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15. Mantle xenoliths from SE China and SE Australia: nature and evolution of the lithospheric mantle

Author

Lu, Jianggu

Subjects
Other education not elsewhere classified
Abstract

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'

Published
2022
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18. Lithospheric memory of subduction in mantle pyroxenite xenoliths from rift-related basalts

Author

Macquarie University, National Natural Science Foundation of China, Lu, Jianggu, Tilhac, Romain, Griffin, William L., Zheng, Jianping, Xiong, Qing, Oliveira, Beñat, O'Reilly, Suzanne Y, Macquarie University, National Natural Science Foundation of China, Lu, Jianggu, Tilhac, Romain, Griffin, William L., Zheng, Jianping, Xiong, Qing, Oliveira, Beñat, and O'Reilly, Suzanne Y

Abstract

Petrological and geochemical studies have revealed the contribution of garnet pyroxenites in basalt petrogenesis. However, whether primary mantle melts are produced with such signature or acquired it subsequently remains somewhat controversial. We here integrate new major-, trace-element and Sr-Nd-Hf isotopic compositions of garnet pyroxenite xenoliths in Holocene alkali basalts from Lakes Bullenmerri and Gnotuk, Southeastern Australia, to relate their petrogenesis to mantle-wedge melt circulation and subsequent lithospheric evolution. Results show that the clinopyroxenites have lower MgO and CrO contents than the associated websterites, and range in compositions from depleted LREE patterns and highly radiogenic Nd and Hf isotopic signatures in relatively low-MgO samples (Type 1), to enriched REE patterns with negative HFSE anomalies, unradiogenic Nd and Hf isotopes, and extremely radiogenic Sr-isotopic ratios in samples with higher MgO (Type 2). Such compositional variabilities suggest that these pyroxenites represent segregates from melts derived from a recycled oceanic lithosphere with a potential contribution from pelagic sediments. Variable LREE contents and isotopic compositions between those of Type 1 and 2 clinopyroxenites are observed in amphibole-bearing samples (Type 3), which are interpreted as Type 1-like protoliths metasomatized by the basaltic and carbonatitic melts, possibly parental to Type 2 clinopyroxenites. The lithosphere beneath Southeastern Australia thus has received variable melt contributions from a heterogeneous mantle-wedge source, which notably includes a subducted oceanic slab package that has retained its integrity during subduction. On this basis, we suggest that the compositional heterogeneity and temporal evolution of the subsequent Southeastern Australian basaltic magmatism were probably affected by the presence of pyroxenite fragments in the basalt source and formed by the tectonic reactivation of this lithosphere during Cenozoic

Published
2020

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