Incongruent melting of garnet during garnet-spinel transition and its implication to the lithospheric exhumation

Phase transition of garnet-spinel is one of the major parameters affecting the lithosphere dynamics. The transition processes of garnet to spinel, however, remain unclear. Spinel-pyroxene symplectites around garnet from one olivine-websterite xenolith hosted by the Cenozoic Xilinhot basalts in northeast China were investigated in this study, to decipher the transition processes of garnet to spinel and its implications to the Cenozoic episodic lithospheric exhumation of northeast China. Symplectites are present as two concentric coronae surrounding relict garnet or as pseudomorphs of garnet grains. The outer symplectite is made of relatively coarse-grained assemblages of orthopyroxene + spinel + clinopyroxene and exhibits large ranges of mineral and bulk chemical compositions. It contains more Mg and less Al content relative to relict garnet. The inner symplectite is a radial arrangement of intergrown fine-grained orthopyroxene + spinel ± glass ± clinopyroxene. Mineral and glass compositions show wide variations within each inner symplectite, but the bulk compositions are relatively homogeneous and show close affinities to the garnet relics. These observations demonstrate that the outer symplectite was formed by a slow reaction between garnet and olivine, whereas the inner symplectite is inferred as a rapid isochemical breakdown product of garnet. The Na- and K-poor glass in inner symplectite could be the in situ melt during the decomposition of garnet, which provides the first physical evidence of incongruent melting of garnet during the garnet-spinel transition. Combined with the geothermobarometer and P-T pseudosection calculations, a slow followed by a fast phase transition process of garnet to spinel was discerned in the lithospheric mantle beneath northeast China during the Cenozoic, which was a result of heating and decompression. These processes were induced by asthenosphere upwelling and lithospheric extension associated with the Pacific subduction during the Cenozoic. We speculate that outer symplectite formed before the Miocene, whereas the inner symplectite has only just begun to form since the Miocene because of the enhanced lithospheric extension. The two-stage phase transitions of garnet to spinel in the lithospheric mantle occurred during the two-stage lithospheric exhumation of northeast China during the Cenozoic. Therefore, the two-stage phase transition in the lithospheric mantle could be one of the factors for the two-stage lithospheric exhumation of northeast China during the Cenozoic.