【作者】 张芳荣；

【导师】 王德滋； 舒良树； 于津海； 沈渭洲；

【作者基本信息】 南京大学 ， 矿物学岩石学矿床学， 2011， 博士

【摘要】 江西省中-南部是华南加里东期花岗岩的重要出露地段,历来为众多学者所关注。然而,长期以来,由于缺乏高质量的定年数据和高精度的地球化学分析数据,导致在花岗岩的分类、物质来源、形成时代及构造背景等方面存在明显分歧。针对上述问题,本研究选取了赣中-南地区付坊、乐安、龙回、上犹、宁冈等五个岩体进行研究。在野外系统地质调查的基础上,采用先进的测试技术和方法,对这些岩体的岩相学、同位素地质学、地球化学、地质年代学等进行了深入的研究,探讨了花岗岩及混合岩的成因、花岗岩与构造的关系,并结合前人研究成果,探讨了华南加里东期花岗岩的地质背景。定年数据表明,付坊岩体为一复式岩体,由付坊岩体与三峰山岩体组成。付坊岩体花岗闪长岩的结晶年龄443.9±3.5Ma,二长花岗岩结晶年龄443.1±4.6Ma；三峰山岩体二长花岗岩结晶年龄433.6±3.9Ma,晚于付坊岩体,与它侵入付坊岩体的地质特征是一致的。乐安岩体二长花岗岩三组锆石结晶年龄分别为401±3.8Ma、405.1±3.3Ma、403.9±2.9Ma,龙回岩体二组锆石结晶年龄分别为440.1±3.7Ma、441.3±3.3Ma,宁冈岩体锆石结晶年龄为433.8±2.2Ma。上犹复式岩体获得4组年龄,分别为440.0±4.7Ma、443.9±3.2Ma、419.2±3.3 Ma和408.2±4.7Ma。其中,细粒似斑状黑云母花岗岩(440.0±4.7 Ma)和中细粒似斑状黑云母二长花岗岩的年龄(443.9±3.2 Ma)属于复式岩体早阶段岩浆活动产物,中粒似斑状黑云母二长花岗岩(419.2±3.3 Ma)系同源不同岩相岩浆结晶的结果,而陡水岩体(408.2±4.7Ma)则是上犹复式岩体中晚期岩浆活动的产物。研究区加里东期花岗岩形成于400-444Ma之间,均位于加里东晚期时间段内。结合野外侵入关系,加里东期花岗岩具有多阶段成因特点。岩相学特征表明加里东期岗岩体中含富铝矿物白云母,角闪石仅少量出现于花岗闪长岩中；没有发现标志岩浆混合的斜长石环带及钾长石的环斑结构。野外也没有观察到岩浆混合作用标志的暗色包体,表明加里东期花岗岩没有发生岩浆混合作用。研究区内加里东期花岗岩体具有相似的地球化学组成,均具有较高的SiO2 (平均为71.31%)、K2O+Na2O(平均为7.37%)、Al2O3(平均为14.05%)及A/CNK比值(平均为1.11),较低的CaO(平均为1.65%)、P2O5(平均为0.13%)及铁镁质组分。微量元素以相对富集大离子元素Cs、Rb、Zi、Th、U和稀土元素La、Ce、Nd,相对亏损Ba、Sr、Nb、P、Ti为特征,具有较高的Rb/Sr[平均为1.54)和Rb/Nb(平均为13.76)比值。岩体轻稀土富集明显,具有较平坦的重稀土配分模式,表现为中等程度的Eu亏损(δEu平均为0.49)。花岗岩的(87Sr/86Sr)i值(0.70353～0.71937)与S型花岗岩的(87Sr/86Sr)i值范围相一致。但根据Nd同位素特征,本区花岗岩可划分为明显不同的两种类型：即具有较低的ENd(t)值(为-8.64～-13.32),较高的Nd模式年龄(1838～2252Ma)和具有较高的εNd(t)值(为-4.06～-7.00),较低的Nd模式年龄(1491～1735Ma)。其同位素地球化学特征反映了不同地区基底组成存在差异,乐安、龙回、上犹岩体具有较低(87Sr/86Sr)i及较低Nd模式年龄的花岗岩可能与其岩浆含有晚中元古代-新元古代含基性火山物质的地壳有关。岩相学、岩石地球化学、同位素地球化学均表明,研究区加里东期花岗岩均属S型花岗岩类,属于壳源型花岗岩范畴,来源于元古代地壳中砂质或泥砂质或泥质源区的部分熔融。其中陡水岩体可能是早期形成上犹岩浆残留的富石榴子石变质岩在后期上升减压条件下再次熔融,石榴子石不稳定而释放出更多的HREE所致。岩浆形成后还可能发生过斜长石、锆石、磷灰石、独居石、褐帘石等副矿物的结晶分离。研究区内武夷山一带混合岩化发育,分布于高角闪岩相变质地区。野外地质特征表明,混合岩起源于部分熔融,代表地壳重熔的初始阶段,其变形属熔融过程中的产生的同构造变形；可能是由于加里东期褶皱叠加使地壳加厚,其地壳厚度提供的热源足以产生高角闪岩相变质作用及深融作用而形成混合岩。但由于地壳厚度的不均一性或在地壳加厚时不均一,部分地段地壳厚度产生的温压条件不足以发生高角闪岩相变质作用,而没有形成混合岩化,使混合岩断续分布。加里东期花岗岩具有后造山花岗岩的岩石地球化学特征。地质特征上,岩体多沿大断裂分布,其展布方向与断裂延伸方向基本一致,岩体侵位多受断裂控制；褶皱造山与相关花岗岩在时间上常表现为褶皱造山在先,花岗岩浆形成在后的特点,形成于后造山背景下。研究表明,早古生代发育有卷入基底的叠加褶皱和强烈的基底变形及广泛的区域绿片岩相变质作用。但未发生明显的火山作用,没有相应的蛇绿岩套或蛇绿混杂岩；也没有超岩石圈断裂；早古生代沉积环境为浅海-半深海环境；加里东期花岗岩总体上属S型花岗岩类或壳源型花岗岩,呈面状分布,其岩相学、同位素组成、地球化学及年代学均没有陆缘造山带那种极性特点。研究结果表明,研究区加里东花岗岩可能形成于陆内造山环境,其形成很可能和发生在早古生代后期的构造-岩浆作用有关,和华夏地块与扬子地块之间以及华夏地块内部各裂解残块之间的拼贴作用有关。更多还原

【Abstract】 Caledonian (Early Paleozoic) granitoids outcropped in the South central of Jiangxi province are one of important parts of granites in South China, which has attracted many geological workers’attentions. Due to the lack of accurate geochronological and geochemical data, the classification, petrogenesis and tectonic settings of granitic rocks are still controversy. In order to answer these questions, this study selected five representative plutons, including Fufang, Le’an, Longhui, Shangyou and Ninggang. Based on the investigation of the field geology and the advanced measurment methods, the thesis studied the petrology, isotopic geology, geochemistry and geochronology of five granitic plutons and discussed the genesis of