Abstract Mafic microgranular enclaves (MMEs) are common in many granitoid plutons and their origin is crucial for understanding the petrogenesis and tectonic setting of their hosts. Most previous workers have studied MMEs by using in situ zircon U-Pb and Hf-O isotopes. However, whether the zircons in the MMEs recorded the primary isotopes of their host MMEs is not well constrained. Here, two MMEs from the Daocheng (DC-2) and Dongcuo (HZS-3) granitic plutons, as well as their host granites, in the Triassic Yidun arc belt, eastern Tibetan Plateau, were studied. The MMEs have zircon U-Pb ages of 219 ± 3 Ma and 225 ± 3 Ma respectively, similar to their host granites. They also have similar or slightly depleted zircon Lu-Hf isotopes to their host granites. Furthermore, zircons from the Dongcuo pluton have indistinguishable Ti-in-zircon temperatures (Tzr) and Th/U ratios from those of their host granites, whereas zircons from the Daocheng pluton have higher Tzr values and Th/U ratios than those of their host granites. Model calculations indicate that the whole-rock composition of the studied MMEs was different from the melt composition from which the zircon grains grew. Additionally, zircon from Daocheng sample DC-2 has larger variations of Zr stable isotopes than its host granite, whereas those of Dongcuo sample HZS-3 have lower δ94/90ZrIPGP-Zr values than its host granite. Previous study suggests that early crystallized zircons will have lighter Zr isotopes. Thus, based on our new data, we propose that zircons from Dongcuo were actually xenocrysts that formed at an early stage in the granitic magma chamber, whereas those from Daocheng crystallized during magma mixing. Thus, none of the zircons in this study have recorded the primary isotopes of the MMEs. In addition, there are also many An-rich plagioclase crystals in the MMEs from both localities and the initial 87Sr/86Sr ratios of these plagioclases are quite heterogeneous. Both of the samples (DC-2 and HZS-3) record three peaks of 0.7078, 0.7064 and 0.7030, and 0.7087, 0.7096 and 0.7105, respectively, which record the primary Sr isotopes of the MMEs, as well as details of the interaction between the mantle-derived and crust-derived magmas. The lowest initial 87Sr/86Sr ratio peak was lower than the arc volcanic rocks but similar to those of MORBs in the Jinshajiang suture zone of the area. Therefore, the primary magma of the MMEs was likely derived from a depleted mantle source. We propose that in situ Sr isotopes of An-rich plagioclase and Zr isotopes of zircon can be powerful tools in deciphering the petrogenesis of MMEs. To simply use the zircon Hf-O isotopes to constrain the primary composition of MMEs is not adequate for determining their origin.

中文翻译：

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花岗岩中镁铁质微粒包体的起源：斜长石原位Sr同位素和锆石Zr-Hf同位素的见解

摘要 镁铁质微粒飞地（MMEs）在许多花岗岩类岩体中很常见，它们的起源对于理解其宿主的岩石成因和构造环境至关重要。大多数以前的工作人员通过使用原位锆石 U-Pb 和 Hf-O 同位素研究了 MME。然而，MMEs 中的锆石是否记录了其宿主 MMEs 的主要同位素并没有得到很好的限制。在这里，研究了青藏高原东部三叠纪义墩弧带中稻城（DC-2）和东厝（HZS-3）花岗质岩体的两个MME及其寄主花岗岩。MME 的锆石 U-Pb 年龄分别为 219 ± 3 Ma 和 225 ± 3 Ma，与其寄主花岗岩相似。它们还具有与其寄主花岗岩相似或略微耗尽的锆石 Lu-Hf 同位素。此外，东厝岩体锆石的锆石温度（Tzr）和 Th/U 比值与其寄主花岗岩难以区分，而稻城岩体锆石的 Tzr 值和 Th/U 比值高于寄主花岗岩。模型计算表明，所研究的 MME 的全岩成分与锆石晶粒生长的熔体成分不同。此外，稻城样品 DC-2 的锆石 Zr 稳定同位素的变化大于其寄主花岗岩，而东厝样品 HZS-3 的锆石δ94/90ZrIPGP-Zr 值低于其寄主花岗岩。先前的研究表明，早期结晶的锆石将具有较轻的 Zr 同位素。因此，根据我们的新数据，我们认为来自东措的锆石实际上是在花岗岩岩浆房早期形成的异晶，而稻城的那些是在岩浆混合过程中结晶的。因此，本研究中的锆石均未记录 MME 的主要同位素。此外，来自两个地方的 MME 中也有许多富含 An 的斜长石晶体，并且这些斜长石的初始 87Sr/86Sr 比率非常不均匀。两个样品（DC-2 和 HZS-3）分别记录了 0.7078、0.7064 和 0.7030 以及 0.7087、0.7096 和 0.7105 的三个峰，它们记录了 MME 的主要 Sr 同位素，以及它们之间相互作用的详细信息地幔源岩浆和地壳源岩浆。最低的 87Sr/86Sr 比值峰值低于弧形火山岩，但与该地区金沙江缝合带 MORBs 的峰值相似。因此，MME 的原生岩浆很可能来自贫化的地幔源。我们建议富含 An 的斜长石的原位 Sr 同位素和锆石的 Zr 同位素可以成为破译 MME 岩石成因的有力工具。简单地使用锆石 Hf-O 同位素来限制 MME 的主要成分不足以确定其来源。