Constraints on a long-lived Paleoproterozoic metamorphic process in a middle- to lower-grade metamorphic complex, Songshan area, southern North China craton: Evidence from minor- and trace-element thermometry and geochronology

Abstract

The Dengfeng Complex and Songshan Group comprise a middle- to lower-grade metamorphic complex with abundant lithological associations in the Songshan area (central Henan, China), representing an excellent opportunity to investigate the thermal history of the North China craton. The polymetamorphic Dengfeng Complex is a typical granite-greenstone belt that records a late Paleoproterozoic metamorphic event (ca. 1.95−1.80 Ga) overprinting on a late Neoarchean metamorphic event (ca. 2.51−2.41 Ga). In contrast, the Songshan Group preserves well-developed stratigraphic sequences and sedimentary structures with greenschist-facies metamorphism. Numerous studies have focused on the late Neoarchean metamorphic event from the Dengfeng Complex, whereas the thermal evolution of ca. 1.95−1.80 Ga units from the Dengfeng Complex and Songshan Group has been largely overlooked. Quartzite of the Songshan Group unconformably overlies schists of the Dengfeng Group, and they show coherent NNE-SSW−striking schistosity structures with dip angles of ∼50°−65°. Here, the metamorphic temperatures from the Dengfeng Group were constrained to be ∼510−550 °C by the Ti-in-biotite geothermometer. The Ti-in-quartz and Zr-in-rutile geothermometers record metamorphic temperatures of ∼480−550 °C or 520−555 °C for the Songshan Group, respectively. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of monazite and rutile constrains the timing of metamorphism of the Songshan Group to be ca. 1.94−1.83 Ga, while zircon constrains the timing of metamorphism of the Dengfeng Complex to be ca. 2.53 Ga and ca. 1.95−1.87 Ga. By integrating geochronologic data from the Dengfeng Complex and Songshan Group, the ca. 2.53 Ga metamorphic age of amphibolite may be connected with Neoarchean subduction-accretion processes, whereas the consistent temperature obtained in this study ca. 1.95−1.83 Ga indicates that the Songshan area may have experienced a long-lived metamorphic event, which could have resulted from the final collision between the Western and Eastern blocks of the North China craton in the late Paleoproterozoic. The persistence of a Paleoproterozoic hot orogen for millions of years was probably the norm in the North China craton, which provides new insights into the tectonic-thermal evolution in the southern North China craton.