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<p>Precise crustal characterisation of the Laxmi Basin (LB) in the northern Indian Ocean has intrigued researchers for several decades. The LB plays a vital role in the final dispersal of Gondwana and the early opening of the Arabian Sea. The International Ocean Discovery Program (IODP) Expedition 355 penetrated igneous basement in the LB for the first time and recovered forearc basalts (FAB) and boninitic forearc basalts (B-FAB) from Site U1457. Here, we investigate crystallisation and differentiation conditions of the source magma using mineral-chemical and thermobarometric constraints. Analyses of these basalts characterise the clinopyroxenes as high-Mg augites with low TiO<sub>2</sub>, high large-ion lithophile element (LILE) concentrations, negative Nb anomalies, and low total rare earth element (REE) contents. Most of the clinopyroxenes exhibit positive Eu anomalies in chondrite-normalised profiles, indicating their crystallisation prior to plagioclase. Plagioclases, on the other hand, range from labradorite to bytownite with flat to slightly light REE (LREE)-enriched patterns and positive Eu anomalies. Furthermore, clinopyroxene thermobarometry suggests crystallisation at ~1.5–6 kbar pressure (i.e., ~9–22&#xa0;km depth) within the estimated temperature range of 1053–1219&#xa0;°C. We infer that crystallisation may have peaked at approximately 3.25–4.25 kbar (~15–20&#xa0;km), aligning with Moho depths (~12–14&#xa0;km), followed by differentiation in sub-crustal and near-surface reservoirs (~10–11&#xa0;km). Overall mineral-chemical signatures and pressure–temperature conditions confirm a nascent subduction origin of LB lavas.</p>

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Mineral-chemical and thermobarometric constraints on magma evolution during relict subduction initiation in the Laxmi Basin, Northern Indian Ocean

  • 期刊论文

  • 发表日期：

    2026年1月4日

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  • Anju Pandey,
  • Dhananjai K. Pandey

摘要

Precise crustal characterisation of the Laxmi Basin (LB) in the northern Indian Ocean has intrigued researchers for several decades. The LB plays a vital role in the final dispersal of Gondwana and the early opening of the Arabian Sea. The International Ocean Discovery Program (IODP) Expedition 355 penetrated igneous basement in the LB for the first time and recovered forearc basalts (FAB) and boninitic forearc basalts (B-FAB) from Site U1457. Here, we investigate crystallisation and differentiation conditions of the source magma using mineral-chemical and thermobarometric constraints. Analyses of these basalts characterise the clinopyroxenes as high-Mg augites with low TiO2, high large-ion lithophile element (LILE) concentrations, negative Nb anomalies, and low total rare earth element (REE) contents. Most of the clinopyroxenes exhibit positive Eu anomalies in chondrite-normalised profiles, indicating their crystallisation prior to plagioclase. Plagioclases, on the other hand, range from labradorite to bytownite with flat to slightly light REE (LREE)-enriched patterns and positive Eu anomalies. Furthermore, clinopyroxene thermobarometry suggests crystallisation at ~1.5–6 kbar pressure (i.e., ~9–22 km depth) within the estimated temperature range of 1053–1219 °C. We infer that crystallisation may have peaked at approximately 3.25–4.25 kbar (~15–20 km), aligning with Moho depths (~12–14 km), followed by differentiation in sub-crustal and near-surface reservoirs (~10–11 km). Overall mineral-chemical signatures and pressure–temperature conditions confirm a nascent subduction origin of LB lavas.