J. metamorphic Geol., 1997, 15, 183–202 Partial transformation of gabbro to coesite-bearing eclogite from Yangkou, the Sulu terrane, eastern China R. Y. ZHANG AND J. G. LIOU Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, USA (E-mail: zhang@pangea.stanford.edu; liou@pangea.stanford.edu) ABSTRACT An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagio- clase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp ( Jd 70–73 )+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite- bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranu- lar coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, tran- sitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution. Key words: China, coesite-bearing eclogite; coronal metagabbro; garnet zoning; metastable persistence; reaction pathway; ultra-high-pressure metamorphism. to mantle depths during UHP metamorphism and have INTRODUCTION subsequently been exhumed to the surface. At least two distinct stages of metamorphic recrystallization at UHP An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region is ( >28 kbar and c. 750–800 °C) and amphibolite facies conditions (8–10 kbar and c. 600–700 °C) occurred (e.g. unique in the occurrence both of intergranular coesite and/or quartz pseudomorph after coesite in the matrix Zhang et al., 1995a; Liou et al. , 1996). During the long journey of subduction and exhum- of eclogite, and the persistence of primary igneous textures and minerals in eclogitized gabbro. A similar ation, most supracrustal rocks have been extensively recrystallized. However, in several 5–30 m gabbroic– step-wise transition from metagabbro to eclogite has been documented in Norway (e.g. Mørk, 1985), in the granitoid blocks from Yangkou Beach, some relict minerals, including primary biotite, clinopyroxene, Greenland Caledonite (Gilotti & Elvevold, 1996), in the NW Bohemian Massif (e.g. O’Brien et al., 1992) and in orthopyroxene and K-feldspar, and some metastable low-P assemblages persist, whereas neoblastic phases the western Alps (Otten & Buseck, 1987; Wayte et al., 1989); the eclogitic rocks from these localities yield such as garnet, omphacite and even coesite occur. In other words, the coesite-bearing metagabbroic block metamorphic pressures of <25 kbar (Meyer, 1983a,b). The preservation of igneous relics and transitional preserves a record of igneous crystallization and various coronal and transitional stages to coesite- assemblages in an UHP block is extraordinary, inas- much as the gabbroic protoliths have been subducted bearing eclogites. The preservation of such gabbroic 183 © Blackwell Science Inc., 0263-4929/97/$14.00 Journal of Metamorphic Geology, Volume 15, Number 2, 1997, 183–202