Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 2. LA-ICP-MS zircon dating and texturally controlled in-situ monazite dating

Abstract

To quantify the Mesoproterozoic pressure–temperature–time evolution of the granulite-facies Ongole domain, a deep crustal Proterozoic magmatic arc in the Eastern Ghats Belt, LA-ICP-MS U–Pb zircon dating and in-situ electron microprobe U–Th–total Pb monazite dating are integrated with the petrologic observations. Zircon grains in the metapelites often preserve oscillatory-zoned, sometimes partly resorbed cores yielding a spread of 207Pb/206Pb ages between ca. 2700 and ca. 1750Ma, interpreted to reflect the age of detrital grains from the surrounding crustal blocks. Metamorphic zircon overgrowths and rims yield Palaeoproterozoic concordia ages from about 1625 to 1600Ma, indicating the timing and duration of an ultrahigh-temperature (UHT) metamorphic event.Texturally controlled in-situ monazite dating however reveals two metamorphic events separated by 60 to 80Ma. In metapelites, the monazite inclusions in garnet and cores of some matrix monazite yield weighted mean ages of ca. 1610Ma, similar to the concordia ages from zircon and indicate the timing of the UHT metamorphism. The monazite grains in the matrix, growing along with a second generation of garnet and in late-stage symplectites are highly recrystallized and yield weighted mean ages of ca. 1540Ma, indicating the timing of a second metamorphism at higher pressures but lower temperatures. The weighted mean ages obtained from metapelites in the adjoining Vinjamuru domain are the same as those obtained from the Ongole domain. The chemical differences between the two generations of monazite (Th, Y and HREE contents) support the above interpretation of monazite growth under different conditions and at different times. Monazite in the charnoenderbites records the second metamorphic event but only rarely the earlier UHT event. In addition, they show evidence for later imprints of ductile to brittle deformation between ca. 1450 and 1360Ma, which may correlate with Mesoproterozoic crustal extension and alkaline plutonism along the western boundary of the Eastern Ghats Belt. Though the Ongole domain was not involved in major crustal reworking during the Neoproterozoic, monazites do record minor disturbances at ca. 730Ma and ca. 510Ma. Hence the combination of zircon and monazite dating unveils the sequence of tectonothermal events in the Ongole domain in a hitherto unknown exactness: UHT metamorphism at ca. 1610Ma due to magmatic heat advection, subsequently, 60 to 80Ma later, a second metamorphic event at medium-pressure and lower-temperature due to the collision of the arc with the Indian continent, and finally minor disturbances due to rifting between ca. 1450 and 1360Ma.