Regional Metamorphic Terranes Research Articles

Determining the amount of overstepping required to nucleate garnet during Barrovian regional metamorphism, Connecticut Valley Synclinorium

AbstractThe novel method of inclusion barometry coupled with the calculation of the required affinity for garnet nucleation is applied to three samples from the previously well‐characterized Connecticut Valley Synclinorium in central Vermont. Raman shifts for quartz inclusions record a range of maximum peak shifts of the quartz 464 cm−1 peak from 2.4 to 3.0 cm−1. Temperature of garnet nucleation was constrained by calculating mineral assemblage diagrams in the MnNCKFMASHT system and plotting the intersection of quartz inclusion in garnet barometry (QuiG, quartz‐in‐garnet) with Zr‐in‐rutile thermometry. Utilizing the intersection of Zr‐in‐rutile thermometry with QuiG barometry, garnet nucleation is inferred to have occurred within a P–T range of ~8.6–9.5 kbar and ~560–575°C. These P–T conditions for garnet nucleation are significantly higher than calculated equilibrium garnet‐in isograds for the three samples. Affinities for garnet nucleation were calculated as the difference between the free energy of a fictive garnet composition based on the matrix assemblage and the free energy of the nucleated garnet. The calculated nucleation affinity varied from 300 to 600 kJ/mol O for St–Ky grade samples. These results suggest that the assumption that metamorphism proceeds as a sequence of near‐equilibrium conditions cannot, in general, be made for regional metamorphic terranes. This body of work agrees with numerous recent studies showing that garnet‐producing reactions must be overstepped in order to for garnet to nucleate.

Periodic precipitation of carbonates in metamorphic veins : Comparison of two examples from contact and regional metamorphic terranes.

Periodic precipitation of carbonates in metamorphic veins : Comparison of two examples from contact and regional metamorphic terranes.

Permian age of orogenic thickening and crustal melting in the Garm Block, South Tien Shan, Tajikistan

In this paper we present new data for the metamorphic Garm Block (150×40km) that was traditionally shown on regional maps as fragment of a Precambrian basement. The Garm Block comprises migmatized gneisses and schists for some of which peak metamorphic conditions (600–660°C at 7–9kbar) characteristic for medium P/T series metamorphic belts were determined. U–Pb ages of overgrown rims of zircon grains and newly grown magmatic zircon grains from four migmatitic samples indicate that melting occurred during peak metamorphic conditions between 299 and 290Ma. Thus the Garm Block is the first Early Permian regional metamorphic terrane established in the southern part of the Central Asian Orogenic Belt. A crosscutting granite intrusion yields a concordia age of 290Ma identical to the age of metamorphism and similar to ages of post-collisional intrusions elsewhere in the Tien Shan. Detrital zircon grains from the Garm samples yield an age spectrum with major peaks at 700–550Ma and 1150–700Ma, and smaller ones at ca. 2.7–2.4Ga and 3.5–3.3Ga. The youngest grains define an Ediacarian maximum depositional age of the sedimentary protoliths. The presence of Grenvillian ages of 1150–800Ma, which are characteristic for the Tarim Craton, and the lack of ca. 1.6–1.2Ga ages, characteristic for the Northern Tien Shan, suggest an origin of the Precambrian zircon grains from southern continental blocks similar to the Tarim Craton. Six granitoid samples yield negative εNd values of −10.6 to −3.7 consistent with the reworking of crustal material with 1.8–1.1Ga average crustal residence times and the formation of the South Tien Shan belt on a continental basement with Paleoproterozoic or older crust. The Garm Block formed as a consequence of lithospheric thickening during the Late Carboniferous collision of two continental blocks that created the South Tien Shan fold belt. Lateral crustal melt flow coupled with extension may have resulted in exhumation of the Permian metamorphic rocks.

