Experimental constraints on the genesis of Jadeite quartzite from Shuanghe, Dabie Mountain ultra-high pressure metamorphic terrane

Abstract

Jadeite quartzite, essentially a two-phase rock made up of jadeite and quartz, is one of the most important UHP lithologies occurring in the Dabie Mountain ultrahigh pressure metamorphic belt and forms layers in biotite-plagioclase gneiss. High pressure-high temperature studies on natural albite from the country rock gneiss were undertaken to reveal the—in parts—complex mineralogical changes that occur in the jadeite quartzite during prograde metamorphism. Experiments were conducted at 800-1200℃, in the pressure range of 2.0-3.5 GPa. One of the most intriguing results shows that the low pressure boundary of the jadeite+coesite stability field is located between about 3.2 GPa at 1000℃ and 3.4 GPa at 1200℃, thus about (0.2-0.3)±0.1 GPa higher than the quartz-coesite transition curve, given the uncertainty in the present study. Minor amounts of sodium and aluminum entering the structure of quartz and the intimate intergrowth texture of the run products may contribute to the observed pressure shift. Combined petrological and mineralogical studies on the run products and the natural rocks yield the following prograde reaction sequence to have occurred: The protolith of the jadeite-quartzite from Dabie Mountain is an albitized siltstone/greywacke characterized by an albite+quartz assemblage. During prograde metamorphism albite breaks down to form jadeite+quartz and thus at this stage two types of quartz can be distinguished whereas type-I-quartz already existed in the protolith, type-Ⅱ-quartz represents a newly formed reaction product of albite. During further P - T -increase the pure type-I-quartz was transformed to coesite, whereas type-Ⅱ-quartz (together with jadeite) was still present as a stable phase because of its impurities of Na and Al. At a later stage during further subduction, type-Ⅱ-quartz also decomposes to form coesite. These studies represent an important puzzlement for a better understanding of the evolution of jadeite-quartzite from the Dabie Mountain during continental crust subduction and thus contribute to a more complete knowledge of the formation of the Dabie Mountain UHP orogenic belt in general.