Petrology of the Non‐mafic UHP Metamorphic Rocks from a Drillhole in the Southern Sulu Orogenic Belt, Eastern‐Central China

Abstract

Abstract The Drillhole ZK703 with a depth of 558 m is located in the Donghai area of the southern Sulu ultrahigh‐pressure (UHP) metamorphic belt, eastern China, and penetrates typical UHP eclogites and various non‐mafic rocks, including peridotite, gneiss, schist and quartzite. Their protoliths include ultramafic, mafic, intermediate, intermediate‐acidic, acidic igneous rocks and sediments. These rocks are intimately interlayered, which are meters to millimeters thick with sharp and nontectonic contacts, suggesting in‐situ metamorphism under UHP eclogite facies conditions. The following petrologic features indicate that the non‐mafic rocks have experienced early‐stage UHP metamorphism together with the eclogites: (1) phengite relics in gneisses and schists contain a high content of Si, up to 3.52 p.f.u. (per formula unit), while amphibolite‐facies phengites have considerably low Si content (<3.26 p.f.u.); (2) jadeite relics are found in quartzite and jadeitite; (3) various types of symplectitic coronas and pseudomorphs after omphacite, jadeite, kyanite and phengite occur extensively in gneisses, schists and quartzites; (4) polycrystalline quartz pseudomorphs after coesite occur as inclusions within garnet of gneisses and schists; (5) coesite, jadeite and omphacite inclusions are found in zircons of gneiss and amphibolite from an adjacent drillhole. Geological, petrological and geochemical data indicate that large volumes of crustal material was buried to depths possibly exceeding 100 km and then exhumed to the Earth's surface.