Abstract Orthopyroxene-bearing granitoids of both magmatic and metamorphic origin occur in various terranes of the world, although their petrogenetic distinction remains equivocal. Here we investigate an example of unmetamorphosed magmatic charnockites (enderbite-charnockite) of the Late Paleoproterozoic Luyashan pluton in the North China Craton, where both charnockitic (granitic) and enderbitic (granodioritic) granitoids occurs in the pluton. In a thin section scale, charnockitic domains (~1–2 cm) are present within the outcrop scale enderbitic granitoid, that are composed of orthopyroxene, biotite, plagioclase, K-feldspar, quartz, ilmenite, magnetite, and pyrrhotite. Whereas the enderbite domain shows amphibole, clinopyroxene and apatite additionally. Mineral textures and orthopyroxene inclusions in amphibole and biotite preserved early stage orthopyroxene from both domains, where they carry pure endmember ilmenite and magnetite as the major oxide phases with similar compositions. Log (aH2O) - Temperature pseudosection modeling at log fO2 = −15 and pressure 800 MPa shows that orthopyroxene might be crystallized at ~900 °C. Subsequently, the mixed system achieved a higher water activity due to CO2 escape during cooling. During cooling, both the enderbitic and charnockitic domain are stable at a water content of >1 wt% or water activity (aH2O) of 0.4 or above. Our results suggest that mixing of immiscible granodioritic and granitic magma caused the formation of the enderbitic and charnockitic domains at similar water activity. Orthopyroxene inclusions in amphibole and biotite, lack of dehydration and oxidation textures, and absence of pyrite and apatite with monazite precipitates in the Luyashan igneous charnockites suggest that these rocks are significantly different from their metamorphic counterparts. Our study suggests that orthopyroxene crystallization in igneous charnockites is mainly dictated by the temperature and pressure at which magma starts crystallizing, rather than water activity. However, in the case of metamorphic charnockites, in the absence of partial melting, metasomatic orthopyroxene formation depends on low water activity of fluids and their dissolution capacity.

中文翻译：

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华北克拉通晚古元古代鹿牙山火成岩炭黑-灰岩岩成因及其与变质岩对比

摘要 岩浆成因和变质成因的含斜方辉石花岗岩出现在世界上不同的地体中，尽管它们的成因区别仍然模棱两可。在这里，我们研究了华北克拉通晚古元古代鲁雅山岩体未变质岩浆岩浆岩（灰岩-岩岩）的一个例子，其中岩岩（花岗岩）和灰岩（花岗闪长岩）花岗岩都出现在岩体中。在薄片尺度上，露头尺度的软岩花岗岩中存在卡诺岩域（~1-2 cm），由斜辉石、黑云母、斜长石、钾长石、石英、钛铁矿、磁铁矿和磁黄铁矿组成。而 enderbite 域还显示角闪石、斜辉石和磷灰石。角闪石和黑云母中的矿物结构和斜方辉石包裹体保留了来自两个域的早期斜方辉石，它们携带纯端元钛铁矿和磁铁矿作为具有相似成分的主要氧化物相。Log (aH2O) - log fO2 = -15 和压力 800 MPa 下的温度假截面模型显示，斜方辉石可能在 ~900 °C 下结晶。随后，由于冷却过程中 CO2 逸出，混合系统实现了更高的水分活度。在冷却过程中，在大于 1 wt% 的水含量或 0.4 或更高的水活度 (aH2O) 下，enderbic 域和 charnockitic 域都是稳定的。我们的结果表明，不混溶的花岗闪长岩和花岗质岩浆的混合导致在相似的水分活度下形成了泥质和炭质域。角闪石和黑云母中的斜方辉石包裹体，缺乏脱水和氧化结构，并且在芦芽山火成岩中没有黄铁矿和具有独居石沉淀物的磷灰石，表明这些岩石与其变质岩显着不同。我们的研究表明，火成岩炭黑中的斜方辉石结晶主要取决于岩浆开始结晶时的温度和压力，而不是水活度。然而，在变质黄铜矿的情况下，在没有部分熔化的情况下，交代斜方辉石的形成取决于流体的低水活度及其溶解能力。我们的研究表明，火成岩炭黑中的斜方辉石结晶主要取决于岩浆开始结晶时的温度和压力，而不是水活度。然而，在变质黄铜矿的情况下，在没有部分熔化的情况下，交代斜方辉石的形成取决于流体的低水活度及其溶解能力。我们的研究表明，火成岩炭黑中的斜方辉石结晶主要取决于岩浆开始结晶时的温度和压力，而不是水活度。然而，在变质黄铜矿的情况下，在没有部分熔化的情况下，交代斜方辉石的形成取决于流体的低水活度及其溶解能力。