Petrology of ultrahigh‐pressure rocks from the southern Su‐Lu region, eastern China

Abstract

Abstract In the Su‐Lu ultrahigh‐P terrane, eastern China, many coesite‐bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet‐quartz‐jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak‐stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high‐Al titanite. The garnet‐quartz‐jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet‐quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith. The Su‐Lu ultrahigh‐P rocks have been subjected to multi‐stage recrystallization and exhibit a clockwise P‐T path. Inclusion assemblages within garnet record a pre‐eclogite epidote amphibolite facies metamorphic event. Ultrahigh‐P peak metamorphism took place at 700–890° C and P>28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P‐T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in‐situ ultrahigh‐P metamorphism during the Triassic collision between the Sino‐Korean and Yangtze cratons.