Outer Aureole Research Articles

Mechanism of metamorphic fluid expulsion from ductile contact aureole: Insights from numerical modeling of a growing mid-crustal magma chamber

Metamorphic fluid flow around an igneous intrusion is governed by temperature, rheology, permeability and fluid pressure condition in the contact aureole. In the middle crust, where lithostatic fluid pressure and ductile deformation dominate, the impulse from incremental magma chamber growth facilitates ductile aureole compaction and induces syn-intrusive metamorphic devolatilization. However, the mechanism of metamorphic fluid expulsion from ductile contact aureole is vaguely understood. In this study, we numerically modeled the two-phase solid-fluid flow process in the ductile aureole above the growing chamber to examine the potential for fluid expulsion, the pattern of fluid flow and fluid pressure condition. The numerical model is constructed based on the geological structure of the Papoose Flat pluton in California, applying three different modes of pluton growth: continuous growth, episodic growth with an interval of 100 years and episodic growth with an interval of 1000 years. The results indicate that the metamorphic fluids propagate in the form of porosity wave in the ductile aureole with solid matrix compaction. Continuous pluton growth, with a slow pressurization of aureole, leads to a positive fluid pressure anomaly in most of the aureole, whereas the episodic growth results in a negative fluid pressure anomaly at the base and positive fluid pressure anomaly at the top of the aureole. Hydrofracturing can occur in the inner aureole, and the outer aureole may retain some fluid with low-frequency, high-flux magma injections. The solid-fluid flow models indicate that porosity wave is an important mechanism for metamorphic fluid expulsion from the pluton and wall rock system.

Limited channelized fluid infiltration in the Torres del Paine contact aureole

AbstractThe metamorphic rocks from the Torres del Paine contact aureole (Patagonia, Chile) show field, petrographic, and geochemical evidence for small amounts of igneous fluid infiltration due to the solidification of the granite complex. Hydrogen isotope ratios (D/H) in the contact aureole first decrease while approaching the intrusion and subsequently increase toward the granite contact. Initial decrease with metamorphic grade is due to preferential loss of the 2H isotopes due to Rayleigh fractionation during prograde dehydration reactions. The infiltration of high-δD fluids from the intrusion increases δD within the last 150 m. In contrast, (18O/16O) ratios show no systematic changes, indicating that neither oxygen loss by Rayleigh fractionation nor oxygen exchange by fluid infiltration was significant enough to dominate original variations seen in the oxygen isotope ratio of the protolith. Calculated volume of fluid using the position of the hydrogen isotope exchange front gives a relatively low time-integrated fluid flux of about 4 m3/m2 at the contact for the igneous fluid. These small amounts of fluid flux are in agreement with whole rock oxygen isotope data that are not affected in the contact aureole. Chlorine content of metamorphic biotite crystals, in contrast to oxygen isotopes, supports infiltration of igneous fluids. Indeed, relatively high-Cl concentrations in biotite were measured in some samples close to the intrusion (up to 0.2 wt%), while chlorine concentrations in biotite are constant everywhere else in the entire contact aureole, having low concentrations (0.01–0.06 wt%). The absence of a well-marked Rayleigh fractionation trend in Cl concentrations with increasing metamorphism is surprising since chlorine strongly fractionates into the fluid. This is best explained by slow diffusive exchange of chlorine in biotite in the cooler outer aureole. Hence recrystallization of biotite would be required to modify its Cl composition. Biotite grains from samples close to the intrusion with high-Cl content also have lower Ti content (0.4 pfu) than biotite (0.5 pfu) from other samples containing biotite with lower Cl content located at the same distance from the contact. Since Ti content in biotite is a function of temperature, this is a good indication that magmatic fluid infiltration started post peak, early during cooling of the metamorphic rocks. Episodes of fluid flow appear to have been nearly continuous during cooling as evidenced by numerous retrogression textures, such as secondary muscovite (above 470 °C) or chlorite + muscovite intergrowth after cordierite or biotite (slightly below 470 °C). This might be related to crystallization of subsequent batches of granites or the onset of minor fluid convection during cooling of the aureole. Nevertheless, only minor secondary muscovite has been found, and fresh cordierite is present throughout the aureole confirming small amounts of fluid infiltration.The time-integrated fluid flux computed from the hydrogen isotope exchange front is two orders of magnitude lower than values computed for metacarbonate in many other contact aureoles, suggesting low permeabilities of pelitic rocks. In conclusion, Cl contents and hydrogen isotope compositions of hydrous minerals provide a sensitive tool to identify small fluid-rock interaction events, much more sensitive than oxygen isotope compositions of the whole rock or minerals.

