香花岭锡多金属矿床同位素年代学及地球化学

华南地区中生代地壳拉张、岩石圈伸展减薄与大规模成岩成矿作用的时代分布格局和地球动力学背景一直是国内外关注的重要科学问题。富碱侵入岩及其有关的成矿作用是伸展构造的直接表现之一。近年来的研究发现，南岭中段花山－西山－香花岭－骑田岭NE向的花岗岩带为一中晚侏罗世形成的具有高εNd(t)和低Nd模式年龄的富碱侵入岩带，并伴有大规模的钨锡多金属成矿作用，是研究大规模地壳拉张、岩石圈伸展减薄与该区大规模的钨锡多金属成矿作用的耦合关系及其深部动力学机制的理想对象。本文以香花岭地区花岗岩及其有关的钨锡多金属矿床为研究对象，运用岩石学、元素地球化学、同位素地球化学等方法手段，对香花岭矿区花岗岩的成因和形成构造环境、成矿物质与成矿流体的来源、成矿年代学及成岩成矿的地球动力学背景等方面进行了系统研究，并初步建立了该矿的成矿模式。论文主要获得以下认识： （1）通过对香花岭矿区癞子岭和尖峰岭岩体的主、微量元素及Sr、Nd同位素研究，查明上述两个岩体均具有高硅、富碱及成矿元素，富LILE及Zr、Ga等部分高场强元素(HFSE)，贫Ca、Mg，P，Eu，较高的εNd及较低的Nd模式年龄等特征，具有相同的岩浆源区，属于后造山背景下形成的铝质A型花岗岩（A2型），具有壳、幔混合来源。 （2）利用白云母Ar-Ar同位素定年手段对香花岭锡多金属矿床、香花铺钨多金属矿床及尖峰岭云英岩型锡多金属矿床进行了成矿年代学研究，并首次对该矿的矿石矿物－锡石进行了U-Pb同位素定年。结果表明，香花岭地区钨锡多金属矿床的成矿时限为154-161 Ma，与矿区花岗岩黑云母K-Ar年龄相一致，并与区域主要钨锡矿床的形成时间一致。 （3）微量元素、稀土元素及同位素地球化学的研究表明，香花岭锡多金属矿床与矿区花岗岩具有密切的成因联系，矿区花岗岩不仅为该区成矿作用提供热动力和介质条件，而且为成矿作用提供了主要的物源。此外，矿区内的赋矿地层可能提供了部分成矿物质。早期成矿流体以岩浆水为主，晚期有大量大气降水的加入。 （4）对香花岭锡多金属矿床锡的超常富集机制作了初步探讨，认为该区锡的超常富集可能归功于如下几个因素：首先，富含F等挥发分的花岗岩浆高度分异演化出富含挥发分及成矿元素的超临界流体，成矿流体在流经赋矿地层时萃取了其中的部分成矿元素；其次，伴随着围压的不断降低，成矿流体发生了沸腾及连续的去气作用，导致了成矿流体的高度浓缩，当流体达到饱和或过饱和时，成矿元素沉淀成矿；最后，早期形成的矿床进一步受到后期的叠加改造，从而形成该区高品位的锡矿石。 （5）根据上述研究，香花岭锡多金属矿床与矿区花岗岩具有密切的时间、空间及成因联系，二者应为同一构造背景下形成的产物。结合区域上的研究，香花岭矿区的成岩成矿作用与南岭中段同时代的其他钨锡多金属矿床具有相同的地球动力学背景。这种构造背景可能与该区中－晚侏罗世构造体制转换，地壳拉张－岩石圈伸展减薄背景下，软流圈地幔上涌及发生壳、幔相互作用密切相关。 （6）初步建立了香花岭锡多金属矿床的成因模式：在该区中晚侏罗世大规模地壳拉张、岩石圈伸展减薄背景下，地幔物质上涌到中下地壳，发生壳幔相互作用，形成壳幔混合的花岗岩浆。花岗岩浆经充分分异演化，形成富含挥发分和成矿元素的一种具超临界性质的成矿流体。成矿流体在上升过程中随着围压的不断降低，发生了连续的去气作用，使得成矿流体高度浓缩并沿有利的构造部位成矿，并在后期受到改造、叠加等成矿作用，以至于形成了该区最富的锡矿床。