Detrital zircon and isotopic constraints on the crustal architecture and tectonic evolution of the northeastern Pamir

New detrital zircon and isotopic (Nd and Sr) analyses from the eastern Pamir provide information on the depositional age and tectonic terrane affiliation of regional metamorphic terranes. Our results show the following. First, detrital zircon analyses from metasedimentary units along the Kongur Shan extensional system dominantly yield Triassic maximum depositional ages, with a similar age distribution to the Tibetan Songpan‐Ganzi terrane. Further, zircon analyses from quartzofeldspathic gneisses in the core of the Muztaghata massif show the protoliths are Triassic granites. These units are interpreted to be part of the Permian‐Triassic Karakul‐Mazar arc‐accretionary complex terrane. Second, εNd(0) compositions of Triassic granites overlap with other metasedimentary units not analyzed for detrital zircons and are also interpreted to be part of the Karakul‐Mazar terrane. Third, schists in the Sares‐Muztaghata gneiss dome structurally above Triassic orthogneisses yield an Ordovician maximum depositional age with a distinct detrital age distribution similar to the Tibetan Qiangtang terrane and are interpreted to be part of the Central Pamir terrane. Finally, Triassic and Ordovician schists along the Muztaghata massif record an Early Jurassic metamorphic event interpreted to date south‐directed subduction of the Karakul‐Mazar terrane beneath the Central Pamir during final closure of the Paleo‐Tethys. These results, integrated with previously published results and field relations, reveal a complex Mesozoic to Cenozoic interleaving of tectonic terranes in the eastern Pamir with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south, respectively.

Epidote Group Minerals in Low-Medium Pressure Metamorphic Terranes

Research Article| January 02, 2004 Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes Rodney H. Grapes; Rodney H. Grapes Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Search for other works by this author on: GSW Google Scholar Paul W. O. Hoskin Paul W. O. Hoskin Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Search for other works by this author on: GSW Google Scholar Author and Article Information Rodney H. Grapes Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Paul W. O. Hoskin Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2004) 56 (1): 301–345. https://doi.org/10.2138/gsrmg.56.1.301 Article history First Online: 03 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 Rodney H. Grapes, Paul W. O. Hoskin; Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes. Reviews in Mineralogy and Geochemistry 2004;; 56 (1): 301–345. doi: https://doi.org/10.2138/gsrmg.56.1.301 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 SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Epidote group minerals are common in metamorphosed mafic to intermediate igneous rocks, quartzofeldspathic sediments and calc-alumina silicate (marl) rocks of higher grade zeolite to medium grade amphibolite facies of low–medium pressure contact and regional metamorphic terranes (i.e., pressure and temperature conditions below the calcite-to-aragonite transition). Within any one rock, epidote composition in terms of Fe3+/(Fe3+ + Al) can be variable, but in general, is limited by whole-rock composition, such that epidote group minerals in metabasite lithologies are more Fe-rich than those in marls that tend to be more Al-rich and typically include zoisite. Because of their wide... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Polymetamorphic origin of muscovite+cordierite+staurolite+ biotite assemblages: implications for the metapelitic petrogenetic grid and for P–T paths

Metapelites containing muscovite, cordierite, staurolite and biotite (Ms+Crd+St+Bt) are relatively rare but have been reported from a number of low‐pressure (andalusite–sillimanite) regional metamorphic terranes. Paradoxically, they do not occur in contact aureoles formed at the same low pressures, raising the question as to whether they represent a stable association. A stable Ms+Crd+St+Bt assemblage implies a stable Ms+Bt+Qtz+Crd+St+Al2SiO5+Chl+H2O invariant point (IP1), the latter which has precluded construction of a petrogenetic grid for metapelites that reconciles natural phase relations at high and low pressure. Petrogenetic grids calculated from internally consistent thermodynamic databases do not provide a reliable means to evaluate the problem because the grid topology is sensitive to small changes in the thermodynamic data. Topological analysis of invariant point IP1 places strict limits on possible phase equilibria and mineral compositions for metamorphic field gradients at higher and lower pressure than the invariant point. These constraints are then compared with natural data from contact aureoles and reported Ms+Crd+St+Bt occurrences. We find that there are numerous topological, textural and compositional incongruities in reported natural assemblages that lead us to argue that Ms+Crd+St+Bt is either not a stable association or is restricted to such low pressures and Fe‐rich compositions that it is rarely if ever developed in natural rocks. Instead, we argue that reported Ms+Crd+St+Bt assemblages are products of polymetamorphism, and, from their textures, are useful indicators of P–T paths and tectonothermal processes at low pressure. A number of well‐known Ms+Crd+St+Bt occurrences are discussed within this framework, including south‐central Maine, the Pyrenees and especially SW Nova Scotia.

