First comprehensive characterization of osumilite from India ( Eastern Ghats Province): Physicochemical characteristics, stability of the mineral and its breakdown products

Journal Pre-proof First report of osumilite from India (Eastern Ghats Province): Physicochemical characteristics, stability of the mineral and its breakdown products

Enakshi Das, Shreya Karmakar, Somdipta Chatterjee, Subrata Karmakar, Pulak Sengupta PII:

S0024-4937(19)30475-X

DOI:

https://doi.org/10.1016/j.lithos.2019.105315

Reference:

LITHOS 105315

To appear in:

LITHOS

Received date:

11 September 2019

Revised date:

16 November 2019

Accepted date:

1 December 2019

Please cite this article as: E. Das, S. Karmakar, S. Chatterjee, et al., First report of osumilite from India (Eastern Ghats Province): Physicochemical characteristics, stability of the mineral and its breakdown products, LITHOS(2019), https://doi.org/10.1016/ j.lithos.2019.105315

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Journal Pre-proof First report of osumilite from India (Eastern Ghats Province): Physicochemical characteristics, stability of the mineral and its breakdown products Enakshi Das1,2*, Shreya Karmakar1, Somdipta Chatterjee1, Subrata Karmakar1, Pulak Sengupta1 1

Department of Geological Sciences, Jadavpur University, Kolkata 700032, India.

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Department of Geology, Shahid Matangini Hazra Government College for Women,

Kulberia 721649, India

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Corresponding author: Enakshi Das

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*[email protected]

Abstract

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A rare occurrence of osumilite has been reported from a metapelite from parts of the Eastern

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Ghats Province, India. Incidentally, this is the first report of osumilite along with its

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composition and textural description from any Indian rock. The presence of osumilite has been confirmed by electron microscopy (BSE images), electron microprobe analyses (and Xray intensity maps) as well as Raman spectroscopy. In the studied sample, osumilite occurs as rare anhedral relicts (~300-350 μm) that are surrounded by a variety of extremely finegrained symplectic intergrowths namely cordierite + K-feldspar, cordierite + K-feldspar + orthopyroxene and cordierite + K-feldspar + orthopyroxene + quartz. The symplectites partially to completely pseudomorph the early formed osumilite grains. An isochemical pressure-temperature phase diagram has been calculated in the Na2O-K2O-FeO-MgO-Al2 O3SiO2-H2O system in order to model the breakdown of osumilite and formation of such symplectitic mineral assemblages, using the average micro bulk compositions calculated with

Journal Pre-proof the XMapTools software. The calculated P-T phase diagram suggests that osumilite was formed during ultra-high temperature metamorphism (at P~8.5 kbar and T~1110C) in an Mg-Al-rich pelitic assemblage and subsequently broke down to produce the symplectitic mineral assemblages during isobaric cooling.

Keywords Osumilite, UHT Mg-Al granulites, K-feldspar+ cordierite+ orthopyroxene+ quartz

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intergrowth, Eastern Ghats Province

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1. Introduction:

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Osumilite is a rare silicate mineral having a general formula

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(K,Na)(Fe2+,Mg)2(Al,Fe3+)3(Si,Al)12O30 and belongs to the Milarite group (Miyashiro, 1956). This mineral is commonly found in volcanic rocks (Miyashiro, 1956) and also in shallow

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(Chinner and Dixon 1973) and deep seated (Berg and Wheeler, 1976) contact metamorphic

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aureoles and regionally metamorphosed ultra-high temperature (UHT) pelites (Adjerid et al.,

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2013; Arima and Gower, 1991; Blereau et al., 2019; Ellis et al., 1980; Grew, 1982; Harley, 2008; Holder et al., 2018; Kawasaki et al., 2011; Sajeev and Osanai, 2004). Osumilite is a key indicator of ultra-high temperature (UHT) metamorphism, but is rarely preserved because of its decomposition (to cordierite + K-feldspar + orthopyroxene ± quartz) during cooling with or without enhancement of the activity of H2O (Lal et al., 1987; Bhattacharya and Kar, 2002; Adjerid et al., 2013; Korhonen et al., 2013; Bial et al., 2015; Kelsey and Hand, 2015 and reference therein). In this communication, we present the compositional characteristics of osumilite (~300-350µm) and its relations with other minerals in an Mg-Al rich metapelitic assemblage from part of the Eastern Ghats Province (Dobmeier and Raith, 2003). Integrating the compositional characteristics of minerals and the textures it has been demonstrated that

Journal Pre-proof the exotic osumilite-bearing assemblages developed in the realm of UHT metamorphism. A textural-modelling study and the computed phase diagram suggest breakdown of osumilite (into a fine symplectite of orthopyroxene + K-feldspar + cordierite + quartz) in a closed system during a phase of isobaric cooling.

