Douglas K. Tinkham

The effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic P-T paths

Abstract P-T pseudosections, constructed in MnNCKFMASH and adjusted for chemical compositional changes resulting from zoned garnet growth (chemical fractionation) in a pelitic rock, show negligible changes in the position of the peak metamorphic mineral assemblage field (garnet + biotite + plagioclase + sillimanite + quartz) compared to the position of this field calculated with the bulk-rock composition. Pelitic rock samples with less than 5% modal garnet were modeled using bulk-rock chemical compositions (unfractionated), and compositions adjusted for 1, 2, and 5% garnet growth, in order to model the effects of changes in effective composition on pseudosection topology. Differences in the location of mineral mode zero lines along the garnet growth P-T path and in the peak mineral assemblage field are generally less than 10 °C and/or less than 0.3 kbar. However, at some P-T conditions, significant changes in topology are observed. For example, at pressures above 9 kbar, large temperature shifts in the zoisite mode zero line change the pseudosection topology so that biotite+zoisite stability in the pseudosection with 5% garnet fractionation has a larger temperature range (>120 °C) than in the unfractionated pseudosection (<50 °C). The effects of porphyroblast growth-induced fractionation of bulk-rock chemistry can be determined from mineral chemistry and mineral modes; pseudosections can be constructed with the adjusted chemical compositions to resolve whether fractionation affects the pseudosection topology in the P-T range of interest. In the case of the North Cascades samples discussed here, garnet fractionation is estimated to have minimal effects on P-T paths determined from pseudosections. Therefore, pseudosection modeling based on bulk-rock chemistry can be used to estimate peak metamorphic P-T conditions and constrain parts of metamorphic P-T-t paths once the effects of fractionating minerals are understood.

Integration of phase equilibria modelling and garnet Sm-Nd chronology for construction of P-T-t paths: examples from the Cordilleran Coast Plutonic Complex, USA

Abstract Integration of petrographic observations, mineral chemistry, garnet Sm-Nd isochrons, and MnNaCaKFMASH pseudosection phase equilibria models constructed with THERMOCALC provides quantitative metamorphic pressure-temperature-time (P-T-t) paths which allow determination of assemblage/reaction history for pelites. Examples are presented for the Cretaceous to Tertiary magmatic arc of the North American Coast Plutonic Complex. Metamorphism in the western Coast Plutonic Complex of southeastern Alaska and in the North Cascades of Washington resulted from at least three widespread events from > 100 Ma to c. 60 Ma, and in both areas partly resulted from crustal thickening, evidenced by local occurrences of kyanite after andalusite. Pressure-temperature pseudosections constructed from bulk rock compositions and the intersection of garnet core composition isopleths provide estimates for the pressure and temperature of garnet core growth. Intersections of these isopleths indicate garnet growth 18 to 85 °C above the predicted garnet-in reaction temperatures. Rim and near-rim garnet compositions and matrix mineral chemistry provide estimates for near-peak metamorphic conditions. Finite pressure-temperature-time paths of garnet zone metamorphism were determined from the combined core growth and pressure-temperature conditions determined from near-rim garnet and matrix mineral compositions. The western Coast Plutonic Complex near the Stikine River, southeastern Alaska, displays a complex pattern of regional metamorphism overprinted by contact metamorphic aureoles. Many of the c. 90 Ma aureoles contain andalusite, andalusite plus sillimanite, or andalusite plus kyanite with complex replacement textures. A pseudosection constructed for a contact metamorphic rock on Kadin Island (95GL11c), predicts that garnet grew c. 555 ± 10 °C and 4.8 ± 0.7 kbar, above the garnet-in line and the aluminium silicate triple-point pressure. These results suggest that andalusite in samples from this aureole likely grew prior to garnet and that the pressure may have increased by ≤ 1 kbar during metamorphism. The southern part of the North Cascades in Washington also contain complex aluminium silicate replacement textures with early andalusite and later kyanite and sillimanite. A sample (96NC67), collected near the andalusite-bearing aureole of the Mt Stuart batholith, contains sillimanite and c. 10 mm garnet crystals containing staurolite inclusions in their cores. Temperatures estimated from the garnet core of this sample are within the pseudosection staurolite stability field, compatible with initial garnet growth significantly above the garnetin line. The garnet rim thermometry estimate of c. 668 ± 59 °C for this sample is c. 85 °C higher than the core growth temperature. The calculated P-T-t path provides important information for interpreting regional and contact metamorphism. An extensive region NE of the Mt Stuart batholith in the North Cascades underwent a significant pressure increase; however, the timing and nature of medium- to high-pressure metamorphism is controversial. Quantitative P-T-t paths constructed for garnet growth along the NE margin of the batholith indicate that 87-85 Ma garnet growth was younger than the nearby Mt Stuart batholith (93.5 ± 1.4 Ma, U-Pb zircon). Garnet core and rim segments are isochronous indicating a short interval for garnet growth. P-T-t paths indicate that garnet growth occurred in the sillimanite stability field during a maximum pressure increase of 1 to 2 kbar, after rocks passed through the andalusite stability field (Mt Stuart contact metamorphism). Careful sampling, hand-picking, acid leaching, and isotopic analysis of garnet provide geologically consistent ages with uncertainties of ≤ 1.0 Ma. Thermodynamic modelling in the MnNaCaKFMASH system provide reasonable P-T predictions for pelite mineral stability that can be integrated with isotope ages to provide quantitative P-T-t paths. The P-T-t paths developed for both regional and contact metamorphic rocks allow critical evaluation of tectonic models and of interpretations for mineral textures.