The effect of Mn on mineral stability in metapelites

Calculations based on a KMnFMASH petrogenetic grid derived using an internally consistent thermodynamic dataset indicate that the principal effect of the presence of Mn in average subaluminous pelite compositions is to stabilize garnet to higher and lower pressures and temperatures over a wide range of bulk compositions. Garnet‐bearing fields expand to lower temperatures and pressures with the addition of Mn, and garnet appears as an extra phase at low pressures. The addition of Mn also increases the number and extent of four AMnFM phase assemblages and stabilizes five AMnFM phases along univariant reactions. The KMnFMASH system predictions for typical subaluminous pelite bulk compositions match the sequence of isograds and assemblages observed in the Barrovian zones. The sequence of assemblages observed in the Stonehaven section can also be predicted if there is variation in bulk composition within the stratigraphic section. Mn appears to be less important in producing the sequence of isograds and garnet‐absent assemblages in the low‐pressure Buchan zones. The addition of Mn to the calculations does not change the sequence of isograds that are predicted to be stable in a regional metamorphic terrane, but the P–T position of these isograds does change. In particular, the predicted temperature of the garnet‐in isograd is lowered by as much as 100 °C by the addition of Mn to KFMASH. Mn also increases the range of metapelite bulk compositions that develop the assemblages traditionally identified as metapelite isograds.

Regional metamorphism in transform zones during supercontinent breakup: Late Proterozoic events of the Scottish Highlands

A new model for regional metamorphism within major strike-slip zones is presented that may be applicable to the generation of many small metamorphic terranes. Long-lived transform zones developed during continental break-up are prone to alternating local compression and extension. Periodic closure of pull-apart basins, formed within such zones, causes internal thickening of the sediment pile and formation of small low- or medium-grade metamorphic terranes, with well-defined, frequently reactivated margins. The Late Proterozoic Dalradian and Moine blocks of the Scottish Highlands are examples of such regional metamorphic terranes generated during the prolonged breakup of the Rodinia supercontinent.

Silicification and metal leaching in semiconformable alteration beneath the Chisel Lake massive sulfide deposit, Snow Lake, Manitoba