Properties of fluids attending variable recrystallization of quartzite during contact metamorphism in the White‐Inyo Range, California

AbstractFluid inclusions in the metamorphic aureole of the Eureka Valley‐Joshua Flat‐Beer Creek (EJB) pluton in the White‐Inyo Range, California, reveal the compositions and origin of fluids that were present during variable recrystallization of quartzite with sedimentary grain shapes to metaquartzite with granoblastic texture. Metamorphosed sedimentary formations, including quartzites, marbles, calcsilicates and schists, became ductile and strongly attenuated in the aureole during growth of the magma chamber. The microstructures of quartzites have an unusual distribution in that within ~250 m from the pluton, where temperatures exceeded 650 °C, they exhibit relict sedimentary grain shapes, only small amount of grain boundary migration (GBM), and crystallographic preferred orientations (CPOs) dominated by <a> slip. At distances >250 m, quartzites were completely recrystallized by GBM and CPOs are indicative of prism [c] slip, characteristics that are typically associated with H2O‐assisted, high‐T recrystallization. The lack of extensive GBM in the inner aureole can be attributed to rapid replacement of H2O by CO2 produced by reaction of quartz grains with calcite cement that also produced interstitial wollastonite. Fluid inclusions in the inner aureole generally occur in margins of quartz grains and are either wholly aqueous (Type 1) or also contain H2S, CO2 and CH4 (Type 2). Type 2 inclusions occur only in some stratigraphic layers. In both inclusion types, NaCl and CaCl2, in variable proportions, dominate the solutes in the aqueous phase, whereas FeCl2 and KCl are less abundant solutes. The solutes indicate attainment of a degree of equilibrium with carbonates and schists that are interbedded with the quartzites. Some Types 1 and 2 inclusions in the inner aureole show evidence of decrepitation due to high amounts of strain and/or heating suffered by the host rocks, which suggests that they represent pore fluids that existed in the rocks prior to contact metamorphism. In addition to Type 1 inclusions, outer aureole quartzites also contain inclusions that contain CO2 vapour bubbles in addition to aqueous phase (Type 3). These inclusions only occur in interiors of granoblastic quartz that was produced by large amounts of GBM. The aqueous phase has identical ranges of first melting and final ice melting temperatures as Type 1 inclusions, suggesting that they have the same solute compositions. These inclusions are thought to represent the interstitial pore H2O that promoted recrystallization of quartz and reacted with graphite to produce CO2. Absence of significant amounts of CH4 in Type 3 inclusions is attributed to elevated fO2 that was buffered by mineral assemblages in interbedded schists. As opposed to the large amount of CO2 that was produced by the wollastonite‐forming reaction in the inner aureole to inhibit GBM, the amount of CO2 produced in the outer aureole by reaction between H2O and graphite was apparently insufficient to inhibit recrystallization of quartz.

Fluid-controlled grain boundary migration and switch in slip systems in a high strain, high temperature contact aureole, California, USA

Within the highly strained aureole surrounding the Eureka Valley–Joshua Flat–Beer Creek (EJB) composite pluton of eastern California, an inversion in microstructures and crystallographic preferred orientations (CPOs) exists with distance from the contact. An inner aureole (<250m from the contact) consists of quartzites that are interbedded with marbles and calc-silicates. These quartzites are incompletely recrystallized. Most grain boundaries have migrated, although it is clear that grain boundary migration (GBM) is not extensive. Multiple data sets indicate that temperatures of deformation were above 650°C. CPOs are indicative of <a> slip in quartz. Within the outer aureole (250m to 1500m from the contact), quartzites are interbedded with pelitic schist and are completely recrystallized and microstructures are indicative of extensive GBM. CPOs are indicative of prism [c] slip. Oxygen isotope ratios in the inner aureole are only slightly shifted from their original values. Oxygen isotopes from the outer aureole are shifted more, which is consistent with equilibration with locally derived fluids. We suggest that recrystallization in the outer aureole was aided by pore water, water derived from fluid inclusions, and water generated by prograde reactions in the schists. The pore fluids in the inner aureole were also probably initially water-rich. However, during prograde reactions in the intervening calc-silicate rocks, and perhaps more importantly, between calcite cement and quartz in the quartzites, the pore fluid composition in the inner aureole changed to become dominated by CO2, which acted as a non-wetting phase and decreased the fugacity of water slowing grain boundary mobility. Low water fugacity also suppressed the activity of prism [c] slip. Therefore, we propose that dry conditions or a grain boundary fluid with a significant non-wetting component (CO2) can result in apparent temperatures of deformation that are more than 100°C lower than the real temperatures of deformation.

Graphite thermometry in a low-pressure contact aureole, Halifax, Nova Scotia

Abstract Intrusion of the late Devonian South Mountain Batholith, southern Nova Scotia, produced a low-pressure contact metamorphic aureole in its metasedimentary host rocks. The effects of contact metamorphism are particularly well developed in pelitic rocks of the Halifax Group on the eastern margin of the batholith. Contact metamorphic isograds and mineral assemblages suggest low-pressure metamorphism, with P–T conditions at the contact estimated at 2.5–3.0 kbar and ca. 650 °C. In this study, Raman spectroscopy of carbonaceous material (RSCM) was used to obtain temperatures from graphite, which is common throughout the contact aureole. Temperature estimates range from ca. 360 °C just outside the cordierite-in isograd to ca. 640 °C in the sillimanite–K-feldspar zone near the contact, the latter consistent with the temperature estimated from the corresponding silicate mineral assemblage. Three different RSCM calibrations produced very similar results except at the high-temperature end of the observed range. A thermal profile constructed from the RSCM data was used to constrain a 2D numerical model for post-intrusion conductive cooling of the batholith along its eastern margin. Comparison of RSCM vs model thermal profiles suggest that observed differences between the thermal structure of the inner and outer aureole were controlled by the subsurface geometry of the pluton contact. The model predicts that peak temperatures in country rocks within 1 km of the contact were reached within 50 ka of intrusion, but that the outer part of the aureole took 250–500 ka to reach peak temperatures. The results confirm the utility of RSCM thermometry for acquiring temperature data over a range of metamorphic grades.