2. Geological Framework: This osumilite bearing assemblage belongs to an Mg-Al-rich metapelitic rock, which consists

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of vividly heterogeneous mineralogical assemblages in different microdomains. One such domain is characterized by the presence of fine symplectitic intergrowths of cordierite + K-

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feldspar ± orthopyroxene ± quartz with or without the presence of relict osumulite grains. The

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rock-sample was collected from Bandavidi village (N 18°03.296'/ E 82°33.434'), which is

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situated south of Paderu (Fig.1). Geologically, Bandavidi is situated within Eastern Ghats Province (EGP, after Dobmeier and Raith, 2003), India. Several studies have reported UHT

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metamorphism, recorded in Mg-Al granulites from different parts of the EGP (reviewed in

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Dasgupta et al., 2013; Bhattacharya and Kar, 2002; Bose et al., 2006, 2000, Das et al., 2017,

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2011; Kelsey et al., 2017; Korhonen et al., 2013; Lal et al., 1987; Mitchell et al., 2018; Mohan et al., 1997; Sengupta et al., 1991, 1990, 1997). However, the presence of fine symplectitic intergrowth of K-feldspar+ cordierite+ orthopyroxene± quartz, presumed to be a breakdown product of osumilite, has been reported in only a few studies from the EGP near our study area (Lal et al., 1987; Bhattacharya and Kar, 2002; Korhonen et al., 2013). Unaltered osumilite grains along with its composition have not yet been reported from the EGP or any part of India to date.

Journal Pre-proof 3. Mesoscopic features of the rock: The dominant lithology of the studied area comprises migmatitic felsic orthogneisses. The Mg-Al metapelitic granulites occur as extremely rare and small (millimeter to centimeter scale) patches or lenses adjacent to quartzose layers within the migmatitic gneiss (Fig. 2a). The Mg-Al-rich patches or pockets have a bluish tinge, where large (millimeter to centimeter scale) orthopyroxene porphyroblasts are discernible (Fig. 2a-b).

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4. Microscopic features of the rock:

Under the microscope the Mg-Al granulites show millimetre thick alternate bands rich in

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quartz and ferro-magnesian minerals ± feldspar (FMM, Fig. 2a, 3a). Megacrystic

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orthopyroxene (Opxpeak) is discernible in some of the FMM patches/layers (Fig. 2b). The

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FMM domains show heterogeneity on the basis of distinctive mineralogy, and hence have been divided into several micro domains:

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(i) osumilite and symplectitic orthopyroxene + cordierite + K-feldspar + quartz ± silimanite;

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(ii) Opxpeak and symplectitic sapphirine + quartz + cordierite + sillimanite + orthopyroxene;

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(iii) coarse lamellar intergrowths of corundum + hematite + ilmenite + spinel + magnetite and coronitic sillimanite ± orthopyroxene± garnet + quartz ± feldspar (plagioclase + K-feldspar). In this study, the focus is only the micro-domains that contain osumilite and a variety of symplectitic assemblages, formed after osumilite. No Opxpeak is present in these domains, only the fine symplectitic orthopyroxene. Thus, in the following description orthopyroxene will refer to fine-grained symplectitic orthopyroxene. Rare relicts of osumilite occur as anhedral grains ~300-350 µm in size (Fig.3b-c). Under transmitted light, osumilite is colourless with a low refractive index (lower than feldspar) and shows 1st order yellow to grey interference colours, making it almost impossible to distinguish from coarse quartz grains (Fig.3b-c). Osumilite can be distinguished only in

Journal Pre-proof back scattered electron images (BSE, Fig.3d) and X-ray intensity maps (K and Al, Fig.3e-f). The relict osumilite is surrounded by a variety of extremely fine-grained (lamellar width  2 m) symplectic intergrowths of K-feldspar + cordierite (KC), K-feldspar + cordierite + quartz (KCQ), orthopyroxene + cordierite + quartz (OCQ) and K-feldspar + cordierite + orthopyroxene + quartz (KCOQ; Fig.3d-f). The contact between the osumilite and the ultrafine symplectite is curved and convex towards the osumulite (Fig.3c, e-f). Sometimes, osumilite is absent and these fine intergrowths are bounded as well as separated from the

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quartz grains of alternating bands by a unique grain boundary (Fig.3g-j). The grain size or

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lamellar thickness of the symplectites varies from <0.5 m to ~2 m, with this variation occuring not only from one microdomain to another (Fig. 3e-f vs. 3g-h), but also within the

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same microdomain (Fig. 3i-j). The relative modal proportion of the constituent phases also

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vary within the same symplectitic domain, from one point to another (Fig. 3e-j). Often ultra fine symplectites of cordierite + quartz (CQ, lamellar width  0.5 m) and fine

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orthopyroxene + sillimanite (OS, lamellar width ~ 2m) is present around the k-feldspar

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bearing symplectites with or without osumilite (Fig.3e-j).

5. Identification of osumilite:

Presence of osumilite has been identified by (a) optical characteristics that distinguishes the mineral from quartz and/ or cordierite. (b) Raman spectroscopy and (c) in-situ composition obtained with an EPMA. Details of the last two procedures are presented in Analytical Techniques (appendix-1 and 2). The spectral peaks obtained from the Raman spectrum shown in Fig. 3k matches exactly with the standard peaks (appendix-1; Fig. 3k) that are characteristic for osumilite.

