Marble coexists with coesite-bearing eclogites in Sulu ultrahigh pressure ( UHP) metamorphic terrane has undergone retrograde metamorphism. Based on the retrograde metamorphic degrees, the marble could be classified in three groups: magnesite-bearing marble ( slightly retrograded) , talc marble ( moderately retrograded) , and tremolite marble ( strongly retrograded ). The magnesite-bearing marble is mainly composed by clinopyroxene, dolomite and calcite. Magnesite only occurs as relics in calcite and coexists with calcite showing inlayed texture. Clinopyroxene shows exsolution texture with quartz rods in clinopyroxene. Clinopyroxene rim was replaced by symplectites of plagioclase and amphibole locally. The moderately and strongly retrograded marbles do not contain any clinopyroxene and magnesite. However, they contain talc or/and tremolite. We report here the first discovery in nature of dolomite decomposition texture in metamorphic marble from Sulu UHP terrane (Mg, Ca)(CO3)2 = MgCO3 + CaCO3) , which demonstrates that the continental materials recycled down to 180 km (possibly 210 km) and came back to surface quickly. This study firstly suggests that continental materials recycled into the forbidden zone in the deep mantle with very low geothermal of -4. 2℃/km. The observation of this deep subducted continental materials means very high speed of exhumation, which is probably the major reason for preservation of dolomite decomposition texture showing progressive metamorphism at UHP conditions.
Abstract Two magnesite‐bearing impure dolomitic marbles from the Dabie–Sulu ultrahigh‐pressure (UHP) region have been investigated to clarify if they had actually attained P–T conditions outside the dolomite stability field, limited by the reaction dolomite = aragonite + magnesite, and to test their potential for recording (U)HP conditions. In both cases, the silicate mineral assemblage records conditions around the terminal amphibole breakdown reaction: amphibole + aragonite ± quartz = clinopyroxene + talc, which is a good geobarometer between at least 2.0 and 2.6 GPa. At higher pressures, the terminal breakdown of talc to clinopyroxene + coesite is the last P–T milestone below 4 GPa recordable by the silicate assemblage, but evidence of former coesite is ambiguous. The dolomite dissociation curve becomes strongly divariant in Fe‐bearing marbles and may thus be attainable during cold subduction near the 5 °C km −1 ‘geotherm’. At least one of the samples (from Xinyan village, near Taihu, Dabie Shan) preserved relicts of both magnesite and aragonite and most likely attained conditions within the aragonite + magnesite stability field. For the second sample from Sanqingge village in the Sulu terrane, no certain evidence has been found in this study. Impure dolomitic marbles have considerable potential to preserve (ultra)high‐pressure relicts. Particularly, massive impure marbles that have not been pervasively infiltrated by fluids during exhumation are considered to preserve high‐pressure relicts better than metapelites, paragneisses and orthogneisses, and perhaps almost as well as eclogites or metaperidotites. The inconspicuous mineral assemblage clinopyroxene + talc or quartz (after former coesite) may in fact record UHP conditions.
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