Mantle xenoliths from SE China and SE Australia: nature and evolution of the lithospheric mantle
thesisposted on 2022-03-28, 19:18 authored by Jianggu Lu
Volcanism brings up mantle xenoliths that provide insights into the composition, structure and tectonic evolution of the ambient mantle. Peridotite and subordinate pyroxenite are the main rock types in the continental lithospheric mantle. In this study, peridotite xenoliths from the South China Craton and pyroxenite xenoliths from southeastern Australia have been investigated using petrographic and geochemical techniques to constrain their origins, evolution and relationships to geodynamic processes in each region. Peridotite xenoliths from the South China Craton are divided into two groups. Group 1 has olivine Mg# ~90 - 92 (relatively refractory) and shows the trace - element signature of "old" carbonatitic metasomatism. Group 2 is fertile with olivine Mg# mainly ~89 - 90, and represents recently accreted lithospheric mantle. The residual refractory lithospheric mantle is inferred to be irregularly eroded by upwelling asthenosphere, leading to its replacement by newly accreted, fertile lithosphere. Peridotite xenoliths of both groups show sequential infiltration by Na - rich and K - rich silicate melts shortly before eruption. The lithospheric - mantle replacement may have been triggered by paleo - Pacific subduction and roll - back during late Mesozoic - Cenozoic time . Garnet pyroxenite xenoliths from southeastern Australia record partial melting of the convective mantle wedge and represent crystallization products of the earliest recognized episodes of lithospheric mantle infiltration by mafic melts in this region . Paleozoic subduction of the proto - Pacific plate triggered melting o f the convective mantle wedge beneath southeastern Australia and generated hydrous tholeiitic melts that crystallized high - MgO garnet pyroxenites at ~1420 - 1460 °C and 75 - 100 k m . Subsequently, fluid - fluxed melting of the mantle wedge metasomatized by subducted sediments generated melts that precipitated low - MgO garnet pyroxenites at 1280 - 1400°C and ~53 - 66 km . Finally, all garnet pyroxenites were uplifted by at least 10~20 km at 40 Ma due to back - arclithospheric extension ; they finally equilibrated at ~950 - 1100°C and 40 - 60 km . The timing of these mantle events coincides with tectonism recorded in the overlying crust'