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Palaeozoic pre- and post-collision calc-alkaline magmatism in the Qinling orogenic belt, central China, as documented by zircon ages on granitoid rocks

Abstract:

Based on large-scale reconnaissance mapping, we identified two calc-alkaline plutonic assemblages from the northern Qinling orogenic belt, central China. The older assemblage of intrusions, closely associated and deformed coevally with their host volcanic arc sequences, seems to represent the fractionation product of basaltic arc magma. It therefore predates the collision of the North China Block with the Central Qinling island-arc system that developed in a SW Pacific-type oceanic domain south of the North China Block. Single-zircon 207Pb/206Pb evaporation dating yielded early to middle Ordovician ages for this assemblage, with a relatively small range from 487.2 ± 1.1 to 470.2 ± 1.3 Ma. Intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of the older assemblage. They are interpreted as post-collisional calc-alkaline granitoids. Single zircon dating provided an age of 401.8 ± 0.8 Ma for the younger assemblage, consistent with earlier work that defines an age range from c. 420 to 395 Ma. Our data favour a tectonic model involving formation and amalgamation of island arc and microcontinent terranes between ca. 490 and 470 Ma ago to create the Central Qinling Zone which subsequently collided with the North China Block prior to c. 400 Ma ago. A late Precambrian age of 762.0 ± 0.7 Ma for a granitoid gneiss at the northern margin of the Yangtze Block supports a Gondwana affinity for this large continental block.