【作者】 余淳梅；

【导师】 郑建平；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 华北克拉通东部显生宙岩石圈破坏及其深部过程研究是我国固体地球科学的重要议题。在经历了中生代岩石圈减薄之后，地幔的性质和时代等对于认识华北克拉通岩石圈自古生代以来的演化历史及约束减薄机制都具有十分重要的意义。论文选择位于华北克拉通内部造山带北缘的汉诺坝、东部郯庐断裂带上昌乐—山旺以及位处南北重力梯度带上的鹤壁地区新生代玄武岩中的橄榄岩捕虏体作为研究对象，通过详细的岩石学、地球化学和Re-Os同位素年代学的对比研究，获得了以下的主要认识：1．汉诺坝橄榄岩捕虏体以二辉橄榄岩为主，有少量方辉橄榄岩（Cpx＜5％）；昌乐—山旺地区幔源捕虏体则仅见二辉橄榄岩，单斜辉石含量相对较高；鹤壁捕虏体以粗粒方辉橄榄岩为主，少量二辉橄榄岩。二辉橄榄岩矿物组合为橄榄石+斜方辉石+单斜辉石±尖晶石，方辉橄榄岩矿物组合为橄榄石+斜方辉石±单斜辉石±尖晶石。2．对比了橄榄岩捕虏体的矿物主量元素和单斜辉石微量元素。汉诺坝橄榄石的Mg^#为86．4-92．8，较该区已发表的分析结果变化范围宽，Mg^#最高值与鹤壁难熔橄榄岩中者相当，方辉橄榄岩中橄榄石Mg^#大于二辉橄榄岩；山旺橄榄岩捕虏体中橄榄石的Mg^#变化范围集中于80．2-90．9之间；鹤壁橄榄石Mg^#则以＞92为主。各地区单斜辉石和尖晶石中Mg^#-Cr^#关系表明鹤壁橄榄岩难熔程度最高，山旺橄榄岩则相对饱满，而汉诺坝橄榄岩则体现了该区岩石圈地幔从饱满到难熔的“混杂”属性。各区橄榄岩捕虏体中微量元素丰度有差异，但单斜辉石微量元素都具有富集Th、U以及亏损高场强元素Nb、Ti、Zr和Hf的特点。稀土元素配分型式上，各区都有LREE富集型和亏损型两种配分型式，体现了研究区陆下岩石圈地幔演化过程中部分熔融和受到熔／流体的交代作用特征。计算了橄榄岩的平衡温度，与橄榄石的Mg^#值之间无相关性。3．橄榄岩捕虏体的主量元素中岩石难熔程度的指标（如Mg／Si、Mg^#及Al₂O₃+CaO等）显示出汉诺坝捕虏体橄榄岩体现出的特点是既有难熔程度较高，与鹤壁橄榄岩捕虏体接近的样品，也有相对饱满，与昌乐—山旺大部分样品特征一致的橄榄岩。全岩与单斜辉石中微量元素反映的信息较为一致。相容元素与MgO的相关性较好，不相容元素与MgO之间的相关性普遍较差，反映地幔橄榄岩可能经历了较为复杂的部分熔融和后期交代过程。4．对研究区部分熔融程度和交代作用介质进行了定性、定量的分析判断。昌乐—山旺橄榄岩所代表的岩石圈地幔是经过较低程度熔融抽取后形成的（1％-5％），后期主要受到硅酸盐熔体的交代作用，具有富集地幔特征；鹤壁是经历了原始地幔高度熔融（15％-20％）抽取后的具有难熔亏损性质的古老岩石圈地幔，后期经历过碳酸岩熔体的强烈改造；汉诺坝地区表现为不同部分熔融程度抽取后的亏损地幔（15％-20％）和富集地幔（＜5％）共存，兼有过渡类型（5％-10％），在交代介质上以硅酸盐熔体交代为主，并同时有碳酸岩熔体交代的痕迹。5．对汉诺坝中包裹体硫化物和填隙状硫化物做了原位的Re-Os同位素分析，并同时分析了汉诺坝和昌乐橄榄岩全岩中Re-Os同位素特征加以对比研究。硫化物较全岩更宽的Re-Os同位素比值范围说明全岩可能中和了不同产状、不同成因的硫化物的Re-Os同位素。汉诺坝橄榄岩捕虏体中硫化物提供了太古代—早元古代（1．8-2．3Ga）—中元古代（0．9-1．4Ga）—晚元古代（0．6Ga）的年龄时段，反映了汉诺坝地区岩石圈地幔存在多发事件，也说明了硫化物原位Re-Os同位素定年在揭示岩石圈地幔演化细节上的优势。昌乐地区橄榄岩全岩给出的最老T_RD模式年龄为1．2Ga，Os代理等时线年龄为早元古代的2．1Ga，结合昌乐大多数橄榄岩所具有的新生饱满、富集的特征推断，岩石圈地幔不均一性可能在华北陆下普遍存在。鹤壁橄榄岩捕虏体中橄榄石内硫化物提供了古老太古代（2．5Ga和3．0Ga）年龄，说明了该区仍然残存了古老岩石圈地幔。6．对华北克拉通中、东部新生代时岩石圈地幔的不均一性进行了探讨，并提出了新生代岩石圈地幔演化过程。自东向西由昌乐—山旺、鹤壁和汉诺坝橄榄岩捕虏体代表的新生代时华北克拉通岩石圈地幔在岩石类型、结构、化学成分、熔融抽取程度、交代介质以及Re-Os同位素特征和年龄结构上存在明显的不均一性特征。这种地幔不均一性最好的解释应该是华北东部岩石圈经历了上涌软流圈物质通过侵蚀、混合和改造实现了对古老岩石圈地幔的不均匀置换，并从而实现了华北东部岩石圈的巨厚减薄。就现有地幔年龄数据来看，华北东部古老太古代岩石圈在早元古代时（1．9-2．3Ga）发生了一次大规模的置换作用，将昌乐—山旺古老岩石圈地幔完全置换，但在鹤壁代表的克拉通南部和汉诺坝所代表的西部地区仅体现了不均匀的置换，因而在这些地区仍然残留了较老的岩石圈地幔。在之后的中、晚元古代汉诺坝地区岩石圈地幔又经历了多次深部热事件作用过程（1．2-1．4Ga，0．6-0．9Ga），但未受中生代华北克拉通东部岩石圈巨厚减薄的影响。早元古代被完全置换的昌乐—山旺岩石圈地幔受中生代形成的郯庐断裂带影响，在中生代岩石圈减薄作用过程中，较易被置换改造，因此其地幔主体表现为新生特点，其个别样品异常高的Re含量和高于原始的Re-Os同位素比值也可以说明昌乐—山旺岩石地幔较新，但新生物质对古老地幔的不均匀置换使得昌乐—山旺地区可能仍保留了一部分元古代岩石圈地幔。更多还原

【Abstract】 The Phanerozoic considerable losing of the Archean keels of the North China lithospherehave attracted many attentions in the past few years.The nature and age of the North Chinalithospheric mantle are especially important to understand the thinning mechanism and evolutionof the lithosphere.Peridotite xenoliths entrained in the Cenozoic basalts from the North ChinaCraton are ideal samples for investigating the nature and evolution of the lithospheric mantle.Inthis paper,we present detailed petrologic,geochemical and Re-Os geochronologicalinvestigations for the peridotite xenoliths captured in the Cenozoic alkaline basalts fromHannuoba （northern edge of the Trans-North China Orogen within the craton）,Changle-Shanwang and Hebi （interior of the craton）,with the aim of constraining the age andnature of lithospheric mantle beneath these areas.Our main conclusions are listed as below:1.The Hannuoba peridotite xenoliths are mainly lherzolites with minor harzburgites （withClinopyroxene content less than 5%）;the Changle-Shanwang mantle-derived xenoliths are allmost lherzolites and characterized by high proportion of