Metamorphic evolution of the contact aureole of the Jhirgadandi pluton, Sonbhadra district, Mahakoshal mobile belt, central India

The metamorphic evolution of the contact aureole around the Late Paleoproterozoic Jhirgadandi pluton in the eastern part of Parsoi Formation of Mahakoshal terrain, central India represents three distinct metamorphic zones, characterized by definite mineral assemblages. The contact-metamorphic event produced the peak-metamorphic mineral assemblages Bt + Qtz + Alb + Sil ± Cd ± Grt ± Mus ± Kfs in the metapelites of inner aureole, Bt + Qtz + And + Mus + Kfs + Plag ± Cd ± Chl in middle aureole and Chl + Mus + Bt ± And + Alb + Qtz ± Ep + Mt ± tourmaline in the outer aureole. The estimated P–T conditions based on detailed geothermobarometric calculations in the thermal metamorphosed rocks are 690 ◦ C/3.4 kbar, 580 ± 15 ◦ C and 487 ± 30 ◦ C in inner aureole, middle aureole and outer aureole, respectively. The variation in metamorphic condition suggests that the shallow crustal level emplacement of Jhirgadandi pluton is responsible for the overprinting of contact metamorphic assemblages (M2) in the low grade metapelites (regional metamorphism M1) of Mahakoshal Group.

Anatectic melt volumes in the thermal aureole of the Etive Complex, Scotland: the roles of fluid‐present and fluid‐absent melting

AbstractPelitic hornfelses within the inner thermal aureole of the Etive igneous complex underwent limited partial melting, generating agmatic micro‐stromatic migmatites. In this study, observed volume proportions of vein leucosomes in the migmatites are compared with modelled melt volumes in an attempt to constrain the controls on melting processes. Petrogenetic modelling in the MnNCKFMASHT system was performed on the compositions of 15 analysed Etive pelite samples using THERMOCALC. Melt modes were calculated at 2.2 kbar (the estimated pressure in the southern Etive aureole) from solidus temperatures to 800 °C for both fluid‐absent and fluid‐present conditions. Volume changes accompanying fluid‐absent melting at 2.2 kbar were also calculated. P–T pseudosections reproduce the zonal sequence of the southern Etive aureole fairly well. The modelled solidus temperatures of silica‐rich pelitic compositions are close to 680 °C at 2.2 kbar and, in the absence of free fluid, melt modes in such compositions rise to between 12 and 29% at 800 °C, half of which is typically produced over the narrow reaction interval in which orthopyroxene first appears. Silica‐poor compositions have solidus temperatures of up to ∼770 °C and yield &lt;11.4% melt at 800 °C under fluid‐absent conditions. For conditions of excess H2O, modelled melt modes increase dramatically within ∼13 °C of the solidus, in some cases to &gt;60%; by 800 °C they range from 61 to 88% and from 29 to 74% in silica‐rich and silica‐poor compositions, respectively. Calculated volume changes for fluid‐absent melting are positive for all modelled compositions and reach 4.5% in some silica‐rich compositions by 800 °C. Orthopyroxene formation is accompanied by a volume increase of up to 1.48% over a temperature increase of as little as 2.7 °C, supporting the arguments for melt‐induced ‘hydrofracturing’ as a viable melt‐escape mechanism in low‐P metamorphism. Mineral assemblages in the innermost aureole support previous conclusions that partial melting took place predominantly under fluid‐absent conditions. However, vein leucosome proportions, estimated by image analysis, do not show the expected correlation with grade, and are locally greatly in excess of melt modes predicted by fluid‐absent models, particularly close to the melt‐in isograd. Melting of interlayered psammites, addition of H2O from interlayered melt‐free rocks, and metastable persistence of muscovite are ruled out as major causes of the excess melt anomaly. The most likely cause, we believe, is that local variations existed in the amount of fluid available at the onset of melting, promoted by focussing of fluid released by dehydration in the middle and outer aureole; however, some redistribution of melt by compaction‐driven flow through the vein channel network cannot be ruled out. The formation of melt‐filled fractures in the inner Etive aureole was assisted by stresses that caused extension at high angles to the igneous contact. The fractures were probably caused either by transient pressure reduction in the diorite magma chamber associated with a second phase of intrusion, or by sub‐solidus thermal contraction in the diorite pluton during the early stages of inner‐aureole cooling.

