Sm–Nd and Rb–Sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China

Abstract

Ultrahigh pressure metamorphic (UHPM) rocks at Shuanghe area in the Dabie Mountains occur as a UHPM block within the regional granitic gneiss. A Sm–Nd isochron defined by garnet + omphacite + rutile from coesite-bearing eclogite yield an age of 226.3 ± 3.2 Ma. Another Sm–Nd isochron defined by garnet + two phengites form UHP gneiss yield a similar age of 226.5 ± 2.3 Ma. The two consistent Sm–Nd ages defined by three UHPM minerals suggest that Nd isotopic equilibrium between UHPM minerals in these rocks during UHP metamorphism has been achieved. This may correspond to the age of peak metamorphism with an average metamorphic temperature of 800°C. It is demonstrated that the retrograde metamorphism of UHPM rocks occurred in an open chemical system, whereas the Nd and Sr isotopic systematics of UHPM minerals may have remained closed. Nd and Sr isotopic disequilibrium between UHPM minerals and retrograde metamorphic minerals has been observed; therefore, the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old Sm–Nd age and a young Rb–Sr age, respectively, with no geologic significance. However, a Rb–Sr ages of 219.0 ± 6.6 Ma defined by phengite + garnet from a UHPM gneiss indicates that the UHPM rocks at Shuanghe cooled down to 500°C at that time. It suggests that the UHPM rocks at Shuanghe experienced the first rapid cooling during 226–219 Ma. On the other hand, the Rb–Sr ages of 174 ± 7.8 to 169.2 ± 3.3 Ma defined by retrograde minerals (amphibole or biotite) with closure temperature ranging from 450° to 300°C and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval. This is consistent with the “rapid cooling” time of 190–170 Ma of the orthogneiss in the Dabie–Su–Lu UHPM belt obtained by Ar–Ar dating method. In contrast to UHPM rocks, the Nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite, which yields a Sm–Nd isochron age of 213 ± 5 Ma indicating the retrometamorphic time corresponding to amphibolite facies. In addition, the biotites from the granitic gneiss yield Rb–Sr ages of 171–173 Ma similar to those of the UHPM rocks. These data suggest that the country rocks (granitic gneiss) may have a similar cooling history to the UHPM rocks at Shuanghe. Two stages of rapid cooling of UHPM rocks at Shuanghe may correspond two stages of fast uplift: the initial rapid uplifting and cooling of UHPM rocks during 226–219 Ma may be caused by compression tectonics during subducting time of the continental crust; whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early–middle Jurassic.