The Middle Neoproterozoic meta-gabbro from the North Lhasa terrane of Tibet and its geological implications
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摘要:
青藏高原各陆块的前寒武纪演化历史及其在冈瓦纳大陆聚合过程中所处的构造位置目前不清楚。通过青藏高原中部北拉萨地块念青唐古拉岩群中变质辉长岩SHRIMP锆石U-Pb定年和锆石Hf同位素组成研究，锆石U-Pb定年结果为663±7Ma，相当于新元古代中期，代表变质辉长岩的原岩形成时代，这是北拉萨地块首次报道该时代的基性岩浆记录。变质辉长岩中锆石具有较低的εHf（t）值（-1.5~+2.3），表明其原岩的岩浆来自富集的地幔源区。结合北拉萨地块已有的变质记录可知，变质辉长岩的原岩可能形成于造山环境。目前定义的"念青唐古拉岩群"实际上是由时代不同、成因不同，甚至来源不同的构造岩片组成，随着工作的深入，有必要对其进行解体。念青唐古拉岩群中的前寒武纪岩浆和变质记录与东非造山带的活动时限较一致，因而北拉萨地块可能与东非造山带具有亲缘性。
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关键词:
- 北拉萨地块 /
- 念青唐古拉岩群 /
- 变质辉长岩 /
- SHRIMP锆石U-Pb定年 /
- 东非造山带
Abstract:The Precambrian evolution of the ancient terranes in the Tibetan Plateau and their roles in the assembly of Gondwana supercontinent remain unclear. This paper reports SHRIMP zircon U-Pb dating and zircon Hf isotopic analysis data of the meta-gabbro from the Nyainqentanglha Group of North Lhasa terrane, Tibet. Zircon U-Pb dating yielded a weighted mean age of 663±7Ma, which represents the crystallization age of the protolith of meta-gabbro and is reported for the first time for the North Lhasa terrane. The meta-gabbro has low εHf(t) values (-1.5 to +2.3), suggesting an enriched magma source. Considering previous Precambrian studies in the North Lhasa terrane, the authors hold that the meta-gabbros were probably formed in an orogenic setting. The Nyainqentanglha Group is actually composed of a suite of tectonic slices with different sizes, ages, and petrogenesises. Thus, it should be disassembled in the future research. The metamorphic and magmatic records in the Nyainqentanglha Group are comparable with those of the East African orogen, suggesting that the North Lhasa terrane probably originated from the northern segment of the East African orogen during the assembly of Gondwana supercontinent.
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致谢: SHRIMP锆石U-Pb定年得到中国地质科学院地质研究所宋彪研究员的帮助；锆石Hf同位素分析得到中国科学院地质与地球物理研究所吴福元研究员的帮助，在此一并致以衷心的感谢。
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图 1 青藏高原板块划分简图（a）和仁错地区区域地质简图（b）
JSSZ—金沙江板块缝合带；LSSZ—龙木错-双湖-澜沧江板块缝合带；NGSZ—北冈底斯板块缝合带；BNSZ—班公湖-怒江板块缝合带；IYZSZ—雅鲁藏布江板块缝合带
Figure 1. Simplified tectonic map of the Tibetan Plateau (a) and geological map of the Ren Co area, Tibet (b)
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图 2 变质辉长岩野外露头照片（a）和镜下照片（正交偏光）（b）
Pl—斜长石；Am—角闪石
Figure 2. Photograph (a) and micrograph (b) of the meta-gabbro
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图 3 变质辉长岩中典型锆石阴极发光（CL）图像和锆石U-Pb谐和图
（实线圈和虚线圈分别代表SHRIMP U-Pb测年位置和Hf同位素分析位置）
Figure 3. Representative CL images and U-Pb concordia diagram of the zircons separated from the meta-gabbro
表 1 北拉萨地块新元古代中期变质辉长岩SHRIMP锆石U-Th-Pb分析结果
Table 1 SHRIMP zircon U-Th-Pb data of the Middle Neoproterozoic meta-gabbros from the North Lhasa terrane, Tibet