Regionally extensive semiconformable zones of silicified, Fe-Mg metasomatized and epidotized, dominantly mafic, volcaniclastic strata and lava flows are exposed 1 to 2 km stratigraphically beneath the Chisel Lake Zn-Cu massive sulfide deposit, Snow Lake district, Manitoba. The alteration zones occur within a Lower Proterozoic medium-grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments, and lie between subvolcanic tonalite sills and the massive sulfide deposit.Semiconformable alteration occurs at two main stratigraphic positions. The lower zone of silicification in pillowed and massive mafic lavas is inferred to have occurred close to the sea floor and probably was not related directly to formation of the Chisel Lake massive sulfide deposit. The stratigraphically higher zone, which may be spatially and temporally associated with massive sulfide deposition, occurs in a approximately 300-m-thick heterolithic mafic volcanic breccia and wacke unit. This volcaniclastic-hosted alteration is zoned laterally from dominantly silicification and epidotization to mainly Fe-Mg metasomatized, garnet-chlorite + or - biotite + or - staurolite rocks nearer the Chisel Lake sulfide deposit. A discordant footwall Fe-Mg alteration zone directly beneath the sulfide deposit extends toward, and may meet, the semiconformable Fe-Mg metasomatized zone.Silicification contributed to partial to complete replacement of volcanic clasts, beds in the volcaniclastic unit, rocks adjacent to some felsic dikes and pillow interiors by quartz and sodic plagioclase. Mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds, dikes, and pillows indicate that SiO 2 increased by up to 50 percent of its initial value, and Na 2 O by up to 30 percent. Up to 80 percent of the FeO, MgO, CaO, and Zn was removed during silicification. Elemental fluxes during Fe-Mg metasomatism are generally opposite those characterizing silicification and are of comparable magnitude. Epidotization resulted in depletion of Na, total Fe (but increased the Fe (super 2+) /Fe (super 3+) ratio), Mg, Mn, K, Zn, and Ba, and enrichment in Ca and Sr relative to least altered rocks. Almost constant interelement ratios of Ti, Zr, and Al in altered and less altered rocks indicate that these elements were essentially immobile during metasomatism and subsequent medium-grade regional metamorphisrm. Limited data suggest that the heavy REE were also immobile during silicification.The subconcordant silicification in the mafic volcaniclastic unit is interpreted to have formed at subsea-floor depths of 1 to 2 km where a felsic dike swarm and subvolcanic tonalite sills heated Si-rich evolved seawater above the temperature of the silica solubility maximum ( approximately 340 degrees -450 degrees C at pressures below 900 bars) causing silica deposition and metal leaching. A portion of the Fe, Mg, and possibly Zn that was leached from the subconcordant silicified zone may have been transported laterally away from this environment, thereby producing the semiconformable Fe-Mg metasomatized zones. Cross stratal structures similar to the hydrothermally altered synvolcanic faults that are known to cut the semiconformable alteration may represent fluid flow paths from a subconcordant metal reservoir in the volcaniclastic unit to the sea floor, where massive sulfides were deposited.Semiconformable pervasively silicified zones, particularly those associated with Fe-Mg metasomatized and/or epidotized rocks, are significantly larger exploration targets than areas of proximal alteration, and indicate large-scale hydrothermal mass transfer. Zonation of silicification to Fe-Mg metasomatism laterally within the alteration may provide a vector toward the discordant Fe-Mg-enriched alteration zones that commonly underlie volcanic-associated massive sulfide deposits.

Fluid flow and stable isotopic alteration in rocks at elevated temperatures with applications to metamorphism

A simple model quantitatively predicts the shifts in stable isotope composition that result from fluid flow through rocks along a temperature gradient in flow systems where local fluid-rock isotope exchange equilibrium is attained. Equilibrium fluid flow along a temperature gradient is a potent mechanism for stable isotopic alteration in rocks: for example, a time-integrated fluid flux of 1 · 10 5 mol H 2O/ cm 2 flowing through a carbonate or quartzo-feldspathic rock at 600° C along a temperature gradient of +25°C/km will causeδ 18O to decrease by ≈5‰. Using the model, measured isotopic shifts in rocks can be quantitatively interpreted as records of time-integrated fluid flux and of flow direction relative to fossil temperature gradients in metamorphic terranes and other high-temperature, deep-crustal fluid flow systems. Inferred whole-rock 18O-depletions of 5–8‰ in contact and regional metamorphic terranes can be explained by flow of 2–50 · 10 4 mol/cm 2 aqueous fluids through rocks in the direction of increasing temperature if local mineral-fluid equilibrium is maintained. Stable isotope alteration in metamorphic rocks does not require infiltration of chemically exotic, non-equilibrium fluids, and therefore does not necessarily provide information about the source of infiltrating fluids.

Orthopyroxenes from granulite rocks of the Wilson Terrane (Victoria Land, Antarctica): crystal chemistry and cooling history

The compositions of 17 orthopyroxenes from granulites facies rocks of the Wilson Terrane (Ross Orogen), determined by microprobe analysis, are consistent with a metamorphic origin. Three groups may be distinguished : a) orthopyroxenes with high Al 2 O 3 content (2.85-5.33 weight%) and X Mg =0.46-0.60, from garnetcordierite granulites; b) orthopyroxenes with low Al 2 O 3 (less than 0.9 wt%) and X Mg =0.49-0.62, from enderbites; c) orthopyroxenes with very low Al 2 O 3 (less than 0.4 wt%) and X Mg =0.70, from two-pyroxene mafic granulites. These differences do not reflect different metamorphic conditions, but are due to local mineral assemblages. Zoning occurs in few orthopyroxenes from garnet-cordierite and mafic granulites, in both cases doe to a retrograde evolution. The crystal chemistry and site populations of two Al-rich and one Al-poor specimen were determined by X-ray single crystal diffraction. The most relevant structural differences are the decrease in M1 and M2 polyhedral dimensions, and an increase in TB, both induced by Al. The high degree of Fe 2+ -Mg order (k D between 0.017 and 0.023) is coherent with slow cooling. Closure temperatures of the ordering process are between 214 ° and 243 ° C, with cooling rates in the range expected for a regional metamorphic terrane

