱锆石巨晶平均加权年龄为8.82±0.27Ma;山东的数据质量较差,有限锆石巨晶的三组年龄为5.00±0.66 Ma,15.98±0.58Ma,20.56±0.80Ma;上述数据显示中国东部锆石巨晶的U-Pb同位素年龄与其寄主玄武岩的喷发时代具有一致性。5、MC-ICPMS分析获得锆石巨晶的¹⁷⁶Hf/¹⁷⁷Hf变化在0.282883±14～0.283151±7,相应的ε_Hf为3.9±0.5～13.4±0.3,落在亏损地幔范围内,其中海南锆石巨晶的¹⁷⁶HF/¹⁷⁷Hf比值最小,并有较大的变化范围,而黑龙江锆石巨晶的¹⁷⁶Hf/¹⁷⁷Hf比值为0.283105～0.283151,变化范围较窄,但其¹⁷⁶Lu/¹⁷⁷Hf的变化范围最大,ε_Hf为+11.8～+13.4,是4个产地锆石巨晶中的最高值。Hf同位素计算显示,我国东部岩石圈地幔的交代作用可能发生在中新生代。综合上述工作,本文认为中国东部新生代新近纪及第四纪玄武岩有关的锆石巨晶是亏损的软流圈地幔（包括流体或熔体）与岩石圈再循环残余物质交代反应的产物;结合前人发现的中国东部中生代期间南北区域所发现存在加厚地壳的证据和现今的岩石圈厚度明显低于古生代,岩石圈主体组成具有“新生”性质的成果,东南沿海和华北及东北部新生代玄武岩刚玉/锆石巨晶的成矿反映了整个中国东部在中新生代均发生过岩石圈的减薄,刚玉/锆石巨晶是经过部分熔融后含金红石榴辉岩下地壳残余在岩石圈拆沉过程中与软流圈流体-熔体相互作用的产物。更多还原

【Abstract】 Zircon is the most important mineral probe in petrology and geochemistry,and the study on zircon has become a new subject,zirconology.The Cenozoic basalts and their xenoliths in southern,northern and northeastern tectonic units along eastern China occurred a set of zircon megacrysts.The research on the genesis and their deep geological process of zircon megacrysts is very significant for understanding the thinning of SCLM and its dynamics.The internal structures,inclusions,main and minor elements,U-Pb dating and hafnium isotopes of zircon megacrysts from several locations along eastern China were studied by using infrared spectroscopy,laser raman microprobe,electron microprobe analysis,LA-ICPMS and MC-ICPMS.Based on the previous research on petrology,geochemisty and isotopes on basalts in eastern China,this paper obtained the following results on the genesis of zircon megacrysts and their significance in revealing the thinning of subcontinental lithosphere mantle:1.In eastern China,zircon megacrysts are closely associated with alkaline basalts.The alkaline basalts are consistently characterized from south to north and northeast China by Si depletion,alkali enrichment,high Fe/Mn,low Nb/Ta,and occur a variety of mantle xenoliths in which the olivine exhibits the features of undergoing mantle metasomatism,e.g.an average Mg^# of 88.63,and enrichment in fusible components（such as FeO,TiO₂,Al₂O₃,CaO and K₂O+Na₂O）.The zircon megacrysts, most having dominant {110} form,show high crystallinity,low metamict degree,and high ZrO₂/HfO₂ ratio.The oscillating internal zones in zircon megacrysts in cathodoluminescence image show that they were probably crystallized in the magma process during the uncontinuous drop of temperature and the obvious absorption between 3500-3700 cm^-1 in infrared images reflects the existence of structural water in zircons.2.All zircon megacrysts from the four locations along eastern China have an inclusion assemblage of mineral-fluid-melt,such as zircon-magnetite-CO₂,still have mineral inclusions of biotite,baddeleyite,apatite,magnesioferrite,calcite,orthoclase, hematite and REE（La,Ce）-enriched minerals（suspected monazite,epidote）,silicate melt inclusions with abundant SiO₂（over 60%）,alkaline and Al,Zn and Ca components,and a plenty of immiscible fluid-melt inclusions,indicating zircon megacrysts were formed fluid-melt environment being enriched in SiO₂,Fe,Zr, alkaline,H₂O and CO₂ at an average temperature between 657-692℃（calculated by Ti geothermometer）.Furthermore,the similarity between the fluid-melt characteristics reflected from inclusions in zircon megacrysts and those of SiO₂ & alkali-rich mantle metasomatic fluid of mantle xenoliths show they both possibly possess similar genesis.3.LA-ICPMS analyses show that zircon megacrysts have total REE between 15-300 ppm and show concordant "right inclined N-Iike"REE distribution pattern with obvious HREE enrichment and positive anomaly in Ce,with or without weak negative anomaly in Eu.The irregular variation of REE and minor element contents from edge to center in every single zircon megacryst demonstrate that the zircons were not crystallized during simple temperature dropping process of magma.But the zircon megacrysts’high Th/U ratio（most above 0.4） is a feature of magmatic origin zircons.Meanwhile,apparent Ta enrichment and low Nb/Ta in zircon megacrysts indicate that they probably crystallized in strongly evolved magma source with low ratios of Nb/Ta and Nb/U（derived from possible crust material addition or Nb-Ta differentiation during zircon crystallization）.That means zircon megacrysts were crystallized during metasomatism process of mantle fluid or melt on recycled crust residual of rutile ecologite.4.The U-Pb dating age of zircon megacrysts are basically the same as those of alkali basalts,e.g.the weighted average age of U-Pb dating of zircon megacrysts for Penglai of Hainan is 4.37±0.24Ma,Mingxi of Fujian 1.27±0.36 Ma and 3.75±0.63Ma,Muleng of Heilongjiang 8.82±0.27Ma,Changle of Shandong 5.00±0.66 Ma,15.98±0.58Ma,20.56±0.80Ma.5.¹⁷⁶Hf/¹⁷⁷Hf ratios range between 0.282883±14～0.283151±7 by using MC-ICPMS and theε_Hf ranges between 3.9±0.5～13.4±0.3,dropping in the depleted mantle area.Among of them,zircon megacrysts of Hainan have the smallest ¹⁷⁶Hf/¹⁷⁷Hf ratios and wide variation range,the zircons of Heilongjiang have narrow range of ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.283105～0.283151 andε_Hf=+11.8～+13.4,but a wide variation of ¹⁷⁶Lu/¹⁷⁷Hf.Hf isotopes dating（THfl） show subcontinental lithospere mantle along eastern China was possiblely metasomatized during Cenozoic to Mesozoic.Based on the results obtained,this paper proposed that the zircon megacryst closely associated with Cenozoic alkali basalts along eastern China were formed during the metasomatism process of depleted mantle fluid and/or melt on the recycled residual of lithosphere.Combined with the previous researches arguing that there existed thicken crust beneath south and north China during Mesozoic,and the present lithospheric mantle is apparently thinner than that in Paleozoic and its principal parts are juvenile,the zircon/corundum megacrysts generally occurred in Cenozoic basalts along eastern China demonstrate that there was a general SCLM thinning beneath eastern China during Mesozoic and Cenozoic,and these megacrysts were the products of the interaction between depleted mantle fluid-melt and the crust residual of rutile-being eclogite after partial melting during lithosphere delamination.更多还原