Metamorphic evolution of a newly identified Mesoproterozoic oceanic slice in the Yuka terrane and its implications for a multi‐cyclic orogenic history of the North Qaidam UHPM belt
Metamorphic evolution of a newly identified Mesoproterozoic oceanic slice in the Yuka terrane and its implications for a multi‐cyclic orogenic history of the North Qaidam UHPM belt
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DOI：
10.1111/jmg.12300
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发表时间：
2018-05
影响因子：
3.4
通讯作者：
Y. Ren;D. Chen;D. Kelsey;X. Gong;L. Liu;X. -. Zhu;S. J. Yang
中科院分区：
文献类型：
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作者：
Y. Ren;D. Chen;D. Kelsey;X. Gong;L. Liu;X. -. Zhu;S. J. Yang
位于青藏高原北缘的柴达木北造山带记录了两个造山旋回：一个是与罗迪尼亚超大陆合并解体有关的元古界旋回，另一个是包括大洋俯冲和大陆深俯冲的早古生代旋回。目前，关于元古代旋回的信息仅有约1000~900 Ma的同时发生的岩浆和变质事件，这限制了人们对NQOB元古代演化的认识以及柴达木地块与其他Rodinia碎屑的关系。在这项研究中，在尤卡地体中发现了一种含蓝晶石的榴辉岩。它具有正斜率球粒陨石归一化稀土元素配分模式，与现今的N-MORB相似。其原岩年龄为1273 Ma，榴辉岩相变质年龄为437 Ma，与尤卡地体的大陆深俯冲年龄基本一致。锆石Lu-Hf分析表明，岩浆锆石岩芯的εHf(T)为8.36-15.98，Tdm1为1,450-1,131 Ma(M=1,303±55 Ma，与原岩年龄误差一致)，表明榴辉岩为年轻地壳原岩。类MORB全岩成分和锆石U-Pb、Lu-Hf分析表明，含蓝晶石榴辉岩的原岩为中元古代洋片。P-T假剖面分析表明，含蓝晶石榴辉岩经历了4个变质阶段：(1)进行性阶段，组合garnet+omphacite+talc+lawsonite+phengite+quartz为22.4kbar~23.2kbar和585°C；(2)高峰期，组合为石榴石+绿辉石+多硅白云母+柯石英，组合为32.5kbar和670°C；(3)早期退变阶段，组合garnet+omphacite+kyanite+phengite+coesite/quartz±lawsonite为27.1~30.0kbar和670~690°C；所建立的顺时针P-T轨迹与本区其他陆型榴辉岩相似。根据地球化学和年代学资料以及P-T轨迹，我们认为含蓝晶石榴辉岩的原岩在Rodinia拼接过程中侵位于柴达木地块的活动边缘，并在早古生代经历了大陆深俯冲。我们的结论是：(1)柴达木地块参与了罗迪尼亚超大陆的拼装。它位于或靠近超大陆的边缘，可能靠近印度、东南极和塔里木；(2)NQOB中存在中元古代和早古生代洋壳切片。因此，在用不含原岩时代的亲洋榴辉岩变质年龄来约束北柴达木超高压变质带的演化历史时，应格外谨慎。
The North Qaidam Orogenic Belt (NQOB), lying at the northern margin of the Tibet Plateau, records two orogenic cycles: A Proterozoic cycle related to the amalgamation and breakup of the supercontinent Rodinia, and an Early Palaeozoic cycle including oceanic subduction and continental deep subduction. At present, the only information about the Proterozoic cycle is the concurrent c. 1,000–900 Ma magmatic and metamorphic events, which limited the understanding of the Proterozoic evolution of NQOB and the relationship between the Qaidam Block and other Rodinia fragments. In this study, a kyanite‐bearing eclogite was identified in Yuka terrane. It has positive‐slope chondrite‐normalized rare earth element distribution patterns, similar to present‐day N‐MORB. LA–ICP–MS zircon U–Pb dating obtained a protolith age of 1,273 Ma and an eclogite facies metamorphic age of 437 Ma, which is similar to the continental deep subduction age of the Yuka terrane. Zircon Lu–Hf analysis show that the magmatic zircon cores have high εHf(t) of 8.36–15.98 and TDM1 of 1,450–1,131 Ma (M = 1,303 ± 55 Ma, consistent with its protolith age within error), indicating a juvenile crust protolith of the eclogite. The MORB‐like whole‐rock composition and zircon U–Pb and Lu–Hf analysis indicate that the protolith of the kyanite‐bearing eclogite was a Mesoproterozoic oceanic slice. P–T pseudosection analysis shows that the kyanite‐bearing eclogite experienced four metamorphic stages: (1) a prograde stage with the assemblage garnet+omphacite+talc+lawsonite+phengite+quartz at 22.4–23.2 kbar and 585°C; (2) a peak stage with the assemblage garnet+omphacite+lawsonite+phengite+coesite at 32.5 kbar and 670°C; (3) an early retrograde stage with the assemblage garnet+omphacite+kyanite+phengite+coesite/quartz±lawsonite at 27.1–30.0 kbar and 670–690°C; and (4) a late retrograde stage with the assemblage garnet+omphacite+epidote+hornblende+phengite+quartz at <18.0 kbar. The established clockwise P–T path is similar with other continental‐type eclogites in this area. On the basis of the geochemical and geochronological data, as well as the P–T path, we suggest that the protolith of the kyanite‐bearing eclogite was emplaced in the active margin of the Qaidam Block during the assembly of Rodinia and underwent continental deep subduction in the Early Palaeozoic. We conclude that (1) the Qaidam Block participated in the assembly of the Rodinia supercontinent. It was situated at or proximal to the margin of the supercontinent and probably close to India, east Antarctica and Tarim; and (2) both Mesoproterozoic and Early Palaeozoic oceanic crust slices occur in the NQOB. Thus, special caution is needed when using the metamorphic ages of oceanic affinity eclogites without protolith ages to constrain the evolution history of the North Qaidam UHPM belt.