滇东南南温河花岗岩年代学和地球化学初步研究

南温河花岗岩分布于中-越接壤处的云南省马关县、麻栗坡县老君山地区穹隆状变形-变质岩系的内核部位，大地构造位置位于华南褶皱带西端与上扬子地块、哀牢山褶皱带和印支地块的接合部位，是研究华南大陆形成与演化及特提斯构造域时空发展的关键地区。前人对本地区花岗岩的研究，更多的集中在燕山期老君山花岗岩的研究上，而对南温河花岗岩的研究还没有引起足够的重视，还停留在初步的岩石学和地球化学阶段。 本文对滇东南老君山地区南温河花岗岩进行了U-Pb、Rb-Sr同位素年代学和主微量、Sr-Nd同位素地球化学的系统研究，取得了以下主要认识： 1、通过对南温河花岗岩 SHRIMP锆石U-Pb、TIMS锆石U-Pb年代学和全岩Rb-Sr年代学的研究，确定了南温河花岗岩主体的结晶年龄为440Ma左右，少量结晶为420Ma左右， 属于广西运动产物。在同位素年代学和岩石学研究基础上，重新对南温河花岗岩进行了岩石单元划分，归并为猛洞河单元和南加河单元两个单元。南捞片麻岩并非如1：50000麻栗坡县幅、都龙幅所认为的新元古代岩浆岩，也同属加里东期岩浆岩，应归属于猛洞河单元。 2、南温河序列花岗岩成分来源主要为上部地壳的泥质岩、粉砂岩、杂砂岩的部分熔融，可能还有少量地幔物质和下部地壳物质的加入。成因上主要属于S型花岗岩，少量属于 I型花岗岩。 形成的构造环境上，主要为同碰撞环境，少部分为造山后环境。 继承锆石年龄和Sr-Nd同位素地球化学特征显示，源区存在古元古代的古老地壳。 3、结合SHRIMP和TIMS锆石U-Pb年代学两种测年方法的差异，确定了后期的主变质年龄为 230Ma左右，提出了确定后期中-浅变质作用年龄的一种新思路。 4、通过南温河花岗岩热年代史反演，恢复了本区加里东期以来的构造历史，并初步探讨了本区加里东期的大地构造环境。

滇东南都龙锡锌多金属矿床矿物学及年代学

都龙锡锌多金属矿床位于云南省马关县都龙镇，是我国最大的锡石硫化物矿床之一。在野外地质和岩矿鉴定基础上,本文对该矿床的绿泥石、鲕状黄铁矿、磁黄铁矿异构交生体、铁闪锌矿的“黄铜矿疾病”等矿物和特殊组构，开展了比较系统的电子探针、拉曼光谱和X光衍射等微束分析，结合对该矿床的黑云母Ar-Ar年代学、锡石TIMS法U-Pb年代学研究，基本明确了矿床各个成矿期次的具体成矿特点，丰富和完善了该矿床的成矿模式。论文取得的主要认识有： （1）绿泥石与锡矿化同为燕山期岩浆热液作用的产物。绿泥石的形成温度（231~304oC）和环境（还原环境）表明，岩浆热液叠加成矿环境为中、低温的还原环境。 （2）鲕状黄铁矿为热水沉积期的特征矿物，是Fe、Zn、S等成矿元素的主要物源。 （3）磁黄铁矿主要在区域变质晚期，由黄铁矿（主要为鲕状黄铁矿）变质脱硫而成。辣子寨矿段单斜磁黄铁矿的出现，说明燕山期岩浆热液叠加成矿温度在该矿段以中低温为主。 （4）铁闪锌矿的“黄铜矿疾病”主要是黄铜矿交代铁闪锌矿的结果，其中的Fe主要来自变质期形成的铁闪锌矿和磁黄铁矿，而Cu主要来自岩浆热液。 （5）自然铋、自然银、锡石、黄铜矿、方铅矿等矿物广泛共生，且交代、穿切其他矿物，表明岩浆热液期的成矿元素组合主要为Bi+Ag+Sn+Cu+Pb。在该矿床首次发现的自然铋和自然银，指示岩浆热液期为低硫、中低温的还原环境。 （6）利用远红外（IR-Laser）阶段加热技术，获得黑云母的低温段和高温段坪年龄，分别为179.0± 5 .0Ma和195.7± 6.6Ma。在黑云母组构显微分析的基础上，认为179.0± 5 .0Ma大致代表了变质成矿作用的年龄。 （7）都龙锡锌多金属矿床属于多成因复成矿床，其形成主要经历了热水沉积、变质改造和岩浆热液叠加三个过程。热水沉积成矿作用形成的鲕状黄铁矿，可能是后期成矿作用中Fe、Zn、S等成矿元素的主要物源。变质改造成矿作用与印支期晚期区域变形-变质作用有关，形成了大量铁闪锌矿、磁黄铁矿，可能对应于造山后伸展事件。岩浆热液叠加成矿作用与燕山晚期老君山花岗岩有关，发生大规模的锡、铜、银、铅、铋矿化。 另外，本次研究在该矿床中首次发现了自然铋、自然银等自然元素，其矿床成因意义有待进一步研究。

