Paleogeotherms of a Midcrustal to Upper‐Crustal Profile Across the Northern North China Block: Implications for the Thermal Structure of Continental Arcs

Abstract

AbstractThe Inner Mongolia Paleo‐uplift along the northern margin of the North China Block represents an exhumed midcrustal to upper‐crustal root of a Late Carboniferous‐Permian Andean‐type continental arc. Here we present the new zircon U‐Pb and hornblende/biotite 40Ar/39Ar ages of seven samples from the exhumed midcrustal to upper‐crustal basement rocks and arc plutons across the northern North China arc, with an aim to examine the thermal structure and paleogeothermal gradients of the arc and its retroarc foreland basin. Our results indicate that the midcrust to upper crust (approx. 14 to 19‐km depth) beneath the northern North China arc remained warm (>530 °C) during arc construction at 320–260 Ma and the average paleogeothermal gradients in midcrustal to upper‐crustal levels in the central part of the arc are 37.0–44.5 °C/km. This warm continental crust and high paleogeothermal gradients resulted in widespread thermal disturbance of the Archean‐Paleoproterozoic basement rocks and slow cooling rates of arc batholiths and may have played dominant roles in the deformation and crustal thickening of the North China arc and other continental arcs around the world. In contrast, the midcrust to upper crust beneath the retroarc foreland basin remained cold (<280 °C) during arc construction, and the average paleogeothermal gradients in midcrustal to upper‐crustal levels are <21.5 °C/km. Our results also show that the average paleogeothermal gradients in midcrustal to upper‐crustal levels during arc construction decrease significantly from the central part to the southern margin of the arc, which was most likely related to the northward migration of the continental arc due to the rollback of the Paleo‐Asian oceanic slab.