Numerical simulation of kinetically-controlled calc-silicate reactions and fluid flow with transient permeability around crystallizing plutons

Numerical simulations were performed to examine the relationships between variable contact-aureole permeability, the kinetics of calc-silicate reactions, and the fluxes of mixed CO 2 -H 2 O fluids around a crystallizing granite laccolith. The role of magmatic water was explicitly considered. The Notch Peak contact-metamor- phic aureole in Utah was used to define the stratigraphy of the model domain and rock compositions. Reactions that were considered include the major isograd-defining reactions that occurred in the Notch Peak aureole. The half-space model domain had the laccolith, 2 km thick at the middle apex. Only 1-phase fluid flow was considered. Results show that the evolution of the fluid flow-field is highly dependent on the pressure (P) boundary condition at the top of the model domain. When P at the top boundary is allowed to increase, P in most of the top half of the domain eventually exceeds the lithostatic pressure plus the assumed tensile strength of rocks of 15 MPa. This boundary condition simulates an unvented flow-system. A more realistic boundary condition, one that simulates a system that is able to vent to the surface, is when Pat the top boundary is held at a hydrostatic pressure. In this case, the flow-field is determined largely by pressure gradients between the overpressured magmatic fluid exsolving from the pluton and the lower pressures at the domain boundaries. Fracturing is predicted to occur early after pluton emplacement as the pore fluid is heated and metamorphic reactions produce CO 2 . Although fracturing and reaction-enhanced porosity and permeability influence the local flow-field, the domain-scale flow-field is controlled by long-distance pressure gradients. The domain-scale flow-field and tem- perature distribution impose the largest control on the distribution of major mineral assemblages in the metamorphic aureole. Transient changes in permeability due to fracturing and volume changes in the solid matrix that accompany reactions have a smaller control on the distribution of minerals. The simulations predict significant overstepping and coeval progress of metamorphic reactions. Reaction rates range from 5x 10 -10 to 10 -14 kmol/m 2 /sec, depending on the actual P-T-XCO 2 f conditions and the abundance of the rate-controlling mineral. Throughout the metamorphic aureole, XCO 2 f approaches 1 as CO 2 evolved by reactions displaces H 2 0. High pore pressures prevent magmatic H 2 O from infiltrating the aureole until pressures drop when reactions are approaching completion. Consequently, only in the inner aureole, which is eventually infiltrated by magmatic H 2 O, can minerals such as wollastonite and vesuvianite be produced. An integrated H 2 O flux of 3×10 4 kmol/m 2 is required to produce the width of the model wollastonite zone by 20 ky. Because of pressure gradients, CO 2 that is produced in the inner aureole flows outward into colder rocks even before these rocks are heated. The T-XCO 2 f paths in outer aureole rocks cut across the H 2 O-CO 2 solvus, which predicts that in nature H 2 O and CO 2 should unmix and behave as two separate fluid phases. The average yearly CO 2 flux at the top of the model domain, 2.3 × 10 3 mol/m 2 /y, is comparable to fluxes of metamorphic carbonic fluids in active geothermal fields.

Concentric slow cooling of a low-P-high-T terrane: Evidence from 1600-1300 Ma mica dates in the 1780-1700 Ma Black Hills Orogen, South Dakota, U.S.A.

The crystalline core of the southern Black Hills, South Dakota, exposes an extensive, low- P –high- T aureole of garnet- to second-sillimanite-zone schists centered on the plutonic core of the 1715 Ma Harney Peak Granite (HPG). This paper demonstrates regional patterns of apparent ages observed for 52 40 Ar/ 39 Ar dates of muscovite and biotite in diverse rocks from across the ~1000 km 2 metamorphic aureole and its plutonic center. About 20 biotite dates, sampled mostly near faults, are influenced by excess 40 Ar and obscure the regional trends. The remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite, with previously unrecognized, elliptical age zones centered on the main HPG pluton and its outliers. The regional pattern of 40 Ar/ 39 Ar cooling ages indicates non-uniform slow cooling of the mid-crust between ~1600–1250 Ma. This scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite (Rb/Sr) and apatite (U/Pb), which range from 1690 to 1550 Ma and from 1700 to ~1500 Ma, respectively. To explain these results, it is likely that ambient pre-granite temperatures of the country rocks were ≥350 °C at the ~10–14 km depth of granite emplacement, as previously proposed, and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years. Alternatively, or in addition, the HPG and inner aureole were not uplifted until ~1480–1330 Ma, whereupon they finally cooled through ~300–350 °C.

Geology of the Cove Mine, Lander County, Nevada, and a Genetic Model for the McCoy-Cove Hydrothermal System

