Two Distinct Types of Pan-African Gabbros from Ulongue Region of Northern Tete Province, Mozambique: Petrogenesis and Tectonic Implication

The olivine gabbro and syenogabbro from the Ulongue district of northern Tete Province， Mozambique have been comprehensively investigated through zircon U-Pb geochronology， mineral chemistry， bulk-rock elemental analysis and zircon Lu-Hf isotopic analysis. Zircon U-Pb dating results indicate emplacement ages of 601 ± 4 Ma for the olivine gabbro and 579 ± 4 Ma for the syenogabbro， which are consistent with the timing of the Pan-African magmatic event. Zircon Hf isotopic signatures of the olivine gabbro and syenogabbro are similar， ranging from +1.4 to +5.6 and +2.4 to +5.1， respectively， suggesting mantle-derived magma sources for both rocks. These gabbroic rocks exhibit typical enrichment in LILE elements （e.g.， Sr， Ba， Rb） and a depletion in HFSE elements （Th， Nb， U， Ta， Zr， etc.）， resembling arc magmatic rocks. The elevated Th/Yb， La/Nb， Rb/Y， and Ba/Zr ratios suggest that these rocks originated from an enriched lithospheric mantle modified by subducted slab fluids. However， the two sets of gabbroic samples exhibit distinct mineral assemblages and geochemical characteristics. The olivine gabbro is composed of plagioclase， Mg-rich biotite， Mg-rich amphibole， clinopyroxene， orthopyroxene， and olivine. It is characterized by low SiO₂ （52.64 - 53.02 wt%） and Na₂O+K₂O （3.95 - 4.07 wt%） contents， but high MgO （5.34 - 5.71 wt%） contents， classifying as a low-K tholeiitic series. In contrast， the syenogabbro is dominated by K-feldspar， plagioclase， Fe-rich amphibole， Fe-biotite， and Fe-rich clinopyroxene. It displays relatively lower MgO （3.35 - 3.86 wt%） and higher Na₂O+K₂O （6.51 - 7.12 wt%） contents， belonging to the shoshonitic series. The low ∑REE （35.8-66.9 ppm） contents and Sm/Yb （1.39-1.93） ratios of the olivine gabbro suggest a possible origin through partial melting of spinel peridotite， while the syenogabbro， with higher ∑REE contents （1246 - 1583 ppm） and high Sm/Yb （12.9 - 13.2） ratios， indicates involvement of garnet peridotite melting and AFC magmatic processes. Overall， we propose that the emplacement of the Ulongue olivine gabbro at 601 Ma records the termination of subduction between the Zambezi block and the Lurio terrane， while the syenogabbro， formed at 579 Ma， represents a subsequent transitional extensional phase following subduction cessation.