云南澜沧老厂大型银多金属矿床成矿年代及地球化学

老厂矿床位于三江成矿带南段昌宁—孟连裂谷的次级澜沧断陷盆地中，昌宁—孟连裂谷是三江成矿带南段的重要成矿段之一，其大地构造位于保山―掸邦微陆块东缘，兰坪—思茅盆地、临沧地块西缘，属东特提斯构造域，是冈瓦纳古陆与欧亚大陆巨型缝合带的组成部分。 矿床开采历史悠久，始于明朝永乐二年（1404年），至今已605年。古时炼银弃铅，最高年产白银30万两。解放后主要开采古人废弃的高铅炉渣，并对深部原生矿体进行了初步勘探，90年代至今对深部银铅矿体进行了详细勘查，深部矿体是矿床主要的开采对象。它以独特的成矿地质特征、富银（铅锌矿石中平均含量为629×10-6，方铅矿中平均2069×10-6）、伴生元素多（In、Se、Te、Ga、Cd、Bi）、规模大（In、Se、Te、Cd已达大型规模）、含Sn等特征而受到广大地质工作者的关注。它是“三江”成矿带南段最具代表性的铅银矿床类型之一，也是昌宁—孟连裂谷内目前探明的唯一大型银铅锌多金属矿床，具有十分重要的研究价值。 虽然前人已从矿床地质、成矿条件、控矿因素等方面对老厂大型银铅锌多金属矿床进行过研究，但研究工作较为零散，在成矿物质与成矿流体来源、矿床成因、花岗斑岩与成矿等方面还存在较大争议。本文在深入细致的野外地质工作基础上，利用多种现代分析测试技术，对矿床进行了较为系统的矿物学、岩石学、年代学和矿床地球化学研究，进而查明了矿床成矿物质与成矿流体来源、揭示了成矿地球动力学背景、探讨了矿床的成因、初步建立了矿床成因模式。论文取得的主要成果如下： 1．查明了硫化物的物质组分及形成阶段。闪锌矿以高铁（早期平均11.51%，晚期平均8.41%）为基本特征，并伴生多种特征的微量元素，早期闪锌矿是Fe、In的主要载体，Cu、Cd、Mn则主要富集在晚阶段闪锌矿中；方铅矿是Ag的主要载体，其早期富Ag、Te、Bi，含Se、Cu，中期主要富Ag，晚期以含As、Bi为特征；其它硫化物成分较单一。 2．揭示了伴生元素赋存状态和富集规律。矿床伴生元素含量高，闪锌矿中Cd平均4293.19×10-6，In平均555.37×10-6，方铅矿中Te平均143.81×10-6，As、Se、Bi、Ga含量也很高。初步估算金属储（万吨）量分别为： Ga 0.097，Cd1.84， In0.13，Se0.06 ，Te0.15，Bi1.28，As4.31。闪锌矿是Ga、In、Cd主要载体，Te、Bi主要赋存在方铅矿中，Se则主要在铅锌、黄铁矿石中富集。 3．首次获得了精确的成矿年代学数据。单颗粒闪锌矿—黄铁矿Rb-Sr法获得矿床成矿年龄t=（45±3.6）Ma，（87Sr/86Sr）i=0.70977±0.00034。证实了矿床成矿与隐伏花岗斑岩关系密切，矿床形成是三江成矿带南段对喜马拉雅碰撞造山成矿运动的响应。 4．首次系统研究了花岗斑岩的地质、地球化学特征。矿床花岗斑岩具高硅、超钾、富碱、贫钠和低镁、钙、铁、磷的特征，岩石为过铝质岩石；其轻稀土富集、Eu为弱负异常、基本无Ce异常、岩石富集大离子亲石元素，具有较高的锶初始值和较低εNd及高的Nd模式年龄，与三江地区富碱斑岩具有相似的地球化学特征。岩浆主要来源于加厚下地壳重熔，成岩过程中有地幔物质的加入，为同碰撞构造环境下形成。 5．首次探讨了容矿沉积建造及矿石、矿物的地球化学特征。矿床碳酸盐岩轻稀土富集、Eu正异常及Ce异常和其微量元素特征与热水沉积岩石相似，表明了碳酸盐岩的热水沉积成因，同时矿石及其硫化物微量和稀土元素特征也指示了在早石炭世矿区发生了广泛的火山喷流热水沉积成矿作用。 6．查明了成矿物质及成矿流体来源。成矿元素研究显示矿床成矿物质具有多来源的特征，Pb同位素研究表明矿床Pb可能来自不同地层岩石的淋滤， S同位素组成显示其可能主要来源于海水对下伏火山岩地层的淋滤与海水硫酸盐的还原，花岗斑岩岩浆也可能提供了部分S。C-O、H-O同位素和矿石及其硫化物稀土元素反映矿床早期成矿流体主要源于深部岩浆，晚期主要来源于喜山期花岗斑岩岩浆热液。 7．初步建立了成矿模式。老厂矿床是长期以来多种地质作用下的综合产物，具有成矿物质多来源，成矿阶段多期次特点。经历了早石炭世火山喷流热水沉积成矿和喜山期花岗斑岩岩浆热液叠加改造成矿作用，热水成矿作用下形成了以黄铁矿为主的块状硫化物矿床，喜山期富碱花岗斑岩岩浆热液流体携带了大量的Cu、Mo、Ag、Pb等成矿元素，在它的叠加改造作用下形成了老厂银多金属矿床。总之矿床为火山喷流沉积—岩浆热液叠加改造成因。

