The role of mafic microgranular enclaves in the generation of Early Cretaceous granitic rocks of SE China: evidence from zircon U–Pb geochronology, geochemistry, and Hf isotopic data for the Liangnong pluton, eastern Zhejiang Province

In order to constrain the timing and petrogenesis of both the hosting rocks and the inner mafic microgranular enclaves (MMEs) of the Liangnong pluton, SE China, we have performed a series of bulk-rock geochemistry, zircon U–Pb, and Hf isotopic analysis, respectively. Zircon laser ablation–inductively coupled plasma–mass spectrometry U–Pb isotopic analysis yielded ages of 106.3 ± 1.1 Ma for the granodiorite and 103.9 ± 1.6 to 105 ± 1.8 Ma for monzogranite phases within the hosting pluton, as well as an age of 104.7 ± 0.8 Ma for the associated MMEs. The host rocks are metaluminous, have A/CNK values of 0.91–1.09, contain relatively high concentrations of SiO2 and K2O, are enriched in Rb, Th, Ba, Zr, and Hf, are depleted of Sr, P, Ti, Nd, and Ta, contain high concentrations of the rare earth elements (REE) and the light REE, and have moderately negative Eu anomalies (Eu*/Eu = 0.6–0.8). In comparison, the MMEs contain high concentrations of Al2O3, FeO, MgO, and TiO2, are relatively enriched in Ba, U, and Sr, and are depleted in Th, Nd, and Zr. They have lower total REE concentrations and higher Eu*/Eu values than the hosting granites. The zircons within the hosting granites have Hf crustal model ages (TDMC) that show a peak at 1.29–1.85 Ga. Zircons within the MMEs have different εHf(t) values (–3.7 to +4.9) than the zircons within the hosting granites (–10.8 to –1.9). The results indicate that the MMEs and the hosting granites crystallized from magmas with different sources, thereby showing that the Early Cretaceous magmatism in the coastal areas of SE China was generated by the widespread injection of mantle-derived magmas caused by rollback of the subducting palaeo-Pacific Plate.

为限定中国东南部凉农岩体及其内部镁铁质微粒包体（mafic microgranular enclaves, MMEs）的形成时代与岩石成因，我们分别开展了一系列全岩地球化学、锆石U-Pb及Hf同位素分析。锆石激光剥蚀-电感耦合等离子体质谱U-Pb同位素定年结果显示，寄主岩体中的花岗闪长岩相形成年龄为106.3±1.1 Ma，二长花岗岩相形成年龄介于103.9±1.6 Ma至105±1.8 Ma之间，伴生的MMEs形成年龄为104.7±0.8 Ma。寄主岩体为准铝质岩石，铝饱和指数（A/CNK）为0.91~1.09，SiO₂与K₂O含量相对较高，富集Rb、Th、Ba、Zr及Hf，亏损Sr、P、Ti、Nd及Ta；稀土元素（REE）总含量及轻稀土元素含量较高，且呈现中等程度的负铕异常（Eu*/Eu=0.6~0.8）。相较而言，MMEs的Al₂O₃、FeO、MgO及TiO₂含量较高，相对富集Ba、U及Sr，亏损Th、Nd及Zr；其总稀土元素含量低于寄主花岗岩，而Eu*/Eu值更高。寄主花岗岩中的锆石Hf地壳模式年龄（TDMC）峰值区间为1.29~1.85 Ga；MMEs中的锆石εHf(t)值介于-3.7~+4.9之间，与寄主花岗岩锆石的εHf(t)值（-10.8~-1.9）存在显著差异。上述结果表明，MMEs与寄主花岗岩分别源自不同的岩浆源区，由此证明中国东南部沿海地区早白垩世岩浆作用是由俯冲古太平洋板块回卷引发的广泛幔源岩浆注入所形成的。