Lawsonite blueschist in Northern Qilian, NW China: P–T pseudosections and petrologic implications

Abstract

The lawsonite-bearing blueschist from the Nine Spring area, Sunan, Northern Qilian Mountains, is a unique well-preserved lawsonite blueschist terrane in China. Based on the dominant Ca–Al hydrosilicate, the lawsonite-bearing blueschists were divided into lawsonite-bearing pumpellyite blueschist, lawsonite blueschist and epidote blueschist with mineral assemblages of Gln (>40%) + Pmp (30%) + Lws (3%) + Chl (10%) + Ab (8%) + Qtz (5%) + Cal (1%), Gln (35–40%) + Lws (35–40%) + Chl (10%) + Ab (10%) + Grt (1–2%) + Zo (<2%) + Qtz (<1%) and Gln (30–35%) + Zo/Czo/Ep (∼30%) + Chl (15%) + Ab (15–15%) + Grt (2%) + Qtz (<2%) and Mag (1%), respectively. P–T pseudosections using internally consistent mineral thermodynamic database and Domino/Theriak software yield P–T conditions of 320–350 °C, 0.75–0.85 GPa for lawsonite-bearing pumpellyite blueschist, 335–355 °C, 0.8–0.95 GPa for lawsonite blueschist and 345–375 °C, 0.75–0.85 GPa for epidote blueschist, respectively. The transition from the lawsonite blueschist facies to epidote blueschist facies is defined by the reaction Lws + Gln = Chl + Czo + Ab + Qtz + H 2O. These high-P rocks of the Northern Qilian Mountains have undergone prograde metamorphic P–T path with no retrograde overprinting suggesting a fast subduction and exhumation which is consistent with channel flow model.