Reaction textures in titanite-bearing mafic granulite: Constraints on the metamorphic evolution of Angul-Tikarpara domain of the Eastern Ghats Province, India

The Angul-Tikarpara domain of the northern Eastern Ghats Province (EGP) of Eastern India is characterized by high-grade rocks like felsic gneiss, khondalite (garnet-sillimanite-K-feldspar-quartz bearing granulite), charnockite, mafic granulite and aluminous granulite (spinel-magnetite-hematite-garnet-cordierite-sillimanite-K-feldspar-quartz-biotite-plagioclase). Two distinct mineralogical varieties of mafic granulite are present as enclaves within felsic gneiss and coarse-grained charnockite. Type-A mafic granulite is composed of orthopyroxene, clinopyroxene, hornblende, plagioclase, magnetite, ilmenite, biotite, and/or garnet with a subordinate amount of quartz. Type-B mafic granulite, on the other hand, contains titanite along with clinopyroxene, plagioclase, hornblende, garnet, calcite, ilmenite, magnetite, and biotite. The present study is focused on the latter variety where the peak (M_A1) assemblage is represented by clinopyroxene, plagioclase, titanite, ilmenite, magnetite, calcite, and hornblende. Porphyroblastic clinopyroxene in the aforesaid assemblage is characterized by well-developed Al-zoning, with Al₂O₃-content being maximum at the core (5.3–3.8 wt.%) and minimum at the rim (3.6–0.22 wt.%). It appears that the high-aluminous core had stabilized at the peak stage, while the low-aluminous rim formed during the retrogressive stage characterized by decompression (M_A1R). From the zoning pattern, the peak M_A1 and M_A1R conditions were estimated to be 8.6‒6.8 kbar, ~850˚C, and 4.2‒3 kbar, 660 ˚C, respectively. During the decompression, subidioblastic titanite broke down to low-Al clinopyroxene (2.5-0.5 wt.% Al₂O₃) + ilmenite intergrowth surrounding the former phase. The decompression was associated with cooling during which coronal garnet grew surrounding porphyroblastic clinopyroxene, plagioclase, and calcite. Zn-bearing spinel was exsolved from magnetite during this stage. Phase diagram modeling of Type-B mafic granulite shows that M_A1 metamorphism occurred along a clockwise P-T trajectory. Further development of grossular-rich coronal garnet around hornblende suggests that the rock underwent a second prograde metamorphic event (M_A2). In the chondrite-normalized rare earth element (REE) diagram, Type-B mafic granulite shows a more fractionated pattern than Type-A mafic granulite.

Textural, thermobarometric, and phase equilibria data indicate that the metamorphic evolution of Type-B mafic granulite is comparable to the recently published P-T evolution of Type-A mafic granulite, aluminous granulite, khondalite, and fine-grained charnockite of the area where the M_A1 metamorphism involved decompression and associated cooling from ∼850^◦C, 7–8 kbar to ∼760^◦C, 4–5.8 kbar at ca. 1200 Ma and followed by subsequent prograde M_A2 metamorphism at ca. 990 Ma. This study provides additional evidence that the geological evolution of the Angul-Tikarpara domain is different than the rest of the EGP lying at the southern part of the Mahanadi Shear Zone (MSZ). The latter segment of the EGP is characterized by ultra-high temperature metamorphism (UHT) at ca. 1030–990 Ma which is absent in the present study area. Such contrasting ages and P-T evolution of high-grade rocks on either side of the MSZ confirm the fact that the shear zone represents a terrane boundary which was formed by juxtaposition of the Angul-Tikarpara domain with the EGP only after ca. 960 Ma.