the granites and migmatites, the relationship between the granites and the tectonics. Finally, combining with the previous works, the thesis suggested the new geological settings of the Caledonian granites in South China.The geochronological data showed that Fufang pluton is a complex pluton, composed of Fufang pluton and Sangfengshan pluton. The crystallization age of the Fufang granodiorite and Fufang monzogranite were 443.9±3.5Ma and 443.1±4.6Ma respectively, belong to the first-stage intrusions. The second-stage intrusion is Sanfengshan monzogranite with the age of 433±3.9Ma. These ages are consistent with field geological characteristics. Three zircon U-Pb ages obtained from Le’an monogranite were 401.1±3.8Ma,405.1±3.3Ma and 403.9±2.9Ma respectively. The ages of Longhui pluton were 401.1±3.7Ma and 441.3±3.3 Ma. Four zircon U-Pb ages of Shangyiu complex pluton were 440.1±4.7Ma,443.9±3.2Ma and 408.2±4.7Ma respectively. The fine-grained porphyritic biotite-granite (440.0±4.7 Ma) and the fine-medium grained porphyritic biotite-monzogranite (443.9±3.2 Ma) from the complex pluton were the products of early-stage magmatism. The medium-grained porphyritic biotite-monzogranite possibly crystallized from the different facies of the homologous source. Doushui pluton (408.2±4.7 Ma) formed by the later stage magmatic activity. The zircon U-Pb age of Ninggang pluton was 433.8±2.2Ma. Above mentioned data showed that all studied granites formed from 444 Ma to 400 Ma, belong to the late Caledonian period. Combining with the field investigation, we considered that Caledonian granites have the characteristics of multi-stage intrusions.The petrology of the Caledonian granites showed that they contained abundant muscovite and a few hornblende only occurred in granodiorites. The zoned plagioclase and rapakivi K-feldspar phenocryst typically in magma mixing phenomena is absent.The micrograined enclaves are not found in Caledonian granites. It is suggested that the magma mixing did not occur in Caledonian granites.Caledonian granites studied here have similar geochemical compositions, high contents of SiO2, K2O+Na2O, Al2O3, A/CNK value with the average of 71.31%,7.37%,14.05% and 1.11, respectively, and low content of CaO, P2O5 as well as mafic components. They enriched in large ion trace elements such as Cs, Rb, Th, Zr, U and rare earth elements in La, Ce, Nd, depleted relatively in Ba, Sr, Nb, P, Ti and had higher Rb/Sr ratio (the average of 1.54) and Rb/Nb ratio (the average of 13.76). These granites have significant LREE enrichment with relatively flat HREE patterns and moderate depletion in Eu (the average 8Eu of 0.49).The values of (87Sr/86Sr) i of granites changed from 0.70353 to 0.71937, in agreement with that of S-type granites. According to Nd isotopes, these granites were divided into two types, one type with the lowerεNd(t) value(-13.32～-8.64) and higher Nd model ages(2252～1838Ma) and the other with higherεNd(t) value(-7.00～-4.06) and lower Nd model ages(1735～1491 Ma). Their isotopic geochemistry reflected the