Mafic Microgranular Enclaves Formed by Gas-driven Filter Pressing During Rapid Cooling: an Example from the Gangdese Batholith in Southern Tibet

Abstract

AbstractMafic 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.