Composite Mantle Xenoliths in Basaltic Pyroclastic Rocks from Tungchihsu, Penghu Islands, Taiwan Strait: Evidence for a Metasomatized Lithospheric Mantle beneath SE China

Three main occurrences of composite mantle xenoliths have been found in outcrops of basaltic pyroclastic rocks from Tungchihsu, the Penghu Islands, Taiwan Strait: (1) an amphibole-rich selvage in contact with carbonated peridotite xenoliths; (2) a thin amphibole-rich spinel websteritic vein crosscutting amphibole-free carbonated peridotite xenoliths; and (3) a clinopyroxene hornblendite band adjacent to the edge of spinel-garnet clinopyroxenite. In the first type, the amphibole-rich selvage commonly contains amphibole, phlogopite, and clinopyroxene, a lesser amount of apatite, and rare orthopyroxene and Ti-Fe oxide minerals. The available major and trace elements, and Sr-Nd isotopic data for the amphibole-rich selvages and their minerals suggest that their genesis may have been similar to that of the Tungchihsu group I pyroxenites. They may represent the products of megacryst precipitation from melts with a similar composition to the Penghu alkali basalt. It was also noted that the mineral composition varies systematically with respect to the distance from the edge of the peridotite xenolith. The iron and titanium contents are higher and magnesium and chromium contents are lower farther from the boundary. These relations indicate that the compositional variation of all phases may have resulted from melt-peridotite interactions. In the second type, the amphibole-rich spinel websterite consists of abundant kaersutite, orthopyroxene, and clinopyroxene with accessory fine-grained spinel. The pyroxenes of the vein have major-element compositions similar to enstatites and diopsides of the ambient peridotite, but are slightly richer in Ti and more depleted in Cr. In addition, the kaersutite in the vein has a higher TiO 2 content when compared with those of interstitial amphiboles of group II pyroxenite and mosaic amphiboles of group I pyroxenite. From petrographic evidence and mineral chemistry, we suggest that the vein may have formed from a relatively magnesium-rich, hydrous basic melt in the fracture of peridotite. In the third type, the spinel-garnet clinopyroxenite contains clinopyroxene, garnet, and spinel. The mineral constituents and compositions are similar to those of the Tungchihsu group II pyroxenites, and may have formed by crystallization of tholeiitic picritic magmas under high pressure. The clinopyroxene hornblendite is composed of interstitial amphibole (pargasite) and clinopyroxene. From mineral chemical data, we suggest that the interstitial amphiboles may have formed by a metasomatic interaction of clinopyroxene , spinel, and garnet of the spinel-garnet clinopyroxenite with a fluid enriched in Ti, Na, K, Ba, Sr, LREE, and H 2O. The lithospheric mantle of SE China was subjected to one or more metasomatic events which formed amphibole-rich selvages/veins and group I pyroxenites, and introduced amphibole and/or phlogopite into both peridotites of composite xenoliths and group II pyroxenites. The enrichment processes modified the chemical composition of the lithospheric mantle beneath SE China.