Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U–Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U–Pb apatite ages are 2.4–2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz–biotite–plagioclase–garnet–staurolite–kyanite in the pelitic schists; (b) quartz–biotite–plagioclase–low-Ca amphiboles–kyanite in Mg–Al-rich schists, and locally (c) hornblende–plagioclase–garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg–Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from >7 kbar to <3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78–1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.     

Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan

Abstract The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc. In Baltistan, this suture has been reactivated by the late 'break-back'Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low- P (2–4 kbar, c. 500° C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent south-verging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8–9-kbar rocks over 5–6-kbar rocks). There is little variation in temperature with structural level (550–625° C). These two different terranes have been juxtaposed after metamorphism by the late south-directed MKT.     

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

A series of ten localized, high-temperature, granulite faciesregions occurs within the regional metamorphic, sillimanitezone of New Hampshire, USA. These regions, or ‘hot spots’,measure 10–30km² in a sillimanite zone that extends 50kmeast-west and over 150km north-south. The hot spots are characterizedby an abrupt increase in temperature over a distance of a fewkilometers, from 550?C in the sillimanite zone to 700?C in thegranulite facies. Mineral assemblages in pelites change rapidlyover the same distance from sillimanite-staurolite-muscovite-garnet-biotite-quartzto sillimanite-K feldspar-cordierite-garnet-biotite-quartz inthe granulite facies. The hot spot located near Bristol, New Hampshire was chosenfor detailed study because it has a network of quartz-graphiteveins in its core. Oxygen isotope analyses of minerals and rocksfrom Bristol suggest a close approach to pervasive exchangeequilibration between disparate rock types throughout the sillimanitezone. A comparison between chlorite zone pelites and calc-silicategranofelses of equivalent stratigraphic age shows a range ofover 7 in ¹⁸O of whole rocks from lower grade, whereas sillimanitezone rocks from Bristol vary by no more than 3, only. Quartzseparated from the Bristol high-grade metasediments varies byno more than 2?3. Superimposed upon the regional, sillimanite-zone oxygen isotopehomogenization is a 1–2 km wide region in the core ofthe hot spot in which the ¹⁸O of metasediments has been furtherhomogenized. Both quartz from quartz-graphite veins and quartzfrom cordierite-bearing wall rocks have exchanged oxygen isotopes;all quartz has ¹⁸O=13?8?0?5 (SMOW). Radiometric ages of quartz-graphite veins and metasedimentarywall rocks show that the hydrothermal event and metamorphismin the hot spot were both of Acadian age. Hydrothermal overgrow-thson zircons from one of the veins give ages of 409 ? 6 Ma. Monazitefrom cordierite bearing pelites from the wall of the same veingives a U-Pb age of 392 ? 3 Ma, using conventional techniques. The coincidence of: (1) an isograd high; (2) an isotherm hotspot; (3) a network of quartz-graphite veins; and (4) an oxygenisotope alteration halo suggests a relationship between hydrothermalactivity and heating of metasediments. The geochronologicalevidence demonstrates that the hydrothermal event and hot spotmetamorphism were not greatly separated in time. It is proposedthat the hot spots represent loci where hot metamorphic fluidswere focused through a fracture system now recorded by quartz-graphiteveins.     

The dynamics of the suture between the Kohistan island arc and the Indian plate in the Himalaya of Pakistan

ABSTRACT The pressure-temperature and temperature-time paths derived for rocks in the Kohistan arc and adjacent Nanga Parbat-Haramosh massif record the dynamics of the collision between the island arc and the Indian plate. Studies of P-T-t paths show that the Kohistan arc was thrust over the Nanga Parbat-Haramosh massif at least 25 Ma ago, but not more than 30–35 Ma ago. Rocks in the Kohistan arc followed decreasing pressure paths, with the early metamorphism beginning at high pressures (9.5 kbar) and later metamorphism occurring at 8.0 kbar. In contrast, rocks in the Nanga Parbat-Haramosh massif (Indian plate) experienced increasing pressure and temperature paths. Prior to thrusting, the massif was at low pressures (4.0 kbar) and low temperatures (450°c). Later, the pressure and temperature increased to 8 kbar and 580°c. The authors interpret the convergence (to approximately the same pressure and temperature) of the P-T paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets. ⁴⁰Ar/³⁹Ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30–35 Ma ago. Temperature-time paths show that after thrusting, during the period 25–10 Ma, the Kohistan arc and Nanga Parbat-Haramosh massif were uplifted at similar rates (0.5 km Ma). However, in the past 10 Ma the Nanga Parbat-Haramosh massif has been uplifted more rapidly than the adjacent Kohistan arc. Rapid uplift has been accommodated by late faults along the edge of the massif.     

