Abstract The Pb isotopic compositions of granulite xenoliths are crucial to constrain the formation and evolution of the lower continental crust (LCC) and are potentially the key to resolve the first Pb paradox of the Earth. However, it remains unclear how host magma infiltration affects the Pb isotopic compositions of LCC xenoliths and how different this effect is for different xenoliths in composition (from mafic to felsic). To address these issues, here we provide an in-situ chemical and Pb Sr isotopic study on feldspars and clinopyroxenes for a suite of fresh mafic and felsic granulite xenoliths from the Hannuoba region in the northern margin of the North China Craton (NCC), which have been previously well characterized for bulk-rock chemical and Sr–Nd–Pb isotopic compositions. Our study shows that the in-situ plagioclase and bulk-rock Pb isotopic compositions of the Hannuoba mafic granulite xenoliths are significantly decoupled. Such decoupling is demonstrated to have been caused by host magma infiltration through contributing the main Pb budget into the grain boundaries and cracks. However, this process appears to have relatively less or negligible effect on the Pb isotopic systematics of feldspar-rich intermediate-felsic granulite xenoliths, which is reflected by their consistent in-situ (feldspar) and bulk-rock Pb isotopic compositions. Such different effects on Pb isotopic systematics for different LCC xenoliths in composition are highly related to the Pb contents or abundances of feldspar in LCC xenoliths. Furthermore, our data suggest that the effect of host magma infiltration on the Pb isotopic systematics of LCC xenoliths may be not easily eliminated by the leaching technique and thus it should be prudent to use the bulk-rock Pb isotopic data of low-Pb LCC and potentially upper mantle xenoliths, such as peridotite, pyroxenite and garnet−/pyroxene-rich mafic granulite. Instead, in-situ analyses of minerals (e.g., feldspar, clinopyroxene) could provide a more convenient and reliable method to obtain the pristine Pb isotopic compositions of these low-Pb xenoliths.

中文翻译：

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寄主岩浆入渗对下地壳捕虏体Pb同位素系统学的影响——华北汉诺坝原位研究

摘要 麻粒岩捕虏体的铅同位素组成对于限制下陆壳（LCC）的形成和演化至关重要，可能是解决地球第一个铅悖论的关键。然而，目前尚不清楚宿主岩浆渗透如何影响 LCC 捕虏体的 Pb 同位素组成，以及这种影响对不同组成的捕虏体（从镁铁质到长英质）有何不同。为了解决这些问题，我们在此对华北克拉通 (NCC) 北缘汉诺坝地区的一组新鲜镁铁质和长英质麻粒岩包体进行了长石和单斜辉石的原位化学和 Pb Sr 同位素研究。之前已经对大块岩石化学和 Sr-Nd-Pb 同位素组成进行了很好的表征。我们的研究表明，汉诺坝镁铁质麻粒岩包体的原位斜长石和大块岩石铅同位素组成显着解耦。这种解耦被证明是由寄主岩浆通过将主要铅预算贡献到晶界和裂缝中而引起的。然而，这一过程似乎对富含长石的中长英质麻粒岩包体的 Pb 同位素系统学的影响相对较小或可以忽略不计，这反映在它们一致的原位（长石）和大块岩石 Pb 同位素组成上。不同组成的 LCC 捕虏体对 Pb 同位素系统学的这种不同影响与 LCC 捕虏体中长石的 Pb 含量或丰度高度相关。此外，我们的数据表明，宿主岩浆渗入对 LCC 捕虏体 Pb 同位素系统的影响可能无法通过浸出技术轻易消除，因此应谨慎使用低 Pb LCC 和潜在的上层岩石 Pb 同位素数据。地幔捕虏体，如橄榄岩、辉石岩和富含石榴石-/辉石的镁铁质麻粒岩。相反，矿物（例如，长石、斜辉石）的原位分析可以提供一种更方便和可靠的方法来获得这些低铅捕虏体的原始铅同位素组成。