Amphibole in UHP eclogite from the Sulu region, eastern China

We examined the stability of amphibole in ultrahigh-pressure (UHP) conditions of about 620-750 °C, 2.9-3.7 GPa based on the mineralogy of UHP eclogites collected from the Qinglongshan and Jianchang areas in Donghai County of the Sulu region. The samples studied are mainly composed of garnet, omphacite, amphibole, rutile, kyanite, quartz, epidote, and phengite. Amphibole can be classified into three types based on its mode of occurrence; Amp I, inclusions in garnet and epidote; Amp II, the matrix phase; Amp III, small (~ 10-30 μm) grains developed along grain boundaries. Amp I from Qinglongshan, mainly barroisite-taramite, is slightly poorer in NaB (Na contents in B site < 1.2 a.p.f.u.) than Amp II. Amp II is barroisite with a mostly homogeneous composition in each grain except for the margins of some grains (NaB = 1.1-1.4 a.p.f.u; Si = 7.0-7.5 a.p.f.u.) and shows the highest NaB values among the types. Amp III, mainly pargasite-taramite, shows clearly lower NaB (= 0.3-1.15 a.p.f.u.) than that of Amp II. On the other hand, Amp II and Amp III from Jianchang are nyböite and taramite, respectively, and the sum of the (NaA + NaB) values from the Jianchang amphibole is distinctly greater than that of the Qinglongshan amphibole. Polycrystalline quartz pseudomorphs after coesite and Amp I with barroisite compositions were found within a large epidote grain of the Qinglongshan samples. These petrological data suggest the following evolutionary history. Amp I with relatively low NaB values crystalized and was involved with other mineral grains during the compression of the eclogite, and then barroisitic Amp II occurred as a stable phase during the peak-P stage. Finally, Amp III with much lower NaB values was formed during the decompression stage. Thus, we concluded that a Ca-Na amphibole (Amp II) can be stably present in an eclogite even in the UHP conditions. In addition, significant amounts of F (up to 1 wt%) were detected from the barroisitic amphibole in the Qinglongshan samples. At the least, some eclogites from the studied areas may have undergone metasomatic infiltration of an F-bearing fluid in the UHP conditions.