Surface displacements resulting from upper-crustal intrusion of melt are a paramount concern for communities and facilities located in or near active volcanic areas (e.g. Campi Flegrei, Yucca Mtn.). Study of active intrusions such as Campi Flegrei, Italy west of Mt. Vesuvius, is limited to remote observations through geophysical/geodetic procedures. While the surface displacement due to melt emplacement at depth can easily be determined, the geometries and depth of intrusions are often based on simplified assumptions (e.g. spheres and prolate or oblate ellipsoids). These models benefit from data constraining both the geometries of the individual intrusions, and the kinematics and mechanics of deformation within the superstructure overlying the intrusions. Mount Ellsworth, a partially exposed sub-volcanic system, is an ideal natural laboratory for the study of near surface intrusions. The intrusions of the Henry Mountains are ideal because they were emplaced into relatively flat-lying stratigraphy of the Colorado Plateau, at a time when the stress field was largely isotropic. Previous geologic work done in the Henry Mountains, conducted by C.B. Hunt (1953) and Marie Jackson and Dave Pollard (1988), presents competing emplacement models (i.e. large batch intrusion or incremental sill growth), as well as, differing geologic map data and interpretations. Through a combination of 1:5000 scale field mapping and profile-oriented gravity study, we have produced detailed geologic maps and cross sections of Mt. Ellsworth assess the previous work done on Mt. Ellsworth with new datasets, as well as, evaluate criteria refining various emplacement models. Mapping results demonstrate that several of the assumptions made in models theorized by Hunt (1953) and Jackson and Pollard (1988), were inappropriately applied on Mt. Ellsworth. These assumptions include the thickness and separation of stratigraphic units, the size and distribution of sills and smaller intrusions, structural attitudes of beds and sills, and the presence of exposure of the main body of the intrusion. Gravity data collected on similar intrusions presented in Corry (1988) demonstrates the difficulty of obtaining a gravity anomaly on the wavelength of the assumed size of the intrusion. Forward gravity modeling of various potential geometries beneath Mount Ellsworth suggests that the anomalies are similar in shape with a magnitude between 16 and 20 mGal. Results from the gravity profiles collected for this study fail to predict an anomaly on the wavelength of the Mount Ellsworth intrusion and record a much more complicated anomaly than is presented by the forward models. By combining the stratigraphic data, structural data, and cross sections, it can be determined that the Mount Ellsworth intrusion is a laccolith with a floor 1.5 kilometers beneath the topographic surface, is 1 kilometer thick at its maximum, and has dimensions of 4 kilometers wide by 6 kilometers long.

Geothermal energy consumption is projected to increase along with other renewable energy in the future. Therefore, it is important to have a better understanding on the evolution of geothermal systems to optimize the exploitation of such resources. Generally, numerical models are used as a fundamental tool to study a potential geothermal field. However, current modeling practices tend to focus on the shallow area around the heat source, while ignoring the deeper part below the heat source. The purpose of this project is to observe the influence of lower boundary at the bottom of intrusion towards the evolution of geothermal system, while changing the permeability and topography of host rock systematically, using a software from USGS called HYDROTHERM. Simulations differed in three main aspects: 1) having a layer below, or having the bottom boundary directly below intrusion, 2) different topographies with volcanic significance, and 3) varying permeabilities of the host rock. The study is based on a fossil geothermal system, the Cerro Bayo laccolith in Chachahuén volcanic complex (Neuquén Basin), Argentina. The input parameters were obtained in several ways. ILMAT Geothermometry analysis provide the temperature value related to the intrusion. The whole rock data is used to determined density of the intrusion by calculating partial molar volume of the oxides. The other parameters, e.g. densities of the host rock and the impermeable layer, permeability, porosity, and thermal conductivity were obtained from literature. The result from numerical modeling shows that the bottom boundary below intrusion strongly affect the entire system evolution. The added layer (with constant permeability) has strong influence on the life-span of the system. Additionally, while taking into account on the variation of topography and permeabilities, the models show two temperature anomalies: 1) A caldera volcano’s geometry “traps” heat below the caldera, whereas shield and strato-volcano geometries “push” heat away from below the volcanic edifice, and 2) a low temperature anomaly develops beneath the intrusion in all high permeability models with an added layer. Finally, this assessment could prove to be useful as prior knowledge for optimizing the extraction of heat from a given geothermal field, as well as future investigations towards geological applicability of numerical models of geothermal systems, hydrothermal alteration, and ore formation processes.