The McCoy Au-Ag skarn and Cove Au-Ag deposits are located in the northern Fish Creek Mountains, Lander County, Nevada. Through the end of mining in 2001, large-scale open-pit and associated underground production at the two deposits yielded 3.3 million ounces (Moz) of Au and 108 Moz of Ag. Most production was from Cove, making it the fourth-largest Ag producer in the history of Nevada. Cove is hosted by the middle to early Late Triassic Augusta Mountain Formation, which consists of limestone with lesser dolostone and clastic units. Ore also is present locally in Eocene porphyritic granodiorite dikes and sills. The deposit comprises two distinct ore types: a central core of polymetallic vein-type ore and an outer aureole of relatively Ag rich Carlin-style ore. Polymetallic veins consist of pyrite-sphalerite-galena–dominated Au- and Ag-bearing veins, veinlets, stockworks, crustifications, and disseminations in clastic and carbonate strata and locally in the intrusions. Carlin-style ore comprises disseminated Fe ± As sulfides with arsenian, argentiferous, and auriferous components ± native Au-electrum in silty to sandy carbonate strata. Polymetallic vein-type ore has Ag/Au ratios of >50/1, and Carlin-style ore has Ag/Au ratios that decrease from ~50/1 near the feeder faults to ~1/1 in one of the more distal ore zones. Both types of ore are associated with decarbonatized, silicified, and illitized rocks. New structural and age data for fresh and altered intrusive rocks indicate that mineralization at Cove occurred during active extension and magmatism at ~39 Ma (40Ar-39Ar). Fluid inclusion and δD and δ18O data for polymetallic vein-type ore indicate that the mineralizing fluids had temperatures of 250° to 370°C and were magmatic in origin. Cove is located 1.6 km northeast of McCoy, where Early Tertiary igneous activity occurred in two pulses. The first pulse consisted of relatively oxidized magnetite-series magma and formed the central stock at McCoy and related dikes that extend to the Cove deposit. This pulse occurred at ~41.5 Ma and produced subeconomic skarn at McCoy. The second pulse consisted of relatively reduced ilmenite-series magma and produced economic skarn ore at McCoy. Adularia from a mineralized skarn assemblage at McCoy was dated at ~39 Ma (K-Ar). The age data indicate that McCoy and Cove formed contemporaneously. The δ34S data for ore-stage sulfides from skarn ore at McCoy, and polymetallic vein-type and Carlin-style ore at Cove indicate a common source for S for the three types of ore. This source is interpreted to have been the pulse of ilmenite-series magma. Although the volume of the Carlin-style ore at Cove is substantially larger than that of the polymetallic vein-type ore, the relatively high concentrations of base metals and clear associations with igneous activity have made Cove difficult to classify. The new data indicate that Cove is a telescoped system consisting of polymetallic vein-type and porphyry-related, distal disseminated ores. The zonation for the McCoy-Cove system is nearly identical to zonations described by other workers for Au-enriched porphyry systems. In this case, the porphyry center and skarn ore at McCoy comprise the proximal component, and polymetallic vein-type and Carlin-style ores at Cove are the intermediate and distal components, respectively. Economic concentrations of Au occur in all three zones, which are separated from one another by subeconomic concentrations of Au. The Carlin-style orebodies at Cove share several important similarities with ores in classic Carlin-type deposits, including ore characteristics, associated alteration styles, and host lithologic units. The late Eocene age of the McCoy-Cove system is also a typical age for Carlin-type deposits in the Great Basin. The proposed genetic model for McCoy-Cove may have important implications for the exploration potential of intrusion-related and Carlin-type deposits worldwide.

Fluid evolution and kinetics of metamorphic reactions in calc-silicate contact aureoles—From H2O to CO2 and back

Research Article| October 01, 2007 Fluid evolution and kinetics of metamorphic reactions in calc-silicate contact aureoles—From H2O to CO2 and back Peter I. Nabelek Peter I. Nabelek 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (10): 927–930. https://doi.org/10.1130/G24051A.1 Article history received: 02 May 2007 rev-recd: 21 May 2007 accepted: 25 May 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter I. Nabelek; Fluid evolution and kinetics of metamorphic reactions in calc-silicate contact aureoles—From H2O to CO2 and back. Geology 2007;; 35 (10): 927–930. doi: https://doi.org/10.1130/G24051A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Mineral assemblages in contact-metamorphic aureoles are the products of the interplay between heat transfer and fluid flow induced by intrusion of magma. In wall rocks containing carbonate and silicate minerals, metamorphic reactions produce CO2, which then becomes part of the hydrodynamic system. Although observed assemblages are the ultimate products of T-XCO2fluid-t paths in aureole rocks, the complexities of the paths, and hence the evolution of a hydrodynamic system, are difficult to decipher from them. Numerical simulations were conducted to model the mineralogical evolution in a hydrodynamic system and to evaluate the extent and effects of reaction retardation. Simulations reveal that fluid composition in the inner aureole evolves rapidly toward high XCO2fluid as the rocks heat up before an appreciable amount of water is exsolved out of the pluton. After local fluid pressure drops when early reactions are coming to completion, infiltration of magmatic water becomes significant and can drive production of typical inner aureole minerals such as wollastonite. Fluid compositions in the outer aureole reflect largely (1) the initial CO2 production, as fluids are driven down the pressure gradients from the inner aureole, and (2) the subsequent infiltration of magmatic H2O. Simulations also suggest temperature overstepping of the onset of reactions and retarded consumption of reactant minerals, which leads to coeval metastable reactions. However, the final simulated mineral assemblage in the inner aureole reflects equilibration with H2O-rich fluids that is usually seen in the field, although evidence for kinetic retardation may be preserved in some rocks, especially in the outer aureole. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contrasting response of monazite and zircon to a high- T thermal overprint

Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage. In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of 207Pb / 206Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This disturbance is correlated with proximity to the pluton implying that Pb was mobile in the zircon lattice during contact metamorphism. Most grains are characterized by apparent Pb loss from low-U domains and apparent Pb gain in higher-U domains. These data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of Pb* that was able to efficiently diffuse into higher-U domains that were partly amorphous prior to rapid reheating in the inner aureole.

