Capturing the Mesoarchean Emergence of Continental Crust in the Coorg Block, Southern India. Issue 15 (3rd August 2018)

• Record Type: Journal Article
• Authors: Roberts, Nick M. W.; Santosh, M.
• Is Part Of: Geophysical research letters. Volume 45:Issue 15(2018)
• Page Start: 7444
• Page End: 7453
• Publication Date: 2018-08-03
• Subjects: continental crust -- zircon -- Hf isotopes -- oxygen isotopes -- continental emergence -- India; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL078114
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The emergence of Earth's continental crust above sea level is debated. To assess whether emergence can be observed at a regional scale, we present zircon U‐Pb‐Hf‐O isotope data from magmatic rocks of the Coorg Block, southern India. A 3.5‐Ga granodiorite records the earliest felsic crust in the region. Younger phases of magmatism at 3.37–3.27 and 3.19–3.14 Ga, comprising both reworked crust and juvenile material, record successive crustal maturation. We interpret an elevation in δ 18 O through time as an increase in both the amount of sediment recycling and hence, crustal thickening, as well as an increase in the emerged area of continental crust available for weathering. Geochemical signatures do not point to any apparent change in geodynamic regime. We interpret the isotopic evolution of these rocks as solely reflecting regional emergence and thickening of the continental crust, assisted by the increasing strength of the lithosphere. Plain Language Summary: The Earth has not always been a mixture of oceans, continents, and mountain ranges. It is generally agreed that early in Earth history the oceans covered the continents almost entirely, but when large landmasses rose above sea level is hotly debated. This process not only altered the chemistry of the oceans and atmosphere but the evolution of life itself. To examine the emergence of the continents, we have studied very small crystals (of the mineral zircon) collected from a suite of rocks in southern India. Using the radioisotopic decay of uranium, we measure their age, and through their chemical composition we learn about the Earth's crust at that time. Over the time period of three and a half to three billion years ago, our data indicate that regionally the continental crust got thicker, and it became significantly emerged above sea level. Critically for understanding Earth evolution, we do not interpret this to be a change in tectonic processes, such as the onset plate tectonics, but simply due to the crust becoming stronger over time. Key Points: Zircon U‐Pb‐Hf‐O isotope data set from the Coorg Block, southern India, represents three phases of magmatism from circa 3.5 to 3.1 Ga Increasing oxygen isotope values through time record an increase in sediment recycling, and continental thickening and emergence above sea level We relate maturation of the continental crust as it thickens and emerges above sea level to the increasing strength of the lithosphere