Partial Melting and Crustal Deformation during the Early Paleozoic Wuyi–Yunkai Orogeny: Insights from Zircon U-Pb Geochronology and Structural Analysis of the Fuhuling Migmatites in the Yunkai Region, South China

Zhang Zhang,Ding Ding,Wang Wang,Liu Liu,Yakymchuk Yakymchuk,Tang Tang,Xu Xu,Sa Sa,Zeng Zeng

doi:10.3390/min9100621

Abstract

：Migmatites record crucial information about the rheology and tectonothermal evolutionof the deep crust during orogenesis. In the Wuyi–Yunkai orogen in South China, migmatites at Fuhuling record Early Paleozoic high temperatures and associated partial melting. However, the absolute timing and implications for the rheology of the deep crust during orogenesis are poorly constrained. In this contribution, we used spatial analysis of migmatitic leucosomes, structural analysis, and U-Pb geochronology of zircon to elucidate the absolute timing of crustal partial melting, the degree of partial melting, and the role of partial melting on the rheology of the crust during the Wuyi–Yunkai orogeny. Partial melting of the Fuhuling migmatites occurred at c. 440 Ma during Early Paleozoic Wuyi–Yunkai orogenesis. Subsequent lower temperature metamorphism associated with Indosinian movement that caused minor zircon recrystallization was temporally associated with the crystallization of nearby biotite monzogranites, but it did not influence the morphology of the Fuhuling migmatites. The migmatites preserve a morphological transition from metatexite to diatexite with an increasing proportion of leucosome. This transition preserves different structural characteristics that represent the response of the solid framework and melt network to variable melt fractions during partial melting. The large proportion of in situ or in source leucosome in the Fuhuling migmatites suggests that it was a melt-rich crustal horizon during orogenesis, and that a substantial proportion of anatectic melt was retained in the deep crust. The rheological transition documented in the Fuhuling migmatites was caused by changes in the melt fraction, and it is an analogue for the rheological transition characteristics of melt-rich crustal horizons in the Yunkai region during Early Paleozoic Wuyi–Yunkai orogenesis and subsequent orogenic collapse.

Highlights

Geological and geophysical studies in modern and ancient orogens demonstrate that the middle to lower continental crust can be partially molten during orogenesis [1,2,3,4,5]
There is still a poor understanding of the relative roles of deformation versus melt fraction during the transition from metatexite to diatexite in migmatite terranes, but this information is crucial for linking structures in migmatites to the broader evolution of orogenic belts
Zircon U-Pb geochronology and trace element analysis indicate that biotite monzogranites are not derived from the migmatites at Fuhuling

Summary

Introduction

Geological and geophysical studies in modern and ancient orogens demonstrate that the middle to lower continental crust can be partially molten during orogenesis [1,2,3,4,5]. Diatexite rarely preserves pre-partial melting structures, and it forms when the melt proportion increases to the extent that the solid framework is no longer connected, resulting in strengths close to that of magmas [14,15,16]. These two types of migmatites are structurally and compositionally heterogeneous, and they preserve deformation features that arise from rheological transitions related to melt-dominated to melt-absent behavior [5,11,17,18]. There is still a poor understanding of the relative roles of deformation versus melt fraction during the transition from metatexite to diatexite in migmatite terranes, but this information is crucial for linking structures in migmatites to the broader evolution of orogenic belts

Results

Discussion

Conclusion