Garnet growth and diffusion zoning during the late Paleoproterozoic metamorphism, North Altyn area, SE Tarim Craton, NW China: Implications for a long-lived hot orogen

Abstract

Metamorphic rocks encode invaluable information on the burial and exhumation history of rocks that is critical to our understanding of orogens. In this work, zircon U−Pb dating, pseudosection, and garnet zoning modeling were performed on samples from the North Altyn area, SE Tarim Craton, NW China. Zircon U−Pb geochronological data indicate that the peak metamorphism occurred at ca. 2.0 Ga. Pseudosection isopleth thermobarometry using garnet growth zoning of the sample from the northern part of the North Altyn area (Kalatashtagh) reveals a decompression and heating path to a peak temperature of 735 °C and 0.74 GPa under the equilibrium crystallization model. The preservation of growth zoning under high temperature requires relatively rapid cooling. In contrast, garnet from the southern part of the North Altyn area (Aktashtagh) preserves diffusion zoning. Metamorphic peak occurs at 670−735 °C and 0.67−0.73 GPa, which suggests a high dT/dP-type metamorphism. Diffusion zoning modeling suggests an extremely slow cooling rate of 1−5 °C/m.y., which indicates prolonged (∼100 m.y.) crustal residence time at high temperature. This requires a long-term heat supply, most likely from radiogenic heat from buried sediments or heat conducted from the asthenosphere. The high dT/dP and slow cooling rate, combined with extensive crustal anatexis from previous studies, indicate that the late Paleoproterozoic North Altyn Orogen is a long-lived hot orogen rather than a subduction−accretionary complex. We propose a tectonic model that involves a ca. 2.1−2.0 Ga arc−back-arc system in the SE Tarim Craton. The back-arc basin was subsequently closed by northward underthrusting due to arc−arc/continental collision at ca. 2.0−1.95 Ga during the integration of the Tarim Craton into the Columbia/Nuna supercontinent.