Coseismic formation of eclogite facies cataclasite dykes at Yangkou in the Chinese Su‐Lu UHP metamorphic belt

Abstract

Eclogite facies cataclasite is recognized at Yangkou in the Chinese Su-Lu ultrahigh-P metamorphic belt. The cataclasite dykes (5À15 cm wide) are bounded by mylonite/ultramylonite zones, cutting through unfoliated metagabbro and/or eclogite. The cataclasite veins (generally 2-4 cm wide) are free of mylonite boundary zones, cutting through the foliation of the high-P host rock. The dykes and veins are dominated by eclogite fragments consisting of debris of omphacite, garnet, quartz, phengite and kyanite, in a matrix of variable amounts of a schist rich in quartz, phengite and kyanite. Garnet clasts in the fragments are welded and overgrown by more Ca-rich garnet containing mineral inclusions different from those in the garnet cores. The micropoikilitic texture of garnet is typical of eclogitic pseudotachylytes. Crack-sealing K-feldspar veinlets in the cataclasite dykes also imply frictional or shock-induced melting of K-mica. The modal abundances in the cataclasite and the schist imply that the dykes formed by flow of the omphacite and garnet-dominated cataclasites into the fractures during seismic faulting, while the lower density minerals (quartz, phengite and kyanite) were largely left in the ultramylonite boundary zones. The dykes have the same composition as their host rocks, except for slightly lower Si and large ion lithophile elements and higher Mg, Ca, Cr, Co and Ni. Chromite, probably spurted from the nearby ultramafic rock, is found as rare particles in the cataclasite fragments. This indicates that material exchange occurred by mechanical mixing between the dykes and the ultramafic rock during seismic faulting. The Cr-rich eclogite minerals grown on the chromite are evidence for coseismic high-P crystallization. Short-lived crystal growth is implied by the fine grain sizes of the eclogite minerals and very limited element diffusion between the garnet clasts and their overgrowths. The fact that the host rocks are more hydrated implies that the dyke formation was not related to fluid infiltration. It appears, therefore, that stress was the key factor inducing the high-P phase transformation in the dykes. Both stress and temperature were only transiently high in the dykes, which have been metastable since they were formed.

Planar microstructures in zircon from paleo-seismic zones

“…To compare, estimations for pressure generated by meteorite impacting the earth surface could vary from 100 to 4000 GPa (Öpik 1958). Although ultrahigh-pressure mineral associations have been previously reported to be coeval for earthquakes (Yang et al 2014a), but shock metamorphism have been not. On the other hand, deformation caused by an earthquake occurs in depth under lithostatic pressure, which may compensate shock pressure; lithostatic pressure is generally absent during impact-related shock deformation.…”

“…Austrheim and Corfu (2009) explain formation of planar microstructures in zircon from pseudotachylytes by very high strain rates. Yang et al (2014a) explain the formation of ultrahigh-pressure mineral assemblages in the dykes as a result of high stresses and frictioninduced high temperatures that were coeval with the earthquake.…”

“…Planar and non-planar fractures and other microstructures have been previously described in garnet (Austrheim et al1996) and zircon (Austrheim and Corfu 2009) and interpreted to be the evidences of seismic activity. More recently, Angiboust et al (2012), Austrheim (2013), Yang et al (2014aYang et al ( , 2014b have interpreted breccia and specific metamorphic structures in eclogites as seismically induced. In our study we investigate zircons from ultramylonites associated with pseudotachylyte veins, and provide microstructural evidence of paleo-seismic activity recorded in zircons from the deep crustal section of the IVZ.…”

Petrological evidence for shock‐induced high‐P metamorphism in a gabbro

“…An alternative hypothesis argues for the role of tectonic overpressure in the formation of high‐ P assemblages (Jamieson, ; Blake et al ., ; Godard et al ., ; Tajčmanová et al ., ). On the other hand, the preservation of intergranular coesite and the growth of fine‐grained Cr‐rich eclogite minerals around exotic chromite particles in the fault rocks at Yangkou in the Chinese Su‐Lu high‐ P metamorphic belt are interpreted to be the result of coseismic high‐ P crystallization in the crust (Yang et al ., ,b).…”

“…The gabbro eclogitized to an increasing extent from metagabbro to coesite‐bearing eclogites at Yangkou was interpreted to have formed in subduction and exhumation processes (Wallis et al ., ; Zhang & Liou, ). In contrast, eclogite facies fault rocks, including a large area of breccia and several cataclasite dykes and veins, have been recently reported from this locality and interpreted to be the result of transient high‐ P events (Yang et al ., ,b). The present study describes dendritic, skeletal, acicular and poikilitic high‐ P microlites and crystallites in the variably eclogitized gabbro at Yangkou (Fig.…”