【作者】 高万里；

【导师】 王宗秀；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2014， 博士

【摘要】 位于华北板块,印支板块,太平洋板块围限下的华南大陆,中生代时期以大规模的构造变形、大面积的火成岩以及巨量的金属成矿作用广为国内外地质学者的关注。但是中生代大面积火成岩形成的构造动力学背景一直以来存在争议。为了更好的理解华南中生代岩浆活动的成因及构造背景,本文选择位于华南大陆东北端的浙江省东南部为研究区,对区内三叠纪、晚侏罗-早白垩世、晚白垩世早期三个关键时期的岩浆活动展开研究,并结合华南中生代构造-沉积-变质事件的研究成果,对华南中生代构造演化进行新的诠释。中国东南部印支期花岗岩对亚洲大陆东部的演化具有重要的构造指示意义。在浙江东阳大爽以及松阳县东部靖剧地区发现两个印支期花岗岩体和胡村、坑口两个花岗岩脉。大爽二长岩和石英二长岩的LA-ICP-MS锆石U-Pb年龄为241±2.1-229.6±2.5Ma,231±2.7Ma,靖剧岩体二长花岗岩和钾长花岗岩的的LA-ICP-MS锆石U-Pb年龄分别为245.7±1.8Ma和241.2±2.7Ma,表明这两个岩体均为早印支期岩浆活动的产物。地球化学特征显示,这两个花岗岩具有高碱(Na2O+K2O=7.73-10.24%),富高场强元素(e.g. Zr=215-471ppm, Y=25.8-36.5ppm, Nb=15-28ppm and Zr+Nb+Ce+Y=293-849ppm),稀土元素总量高(total REE=299-701ppm),具有高的Ga/Al比值(10000×Ga/Al=2.44-2.9)的特点。其锆石结晶温度800-837℃,具有A型花岗岩的特征。花岗岩的εHf(t)变化于-20-6,两阶段亏损地幔Hf模式年龄集中在1.6Ga-2.6Ga,表明其为古元古代地壳物质部分熔融的结果。胡村花岗岩脉的LA-ICP-MS锆石U-Pb年龄为242.1±1.5Ma,坑口花岗岩脉LA-ICP-MS锆石U-Pb年龄为232.2±3.6Ma,表明这两个脉体也为早印支期岩浆活动的产物。花岗岩脉地球化学特征显示,其富钾、高铝,富集轻稀土LREE及Rb、Th、U、K,亏损Ba、Nb、Sr、P、Ti,(La/Yb)N值为28.46～38.07及弱负铕异常(Eu/Eu*=0.65-0.73)。胡村脉岩的锆石Hf同位素变化较小,εHf(t)值变化于-6.4～13.9,TDMC模式年龄为1.68-2.15Ga,反应胡村脉岩主要来源于元古代地壳物质的部分熔融。坑口脉岩的锆石Hf同位素值变化较大,εHf(t)变化于-40.7～31.5,表明坑口脉岩是由不同源岩组成的。近年中国东南部发现的印支期花岗岩,以及中国东南部一系列A型花岗岩及碱性正长岩,表明东南沿海在早印支期处于区域伸展构造环境下。两个花岗岩脉的发现进一步证明华南板块与周边板块碰撞之后的转换伸展,使得古老地壳物质发生部分熔融,并沿区域上断裂上侵,是东南沿海早印支岩浆活动的主要原因。晚侏罗-早白垩世时期华南构造环境发生重要改变的时期,该时期岩浆活动的规律可以为构造环境的变化提供关键制约。本次研究选取浙江中部金华和龙游附近的浅成侵入体进行研究,锆石LA-ICPMS U-Pb年代学研究表明,浙东南地区晚侏罗花岗斑岩和钾长花岗岩的主要形成在144±1.8～150.6±1.9Ma,早白垩世花岗质岩石主要形成在133.1±1.4～134.5±1.6Ma。主、微量元素地球化学特征显示,两阶段花岗质岩石均具有准铝,亚碱性和富钾的特点,并富集Rb、Th、Ba、Zr、Hf,贫Sr、P、Ti、Nd、Ta,岩石的稀土元素总量集中在95-216ppm,富集轻稀土LREE/HREE(3.0-14.8),(La/Yb)N的值在2.1～14之间变化,铕负异常,δEu介于0.1～0.7。晚侏罗花岗岩的εHf(t)值集中在-3.2～-11.4,其二阶段Hf模式年龄为1.37-1.92Ga,早白垩世的花岗岩εHt(t)介于-13.2～4.3,其二阶段Hf模式年龄主要集中在0.91-2.02Ga,暗示早白垩世花岗质岩石具有显著的地幔物质的加入。结合区域地层以及岩浆活动爆发的特点,本次研究认为,晚侏罗世浙东南地区可能处在整体挤压环境下的局部伸展背景下,岩石圈开始发生伸展减薄,出现岩浆侵入和火山喷发,而到早白垩时期,岩石圈发生了大规模的伸展作用,从而形成了东南沿海白垩纪大规模的火山侵入杂岩,这一变化最有可能与俯冲的古太平洋板块开始发生回撤有关。梁弄岩体是浙江沿海晚中生代一个典型的复合岩体,其主体岩性为二长花岗岩和花岗闪长岩。锆石LA-ICP-MS定年测得二长花岗岩的锆石U-Pb年龄为106.3±1.1Ma,花岗闪长岩的锆石U-Pb年龄为103.9±1.6Ma和105±1.8Ma,表明该岩体属早白垩晚期岩浆活动的产物。地球化学特征上,该岩体具有准铝,亚碱性和富钾的特点,并富集Rb、Th、Ba、Zr、Hf,贫Sr、P、Ti、Nd、Ta,岩石的稀土元素总量较高(REE=150.2-227.9),富集轻稀土(LREE/HREE=12-16.1),具有中等铕负异常(Eu*/Eu=0.6-0.8)。岩石普遍发育花斑结构,岩体中可见暗色暗色包体,其LA-ICP-MS锆石U-Pb年龄为104.67±0.82Ma。暗色包体的地球化学特征显示,其富铝、富铁,高镁、高钛,并相对富集Ba、U、Sr、Ti,亏损Th、Nd、Zr,其稀土元素总量、轻重稀土元素比值较主体花岗岩为低,显示其分异演化程度较主体花岗岩为低,其Eu*/Eu=1.0-1.1,表明其源区存在显著的斜长石残留。卞体花岗岩与暗色包体的锆石Hf同位素分析表明,其锆石Hf同位素组成变化范围大,εHf(t)值主要变化于-10.8～4.9,暗色包体(104.67±0.82Ma)正的εHf(t)(0.1～4.9)显示有新生地幔物质加入到地壳。锆石Hf同位素二阶段模式年龄变化于TDMc=1.4-1.6Ga,暗示岩体主要源于中元古代地壳物质的部分熔融。结合区域上同期花岗岩的特征,它们可能都是俯冲的太平洋板块发生后撤,使该区域整体上处于岩石圈伸展构造背景下,幔源岩浆上侵导致地壳物质发生部分熔融产生花岗质岩浆并与之发生岩浆混合作用。综合华南早、晚中生代岩浆-变质-构造变形特征的研究,对华南现有的大地构造演化模式进行了新的诠释,认为华南早中生代主要受到了华南板块与周边板块碰撞作用的制约,早中生代主要表现出陆内造山的特征,板块碰撞之后的岩石圈圈层之间的拆离作用可能是导致华南构造热事件形成的主要原因；晚中生代构造演化主要受古太平洋板块俯冲作用的影响,早中侏罗世处于受到俯冲应力传递影响的板内裂谷环境下；中侏罗世开始,俯冲的古太平洋板块片开始发生回撤,引发了大规模的岩石圈伸展,幔源岩浆开始卷入花岗岩的形成；白垩纪大规模的火山侵入杂岩的形成受俯冲作用与岩石圈伸展的双重影响。更多还原