Petrochemistry, oxygen isotopes and U-Pb SHRIMP geochronology of mafic–ultramafic bodies from the Sulu UHP terrane, China

Two Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ¹⁸O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ¹⁸O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ～700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.     

Mapping Submerged Aquatic Vegetation with GIS in the Caloosahatchee Estuary: Evaluation of Different Interpolation Methods

This article evaluates different spatial interpolation methods for mapping submerged aquatic vegetation (SAV) in the Caloosahatchee Estuary, Florida. Data used for interpolation were collected by the Submersed Aquatic Vegetation Early Warning System (SAVEWS). The system consists of hydro-acoustic equipment, which operates from a slow-moving boat and records bottom depth, seagrass height, and seagrass density. This information is coupled with geographic location coordinates from a Global Positioning System (GPS) and stored together in digital files, representing SAV status at points along transect lines. Adequate spatial interpolation is needed to present the SAV information, including density, height, and water depth, as spatially continuous data for mapping and for comparison between seasons and years. Interpolation methods examined in this study include ordinary kriging with five different semivariance models combined with a variable number of neighboring points, the inverse distance weighted (IDW) method with different parameters, and the triangulated irregular network (TIN) method with linear and quintic options. Interpolation results were compared with survey data at selected calibration transects to examine the suitability of different interpolation methods. Suitability was quantified by the determination coefficient (R2) and the root-mean-square error (RMSE) between interpolated and observed values. The most suitable interpolation method was identified as the one yielding the highest R2 value and/or the lowest RMSE value. For different geographic conditions, seasons, and SAV parameters, different interpolation methods were recommended. This study identified that kriging was more suitable than the IDW or TIN method for spatial interpolation of all SAV parameters measured. It also suggested that transect data with irregular spatial distribution patterns such as SAV parameters are sensitive to interpolation methods. An inappropriate interpolation method such as TIN can lead to erroneous spatial representation of the SAV status. With a functional geographic system and adequate computing power, the evaluation and selection of interpolation methods can be automated and quantitative, leading to a more efficient and accurate decision.     

Evidence for the Repeated Folding of Isotherms during Regional Metamorphism

Petrologic and structural investigations in a polydeformed terranein southwestern New Hampshire show that more than one stageof noncoaxial folding has significantly affected the metamorphichistory of this region. Detailed examinations of both isograd-isothermpatterns and mineral reaction histories within pelitic rockssuggest that pre-existing, nearly horizontal isotherms werefolded during an early stage of folding about north-south axes.After folding, thermal relaxation resulted in cooling in theanticlinal portions of the early folds and heating in the synclinalregions. This process of isotherm folding and re-equilibrationwas repeated during a later stage of east-west folding, resultingin complex isograd patterns and mineral reaction histories. Detailed mapping of isograd and isotherms in the pelitic rocksof this region shows a complex patten. High grade and high temperatureassemblages occur along early and late synclinal axes, withthe highest grade assemblages occurring at the intersectionof the two synclinal axes. Conversely, the lowest grade assemblagesare found at the intersection of the early and late anticlinalfolds. Four different types of fold intersections result from the twostages of noncoaxial folding. Pelitic rocks at each of thesefour different fold intersections show different and contrastingmineral reaction histories. At the intersections of early andlate synclines the rocks show evidence for continuous heating,while continuous cooling trends are seen at intersections ofearly and late anticlines. Complex reaction histories are observedat the intersections of early synclines and late anticlinesand early anticlines and late synclines, which show heating-coolingand cooling-heating trends, respectively. These results show that over small areas of a metamorphic terranedifferent samples may show widely different P-T paths. Therefore,without careful structural analysis and thorough sampling, tectonicinterpretations based solely on paths determined from a fewwidely scattered samples may lead to erroneous conclusions.