L'étude de ce complexe plutonique alcalin fut multidisciplinaire mêlant, observations et mesures sur le terrain, ASM, pétrographie, minéralogie, géochimie (majeur et traces), géochimie isotopique Rb/Sr et Sm/Nd et géochronologie U-Pb sur zircons. La synthèse des résultats démontre la participation minoritaire de plusieurs sources (manteau métasomatisé et fragments continental) en plus de la source prédominante qui est le manteau typique du panache de Kerguelen. Dans un environnement magmatique ouvert, les magmas contaminés ont formé la plus ancienne série magmatique calcique du complexe. La seconde et plus jeune série magmatique a produit (dans un environnement clos) des roches de plus en plus agpaïtique au cours de la différenciation. La mise en place progressive mais diachrone des intrusions s'est faite entre 11,7 (au sud) et 7,9 Ma (au nord). Enfin, le mode de construction de ce pluton en fait un exemple unique connu de laccolithe en contexte intraplaque océanique. / The study of this alkaline plutonic complex was multidisciplinary, mixing field observations and measurements, AMS, petrography, mineralogy, geochemistry (major and traces), isotopic geochemistry Rb / Sr and Sm / Nd and geochronology U-Pb on zircons. The synthesis of the results shows the minor participation of several sources (metasomatised mantle and continental fragments) in addition to the predominant source which is the typical asthenospheric mantle of the Kerguelen plume. In a dynamic and open magmatic system, the contaminated magmas formed the oldest magmatic and calcic series of the complex. The second and younger magmatic series produced (in a closed environment) more and more agpaitic rocks during differentiation. The incremental emplacement of the intrusions occurred between 11.7 (in the south) and 7.9 Ma (in the north). Finally, the type of construction of this pluton makes it a unique known example of laccolith in oceanic within-plate settings.

The Round Top laccolith is considered to be one of the youngest laccoliths in a series of five known as the Sierra Blanca peaks, located in Hudspeth county, Texas. The laccolith is anomalous within the region in that it is peraluminous and enriched in HREEs, F, and U, and is comprised of intermingled discrete packages of various rhyolite types. The laccolith rhyolite varies in color from gray, purple, red, and tan, which combine locally to form distinct geometric mottled textures. The general composition of the rhyolite is 48-52% potassium feldspar, 28-30% quartz, 8-14% plagioclase feldspar, 4-5% annite biotite, 2-3% magnetite-hematite, 1% zircon, and 1% trace phases. The morphology of the trace phases suggests quenching of a late-stage volatile-rich vapor phase at the time of the laccolith formation. The rhyolite displays a wide array of unique mineralogical characteristics indicative to rapid emplacement and metastable crystallization conditions, including three-part quartz phenocrysts, hourglass sector-zoned potassium feldspars, and late-stage anhedral zircons. Unique accessory and trace phases include cassiterite, cerianite-(Ce), changbaiite, columbite, cryolite, tantalite, thorite, yttrofluorite, yttrocerite, and two unidentified minerals named (W) and (X). Initial alteration of the laccolith by high temperature volatile-rich vapor during the late stages of crystallization caused the partial dissolution of the feldspars and quartz. Subsequent quenching of this high temperature vapor phase produced the abundant interstitial, and pore filling REE-fluorides common to the laccolith. The variation in rhyolite color and the presence of the mottled textures are a direct result of partial oxidation of the laccolith by secondary fluids. The oxidizing fluids migrated within the laccolith along an extensive fracture network, altering the adjacent wallrock by oxidizing magnetite phenocrysts to hematite. The gray, purple, and red rhyolite types reflect an increase in turbidity caused by hematitic inclusions primarily within the pore spaces of the potassium feldspar portions of the groundmass. The tan rhyolite is locally restricted to the base of the laccolith and has been subjected to an intense degree