Journal Pre-proof 6. Compositional features:

6.1. Phase chemistry Salient compositional features of minerals in the studied domains are presented in Tables -1 and 2. All abbreviations of mineral names in data tables as well as in figures have been used after Siivola and Schmid (2007). Analytical details are given in appendix-2. Osumilite grains are compositionally homogeneous being highly magnesian (XMg=

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0.88-0.90; Table-1), with negligible Na and Ca content (Na~0.1 apfu, Ca<0.1 apfu). The crystal chemical formula of osumilite can be written as CM2(T2)3(T1)12O30 and has four

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distinct sites for cations as described in detail in Grew (1982). The tetrahedral T1 site is

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predominantly occupied by Si and Al. The tetrahedral T2 site is occupied by Al and a minor

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amount of Mg, Mn, Fe+2 and often Fe+3 (if present). The octahedral M site is occupied by Mg, Fe+2, Mn and sometimes Ti (if present). Lastly, the 12 co-ordinated cage site is occupied by

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large alkali ions like K, Na, Ca (Arima and Gower, 1991; Armbruster and Oberhaesnsli,

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1988; Grew, 1982). Measured osumilite compositions plot along the [(Fe+Mg+Mn) in M site + Si in T1 site]↔[Al (T1+T2 site)] substitution vector, close to the end member

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KMg2Al2.75Si10.25Al1.75O30 (point B, Fig. 4a), with a slight spread towards the end member KMg2Al2.5Si10.5 Al1.5O30 (Fig.4a, shown as point C) synthesized by Olesch and Seifert (1981). Thus, partial Al substitution has taken place in osumilite, approaching theoretical endmember of KMg2Al3Si10Al2O30 (Fig.4a, shown as point A), synthesized by Schreyer and Seifert (1967). A plot of total cations on the C site versus all cations present on the M site shows that all the compositions tend to be ideal and the C site has its full occupancy which is near 1 (varies from 0.99-1.05 as shown in Fig.4b). The composition of osumilite from this study is similar to the compositions of other naturally occurring osumilites (Fig. 4) (Adjerid et al., 2013; Arima and Gower, 1991; Ellis et al., 1980; Grew, 1982).

Journal Pre-proof Orthopyroxene (symplectitic) is overall highly aluminous and magnesian (XMg= 0.70.75; Table1). The Al2O3 content of orthopyroxene in this domain ranges from 9.46-6.67 wt% (MgTs component =0.20-0.15). The core of coarse orthopyroxene (Opxpeak ; Fig.3a) is much more aluminous (Al2O3=10.3 wt%, MgTs component=0.23; Table1). Sillimanite contains slight amount of Fe2O3 (~0.6 wt%; Table1). Cordierite composition in this domain is fairly homogeneous and is highly magnesian (XMg = 0.86-0.93; Table2) with low volatile content as indicated by their high

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totals (>98 wt%).

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6.2. Calculated bulk chemistry

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K-feldspar is mostly orthoclase rich with composition Or0.68-78Ab0.32-0.22 An0 (Table2).

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Osumilite in the studied domains is partially to completely replaced by fine-grained symplectitic intergrowths of K-feldspar + cordierite + orthopyroxene ± quartz (Fig.3e-j).

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Also, the relative abundance of the constituent phases vary locally, within the same

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symplectitic domain, as is seen in the variation in the K-intensity maps (Fig. 3e, g, i). The

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bulk composition of the symplectite domains were estimated using X-ray intensity maps of the relevant major elements Si, Al, Mg, Fe, K and Na of several osumilite present and absent symplectite domains, that were produced using EPMA. The elemental distribution maps for each domain were integrated using the software XMapTools (Lanari et al., 2014), to produce phase distribution maps (Fig.5). The process of quantification of X-ray intensity maps of different elements in order to get bulk composition of a specific micro-domain is described elaborately in the user manual of XMapTools and as well as in Lanari et al. (2014). Bulk compositions were calculated in two different modes: that of the entire symplectite domain (squares in Fig. 6; Table2), and also of several micro-domains from each symplectitic pocket based on the variation in the K-intensity maps (triangles in Fig. 6), and plotted in ternary

Journal Pre-proof chemographic projections in the SiO2-Al2O3-[FeO+MgO] and Al2O3-[K2O+Na2O][FeO+MgO] systems. Fig. 6 indicates the following: (i) the calculated bulk compositions of each of the symplectitic domains (squares in Fig. 6) plot close to the measured composition of the relict osumilites, supporting the interpretation that the fine symplectites replaced osumilite. (ii) the calculated bulk compositions of the different micro-domains from the same symplectitic pocket (triangles in Fig. 6) show a slightly wider spread, but are restricted within

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the polygons defined by K-feldspar + cordierite + orthopyroxene ± quartz (Fig. 6; Table-2). This reflects the heterogeneous distribution of constituent phases in the pseudomorphs, as has

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also been documented from other reported occurrences (Fig. 5 of Adjerid et al., 2013; Ellis et

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7. Modelling osumilite breakdown:

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al., 1980; Grew, 1982).

