Neoproterozoic evolution and Cambrian reworking of ultrahigh temperature granulites in the Eastern Ghats Province, India

Abstract

The time‐scales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel, and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole‐rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1,000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with <1 kbar of decompression. Concordant U–Pb ages obtained by SHRIMP from zircon (spread 1,050–800 Ma) and monazite (spread 950–800 Ma) demonstrate that crystallization of these minerals occurred during an interval of c. 250 Ma. By combining LA‐ICP‐MS U–Pb zircon ages with Ti‐in‐zircon temperatures from the same analysis sites, we show that the crust may have remained above 900°C for a minimum of c. 120 Ma between c. 1,000 and c. 880 Ma. Overall, the results suggest that, in the interval 1,050 to 800 Ma, the evolution of the Vizianagaram granulites culminated with UHT conditions from c. 1,000 Ma to c. 880 Ma, associated with minor decompression, before further zircon crystallization at c. 880–800 Ma during cooling to the solidus. However, these rocks are adjacent to the Paderu–Anantagiri–Salur crustal block to the NW that experienced counterclockwise P–T–t paths, and records similar UHT peak metamorphic conditions (7–8 kbar, ~950°C) followed by near‐isobaric cooling, and has a similar chronology during the Neoproterozoic. The limited decompression inferred at Vizianagaram may be explained by partial exhumation due to thrusting of this crustal block over the adjacent Paderu–Anantagiri–Salur crustal block. The residual granulites in both blocks have high concentrations of heat‐producing elements and likely remained hot at mid‐crustal depths throughout a period of relative tectonic quiescence in the interval 800–550 Ma. During the Cambrian Period, the EGP was located in the hinterland of the Denman–Pinjarra–Prydz orogen. A later concordant population of zircon dated at 511 ± 6 Ma records crystallization at temperatures of ~810°C. This age may record a low‐degree of...

Time's arrow, time's cycle: Granulite metamorphism and geodynamics

“…Santosh et al, 2012) and the late Mesoproterozoic-early Neoproterozoic Eastern Ghats Province-Rayner Province of India and East Antarctica (e.g. Korhonen et al, 2013Korhonen et al, , 2014; see also Mitchell et al, 2018), while other Proterozoic orogens record more complex metamorphic P-T paths, such as recorded in the Central Indian Tectonic Zone (Bhowmik and Chakraborty, 2017). These orogens might form by renewed convergence and thickening of the extensional basin formed by truncated hot collision (Sizova et al, 2014).…”

Geology of the Proterozoic Eastern Ghats Belt: recent developments and outstanding issues

“…In the case of two-cycle orogenesis, a major phase of isothermal decompression occurred as recorded in the study of Kelsey et al (2017). Mitchell et al (2018) ruled out this decompression event but reserved comments on how the hot lower crustal granulites resided in the middle crust up to ca. 0.55 Ga before exhumation.…”

“…There is a recent discrepancy regarding the nature of P-T loop for the Eastern Ghats Province. The existing counter-clockwise P-T path which has been validated in the study of Kelsey et al (2017) from the Visakhapatnam domain has been contradicted by Mitchell et al (2018) from the same domain. The latter study also refuted the model of short-lived two-cycle orogeny for this province (ca.…”

“…Although it is more or less agreed upon that the time frame is ca. 1050-990 Ma 90 Sankar Bose Mitchell et al, 2018), reported ca. 1.17 Ga UHT event based on monazite data.…”

“…0.55 Ga before exhumation. The latter age represents amphibolite facies Kelsey et al, 2017) to locally granulite facies (Mitchell et al, 2018) reworking of the cooled granulites during the Pan African orogeny.…”