江西付坊花岗岩体的年代学、地球化学特征及其成因研究

付坊岩体出露于华南武夷山中段,属早古生代花岗岩体。在大量地质资料的基础上,对付坊岩体的岩石学、岩石地球化学、同位素地球化学及LA-ICM-MS 锆石U-Pb 定年进行了系统研究。研究表明, 付坊岩体主要由二长花岗岩、花岗闪长岩及少量英云闪长岩组成,岩石中含有石榴石、矽线石等富铝矿物。A/CNK值为0.99~1.14,属弱过铝质－强过铝质。微量元素以富集大离子元素Rb,Th,Cs 和稀土元素La,Ce,Nd, 亏损Ba,Sr,Nb,Ta,P,Ti 为特征,总体上属低Ba-Sr 花岗岩范畴。Rb/Sr（0.56~1.19,平均值为0.92）和Rb/Nb比值（6.85~25.18,平均值为15.43）高于中国东部及全球地壳平均值。二长花岗岩和花岗闪长岩均具有较高的稀土总量（平均值分别为243.7×10^-6和251.6×10^-6）,轻稀土富集明显,配分模式明显呈右倾型。Eu亏损相对明显,Eu/Eu* = 0.44~0.61（平均值为0.53）。对付坊岩体中的一个花岗闪长岩和两个二长花岗岩进行了LA-ICP-MS 锆石U-Pb 年龄测定,其结果分别为(443.9±3.5 ） Ma（MSWD = 1.09）,（443.1±4.6）Ma（MSWD = 0.25） 和（433.6±3.9） Ma（MSWD = 0.61）。其中前两组年龄在误差范围内一致,属奥陶期末同源岩浆分异演化产物。后一组年龄略晚,属早志留世花岗岩。研究表明,付坊岩体属S 型花岗岩类,岩浆来源于古元古代(Nd 模式年龄为2 153 Ma~2 249 Ma）地壳组分的低程度的部分熔融,其源岩主要由含少量泥质的砂质岩组成。形成于奥陶纪末的花岗岩属同碰撞（同造山）期花岗岩,形成于挤压环境,区域上分布局限。志留纪花岗岩形成于后碰撞（后造山）伸展环境,区域上分布广泛。     

江西云山复式岩体的地球化学特征及与锡成矿关系探讨

江西永修县云山岩体位于九岭隆起北侧,为二云母花岗岩复式岩体,其地球化学特征表明它是一种富硅、富碱、贫钙、过铝且富轻稀土的陆壳重熔型花岗岩,岩浆分异程度较高。云山岩体中Sn、W含量较高,为锡矿床提供了丰富的成矿物质来源,岩浆侵入与构造活动为成矿作用创造了条件。岩浆在侵位过程中进行了较为彻底的结晶分异,最终成矿物质在云山复式岩体呈岩舌、岩瘤状突出转折部位内接触带构造及各次级构造中富集成矿。在总结区内找矿标志的基础上,根据区内地、矿、物、化、遥等找矿信息,共圈定邓家山-桐木坑、余白田-红花脑、周田-王家岭、大杨-刘家坪、庵塘-马颈、板家塘-板家坳、耕源、佛祖山-焦冲等预测区8处,具有一定找矿潜力。     

REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt,SW Japan and the Southern Jiangxi Province,China