The effect of whole‐rock MnO content on the stability of garnet in pelitic schists during metamorphism

Garnet‐bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium‐ to high‐grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole‐rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O for a moderate range of MnO contents from a set of non‐linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K‐feldspar and H2O fluid. In the MnO‐free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with XMn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet‐bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole‐rock MnO content. The small MnO contents of normal pelitic schists (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.

Oxygen Isotope Zoning in Garnet

Oxygen isotope zoning was examined within garnet with the use of the stable isotope laser probe. Four metamorphic garnets from the regional metamorphic terrane in Vermont and the skarn deposit at Carr Fork, Utah, were examined and were found to be concentrically zoned in delta(18)O values. The largest variations in delta(18)O values were observed in the regional metamorphic garnets, where delta(18)O values change by 3 per mil from core to rim. These oxygen isotope zoning profiles reflect the changes in the delta(18)O values of the rocks during garnet growth, which are caused by infiltration of fluids and by dehydration reactions during metamorphism.

Evidence from mineral assemblages for infiltration of pelitic schists by aqueous fluids during metamorphism

Prograde mineral assemblages and compositions have been predicted for pelitic schist in the 10 component system Na2O−K2O−CaO−MnO−FeO−MgO−Al2O3−SiO2−CO2−H2O for three cases of prograde metamorphism and fluid-rock interaction: (1) increasing temperature (T) at constant pressure (P) and constant pore fluid volume (1%) without infiltration (no-infiltration case); (2) increasing T at constant P accompanied by sufficient fluid infiltration that fluid composition is at all times constant (large-flux case); and (3) increasing T at constantP accompanied by a timeintegrated fluid flux f 104 cm3 cm 2 (intermediate-flux case). Stable mineral assemblages and compositions were calculated by solving a system of non-linear equations that specify mass balance and chemical equilibrium between minerals and fluid. The model pelitic system includes quartz, muscovite, plagioclasc, chlorite, ankerite, siderite, biotite, garnet, staurolite, andalusite, kyanite, sillimanite, K-feldspar, and a coexisting, binary H2O−CO2 fluid. Specifically, prograde thermal metamorphism was modelled for Shaw's (1956) average low-grade pelite and for a moderate range of bulk rock compositions at P=3, 5, and 7 kb and initial fluids with Xco 2 o =0.02–0.40. The model predicts a carbonate-bearing mineral assemblage for average pelite under chlorite zone conditions composed of quartz, muscovite, albite, chlorite, ankerite, and siderite. The mineral assemblages predicted for the noinfiltration case are unlike those typically observed in regional metamorphic terranes. Simulations of metamorphism for the large-flux and intermediate-flux cases, however, reproduce the sequence of mineral assemblages observed in normal Barrovian regional metamorphic terranes. These results suggest that regional metamorphism of pelitic schists is typically associated with infiltration of significant quantities of aqueous fluid.

Hercynian low-pressure-high-temperature regional metamorphism and subhorizontal foliation development in the Canigou massif, Pyrenees, France—Evidence for crustal extension

A detailed study has been made of the structural and metamorphic history of part of the Hercynian low-pressure-high-temperature regional metamorphic terrane exposed in the Canigou massif, France. Microtextural relations between, and compositional zoning within, porphyroblast phases in pelitic rocks indicate that the pressure-temperature-time ( P-T-t )trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions (clockwise P-T-t trajectory). The geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation (S3). The S3 foliation formed in response to noncoaxial bulk strain. The absence of a marked hiatus in sedimentation in the Pyrenean region during Hercynian metamorphism and deformation and the widespread preservation of low-grade Hercynian metasedimentary rocks suggest that low- P -high- T metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top-to-the-northwest shear during D3.