Complex patterns of fluid flow during wollastonite formation in calcareous sandstones at Laurel Mountain, Mt. Morrison Pendant, California

Research Article| January 01, 2004 Complex patterns of fluid flow during wollastonite formation in calcareous sandstones at Laurel Mountain, Mt. Morrison Pendant, California Jade Star Lackey; Jade Star Lackey 1Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar John W. Valley John W. Valley 1Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jade Star Lackey 1Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA John W. Valley 1Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA Publisher: Geological Society of America Received: 14 Aug 2002 Revision Received: 29 Apr 2003 Accepted: 12 Jun 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2004) 116 (1-2): 76–93. https://doi.org/10.1130/B25239.1 Article history Received: 14 Aug 2002 Revision Received: 29 Apr 2003 Accepted: 12 Jun 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jade Star Lackey, John W. Valley; Complex patterns of fluid flow during wollastonite formation in calcareous sandstones at Laurel Mountain, Mt. Morrison Pendant, California. GSA Bulletin 2004;; 116 (1-2): 76–93. doi: https://doi.org/10.1130/B25239.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Late Cretaceous contact metamorphism at Laurel Mountain, east-central California, resulted in growth of wollastonite in calcareous metasandstones during infiltration of water-rich [X(CO2) = 0.03–0.14] fluids. Fluids originated from the Laurel Mountain pluton and other magmatic sources. The mapped distribution of wollastonite versus calcite + quartz in the aureole reveals geometrically complex fluid flow paths at a range of scales. An inner aureole, concentric to the pluton, extends up to 500 m from exposed contacts and is characterized by ubiquitous wollastonite. An outer aureole has irregular distribution of wollastonite, chiefly at faults and stratigraphic contacts.Fluid flow was largely vertical during metamorphism, and permeability varied systematically. As fluids migrated up from crystallizing magmas into pendant rocks, they were variably channeled along a hierarchy of structural features. Faults in the aureole transported the largest fluid fluxes as recorded by δ18O(Wo) values (7–8‰) that approach equilibrium with the pluton. These faults distributed fluids up and away from the Laurel Mountain pluton. Of the total fluid flow, 2–4% was across layers, which mixed low-δ13C carbon (−25‰) from pelites into wollastonite metasandstones. Textural and isotopic data indicate that mineral reaction in the inner aureole was controlled by availability of fluids, permeability, and likely temperature; reaction in the outer aureole was driven mostly by access to water-rich fluids. Magmatic fluids in the outer aureole were isotopically shifted farther from magmatic values than fluids in the inner aureole and are inferred to have traveled from deeper sources.Isotope disequilibrium is widespread at Laurel Mountain. Values of Δ18O(Qt–Cc) in calcareous sandstones (1.78 ± 0.3‰) indicate exchange, but not equilibration at peak metamorphic temperatures (560 °C). Most Δ18O(Qt–Wo) values (2.0–6.5‰) indicate kinetically controlled exchange and disequilibrium during fluid-rock interaction. Minor isotopic resetting in the aureole was caused by polymetamorphism related to younger plutonism in the eastern Sierra Nevada. This study highlights the complexities of modeling fluid flow during contact metamorphism in composite batholith settings. These complexities include multiple fluid and heat sources, plus variable permeability, temperature, mineral-fluid exchange, and flow path. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Reactive flow of mixed CO2–H2O fluid and progress of calc‐silicate reactions in contact metamorphic aureoles: insights from two‐dimensional numerical modelling

AbstractPrevious models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO2 and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO2–H2O fluid and kinetically controlled progress of calc‐silicate reactions using a two‐dimensional, finite‐element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO2 strongly affects fluid‐flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO2–H2O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO2‐poor fluid in the outer aureole. Metamorphic CO2‐rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P‐T‐XCO2conditions of calc‐silicate reactions. The progress of low‐ to medium‐grade (phlogopite‐ to diopside‐forming) reactions is mainly driven by heat as the CO2 concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high‐grade wollastonite‐forming reaction is mainly driven by infiltration of chemically out‐of‐equilibrium, CO2‐poor fluid during late‐stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO2‐rich fluid dominates in the inner aureole during early heating, whereas CO2‐poor fluid prevails at or after peak temperature is reached. Low‐grade metamorphic rocks are predicted to record the presence of CO2‐rich fluid, and high‐grade rocks reflect the presence of CO2‐poor fluid, consistent with geological observations in many calc‐silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.

Relationships between contact metamorphism and magnetite formation and destruction in a plutonʼs aureole, White-Inyo Range, eastern California

Magnetic-petrologic studies on metasedimentary rocks from the low-pressure–high-temperature contact-metamorphic aureole of the Joshua Flat–Beer Creek pluton at the southern rim of the Inyo batholith, California, reveal the effect of contact metamorphism on magnetic minerals. The regional metamorphic biotite zone and three different contact-metamorphic zones (andalusite- cordierite, transition, and cordierite–K- feldspar) in the aureole are identified by silicate petrology and their rock magnetic behavior. Phengite barometry yields pressure data in the range of 0.2–0.3 GPa, and Na-in-cordierite thermometry indicates a peak temperature of ∼730 °C for the highest-temperature contact-metamorphic zone. Compositional changes of iron-magnesium silicates and iron oxides correspond to changes in magnetic susceptibility, temper ature dependence of magnetic susceptibility, and AF (alternating field) demagnetization of NRM (natural remanent magnetization) and can be correlated with the metamorphic history of the aureole. Magnetic susceptibility ranges from 0.2 × 10 –3 to 0.5 × 10 –3 SI in the biotite zone and is highly variable in the andalusite-cordierite and transition zones, for which the values range from 0.35 × 10 –3 to 28 × 10 –3 SI. In prograde metamorphism, the assemblage ilmenite + rutile ± hematite transforms to magnetite and ilmenite-hematite solid solutions, producing an increase in magnetic susceptibility that can be correlated with the cordierite-in isograd. In the cordierite–K-feldspar zone and in samples having undergone retrograde metamorphism, prograde magnetite reacts to ilmenite and hematite. This breakdown is associated with decreased magnetic susceptibility (0.2–0.8 × 10 –3 SI) and blurs the magnetite isograd that formed during prograde metamorphism in the outer contact aureole. Retrograde metamorphism in the contact aureole is defined by chlorite composition [ T : 280 and 420 °C, log f (O 2 ):−35 and−28]. Contact-metamorphic aureoles, as long as they are not strongly altered, may contribute significantly to magnetic anomalies around magnetite- series plutons. This fact has to be considered when interpreting aeromagnetic data of granitoid rocks.