Rare earth element (REE) geochemistry and mineralogy have been studied in the weathered crusts derived from the Early Yanshanian (Jurassic) biotite granites of Dabu and Dingnan, as well as in the Indosinian (Permian) muscovite–biotite granite of Aigao in southern Jiangxi province, China, and the weathered crusts and clay sediments on biotite granites in the Sanyo belt, SW Japan, that is, Okayama, Tanakami, and Naegi areas. In all of the weathered crusts, biotite and plagioclase commonly tend to decrease toward the upper part of the profile, whereas kaolinite and residual quartz and K‐feldspar increase. The weathered crusts of the Dingnan granites and some Naegi granites, which are characterized by the enrichment in light REE (LREE) in C horizons, have higher total REE (ΣREE) content than the parent REE‐enriched granites. Weathering of LREE‐bearing apatite and fluorocarbonates in the Dingnan granites and allanite and apatite in some Naegi granites may account for the leaching of LREE at the B horizons. The leached LREE must result in subsequent enrichment of LREE in the C horizons. The enrichment is probably associated with mainly adsorption onto kaolinite and partly formation of possible secondary LREE‐bearing minerals. In Japan it was found that REE mineralization occurs not in the weathered granitic crusts but in reworked clay sediments, especially kaolinite‐rich layers, derived mainly from the weathering materials of REE‐enriched granitic rocks. The clay sediments are more enriched in LREE, which likely adsorbed onto kaolinite. Concentration of heavy REE within almost all the weathered crusts and clay sediments, however, may reflect mainly residual REE‐bearing minerals such as zircon, which originated in the parent granitic rocks. The findings of the present study support the three processes for fractionation of the REE during weathering: (i) selective leaching of rocks containing both stable and unstable REE‐bearing minerals; (ii) adsorption onto clay minerals; and (iii) presence of possible secondary LREE‐bearing minerals.     

The Kuiqi Peralkaline Granitic Complex (SE China): Petrology and Geochemistry

The Mesozoic volcano-plutonic belt of SE China is characterized,in the Kuiqi area, by acid volcanics followed by the Yanshangranites. The Kuiqi granitic complex, which belongs to the latterunit, is made up of a calc-alkaline and a peralkaline group.The calc-alkaline group consists of two intrusions, the Danyangmonzogranite and the Fuzhou syenogranite, emplaced 10310 Maand 104 5 Ma ago, respectively (Rb-Sr whole-rock isochrons).Formation of the Danyang monzogranite can be explained by athree-stage model: (1) partial melting of a metasomatized mantlegenerated a dioritic magma known in the area as the Nanyu diorite;(2) the magma was contaminated (25%) by lower continental crust;(3) large amounts (70–80%) of fractional crystallizationof hornblende and plagioclase at depth gave rise to the magmaticsuite. The Fuzhou syenogranite is more fractionated and itsformation involved crystallization of plagioclase + biotite+ K-feldspar + apatite. Intrusion of the peralkaline group isdated at 93 1 Ma (Kuiqi peralkaline granite) and at 91.80.9(Bijiashan peralkaline granite). These units are homogeneousand their petrogenesis is less constrained than for the calc-alkalinesuite. Nevertheless, a multistage process can be proposed: (1)partial melting of a metasomatized mantle produced a dioriticmagma; (2) fractional crystallization began with segregationof hornblende + plagioclase ilmenite and/or magnetite; subsequently,hornblende no longer crystallized; (3) the last stage of fractionationcorresponded to the crystallization of K-feldspar + plagioclase+ REE-rich accessory phases. Mineralogical study indicates thatduring the last stage, fluids played a prominent role and controlledthe nature of the crystallizing minerals. The magma evolvedfrom F- and S-rich, and water-undersaturated to water-oversaturated,leading to the exsolution and dissociation of an H₂O vapourphase and to the loss of H₂. The change from calc-alkaline toperalkaline magmatism is related to inferred changes in thetectonic environment. The calc-alkaline granites were generatedin a subduction setting in which water was supplied by dehydrationof the downgoing slab. The peralkaline granites were producedin a crustal thinning environment where little water was available,thus necessitating high temperatures to initiate partial melting.On the other hand, petrogenetic modelling shows that both calc-alkalineand peralkaline granites could have been derived from the samesource which is metasomatized mantle. This possibly indicatesthat the volcanic are source, active during subduction, persistedbeneath the continent and was reactivated during the post-orogenicmagmatism. The reactivation was caused by a deep crustal fault.     