Radiocarbon wiggle-match dating of proglacial lake sediments - Implications for the 8.2 ka event

The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the C-14 calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event. (C) 2009 Elsevier Ltd. All rights reserved.

High precision C-14 dating of Holocene peat deposits: A comparison of Bayesian calibration and wiggle-matching approaches

The chronologies of five northern European ombrotrophic peat bogs subjected to a large ANIS C-14 dating effort (32-44 dates/site) are presented here. The results of Bayesian calibration (BCal) of dates with a prior assumption of chronological ordering were compared with a Bayesian wiggle-match approach (Bpeat) which assumes constant linear accumulation over sections of the peat profile. Interpolation of BCal age estimates of dense sequences of C-14 dates showed variable patterns of peat accumulation with time, with changes in accumulation occurring at intervals ranging from 20 to 50 cm. Within these intervals, peat accumulation appeared to be relatively linear. Close analysis suggests that some of the inferred variations in accumulation rate were related to the plant macrofossil composition of the peat. The wiggle-matched age-depth models had relatively high chronological uncertainty within intervals of closely spaced 14 C dates, suggesting that the premise of constant linear accumulation over large sections of the peat profile is unrealistic. Age models based on the assumption of linear accumulation over large parts of a peat core (and therefore only effective over millennial timescales), are not compatible with studies examining environmental change during the Holocene, where variability often occurs at decadal to centennial time-scales. Ideally, future wiggle-match age models should be constrained, with boundaries between sections based on the plant macrofossil composition of the peat and physical-chemical parameters such as the degree of decomposition. Strategies for the selection of material for dating should be designed so that there should be enough C-14 dates to accurately reconstruct the peat accumulation rate of each homogeneous stratigraphic unit. (c) 2006 Elsevier Ltd. All rights reserved.