of alteration independent of the other rhyolite types, primarily indicated by the conversion of feldspars to clay. Petrographic, microbeam, and geochemical studies have determined little variation in REE concentration between the three rhyolites of similar alteration intensity, but have indicated a depletion in LREEs within the more altered tan rhyolite. The average REE+Y content for the rhyolites sampled (n=11) ranges between 249 ppm and 518 ppm. The REE+Y concentrations between rhyolite samples of the same type show some variation, possibly indicating a correlation between alteration and REE+Y abundance and/or innate heterogeneity in the vapor phase during the initial laccolith formation. The magma emplaced at Round Top underwent a prolonged evolutionary process of fractionation/differentiation as evident by the unusual mineral assemblage and geochemical enrichment associated with the laccolith (e.g. extremely negative europium anomaly, and the positive La/Yb correlation). Future exploration for Round Top style REE-deposits should center within long-lived, tectonically active and complex regions where laccoliths are likely to exist. Specifically, exploration should focus on identifying the youngest laccolith in a felsic series, as this is the most likely to contain the greatest abundance of incompatible elements within the laccolithic group. The early alteration of feldspars by the high temperature vapor phase was crucial in the development of the REE+Y enrichment at Round Top. The feldspar dissolution provided abundant open pore space that was subsequently filled by the REE-fluorides. Thus, exploration should additionally seek laccoliths that have undergone a similar early alteration process, and expand to potential laccolith groups not yet exposed by erosional processes. / text

The Chachahuén volcanic complex, with the Cerro Bayo laccolith as one of the largest intrusions, is part of back-arc Payenia volcanic province in western central Argentina. Laccoliths show potential for generating oil in sedimentary basins and producing ore deposits. It is crucial to put more effort into understanding the magma plumbing system beneath the Chachahuén volcanic complex, which the Cerro Bayo laccolith is part of. Thus this project present the first thermobarometric modelling for the Cerro Bayo laccolith and the Chachahuén volcanic complex. Several thermometers and barometers were applied to plagioclase, amphibole and clinopyroxene to understand magma evolution in the underlying plumbing system. The dacitic to rhyodacitic laccolith rocks from Cerro Bayo have porphyric textures with plagioclase in the form of both clots (glomerocrysts) and single crystals, amphibole and clinopyroxene as main phenocrysts, while enclaves are more mafic in composition and have equigranular textures. Plagioclase in the sample varies from andesine to labradorite with a composition of An22-An68, while the clinopyroxene can be classified into diopside to augite with cores (Mg#=76-84) and rims (Mg#62-72). Amphibole, varies from pargasite, ferropargasite to magesiohastingsite and hastingsite, which can be grouped into three groups according to the Mg#: low Mg# amphibole (Mg# = 40-51), medium Mg# amphibole (Mg# = 52-61) and high Mg# amphibole (Mg# = 62-78). Besides, most of the Fe-Ti oxides in the samples are titanomagnetite with Usp mol.% in the range of 0.04-0.54. Results from clinopyroxene-liquid thermobarometers suggest a crystallization depth of 44 to 51 km for clinopyroxene cores and a crystallization depth of 19 to 31 km for rims, of which the derived crystallization temperatures are in the range of 1144 to 1170 °C and 973 to 1002 °C respectively. On the other hand, different thermobarometers of amphibole give consistent results of crystallization temperatures and depths. According to the amphibole-liquid thermometer, low Mg# (Mg#= 40-51) amphiboles have the lowest crystallization temperature in the range of 898 to 931°C, while medium Mg# (Mg#=52-61) amphiboles have higher crystallization temperatures in the range of 951 to 972°C and high Mg# (Mg# = 62-78) amphiboles have highest crystallization temperatures between 991 and 1013°C. The depth