Zircon U-Pb-Hf isotopes and geochemistry of Jurassic igneous rocks from the southern Zhangguangcai Range, NE China: constraints on magmatism, petrogenesis and tectonic implications

In this study, we present new zircon U-Pb and Hf isotope data and geochemical analyses for Jurassic igneous rocks from the southern Zhangguangcai Range, NE China. In the context of our new U/Pb data together with other published data (n = 165), we recognize three major magmatic pulses in the eastern Songnen Block during the Jurassic: earliest Jurassic (200–195 Ma), late Early Jurassic (183–175 Ma) and early Late Jurassic (ca. 163 Ma). According to the petrological classification scheme, rocks are divided into two categories of andesite and I-type granitoids. Generally, Early Jurassic monzogranites have more evolved compositions with SiO2, Al2O3, and K2O+Na2O concentrations of 67–74 wt.%, 13.28–17.04 wt.% and 7.25–9.31 wt.%, respectively than Late Jurassic granodiorite. Early Jurassic andesite has SiO2, Al2O3, Na2O+K2O contents of 54.64–56 wt.%, 17.27–17.44 wt.% and 3.47–4.06 wt.%, respectively with low Mg# values of 0.18–0.26. In addition, all samples show enrichment in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depletion in high strength field elements (HSFEs, such as Nb, Ta, P and Ti) and low Sr/Y and (La/Yb)N ratios, all of which are characteristic of arc-type magmatic signatures. In view of the Hf isotope signatures, these rocks show high initial 176Hf/177Hf ratios (0.282711–0.283069) corresponding to positive εHf(t) values (+0.39 to +13.66) and Neoproterozoic two-stage model ages (1154–550 Ma). In summary, the geochemical results suggest that the primary magmas of Jurassic granitoids were derived from variable degrees of partial melting of juvenile lower crust, whereas Early Jurassic andesite was derived from the partial melting of mantle wedge peridotite metasomatized by hydrous melts from subducting sediments and/or oceanic crust. The Jurassic arc-type magmatism in the eastern Songnen Block indicates that the Mudanjiang oceanic plate was still subducting in the Jurassic.

本研究针对中国东北张广才岭南部的侏罗纪火成岩，提供了全新的锆石U-Pb（zircon U-Pb）、Hf同位素（Hf isotope）数据以及地球化学分析结果。结合本次获得的U-Pb数据与已发表的165组相关数据，我们识别出松嫩地块东部侏罗纪时期的三期主要岩浆活动脉冲：早侏罗世早期（200–195 Ma）、早侏罗世晚期（183–175 Ma）以及晚侏罗世早期（约163 Ma）。依据岩石学分类方案，本次研究的岩石可划分为安山岩与I型花岗岩类（I-type granitoids）两大类。总体而言，早侏罗世二长花岗岩的成分更为演化，其SiO₂、Al₂O₃以及K₂O+Na₂O含量分别为67–74 wt.%、13.28–17.04 wt.%与7.25–9.31 wt.%，均高于晚侏罗世花岗闪长岩。早侏罗世安山岩的SiO₂、Al₂O₃、Na₂O+K₂O含量分别为54.64–56 wt.%、17.27–17.44 wt.%与3.47–4.06 wt.%，且镁值（Mg#）较低，介于0.18–0.26之间。此外，所有样品均表现出轻稀土元素（light rare earth elements，LREEs）与大离子亲石元素（large-ion lithophile elements，LILEs）富集、高场强元素（high strength field elements，HSFEs，如Nb、Ta、P及Ti）亏损的特征，同时具备较低的Sr/Y与(La/Yb)ₙ比值，这些均为弧型岩浆作用的典型标识。从Hf同位素特征来看，这些岩石的初始¹⁷⁶Hf/¹⁷⁷Hf比值较高（0.282711–0.283069），对应正的εHf(t)值（+0.39至+13.66），且呈现新元古代两阶段模式年龄（1154–550 Ma）。综上，地球化学结果表明，侏罗纪花岗岩类的原始岩浆起源于新生下地壳不同程度的部分熔融，而早侏罗世安山岩则源自被俯冲沉积物及/或洋壳的含水熔体交代的地幔楔橄榄岩的部分熔融。松嫩地块东部的侏罗纪弧型岩浆活动表明，牡丹江洋板块在侏罗纪时期仍处于俯冲状态。