【作者】 刘锐；

【导师】 钟增球；

【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】 华夏地块位于华南板块东南部，其广泛出露的前寒武纪变质基底曾遭受后期地质事件的强烈改造。浙西南和闽西北地区地处华夏地块腹地，此地区分布的基底变质岩经历了强烈的后期构造变形和区域混合岩化作用，形成大范围出露的花岗质杂岩系。这些分布于变质基底中的花岗质岩石是研究华夏地块前海西期地壳深熔作用及构造演化的窗口。本文通过详细的野外观察、系统采样，以详细的岩相学研究为基础，结合岩石元素地球化学，以先进的LA-（MC）-ICP-MS锆石U-Pb定年、微量元素及Hf同位素测试为手段，对华夏地块浙西南和闽西北变质基底中的花岗质岩石进行了系统的研究，来探讨华夏地块前海西期（早元古代和加里东期）地壳深熔作用的时间、成因和构造属性。获得的主要认识如下：1．浙西南地区出露于八都群中的淡竹和三枝树花岗岩分别为黑云钾长花岗和二长花岗岩。主量和微量元素研究表明，淡竹和三枝树花岗岩高Si富碱，高K贫Al、Ca和Mg，准铝质，高FeO^*／MgO和10000Ga／Al值，HFSE元素组合（zr+Nb+Ce+Y）值也较高，表明淡竹和三枝树花岗岩为典型的铝质A型花岗岩，而不同于以往认为的S型（或改造型）花岗岩。LA-ICP MS锆石U-Pb年代学研究表明，淡竹和三枝树花岗岩的成岩年龄分别为1844±10Ma和1860±13 Ma。岩浆锆石ε_Hf（t）值均为较低的负值（分别为-5.4～-3.6和-15.6～-10.0），表明花岗岩均为古老地壳深熔作用的产物。2．以前被认为是花岗闪长岩的大柘花岗质岩石实际为含紫苏辉石花岗质麻粒岩，其变质年龄为1851±11 Ma。变质锆石ε_Hf（t）值也为较低的负值（-13.4～-9.5），另外岩相学观察表明岩石中发生了黑云母的脱水熔融变质反应，因此麻粒岩是古老地壳在遭受麻粒岩相变质作用过程中经深熔作用而成。淡竹和三枝树花岗岩具A型花岗岩特征及大柘花岗质麻粒岩显示近等压冷却退变质（IBC）演化轨迹，暗示华夏地块～1.85 Ga基性岩浆底侵作用可能导致了麻粒岩相变质（可能伴随地壳拉张）以及古老地壳熔融形成花岗岩。华夏地块～1.85 Ga岩浆-变质事件很可能记录了Columbia超大陆聚合向裂解转折的信息。3．详细的地球化学及锆石年代学研究表明，闽西北地区基底变质岩-混合岩-花岗岩存在成因联系。基底变质岩中的黑云母在较低温（～800℃）、H₂O不饱和的条件下发生脱水熔融反应产生初始熔体，初始熔体发生结晶分异作用，堆晶产物形成了混合岩的浅色体，而残余熔体继续演化形成花岗岩。基底变质岩的变质年龄为455～443 Ma，浅色体结晶年龄为～437 Ma，花岗岩结晶年龄为～440 Ma。锆石Hf同位素研究表明，基底变质岩中先存（继承）锆石以及其它富Hf矿物的溶解释放出的Hf是形成新生变质（岩浆）锆石重要的Hf来源。同时，变质／岩浆锆石均具有负的ε_Hf（t）值和较高的二阶段Hf模式年龄，指示基底变质岩、混合岩和花岗岩均为加里东期地壳深熔再造的产物。4．闽西北变质基底中变质-深熔-岩浆作用时间持续时间至少为15 Mya，混合岩和花岗岩可能是华南加里东期陆内碰撞造山作用的产物。结合前人的研究，华南加里东造山作用开始时间不晚于458 Ma，主碰撞时间可能为～443 Ma，造山带垮塌时间可能稍晚于423 Ma。5．闽西北花岗质岩石中继承锆石1.4～1.3 Ga的年龄与海南岛中元古代岩浆岩的年龄（1.44～1.43 Ga）接近，也与劳伦古陆南部的1500～1350 Ma花岗岩-流纹岩同时。另外闽北地区新元古代沉积岩碎屑锆石年龄谱与劳伦古陆西部（现今北美）的年龄谱极为相似，似乎表明了华南地块与劳伦古陆的亲缘性。另外，华夏地块南部是否存在Grenville期（1.0～0.9Ga）造山带值得商榷。更多还原

