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Source of Mesozoic intermediate-felsic igneous rocks in the North China craton:Granulite xenolith evidence

Richard W.Carlson  
【摘要】:正Four intermediate to felsic igneous rocks from the Zhangjiakou region,along the northern margin of the North China craton,have magmatic zircon U-Pb ages from 122 to 144 Ma.Two of these samples have inherited zircon U-Pb ages of~2.5 Ca,similar to the zircon ages of rocks from the surrounding granulite terrain.Zircons from two intermediate composition granulite xenoliths(JN0811 and JN0919) in the nearby Cenozoic Hannuoba basalts yield two groups of ages.The rims have concordant Mesozoic ages mostly between 120 and 145 Ma,coeval with the Mesozoic intermediate-felsic magmatism in the region,while the cores have discordant U-Pb ages with upper-intercepts of~2.5 Ga,overlapping the zircon ages of granulite terrain rocks, and lower-intercept ages of~130 Ma,approximating the ages of the Mesozoic intermediate-felsic magmatism.The Sr-Nd isotopic compositions of the Mesozoic intermediate-felsic igneous rocks are completely different from those expected for basaltic melts from either the lithospheric mande or the asthenospheric mantle,precluding a derivation by extensive fractional crystallization of mantle-derived magmas.The lack of correlation between(~(86)Sr/~(87)Sr)_i,ε_(Nd(t)) and SiO_2 for the Mesozoic igneous rocks,the very narrow range of zirconε_(Hf(t)) for individual intermediate-felsic igneous rocks,and simple binary mixing calculations argue against them being formed by mixing between mande-derived magma and preexisting crust that has extremely evolved Sr-Nd isotopic compositions like granulite xenoliths JNOSl 1 and JN0919.Hf isotopic compositions of the Mesozoic zircons and whole-rock geochemistry show that the granulite xenoliths with extremely evolved Sr-Nd isotopic compositions have not undergone partial melting during the Mesozoic and thus do not contribute to the Mesozoic intermediate-felsic magmas.Further comparisons show that the source rocks for the Mesozoic intermediate-felsic magmas likely were late Archean lower crustal rocks similar in chemical and isotopic compositions to rocks with SiO_262 wt.%from the granulite terrain.If so,pyroxene-rich mafic granulite xenoliths found in the Hannuoba basalts could be restites left behind after the partial melting of the late Archean lower crust.The results shed new light on the origin for Mesozoic zircons in granulite xenoliths from Nushan,at the southern margin of the North China craton.We suggest that the widespread Mesozoic intermediate-felsic igneous rocks in the eastern North China craton are most likely derived from partial melting of the intermediate-mafic rocks of the late Archean lower crust.

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