Analogous diagenetic conditions of dark enclave and its host granite derived by magma mixing: Evidence for a post-mixing magmatic process
Analogous diagenetic conditions of dark enclave and its host granite derived by magma mixing: Evidence for a post-mixing magmatic process
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暗色包体与其由岩浆混合形成的寄主花岗岩相似的成岩条件：混合后岩浆作用的证据
DOI：
10.1016/j.lithos.2020.105373
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发表时间：
2020-01
期刊：
影响因子：
3.5
通讯作者：
Saijun Sun;Rongqing Zhang;Y. Cong;Lipeng Zhang;Wei-dong Sun;Cong-ying Li;Xing Ding
中科院分区：
文献类型：
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作者：
Saijun Sun;Rongqing Zhang;Y. Cong;Lipeng Zhang;Wei-dong Sun;Cong-ying Li;Xing Ding
镁铁质微粒包体（MMEs）通常是在岩浆混合过程中形成的，通常记录相对于其寄主岩石的物理化学成岩特征。研究发现，白垩纪千家岩体中的一些MMEs（被认为是岩浆混合的产物）表现出与寄主花岗岩相似的成岩条件。基于矿物原位成分的综合计算（例如，结果表明，千家二长质MMEs及其寄主花岗岩的成岩温度（700-800 °C）和压力（0.20-0.28 GPa）相似，但氧逸度和水活度不同。MMEs具有较高的氧逸度和水活性，以及较低的Th/U比，相比，寄主花岗岩，这表明更多的氧化和富H2O的情况。此外，MMEs和寄主花岗岩及其母岩浆中磷灰石的高F浓度和F/Cl比值表明，岩浆源区相对富F，可能是板块俯冲过程中含F矿物脱水的产物。MMEs和其寄主花岗岩之间相似的物理化学成岩条件似乎揭示了它们的同源特征;然而，某些矿物（例如，磷灰石和钛铁矿）提供了岩浆混合的线索。因此，我们建议，所涉及的MMEs可能是来自本地异质混合熔体或早期结晶过程中的后混合过程。在这种情况下，由于地球化学再平衡，有关岩浆混合的信息几乎被消除了。因此，在岩浆混合型侵入体中，MMEs不仅可以在岩浆混合过程中形成，而且可以在混合后阶段形成，其中某些矿物很可能保留了岩浆混合的一些地球化学特征。
Mafic microgranular enclaves (MMEs) are commonly formed during magma mixing and usually record contrasting physicochemical diagenetic characteristics relative to their host rocks. We present a finding that some MMEs from the Cretaceous Qianjia pluton, which is believed to be the product of magma mixing, exhibit diagenetic conditions analogous to those of their host granites. Integrated calculations based on in situ compositions of minerals (e.g., zircon, titanite, amphibole, feldspar, and apatite) from the Qianjia monzonitic MMEs and their host granites indicate that they exhibit similar diagenetic temperatures (700–800 °C) and pressures (0.20–0.28 GPa), but discrepant oxygen fugacity and water activity. The MMEs have higher oxygen fugacity and water activity, as well as lower Th/U ratios, compared to the host granites, suggesting more oxidized and H2O-rich circumstances. Besides, high F concentrations and F/Cl ratios in apatites from both the MMEs and host granites and their parent magma indicate a relatively F-rich magma source that was probably derived by F-bearing mineral dehydration during plate subduction. Similar physicochemical diagenetic conditions between the MMEs and their host granites seemingly reveal their cognate features; however, slight differences in the oxygen fugacity, water activity, and chemistry of certain minerals (e.g., apatite and titanite) provide clues on magma mixing. We therefore propose that the MMEs involved might have been derived from local heterogeneous hybrid melts or by early crystallization during a post-mixing process. In this case, information about the magma mixing has been almost eliminated due to the geochemical re-equilibration. Therefore, in the case of magma-mixing derived intrusions, the MMEs could be formed not only during magma mixing but also during a post-mixing stage, in which certain minerals most likely preserved a few geochemical characteristics of magma mixing.