P–T evolution of a spinel + quartz bearing khondalite from the Highland Complex, Sri Lanka: Implications for non-UHT metamorphism

doi:10.1016/j.jseaes.2014.05.003

Journal of Asian Earth Sciences

Volume 95, 1 December 2014, Pages 99-113

https://doi.org/10.1016/j.jseaes.2014.05.003 Get rights and content

Highlights

•
We present petrology of a Zn rich-spinel + quartz bearing khondalite.
•
Pseudosection and geothermobarometry show the rock has not reached UHT metamorphism.
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Addition of Zn from exotic fluid into spinel at high oxygen fugacity is discussed.

Abstract

Here, we report a natural field example for the coexistence of spinel + quartz as a non-UHT assemblage in spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (khondalite) interbedded with orthopyroxene-garnet-biotite bearing intermediate granulites from the Highland Complex (HC) in Sri Lanka. The khondalite contains Zn-rich spinel mainly in four textural assemblages namely: (a) spinel co-existing with tiny quartz (ZnO = 12.67–12.85 wt%), (b) spinel surrounded by sillimanite moates and in intergrowth with skeletal sillimanites (ZnO = 9.03–9.17 wt%), (c) symplectitic spinels at the margin of sillimanite (ZnO = 4.09–4.28 wt%) and (d) spinel co-existing with ilmenite or as isolated grains (ZnO = 7.61–7.97 wt% and Cr₂O₃ = 5.99–6.27 wt%). Assemblage (a) and (b) occur within garnet while assemblages of (c) and (d) are present within cordierite moates after garnet in the matrix. Pseudosections calculated in the NCKFMASHTMnO system and conventional geothermobarometry suggest that the metamorphic peak conditions attained by the spinel + quartz bearing khondalites and associated intermediate granulites did not exceed T of 900 °C and P of 7.5–8.5 kbar. Post-peak evolution was characterized by a stage of nearly-isobaric cooling down to T of 770 °C and P of 7.5 kbar, followed by a late stage of isothermal decompression down to P < 6.5 kbar and T of 770 °C.

We propose that the incorporation of large amount of Zn into spinel from exotic, metasomatic fluids and possibly incorporation of Fe³⁺ into spinel under high oxidizing conditions may have shifted the stabilization of co-existing spinel + quartz to T < 900 °C. Hence, this study provides insights into the occurrence of spinel + quartz as a non- UHT assemblage suggesting that the coexistence of spinel + quartz should be treated with care and considered only as indicative, but not diagnostic of UHT metamorphism.

Introduction

Aluminium and magnesium rich granulites are essential to understand high-grade metamorphism, because they commonly preserve peak or near-peak mineral phases, including sapphirine, aluminous orthopyroxene, sillimanite, garnet, spinel or corundum, and display a range of post-peak reactions, which permit to unravel their P–T evolution (Droop and Bucher-Nurminen, 1984, Waters, 1986, Hensen, 1987, Droop, 1989, Su et al., 2012a). The mineral assemblage spinel + quartz has been documented from many well-known ultra-high-temperature (UHT) terrains across the globe and often used to infer extreme crustal UHT metamorphic conditions (e.g. Kawakami and Motoyoshi, 2004, Morimoto et al., 2004, Sajeev and Osanai, 2004b, Barbosa et al., 2006, Santosh et al., 2006, Tsunogae et al., 2008, Kawasaki et al., 2011, Zhang et al., 2012).

Despite the fact that spinel + quartz assemblage is reported from numerous UHT granulites, the influence of ZnO on the stability of spinel in the P–T space is still a matter of debate (Kelsey, 2008, Kawasaki et al., 2011). Many experimental studies have suggested that the stability of spinel + quartz assemblages may be shifted to lower temperatures and relatively higher pressures through the incorporation of Zn into spinel (e.g. Shultere and Bohlen, 1988, Nichols et al., 1992, Dasgupta et al., 1995, Das et al., 2001, Das et al., 2003). Further, incorporation of minor elements such as Cr, Ti, Ni, V (Harley and Hensen, 1990, Waters, 1991, Nichols et al., 1992, Dasgupta et al., 1995, Harley, 1998, Harley, 2008, Das et al., 2001, Kelsey, 2008) or Fe³⁺ ions under oxidizing conditions (Hensen, 1986, Dasgupta et al., 1995) may strongly influence the stability field of spinel + quartz to lower temperatures.