江西中部新丰街花岗质岩体LA-ICP-MS锆石U-Pb年代学、元素和Sr-Nd-Hf同位素地球化学特征及岩石成因

对江西中部新丰街花岗质岩体开展了岩相学、LA-ICP-MS锆石U-Pb年代学以及元素和Sr-Nd-Hf同位素地球化学研 究,并探讨了岩石成因及其构造意义。结果表明,新丰街岩体由二云母花岗岩和黑云母花岗组成;两者均形成于晚侏罗世 （~148 Ma）;二云母花岗岩SiO2含量为75.71%~78.36%,为弱过铝质-强过铝质岩石,属高钾钙碱性系列,Mg#变化于0.26~ 0.34,具有较低的Ga/Al比值（绝大部分<2.6×10-4） 和较低的Zr+Nb+Ce+Y含量（<350×10-6）,全岩εNd(t)为-10~-8.2,锆石原 位εHf(t)为-15.7~-9.4;黑云母花岗岩SiO2含量为71.25%~74.41%,主要为准铝质-弱过铝质岩石,也属于高钾钙碱性系列, Mg#变化于0.32~0.37,同样具有较低的Ga/Al比值（绝大部分<2.6×10-4） 和较低的Zr+Nb+Ce+Y含量（<350×10-6）,全岩初始 87Sr/86Sr比值为0.7136~0.7153,εNd(t)为-10.0~-8.9,锆石原位εHf(t)值为-16.5~-10.9。通过综合研究认为二云母花岗岩具有S 型花岗岩特征,是由下地壳中变质泥岩在相对较低温度下发生部分熔融而形成的;黑云母花岗岩具有I型花岗岩特征,是由下 地壳中长英质火成岩在相对较高温度下发生部分熔融而形成的。岩体侵位于由古太平洋板块俯冲引起的陆缘弧构造环境。     

江西九岭花岗岩类复式岩基锆石SHRIMP U-Pb年代学

江西九岭花岗岩体产于扬子地块东南缘, 是我国华南的一个规模巨大的复式花岗岩基.该岩体一直被认为是在晋宁期、海西期和燕山期多期次岩浆侵入活动的产物.在对该复式岩基进行详细野外调查的基础上, 开展了锆石SHRIMPUPb年龄测定, 过去认为是晋宁期形成的花岗岩样品, 测出的206Pb/238U年龄为(9828±8) Ma; 而原来认为是海西期形成的甘坊岩体测出的年龄为(820±10) Ma, 说明该区不存在海西期的花岗岩, 结合其他新的年代学资料推测扬子板块内可能不存在加里东—海西期的岩体; 原认为是燕山中期的样品测出的年龄为(151.4±2.4) Ma, 研究结果与原来的一致.此外, 还获得了1.4~1.9 Ga的继承锆石的年龄, 可能代表九岭新元古代花岗岩源岩的年龄.      

祁连山地区花岗质岩浆作用及构造演化

祁连山在构造上是一条经历了多期构造旋回叠加的早古生代复合型造山带,花岗质岩浆作用研究对揭示其构造演化具有重要意义。锆石U-Pb年代学统计结果表明,祁连地区花岗质岩浆活动可以分为7个大的阶段,包括古元古代早期(2 470~2 348 Ma)、古元古代晚期(1 778~1 763 Ma)、中元古代晚期-新元古代早期(1 192~888 Ma)、新元古代中期(853~736 Ma)、中寒武世-志留纪(516~419 Ma),泥盆纪-早石炭世(418~350 Ma)以及中二叠世-晚三叠世(271~211 Ma)。其中古元古代早期发育强过铝质高钾钙碱性S型和准铝质低钾拉斑-高钾钙碱性I型花岗岩,记录了早期的陆壳增生及改造事件。古元古代晚期为准铝质-弱过铝质高钾钙碱性-钾玄质A型花岗岩,是Columbia超大陆裂解事件的产物。中元古代晚期-新元古代早期以过铝质-强过铝质钙碱性-钾玄质S型花岗岩为主,新元古代中期以准铝质-强过铝质钙碱性-高钾钙碱性A型花岗岩为主,分别对应Rodinia超大陆的汇聚和裂解事件。中寒武世-志留纪花岗岩是洋陆转换过程中的产物,约440 Ma加厚基性下地壳部分熔融形成的低Mg埃达克岩的广泛出现指示祁连地区全面进入碰撞造山阶段。泥盆纪-早石炭世花岗岩代表后碰撞伸展阶段岩浆岩组合,发育准铝质-强过铝质低钾拉斑-钾玄质等一系列花岗岩。中二叠世-晚三叠世花岗岩以准铝质-弱过铝质钙碱性-高钾钙碱性I型花岗岩为主,有少量弱过铝质高钾钙碱性A型花岗岩,是宗务隆洋俯冲消减以及碰撞后伸展过程的产物。     

