Petrogenesis of garnet‐bearing ultramafic rocks and associated eclogites in the Su‐Lu ultrahigh‐P metamorphic terrane, eastern China

Abstract

ABSTRACTUltramafic blocks that themselves contain eclogite lenses in the Triassic Su‐Lu ultrahigh‐P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al2O3 (<3.8wt%), CaO (<3.2%) and TiO2 (<0.11 wt%), and are low in total REE contents (several p.p.m.), suggesting that the rocks are residual mantle material that was subjected to low degrees of partial melting. The eclogite lenses or layers within the ultramafic rocks are characterized by higher MgO and CaO, lower Al2O3 and TiO2 contents, and a higher CaO/Al2O3 ratio compared to eclogites enclosed in the quartzofeldspathic gneiss. Scatter in the plots of major and trace elements vs. MgO, REE patterns and La, Sm and Lu contents suggest that some eclogites were derived from melts formed by various degrees (0.05–0.20) of partial melting of peridotite, and that other eclogites formed by accumulation of garnet and clinopyroxene ± trapped melt in the upper mantle.Both ultramafic and eclogitic rocks have experienced a complex metamorphic history. At least six stages of recrystallization occurred in the ultramafic rocks based on an analysis of reaction textures and mineral compositions. Stage I is a high temperature protolith assemblage of Ol + Opx + Cpx + Spl. Stage II consists of the ultrahigh‐pressure assemblage Ol + Cpx + Opx + Grt. Stage III is manifested by the appearance of fine‐grained garnet after coarse‐grained garnet. Stage IV is characterized by formation of kelyphitic rims of fibrous Opx and Cpx around garnet, and replacement of garnet by spinel and pargasitic‐hornblende. Stage V is represented by the assemblage Ol + Opx + Prg‐Hbl + Spl. The mineral assemblages of stages VIA and VIB are Ol + Tr‐Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na2O in clinopyroxene also decreases throughout this history. Eclogites enclosed within ultramafic blocks consist of Grt + Omp + Rt ± Qtz ± Phn. A few quartz‐bearing eclogites contain rounded and oval inclusion of polycrystalline quartz aggregates after coesite in garnet and omphacite. Minor retrograde features include thin symplectic rims or secondary amphiboles after Cpx, and ilmenite after rutile. P‐T estimates indicate that the ultrahigh‐metamorphism (stage II) of ultramafic rocks occurred at 820‐900d̀ C and 36‐41 kbar and that peak metamorphism of eclogites occurred at 730‐900d̀ C and >28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh‐pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino‐Korean and Yangtze cratons.