This study was carried out in the Highland Complex (HC) of Sri Lanka where evidences of UHT metamorphism have been reported from several localities in the central part and rarely in the southwestern part (Fig. 1), from pelitic, mafic and quartzofeldspathic granulites (Osanai, 1989, Kriegsman and Schumacher, 1999, Osanai et al., 2000, Osanai et al., 2003, Osanai et al., 2006, Sajeev and Osanai, 2004a, Sajeev and Osanai, 2004b, Sajeev et al., 2003, Sajeev et al., 2007, Sajeev et al., 2010).The reason for local occurrences of UHT granulites in the HC is still under debate. In this paper, we report P–T evolution of spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (khondalite) from the HC. Further, we discuss the stability field of co-existing Zn-rich spinel and quartz within porphyroblastic garnets of the studied khondalite.

Section snippets

Geological setting

Based on Nd-model age determinations and zircon geochronology Proterozoic basement of Sri Lanka has been subdivided in to four units (Milisenda et al., 1988, Milisenda et al., 1994, Kröner et al., 1991; Cooray, 1994, see Fig. 1): the Wanni Complex (WC), the Kadugannawa Complex (KC), the Vijayan Complex (VC), and the Highland Complex (HC).

The WC consists of amphibolite to granulite facies meta-igneous and minor meta-sediments rocks displaying Nd-model ages of 1.1–1.8 Ga (Milisenda et al., 1988,

Sample description and field relations

We collected spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneisses (khondalite) and interbedded intermediate granulites exposed in an excavated embankment for construction close to the city of Horana (Fig. 1). This area lies within the southwestern part of the HC and is mainly composed of cordierite-bearing metapelites, wollastonite-bearing calcsilicates, massive charnockites, and charnockitic pegmatites (Cooray, 1965, Hapuarachchi, 1968). In our sampling area, quartzites,

Khondalite

The rock contains two petrographic domains: (i) medium- to coarse-grained garnet-bearing (1–8 cm in diameter) domain; (ii) fine- to medium-grained garnet-bearing (0.25–1 cm) domain.

Whole rock chemistry

Major and trace element compositions of the khondalite and intermediate granulites are taken from XRF analysis, performed using a Panalytical Axios wave-length dispersive XRF spectrometer (WDXRF, 2.4 kV) at the ETH Zurich, Switzerland. The obtained major and trace element data are presented in the Table 1.

Both khondalite and intermediate granulites show high Si (up to 57.2 wt% and 56.8 wt%, respectively). Khondalite is Al-richer than interbedded intermediate granulite (20.2 wt% vs. 14.8 wt%). X_Mg of

Khondalite

Mineral assemblage(s) preserved as inclusions in large porphyroblasts, commonly garnet, may preserve at least part of the prograde evolution of a rock. In the studied khondalite, spinel + quartz inclusions within garnet completely surrounded by alkali-feldspar (Fig. 4a and b), together with Ti-rich biotite and rare sillimanite inclusions within alkali-feldspar grains located next to spinel may suggest the occurrence of the prograde reaction: $Bt + Sil = Spl + Qtz + Kfs + Melt$

Above minerals together with

P–T estimates

In order to estimate peak metamorphic conditions of the studied khondalite, we applied two complementary approaches: pseudosections and conventional geothermobarometry. The pseudosection approach is independent from mineral compositions that may have been modified during cooling. Nevertheless, the lack of thermodynamic data to include Zn in the modeling and hence to quantify its role on the stability of spinel, which is the crucial phase of the investigated khondalite, may represent a major

Discussion

Spinel + quartz may assemblage mislead many to interpret non-UHT textures as UHT features. For example, textures such as spinel-quartz inclusions in garnet porphyroblasts where the quartz is a late crystallization product formed from either melt rather than part of an equilibrated assemblage, or post-peak decompositions of magnetite-ulvospinel solid solution (e.g. Harley, 2008). Therefore, care must be taken in the interpretation of P–T evolution of Zn-rich spinel in equilibrium with quartz.