The Geochemical Evolution of the Proterozoic Continental Crust in Northwestern Jiangxi and Western Zhejiang,China

The pertochemistry and Sm-Nd isotopic compositions and the geochemical characteristics of REE,U,Th,etc..in the Late Proterozoix and Early Palaeozoic strata in northwestern Jiangxi and western Zhejiang provinces are described in this paper.It seems to be sure that the middle Proterozoic strata of southeastern China are low in the degree of matrration .The strata contain much mantle-derived material.At the end of Late Proterozoic there was an abrupt turn with respect to the crustal geochemical evolution of the eastern part of South China .Since then ,the geochemical environment has undergone a change from a simple reducing environment to a complex oxidizing-reducing environ-ment, which would be geochemically beneficial to the formation of Late Sinian to Early Cambrian U-bearing formations.     

赣北燕山期花岗岩浆的底辟伸展造山作用

以赣北中生代伸展造山体制为例,提出了“陆内花岗岩浆底辟伸展造山模式”。上地幔隆起区的中、下地壳向周围山根地带拆离下滑而减薄。壳幔间及吓地壳内的韧性剪切拆离断层带,不但是陆壳迁移的传送带,而且也是热流体的传导及热源产生带。山根下部的地壳在增厚及热流体的不断富集过程中,导致重 生大量花岗岩浆。在浮力作用下花岗岩浆下断上升扩和在上地壳内以底辟形式就位,使上地壳产生伸展造山。     

Granitic Magmatism in Eastern Tethys Domain (Western China) and their Geodynamic Implications

Western China locates in the eastern section of the Tethys domain, granitic rocks in this region with variable formation ages and geochemistry record key information about the crust-mantle structure and thermal evolution during the convergent process of Tethys. In this study, we focus on some crucial granitic magmatism in the western Yangtze, Qinling orogen, and western Sanjiang tectonic belt, where magma sequence in the convergent orogenic belt can provide important information about the crust-mantle structure, thermal condition and melting regime that related to the evolution processes from Pre- to Neo-Tethys. At first, we show some features of Pre-Tethyan magmatism, such as Neoproterozoic magmatism (ca. 870–740 Ma) in the western margin of the Yangtze Block were induced by the assembly and breakup of the Rodinia supercontinent. The complication of voluminous Neoproterozoic igneous rocks indicated that the western Yangtze Block underwent the thermodynamic evolution from hot mantle-cold crust stage (ca. 870–850 Ma) to hot mantle and crust stage (ca. 850–740 Ma). The Neoproterozoic mantle sources beneath the western Yangtze Block were progressively metasomatized by subduction-related compositions from slab fluids (initial at ca. 870 Ma), sediment melts (initial at ca. 850 Ma), to oceanic slab melts (initial at ca. 825–820 Ma) during the persistent subduction process. Secondly, the early Paleozoic magmatism can be well related to three distinctive stages (variable interaction of mantle-crust to crustal melting to variable sources) from an Andeans-type continental margin to collision to extension in response to the evolution of Proto-Tethys and final assembly of Gondwana continent. Thirdly, the Paleo-Tethys magmatism, Triassic granites in the Qinling orogenic display identical formation ages and Lu-Hf isotopic compositions with the related mafic enclaves, indicate a coeval melting event of lower continental crust and mantle lithosphere in the Triassic convergent process and a continued hot mantle and crust thermal condition through the interaction of subducted continental crust and upwelling asthenosphere. Finally, the Meso- and Neo-Tethyan magmatism: Early Cretaceous magmatism in the Tengchong Block are well responding to the subduction and closure of Bangong-Nujiang Meso-Tethys, recycled sediments metasomatized mantle by subduction since 130 Ma and subsequently upwelling asthenosphere since ca. 122 Ma that causes melting of heterogeneous continental crust until the final convergence, this process well recorded the changing thermal condition from hot mantle-cold crust to hot mantle and crust; The Late Cretaceous to Early Cenozoic magmatism well recorded the processes from Neo-Tethyan ocean slab flat subduction, steep subduction, to initial collision of India-Asia, it resulted in a series of continental arc magmatism with enriched mantle to crustal materials at Late Cretaceous, increasing depleted and/or juvenile materials at the beginning of early Cenozoic, and increasing evolved crustal materials in the final stage, implying a continued hot mantle and crust condition during that time. Then we can better understand the magmatic processes and variable melting from the mantle to crust during the evolution of Tethys, from Pre-, Paleo-, Meso-, to Neo-, both they show notably intensive interaction of crust-mantle and extensive melting of the heterogeneous continent during the final closure of Tethys and convergence of blocks, and thermal perturbation by a dynamic process in the depth could be the first mechanism to control the thermal condition of mantle and crust and associated composition of magmatism.     