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7.1. Textural modelling: a mass balance approach Textural modelling study is a powerful tool to identify mass balanced reaction among a set of

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minerals ( Fisher, 1989; Karmakar et al., 2017; Lang et al., 2004). Furthermore, this study also helps to identify if a given reaction operated in an open or closed system. Details of the textural modelling process can be found in Karmakar et al. (2017). The osumilite breakdown reaction textures have been modelled using the measured composition of the phases with the computer program C-Space (Torres-Roldan et al., 2000) to obtain the following balanced chemical reactions: 1.74 osumilite = 3.30 K-feldspar + 1.00 cordierite + 1.31 orthopyroxene

(1)

1.91 osumilite = 3.25 K-feldspar + 1.27 cordierite + 1.38 orthopyroxene + 1.00 quartz (2)

Journal Pre-proof As the above reactions could be balanced without considering any mobile components, it follows that the decomposition of osumilite to form the symplctitic K-feldspar + cordierite + orthopyroxene (reaction 1) and K-feldspar + cordierite + orthopyroxene + quartz (reaction 2) occurred in a closed system. The heterogeneous distribution of the phases within the symplectites is probably controlled by contrasting mobility of the different elements. Similar heterogeneity in phase proportion of symplectites from closed system breakdown of natural

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osumilite has been also reported from Hoggar, Algeria (Figure 5 of Adjerid et al., 2013).

7.2. P-T modelling

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Phase equilibria modelling (pseudosection approach) using the effective average bulk

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composition of the symplectite domains (Fig. 3c-f) has been done to estimate the stability of

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osumilite and the post-peak evolution of the symplectitic assemblages. As has been described previously, the studied textural microdomains alternate with quartz. Accordingly, an effective

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bulk composition was obtained by adding ~6-7 volume% quartz to the bulk composition of

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the symplectitic domains calculated with XMapTools. The pseudosection was calculated

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using PERPLE_X 6.8.0 version (Connolly, 2009, 2005) with the thermodynamic dataset of Kelsey et al. (2004), as no model for osumilite is available in recent thermodynamic dataset of Holland and Powell (2011) or White et al. (2014). The following solution models were used: Holland and Powell (1998) for garnet and spinel, Holland and Powell (1996) for orthopyroxene, Powell and Holland (1999) for biotite, Fuhrman and Lindsley (1988) for feldspar, ideal solid solution for hydrous and anhydrous cordierite (hCrd), Kelsey et al. (2004) for sapphirine, Holland and Powell (2001)- White et al (2001) for melt and DQF corrected version of Holland et al. (1996) as used by Korhonen et al. (2013) for osumilite. The pseudosection has been calculated in system Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O (NKFMASH; Fig.7a) as no Ca-Ti-Fe+3 bearing major phases are present in the studied

Journal Pre-proof domains. Studies have shown that the stability of cordierite-bearing assemblages is extremely sensitive to the presence of fluid in the cordierite channel (Bhattacharya and Sen,1985; Carrington, 1995; Mitchell et al., 2018). The sum totals of the cordierite analyses suggest the presence of very low amount of fluid (<2 wt%, Table-1). This small amount of fluid can still exert some influence on the stability of the osumilite, as hydrous fluid is preferentially partitioned into cordierite relative to osumilite (Adjerid et al., 2013; Bhattacharya and Sen, 1985). Hence, H2O is included as a system component. The computed P-T phase diagram in

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the NKFMASH system is contoured for volume% of K-feldspar and MgTs component (AlM1) of orthopyroxene (Fig.7a). Fig. 7a indicates that osumilite has tempareture stability

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restricted to ~910-1140C and is limited to pressure below ~11 kbar. As cordierite and K-

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feldspar in the studied domains occur as breakdown products of osumilite, the cordierite and

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K-feldspar-absent field of “Opx-Osu-Qtz-Melt” is considered to represent the peak metamorphic conditions (Fig. 7a). It has been mentioned earlier under section-4, that

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orthopyroxene (Opxpeak) is present in the peak assemblage of the rock though not in this

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particular domain. The Al content in this porphyroblastic orthopyroxene (Opxpeak, Al-M1

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~0.23) gives temperature of ~1050C from the Al-in orthopyroxene thermometer (Harley, 2004; Harley and Motoyoshi, 2000). This isopleth passes through the stability field of “OpxOsu-Qtz-Melt” at ~1110C (Fig. 7a), and thus corroborates that this field represents the peak metamorphic conditions. Peak pressure of metamorphism cannot be deduced directly from this assemblage due to lack of suitable mineral assemblages, but is considered to be in the range of ~8.5 kbar because: (i) similar pressures have been constrained for the stability of osumilite (at ~950-1000C) in rocks having very similar mineral assemblages by Adjerid et al. (2013); and (ii) Korhonen et al. (2013) deduced peak pressure to be ~8-8.5 kbar from MgAl rocks in a nearby area. Fig. 7a and b further indicate that with near isobaric cooling, osumilite disappears and the volume proportion of K-feldspar, cordierite, quartz and

Journal Pre-proof orthopyroxene increase (Fig.7b). The isopleths for relatively low Al contents in symplectitic orthopyroxene (Al-MI~0.2-0.15; Fig.7a) pass through the fields of “Opx-Crd-Kfs-Qtz” and “Opx-Osu-Crd-Kfs-Qtz” at temperature of ~850-950C (Fig. 7a). The calculated volume proportion of the product phases from pseudosection matches well with the observed modal distribution of the intergrown phases. Fig. 7b further indicates that with cooling below ~850C, cordierite eventually breaks down to give way to sillimanite + orthopyroxene, explaining the formation of the orthopyroxene-sillimanite intergrowths which are found very

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often as in Fig.3j.