Methods and code for 'classical' age-modelling of radiocarbon sequences.

Age–depth models form the backbone of most palaeoenvironmental studies. However, procedures for constructing chronologies vary between studies, they are usually not explained sufficiently, and some are inadequate for handling calibrated radiocarbon dates. An alternative method based on importance sampling through calibrated dates is proposed. Dedicated R code is presented which works with calibrated radiocarbon as well as other dates, and provides a simple, systematic, transparent, documented and customizable alternative. The code automatically produces age–depth models, enabling exploration of the impacts of different assumptions (e.g., model type, hiatuses, age offsets, outliers, and extrapolation).

High-precision ultra-distal Holocene tephrochronology in North America

Far-travelled volcanic ashes (tephras) from Holocene eruptions in Alaska and the Pacific northwest have been traced to the easternmost extent of North America, providing the basis for a new high-precision geochronological framework throughout the continent through tephrochronology (the dating and correlation of tephra isochrons in sedimentary records). The reported isochrons are geochemically distinct, with seven correlated to documented sources in Alaska and the Cascades, including the Mazama ash from Oregon (w7600 years old) and the eastern lobe of the White River Ash from Alaska (~1150 years old). These findings mark the beginning of a tephrochronological framework of enhanced precision across North America, with applications in palaeoclimate, surface process and archaeological studies. The particle travel distances involved (up tow7000 km) also demonstrate the potential for continent-wide or trans-Atlantic socio-economic disruption from similar future eruptions.

Sediment accumulation rates in subarctic lakes: insights into age-depth modeling from 22 dated lake records from the Northwest Territories, Canada

Age-depth modeling using Bayesian statistics requires well-informed prior information about the behavior of sediment accumulation. Here we present average sediment accumulation rates (represented as deposition times, DT, in yr/cm) for lakes in an Arctic setting, and we examine the variability across space (intra- and inter-lake) and time (late Holocene). The dataset includes over 100 radiocarbon dates, primarily on bulk sediment, from 22 sediment cores obtained from 18 lakes spanning the boreal to tundra ecotone gradients in subarctic Canada. There are four to twenty-five radiocarbon dates per core, depending on the length and character of the sediment records. Deposition times were calculated at 100-year intervals from age-depth models constructed using the ‘classical’ age-depth modeling software Clam. Lakes in boreal settings have the most rapid accumulation (mean DT 20 ± 10 years), whereas lakes in tundra settings accumulate at moderate (mean DT 70 ± 10 years) to very slow rates, (>100 yr/cm). Many of the age-depth models demonstrate fluctuations in accumulation that coincide with lake evolution and post-glacial climate change. Ten of our sediment cores yielded sediments as old as c. 9,000 cal BP (BP = years before AD 1950). From between c. 9,000 cal BP and c. 6,000 cal BP, sediment accumulation was relatively rapid (DT of 20 to 60 yr/cm). Accumulation slowed between c. 5,500 and c. 4,000 cal BP as vegetation expanded northward in response to warming. A short period of rapid accumulation occurred near 1,200 cal BP at three lakes. Our research will help inform priors in Bayesian age modeling.