江西雅山花岗岩岩浆演化及其Ta-Nb富集机制

雅山岩体是华南地区著名的富含钽铌矿的稀有金属花岗岩。从早阶段到晚阶段花岗岩中的云母的Li、F和Rb₂O含量逐渐升高,其类型变化为“黑鳞云母→Li-云母→锂云母”。锆石的Zr元素被Hf、U、Th、Y和P等元素的置换比例随着岩浆演化程度升高而增大。云母和锆石矿物成分变化特征与全岩体系的Zr/Hf、Nb/Ta比值不断下降而F、Li和P₂O₅含量逐渐升高的趋势一致,将可以用于指示岩浆演化程度。在岩浆演化过程中不断富集的P、F、Li元素增加了熔体中非桥氧数（NBO）,促使钽-铌元素在岩浆中的溶解度加大而逐渐富集,在最晚阶段的黄玉锂云母花岗岩具有最高的Ta、Nb元素含量。因此,雅山花岗岩具有较高的F、Li、P₂O₅含量是其岩浆演化及其Ta-Nb富集的重要机制。西华山花岗岩中的云母与雅山花岗岩中的锂云母相比,具有明显较低的F、Li、Rb₂O含量,表明西华山花岗岩的岩浆演化程度相对低于雅山花岗岩。西华山花岗岩中的钨富集与流体作用密切相关,体系氧逸度的降低促使了钨成矿。因此,岩浆演化程度的不同可能是造成华南稀有金属花岗岩发生不同成矿作用（如Ta-Nb矿和W矿）的重要原因。     

福建南平31号花岗伟晶岩脉中绿柱石的电子探针成分与成因

福建南平花岗伟晶岩是中国重要的稀有金属花岗伟晶岩之一,其中31号脉是分异程度最高、稀有金属成矿作用最重要的岩脉,绿柱石是其中最主要的铍矿物。利用电子探针和背散射电子成像技术对该岩脉中绿柱石进行了较系统分析。结果表明,南平31号花岗伟晶岩中绿柱石可分为原生绿柱石和晚期绿柱石。原生绿柱石形成于早期岩浆阶段,与造岩矿物石英、白云母、钠长石、锂辉石共生。晚期绿柱石与原生绿柱石密切伴生,是富铍流体沿原生绿柱石的边缘结晶而成,或热液沿原生绿柱石的解理、裂隙或孔隙度较大部位进行交代的产物。电子探针成分分析显示,南平31号花岗伟晶岩脉中原生绿柱石除含主要化学成分SiO2,Al2O3和BeO外还含Na,Cs,Fe,Mg等元素;FeO,MgO,Na2O和Cs2O的最高含量分别为0.28%,0.89%,1.36%和3.92%。总体上,从I带至IV带,原生绿柱石的FeO和MgO含量逐渐降低,Na2O和Cs2O的含量逐渐升高,尤其是Cs2O的含量变化较大。III—IV带中少量原生绿柱石分布于锂辉石中,具有富FeO和MgO的特征。晚期绿柱石的Cs2O含量较低,可能是由于铯沸石和南平石（铯云母）等铯矿物的结晶,导致晚期熔体-热液中相对贫铯,而热液流体与富铯绿柱石之间可能存在铯的化学再平衡。     

