Paleogeotherms of a Midcrustal to Upper‐Crustal Profile Across the Northern North China Block: Implications for the Thermal Structure of Continental Arcs. Issue 2 (22nd February 2019)

• Record Type: Journal Article
• Authors: Zhang, Shuan‐Hong; Zhao, Yue; Pei, Jun‐Ling; Zhang, Qi‐Qi; Miggins, Daniel P.; Koppers, Anthony A. P.
• Is Part Of: Tectonics. Volume 38:Issue 2(2019)
• Page Start: 706
• Page End: 721
• Publication Date: 2019-02-22
• Subjects: 40Ar/39Ar dating -- thermal structure -- midcrustal to upper‐crustal profile -- crustal deformation -- continental arc -- North China Block; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2018TC005154
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: The 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 40 Ar/ 39 Ar 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. Plain Language Summary: A continental arc represents one of the most important sites for magmatism and crustal deformation in a continental crust. Thermochronometric and thermobarometric data on exhumed midcrustal rocks from different portions of ancient continental arcs can provide valuable insight into the thermal structure and paleogeothermal gradients of these arcs. In this contribution, we present thermochronometric data of the exhumed midcrustal to upper‐crustal samples across the continental arc on the northern margin of the North China Block. The results show that the northern North China continental arc was characterized by high average paleogeothermal gradients in midcrustal to upper‐crustal levels and the midcrustal to upper crust beneath the arc remained warm during arc construction at 320–260 Ma. In contrast, the midcrust to upper crust beneath the retroarc foreland basin south to the arc remained cold during arc construction and was characterized by low average paleogeothermal gradients in midcrustal to upper‐crustal levels. Our results demonstrate that continental arcs are characterized by high average paleogeothermal gradients and warm midcrust to upper crust, which play an important role in crustal thickening and surface uplift of the active and ancient continental arcs. Key Points: The middle to upper crust beneath northern North China continental arc remained warm (>530 °C) during arc construction at 320–260 Ma The midcrust to upper crust beneath the retroarc foreland basin south to the North China continental arc remained cold during arc construction High geothermal gradients play important role in crustal thickening of continental arcs