Numerical modeling of fluid flow and oxygen isotope exchange in the Notch Peak contact-metamorphic aureole, Utah

We investigated the macroscale fluid flow and oxygen isotope exchange in contact-metamorphic aureoles by using two-dimensional finite-element modeling. The model parameters are assigned according to geologic observations in the Notch Peak metamorphic aureole, Utah. The results show that fluid flow in contact aureoles depends strongly on time and space, unlike the commonly assumed uniform and unidirectional fluid flow in previous studies. In an aureole with homogeneous permeability, a simple convection cell develops in the inner aureole, causing δ 1 8 O values to decrease in the inner aureole and increase in the upper outer aureole. However, when the horizontally layered permeability structure of the Notch Peak aureole is represented in the model, the resulting fluid flow is concentrated within high-permeability aquifers, and local convection cells develop near the contact within each aquifer. Sub-horizontal down-temperature flow and up-temperature flow can coexist in the same aquifer. The up-temperature flow in the lower part of each aquifer mainly causes depletion of 1 8 O in the inner aureole. Release of magmatic fluid significantly enhances the depletion of 1 8 O in the inner aureole at early stages, although sub-horizontal up-temperature flow at later stages tends to reduce the effects of the early-stage exchange. The isotopic features of the Notch Peak aureole are best explained by (1) infiltration of magmatic fluid having a δ 1 8 O value of 8‰, (2) ∼10% of the wall rocks involved in the oxygen isotope exchange with fluids, (3) no isotope exchange below 250 °C and during retrograde cooling, and (4) enhanced permeability in the inner aureole in addition to the litho-logically layered permeability structure.

The contact aureole on Tinos (Cyclades, Greece): tourmaline-biotite geothermometry and Rb-Sr geochronology

In order to shed some light on the timing of metamorphic and deformational processes operating during the late stages of exhumation in the Attic-Cycladic Crystalline Belt, the contact metamorphism around Miocene granitoids was studied on Tinos. Based on the occurrence of biotite, plagioclase, diopside and scapolite, four mineral zones were mapped in the thermally overprinted area. An attempt was made to constrain relative temperature variations across the contact aureole by application of tourmaline-biotite geothermometry. While part of the calculated temperatures are broadly in accordance with the mapped mineral isograds of the aureole, temperatures far too low were obtained for certain areas. The latter may be explained by retrograde cation exchange in biotite and refractory behaviour of tourmaline but may also be attributed to systematic shortcomings of this method. New petrological and isotopic data suggest that samples from the outer aureole mostly consist of regional metamorphic phengite with ages unaffected by the thermal overprint. This renders phengite dating unsuitable to determine timing of the contact metamorphism. The hypothesis of cryptic contact metamorphic effects on the radiogenic isotope systems of white mica outside the mapped contact aureole could not be substantiated. Reliable age information for this event is provided by biotite, because most of this phase formed during contact metamorphism and inherited components only cause minimal error on the ages. In the eastern aureole, the thermal overprint is dated at about 14 Ma, corresponding to previously published geochronological results for the granitoids. Younger biotite ages of samples from the western aureole (8–10 Ma) possibly date deformation that affected the marginal parts of the main intrusion. Alternatively, the younger ages may indicate a different cooling history in this part of the contact aureole, caused by an undated magmatic pulse within a composite pluton. The Sr isotope characteristics of regional and contact metamorphic metabasic rocks cannot be distinguished. Thus, the intrusion-related infiltration of large amounts of fluids having a 87Sr/86Sr ratio different from the country rocks can be ruled out.