Ore-Forming Mechanism of the Asikaerte Granitic Pegmagtite Beryllium Deposit in Xinjiang, China

正Objective Granitic pegmatite has great significance for studying magmatic-hydrothermal evolution,which is the main formation mechanism of rare metal deposits.Conventionally,granitic pegmatite rare metal deposits are     

邹家山铀矿床含矿裂隙系统研究

邹家山铀矿床位于相山铀矿田西部,是相山铀矿田最重要的矿床,该矿床属于受断裂裂隙构造控制的脉状中-低温热液矿床,以规模大、品位高而著称。对邹家山铀矿床露天采场含矿构造的详细调查显示,含矿裂隙系统由许多弧形小断层及其所夹持的透镜状岩块构成;含矿裂隙主要为北北东向和北北西向,次为其他方向;含矿裂隙是由"X"剪节理基础上发展起来的,经过持续变形而形成;矿液充填交代时NNE向断裂具有一定的左行特点,近南北向裂隙往往是矿脉膨胀部位;成矿流体压力比较大,具有明显的水压致裂作用,形成同一露头上多个方向弧形和不规则铀矿脉,并且有从高一级裂隙向低级别裂隙、微裂隙边矿化边扩展的特点。邹家山铀矿床矿带(矿体群)呈斜列状分布,平面上呈左列式,单体北东走向,轴线北东东走向;剖面上单体中-陡倾角,轴线中-低倾角倾向西。矿带(矿体群)的空间分布与邹家山-石洞断裂带明显不一致,反映出矿带可能受一组走向北东东(50°～65°)、中-缓倾角(30°～45°)倾向北西的隐性构造控制,而不是受陡倾角的北东向邹家山-石洞断裂带控制。就邹家山铀矿区而言,进一步找矿方向在现有矿带的南西西方向深部。     

辽东半岛中生代花岗质岩浆作用的年代学格架

辽东半岛是我国东部中生代花岗质岩石较为发育的地区。传统观点认为,该区花岗岩以三叠-早中侏罗世（印支-早燕山期）为主。采用SHRIMP、TIMS和LA—ICPMS三种方法,对60余个样品中的锆石进行了U—Pb同位素年代学测定。根据这些资料,目前可以将该区中生代花岗质岩浆作用划分为3个阶段：三叠纪（233～212Ma）、侏罗纪（180～156Ma）和早白垩世（131～117Ma）。与以前认识不同的是,区内的花岗质岩石以早白垩世为主。对比华北地台其它地区中生代岩浆作用的年代学格架发现,三叠纪一侏罗纪花岗岩主要出现在华北东部,而早白垩世花岗岩在全区均较发育,这一分布特征为探讨华北中生代地质演化提供了重要信息。     

