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<p>Understanding the evolution of long-lived orogenic systems requires tools capable of deciphering overlapping tectono-magmatic regimes across deep geological time. Although Th/U ratios in zircon are routinely acquired in U–Pb geochronology, their tectonic significance remains underexplored. Here, we compile and analyze an extensive zircon Th/U database from igneous and sedimentary rocks related to the Paleozoic Famatinian Orogen, a key segment of the Terra Australis Orogen, to evaluate the reliability of Th/U as a proxy for reconstructing tectonic regimes and to constrain the evolution of the Famatinian Orogenic Cycle at 31°S. The results reveal that zircon Th/U, when integrated with magmatic timing, provides a robust framework for decoding the temporal and geodynamic evolution of protracted orogenic systems. Based on our results and previously published data, the Famatinian Orogenic Cycle can be subdivided into three distinct stages: 1) subduction-related magmatism (ca. 492–460&#xa0;Ma) with the development of a long-lived continental arc under a contractional regime, characterized by hydrous, oxidized magmas and consistently low mean Th/U ratios (Th/U &lt; 1; 0.67–0.78). A pronounced magmatic flare-up occurred between 474 and 467&#xa0;Ma, followed by a gradual decline in magmatic productivity toward 460–455&#xa0;Ma. 2) Collisional orogeny and crustal thickening (ca. 460–400&#xa0;Ma). The collision of the Cuyania Terrane with the southwestern margin of Gondwana marks a prolonged phase of reduced magmatism and persistently low Th/U (&lt; 1), consistent with a contractional regime. Between 420 and 400&#xa0;Ma, Th/U ratios show a progressive increase (up to <i>ca.</i> 0.8), reflecting the transition to the onset of extensional regime following crustal thickening. 3) Post-collisional extensional magmatism (ca. 400–360&#xa0;Ma) associated with emplacement of the Achala Batholith and related intrusions. The Achala magmas crystallized at high temperatures (≥ 800&#xa0;°C), yielding high and scattered Th/U ratios (&gt; 1; 1.09–1.56). Our findings validate zircon Th/U ratios as a diagnostic proxy for discriminating major tectono-magmatic regimes and demonstrate their potential to resolve the evolution of protracted orogenic systems in deep geological time.</p> Graphical abstract <p></p>

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Deciphering long-lived tectonic processes through zircon Th/U ratios: a case study from the Famatinian Orogen in the southwestern Gondwana margin

  • 期刊论文

  • 发表日期：

    2026年4月8日

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  • Julián D. Rolan,
  • David Chew,
  • Francisco I. Lugo,
  • Jonatan A. Arnol,
  • Norberto J. Uriz,
  • Carlos A. Cingolani,
  • Maximiliano Naipauer

摘要

Understanding the evolution of long-lived orogenic systems requires tools capable of deciphering overlapping tectono-magmatic regimes across deep geological time. Although Th/U ratios in zircon are routinely acquired in U–Pb geochronology, their tectonic significance remains underexplored. Here, we compile and analyze an extensive zircon Th/U database from igneous and sedimentary rocks related to the Paleozoic Famatinian Orogen, a key segment of the Terra Australis Orogen, to evaluate the reliability of Th/U as a proxy for reconstructing tectonic regimes and to constrain the evolution of the Famatinian Orogenic Cycle at 31°S. The results reveal that zircon Th/U, when integrated with magmatic timing, provides a robust framework for decoding the temporal and geodynamic evolution of protracted orogenic systems. Based on our results and previously published data, the Famatinian Orogenic Cycle can be subdivided into three distinct stages: 1) subduction-related magmatism (ca. 492–460 Ma) with the development of a long-lived continental arc under a contractional regime, characterized by hydrous, oxidized magmas and consistently low mean Th/U ratios (Th/U < 1; 0.67–0.78). A pronounced magmatic flare-up occurred between 474 and 467 Ma, followed by a gradual decline in magmatic productivity toward 460–455 Ma. 2) Collisional orogeny and crustal thickening (ca. 460–400 Ma). The collision of the Cuyania Terrane with the southwestern margin of Gondwana marks a prolonged phase of reduced magmatism and persistently low Th/U (< 1), consistent with a contractional regime. Between 420 and 400 Ma, Th/U ratios show a progressive increase (up to ca. 0.8), reflecting the transition to the onset of extensional regime following crustal thickening. 3) Post-collisional extensional magmatism (ca. 400–360 Ma) associated with emplacement of the Achala Batholith and related intrusions. The Achala magmas crystallized at high temperatures (≥ 800 °C), yielding high and scattered Th/U ratios (> 1; 1.09–1.56). Our findings validate zircon Th/U ratios as a diagnostic proxy for discriminating major tectono-magmatic regimes and demonstrate their potential to resolve the evolution of protracted orogenic systems in deep geological time.

Graphical abstract