Reaction textures in ultramafic rocks: Evidence for supra to sub solidus reaction continuum скачать в хорошем качестве

Reaction textures in ultramafic rocks: Evidence for supra to sub solidus reaction continuum

Reaction textures in ultramafic rocks: Evidence for supra to sub solidus reaction continuum Adak, V. and Dutta, U. Reaction textures like coronas and symplectitic textures are a common occurrence in ultramafic-mafic rocks, where they replace earlier minerals. Corona textures surrounding olivine (Ol) are a common feature in such rocks. A longstanding debate ensues about the origin of such coronas. While some research has indicated the process to be of supra-solidus in nature with an active participation of melt, others have suggested the coronas to form entirely by sub-solidus metamorphic processes [1]. Similarly, symplectites although mostly considered to be the result of sub-solidus reactions [2], have also been attributed to late-stage supra-solidus replacive reactions involving interstitial fluids [3]. Ultramafic rocks from the Chotanagpur Granite Gneissic Complex, in eastern India, contain Ol, orthopyroxene (Opx), clinopyroxene (Cpx), plagioclase (Pl) and spinel (Spl), along with amphibole (Amp) and biotite (Bt). In addition to primary euhedral Opx, petrographic studies show Opx to occur in four different textural varieties with respect to Ol. Firstly, they occur as large irregular Opx oikocrysts with resorbed Ol inclusions. These Opx grains are characterised by continuous cleavages along most of their length. The second variety occurs as coronas around Ol with cuspate boundaries against the central Ol grains. These Opx grains may or may not contain cleavages. Opx grains belonging to the third variety occur as granular aggregates beside the Opx coronas of the second variety. These are identified as the third variety. Lastly, they occur as layers of uniform thickness around Ol mimicking the Ol grain boundaries. Major oxide data shows no difference in the composition of these textural varieties. Textural studies indicate that this replacement of Ol by Opx occurred through melt-involved reactions for the first two varieties. The third variety is interpreted to have formed through interaction between the Opx coronas with melt [4]. The last variety is typical of diffusion-controlled growth and is interpretated to be sub-solidus in nature. Symplectites in the current rock are present either as anhydrous symplectitic texture of Cpx + Pl or as Amp + Spl. Symplectites of Cpx and Pl are often found present along triple junctions of Pl grain boundaries. They might also occur as replacement products around primary Opx, formed by the reaction between Opx and Pl (relatively albitic compared to product Pl). Compositionally both this varieties are identical. X-ray maps show the preservation of Opx grains within Cpx symplectites of the second type. For the anhydrous symplectites, while the first occurrence is representative of fish-hook pyroxenes [4] formed from interstitial fluids, the latter is interpreted to be a sub-solidus reaction replacing Opx. All of these minerals were later variably replaced by biotite (Bt) and/or amphibole (Amp) ± spinel (Spl). Textural and chemical data therefore, indicate that both the Ol to Opx and the Cpx + Pl forming reactions occurred both at supra and sub-solidus conditions. The sub-solidus reactions possibly took place in near-solidus conditions. Thermodynamic calculations