On the survival of intergranular coesite inUHPeclogite

Abstract

Coesite is typically found as inclusions in rock‐forming or accessory minerals in ultrahigh‐pressure (UHP) metamorphic rocks. Thus, the survival of intergranular coesite in UHP eclogite at Yangkou Bay (Sulu belt, eastern China) is surprising and implies locally “dry” conditions throughout exhumation. The dominant structures in the eclogites at Yangkou are a strong D2 foliation associated with tight‐to‐isoclinal F2 folds that are overprinted by close‐to‐tight F3 folds. The coesite‐bearing eclogites occur as rootless intrafolial isoclinal F1 fold noses wrapped by a composite S1–S2 foliation in interlayered phengite‐bearing quartz‐rich schists. To evaluate controls on the survival of intergranular coesite, we determined the number density of intergranular coesite grains per cm2 in thin section in two samples of coesite eclogite (phengite absent) and three samples of phengite‐bearing coesite eclogite (2–3 vol.% phengite), and measured the amount of water in garnet and omphacite in these samples, and also in two samples of phengite‐bearing quartz eclogite (6–7 vol.% phengite, coesite absent). As coesite decreases in the mode, the amount of primary structural water stored in the whole rock, based on the nominally anhydrous minerals (NAMs), increases from 107/197 ppm H2O in the coesite eclogite to 157–253 ppm H2O in the phengite‐bearing coesite eclogite to 391/444 ppm H2O in the quartz eclogite. In addition, there is molecular water in the NAMs and modal water in phengite. If the primary concentrations reflect differences in water sequestered during the late prograde evolution, the amount of fluid stored in the NAMs at the metamorphic peak was higher outside of the F1 fold noses. During exhumation from UHP conditions, where NAMs became H2O saturated, dehydroxylation would have generated a free fluid phase. Interstitial fluid in a garnet–clinopyroxene matrix at UHP conditions has dihedral angles >60°, so at equilibrium fluid will be trapped in isolated pores. However, outside the F1 fold noses strong D2 deformation likely promoted interconnection of fluid and migration along the developing S2 foliation, enabling conversion of some or all of the intergranular coesite into quartz. By contrast, the eclogite forming the F1 fold noses behaved as independent rigid bodies within the composite S1–S2 foliation of the surrounding phengite‐bearing quartz‐rich schists. Primary structural water concentrations in the coesite eclogite are so low that H2O saturation of the NAMs is unlikely to have occurred. This inherited drier environment in the F1 fold noses was maintained during exhumation by deformation partitioning and strain localization in the schists, and the fold noses remained immune to grain‐scale fluid infiltration from outside allowing coesite to survive. The amount of inherited primary structural water and the effects of strain partitioning are important variables in the survival of coesite during exhumation of deeply subducted continental crust. Evidence of UHP metamorphism may be preserved in similar isolated st...

Archean eclogite-facies oceanic crust indicates modern-style plate tectonics

“…S11 C ). This study shows that the preservation of eclogite-facies rocks is critically dependent on having inherited dry rocks, which favors the preservation of the higher-grade metamorphic assemblages ( 47 ), and further shows that eclogite-facies rocks were produced on the early Earth and preserved under these special dry conditions. Further work is required on even older samples globally less affected by later overprinting metamorphism or fluid/melt metasomatism to better understand the geodynamics and tectonics of the early Earth.…”

Ultrahigh-Pressure Metamorphism and P-T Path of Xiaoxinzhuang Eclogites from the Southern Sulu Orogenic Belt, Eastern China, Based on Phase Equilibria Modelling

“…The Sulu HP-UHP belt is further divided into a HP terrane in the south and an UHP terrane in the north (Figure 1a; [21][22][23][24][25]), which were unconformably overlain by the Cretaceous sedimentary strata and volcanoclastic rocks and were intruded by postorogenic Mesozoic granitic plutons [26,27]. The Sulu HP terrane predominantly consists of quartzites, quartz schists, blueschists, marbles and albite gneisses [4,21,26,28,29].…”

“…Orthogneisses are monzonite and granite in composition (Figure 1b). [25]). S = southern part of Sulu UHP terrane; N = northern part of Sulu UHP terrane.…”

Factors affecting preservation of coesite in ultrahigh‐pressure metamorphic rocks: Insights from TEM observations of dislocations within kyanite

“…Nishiyama, ). The pressure‐vessel model suggests that the preservation of a high‐ P phase as an inclusion is controlled by the stiffness of the host minerals (Gillet, Ingrin, & Chopin, ; Guiraud & Powell, ; Nishiyama, ; Perrillat, Daniel, Lardeaux, & Cardon, ; Tajčmanová et al., ; Van der Molen & Van Roermund, ); however, this model cannot simply explain the presence of intergranular coesite (Liou & Zhang, ; Liu, Massonne, Zhang, Wu, & Jin, ; Wang et al., ; Ye, Hirajima, Ishiwatari, Guo, & Zhai, ). The occurrences of intergranular coesite aggregates suggest that reaction kinetics related to the activity of an aqueous fluid could play a key role in its preservation (Liu et al., ).…”