The origin of mafic microgranular enclaves in granitoids: Insights from in situ Sr isotope of plagioclases and Zr-Hf isotopes of zircons

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.