Abrupt Climate Changes: Climate and Biogeochemical consequences

Hosted in a wide depression within the Berici Hills (Venetian Plain), outside the maximum extent reached by LGM glaciers, Lake Fimon preserves an almost continuous archive of landscape and climate changes from the penultimate glacial maximum onwards. The stratigraphic succession deposited at the lake bottom has been investigated in three deep cores by means of pollen analysis, petrographic composition, magnetic susceptibility, LOI, and geochronology. Tephra layers have been identified and are currently under study.
Pollen data provide the first continuous vegetation record in northern Italy for the last 150 ky. Terrestrial vegetation varied from interglacial warm-temperate broad leaved to oceanic mixed forests, from boreal conifer forests to open forest-steppes of colder climate. Phases of major forest expansion and reduction have been correlated to isotopic events described in ice (NGRIP), stalagmite (Antro del Corchia) and marine records. Persistent afforestation recorded in northern Italy even during cold phases of the full pleniglacial is consistent with mesoscale paleoclimate simulations suggesting that a sharp rainfall gradient across the Alps enabled the survival of woody species in the southern alpine foreland.
Integrating litho- and biostratigraphical data, we identified sedimentation regìmes, accumulation rates, sediment sources and supply both for the Lake Fimon cores and the adjacent Venetian Plain, allowing a direct comparison with major glacial advances in the Alpine area, deglaciation pulses, and glacio-eustatic displacements of the northern Adriatic shoreline.

Diatom-based characterization of iberian coastal environments at different time scales

Tese de doutoramento, Geologia (Geologia Económica e do Ambiente), Universidade de Lisboa, Faculdade de Ciências, 2014

