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Pelites (shales and mudstones) are arguably the most important rock type for interpreting metamorphism. Their significance derives from their widespread occurrence and the range of mineral assemblages they develop at different conditions of pressure and temperature. We compiled a global database of 5729 major-element whole-rock analyses of pelites from different metamorphic grades (shales to granulite-facies paragneisses) to (1) determine an average composition, (2) examine the range and variability in their composition, and (3) assess if there is evidence for grade-related geochemical changes. Median values are given instead of average values to eliminate the effect of extremes. The median worldwide pelite is as follows (anhydrous, values in wt%): SiO2 = 64.13, TiO2 = 0.91, Al2O3 = 19.63, FeOtotal = 6.85, MnO = 0.08, MgO = 2.41, CaO = 0.65, Na2O = 1.38, and K2O = 3.95. The median XMg = MgO/(MgO + FeOtotal) in moles is 0.39. The median XFe3+ = 2 Fe2O3/(2 x Fe2O3 + FeO) in moles was measured in 1964 samples and is 0.23. On an Al2O3-FeO-MgO (AFM) diagram, the median worldwide pelite plots within a strong clustering of analyses between XMgproj = projected molar MgO/(MgO + FeOtotal) = 0.30--0.55 (median = 0.42) and AMs = molar [Al2O3 -- (3 x K2O)]/[Al2O3 -- (3 x K2O) + FeOtotal + MgO] = 0.0--0.4 (median = 0.19). Pelites show a continuous decrease in volatile content with increasing metamorphic grade and a decrease in XFe3+ from the diagenetic to biotite zone. Lower median SiO2 values and higher median Al2O3 and AMs values in the porphyroblast and subsolidus sillimanite or K-feldspar zones, as well as higher median MnO values in the garnet zone, may reflect sampling bias or metasomatism. [ABSTRACT FROM AUTHOR]

The Kohistan island arc represents a >50 km thick and completely exposed Late Cretaceous–Eocene Neo‐Tethyan island arc sequence between the Indian and Eurasian plates in the western Himalaya. At the base of the Kohistan Arc, the Jijal Complex exposes a continuous sequence from ultramafic upper mantle rocks to mafic rocks of the lower crust across the Moho transition zone. In the shallower mafic section of the Jijal Complex, widespread garnet–clinopyroxene gabbros are uniform in assemblage but show a spectrum of microstructural variation involving two distinguishable styles of garnet growth. Uncertainty surrounds the genetic origin of these equilibria across the sequence which have been variably proposed to derive either from (a) dehydration melting of amphibole‐bearing precursor rocks, (b) subsolidus recrystallization of gabbronorite, or (c) primary magmatic crystallization. Owing to extensive high‐T re‐equilibration, compositional records of magmatic precursors and the pre‐peak evolution across the complex have been erased. However, quantitative textural analysis evidences a progressively increasing departure from microstructural randomness from deep to shallow levels of the upper Jijal Complex. This is interpreted to reflect the increased propensity for shallow garnet–clinopyroxene gabbros to have been derived from solid‐state recrystallization of a cumulate gabbronorite precursor, resulting in the development of coronitic garnet microstructures. In the deepest portions of the mafic sequence, both cumulate textures and thermodynamic modelling attest to the basal levels of the Kohistan arc crust at ~10–14 kbar crystallizing primary magmatic garnet and clinopyroxene from low H2O (<6 wt%) basaltic–andesitic melts. Granulite facies metamorphism and re‐equilibration across the whole upper Jijal Complex—shown via phase equilibrium modelling to be associated with over‐accretion in the mid‐arc crust and a pressure increase of 2–3 kbar to 850–1,050℃ and 11–14 kbar—was followed by a period of approximately isobaric cooling across ~200℃ and amphibolitization. Compositions of low‐volume leucosome‐hosted garnet indicate melting occurred prior to, or during, this re‐equilibration period and melt reintegration simulations suggest that dehydration melting of amphibole‐bearing precursors played only a minor role in the genesis of the Jijal garnet–clinopyroxene gabbros. By combining field relationships, petrological and textural analysis, interpretations of mineral compositions, and phase equilibrium modelling, we therefore provide detailed constraints on crustal thickening and the thermal evolution of the lowermost arc crust. [ABSTRACT FROM AUTHOR]

