A multivariate approach to study the shape modifications of feldspar exsolution lamellae during cooling: A case study from Koraput Alkaline Complex, India

Abstract

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.  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.