Contact metamorphism and emplacement of the western Adamello tonalite

The geodynamic forces acting in the Earth's interior manifest themselves in a variety of ways. Volcanoes are amongst the most impressive examples in this respect, but like with an iceberg, they only represent the tip of a more extensive system hidden underground. This system consists of a source region where melt forms and accumulates, feeder connections in which magma is transported towards the surface, and different reservoirs where it is stored before it eventually erupts to form a volcano. A magma represents a mixture of melt and crystals. The latter can be extracted from the source region, or form anywhere along the path towards their final crystallization place. They will retain information of the overall plumbing system. The host rocks of an intrusion, in contrast, provide information at the emplacement level. They record the effects of thermal and mechanical forces imposed by the magma. For a better understanding of the system, both parts - magmatic and metamorphic petrology - have to be integrated. I will demonstrate in my thesis that information from both is complementary. It is an iterative process, using constraints from one field to better constrain the other. Reading the history of the host rocks is not always straightforward. This is shown in chapter two, where a model for the formation of clustered garnets observed in the contact aureole is proposed. Fragments of garnets, older than the intrusive rocks are overgrown by garnet crystallizing due to the reheating during emplacement of the adjacent pluton. The formation of the clusters is therefore not a single event as generally assumed but the result of a two-stage process, namely the alteration of the old grains and the overgrowth and amalgamation of new garnet rims. This makes an important difference when applying petrological methods such as thermobarometry, geochronology or grain size distributions. The thermal conditions in the aureole are a strong function of the emplacement style of the pluton. therefore it is necessary to understand the pluton before drawing conclusions about its aureole. A study investigating the intrusive rocks by means of field, geochemical, geochronologi- cal and structural methods is presented in chapter three. This provided important information about the assembly of the intrusion, but also new insights on the nature of large, homogeneous plutons and the structure of the plumbing system in general. The incremental nature of the emplacement of the Western Adamello tonalité is documented, and the existence of an intermediate reservoir beneath homogeneous plutons is proposed. In chapter four it is demonstrated that information extracted from the host rock provides further constraints on the emplacement process of the intrusion. The temperatures obtain by combining field observations with phase petrology modeling are used together with thermal models to constrain the magmatic activity in the immediate intrusion. Instead of using the thermal models to control the petrology result, the inverse is done. The model parameters were changed until a match with the aureole temperatures was obtained. It is shown, that only a few combinations give a positive match and that temperature estimates from the aureole can constrain the frequency of ancient magmatic systems. In the fifth chapter, the Anisotropy of Magnetic Susceptibility of intrusive rocks is compared to 3D tomography. The obtained signal is a function of the shape and distribution of ferromagnetic grains, and is often used to infer flow directions of magma. It turns out that the signal is dominated by the shape of the magnetic crystals, and where they form tight clusters, also by their distribution. This is in good agreement with the predictions made in the theoretical and experimental literature. In the sixth chapter arguments for partial melting of host rock carbonates are presented. While at first very surprising, this is to be expected when considering the prior results from the intrusive study and experiments from the literature. Partial melting is documented by compelling microstructures, geochemical and structural data. The necessary conditions are far from extreme and this process might be more frequent than previously thought. The carbonate melt is highly mobile and can move along grain boundaries, infiltrating other rocks and ultimately alter the existing mineral assemblage. Finally, a mineralogical curiosity is presented in chapter seven. The mineral assemblage magne§site and calcite is in apparent equilibrium. It is well known that these two carbonates are not stable together in the system Ca0-Mg0-Fe0-C02. Indeed, magnesite and calcite should react to dolomite during metamorphism. The presented explanation for this '"forbidden" assemblage is, that a calcite melt infiltrated the magnesite bearing rock along grain boundaries and caused the peculiar microstructure. This is supported by isotopie disequilibrium between calcite and magnesite. A further implication of partially molten carbonates is, that the host rock drastically looses its strength so that its physical properties may be comparable to the ones of the intrusive rocks. This contrasting behavior of the host rock may ease the emplacement of the intrusion. We see that the circle closes and the iterative process of better constraining the emplacement could start again. - La Terre est en perpétuel mouvement et les forces tectoniques associées à ces mouvements se manifestent sous différentes formes. Les volcans en sont l'un des exemples les plus impressionnants, mais comme les icebergs, les laves émises en surfaces ne représentent que la pointe d'un vaste système caché dans les profondeurs. Ce système est constitué d'une région source, région où la roche source fond et produit le magma ; ce magma peut s'accumuler dans cette région source ou être transporté à travers différents conduits dans des réservoirs où le magma est stocké. Ce magma peut cristalliser in situ et produire des roches plutoniques ou alors être émis en surface. Un magma représente un mélange entre un liquide et des cristaux. Ces cristaux peuvent être extraits de la source ou se former tout au long du chemin jusqu'à l'endroit final de cristallisation. L'étude de ces cristaux peut ainsi donner des informations sur l'ensemble du système magmatique. Au contraire, les roches encaissantes fournissent des informations sur le niveau d'emplacement de l'intrusion. En effet ces roches enregistrent les effets thermiques et mécaniques imposés par le magma. Pour une meilleure compréhension du système, les deux parties, magmatique et métamorphique, doivent être intégrées. Cette thèse a pour but de montrer que les informations issues de l'étude des roches magmatiques et des roches encaissantes sont