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<p>The petrogenesis of Middle Eocene Himalayan adakitic rocks is debated. This study investigates middle Eocene (42.4 ± 1.4&#xa0;Ma) granite porphyries from Lianxiang, southern Tibet, using zircon U–Pb geochronology, whole-rock major-trace elements, and Sr–Nd isotopes, together with zircon Hf isotope to constrain their petrogenesis and geodynamic implications. The rock displays adakitic signatures (high SiO₂, Al₂O₃, Sr/Y, and La/Yb ratios) but enriched isotopes (high ⁸⁷Sr/⁸⁶Sr(i), ε<sub>Nd</sub>(t) = –8.2 to –7.7; ε<sub>Hf</sub>(t) = –24.4 to –14.9 and ancient crustal model ages (Nd T<sub>DM2</sub> ~ 1.47&#xa0;Ga, Hf T<sub>DM</sub><sup>C</sup> ~ 1.49&#xa0;Ga). The samples exhibit ultralow MgO (0.28–0.46 wt.%), Cr (4.54–8.59&#xa0;ppm), and Ni (2.47–3.76&#xa0;ppm), precluding mantle-derived or subducted slab origins. Modeling shows predominant (&gt; 85%) Paleoproterozoic crustal recycling with minimal mantle input (&lt; 15%). Geochemical systematics (e.g., CaO/Na₂O = 0.04–0.06, Rb/Sr–Rb/Ba trends) indicate derivation from partial melting of thickened (&gt; 50&#xa0;km) ancient lower crust dominated by pelitic metasediments. We attribute magma generation to Neo-Tethyan slab break-off at ~ 45&#xa0;Ma, which triggered asthenospheric upwelling and thermally facilitated anatexis of the Indian continental margin. This process records crustal thickening during India-Asia collision and highlights slab break-off as a key mechanism for syn-collisional adakitic magmatism in the Himalayas.</p>

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Slab break-off triggered remelting of ancient thickened crust: petrogenesis of middle Eocene adakitic rocks in southern Tibet and implications for crustal growth

  • 期刊论文

  • 发表日期：

    2026年1月7日

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  • Sheng-Sheng Chen,
  • Ze Zhang

摘要

The petrogenesis of Middle Eocene Himalayan adakitic rocks is debated. This study investigates middle Eocene (42.4 ± 1.4 Ma) granite porphyries from Lianxiang, southern Tibet, using zircon U–Pb geochronology, whole-rock major-trace elements, and Sr–Nd isotopes, together with zircon Hf isotope to constrain their petrogenesis and geodynamic implications. The rock displays adakitic signatures (high SiO₂, Al₂O₃, Sr/Y, and La/Yb ratios) but enriched isotopes (high ⁸⁷Sr/⁸⁶Sr(i), εNd(t) = –8.2 to –7.7; εHf(t) = –24.4 to –14.9 and ancient crustal model ages (Nd TDM2 ~ 1.47 Ga, Hf TDMC ~ 1.49 Ga). The samples exhibit ultralow MgO (0.28–0.46 wt.%), Cr (4.54–8.59 ppm), and Ni (2.47–3.76 ppm), precluding mantle-derived or subducted slab origins. Modeling shows predominant (> 85%) Paleoproterozoic crustal recycling with minimal mantle input (< 15%). Geochemical systematics (e.g., CaO/Na₂O = 0.04–0.06, Rb/Sr–Rb/Ba trends) indicate derivation from partial melting of thickened (> 50 km) ancient lower crust dominated by pelitic metasediments. We attribute magma generation to Neo-Tethyan slab break-off at ~ 45 Ma, which triggered asthenospheric upwelling and thermally facilitated anatexis of the Indian continental margin. This process records crustal thickening during India-Asia collision and highlights slab break-off as a key mechanism for syn-collisional adakitic magmatism in the Himalayas.