8. Discussion and Conclusion:

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Fine symplectitic intergrowths of K-feldspar + cordierite + orthopyroxene ± quartz, presumed

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to be a breakdown product of osumilite, have been reported from different places in the central Eastern Ghats Province, mainly from Paderu and its adjoining areas (Bhattacharya

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and Kar, 2002; Korhonen et al., 2013; Lal et al., 1987). This study reports for the first time

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the direct presence and composition of osumilite, from not only the Eastern Ghats Belt but

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also India, wherein lies the importance of this study. Furthermore, detailed compositional and textural modelling (using XMapTools and C-Space respectively) indicates closed system breakdown of osumilite. The analyzed relict osumilite grain, as well as the calculated bulk compositions of the symplectites that pseudomorph osumilite are found to be highly magnesian. This could be the reason that the rock is devoid of garnet or any other ferroan minerals and rather stabilizes highly magnesian and aluminous orthopyroxene upon the breakdown of osumilite. The experimental results of phase stability (in KFMASH P-T grid of Das et al., 2003,2001) in high magnesian bulk composition of Das et al. (2001) are quite similar to our studied natural assemblage.The composition of relict osumilite suggest it is approaching to the experimentally deduced composition of Schreyer and Seifert (1967). It is

Journal Pre-proof described as the characteristic composition that may form under granulite facies condition rather than in contact aureoles (Grew, 1982). Isochemical pressure-temperature phase diagrams modelled in the NKFMASH system (Fig. 7a) using the effective bulk compositions calculated with XMapTools indicate that osumilite was stable at a temperature >950°C and a pressure of ~8.5 kbar. Pseudosection modelling further indicates that the complete breakdown of osumilite to produce K-feldspar + cordierite + orthopyroxene ± quartz occurred during isobaric cooling to ~850-950C from peak (~1110°C) UHT conditions (Fig.

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7a, b). The inferred near isobaric cooling path has also been documented from other assemblages reported earlier from adjoining areas (Korhonen et al., 2013). The preservation

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of extremely rare relict osumilite in only some areas of the studied microdomain possibly

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results from significant overstepping of the reaction due to microdomainal variation in the

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presence of hydrous fluid. Thus, the characteristic breakdown product of osumilite (K-

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Acknowledgements

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feldspar + cordierite + orthopyroxene + quartz) is much more abundant than unaltered one.

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ED acknowledges state government fellowship scheme and RUSA 2.0 of Jadavpur University for financial support. SK (1st) acknowledges the University Grants Commission (UGC). SC acknowledges the Council for Scientific and Industrial Research (CSIR). SK (2 nd) and PS acknowledge the grants received from the programs awarded to the Department of Geological Sciences, Jadavpur University: Potential for Excellence (UPE-Phase II) and Center of Advance Studies (CAS Phase VI) from UGC (University Grants Commission); Fund for the Improvement of Science and Technology (FIST-Phase II) and RUSA 2.0 from DST (Department of Science and Technology, India). We sincerely acknowledge the co-operation of Prof. James Connolly and Dr. Fawna Korhonen for helping out with scientific corrections in the solution model of osumilite. We thank Dr. Kathryn Cutts and the anonymous reviewer

Journal Pre-proof for their constructive suggestions that helped us to improve the clarity of this manuscript significantly. We also thank Prof. Marcos Scambelluri for his editorial works.

Appendix 1. For the identification of osumilite, analyses were done using Laser Raman Spectroscopy at Central Petrological Laboratories, Geological Survey of India, Kolkata. Analytical

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environment for the analyses were setup to 785 nm edge diode laser (gratings: 1200 lines/nm), having a ~1.2 µm spot beam diameter, with focus energy in between 15-18 mW.

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Accumulation time for the Raman spectra varies from 10-50s with a room temperature

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maintained at 22 ± 1 °C. The peak positions of the Raman Spectra were determined by the

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Wire program (v. 3.4). The peak of analysed sample matched with the standard peak of library index-641 from Inorgan.lib used by Geological Survey of India, Kolkata.

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2. Major elemental analyses were performed to identify the different mineral compositions

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and to construct elemental X-ray intensity maps of different micro domains. These analyses

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were done using a CAMECA SX5 microprobe at the Central Research Facility of the Indian Institute of Technology (Indian School of Mines), Dhanbad, India. For analysis 15kV accelerating voltage and 15 nA beam current, with a beam diameter of ~1-2 µm was used for all the points. Well characterized, natural and synthetic compounds are used as standards to calibrate the instruments for the analyses.

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Harley, S.L., 2008. Refining the P-T records of UHT crustal metamorphism. J. Metamorph. Geol. 26, 125–154. https://doi.org/10.1111/j.1525-1314.2008.00765.x

