Petrology of the nyböite-bearing eclogite in the Donghai area, Jiangsu Province, eastern China

Abstract

Nyb6ite occurs as porphyroblasts in the Jianchang eclogite in the Donghai area, northeastern Jiangsu Province, eastern China. The Jianchang eclogite contains some inclusions of quartz after coesite in clinopyroxene, garnet and epidote. It has colourless to pale-violet pleochroism. A thin rim with violet pleochroism often develops around nyb6ite and is taramitic. It is further retrogressed by the symplectite which is mainly composed of hornblende, aegirine-augite and albite. Nyb6ite is associated with jadeitic pyroxene in the Jianchang eclogite, although other porphyroblastic amphiboles in other Donghai eclogites are barroisitic to katophoritic and are associated with omphacite.Fe-Mg partitioning between garnet and clinopyroxene and the presence of coesite pseudomorphs indicate P-T conditions in the Jianchang eclogite of about 740 + 60 ~ and more than 28 kbar. Similar P-T conditions were estimated for other porphyroblastic amphibole-bearing eclogites in the Donghai area. Nyb0ite can occur in the Na-A1-Fe-rich local bulk composition under the medium to high temperature and very high-pressure conditions. Retrograde rim amphibole is poorer in NaB, variable in Si content, and richer in Nan variable than the porphyroblastic amphibole in the Donghai area. This roughly implies a P-T path where P decreases without a large decrease of T.

Epidote Minerals in High P/T Metamorphic Terranes: Subduction Zone and High- to Ultrahigh-Pressure Metamorphism

“…18A and B) with X Fe = 0.23 from Sulu and suggested that epidote was stable together with coesite, garnet, and omphacite at about 850°C and 3.2 GPa. Inclusions of coesite pseudomorphs in epidote were then described by Hirajima et al (1992) in nyböite eclogite in Donghai of the southern Sulu belt and recently in Himalayan coesitebearing eclogite by Massonne and O'Brien (2003). These fi ndings established the stability of epidote at UHP conditions.…”

“…Epidote and zoisite with inclusions of coesite or coesite pseudomorphs in Dabie-Sulu. Inclusions of coesite or coesite pseudomorphs in epidote and zoisite have been described from several Dabie-Sulu eclogites (Zhang 1992;Hirajima et al 1992;Zhang and Liou 1994;Zhang et al 1995a;Rolfo et al 2000;Yao et al 2000). Most of these eclogites contain kyanite ± talc in addition to the common assemblage of garnet, omphacite, rutile, and coesite/quartz.…”

Effect of clinopyroxene composition on Fe–Mg distribution coefficient between garnet and clinopyroxene

“…Nyböite [Na-rich amphibole: ideal formula is NaNa 2 Mg 3 Al 2 Si 7 AlO 22 (OH) 2 ] was found in one of the eclogite samples (88J4) from Jianchang (Hirajima et al, 1992). Two kyanite-and epidote-bearing eclogite samples (88J3 and 89J9) from this locality were selected for this study, because retrograde low-pressure modifications of UHP minerals are not intense in these samples.…”

“…Petrological and mineralogical characters of eclogite from Jianchang have been described in detail by Hirajima et al (1992), and therefore we briefly describe the two studied eclogite samples (88J3 and 89J9). Both samples consist of garnet, omphacite, epidote, phengite, kyanite, quartz, rutile, apatite and secondary symplectites of amphibole and plagioclase, and show weak foliations that can be defined by variations in abundance of constituent minerals.…”

Ultra-high pressure aluminous titanites in carbonate-bearing eclogites at Shuanghe in Dabieshan, central China

“…Such 'high-aluminium' titanites, with XAI = AI/(AI + Ti + Fe 3 § i> 0.25 (Oberti et al, 1991), have since been reported from eclogites and associated rocks in several other high-pressure metamorphic terranes (e.g. Franz and Spear, 1985;Sobolev and Shatsky, 1990;Oberti et al, 1991;Hirajima et al, 1992). On the other hand, Enami et al (1993) have demonstrated the existence of aluminous titanites in lowgrade metasediments from the Salton Sea geothermal system, USA and in granitoids and associated fluorite-bearing skarns from southwest Japan.…”