ALBERT

Description: This contribution examines the melting behaviour of feldspar clasts in 9 th century CE Sue ware from the Nakadake Sanroku kiln site, Kagoshima prefecture (Kyūshū), southern Japan. Sue ware is stoneware of typical grey to brownish-grey appearance fired at high temperature; its surfaces may be untreated or covered by natural ash glaze. Firing was performed in sloping single-chamber tunnel kilns at reducing conditions and temperatures exceeding 1000 °C. Set in a vitrified matrix (glass, mullite, minor spinel and cristobalite), quartz and feldspar constitute ubiquitous clast components. The varying degree of vitrification (15–60 wt.% glass phase) and reversely correlated modal proportions of quartz and feldspar clasts likely reflect superposed effects of varying exposure time to peak-firing temperature and temperature gradients in the kilns. Sodic plagioclase clasts (An 18–25 ) melted congruently, producing sieve-textured domains of vitreous phase and residual calcic plagioclase, while calcic plagioclase clasts (An 35–63 ) were dissolved incongruently by reaction with matrix melt, generating narrow vitreous rims embedding residual calcic plagioclase laths at the immediate contact with the host plagioclase and acicular mullite towards the ceramic matrix. Elevated Fe, Mg, and K contents of the melt phase with development of concentration gradients indicate bi-directional diffusive element transfer between reaction domains and the ceramic matrix. Melting phenomena of alkali feldspar clasts, depending on the intensity of vitrification, range from few micrometre-thin vitreous margins to completely melted blistered clasts. With advanced melting, pronounced chemical zonations are developed with continuous gradation from potassic core compositions (Or 90–80 Ab 10–20 An 〈1 ) to sodic compositions (Or 68–51 Ab 30–46 An 2–3 ) in domains adjoining the vitreous domain. The chemically homogeneous vitreous phase retains the 1:3 Al:Si-stoichiometry of the feldspar framework, whereas K and Na are depleted and Ca, Fe and Mg enriched relative to the precursor feldspar. These findings demonstrate that (1) alkali feldspar clasts did not melt through eutectic reaction with silica/quartz nor via the incongruent reaction K-feldspar -〉 leucite + liquid; (2) melting was initiated by diffusive K-Na exchange with the matrix, thereby shifting the pristine clast compositions towards the thermal minimum (Or 40 Ab 60 ) in the Ab-Or-An ternary system; (3) while sodic marginal domains (Or 68–51 Ab 30–46 An 2–3 ) melted congruently, residual core domains (Or 75–66 Ab 24–33 An 1 ) melted incongruently producing micrometre-scale intergrowths of skeletal plagioclase and melt; (4) melting involved diffusive cation exchange through a quasi-stationary aluminosilicate framework, viz . influx of Ca + Fe + Mg to the feldspar melt domains and concomitant release of the fluxing components K + Na to the ceramic matrix. The evaluation of feldspar melting phenomena suggests firing temperatures did not exceed ca . 1150 °C.