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Harley, S.L., 2004. Extending our understanding of Ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158. https://doi.org/10.2465/jmps.99.140 Harley, S.L., Motoyoshi, Y., 2000. Al zoning in orthopyroxene in a sapphirine quartzite: Evidence for >1120 °C UHT metamorphism in the Napier Complex, Antarctica, and implications for the entropy of sapphirine. Contrib. to Mineral. Petrol. 138, 293–307. https://doi.org/10.1007/s004100050564 Holder, R.M., Hacker, B.R., Horton, F., Rakotondrazafy, A.F.M., 2018. Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-temperature orogen. J. Metamorph. Geol. 36, 855–880. https://doi.org/10.1111/jmg.12316 Holland, T., Powell, R., 1996. Thermodynamics of order-disorder in minerals; II, Symmetric formalism applied to solid solutions. Am. Mineral. 81, 1425–1437. Holland, T.I.M., Powell, R., 2001. Calculation of phase relations involving haplogranitic melts using an internally consistent thermodynamic dataset. J. Petrol. 42, 673–683. Holland, T.J.B., Babu, E.V.S.S.K., Waters, D.J., 1996. Phase relations of osumilite and dehydration melting in pelitic rocks: A simple thermodynamic model for the KFMASH system. Contrib. to Mineral. Petrol. 124, 383–394. https://doi.org/10.1007/s004100050198 Holland, T.J.B., Powell, R., 2011. An improved and extended internally consistent thermodynamic

Journal Pre-proof dataset for phases of petrological interest, involving a new equation of state for solids. J. Metamorph. Geol. 29, 333–383. Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic data set for phases of petrological interest. J. Metamorph. Geol. 16, 309–343. Karmakar, S., Mukherjee, S., Sanyal, S., Sengupta, P., 2017. Origin of peraluminous minerals (corundum, spinel, and sapphirine) in a highly calcic anorthosite from the Sittampundi Layered Complex, Tamil Nadu, India. Contrib. to Mineral. Petrol. 172. https://doi.org/10.1007/s00410017-1383-8 Kawasaki, T., Nakano, N., Osanai, Y., 2011. Osumilite and a spinel + quartz association in garnetsillimanite gneiss from Rundvågshetta, Lützow-Holm Complex, East Antarctica. Gondwana Res. 19, 430–445. https://doi.org/10.1016/j.gr.2010.07.008

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Kelsey, D.E., White, R.W., Holland, T.J.B., Powell, R., 2004. Calculated phase equilibria in K2O‐FeO‐ MgO‐Al2O3‐SiO2‐H2O for sapphirine‐quartz‐bearing mineral assemblages. J. Metamorph. Geol. 22, 559–578.

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Journal Pre-proof Olesch, M., Seifert, F., 1981. The Restricted Stability of Osumilite Under Hydrous Conditions in the System K20 - MgO - Al203 - SiO2 - H20. Contrib. to Mineral. Petrol. 76, 362–367. https://doi.org/10.1007/s00410-011-0686-4 Powell, R., Holland, T., 1999. Relating formulations of the thermodynamics of mineral solid solutions; activity modeling of pyroxenes, amphiboles, and micas. Am. Mineral. 84, 1–14. Sajeev, K., Osanai, Y., 2004. Osumilite and spinel+quartz from Sri Lanka: Implications for UHT conditions and retrograde P-T path. J. Mineral. Petrol. Sci. 99, 320–327. https://doi.org/10.2465/jmps.99.320 Schreyer, W., Seifert, F., 1967. Metastability of an osumilite end member in the system K2O-MgOAl2O3-SiO2-H2O and its possible bearing on the rarity of natural osumilites. Contrib. to Mineral. Petrol. 14, 343–358. https://doi.org/10.1007/BF00373812

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Sengupta, P., Dasgupta, S., Bhattacharya, P.K., Fukuoka, M., Chakraborti, S., Bhowmick, S., 1990. Petro-tectonic imprints in the sapphirine granulites from anantagiri, Eastern Ghats mobile belt, India. J. Petrol. 31, 971–996. https://doi.org/10.1093/petrology/31.5.971

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Sengupta, P., Karmakar, S., Dasgupta, S., Fukuoka, M., 1991. Petrology of spinel granulites from Araku, Eastern Ghats, India, and a petrogenetic grid for sapphirine-free rocks in the system FMAS. J. Metamorph. Geol. 9, 451–459.

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Sengupta, P., Dasgupta, S., Ehl, J., Raith, M.M., 1997. Thermobaric evolution of a suite of Mg-Al granulites from Paderu: further evidence for a ACW PT path in the Eastern Ghats Belt, India. Eur. J. Mineral. 9, 331. Siivola, J., Schmid, R., 2007. List of Mineral abbreviations. IUGS Subcomm. Syst. Metamorph. Rocks 1–14.

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Torres-Roldan, R.L., Garcia-Casco, A., Garcia-Sanchez, P.A., 2000. CSpace: An integrated workplace for the graphical and algebraic analysis of phase assemblages on 32-bit wintel platforms. Comput. Geosci. 26, 779–793. https://doi.org/10.1016/S0098-3004(00)00006-6

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White, R.W., Powell, R., Holland, T.J.B., 2001. Calculation of partial melting equilibria in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH). J. Metamorph. Geol. 19, 139–153. White, R.W., Powell, R., Holland, T.J.B., Johnson, T.E., Green, E.C.R., 2014. New mineral activitycomposition relations for thermodynamic calculations in metapelitic systems. J. Metamorph. Geol. 32, 261–286. https://doi.org/10.1111/jmg.12071

Figure Captions

Fig. 1. Simplified geological map of the Eastern Ghats Belt (EGB) showing the different crustal provinces based on different geological evolution after Dobmeier and Raith (2003). Study area- Bandavidi is shown in the map.