【Abstract】 South China Block, under conformed by the North China Plate, Indochina plate and the Pacific plate, which is features by large-scale Mesozoic tectonic deformation, metal mineralization as well as a large area igneous rocks, has stimulated the attention of many geologists. However, dynamics background of large-scale magmatism has been controversial. To better understand the causes and tectonic setting of Mesozoic magmatism, detailed geochronology and geochemistry studies of Triassic, Late Jurassic-Early Cretaceous, early Late-Cretaceous magmatism have been conducted in SE Zhejiang. Combined with Mesozoic tectonic-sedimentary-metamorphic event studies, a new interpretation on the Mesozoic tectonic evolution is conducted.The late Permian-Triassic granites in southeastern China have important tectonic significance for the evolution of East Asia. A detailed study utilizing zircon U-Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry has been carried out for the Dashuang granite, Jingju granite, Hucun dyke and Kengkou dyke in Zhejiang Province, South China. LA-ICP-MS zircon U-Pb analyses yielded age of monzonite and Qutarz-monzonite is241±2.1-229.6±2.5Ma,231±2.7Ma from Dashuang pluton, and LA-ICP-MS zircon U-Pb ages of Qutarz-monzonite and moyite are245.7±1.8Ma,241.2±2.7Ma from Jingju pluton, which indicates that the magmatism took place during from early Triassic to Mid-Triassic. The two granites show high contents of total alkalis (Na2O+K2O=7.73-10.24%), high field strength elements (e.g. Zr=215-471ppm, Y=25.8-36.5ppm, Nb=15-28ppm and Zr+Nb+Ce+Y=293-849ppm) and rare earth elements (total REE=299-701ppm) as well as high Ga/Al ratios (10000×Ga/Al=2.44-2.9). The lowest magmatic temperatures estimated from zircon saturation thermometer were800-837℃for the two granites, which suggested that they have the petrographic and geochemical characteristics of A-type granites. In-situ Hf isotopic analyses indicated that two granites have εHf(t) values ranging from-20to-6and two-stage depleted mantle Hf model ages from1.6Ga-2.6Ga, which was inferred that the two granites magma formed by partial melting of Paleoproterozoic metaigneous in Cathaysia Block. A series of A-type granites distribution along the NE-trending fracture show extensional tectonic setting of southeastern China from early Triassic to Mid-Triassic, which may be caused by strike-slip conversion stretching after collision between SCB (South China block) and surrounding block. Granitic dykes were commonly developed in Chencai Group of Zhejiang provinces, which were important symbol of post-magmatism activity. LA-ICP-MS zircon U-Pb analyses yielded age about242.1±1.5Ma and232.2±3.6Ma for