江西武功山热穹隆变质--岩浆作用及演化

处于扬子与华夏古板块加里东期拼贴带的武功山热穹隆,以造山期变形花岗岩为中心,南华一寒武系遭受绿片岩相一低角闪岩相变质并形成变质矿物分带,环绕岩体分布,与中生代以来伸展形成的脆性断裂系统构成外环,共同组合形成屹立于晚古生代地层分布区的穹隆状构造。在区域递增变质带基础上,造山早期,深部初始重熔岩浆与深埋的变质岩发生交代形成原地型英云闪长质“混染岩浆”,岩浆上浸过程中,活动组分不断作用于围岩,致使围岩的成分、结构构造产生重新调整,形成岩体边缘混合岩。同时,由于硅铝质围岩组分加入,酸度增加,而渐变过渡为花岗闪长质岩浆。造山晚期,进一步演化为岩浆型正常花岗岩,并呈套叠状侵入于早期岩体之中。从早到晚构成同源岩浆演化系列,并发育明显构造应力变形。热穹隆区加里东期形成递进渐增变质一岩体边缘混合一变形花岗岩“三位一体”分布格局。中生代伸展一岩浆作用的复合叠加,进一步强化了热穹隆的构造背景。大致在旱白垩世末基本定型。     

REE Abundance and REE Minerals in Granitic Rocks in the Nanling Range,Jiangxi Province,Southern China,and Generation of the REE‐rich Weathered Crust Deposits

Studies of Mesozoic granites associated with rare earth element (REE)‐rich weathered crust deposits in southernmost Jiangxi Province indicate that they have high‐K to shoshonite compositions and belong to ilmenite‐series I‐type granites. Of the studied rocks at 59–292 ppm of bulk REE content, the highest are seen in the biotite granites of Dingnan (358, 429 ppm) and mafic biotite granite of the Wuliting Granite (344 ppm) near the Dajishan tungsten mine, both areas where weathered‐crust REE deposits occur. REE‐bearing accessory minerals in these granites are mainly zircon, apatite and allanite, and REE‐fluorocarbonates are common. REE enrichment occurs in the rims of apatite crystals, and in fluorocarbonates that occur along grain boundaries of and cracks in major silicate minerals, and in fluorocarbonates that replaced altered biotite. It is therefore thought that a major part of the REE content of these granites was concentrated during deuteric activity, rather than during magmatic crystallization. The crack‐filling REE‐fluorocarbonates could subsequently have been easily leached out and deposited in weathered crust developed during a long period of exposure.     

Carbonate platform evolution: from a bioconstructed platform margin to a sand-shoal system (Devonian, Guilin, South China)

ABSTRACT The depositional organization and architecture of the middle–late Devonian Yangdi rimmed carbonate platform margin in the Guilin area of South China were related to oblique, extensional faulting in a strike‐slip setting. The platform margin shows two main stages of construction in the late Givetian to Frasnian, with a bioconstructed margin evolving into a sand‐shoal system. In the late Givetian, the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies (mostly Renalcis and Epiphyton). These grew upwards and produced an aggradational (locally slightly retrogradational) architecture with steep foreslope clinoforms. Three depositional sequences (S3–S5) are recognized in the upper Givetian strata, which are dominated by extensive microbialites. Metre‐scale depositional cyclicity occurs in most facies associations, except in the platform‐margin buildups and upper foreslope facies. In the latest Givetian (at the top of sequence S5), relative platform uplift (± subaerial exposure) and associated rapid basin subsidence (probably a block‐tilting effect) caused large‐scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups. Subsequently, sand shoals and banks composed of ooids and peloids and, a little later, stromatoporoid buildups on the palaeohighs, developed along the platform margin, from which abundant loose sediment was transported downslope to form gravity‐flow deposits. Another strong tectonic episode caused further platform collapse in the early Frasnian (at the top of sequence S6), leading to large‐scale breccia release and the death of the stromatoporoid buildups. Siliceous facies (banded cherts and siliceous shales) were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids. The platform‐margin sand‐shoal/bank system, possibly with gullies on the slope, persisted into the latest Frasnian until the restoration of microbial buildups. Four sequences (S6–S9), characterized by abundant sand‐shoal deposits on the margin and gravity‐flow and hemipelagic deposits on the slope, are distinguished in the Frasnian strata. Smaller‐scale depositional cyclicity is evident in all facies associations across the platform–slope–basin transect. The distinctive depositional architecture and evolution of this Yangdi Platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy, environmental factors, oceanographic setting, biotic and sedimentary fabrics.