Journal Pre-proof Fig. 2. Photographs showing relations between quartzose band and Mg-Al patches of migmatitic gneiss. (a, b) Layers or patches of pyroxene bearing blue Mg-Al pelites (marked using yellow dotted lines) within quartzose rock (marked using red dotted lines). Orthopyroxene porphyroblasts are marked with red arrows. Fig. 3. Photomicrographs, Back Scattered Electron (BSE) images and X-ray intensity maps of elements showing textural relations. (a) PPL image showing recrystallized coarse quartzose layers alternating with porphyroblastic orthopyroxene (Opxpeak) and discrete pockets of ferro-

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magnesian minerals (FMM). (b-f) PPL image (b), CPL image (c), BSE image (d), K-intensity map (e) and Mg-intensity map (f) of the same microdomain, showing osumilite (low

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refractive index, colorless under PPL, first order grey interference colour under CPL) is

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surrounded by a mat of very fine symplectitic intergrowths of k-feldspar + cordierite (KC)

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and k-feldspar + cordierite + quartz (KCQ) which are separated from adjacent coarse quartz by discrete patches of orthopyroxene + cordierite (OC) or k-feldspar + cordierite +

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orthopyroxene + cordierite (KCOQ). Fine symplectites of cordierite + quartz (CQ) are also

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present. CPL image of (b) reveals the corrugated grain boundary of osumilite. The BSE

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image (b) and X-ray intensity maps (e, f) show that the fine grained symplectitic intergrowths are bounded by a unique boundary and separated from adjoining coarse elongated quartz ribbon. (g, h) X-ray intensity map of K (g) and Mg (h) showing another discrete patch of symplectites of k-feldspar + cordierite + quartz (KCQ), orthopyroxene + cordierite (OC) and cordierite + quartz (CQ), bounded by a unique boundary and separated from coarse matrix quartz. The symplectites in this domain are slightly coarser than in the previous domain (b-f). (i, j) X-ray intensity map of K (i) and Mg (j) showing similar types of symplectitic intergrowth of in different pocket like g and h. Additionally some patches of symplectitic intergrowth of orthopyroxene + sillimanite (OS) are present.

Journal Pre-proof (k) Different peaks (wave numbers) which are characteristic for identification of phase in Raman spectrum are shown in red for osumilite in studied sample, and in blue for the standard. Fig. 4. Compositional variation of osumilite. (a) Al in T1+T2 site plotted versus all bivalent cations that could occupy M1 site of osumilite (calculated in 30 oxygen basis). Line refers to ideal substitution of Al in T2 + T1 = M + Si in T1. A, B and C refer to the composition of synthetically derived osumilite by Schreyer and Seifert (1967) and Olesch and Seifert (1981)

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f

respectively. Osumilite composition of the studied assemblage lie along a linear array near end member B. (b) Total cation in the C1 site versus all cations that could occupy the M site

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in osumilite (calculated in 30 oxygen basis) is plotted. Line is for ideal osumilite with full

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occupancy in C1 site.

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Fig.5. Merged map or mineral distribution map calculated using XMapTools (Lanari et al., 2014) showing distribution of different phases in the symplectitic domains. (a) showing phase

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distribution of Fig.3i-j.

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distribution of Fig. 3b-f. (b) showing phase distribution of Fig. 3g-h. (c) showing phase

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Fig. 6. Ternary projections in the systems SiO2-Al2 O3-[FeO+MgO] and Al2O3-[K2O+Na2O][FeO+MgO] showing the co-existing phases and their reaction relationships with each other. Fig. 7. a) Isochemical P-T pseudosection, calculated in the NKFMASH system. The pseudosection is contoured with isopleths of orthopyroxene [MgTs component] and Kfeldspar [vol%]. Arrow indicates the inferred P-T path. (b) Variation in modes of phases along the inferred P-T path during isobaric cooling (at ~8.5 kbar) from 1100C to 825C.

Journal Pre-proof Table1: Representative oxide analyses and calculated cations of osumilite, orthopyroxene and sillimanite Phase

Osu

Osu

Osu

Osu

Osu

Opx

Opx

Opx

Sil

Point

1/1.

65 / 1 .

66 / 1 .

68 / 1 .

69 / 1 .*

2/1.

11 / 1 *.

27 / 1 .

19 / 1 .

