<p>Symplectitic intergrowth of orthopyroxene (host) with magnetite and minor ilmenite (lamellae) are recorded from Atlantis Bank, an Oceanic Core Complex on Southwest Indian Ridge. The texture is typically developed in oxide-rich gabbros recovered from the lower crustal section in this region and formed at the expense of olivine. Usually, the intergrowth primarily occurs where the olivine is in contact with magmatic magnetite and/or ilmenite. The maximum length and width of the lamellae in thin section go up to 350 &#181;m and 20 &#181;m respectively. The vermicular lamellae commonly maintain orthogonal relationship to the olivine grain boundary. A significant characteristic feature of this symplectitic intergrowth is the presence of domain structure, where each domain is characterized by the different orientation of vermicular lamellae. Locally, occurrences of amphibole rim abutting the intergrowth are noted. Development of orthopyroxene + Fe-Ti oxide symplectite is linked to the olivine oxidation endorsed by the interstitial Fe-Ti oxides during the progressive evolution of the lower oceanic crust at Atlantis Bank. Consequently, the occurrence of this texture dominantly from the oxide-rich lithologies bears significance. A phase equilibria modelling adopted to better comprehend the relationship between the state of oxidation and symplectite formation at Atlantis Bank indicates that this texture can be developed at relatively lower oxidation state at lower temperature for the observed range in olivine compositions. The formation temperature of the oxy-symplectites estimated from ilmenite-magnetite pairs ranges from ~730&#176; to 450&#176;C over an oxygen fugacity range of -0.86 to +3.83 (FMQ buffer). We suggest that, the development of this intergrowth took place under the influence of late-stage Fe-Ti oxides at subsolidus condition, which fostered the olivine oxidation. Temperature and oxygen fugacity ranges estimated from majority of the discrete ilmenite and magnetite pairs away from the symplectites however suggest relatively higher temperature and lower oxidation state (temperature ranges from 720&#176; to 550&#176;C, and oxygen fugacity ranges from -1.91 to +2.77 (FMQ buffer)). Higher oxygen fugacity values recorded from the symplectites further validate the effect of oxidation.</p>
<p>Like all the alkaline complexes, feldspar exsolution texture is common within different rock types of Koraput Alkaline Complex (KAC), India. Both perthite and mesoperthite exsolution texture are common in this alkaline complex. The former is present within nepheline syenite and granite whereas the latter is observed within alkali gabbro, syenite, granite and nepheline syenite. Shapes of plagioclase feldspar lamellae of perthite and mesoperthite texture vary widely. Plagioclase feldspar lamellae in mesoperthite at places bifurcate and merge with each other forming anastomosing pattern. On the other hand, plagioclase feldspar lamellae in perthite are commonly needle-like. Using two-feldspar and one-feldspar thermometry we have estimated the temperature of formation of these textures. Compositions of exsolved alkali feldspar and the adjacent plagioclase feldspar pairs are used in two-feldspar thermometry. In one-feldspar thermometry, we have used the reintegrated compositions of exsolved alkali feldspars. In two-feldspar thermometry compositions of alkali feldspars immediately after exsolution in these rocks are also estimated. Here we quantify the shape of perthite and mesoperthite grains to objectively understand how effects of different factors (temperature and composition) determines lamellae shape. To solve this problem, we use a multivariate shape analysis approach via geometric morphometrics tool supplemented by statistical methods such as ordinations and statistical analyses [Principal Components Analysis (PCA) and Canonical Variates Analysis (CVA)]. Our result suggests that there is no influence of composition on the shape of plagioclase feldspar lamellae. On the other hand, a strong influence of temperature on the shape of lamellae is distinctly observed. Plagioclase feldspar lamellae showing anastomosing pattern formed at higher temperature. On the other hand, needle like plagioclase feldspar lamellae formed at the lower temperature. &#160;Our data analytics-driven results provide a new perspective to understand the relation between the feldspar exsolution shape and formation temperature of the lamellae. The shape modification of the feldspar lamellae implies that the exsolution texture in alkali gabbro formed early followed by nepheline syenite, syenite and alkali feldspar granite. This finding is also coeval with the crystallisation history of the lithological units of this alkaline complex.</p>
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