Chemical petrology of ultramafites and the origin of magnesite deposites of Musorre District, Karnataka

Abstract

Any attempt to classify the ultramafites of the Sindhuvalli–Talur area should take into consideration the field features, petrography, and the chemistry of the whole rock as well as the individual mineral phases. Field features suggest that the ultramafites and the associated metabasic rocks show concordant relations. Geophysical studies indicate that the ultramafites are connected to larger masses below a cover of the gneisses. Petrographic studies show that the ultramafites are unmetamorphosed and the olivines in them have no preferred orientation and cannot be assigned to the Alpine type. Since no quench features such as spinifex textures are present, they cannot be assigned to the Komatiite group either. On the other hand, the presence of corona structures around olivine indicates a higher confining pressure during crystallization. The relative abundance of hornblende peridotite is again characteristic of this area, unlike in the Alpine type ultramafic association. The chemical data suggest that the ultramafites and the associated metabasic rocks are derived from the same magmatic source. All the above points indicate that the present ultramafites cannot be assigned to any one of the orogenic rock suites, namely Alpine, Komatiite, or Alaskan. Taking all evidence into consideration, the following model has been suggested for the origin of the ultramafites and associated metabasic rocks of this area: The parent magma must have been of silica poor (olivine) basalt composition, which differentiated under hydrous magmatic conditions, forming a suite of rocks ranging from dunite, chromitite, harzburgite, hornblende peridotite, and picrite. The Mg rich hornblendites and pyroxenites form the intermediate differentiates. The late stage differentiates are represented by the anorthosites, plagioclase peridotite, and gabbros. The present basic granulites were probably of plagioclase peridotite composition.

The differentiation of the parent magma was accompanied by the ascent of the magma. Tectonic events resulted in the separation and emplacement of plagioclase peridotite, gabbro, and anorthosites into the supracrustal sequence. Subsequently, they underwent metamorphism in the upper amphibolite to pyroxene granulite facies. A powerful orogeny pushed up the remaining part of the rock suite-namely the ultramafites and associated rocks-and emplaced them into the metamorphic rock sequence under katazonal conditions. Because of the granulite grade metamorphism, the ultramafites preserved most of their igneous character. Besides, the poverty of plagioclase and quartz in the ultramafites inhibited metamorphic reactions under such conditions. The high MgO, high Ca/Al, MgO/Na O and Na O/K O ratios indicate the komatiitic affinities of these rocks. The komatiitic chemistry suggests that the Archaean mantle was undepleted and that the protocontinent was thin.

The variation in the mineral chemistry of the metamorphic series suggests that, apart from P–T conditions of metamorphism, the chemistry of the parent rock plays an important role. The orderly distribution of cations between coexisting minerals indicates that they form equilibrium assemblages. Higher K_d (Fe–Mg distribution) in mineral pairs occurring in garnetiferous rocks compared to nongarnetiferous ones indicates equilibrium at higher temperatures, with negligible pressure variation considering the small size of the area. The P–T conditions indicated by coexisting hornblende–orthopyroxene, hornblende–clinopyroxene, garnet–clinopyroxene, and garnet–orthopyroxene mineral pairs suggest that the highest conditions attained are around 700 °C and 8–10 kbar. Petrological and geochemical observations further restrict the P–T regime to the transition between low pressure granulites and intermediate pressure granulites, possibly 6–9 kbar and 650–750 °C. Local variations in water pressure (P_H O) relative to total pressure explain the intermingling of amphibolite and pyroxene granulite assemblages.

The distinct mineral chemistry of the ultramafites and the orderly cation distribution in the coexisting phases suggest that hornblende, orthopyroxene, and phlogopite are typical igneous assemblages. The presence of Mg rich hornblende and phlogopite suggests a pressure of 25 kbar and a depth of about 75 km for the origin of the basaltic magma, considering a higher value for the Precambrian geotherm. Serpentinization of dunites and peridotites began after emplacement of the ultramafic suite. The serpentinites (mostly antigorite) were further converted to magnesite and quartz by the action of CO rich meteoric water under subsurface conditions. Petrographic studies show that magnesite and quartz coexist in equilibrium. The transformation from antigorite to magnesite and quartz is attributed to the reaction: Mg Si O (OH) + 3 CO 3 MgCO + 2 SiO + 2 H O Limited talc–magnesite occurrences can be attributed to: 2 Mg Si O (OH) + 3 CO 3 MgCO + Mg Si O (OH) + 3 H O Slow crystallization of magnesite at 1 atm and 35–100 °C is due to the stability of hydrated Mg² ions. Dissolved salts enhance magnesite precipitation by reducing double layer thickness and lowering water activity. Hydrothermal phase equilibrium studies in the system MgO–SiO –CO –H O suggest antigorite transforms to magnesite and quartz. Magnesite begins to crystallize at 150 °C at 500–1000 atm. With clinopyroxene and hornblende starting materials, calcite and aragonite form along with magnesite and quartz.

The preferential retention of Ca and release of Mg during alteration of hornblende can be explained by higher mobility and lower ionic radius of Mg² compared to Ca² . Further phase separation stabilises calcite, aragonite, and quartz, with aragonite favoured by the presence of Sr² ions. Archaean ultramafites (>2.5 Ga) have a restricted distribution. Ultramafites of Karnataka have often been assigned to the Alpine type without evidence. Alpine type ultramafites are typically Mesozoic–Tertiary and occur in different tectonic settings. Komatiites have now been recognized in many greenstone belts, but some Archaean–Proterozoic ultramafic suites such as those of the Sindhuvalli–Talur area do not fit Alpine or Komatiite classifications, making them a unique group. The present study is an initial attempt to systematically understand these ultramafic suites from a restricted region. Broader generalisations must await similar studies of ultramafites across the Karnataka Craton.