Tectonic evolution of early Paleozoic HP metamorphic rocks in the North Qilian Mountains, NW China: New perspectives

Abstract

The North Qilian Mountain range is one of the three oldest orogenic belts preceded with a cold oceanic subduction-zone so far recognized on Earth with a subduction history from 490 Ma to 440 Ma in rock record. This orogenic belt has received much attention over the past decades. A significantly improved understanding of the tectonic evolution of the orogenic belt is now emerging thanks to detailed mineralogical, petrologic, geochemical and geochronological studies as well as field observations carried out over the past few years. In this paper, we offer new perspectives on the tectonic evolution of the North Qilian orogenesis in terms of subduction-zone metamorphism by integrating existing observations and our new data. We provide fairly comprehensive documentation of various observations to support our interpretations, which will also serve as a convenient guide for the field excursion of the 8th International eclogite conference. In the North Qilian subduction-zone complex (suture zone), two sub-belts of high-pressure (HP) metamorphism have been distinguished in terms of mineral assemblages, i.e., the low-grade blueschist belt and high-grade blueschist–eclogite belt. The low-grade blueschist belt consists of lawsonite–pumpellyite–glaucophane schist, lawsonite–glaucophanite and minor lawsonite-bearing felsic blueschist with a typical mineral assemblage of Lws + Pmp + Gln + Ab + Chl ± Arg. This assemblage constrains the metamorphic conditions of T = ∼250–350 °C and P = 0.6–1.1 GPa. Protoliths of the lawsonite-bearing mafic blueschist are similar to present-day N-type MORB. The high-grade blueschist belt occurs as three slices within the island-arc volcanic complex and consists of blueschist- to eclogite-facies metamorphosed greywacke, basite, serpentinite, meta-pelite, chert and marble. Lawsonite and Mg-carpholite occur in eclogites and meta-pelites, respectively. P− T estimation yields peak metamorphic conditions of P = 2.2–2.6 GPa and T = 460−540 °C for eclogite and carpholite meta-pelite. Geochemical data suggest that protoliths of mafic blueschist and eclogite in both belts have features of present-day MORB or OIB. These observations indicate that the North Qilian HP metamorphic rocks have experienced a history of seafloor subduction in a cold subduction-zone with a geotherm of ∼6–7 °C/km in the early Paleozoic before exhumed at the Late Silurian and Devonian in response to continental collision.