青藏高原纳木错西缘新元古代中期岩浆事件:对北拉萨地块起源的约束

Middle Neoproterozoic magmatic event in the western Nam Tso area,Tibetan Plateau: Constraint on the origin of the North Lhasa terrane

本文报道了青藏高原北拉萨地块纳木错西缘变质辉长岩和花岗片麻岩的锆石U-Pb定年、岩石地球化学和锆石Hf同位素分析结果。锆石LA-ICP-MS定年结果表明,变质辉长岩和花岗片麻岩的原岩形成时代分别为720±6Ma和732±7Ma,相当于新元古代中期。变质辉长岩为钙碱性系列,具有Nb、Ta和Ti负异常,与岛弧玄武岩类似。变质辉长岩中锆石具有较高的εHf(t)值(+5. 2^+9. 7),应当是源自俯冲环境下相对亏损的地幔楔。花岗片麻岩原岩为I型花岗质岩石,并且具有较为均一的锆石εHf(t)值(-3. 3^+0. 3),可能形成于地壳内古元古代变质火成岩的部分熔融作用。结合区域地质资料,变质辉长岩和花岗片麻岩的原岩应当形成于新元古代中期的洋壳俯冲消减过程。北拉萨地块上的前寒武纪岩浆和变质记录与东非造山带的活动时限较为一致,因而北拉萨地块可能与东非造山带具有亲缘性。

The Lhasa terrane is located in the central Tibetan Plateau and is traditionally considered to originate from the northeastern India or northern Australia. However,a Paleo-Tethyan suture zone,named the North Gangdese suture zone,was recently identified in the middle of the Lhasa terrane,meaning that the Lhasa terrane should be subdivided into the South and North Lhasa terranes along this suture zone and the origins of these two newly identified terranes should be revisited. For this problem,this paper reports new zircon U-Pb age and Lu-Hf isotope,and whole-rock major and trace element data from the meta-gabbros and granitic gneisses located in the western Nam Tso area,North Lhasa terrane. The zircons from the meta-gabbros and granitic gneisses have relatively high Th/U ratios of 0. 22 to 1. 05( > 0. 1),indicating an igneous origin. LA-ICP-MS zircon U-Pb dating reveals that the protoliths of the meta-gabbros and granitic gneisses were formed at 720 ± 6 Ma and 732 ± 7 Ma,respectively. The meta-gabbros are calcalkaline,have elevated Th/Yb ratios,and display depletions of Nb,Ta,and Ti,geochemically similar to island arc basalts. They also have high zircon εHf( t) values( + 5. 2 to + 9. 7),and are probably derived from partial melting of a depleted mantle wedge in a subduction-related environment. The protoliths of granitic gneisses are I-type granite,with homogeneous zircon εHf( t) values(-3. 3 to + 0. 3),and were probably generated by partial melting of Palaeoproterozoic meta-igneous rocks. Their depletions of Nb,Ta,and Ti also suggest an island arc affinity. The protoliths of these meta-gabbros and granitic gneisses were probably related to Middle Neoproterozoic oceanic subduction process. The Precambrian magmatic and metamorphic records in the North Lhasa terrane are comparable with those of the East African Orogen( EAO). Integrating previous studies with the data presented in this contribution,we suggest that the North Lhasa terrane was most likely separated with the South Lhasa terrane during the Precambrian and originated from the northern EAO.