Clinopyroxene;the Hebi xenoliths aremainly comprised of coarse-grained harzburgites and minor lherzolites.The lherzolites xenolithshave mineral assemblage of olivine+orthopyroxene+Clinopyroxene±Spinel,whereas theharzburgites are characterized by occurrence of olivine+ orthopyroxene±Clinopyroxene±Spinel.2.The olivine Mg^# of the Harmuoba xenoliths range from 86.4 to 92.8,which are slightlywider than the published data for this area.The highest olivine Mg^# of the Hannuoba samples issimilar to that of the Hebi refractory peridotites,which are interpreted as shallow relics of theArchean cratonic mantle;Mg^# of the harzburgites is higher than the lherzolites in the Hannuobaxenoliths.The olivine Mg^# of the Shanwang xenoliths is mainly concentrated in 80.2-90.9.However,the Mg^# of most Hebi olivine is higher than 92.The Mg^# and Cr^# correlation in theclinopyroxene and spinel indicate that the Hebi peridotites are the most refractory and theChangle-Shanwang samples exhibit more fertile characteristics.The Hannuoba peridotites,however,display heterogeneous characteristics,suggesting the co-existence of both refractoryand fertile mantle components underneath this region.The trace element for the studied xenolithshave differences between each other,however,their clinopyroxenes are all characterized byenrichment of Th,U,and depletion of HFSE such as Nb,Ti,Zr and Hf.Xenoliths from all of the studied areas have two distinct REE distribution patterns including LREE enrich type and LREEdepleted type,further indicating a comprehensive melts/fluids metasomatism within thelithospheric mantle beneath the studied areas.The balance temperatures of the peridotites havebeen calculated,which have no relevance with the Mg^# value in olivine.3.Some major element,such as MgO,Mg^# and （Al₂O₃+CaO） which are commonly regardedas indicator of lithospheric mantle refractory,also reveal that the Hannuoba peridotitic xenolithsdisplay both refractory and fertile characteristics.Among the Hannuoba peridotites,the much morerefractory samples have similar geochemical features to those of the Hebi peridotites,and therelatively fertile xenoliths exhibit nearly consistent characteristics with those of theChangle-Shanwang.The peridotites whole-rock trace element composition is in agreement withthat of the clinopyroxenes,both are characterized by U,Th enrichment and HFSE depletion.Meanwhile,the Shanwang and minor Hannuoba peridotites have positive Sr anomaly.The MgOexhibits excellent liner correlation with the compatible element but relatively weak linercorrelation with the incompatible element,suggesting that the peridotites may have undergonecomplicated modification.4.The lithospheric mantle beneath Shanwang area is characterized by enriched feature andhas undergone silicate melts metasomatism,such mantle represents a relatively lower partialmelting （1%-5%） residue and mainly comprises of lherzolites.Most of the Hebi peridotitesrepresent refractory Archean relics that have undergone high proportion of melts exaction（15%-20%） and a later stage carbonatitic melts modification.Lithospheric mantle underneath theHannuoba area,however,exhibits heterogeneous feature,that is,with the coexistence of fertilecomponents （＜5%） and refractory mantle（15%-20%） of different melts exaction proportion,andmainly undergone silicate melts metasomatism with minor carbonatitic metasomatism.5.Re-Os isotope of the sulfides in peridotites and the whole