translated from pressure derived from amphibole-liquid barometers give a shallower crystallization depth range from 19 to 30 km for low Mg# amphiboles, a deeper crystallization depth in the range of 22 to 39 km for high Mg# amphiboles and deepest crystallization depth range of 27 to 41 km for medium Mg# amphiboles. Moreover, a depth from 2 to 20 km could be gained from the plagioclase-liquid thermobarometry which crystallized at 883 to 910°C. It is worth to point out that crystallization depth of clinopyroxene cores is deeper than the Moho in the Chachahuén area, while crystallization depth and temperatures of clinopyroxene rims are consistent with those of high Mg# amphiboles. Consistent temperatures and depths are also found for crystallization of plagioclase and low Mg# amphiboles which intergrow with each other. Thus, a multi-level magma plumbing system began from below the Moho, with a mushy zone in the upper crust, could be suggested to exist beneath the Cerro Bayo laccolith. Evidence from both petrology and geochemistry also indicate fractional crystallization as well as processes of magma mixing and recharge during magma evolution. / Chachahuén火山综合体，包含其最大的侵入体Cerro Bayo岩盘，是位于阿根廷中西部Payenia弧后火山区的一部分。前人研究多着重于岩石学、地球化学和地层学方面，因而针对下伏岩浆涌升系统的研究在该地区极具前瞻性，需要投入更多工作。本文首次尝试对Chachahuén火山复合体应用温压计进行研究，针对斜长石、角闪石和单斜辉石应用了不同的温度计和压力计，以揭示下伏于Cerro Bayo岩盘的岩浆涌升系统中的岩浆演化。 来自Chachahuén火山复合体Cerro Bayo岩盘的英安质-流纹英安质火成岩具有板状-似斑状结构，其斑晶主要为单晶或聚合物形式存在的斜长石、角闪石及单斜辉石。此外，岩盘中还发现具等粒结构的酸性捕虏体的存在。样品中斜长石（An=22-68）主要成分为中长石至拉长石.单斜辉石主要为透辉石和普通辉石，常发育高镁值核部（Mg#=76-84）和低镁值环带（Mg#62-72）。角闪石主要为韭闪石、铁韭闪石、镁绿钙闪石和绿钙闪石，可根据镁值进一步划分为三个组：低镁值组（Mg# = 40-51），低镁值组（Mg# = 52-61）和高镁值组（Mg# = 62-71）。此外，样品中的钛铁氧化物重要为钛磁铁矿（Usp%=0.04-0.54）。 单斜辉石-熔体温压计的结果表明单斜辉石的核部和环带的结晶温度和深度有所差异：核部具有较高的结晶温度（1144-1170 °C）和较深的结晶深度（44-50km），而环带具有较低的结晶温度（973-1002 °C）和较浅的结晶深度（18-31 km）。与角闪石相关的不同温压计给出了类似的结晶温度和压力，其中存在的微小差异均在误差范围内。因此本文将角闪石-熔体温压计的结果作为最终结果，其显示低镁值角闪石具有较低结晶温度（898-931°C），中镁值角闪石具较高结晶温度（951-972°C），而髙镁值角闪石具最高结晶温度（991-1013°C）。角闪石的结晶深度特征有所不同：低镁值角闪石的结晶深度最浅（19-30km），髙镁值角闪石的结晶深度较深（22-39 km），反而中镁值角闪石结晶深度最深（27-41 km）。斜长石-熔体温压计给出的斜长石结晶温度和深度分别为883-910°C及1-20km。 值得一提的是单斜辉石核部的结晶深度位于研究区莫霍面之下。同时，单斜辉石的环带与髙镁值角闪石具有相似的结晶温度和深度，而斜长石及共生的低镁值角闪石具有相似的结晶温度。因此可以推测，Cerro Bayo岩盘之下，发育一个自莫霍面之下开始的多层岩浆上涌系统。而岩石学和地球化学的证据还表明在岩浆发育过程中岩浆分化结晶和岩浆重注和混合作用并存。 / En lakkolit är en grund magmaficka och en vanligt förekommande del av en vulkan, vilken kan bilda en länk mellan djupa magmatiska aktiviteter och vulkanutbrott. Forskning om kompositionen och bildandet av grunda magmafickor kan hjälpa till att öka förståelsen om geologiska faror och naturresurser. Vid Cerro Bayo exponeras en grund magmakammare vilken utgör en del av Chachahuén vulkankomplexet i Argentina. Det här projektet fokuserar på det magmatiska lagringssystemet och magmautvecklingen vid Cerro Bayo. Magmasammansättningen och magmakammarens förhållanden såsom tryck, temperatur och vattenhalt samt vattensystemets struktur är viktiga faktorer som styr magmatiska processer. För att förstå magmautvecklingen under Cerro Bayo inleddes projektet med en kompositionsanalys av bergprover och mineral. Därefter genomfördes termobarometrisk modellering för att bestämma temperatur- och tryckförhållandena för kristalliseringen av mineraler i det magmatiska systemet. Resultaten av modelleringen tyder på att ett magmatiskt rörsystem av flera nivåer existerar under Cerro Bayo lakkoliten. Det djupaste magmamagasinet ligger på 44-50 km och når upp till 22-41 km i nedre jordskorpan, medan de grunda reservoarerna av magma ligger på ett djup av 2-20 km i övre jordskorpan som slutligen når den grunda magmafickan. Vidare indikerar analysen av mineralstruktur och mineralkemi processer av fraktionerad kristallisering samt påfyllning och blandning av magma under magmautvecklingen. Magman genomgick fraktionerad kristallisering under rörelser uppåt från de djupaste magmakammarna. Magmans temperatur varierar från högre än 1100 ° C till ~ 900 ° C. Ansamlingar av magma fortsatte att fylla på magmamagasin i nedre och övre jordskorpan. Blandning av magma inträffade när en ansamling varmare magma rörde sig från nedre till övre jordskorpreservoarer och påverkade såväl migrering av magma som placeringen av Cerro Bayo lakkoliten