Conclusions

Petrographical and two complementary geothermobarometric approaches (pseudosections and conventional geothermobarometry) reveal that peak metamorphic mineral assemblage of the studied khondalite from the southwestern HC, Sri Lanka, comprised of Zn-rich-spinel + quartz ( + sillimanite-plagioclase-alkali-feldspar-biotite-garnet) assemblage. The rock has records of maximum P–T conditions of 7.5–8.5 kbar and 870–900 °C, respectively, thus has not reached UHT metamorphic conditions. Coexistence of spinel +

Acknowledgements

We are grateful to the National Research Council (NRC) of Sri Lanka (Grant No. NRC-11-180) for funding the project. Additional financial supports from National Natural Science Foundation of China for EPMA (Grant No. 41173011), ETH Zurich for XRF analyses and University of Peradeniya (Grant No. RG/2012/41/S to S.P.K.M) are gratefully acknowledged. Our thanks are due to L.R.K. Perera of the Department of Geology, University of Peradeniya, Sajeev Krishnan of the Indian Institute of Science and to

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Isothermal decompression (ITD): In the studied khondalites there is no direct textural evidence for isothermal decompression. However, several studies on the HC granulites inferred a stage of ITD after IBC (e.g. Perera, 1987, 1994; Prame, 1991; Raase and Schenk, 1994; Mathavan and Fernando, 2001; Dharmapriya et al., 2014, 2015b) and evidence for decompression is very strong in interlayered metabasites (e.g. Schumacher and Faulhaber, 1994; Sajeev et al., 2007) and sapphirine granulites (Kriegsman and Schumacher, 1999; Sajeev and Osanai, 2004b; Dharmapriya et al., 2015b). White et al. (2007) suggested that mineral assemblages in dry rocks would hardly react in response to P–T changes.
We report here rare evidence for the early prograde P–T evolution of garnet–sillimanite–graphite gneiss (khondalite) from the central Highland Complex, Sri Lanka. Four types of garnet porphyroblasts (Grt₁, Grt₂, Grt₃ and Grt₄) are observed in the rock with specific types of inclusion features. Only Grt₃ shows evidence for non-coaxial strain. Combining the information shows a sequence of main inclusion phases, from old to young: oriented quartz inclusions at core, staurolite and prismatic sillimanite at mantle, kyanite and kyanite pseudomorph, and biotite at rim in Grt₁; fibrolitic sillimanite pseudomorphing kyanite ± corundum, kyanite, and spinel + sillimanite after garnet + corundum in Grt₂; biotite, sillimanite, quartz ± spinel in Grt₃; and ilmenite, rulite, quartz and sillimanite in Grt₄.
The pre-melting, original rock composition was calculated through stepwise re-integration of melt into the residual, XRF based composition, allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections. The earliest recognizable stage occurred in the sillimanite field at around 575 °C at 4.5 kbar. Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P–T trajectory with an average dP/dT of ∼30 bar/°C in the kyanite field, up to ∼660 °C at 6.5 kbar, before crossing the wet-solidus at around 675 °C at 7.5 kbar. The highest pressure occurred at P > 10 kbar and T around 780 °C before prograde decompression associated with further heating. At 825 °C and 10.5 kbar, the rock re-entered into the sillimanite field. The temperature peaked at 900 °C at ca. 9–9.5 kbar. Subsequent near-isobaric cooling led to the growth of Grt₄ and rutile at T ∼880 °C. Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T < 400 °C, which probably occurred after uplift to upper crustal levels. U–Pb dating of zircons by LA-ICPMS of the khondalite yielded two concordant ²⁰⁶Pb/²³⁸U age groups with mean values of 542 ± 2 Ma (MSWD = 0.24, Th/U = 0.01–0.03) and 514 ± 3 Ma (MSWD = 0.50, Th/U = 0.01–0.05) interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling.

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P–T evolution of a spinel + quartz bearing khondalite from the Highland Complex, Sri Lanka: Implications for non-UHT metamorphism

Highlights

Abstract

Introduction

Section snippets

Geological setting

Sample description and field relations

Khondalite

Whole rock chemistry

Khondalite

P–T estimates

Discussion

Conclusions

Acknowledgements

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