Magma mixing revealed from in situ zircon U–Pb–Hf isotope analysis of the Muhuguan granitoid pluton, eastern Qinling Orogen, China: implications for late Mesozoic tectonic evolution
Magma mixing revealed from in situ zircon U–Pb–Hf isotope analysis of the Muhuguan granitoid pluton, eastern Qinling Orogen, China: implications for late Mesozoic tectonic evolution
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DOI：
10.1007/s00531-013-0900-x
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发表时间：
2013-05
影响因子：
2.3
通讯作者：
Rui Liu;Jian-wei Li;Shi-Jian Bi;Hao Hu;Mi Chen
中科院分区：
文献类型：
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作者：
Rui Liu;Jian-wei Li;Shi-Jian Bi;Hao Hu;Mi Chen
本文通过对东秦岭晚侏罗世花岗岩类侵入体的锆石U-Pb年龄和Hf同位素研究，探讨了岩浆混合作用在花岗岩类侵入体形成中的意义。该造山带的木虎关花岗岩体由二长花岗岩和少量黑云母花岗岩及花岗闪长岩组成，均含有丰富的角闪石堆积岩、闪长质捕虏体和镁铁质岩浆包体。在二长花岗岩和花岗闪长岩中有多条层状岩墙侵入。堆晶岩和闪长质捕虏体样品的锆石U-Pb年龄一致，约为0.0001 μ m。161 ± 1 Ma，但Hf同位素组成差异较大。堆晶岩具有较多的放射成因锆石Hf同位素，εHf（t）值为负值（−7.9 ~ −2.5），TDM 1年龄为0.9- 1.1Ga，表明其可能来源于本区新元古代至早古生代宽坪群的玄武质岩石。闪长岩捕虏体的锆石εHf（t）值低得多，为-19.5至-8.8，TDM 2年龄为2.4-1.7 Ga，与推测以该地区秦岭群变基性岩为代表的幼年古元古代地壳源相一致。二长花岗岩、MME和层状岩墙的单个样品的U-Pb年龄分别为150 ± 1、152 ± 1和152 ± 1 Ma，表明晚侏罗世同时代的镁铁质和长英质岩浆活动。层状岩墙具有均一的、高度负的锆石εHf（t）值（−29.8 ~ −24.8）和太古代TDM 2年龄（3.0-2.7 Ga），其成因被解释为古富集的陆下地幔源区的部分熔融。来自细粒MME的锆石显示εHf（t）在−29.1和−9.8之间的大范围，与二长花岗岩和层状岩墙样品的值重叠。MME的锆石U-Pb年龄和Hf同位素与其壳源岩浆和富集幔源岩浆混合形成的结果一致。二长花岗岩中的主群锆石的εHf（t）值（−17.9 ~ −9.3）和TDM 2年龄（2.3- 1.8Ga）与闪长质捕虏体一致，表明二长花岗岩是由古元古代壳源部分熔融并有幔源岩浆参与的产物。木虎关岩体中的镁铁质岩浆活动表明，东秦岭造山带在晚侏罗世以岩石圈伸展作用为主。现有的地质和年代学数据表明，这一伸展事件开始于晚侏罗世（约。160 Ma），并持续到早白垩世，直到约。110毫安侏罗纪-白垩纪伸展作用可能是晚中生代太平洋板块向西俯冲到东亚大陆边缘之下的结果。
In this paper, we present zircon U–Pb age and Hf isotope data to document the significance of magma mixing in the formation of Late Jurassic granitoid intrusions in the eastern Qinling Orogen, China. The Muhuguan granitoid pluton from this orogen consists of monzogranite and lesser biotite granite and granodiorite, all containing abundant hornblende-rich cumulates, dioritic xenoliths, and mafic magmatic enclaves (MMEs). The monzogranite and granodiorite are intruded by a number of lamprophyre dykes. Both a cumulate and a dioritic xenolith samples have concordant zircon U–Pb ages of ca. 161 ± 1 Ma, but possess contrasting Hf isotopic compositions. The cumulate has more radiogenic zircon Hf isotopes with negativeεHf(t) values (−7.9 to −2.5) andTDM1ages of 0.9–1.1 Ga, indicating its derivation likely from basaltic rocks of the Neoproterozoic to early Paleozoic Kuanping Group in the area. The dioritic xenolith has much lower zirconεHf(t) values of −19.5 to −8.8 andTDM2ages of 2.4–1.7 Ga, consistent with a juvenile Paleoproterozoic crust source presumably represented by the metabasic rocks of the Qinling Group in the area. Individual samples of the monzogranite, MME, and a lamprophyre dyke have U–Pb ages of 150 ± 1, 152 ± 1, and 152 ± 1 Ma, respectively, demonstrating coeval mafic and felsic magmatism in the Late Jurassic. The lamprophyre dyke has homogeneous, highly negative zirconεHf(t) values (−29.8 to −24.8) and ArcheanTDM2ages (3.0–2.7 Ga), and its genesis is interpreted as partial melting of an ancient enriched subcontinental mantle source. Zircons from the fine-grained MME show a large range ofεHf(t) between −29.1 and −9.8, overlapping values of the monzogranite and lamprophyre dyke samples. Zircon U–Pb age and Hf isotopes of the MMEs are consistent with their formation by mixing of crustal- and enriched mantle-derived magmas. The main group of zircons from the monzogranite hasεHf(t) values (−17.9 to −9.3) andTDM2ages (2.3–1.8 Ga) that are compatible with the dioritic xenoliths, indicating that the former was produced by partial melting of Paleoproterozoic crustal source with involvement of mantle-derived magmas. Mafic magmatism revealed from the Muhuguan pluton indicates that the eastern Qinling Orogen was dominated by lithospheric extension during the Late Jurassic. Compilation of existing geological and geochronological data suggests that this extensional event started in Late Jurassic (ca. 160 Ma) and persisted into the Early Cretaceous until ca. 110 Ma. The Jura-Cretaceous extension may have resulted from the late Mesozoic westward subduction of the Pacific plate beneath the East Asian continental margin.