Mafic microgranular enclaves (MMEs), widespread in intermediate to felsic arc plutons, carry significant information on the genesis and evolution of arc magmas, yet their origin remains debatable. Here, we examine MME-host diorite pairs from the c.200 Ma Cuijiu Igneous Complex in the eastern Gangdese Batholith, southern Tibet, to constrain the petrogenesis of MMEs and the evolution of arc magmas. Within the complex, MMEs are essentially similar to their host diorites with similar emplacement ages (∼200 Ma), mineral assemblages and mineral compositions, as well as whole-rock Sr–Nd–Hf and zircon Hf isotopic compositions. However, MMEs have higher modal contents of hornblende and biotite, and are enriched in compatible elements and depleted in incompatible elements. Zircons from some MME samples are characterized by dark cathodoluminescence (CL) cores overgrown by light-CL rims of varying thickness. The dark-CL cores show higher Th, U and rare earth elements (REE) abundances than the light-CL rims. Based on comparison with co-genetic mafic melts and mass-balance calculations, we propose that the MMEs were early-crystallized cumulates (autoliths) related to their host diorites. The chilled textures, flow microstructures and pillow shapes suggest that the MMEs experienced rapid cooling before being captured by the host magmas. The rapid cooling may result from contact between ascending diorite magmas and cooler wall rocks. As the magmas quickly crystallized, they reached second boiling and vesiculation, and separated into fine-grained crystal-rich margins and melt-rich centres. Gradients in crystallinity and pressure expelled interstitial melts from the crystal-rich margins to the crystal-poor centres, leading to crystal-liquid separation (gas-driven filter pressing). The dark-CL zircon cores with high Th and U abundances may crystallize from highly evolved interstitial melts within the crystal-rich margins. The fine-grained crystal-rich margins were subsequently captured and dragged as MMEs before their complete crystallization by later ascending host magmas. This differentiation process could have occurred over several kilometres of magma ascent, and have played an important role in the polybaric fractional crystallization of the Cuijiu Igneous Complex, feeding more differentiated andesitic magmas to upper crustal mushes.

中文翻译：

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快速冷却过程中气压压滤形成的镁铁质微粒包体——以藏南冈底斯岩基为例

广泛存在于中长英质弧岩体中的镁铁质微粒飞地 (​​MME) 携带了有关弧形岩浆成因和演化的重要信息，但其起源仍有争议。在这里，我们研究了藏南冈底斯岩基东部 c.200 马翠酒火成杂岩中的 MME 主体闪长岩对，以约束 MME 的成因和弧形岩浆的演化。在杂岩体中，MME 与其宿主闪长岩基本相似，具有相似的侵位年龄（~200 Ma）、矿物组合和矿物组成，以及全岩 Sr-Nd-Hf 和锆石 Hf 同位素组成。然而，MMEs 具有较高的角闪石和黑云母模态含量，并且富含相容元素并缺乏不相容元素。来自一些 MME 样品的锆石的特征是深色阴极发光 (CL) 核心长满了不同厚度的浅色 CL 边缘。暗 CL 核心显​​示出比亮 CL 边缘更高的 Th、U 和稀土元素 (REE) 丰度。基于与共生镁铁质熔体的比较和质量平衡计算，我们提出 MME 是与其宿主闪长岩相关的早期结晶堆积物（自石）。冰冷的纹理、流动微观结构和枕形表明 MME 在被宿主岩浆捕获之前经历了快速冷却。快速冷却可能是由于上升的闪长岩岩浆和较冷的围岩之间的接触造成的。随着岩浆迅速结晶，它们达到第二次沸腾和泡状，并分离成细粒富含晶体的边缘和富含熔体的中心。结晶度梯度和压力将间隙熔体从富含晶体的边缘驱赶到晶体贫乏的中心，导致晶体-液体分离（气体驱动压滤）。具有高 Th 和 U 丰度的暗 CL 锆石核心可能从富含晶体的边缘内高度演化的间隙熔体中结晶。随后，细粒富含晶体的边缘在它们完全结晶之前被后来上升的宿主岩浆捕获并拖拽为 MME。这种分异过程可能发生在数公里的岩浆上升过程中，并且在翠酒火成杂岩的多元分馏结晶中发挥了重要作用，将更多分异的安山岩浆输送到上地壳糊状物中。导致晶液分离（气体驱动压滤）。具有高 Th 和 U 丰度的暗 CL 锆石核心可能从富含晶体的边缘内高度演化的间隙熔体中结晶。随后，细粒富含晶体的边缘在它们完全结晶之前被后来上升的宿主岩浆捕获并拖拽为 MME。这种分异过程可能发生在数公里的岩浆上升过程中，并且在翠酒火成杂岩的多元分馏结晶中发挥了重要作用，将更多分异的安山岩浆输送到上地壳糊状物中。导致晶液分离（气体驱动压滤）。具有高 Th 和 U 丰度的暗 CL 锆石核心可能从富含晶体的边缘内高度演化的间隙熔体中结晶。随后，细粒富含晶体的边缘在它们完全结晶之前被后来上升的宿主岩浆捕获并拖拽为 MME。这种分异过程可能发生在数公里的岩浆上升过程中，并且在翠酒火成杂岩的多元分馏结晶中发挥了重要作用，将更多分异的安山岩浆输送到上地壳糊状物中。随后，细粒富含晶体的边缘在它们完全结晶之前被后来上升的宿主岩浆捕获并拖拽为 MME。这种分异过程可能发生在数公里的岩浆上升过程中，并且在翠酒火成杂岩的多元分馏结晶中发挥了重要作用，将更多分异的安山岩浆输送到上地壳糊状物中。随后，细粒富含晶体的边缘在它们完全结晶之前被后来上升的宿主岩浆捕获并拖拽为 MME。这种分异过程可能发生在数公里的岩浆上升过程中，并且在翠酒火成杂岩的多元分馏结晶中发挥了重要作用，将更多分异的安山岩浆输送到上地壳糊状物中。