Petrogenesis of two groups of pyroxenite from Tungchihsu, Penghu Islands, Taiwan Strait: implications for mantle metasomatism beneath SE China

Abstract

Type II pyroxenite xenoliths are a common form of mantle material found in Miocene basaltic pyroclastic rocks at Tungchihsu in the Penghu Islands, Taiwan Strait. Two groups are identified from mineralogy, petrology and chemistry: Group I have mineral assemblages of clinopyroxene+amphibole (mosaic, generally kaersutite)±apatite±Ti–Fe oxide mineral±iron sulfide. These xenoliths are characterized by high abundances of large-ion lithophile elements (including Ba, K, Rb, Sr and Th), high field strength elements (Zr, Nb and Ti), and light rare-earth elements. Chemical compositions of this group of pyroxenite resemble those of Penghu alkali basalts. Likewise, the Sr–Nd isotopic compositions ( 87Sr/ 86Sr=0.70370–0.70385; 143Nd/ 144Nd=0.51285–0.51295) of clinopyroxene, amphibole and apatite separates from this group falls within the field for late Cenozoic Penghu basaltic rocks and megacrysts, and it is suggested that this group of pyroxenite was formed by the crystallization of alkali basaltic magma in the upper mantle. The second group pyroxenites (group II) contain clinopyroxene, garnet, spinel, amphibole (interstitial, generally pargasite/hastingsite) and/or orthopyroxene and a Ti–Fe oxide mineral. They have low Na 2O, K 2O, TiO 2, LREE, and high Al 2O 3 relative to group I pyroxenites, and compositionally are similar to tholeiitic picrites. From major- and trace elements evidence, we suggest that the second group of pyroxenites may have formed by crystallization of tholeiitic picritic magmas under high pressure. Isotopic differences between clinopyroxene, amphibole and garnet or apatite separates from the two groups (group II: 87Sr/ 86Sr=0.70460–0.70640; 143Nd/ 144Nd=0.51269–0.51290) indicate the two groups of pyroxenite are not related. The coexisting mineral assemblages (clinopyroxene, amphibole and garnet) of group II pyroxenite are not in isotopic equilibrium suggesting that the minerals crystallised from different melt phases, possibly in a magma chamber undergoing fractionation and recharge or by progressive crystallisation in a vein system. The pyroxenite thus formed was then subsequently fragmented and/or re-cemented by a subsequent volatile bearing-melt that crystallised the interstitial amphibole.