Thermal Anomalies In A Regional Metamorphic Terrane: An Isotopic Study of the Role of Fluids

Thermal Anomalies In A Regional Metamorphic Terrane: An Isotopic Study of the Role of Fluids

The Role of Volatiles in the Thermal History of Metamorphic Terranes

Analytical and numerical solutions to the differential equations for the conduction of heat with heat production or with fluid flow have been used to evaluate the role of volatiles in the thermal history of regional metamorphic terranes. The maximum thermal effect from pervasive, single-pass, regional volatile flow may be predicted from a steady-state solution given by Bredehoeft & Papadopoulos (1965). For fluid velocity vF (m/s) and connected porosity Ψ, combinations of volatile flux vFΨ (m3 of fluid/m2s) and transport distance L(m) such that vΨL is greater than 3·6×10−7 should produce regional temperature increases due to fluid flow, if the flow persists for l05–106 a (depending on the transport distance L). The absolute value of the temperature increase due to volatile flow will be greater in regions with higher ambient geothermal gradients. For L=20 km, a volatile flux of 1·8 × 10−11 (m3 of fluid/m2s) or greater is required to achieve a temperature effect. Few geologic processes release volatiles at this rate for extended periods of time, so regional thermal effects from the single-pass, pervasive flow of volatiles are unlikely. A new analytical solution for the steady state temperature distribution between idealized parallel channels of fluid flow is presented along with the results of two-dimensional numerical models of channelized fluid flow. Both approaches show that little temperature increase is expected near channels of fluid flow relative to the rocks between the channels, unless the channels exceed 100 m in width or unless the fluid fluxes are very large and transient. A possible thermal effect of volatile flow in metamorphic terranes is the production of metamorphic hot spots due to focusing of volatiles into widely spaced channels or conduits exceeding 1 km in width. Given a sufficient fluid flux (exceeding 10−10 m3 of fluid/m2s), thermal gradients of over 100K from center to edge may be produced in such channels during relatively short time intervals (105–106a).

Ductile strain and metamorphism in an extensional tectonic setting: a case study from the northern Snake Range, Nevada, USA

Summary In recent years considerable attention has focused on metamorphic core complexes of the Basin and Range Province of the western US Cordillera. These highly extended areas are characterized by an upper plate that has been brittlely attenuated by normal faults separated by a sub-horizontal detachment surface from a lower plate that has been ductilely thinned and stretched. A study of mesoscopic structures, finite strain, microstructures, quartz c -axis fabrics and 40 Ar/ 39 Ar geochronology was undertaken in order to characterize the nature, geometry, kinematic history and timing of ductile extension in the lower plate of the northern Snake Range metamorphic core complex in E-central Nevada. These data provide new insights into the processes of deep-seated ductile strain beneath supracrustal normal fault mosaics in highly extended regions. Mesoscopic structures and finite strain analyses indicate that the lower plate underwent plane strain with a sub-vertical Z-axis and a sub-horizontal WNW-ESE X -axis. The magnitude of strain increases dramatically from W-to-E, and is constant vertically. Bedding and foliation are everywhere parallel and bedding is thinned 30–90%. The nature and geometry of microstructures and c -axis fabrics changes progressively from W-to-E. Lower strain rocks on the W flank of the range are characterized by a single bedding-parallel foliation, defined by flattened detrital grains, globular grains which have their c -axes parallel to the Z -axis and symmetrical cross-girdle c -axis fabrics. Further E to slightly higher strains, the rocks are characterized by asymmetrical cross-girdle c -axis fabrics, although globular grains with their c -axes parallel to Z are still present. The high-strain rocks on the E flank of the range are characterized by C -planes, asymmetrical mica ‘fish’ and oblique quartz foliations, and asymmetrical single-girdle c -axis fabrics. These data suggest that the lower plate has not deformed entirely by either coaxial or non-coaxial strain, and we propose an evolutionary model whereby the lower plate deformed during an early period of coaxial strain followed by a later component of non-coaxial strain accompanied by coaxial strain on the E flank of the range. 40 Ar/ 39 Ar geochronology on lower-plate hornblendes and micas indicates that temperatures of deformation were >280°C, but <530°C, and increased with depth. Geochronological data suggest that lower-plate ductile strain is post-latest Cretaceous, and biotite and muscovite data indicate that ductile deformation was probably ongoing 22–26 Ma. Our data and regional geological relations suggest that ductile extensional deformation in the northern Snake Range occurred because of localized heat input to shallow levels of the crust, and together with seismic reflection data, indicate that lower-plate rocks probably represent the top of a regional metamorphic terrane of Tertiary age.