Description: An exposure of polyphase, Neoarchaean–early Palaeoproterozoic basement rocks has been identified in the northeastern part (10°53·44'N, 78°22·88'E) of the Madurai Province. The dominant migmatitic charnockite contains older enclaves of isoclinally folded syenitic biotite gneiss and boudinaged mafic dykes. A highly evolved (Mg# 10) and high field strength element (HFSE) + rare earth element (REE)-enriched ferroan basic dyke body exhibits a distinct mineralogical zonation produced during intense syn-boudinage infiltration metasomatism, coeval with high-grade metamorphism and anatexis of the host charnockite. Core domains that preserve an anhydrous granulite assemblage (domain A: garnet, clinopyroxene and minor plagioclase) grade through a narrow mineralogical transition zone (domain B) into a broad amphibole-rich rind (domain C: ferropargasite + K-feldspar ± plagioclase, quartz). Pseudosection modelling and thermobarometry yields P – T estimates of ~800 °C, 8 kbar for the granulite-facies metamorphism (M 1 ) and ~730 °C, 7 kbar for the high-grade metasomatism (M 2 ). The changes in fabric, mineral assemblage and whole-rock chemistry across domains A to C reveal near-isovolumetric–isochoric conditions of infiltration-driven metasomatism and an unusual enrichment in large ion lithophile elements (LILE), REE, and HFSE, causing prolific neoblastesis of apatite and zircon, and attest attainment of chemical equilibrium. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses for major constituents and accessory phases in the metasomatic domains provide a new set of equilibrium distribution coefficients for the REE. LA-ICP-MS spot analyses of zircon in well-documented microstructural settings identified the following events: ~2·67 Ga: intrusions of foid-bearing syenite and ferroan basic dykes during crustal extension; ~2·6 Ga: high-grade metamorphism M 1 ; ~2·6 Ga: emplacement of voluminous arc-type intrusions (the protolith of charnockite); ~2·48 Ga: high-grade metamorphism and anatexis M 2 of the charnockite protolith and metasomatism of the ferroan metabasite. A distinct Pan-African overprint (M 3 , ~610 Ma, 520 Ma) of the Palaeoproterozoic high-pressure rocks is manifested by hydration related to the exhumation of the deep-seated granulites to mid-crustal levels. This study confirms the continuation of Neoarchaean crust farther south beyond the perceived Palghat Cauvery shear zone system and contradicts the view that this shear zone system represents the Neoproterozoic–Cambrian suture zone along which the Mozambique Ocean was closed.

Description: An exposure of polyphase, Neoarchaean–early Palaeoproterozoic basement rocks has been identified in the northeastern part (10°53·44'N, 78°22·88'E) of the Madurai Province. The dominant migmatitic charnockite contains older enclaves of isoclinally folded syenitic biotite gneiss and boudinaged mafic dykes. A highly evolved (Mg# 10) and high field strength element (HFSE) + rare earth element (REE)-enriched ferroan basic dyke body exhibits a distinct mineralogical zonation produced during intense syn-boudinage infiltration metasomatism, coeval with high-grade metamorphism and anatexis of the host charnockite. Core domains that preserve an anhydrous granulite assemblage (domain A: garnet, clinopyroxene and minor plagioclase) grade through a narrow mineralogical transition zone (domain B) into a broad amphibole-rich rind (domain C: ferropargasite + K-feldspar ± plagioclase, quartz). Pseudosection modelling and thermobarometry yields P–T estimates of ∼800 °C, 8 kbar for the granulite-facies metamorphism (M1) and ∼730 °C, 7 kbar for the high-grade metasomatism (M2). The changes in fabric, mineral assemblage and whole-rock chemistry across domains A to C reveal near-isovolumetric–isochoric conditions of infiltration-driven metasomatism and an unusual enrichment in large ion lithophile elements (LILE), REE, and HFSE, causing prolific neoblastesis of apatite and zircon, and attest attainment of chemical equilibrium. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses for major constituents and accessory phases in the metasomatic domains provide a new set of equilibrium distribution coefficients for the REE. LA-ICP-MS spot analyses of zircon in well-documented microstructural settings identified the following events: ∼2·67 Ga: intrusions of foid-bearing syenite and ferroan basic dykes during crustal extension; ∼2·6 Ga: high-grade metamorphism M1; ∼2·6 Ga: emplacement of voluminous arc-type intrusions (the protolith of charnockite); ∼2·48 Ga: high-grade metamorphism and anatexis M2 of the charnockite protolith and metasomatism of the ferroan metabasite. A distinct Pan-African overprint (M3, ∼610 Ma, 520 Ma) of the Palaeoproterozoic high-pressure rocks is manifested by hydration related to the exhumation of the deep-seated granulites to mid-crustal levels. This study confirms the continuation of Neoarchaean crust farther south beyond the perceived Palghat Cauvery shear zone system and contradicts the view that this shear zone system represents the Neoproterozoic–Cambrian suture zone along which the Mozambique Ocean was closed.