two samples from Hucun and Kengkou granitic dykes, which were products of early Indosinian magmatism activity. The two dykes were enriched in K, Al, LREE and Rb, Th, U, Pb, and were depleted in Ba, Nb, Ta, Sr, P, Ti. Their REE patterns were highly fractionated, with (La/Yb).v ratios of28.46-38.07and weakly negative Eu anomalies (Eu/Eu*=0.65-0.73). In-situ Hf isotopic analyses indicated that Hucun dykes have εHf(t) values ranging from-13.9to-6.4and two-stage depleted mantle Hf model ages from1.68Ga-2.15Ga, which was inferred that the Hucun dyke magma formed by partial melting of Paleoproterozoic metasedimentary in Cathaysia Block. Kengkou dykes have εHf(t) values ranging from-40.7-31.5and two-stage depleted mantle Hf model ages from0.99Ga-2.49Ga, indicated its source were complicated. The two Indosinian dykes (242Ma and232Ma), together with discovery of Indosinian A-type granites in the province of Zhejiang and Fujian, difined important extensional events in Triassic. We suggested that Indosinian dykes and A-type granites in the coastal reigion of the SCB probably formed under an extensional regime related to collision between SCB and surrounding block, but not with oblique subduction of the paleo-Pacific plate.It is commonly accepted that tectonic regimes underwent a tremendous change, during the Late Jurassic-Early Cretaceous period. The feature of magmatism can provide a critical constraint to the changes in the period. Zircon LA-ICPMS U-Pb geochronology studies have shown that granite porphyry and K-feldspar granite of Late Jurassic yielded age of144±1.8-150.6±1.9Ma, and early Cretaceous granitic-rocks are implaced at133.1±1.4-134.5±1.6Ma. Geochemical characteristics show that the two-stage granitic-rocks have high contents of metaluminous, potassium and sub-alkaline, and are enrichment of Rb, Th, Ba, Zr, Hf, depleted in Sr, P, Ti, Nd, Ta. The total REE is ranging from95-216ppm, and have high LREE (LREE/HREE=3.0-14.8), and (La/Yb) N changes between2.1to14, and negative Eu anomaly (δEu=0.1-0.7). Late Jurassic granitic rocks have εHf(t) values ranging from-3.2--11.4, and the two-stage depleted mantle Hf model age of1.37-1.92Ga, while Early Cretaceous granitic-rocks have εHf (t) ranging from-13.2to4.3, and the two-stage depleted mantle Hf model ages ranging from0.91-2.02Ga. The result suggests that mantle-drived magma injection have significantly occured in early Cretaceous than that in Late Jurassic. Combined with regional stratigraphic and magmatic characteristics, this study suggests that lithospheric thinning occurred under the local extension environment background in Late Jurassic. To the early Cretaceous period, large-scale lithospheric extension occurred and triggered a massive magmatism. The change is most likely to cause by roll-back of