What Drives the Continental Crust To Be Extremely Hot So Quickly?

Abstract

The widespread spatio-temporal occurrence of ultrahigh-temperature (UHT) metamorphism in continental crust has been widely documented, but the heat source responsible for generating these extreme conditions over a short period of time remains enigmatic. Here, garnet orthogneiss, spinel-bearing granulite, and gabbronorite from the east Khondalite belt, China, have been analyzed via integrated geochemistry, U-Pb zircon isotope geochronology, and phase equilibria modeling to shed light on the timing and nature of rapid UHT metamorphism in this area. Peak metamorphism is characterized in garnet orthogneiss by the assemblage garnet + orthopyroxene + plagioclase + K-feldspar + quartz, which is calculated to represent pressure-temperature conditions of 0.83-0.88 GPa at 930-990°C. Zircon ages from this sample yield an igneous (protolith) emplacement age of approximately 1.93-1.95 Ga, which matches the timing of metamorphism in adjacent (pre-UHT) metapelites. These garnet orthogneiss were then metamorphosed to UHT conditions at approximately 1.92 Ga, coincident with emplacement of mantle-derived gabbronorites, as supported by REE patterns in zircon, and evidence for a rapid thermal pulse in the east Khondalite belt lower crust at that time. Heat-source and heat-transfer calculations indicate that advected heat from mafic magmas is the most likely cause of this regional-scale event, although ductile shearing might have contributed to a lesser degree.

Paleoproterozoic ultrahigh‐temperature metamorphism in the Alxa Block, the Khondalite Belt, North China Craton: Petrology and phase equilibria of quartz‐absent corundum‐bearing pelitic granulites

“…The origin of the UHT rocks in the KB is generally suggested to be related to extra and rapid heating by mantle‐derived mafic magmatism (Gou et al, 2018; Guo et al, 2012; Huang et al, 2019; Li & Wei, 2018; Peng et al, 2010), whereas the tectonic regime of UHT metamorphism is controversial, including a plume event (Santosh et al, 2008), ridge subduction (Peng et al, 2010; Santosh & Kusky, 2010) and post‐collisional mantle upwelling (Zhao, 2009). A one‐dimensional heat flow equation (equation 1 in Clark et al, 2011) shows that conductive heat, vertical heat transport by advection and creating heat (including radioactive heat, mechanical heating by ductile shear and chemical reaction heat) are the main heat sources of UHT metamorphism.…”

Paleoproterozoic UHT metamorphism with isobaric cooling (IBC) followed by decompression–heating in the Khondalite Belt (North China Craton): New evidence from two sapphirine formation processes

“…Recent study has further certified rapid heat input from mantle‐derived magma at c . 1.91 Ga in Jining Terrane (Huang, Guo, Jiao, & Palin, ). The long‐lived timescale of UHT metamorphism documented here seems incompatible with the rapid UHT metamorphism proposed by Huang et al ().…”

“…1.91 Ga in Jining Terrane (Huang, Guo, Jiao, & Palin, ). The long‐lived timescale of UHT metamorphism documented here seems incompatible with the rapid UHT metamorphism proposed by Huang et al (). We acknowledge that the timing of UHT metamorphism in Jining Terrane was well‐constrained to c .…”

“…1.93–1.91 Ga by zircon U–Pb dating (e.g. Santosh, Wilde, et al, ; Santosh, Wan, et al, ; Santosh, Liu, Shi, & Liu, ; Huang et al, ), with rare <1.88 Ga zircon records. However, monazite chemical dating results from UHT granulites in Jining Terrane show a large age range of 1.93–1.82 Ga (Santosh, Tsunogae, et al, ).…”

Phase equilibria modeling of anatexis during ultra-high temperature metamorphism of the crust

“…As such, several possible alternative heat sources have been proposed for raising the temperature of the crust at a regional scale, including advected heat from mafic magmatic intrusion, elevated concentrations of heat-producing elements coupled with slow erosion, and ductile shearing (e.g. Clark et al, 2011;Huang, Guo, Jiao, & Palin, 2019;Nabelek, Whittington, & Hofmeister, 2010;Vielzeuf, Clemens, Pin, & Minet, 1990).…”