XRMapAnal: A program for analysis of quantitative X-ray maps

Abstract Software for processing quantitative X-ray maps collected on an electron probe microanalyzer is described. The techniques used in collection of maps and capabilities of the software to produce data useful for petrologic analysis are illustrated using three examples from pelitic schists from Mica Creek, British Columbia. The program utilizes a Bence-Albee algorithm to convert raw X-ray counts at each map pixel to wt% oxide, and allows for spectrometer deadtime corrections and background subtraction. Additional features include phase identification, modal analysis, calculation of cations, calculation of integrated rock composition over specified regions of the map, display of maps and compositional graphs, output of compositional statistics, and the ability to remove pixels and screen for bad analyses. The first example utilizes a sample lacking phases with significant chemical zonation, and outlines the basic functionality of the program. The second example compares an integrated map composition to an X-ray fluorescence analysis on the same rock chip. The results indicate it is possible to obtain integrated map compositions approaching that obtained from an X-ray fluorescence analysis. The third example compares mean X-ray map compositions for individual phases to means obtained using routine point analysis procedures, and the results indicate mean phase compositions obtained from the X-ray maps are of generally high quality. The sample in this example contains compositionally zoned garnet, and is used to show the ability to remove selective portions of garnet to model the approximate change in effective bulk composition during garnet growth.

Low-pressure and high-temperature metamorphism of basalts: insights from the Sudbury impact melt sheet aureole and thermodynamic modelling

Editor: Donna Whitney Abstract Low‐pressure and high‐temperature (LP–HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high‐ grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least ~925°C at ~1–3 kbar near the SIC contact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase–orthopyroxene–clinopyroxene–ilmenite‐magnetite±melt assemblage. Peak temperatures reached ~875°C up to 500 m from the SIC lower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts ~500 to 750 m from the SIC contact are characterized by a similar two‐pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino‐ and orthopyroxene along an isobaric cooling path. Metabasalts ~750 to 1,000 m from the SIC contact are characterized by a hornblende–plagioclase–quartz–ilmenite assemblage indicating temperatures up to ~680°C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10–20% melt generation in the inner ~500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high‐Ca amphibole and silicate melt at relatively low pressures (~1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes at LP–HT conditions. Received: 21 March 2018 | Accepted: 14 October 2018 DOI: 10.1111/jmg.12460