Texture

Core

Core

Rim

Rim

Rim

Peak-Core

Symp

Symp

Symp

61.94

61.35

61.58

61.98

61.8

48.36

49.54

51.75

36.85

0.06

0.06

0.04

0.05

0.04

0.16

0.16

0.14

b.d.l

Al2O3

22.72

22.58

22.64

22.54

22.76

10.3

9.46

6.67

61.94

Cr2O3

b.d.l

b.d.l

0.01

0.01

0.02

0.01

0.02

0.02

b.d.l

Fe2O3

-

-

-

-

-

-

-

-

0.59

FeO

1.82

1.63

1.78

1.93

1.75

18.52

16.38

15.53

b.d.l

MnO

b.d.l

0.11

b.d.l

b.d.l

0.01

0.04

0.03

0.18

0.01

MgO

8.07

8.08

8.36

8.14

8.13

21.97

24.74

26.03

0.05

CaO

b.d.l

0.04

0.01

b.d.l

0.05

b.d.l

0.04

0.02

b.d.l

Na2O

0.41

0.31

0.37

0.34

0.34

0.05

b.d.l

b.d.l

b.d.l

K2 O

4.17

4.37

4.38

4.4

0.03

0.03

0.03

b.d.l

Total

98.74

99.2

99.46

98.69

99.29

99.62

100.42

100.34

99.43

Si

10.26

10.30

10.26

10.25

10.26

1.77

1.78

1.85

1.00

Ti

0.01

0.01

0.00

0.01

0.00

0.00

0.00

0.00

-

Al

4.46

4.42

4.42

-

-

0.00

-

-

-

0.25

0.23

0.25

Mn

-

0.02

Mg

2.00

2.00

Ca

-

0.01

Na

0.13

0.10

K

0.89

Total Oxygen XMg

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pr

Pr 4.44

4.45

0.45

0.40

0.29

1.98

0.00

0.00

0.00

0.00

0.00

0.00

-

-

-

-

-

0.01

0.27

0.57

0.49

0.47

-

al

Fe

0.24

-

-

0.00

0.00

0.00

0.01

0.00

2.06

2.03

2.01

1.20

1.33

1.39

0.00

0.00

-

0.01

-

0.00

0.00

-

0.12

0.11

0.11

0.00

-

-

-

0.93

0.93

0.94

0.91

0.00

0.00

0.00

-

18.00

18.00

18.01

18.00

18.01

4.00

4.01

4.00

3.00

30.00

30.00

30.00

30.00

30.00

6.00

6.00

6.00

5.00

0.89

0.90

0.89

0.88

0.89

0.68

0.73

0.75

rn

Fe

+3

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Cr

4.31

f

TiO2

e-

SiO2

-

Note: Fe+3 recalculated on the basis of stochiometry; 'b.d.l' implies below detection limit; X Mg =Mg/Mg+Fe+2; * Analyses used for reaction modelling.

Table2: Representative oxide analyses and calculated cations of cordierite, k-feldspar and calculated bulk composition of pseudomorph after osumilite Phase

Crd

Crd

Kfs

Kfs

Pseud.

Pseud.

Pseud.

Point

38 / 1 . *

29 / 1 .

9/1.*

5/1.

1

2

3

Symp

Symp

Symp

Symp

Bulk

Bulk

Bulk

Texture SiO2

50.66

50.17

65.7

66.22

63.06

60.73

62.40

TiO2

0.01

0.00

0.04

0.02

-

-

-

33.89

33.17

18.83

18.57

22.92

21.47

21.97

Cr2O3

0.01

0.01

b.d.l

0.02

-

-

-

Fe2O3

0.00

0.97

-

-

-

-

-

FeO

3.24

1.58

b.d.l

0.09

2.83

4.20

3.94

MnO

0.11

0.00

b.d.l

0.08

-

-

-

MgO

11.50

12.41

b.d.l

b.d.l

8.67

4.01

5.33

CaO

0.02

0.00

0.03

0.05

-

-

-

Na2O

0.03

0.07

2.25

3.52

0.50

2.67

1.94

K2 O

0.02

0.03

12.43

11.55

2.02

6.92

4.43

Total

99.49

98.42

99.19

100.12

100.00

100.00

100.00

Si

5.04

5.02

3.01

3.01

10.30

10.36

10.42

Ti

0.00

0.00

0.00

0.00

-

-

-

Al

3.97

3.91

1.02

0.99

4.41

4.32

4.33

0.00

0.00

-

0.00

-

-

-

0.00

0.07

-

-

Fe

0.27

0.13

-

Mn

0.01

0.00

-

Mg

1.70

1.85

-

Ca

0.00

0.00

0.00

0.00

-

-

-

Na

0.01

0.01

0.20

0.31

0.16

0.88

0.63

K

0.00

0.00

0.73

0.67

0.42

1.50

0.94

Total

11.00

11.00

4.95

4.99

17.79

18.68

18.20

Oxygen

18.02

18.00

8.00

8.00

30.00

30.00

30.00

XMg

0.86

0.93

-

-

0.85

0.63

0.71

XOr

-

-

0.78

0.68

-

-

-

XAb

-

-

0.22

0.32

-

-

-

XAn

-

-

0.00

0.00

-

-

-

oo

pr

-

-

-

0.00

0.39

0.60

0.55

0.00

-

-

-

-

2.11

1.02

1.33

e-

al

rn

Fe

+3

Jo u

Cr

f

Al2O3

Pr

Journal Pre-proof

Note: Fe+3 recalculated on the basis of stochiometry; 'b.d.l' implies below detection limit; X Mg =Mg/Mg+Fe+2; * Analyses used for reaction modelling.

Journal Pre-proof Highlights

rn

al

Pr

e-

pr

oo

f

First report of osumilite from India (Eastern Ghats Province). Osumilite occurs in Mg-Al rich UHT metamorphic rocks. Textural and compositional modelling implies close system breakdown of osumilite. The breakdown forms very fine-grained symplectites of Kfs + Crd + Opx ± Qtz. The symplectites formed due to isobaric cooling from ~1100°C at 8.5 kbar.

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1. 2. 3. 4. 5.

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 4

Figure 5

Figure 6

Figure 7