【Abstract】 The Cathaysia Block, located in the southeast part of South China, expose extensive Precambrianmetamorphic basement rocks that were heavily reformed by later geological events. Thebasement metamorphic rocks experienced later strong structural deformation and migmatizationin SW Zhejiang and NW Fujiang Provinces in the interior of the Cathaysia Block, with extensiveexposed granitic complex in it. The granitic rocks of the metamorphic basement provide awindow into the study of pre-Hercynian crustal anatexis and tectonic evolution of the CathaysiaBlock. Based on field observation, systematic sampling and petrographic investigation, acombined study of rock geochemistry, zircon trace elements, U-Pb and Lu-Hf isotopes wascarried out on the granitic rocks in basement rocks of SW Zhejiang and NW Fujian Provinces,aiming to elucidate the timing and genesis of pre-Hercynian crustal anatexis in the CathaysiaBlock and its tectonic significance.1. The Danzhu and Sanzhishu granites, exposed in the Badu Group of SW ZhejiangProvince, are biotite K-feldspar granite and adamellite, respectively. They are high in Si, alkaliand K, and low in Al, Ca and Mg. The granites are metaluminous with high FeO^*/MgO and10000Ga/Al ratios and high （Zr+Nb+Ce+Y） content, implying that they are typical aluminousA-type granites rather than S-type granite as previously thought. LA-ICP MS U-Pb datinganalyses of magmatic zircons reveal that the formation ages of the Danzhu and Sanzhishugranites are 1844±10 Ma and 1860±13 Ma, respectively. The magmatic zircons had negativeε_Hf（t）values of -5.4--3.6 and -15.6 to -10.0 for each, suggesting the granites are anatetic products ofancient crust.2. The granitic rock in Dazhe area of SW Zhejiang Province, previously regarded asgranodiorite, is actually orthopyroxene-bearing granitic granulite, with metamorphic age of1851±11 Ma. The metamorphic zircons had negativeε_Hf（t） values of -13.4--9.5. Along with thegeographic observation of biotite dehydration melting reactions in the granulite, we suggest thatthe Dazhe granitic granulite was likely formed by anatexis of ancient crust during the granulite facies metamorphism. Combining A-type affinities of the Danzhu and Sanzhishu granites withthe retrogressive metamorphism evolution of the Dazhe granulite characterized by near-isobariccooling （IBC）, the～1.85 Ga mafic underplating in the Cathaysia Block might be responsible forthe granulite facies metamorphism （probably accompanied by crustal extension） and the meltingof ancient crust that produced the granites. The～1.85 Ga magmatic-metamorphic events in theCathaysia Block may mark the transition from assembly to break-up of the Columbiasupercontinent.3. On the basis of detailed studies of geochemistry and zircon geochronology, there existed agenetic link among the basement rocks, migmatites and granites in NW Fujian Province. Theoriginal granitic melts were likely produced by dehydration-melting of the biotite frommetasedimentary rocks in basement under relatively low temperature （～800℃） andH₂O-undersaturated conditions, then the leucosomes of migmatites were probably formed by thefractional crystallization and crystal accumulation of the initial granitic melts mentioned above,and the remaining melts continued to evolve and ultimately crystallized as granites. Themetamorphic ages of the basement metamorphic rocks fall in the range of 455-443 Ma, and thecrystallization ages are～437 Ma for the leucosomes of migmatites and～440 Ma for the granites.The zircon Hf isotope data reveal that breakdown of pre-existing zircons and minerals other thanzircon in the source played a key role in new zircon formation by release of Hf into the melt.Meanwhile, the newly formed zircons had negativeε_Hf（t） values and high two-stage Hf modelages, indicating that the basement metamorphic rocks, migmatites and related granites in NWFujian were formed by the Caledonian crustal anatexis and reworking event in South China.4. The Caledonian metamorphism, anatexis and magmatism observed in the metamorphicbasement of NW Fujian Province was a protracted tectonothermal event which lasted for ca. 15Mya, and the migmatites and related granites might be a result from an intracontinental collisionbetween the Yangtze and Cathaysia blocks. Combined with previous research, the initiation ofCaledonian orogeny in South China began no later than 458 Ma, with climax of collision at～443Ma and collapse of the orogenic belt slightly later than 423 Ma.5. The ages of 1.4～1.3 Ga recorded by inherited zircons from the granitic rocks in NWFujian Province are identical to the formation ages （1.44～1.43 Ga） of the magmatic rocks onHainan Island at the southern end of the Cathaysia Block, and also synchronous with the1500-1350 Ma granite-rhyolite in southern Laurentia. In fact, the zircon age spectra ofNeoproterozoic sediments in NW Fujian Province is quite similar to that of river sands fromwestern Laurentia （North America）, providing good evidence for a connection between Cathaysiaand Laurentia in Rodinia. Besides, whether there was a Grevillian orogenic belt in the southernCathaysia Block is open to discussion.更多还原