complémentaires. C'est un processus itératif qui utilise les contraintes d'un domaine pour améliorer la compréhension de l'autre. Comprendre l'histoire des roches encaissantes n'est pas toujours aisé. Ceci est démontré dans le chapitre deux, où un modèle de formation des grenats observés sous forme d'agrégats dans l'auréole de contact est proposé. Des fragments de grenats plus vieux que les roches intru- sives montrent une zone de surcroissance générée par l'apport thermique produit par la mise en place du pluton adjacent. La formation des agrégats de grenats n'est donc pas le résultat d'un seul événement, comme on le décrit habituellement, mais d'un processus en deux phases, soit l'altération de vieux grains engendrant une fracturation de ces grenats, puis la formation de zone de surcroissance autour de ces différents fragments expliquant la texture en agrégats observée. Cette interprétation en deux phases est importante, car elle engendre des différences notables lorsque l'on applique des méthodes pétrologiques comme la thermobarométrie, la géochronologie ou encore lorsque l'on étudie la distribution relative de la taille des grains. Les conditions thermales dans l'auréole de contact dépendent fortement du mode d'emplacement de l'intrusion et c'est pourquoi il est nécessaire de d'abord comprendre le pluton avant de faire des conclusions sur son auréole de contact. Une étude de terrain des roches intrusives ainsi qu'une étude géochimique, géochronologique et structurale est présente dans le troisième chapitre. Cette étude apporte des informations importantes sur la formation de l'intrusion mais également de nouvelles connaissances sur la nature de grands plutons homogènes et la structure de système magmatique en général. L'emplacement incrémental est mis en évidence et l'existence d'un réservoir intermédiaire en-dessous des plutons homogènes est proposé. Le quatrième chapitre de cette thèse illustre comment utiliser l'information extraite des roches encaissantes pour expliquer la mise en place de l'intrusion. Les températures obtenues par la combinaison des observations de terrain et l'assemblage métamorphique sont utilisées avec des modèles thermiques pour contraindre l'activité magmatique au contact directe de cette auréole. Au lieu d'utiliser le modèle thermique pour vérifier le résultat pétrologique, une approche inverse a été choisie. Les paramètres du modèle ont été changés jusqu'à ce qu'on obtienne une correspondance avec les températures observées dans l'auréole de contact. Ceci montre qu'il y a peu de combinaison qui peuvent expliquer les températures et qu'on peut contraindre la fréquence de l'activité magmatique d'un ancien système magmatique de cette manière. Dans le cinquième chapitre, les processus contrôlant l'anisotropie de la susceptibilité magnétique des roches intrusives sont expliqués à l'aide d'images de la distribution des minéraux dans les roches obtenues par tomographie 3D. Le signal associé à l'anisotropie de la susceptibilité magnétique est une fonction de la forme et de la distribution des grains ferromagnétiques. Ce signal est fréquemment utilisé pour déterminer la direction de mouvement d'un magma. En accord avec d'autres études de la littérature, les résultats montrent que le signal est dominé par la forme des cristaux magnétiques, ainsi que par la distribution des agglomérats de ces minéraux dans la roche. Dans le sixième chapitre, une étude associée à la fusion partielle de carbonates dans les roches encaissantes est présentée. Si la présence de liquides carbonatés dans les auréoles de contact a été proposée sur la base d'expériences de laboratoire, notre étude démontre clairement leur existence dans la nature. La fusion partielle est documentée par des microstructures caractéristiques pour la présence de liquides ainsi que par des données géochimiques et structurales. Les conditions nécessaires sont loin d'être extrêmes et ce processus pourrait être plus fréquent qu'attendu. Les liquides carbonatés sont très mobiles et peuvent circuler le long des limites de grain avant d'infiltrer d'autres roches en produisant une modification de leurs assemblages minéralogiques. Finalement, une curiosité minéralogique est présentée dans le chapitre sept. L'assemblage de minéraux de magnésite et de calcite en équilibre apparent est observé. Il est bien connu que ces deux carbonates ne sont pas stables ensemble dans le système CaO-MgO-FeO-CO.,. En effet, la magnésite et la calcite devraient réagir et produire de la dolomite pendant le métamorphisme. L'explication présentée pour cet assemblage à priori « interdit » est que un liquide carbonaté provenant des roches adjacentes infiltre cette roche et est responsable pour cette microstructure. Une autre implication associée à la présence de carbonates fondus est que la roche encaissante montre une diminution drastique de sa résistance et que les propriétés physiques de cette roche deviennent comparables à celles de la roche intrusive. Cette modification des propriétés rhéologiques des roches encaissantes peut faciliter la mise en place des roches intrusives. Ces différentes études démontrent bien le processus itératif utilisé et l'intérêt d'étudier aussi bien les roches intrusives que les roches encaissantes pour la compréhension des mécanismes de mise en place des magmas au sein de la croûte terrestre.

Metamorphism and Fluid Inclusion Studies of Charnockites and Associated Rocks from the Madurai Granulite Block , Southern India with Special Reference to Graphite Mineralization

The present research is aimed at studying the charnockites and associated rocks of the Madurai Granulite Block (MGB), especially in terms of their field settings, texture, mineralogy, and mineral chemistry analyzing their petrogenesis with the help of thermobarometrical studies and geochronological constraints. The mechanism of charnockitization by the influx of CO2 rich fluids and its relation to the graphite mineralization is actually a matter of discussion and study. The objectives of the present study are, to delineate petrological and structural relationship of charnockites and associated gneissic rocks, to study the field and petrogenetic aspects of graphite mineralization in the MGB, to establish and re-evaluate the P-T conditions of formation of the rocks with the aid of thermbarometric computations and to compare with the earlier studies, characterization of graphite with XRD, Raman spectroscopy and isotope studies together with a search in to its genesis and its relation to the high-grade metamorphism of the terrain, to evaluate the role of CO2 bearing fluids in the processes of charnockitization as well as in the genesis of graphite within the high-grade terrain and to delineate the metamorphic geochronology of selected rocks using ‘monazite dating’ technique with EPMA.