Source of Mesozoic intermediate-felsic igneous rocks in the North China craton: Granulite xenolith evidence
Source of Mesozoic intermediate-felsic igneous rocks in the North China craton: Granulite xenolith evidence
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DOI：
10.1016/j.lithos.2011.02.017
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发表时间：
2011-07
期刊：
影响因子：
3.5
通讯作者：
Neng Jiang;R. Carlson;Jinhui Guo
中科院分区：
文献类型：
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作者：
Neng Jiang;R. Carlson;Jinhui Guo
华北克拉通北缘张家口地区4块中英质火成岩的岩浆锆石U-Pb年龄在122 ~ 144 Ma之间。其中2个样品继承了~ 2.5 Ga的锆石U-Pb年龄，与周围麻粒岩地形岩石的锆石年龄相似。新生代汉诺坝玄武岩中两个中等成分麻粒岩捕虏体（JN0811和JN0919）的锆石显示出两组年龄。岩心的U-Pb年龄不一致，上截距为~ 2.5 Ga，与麻粒岩地质体锆石年龄重叠，下截距为~ 130 Ma，与中生代中长英质岩浆活动年龄相近。中生代中长英质火成岩的Sr-Nd同位素组成与岩石圈地幔或软流圈地幔的玄武岩熔体的Sr-Nd同位素组成完全不同，排除了地幔岩浆广泛分离结晶的推导。中生代火成岩的（86Sr/87Sr）i、εNd(t)和sio2之间缺乏相关性，个别中英质火成岩的锆石εHf(t)范围很窄，以及简单的二元混合计算表明，中生代火成岩不是由幔源岩浆和原存地壳混合形成的，原存地壳的Sr-Nd同位素组成如麻粒岩包体JN0811和JN0919。中生代锆石Hf同位素组成和全岩地球化学特征表明，Sr-Nd同位素组成极演化的麻粒岩包体在中生代未发生部分熔融，不属于中生代中长英质岩浆的成因。进一步比较表明，中生代中长英质岩浆的烃源岩可能为晚太古代下地壳岩石，其化学和同位素组成与麻粒岩地区SiO2< 62 wt.%的岩石相似。如果是这样，那么汉诺坝玄武岩中发现的富辉石基性麻粒岩捕虏体可能是太古宙晚期下地壳部分熔融后遗留下来的残岩。研究结果对华北克拉通南缘庐山麻粒岩捕虏体中中生代锆石的成因有了新的认识。华北克拉通东部广泛分布的中生代中长英质火成岩极有可能是由晚太古代下地壳中基性岩的部分熔融作用形成的。
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 Ga, 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 mantle or the asthenospheric mantle, precluding a derivation by extensive fractional crystallization of mantle-derived magmas. The lack of correlation between (86Sr/87Sr)i, εNd(t)and SiO2for the Mesozoic igneous rocks, the very narrow range of zircon εHf(t)for individual intermediate-felsic igneous rocks, and simple binary mixing calculations argue against them being formed by mixing between mantle-derived magma and preexisting crust that has extremely evolved Sr–Nd isotopic compositions like granulite xenoliths JN0811 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 SiO2< 62 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.