点号 206Pbc/% U/10-6 Th/10-6 232Th/238U 206Pb*10-6 207Pb*/206Pb* ±% 207Pb*/235U ±% 206Pb*/238U ±% 206Pb/238U年龄/Ma 1.1 -- 504 204 0.42 47.0 0.0648 2.2 0.972 3.2 0.1088 2.4 666 ±15 2.1 0.26 521 204 0.40 49.0 0.0611 1.9 0.92 3 0.1092 2.4 668 ±15 3.1 0.17 461 138 0.31 40.9 0.0624 1.7 0.888 2.9 0.1032 2.4 633 ±14 4.1 0.10 410 142 0.36 39.5 0.0627 1.7 0.968 2.9 0.112 2.4 684 ±15 5.1 0.39 405 128 0.33 36.3 0.0593 2.1 0.848 3.2 0.1038 2.4 637 ±14 6.1 0.33 468 148 0.33 40.8 0.0619 2 0.863 3.1 0.1012 2.4 621 ±14 7.1 -- 495 134 0.28 44.7 0.0635 1.6 0.921 2.9 0.1052 2.4 645 ±15 8.1 0.09 582 237 0.42 56.7 0.0614 1.3 0.959 2.7 0.1132 2.3 691 ±15 9.1 -- 557 236 0.44 53.5 0.06129 1.1 0.945 2.5 0.1119 2.3 684 ±15 10.1 0.16 553 204 0.38 52.1 0.064 1.9 0.97 3 0.1099 2.3 672 ±15 11.1 0.13 1623 168 0.11 151 0.0608 0.94 0.904 2.5 0.1079 2.3 661 ±14 12.1 0.79 496 180 0.37 45.6 0.0605 3 0.899 3.8 0.1077 2.3 660 ±15 13.1 1.17 479 184 0.40 44.8 0.0594 3.1 0.902 3.9 0.1102 2.3 674 ±15 14.1 0.52 561 209 0.38 54.7 0.061 1.8 0.959 2.9 0.1141 2.3 696 ±15 15.1 0.54 540 200 0.38 49.6 0.0591 2.5 0.875 3.4 0.1074 2.3 658 ±14 16.1 0.59 515 205 0.41 48.6 0.0588 5.6 0.896 6.1 0.1105 2.3 676 ±15 
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表 2 北拉萨地块新元古代中期变质辉长岩锆石Hf同位素分析结果
Table 2 Hf isotopic data of zircon of the Middle Neoproterozoic meta-gabbros from the North Lhasa terrane, Tibet
点号 年龄/Ma 176Yb/177Hf 2σ 176Lu/177Hf 2σ 176Hf/177Hf 2s 176Hf/177Hfi εHf(0) εHf(t) 2σ TDM /Ma TDMC /Ma fLu/Hf 1.1 666 0.001565 0.000013 0.000059 0.000001 0.282422 0.000030 0.282421 -12.4 2.3 1.1 1145 1449 -1.00 2.1 668 0.001423 0.000011 0.000063 0.000000 0.282377 0.000020 0.282376 -14.0 0.7 0.7 1208 1550 -1.00 3.1 633 0.000419 0.000011 0.000015 0.000000 0.282391 0.000017 0.282391 -13.5 0.5 0.6 1186 1538 -1.00 4.1 684 0.000449 0.000032 0.000019 0.000002 0.282338 0.000018 0.282338 -15.4 -0.3 0.6 1259 1625 -1.00 5.1 637 0.000464 0.000006 0.000018 0.000000 0.282349 0.000016 0.282349 -14.9 -0.9 0.6 1244 1629 -1.00 6.1 621 0.003762 0.000184 0.000158 0.000007 0.282348 0.000020 0.282346 -15.0 -1.4 0.7 1250 1646 -1.00 7.1 645 0.001646 0.000054 0.000074 0.000002 0.282344 0.000018 0.282343 -15.2 -1.0 0.6 1253 1639 -1.00 8.1 691 0.000934 0.000015 0.000035 0.000001 0.282401 0.000022 0.282401 -13.1 2.1 0.8 1173 1479 -1.00 9.1 684 0.001658 0.000022 0.000078 0.000001 0.282369 0.000020 0.282368 -14.3 0.8 0.7 1219 1558 -1.00 10.1 672 0.002666 0.000188 0.000115 0.000008 0.282369 0.000016 0.282367 -14.3 0.5 0.6 1221 1567 -1.00 12.1 660 0.002276 0.000042 0.000096 0.000002 0.282325 0.000015 0.282324 -15.8 -1.3 0.5 1279 1671 -1.00 13.1 674 0.001705 0.000010 0.000064 0.000000 0.282388 0.000021 0.282387 -13.6 1.3 0.8 1192 1520 -1.00 15.1 658 0.011370 0.000981 0.000486 0.000040 0.282326 0.000018 0.282320 -15.8 -1.5 0.6 1292 1682 -0.99
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