Phase relations and monazite geochronology of two-pyroxene granulite and high iron oxide bearing garnet-orthopyroxene granulite from Karimnagar Granulite Belt: its importance in Neoarchean-Paleoproterozoic metamorphism

This study reports phase relations and U – Th – Pb_totalin-situ monazite geochronology of two-pyroxene granulite and high iron oxide bearing garnet-orthopyroxene granulite from the Karimnagar Granulite Belt, Eastern Dharwar Craton, India. The two-pyroxene granulite samples with granoblastic texture are composed of quartz, plagioclase, orthopyroxene, and clinopyroxene as major mineral phases, and biotite, zircon, ilmenite, and monazite occur as accessory phases. Small anhedral coronal orthopyroxenes surround the clinopyroxene megacryst and inclusions of clinopyroxene occur in orthopyroxene. Locally, biotites overgrow orthopyroxene and clinopyroxene along their rims. Garnet-orthopyroxene granulites are composed of orthopyroxene, garnet, spinel, quartz, plagioclase feldspar, hematite-magnetite, and ilmenite. Small, rounded orthopyroxene inclusions in garnet and anhedral coronal garnets surrounding orthopyroxene were observed. Spinels are hercynitic in composition. Ilmenite and tiny grains of spinel (~30 μm) occur as exsolve phases in magnetite. Magnetite separates spinel from garnet and orthopyroxene. The following metamorphic reactions can be proposed from the mineral assemblage:

• Cpx megacryst + Pl = metamorphic Opx + Cpx (Two-pyroxene granulite)
• (a) Opx ± Ilm + Pl → Coronal Grt + Qtz, (b) Al-rich magnetite→ Magnetite + Spinel (Hercynite), Ti-rich magnetite → Magnetite+ Ilmenite  (High iron oxide garnet-orthopyroxene granulite).

The result from petrography, phase equilibria, and thermometry of the two-pyroxene granulite predicts that metamorphic orthopyroxene becomes stable at 0.65 Gpa - 950^oC along a cooling path, and biotites overgrow orthopyroxene at 650 ^oC (0.65 GPa). Similarly, the result from grt-opx granulite implies a cooling path where coronal garnet becomes stable at 800 ^oC-0.65 Gpa.

The U-Th-Pb analysis of monazite grains in pyroxene (38 analysis) and feldspar (25 analysis) from two-pyroxene granulite shows the oldest and youngest peak at 2635 ± 90 Ma and 2449 ± 44 Ma, respectively. The monazite in pyroxene (46 analysis) and quartz (14 analysis) from garnet-orthopyroxene granulite exhibits the oldest peak at 2449 ± 14 Ma and a tiny youngest peak at 1987 ± 29 Ma. Another sample of garnet-orthopyroxene (40 analyses) displays the oldest and youngest peaks at 2574 ± 32 Ma and 2448 ± 25 Ma, respectively.

The late Neoarchean peak at ~2600 Ma retrieved from the pyroxene granulite and high iron oxide bearing garnet-orthopyroxene granulite is probably the protolith emplacement age of granulite, correlated with the accretion of Eastern Dharwar Craton and Bastar Craton as a part of extended Ur assembly. The early Paleoproterozoic peak at ~2450 Ma implies the formation of coronal garnet and metamorphic orthopyroxene during a cooling path.