Fe3+ and XFe3+, defined as Fe3+/(Fe2+ + Fe3+) on a molar basis, are now recognised as key parameters in phase equilibrium modelling. A hindrance is that it is only possible to routinely measure total Fe, and not Fe3+ and Fe2+, in minerals using the electron microprobe. Charge balance techniques can be used to estimate Fe3+ and Fe2+ for some minerals, but not for those that contain vacancies. Whilst other analytical techniques can determine XFe3+ in minerals, these are not commonly applied by metamorphic petrologists. Therefore, researchers must rely on estimates. This study collates wet chemical, Mössbauer spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy analyses of XFe3+ in metapelitic minerals and rocks from the literature. The resulting database of 77 studies contains 591 samples, of which 261 have XFe3+ determined for the whole rock. There are XFe3+ measurements for 483 biotites, 192 white micas, 78 chlorites, and 32 staurolites. Average (± 1σ) XFe3+ values in whole rock, biotite, white mica, chlorite, and staurolite are 0.23 ± 0.16, 0.11 ± 0.08, 0.55 ± 0.18, 0.08 ± 0.07, and 0.07 ± 0.06, respectively. The average (± 1σ) number of Fe3+ cations in biotite, white mica, chlorite, and staurolite is 0.28 ± 0.19 (22 O + Ti cations per formula unit, pfu), 0.17 ± 0.13 (22 O pfu), 0.31 ± 0.27 (28 O pfu), and 0.20 ± 0.17 (46 O pfu), respectively. The mean whole rock XFe3+ is similar for metapelites containing ilmenite and magnetite, as well as those that report no Fe-oxide, but is considerably higher for hematite-bearing rocks. Whilst there is little variation with pressure and temperature, there is an increase in the number of Fe3+ cations and XFe3+ of both white mica and biotite with the type of Fe-oxide present. Our observations are compared with the predictions of phase equilibrium modelling using thermodynamic dataset 6.2 (Holland and Powell, J Metamorph Geol 29:333–383, 2011) and the solution models of White et al. (J Metamorph Geol 32:261–286, 2014a) for Fe3+ and XFe3+ in these minerals. The predicted XFe3+ and number of Fe3+ cations in biotite, chlorite, and staurolite broadly match natural observations, but for white mica the predicted mean XFe3+ is underestimated by 0.2–0.4 and the number of Fe3+ cations by 0.05–0.2. Whilst modelling correctly predicted increases in the XFe3+ of white mica and biotite with whole rock XFe3+, it also predicted variations in mineral XFe3+ as a function of pressure and temperature which are not observed in the natural samples. [ABSTRACT FROM AUTHOR]

Recently published activity–composition (a–x) relations for minerals in upper amphibolite‐ and granulite facies intermediate and basic rocks have expanded our ability to interpret the petrological evolution of these important components of the lower continental crust. If such petrological modelling is to be reliable, the abundances and compositions of phases calculated at the interpreted conditions of metamorphic equilibration should resemble those in the sample under study. Here, petrological modelling was applied to six granulite facies rocks that formed in different tectonic environments and reached different peak metamorphic pressure–temperature (P–T) conditions. While phase assemblages matching those observed in each sample can generally be calculated at P–T conditions that approximate those of peak metamorphism, a consistent discrepancy was found between the calculated and observed compositions of amphibole and clinopyroxene. In amphibole, Si, Ca and A‐site K are underestimated by the model, while Al and A‐site Na are overestimated; comparatively, in clinopyroxene, Mg and Si are generally underestimated, while Fe2+ and Al are typically overestimated, compared to observed values. One consequence is a reversal in the Fe–Mg distribution coefficient (KD) between amphibole and clinopyroxene compared to observations. Some of these mismatches are attributed to the incorrect partitioning of elements between the predicted amphibole and clinopyroxene compositions; however, other discrepancies are the result of the incorrect prediction of major substitution vectors in amphibole and clinopyroxene. These compositional irregularities affect mineral modal abundance estimates and in turn the position and size (in P–T space) of mineral assemblage fields, the effect becoming progressively more marked as the modal abundance of hornblende increases; hence, this study carries implications for estimating P–T conditions of high‐temperature metabasites using these new a–x relations. [ABSTRACT FROM AUTHOR]