In-situ geochemical and isotopic analyses of zircon and monazite can provide compelling and detailed insights into the source and evolution of their hosting granite. Here we investigated the U-Th-Pb and Hf/Nd-O isotopic signatures of zircon and monazite in two petrological zones (i.e., fine- to medium-grained zone and medium-grained zone) of the Laojunshan granite pluton in the Qinling Orogen, central China, to better constrain their source nature and evolution processes. Co-existing magmatic zircon and monazite in both zones gave identical crystallization ages of ca. 113 Ma. In the fine- to medium-grained zone, both magmatic and xenocrystic zircons were identified, which respectively have ε(t) values of −6.5 to −5.0 and − 17.4 to +5.1, and δO values of +5.25‰ to +5.78‰ and + 5.17‰ to +9.15‰. The age distribution pattern and HfO isotopic characteristics of the xenocrystic zircons are similar to those of detrital zircons from the wall rock, i.e., meta-sedimentary rocks in the Kuanping unit. Monazite grains in this zone have relatively low SiO, ThO but high CaO contents, dominated by cheralite-type substitution. These monazites have ε(t) values of −5.8 to −4.6 and δO values of +3.79‰ to +5.66‰, except one grain (ε(t) = −10.4 to −9.6, δO = +9.22‰ to +9.81‰), which is likely a xenocrystic crystal assimilated from wall rocks. The identical UPb age but contrasting NdO isotopic compositions between the xenocrystic monazite and the magmatic monazite grains indicate that the former might have experienced solid-state recrystallization process and completely lost radiogenic Pb. Both zircon and monazite geochemical and isotopic signatures consistently indicate that crustal assimilation occurred in the fine- to medium-grained granite zone. In contrast, in the medium-grained zone, only magmatic zircons and monazites were identified. The magmatic zircons have ε(t) values of −3.2 to +2.7, and δO values of +4.65‰ to +6.36‰. The magmatic monazites are characterized by huttonite-type substitution with high SiO, ThO but low CaO contents. Their ε(t) values vary from −5.7 to +1.6 (mostly < −4.4) and δO values from +4.65‰ to +6.36‰. Given that the isotopic features of zircons and monazites in this zone are consistent with their whole-rock isotope features and no inherited/xenocrystic grains were observed, the zircons and monazites should record the source isotope compositions. The presence of highly radiogenic Nd isotopes in monazite coupled with its normal mantle-like O isotopes suggest minor involvement of juvenile mantle-derived components in the magma source. Accordingly, we advocate combined study of zircon and monazite, which can better track granite source nature and evolution processes than individual mineral.

中文翻译：

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通过询问锆石和独居石 U-Th-Pb 和 Hf/Nd-O 同位素来精炼花岗岩的生成

锆石和独居石的原位地球化学和同位素分析可以为它们的花岗岩的来源和演化提供令人信服且详细的见解。本文研究了秦岭造山带老君山花岗岩体两个岩石学带（即细粒到中粒带和中粒带）中锆石和独居石的U-Th-Pb和Hf/Nd-O同位素特征华中地区，以更好地约束其来源性质和演化过程。两个区域共存的岩浆锆石和独居石给出了相同的约 100 年代的结晶年龄。 113马。细粒至中粒带中同时发现岩浆锆石和异晶锆石，其ε(t)值分别为-6.5～-5.0和-17.4～+5.1，δ18O值为+5.25‰～+5.78‰。 + 5.17‰至+9.15‰。异晶锆石的年龄分布规律和HfO同位素特征与宽平单元围岩变质沉积岩中的碎屑锆石相似。该区独居石晶粒SiO、ThO含量较低，而CaO含量较高，以白铍沸石型取代为主。这些独居石的 ε(t) 值为 -5.8 至 -4.6，δ18O 值为 +3.79‰ 至 +5.66‰，除了一颗晶粒（ε(t) = -10.4 至 -9.6，δ18O = +9.22‰ 至 +9.81‰ ），这可能是从围岩中同化的异种晶体。异晶独居石与岩浆独居石颗粒的UPb年龄相同，但NdO同位素组成不同，表明前者可能经历了固态再结晶过程，放射性Pb完全消失。锆石和独居石的地球化学和同位素特征一致表明，地壳同化作用发生在细粒至中粒花岗岩区域。相反，在中粒带中，仅发现了岩浆锆石和独居石。岩浆锆石的ε(t)值为-3.2至+2.7，δO值为+4.65‰至+6.36‰。岩浆独居石具有高SiO、ThO、低CaO含量的钠辉石型替代特征。它们的 ε(t) 值从 -5.7 到 +1.6（大部分 < -4.4）变化，δ18O 值从 +4.65 ‰ 到 +6.36 ‰ 变化。鉴于该带锆石和独居石的同位素特征与其全岩同位素特征一致，且未观察到继承/异晶颗粒，因此锆石和独居石应该记录了源同位素组成。独居石中存在高放射性 Nd 同位素及其正常的类地幔 O 同位素，表明岩浆源中存在少量来自幼年地幔的成分。因此，我们主张对锆石和独居石进行联合研究，这比单个矿物更能追踪花岗岩的物源性质和演化过程。