rocks have been analyzed andcontrasted in this paper.The wider range of Re-Os isotopic ratios in sulfides than those in thewhole rock indicates that the Re-Os isotopes have mixed those in sulfides with differentoccurrence and origin.The maximum Re depleted model age （TRD） of the enclosed sulfides inHannuoba peridotitic xenoliths is 2.1 Ga.The enclosed sulfides,with Re-Os isotopic ratios lowerthan primitive mantle,give an isochron age of 2.3±1.2 Ga.However it is Archean era when theisochron line is fitted without the primitive mantle.All of the interstitial sulfides and some of theenclosed sulfides,with higher Re-Os isotopic ratios than primitive mantle,yield an isochron age of645±225 Ma,which might record mantle melt/fluid activity in SCLM in Neoproterozoic.1.3Gaobtained from an interstitial sulfide is interpreted as a thermal event in Mesoproterozoic.Therefore,several periods of the sulfide Re-Os ages in the Cenozoic Hannuoba peridotitic xenolithssuccessfully reveal the frequently-occurring mantle events and thus complex evolution oflithosphere beneath the North China Block.The oldest TRD age calculated by the Re-Os isotopes inperidotitic xenoliths from Changle is 1.2Ga and the Os proxy isochron is 2.1Ga.Therefore,themantle heterogeneity probably is common beneath the North China Craton according to the fertilecharacters of peridotites from Changle.The Archean age （2.5Ga and 3.0Ga） acquired by the Re-Osisotopes of sulfides in Hebi peridotitic xenoliths implies that the present mantle beneath Hebi is the relic refractory mantle.This mantle heterogeneity is mostly the result of the asthenospheric mantlereplacing the aged craton mantle through erosion,intermingling and modification.6.The heterogeneous nature of the lithospheric mantle beneath the central and eastern NorthChina Craton has been discussed,and the simply evolutional processes of the lithospheric mantleduring the Cenozoic period have been proposed in this paper.Such lithospheric mantle architectureis consistent with the mantle evolution model that the eastern North China lithospheric mantle hasundergone heterogeneous asthenospheric materials replacement via erosion and modification.Thepresent age data also indicate that the ancient Archean refractory lithospheric mantle beneath theNorth China has undergone a large scale replacement during 2.3-1.9 Ga in the Paleoprotozoic,with shallow relics preserved in Hebi of the central craton and Hannuoba of the northern edge ofthe Trans-North China Orogen within the craton.This is supported by the geochemical data of theperidotites xenoliths.The Hannuoba lithospheric mantle have undergone frequently occurred deepmantle thermal modification at 1.4-1.2 Ga and 0.9-0.6 Ga in the follow Mesoproterozoic andNeoproterozoic periods,but not affected by the considerable lithospheric mantle replacementunderneath the eastern North China Craton during Phanerozoic.Lithospheric mantle beneathChangle-Shanwang area within the Tanlu big fault has undergone at least two stages of mantlereplacement happened the Paleoprotozoic and Mesozoic.The Tanlu big fault provided a prefectway for the asthenospheric materials upwelling,through which finally replaced the Paleoprotozoiclithospheric mantle underneath Changle-Shanwang area and given formation of a newly accretedfertile mantle,with only minor Proterozoic mantle components preserved.This conclusion ispartly supported by the whole-rock Re-Os isotopic data.更多还原