Short-lived high-temperature prograde and retrograde metamorphism in Shaerqin sapphirine-bearing metapelites from the Daqingshan terrane, North China Craton

Abstract

The tectonic mechanism responsible for the formation of high-temperature (HT) and ultrahigh-temperature (UHT) granulites (>850°C) at moderate pressure (7–13kbar) is diverse and contentious, ranging from hot back-arc inversion and thickening, to lithospheric extension and underplating/intrusion of mafic magma, long-lived orogenic self-heating, and ridge subduction. The fundamental reason for this is partly related to the fact that the prograde mineral assemblage and growth zoning are difficult to perserve due to HT diffusion, strong deformation and fluid/melt existence. However, the prograde heating reaction footprint is most likely preserved during a rapid tectonothermal process. In the current paper, we discovered large-scale Spr-bearing metapelites that were originally Sil-Bt metapelite (M1) and preserved secondly Spr+Mag (M2) assemblage at the Shaerqin locality in the Daqingshan terrane of the North China Craton. Sapphirine is later replaced by spinel and cordierite corona (M3), all of which were further back-reacted and overprinted by fibrous sillimanite and retrograde biotite (M4). The well-preserved multistage prograde and retrograde mineral assemblages suggest a rapid tectonothermal process. A clockwise P-T path is confined with peak conditions of 860–890°C and 7.5–8.5kbar, by phase equilibria modeling in the NCKFMASHTO system. It involves a prograde heating evolution (possibly with slight decompression), a post-peak limited isothermal decompression, and a final isobaric cooling segments. SIMS zircon and monazite U-Th-Pb dating constrained the near-peak metamorphic age of ca. 1.86Ga. The rapid process recorded in the Spr-bearing metapelite at ca. 1.86Ga is likely caused by heating of the mantle-derived magma and controlled by an extension mechanism at the termination of the Paleoproterozoic continental–continental (or arc) collision during the amalgamation of the NCC.