FORMATION AND EVOLUTION OF A PROTEROZOIC MAGMATIC ARC – ONGOLE DOMAIN OF THE EASTERN GHATS BELT, INDIA

The Ongole domain in the SW part of the Eastern Ghats Belt represents a Palaeoproterozoic granulite terrain and is mainly composed of a suite of charnockites and enderbites within which metapelitic granulites and basic granulites occur as enclaves. Geochemical data suggest that the intrusive meta-igneous rocks (mafic granulites and charnoenderbites) possess trace and REE compositions that are typical of magmatic arcs. They are subalkaline, enriched in LREE and LILE and depleted in HREE and HFSE like Nb and Ta. The metapelitic granulites are Fe-Al rich hercynite-quartz bearing rocks showing reaction textures indicating two separate metamorphic events. An initial MP-HT event (spinel-quartz, >900°C at 6.5-7.0 kbar) was followed by a MT-HP event (grossular-rich Grt2 overgrowth, Opx-Ky-Sil symplectite ca. 700°C at 9 kbar).

Zircon from charnockites shows oscillatory-zoned magmatic cores with broad metamorphic rims. LA-ICPMS dating revealed concordant magmatic intrusion ages between 1760 – 1710 Ma and a concordant metamorphic age between 1620 – 1600 Ma. In addition, syn-metamorphic intrusive enderbites showing magmatic and metamorphic ages of ca. 1600 Ma is present. Metamorphic rims of zircons from metapelites show concordant ages between 1620 - 1600 Ma, complying with the metamorphic age from charnockites.

In contrast, texturally controlled in situ EMP monazite dating of two chemically distinct monazite generations in metapelites revealed two metamorphic events separated by 60-80 Ma. Small, rounded, unzoned monazite grains that are armored within porphyroblastic garnet show older ages of ca. 1620 Ma. The larger, patchy zoned matrix monazite and inclusions in Grt2 show younger ages of ca. 1540 Ma, only occasionally preserving older cores.

In summary, the presented geochemical, petrological and geochronological evidence suggest that the Ongole domain was formed as a magmatic arc near the Indian continent during the Palaeoproterozoic (ca. 1750-1700 Ma). Subsequently, it experienced UHT metamorphism accompanied by syn-metamorphic intrusions during the late Palaeoproterozoic (ca. 1600 Ma), most likely due to magmatic underplating. This was followed by a crustal thickening event 60-80 Ma later during early Mesoproterozoic (ca. 1540 Ma) due to the attachment of the Ongole magmatic arc with the Indian craton.