Sphalerite geobarometry in metamorphic terranes: an appraisal with implications for metamorphic pressure in the Otago Schist

Abstract Sphalerite geobarometry has long been known to give poor results when applied to regional metamorphic terranes. Application of the sphalerite geobarometer to three low‐to medium‐grade sulphide deposits—the Moke Creek and Waitahuna deposits, Otago, and the Goose Cove deposit, Newfoundland—yields pressures up to 9 kbar, which appear to be too high when compared with other geological data. Textural and mineralogical relationships suggest that the Goose Cove and, possibly, the Moke Creek deposits lacked the required equilibrium assemblage (pyrite + hexagonal pyrrhotite + sphalerite) during peak metamorphic conditions, rendering the geobarometer inapplicable. In addition, all three deposits show evidence of re‐equilibration at T < 300°C, which has resulted in decreased FeS contents and high apparent pressures. Analyses of sphalerites from very low‐grade metachert from South Georgia Island, which contains the assemblage sphalerite + pyrite + monoclinic pyrrhotite + chalcopyrite, confirm that low‐temperature equilibration of this assemblage results in approximately 10–11 mol. % FeS in sphalerite. Comparison of these results with published descriptions of other deposits suggests that lack of the appropriate assemblage and retrograde re‐equilibration of sphalerite probably account for most anomalously high‐pressure estimates. Erratic compositions of sphalerites containing chalcopyrite inclusions may result from replacement of high‐temperature intermediate solid‐solution by chalcopyrite during cooling. Strain may enhance retrograde re‐equilibration of sphalerite by grain‐size reduction or dislocation‐assisted diffusion and/or nucleation. Re‐evaluation of the data from Moke Creek suggests that the sulphides experienced pervasive greenschist facies re‐equilibration at pressures of about 4.5 kbar, with late stage mobilization at about 2.8 kbar, and thus sphalerite compositions are not likely to reflect blueschist facies conditions. Pressure estimates based on sphalerite geobarometry should take into account at what stage in the history of a metamorphic terrane the sphalerite composition equilibrated.

Stable isotopic evidence for large-scale seawater infiltration in a regional metamorphic terrane; the Trois Seigneurs Massif, Pyrenees, France

Oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a Hercynian metamorphic sequence in the Trois Seigneurs Massif, Pyrenees, France, indicate that all lithologies at higher metamorphic grades than the “andalusite in” isograd have relatively homogeneous δ18O values. The extent of homogenization is shown by the similarity of δ18O values in metacarbonates, metapelites and granitic rocks (+11 to +13), and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies. These values contrast with the δ18O values of metapelites of lower metamorphic grade (δ18O about +15). Homogenization was caused by a pervasive influx of hydrous fluid. Mass-balance calculations imply that the fluid influx was so large that its source was probably high-level groundwaters or connate formation water. Hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at δD=−25 to −30, suggesting a seawater origin. Many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6–12 km in the presence of this water-rich fluid, the composition of which was externally buffered. Deep penetration of surface waters in such environments has been hitherto unrecognized, and may be a key factor in promoting major anatexis of the continental crust at shallow depth. Three types of granitoid are exposed in the area. The leucogranites and the biotite granite-quartz diorite are both mainly derived from fusion of local Paleozoic pelitic metasediment, because all these rocks have similar whole-rock δ18O values (+11 to +13). The post-metamorphic biotite granodiorite has a distinctly different δ18O (+9.5 to +10.0) and was probably derived from a deeper level in the crust. Rare mafic xenoliths within the deeper parts of the biotite granite-quartz diorite also have different δ18O (+8.0 to +8.5) and possibly represent input of mantle derived magma, which may have provided a heat source for the metamorphism.