Mineralogical Evidence for Fluid–Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern part of agrees well with the q m TIFF (3·4 × 10 7 mol/m 2 ) determined by the metamorphic aureole of the Alta stock (Utah, USA) produced numerical simulation of the temperature and 18 O depletion profiles the prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite preserved in the southern aureole. The estimates of q m TIFF for the (Fo), and periclase (Per). Calcite (Cc)–dolomite (Do) ge- forsterite and tremolite zones have much greater uncertainty, but othermometry and phase equilibria define a general prograde T– may indicate that fluid flux was considerably lower in these zones X(CO2) path of decreasing X(CO2) with rising temperature for than in the periclase zone. Given the outward (down-temperature), the dolomite. High-variance assemblages typify the aureole. Per + subhorizontal flow geometry indicated by a variety of petrologic, Cc and Fo + Cc + Do characterize the inner aureole (Per and geochemical, and geothermometry evidence presented here and elseFo zones), and Tr + Do + Cc and Tc + Do + Cc are where, this decrease implies that fluid has leaked from the flow widespread in the outer aureole (Tr and Tc zones). Low-variance system between the periclase and tremolite zones. assemblages are rare and the thickness of reaction zones (coexisting reactant and product minerals) at the isogradic reaction fronts are narrow (tens of metres or less). The mineral assemblages, calculated progress of isograd reactions, and the prograde T–X(CO2) path all KEY WORDS: Alta; fluid flow; infiltration; marble; mineralogical indicate that massive dolomite was infiltrated by significant fluxes of water-rich fluids during prograde metamorphism, and that the fluid flow was down-temperature and laterally away from the igneous contact. Fluid infiltration continued through at least the initial retrograde cooling of the periclase zone. Down-T fluid flow INTRODUCTION is also consistent with the results of Cc–Do geothermometry and Both mineral assemblages and stable isotopes have been patterns of 18 O depletion in this area. The close spatial association

Pluton emplacement in arcs: a crustal-scale exchange process

ABSTRACT:Buddington (1959) pointed out that the construction of large crustal magma chambers involves complex internal processes as well as multiple country rock material transfer processes (MTPs), which reflect large horizontal, vertical and temporal gradients in physical conditions. Thus, we have attempted to determine the relative importance of different magmatic and country rock MTPs at various crustal depths, and whether country rock MTPs largely transport material vertically or horizontally, rather than seeking a single model of magma ascent and emplacement.Partially preserved roofs of nine plutons and in some cases roof–wall transitions with roof emplacement depths of 1·5–11 km were mapped. During emplacement, these roofs were not deformed in a ductile manner, detached or extended by faults, or significantly uplifted. Instead, sharp, irregular, discordant contacts are the rule with stoped blocks often preserved immediately below the roof, even at depths of 10 km. The upper portions of these magma chambers are varied, sometimes preserving the crests of more evolved magmas or local zones of volatile-rich phases and complex zones of dyking and magma mingling. Magmatic structures near roofs display a wide variety of patterns and generally formed after emplacement. Transitions from gently dipping roofs to steep walls are abrupt. At shallow crustal levels, steep wall contacts have sharp, discordant, stepped patterns with locally preserved stoped blocks indicating that the chamber grew sideways in part by stoping. Around deeper plutons, an abrupt transition (sometimes within hundreds of metres) occurs in the country rock from discordant, brittle roofs to moderately concordant, walls deformed in a ductile manner defining narrow structural aureoles. Brittle or ductile faults are not present at roof–wall joins.Near steep wall contacts at shallow to mid-crustal depths (5–15 km), vertical and horizontal deflections of pre-emplacement markers (e.g. bedding, faults, dykes), and ductile strains in narrow aureoles (0·1–0·3 body radii) give a complete range of bulk strain values that account for 0–100% of the needed space, but average around 30%, or less, particularly for larger batholiths. A lack of far-field deflection of these same markers rules out significant horizontal displacement outside the aureoles and requires that any near-field lateral shortening is accommodated by vertical flow. Lateral variations from ductile (inner aureole) to brittle (outer aureole) MTPs are typically observed. Compositional zoning is widespread within these magma bodies and is thought to represent separately evolved pulses that travelled up the same magma plumbing system. Magmatic foliations and lineations commonly cross-cut contacts between pulses and reflect the strain caused either by the late flow of melt or regional deformation.Country rocks near the few examined mid- to deep crustal walls (10–30 km) are extensively deformed, with both discordant and concordant contacts present; however, the distinction between regional and emplacement-related deformation is less clear than for shallower plutons. Internal sheeting is more common, although elliptical masses are present. Lateral compositional variations are as large as vertical variations at shallower depths and occur over shorter distances. Magmatic foliations and lineations often reflect regional deformation rather than emplacement processes.The lack of evidence for horizontal displacement outside the narrow, shallow to mid-crustal aureoles and the lack of lateral or upwards displacement of pluton roofs indicate that during emplacement most country rock is transported downwards in the region now occupied by the magma body and its aureole. The internal sheeting and zoning indicate that during the downwards flow of country rock, multiple pulses of magma travelled up the same magma system. If these relationships are widespread in arcs, magma emplacement is the driving mechanism for a huge crustal-scale exchange process.

Magnetic fabric in some granitic aureoles, Southern Uplands, Scotland

The Loch Doon, Cairnsmore of Fleet and Criffell granites and aureoles show considerable variation in their magnetic mineralogy and fabrics. The Loch Doon area has magnetite as the dominant magnetic mineral, while the Cairnsmore of Fleet area shows magnetite and hematite in different areas. The Criffell granite and immediately adjacent aureole are characterized by magnetite, but hematite predominates in the outer aureole and country rocks. Despite such differences, the magnetic fabric only shows effects of granitic emplacement within the thermal aureole, mostly within 200–300 m of the contact. Considerable variation in magnetic fabric is observed over small distances, but the granitic contacts are generally marked by a strong foliation parallel to the granite contact.