北苏鲁变辉长岩锆石U-Pb年龄和Lu-Hf同位素组成及其对源区的指示

Zircon U-Pb Ages and Lu-Hf Isotopic Compositions of Meta-gabbro in the Northern Sulu UHP Metamorphic Belt and its Provenance

在北苏鲁经历超高压变质的花岗片麻岩中零星分布着一些变质表壳岩包体或者残片,通常称之为荆山群。北苏鲁荆山群的年代学特征以及构造亲缘性仍不清楚。本文报道了威海市郊和海阳所附近的荆山群露头,有脉状变质辉长岩侵入到荆山群岩系中。对上述两处变质辉长岩中锆石的阴极发光照片、稀土元素配分模式和Th/U比值的综合分析表明,它们均为岩浆锆石,部分岩浆锆石具有弱发光效应和岩浆韵律环带的核部、以及被改造的强发光效应的边部,但未见新生的变质锆石。未被改造的岩浆锆石微区的LA-ICP-MS U-Pb定年结果表明,威海市郊变辉长岩(S4-2)的^(207)Pb/^(206)Pb年龄值变化范围是1831~1966 Ma,加权平均年龄为1870±34 Ma(MSWD=0.6);海阳所变辉长岩(H3)中绝大部分锆石的^(207)Pb/^(206)Pb年龄值变化范围是1769~1887 Ma,加权平均年龄为1839±37 Ma(MSWD=0.5),两件样品共同记录了一致的ca.1.85 Ga岩浆事件。另外,海阳所变辉长岩中3颗捕获的岩浆锆石分别测得ca.2.9 Ga、ca.2.3 Ga和ca.2.1 Ga等三组^(207)Pb/^(206)Pb年龄。两件样品ca.1.85 Ga岩浆锆石具有相似的Hf同位素组成,其ε_(Hf)(t)=-6.1^-10.7,t_(DM2)=2.74~2.98 Ga,捕获ca.2.1 Ga和ca.2.9 Ga锆石的ε_(Hf)(t)值和t_(DM2)年龄分别是-1.9和2.7 Ga、-4.8和3.5 Ga。锆石U-Pb年龄、Lu-Hf同位素特征表明,北苏鲁两处变辉长岩记录的岩浆事件与胶北前寒武纪陆壳的多期演化历史一致,变辉长岩及其围岩荆山群可能是源自胶北的外来地质体。

Several meta-supracrustal rock units occurring as irregular enclaves or tectonic blocks are identified as the Jingshan Group, which are distributed within the gneissic rocks in the Sulu Ultrahigh Pressure （UHP） metamorphic belt. The assemblages, formation ages and tectonic affinity of the Jingshan Group remain unclear. In this study, we carried out zircon U-Pb dating and Hf isotope investigations on two outcrops of Jingshan Groups in the Weihai and Haiyangsuo areas in the northern Sulu UHP belt and the Jingshan Groups, which were intruded by meta-gabbro-dikes. A combined study of CL image, chondrite-normalized RRE patterns and Th/U rations suggests that zircon grains from the meta-gabbros are of magmatic origin. Although some zircon grains have obviously zoned, dark-luminescent cores and weakly or not zoned, grey-white luminescent rims, but no newly formed metamorphic zircon grains have been found in the two samples. The magmatic zircon grains from Weihai （S4-2） meta-gabbro have 207pb/206pb age of 1831-1966 Ma with a weighted mean of 1870±34 Ma （MSWD=0.6）, and the most 207pb/206pb ages of the Haiyangsuo （H3） sample are 1769-1887 Ma, yield a weighted mean of 1839±37 Ma （MSWD=0.5）. Zircon grains from the two meta-gabbro samples recorded the same magmatic ages of ca. 1.85 Ga. Meanwhile, three inherited zircons form sample H3 recorded another three magmatic ages of ca. 2.9 Ga, ca. 2.3 Ga and ca. 2.1 Ga. Zircon grains with U-Pb ages of ca. 1.85 Ga have εHf（t） values varying from -6.1 to -10.7 and two-stage Hf model ages （tDM2） of 2.74-2.98 Ga. Inherited zircon grains with U-Pb ages of ca. 2.1 Ga and ca. 2.9 Ga have εHf（t） values of -1.9 and -4.8, the tDM2 ages of 2.7 Ga and 3.5 Ga, respectively. The zircon U-Pb ages and Hf isotopic compositions indicate that the magmatic events recorded in two meta-gabbros from the Sulu UHP metamorphic belt are accordant with polyphase ancient continental crustal evolution of the Jiaobei terrane. Thus, we suggest that the two meta-gabbros together with their country rocks of the Jingshan Group may represent a fragment of the Jiaobei terrane, which was juxtaposed with the Sulu terrane prior to the Triassic collision-subduction.