242-10 The La/Yb, Sr/Y and Eu Characteristics of Continental-arc Granitoids as Proxies for Lithospheric Thickness, Intracrustal Melting and Differentiation Mechanisms of Continental Crust

Session: Crustal Petrology, Part I

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Tonalite-trondhjemite-granodiorite (TTG) suites, the dominant component of Earth's first continents that build the upper and middle continental crusts are products of melt extraction from the mantle. However, the Si-rich and Mg-poor nature of these granitoids suggest it is unlikely to derive these magma suites directly from an ultramafic mantle. Therefore, it is important to decipher the mechanisms of formation and source depth characteristic of the granitoid magma to understand the genesis and growth of continental crust, though quantifying crustal thickness throughout the geological record is a challenging task. It is well-known that the compositions of TTGs are source-controlled; therefore, trace element geochemistry of these rocks can provide insights into their source depths characteristics and geodynamic settings. Here, an effort is made to explore the connection between geochemical features of granitoids rocks and crustal thickness, intracrustal melting processes and differentiation mechanisms of continental crust. The La/Yb, Sr/Y and Eu characteristics of continental-arc granitoids of southern India are analysed to demonstrate their systematic variations as an indicator of shallow versus deep fractionation processes, thereby providing avenues to estimate the lithospheric thickness during magma genesis. The calc-alkaline granitoids, with contrasting geochemical compositions are recognized as mixtures of large-scale leucosomes (granites) and entrained melanosomes (tonalites). The tonalites are characterised by high Sr/Y ratios (avg. = 90), whereas, the ratios are low in granites (avg. = 3). This along with other features such as high (La/Yb)_N (avg. 45) ratios and strongly positive and/or no europium (Eu/Eu* = 0.7–1.47) anomalies suggests TTGs, derived through partial melting of a thickened lower arc-crust. On the other hand, moderate REE fractionation (avg. (La/Yb)_N = 20), with significant negative Eu anomalies (avg. Eu/Eu* = 0.25) shown by the granites suggest remelting of TTG dominated arc-crust, leaving plagioclase enriched tonalites as residue during granite genesis. The conclusion of this investigation proposes that the processes involved in secular evolution of the Earth, on a depth source-integrated manner, influence and define the composition and growth-rate of the continental crust.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4386

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