Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

Central China Orogenic Belt and amalgamation of East Asian continents

Continental subduction and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China)

http://directory.umm.ac.id/Data%20Elmu/jurnal/P/Precambrian%20Research/Vol102.Issue3-4.2000/1996.pdf

The roots of the Dabieshan ultrahigh-pressure metamorphic terrane: constraints from geochemistry and Nd–Sr isotope systematics

Migmatite gneisses are widespread in the Dabie orogen, but their formation ages are poorly constrained. Eight samples of migmatite, including leucosome, melanosome, and banded gneiss, were selected for U-Pb dating and Hf isotope analysis. Most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains. Morphological and internal structure features suggest that their growth is associated with partial melting. According to the Hf isotope ratio relationships between metamorphic zircon and inherited cores, three formation mechanisms for metamorphic zircon can be determined, which are dissolution-reprecipitation of pre-existing zircon, breakdown of Zr-bearing phase other than zircon in a closed system and crystallization from externally derived Zr-bearing melt. Four samples contain magmatic zircon cores, yielding upper intercept U-Pb ages of 807 +/- 35-768 +/- 12 Ma suggesting that the protoliths of the migmatites are Neoproterozoic in age. The migmatite zircon yields weighted mean two-stage Hf model ages of 2513 +/- 97-894 +/- 54 Ma, indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation. The metamorphic zircons give U-Pb ages of 145 +/- 2-120 +/- 2 Ma. The oldest age indicates that partial melting commenced prior to 145 Ma, which also constrains the onset of extensional tectonism in this region to pre-145 Ma. The youngest age of 120 Ma was obtained from an undeformed granitic vein, indicating that deformation in this area was complete at this time. Two major episodes of partial melting were dated at 139 +/- 1 and 123 +/- 1Ma. The first episode of partial melting is obviously older than the timing of post-collision magmatism, corresponding to regional extension. The second episode of partial melting is coeval with the widespread post-collision magmatism, indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the Dabie orogen, which were possibly triggered by the uprising of the Cretaceous mid-Pacific superplume.

Zircon U-Pb ages and Hf isotope compositions of migmatite from the North Dabie terrane in China: constraints on partial melting

Petrological, geochronological and structural data show that the eastern Dabie metamorphic complex resulted from two orogenic stages. Precursor rocks of the ultrahigh-pressure (UHP) and high-pressure (HP) units in the present hanging wall tectonic position were buried, penetratively deformed and subsequently exhumed by distributed, ESE-directed shearing during Triassic time. In contrast, rocks of the Dabie orthogneiss domes, now in a footwall tectonic position, were penetratively deformed during temperature-dominated, Early Cretaceous tectonic events, that are likely related to magmatic underplating. The Dabie orthogneiss domes and the UHP/HP units were juxtaposed during Early Cretaceous exhumation of Dabie orthogneiss domes by the formation of an ESE-directed low-angle ductile normal shear zone. Consequently, the UHP/HP units now represent an extensional allochthon in the hanging wall of the younger normal shear zone. The Cretaceous extensional structures are limited by boundary strike-slip faults. Consistent with the South China regional tectonic framework, ESE-directed lateral extrusion is considered to be the driving mechanism for extension and the present-day structure of the Dabie metamorphic complex.

The Dabie UHP unit, Central China: a Cretaceous extensional allochthon superposed on a Triassic orogen

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The Dabie UHP unit, Central China: a Cretaceous extensional allochthon superposed on a Triassic orogen