Hydration, melt production and rheological weakening within an intracontinental gneiss dome
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Migmatite
Pegmatite
Orogeny
Anatexis
Metamorphic core complex
Protolith
Basement
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Abstract Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c . 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c . 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c . 455–400 Ma. The migmatization was c . 50 Ma later than the c . 490 Ma ultra‐high‐ P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.
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The Tongbai orogen is a key region to investigate the evolution of the Qingling-Tongbai-Dabie-Sulu metamorphic belt.However,the age of the crustal anatexis in the Tongbai high-grade metamorphic complex has not been well constrained,and thus hinders our understanding of the tectonic evolution of this area.Three migmatite samples from the Tongbai complex,including one melanosome and two leucosomes,were selected for LA-ICPMS zircon U-Pb dating.Zircon from the melanosome defines upper and lower intercept U-Pb ages of 859±73Ma and 135±250Ma,respectively.Six spot analyses near the upper intercept yield a weighted mean 206Pb/238U age of 828±7Ma (MSWD=0.57).This age consists with the upper intercept age,interpreting as the age of the protolith crystallization.The protolith formation age might correspond to magmatism during the Mid-Neoproterozoic in the northern margin of the South China Block.On the other hand,new growth zircons in the leucosome samples show planar or weakly oscillatory zoning,low Th/U ratios.The morphological and internal structure features of zircons suggest that their growths were associated with partial melting.They yielded 206Pb/238U weighted mean ages of 135Ma and 131Ma,which were interpreted to register the ages of migmatization.The timing of the anatexis in the Tongbai high-grade metamorphic complex is coeval with the widespread post-collision magmatism in the Tongbai-Dabie orogen.It suggests that the onset of extensional tectonism in the Tongbai orogen is no later than 135Ma.
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ABSTRACT The current classification of granitic pegmatites, originally introduced by Černý (1991a), has been the accepted system for grouping pegmatites of diverse mineralogy and chemistry for nearly three decades. Despite its general acceptance, several issues have been highlighted (Müller et al. 2022) which have imposed some limitations on its use and therefore necessitated the need to reevaluate its methodology. A new classification for granitic pegmatites is proposed in an attempt to be more inclusive of pegmatite types omitted in previous classification schemes. The new approach utilizes a more comprehensive suite of accessory minerals and defines three pegmatite groups which are genetically related to granite plutons and the anatexis of metaigneous and metasedimentary protoliths. Pegmatites belonging to Groups 1 and 2 are generated from the residual melts of S-, A-, and I-type granite magmatism (RGM) as well as being direct products of anatexis (DPA), whereas Group 3 pegmatites are only derived by anatexis.
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