different composition of the distinctive basements. Granites from Le’an, Longhui and Shangyiu pluton with the characteristics of higherεNd(t) value and lower Nd model ages possibly indicated that their source was related to the Mid-to Neo-proterozoic basement containing the basic volcanic rocks.The petrology, geochemistry isotopic geology of the Caledonian granites showed that they could be classified into S-type granite or crust-derived granites. They were originated from partial melting of meta-greywacke, meta-greywacke-pelite or meta-pelite in low mature Proterozoic crust. Doushui pluton was possibly originated from remelting of garnet-rich metamorphic rocks residue after the formation of Shangyou pluton at the early stage, because unstable garnet released more HREEs in the condition of decompression. Moreover, the fractionation of plagioclase and accessory minerals such as zircon, apatite, monazite and allanite occured. Fufang pluton was originated from partial melting of the meta-greywacke.Migmatites well developed in Wuyi Mountain and distributed in HP amphibolite facies region. The field geological characteristics suggested that migmatites formed by partial melting and represented the initial stage of crustal remelting with the syn-tectonic deformations during partial melting. The superimposed fold leaded the crust to be thicken, so the heat supplied by the thick crust is hot enough to form HP amphibolite facies metamorphism, deeply re-melting and finally form the migmatites. However because of dissimilar thickness of the crust or crust thickening in different degrees, HP amphibolite facies in some areas have not been happened to produce the migmatites. Also this is the reason that leaded the discontinuous distribution of the migmatites.Caledonian granites have similar geochemical characteristics of post-orogenic granite. Tectonically, the rocks distributed along large fault belt, which suggested that the intrusions were controlled by the fault. Temporally, Caledonian granitic magmatism formed after the fold-orogenesis and in the post-orogenic tectonic settings.It is showed that during Early Paleozoic the superimposed fold developed with the basement involved, the strong deformation of the basement and broad regional greenschist phase metamorphism also happened. There were absent of obvious volcanism, ophiolite suites and immingling ophiolite rocks or super lithosphere fault in studied area have not been found. The sedimentary environments of Early Paleozoic stratum are shallow to half-deep sea. As a whole, the Caledonian granites in this area were classified into S-type granite or crust-derived granite, dispersively distributing. The petrology, isotopic geology, geochemistry and geochronology are not same with that in the granites of the epicontinental orogenic belt. The Caledonian granites formed in the intracontinental orogenic setting, possibly related to the amalgamation and collision between Cathaysian block and the Yantze block, and the amalgamation amongst many Cathaysian remnant blocks.更多还原