subducting Paleo-Pacific plate.Liangnong pluton, lithologically consisting mainly of monzogranite and granodiorite suited, is a typical late Mesozoic complex in the coastal area of Zhejiang province. Zircon LA-ICP-MS dating results imply that granodiorite was emplaced at106.3±1.1Ma, and monzogranites were emplaced at103.9±1.6Ma,105±1.8Ma, indicating that they were generated during early Late-Cretaceous. The major rock is metaluminous with A/CNK range from0.91to1.09. The rock shows relatively high SiO2, K2O and is enriched in Rb Th, Ba, Zr, Hf, depleted in Sr, P, Ti, Nb, Ta, which also have high REE, LREE and moderate negative Eu anomalies (Eu*/Eu=0.6～0.8). The pluton is general developed spotted rock structure with dark dioritic enclaves visible. LA-ICP-MS analyses yield age of104.67±0.82Ma for dioritic enclaves. Dioritic enclaves geochemical characteristics show that it’s aluminum-rich, iron-rich, high-magnesium, high titanium, and relative enrichment of Ba, U, Sr, Ti, loss of Th, Nd, Zr. It has lower∑REE ratio and higher Eu*/Eu than that of major-rock, indicating the extent of its differentiation is lower. Zircon Hf isotope composition of major-rock and dioritic enclaves is different, εHf (t)(-10.8～4.9) and Zircon Hf two-stage model ages (TDMC=1.3～1.6Ga), suggest that they are mainly generated from partial melting of Mesoproterozoic crust and have took place crust-mantle interaction obviously. Combined with the characteristics of igneous-rocks in the region, we infer that a widespread mantle-drived magma injection have happened in SE coastal area in early Late-Cretaceous, lithospheric extension caused by roll-back of subducting Paleo-Pacific plate plays a critical role.Comprehensive analysis of early and late Mesozoic magma-metamorphic-tectonic deformation characteristics in South China, a new interpretation of tectonic evolution of South China was conducted. It is suggested that tectonic evolution of South China in early Mesozoic mainly exhibit intraplate orogenic feature, constrainted by collision between SCB and surrounding blocks. The main cause of of early Mesozoic tectonic thermal event is lithospheric detachment after plates collision. Late Mesozoic tectonic evolution is mainly affected by the subduction of the Pacific plate. It may just be under rift environment influenced by stress transfer of plate subduction in early-middle Jurassic. Paleo-Pacific plate begin subduct in Middle-Late Jurassic. Roll-back of plate have happened, subsequently, which trigger a massive lithospheric extension, and cause mantle-derived magma underplating involved in the formation of granitic rocks from late Jurrasic to early Cretaceous. Massive volcanic-intrusive rocks were formed by the double impact of subduction and lithospheric extension in Late-Cretaceous.更多还原