The Newly Discovered Cambrian Gabbro-Diorite in Bange, Tibet and Their Tectonic Implications
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摘要: 西藏班戈地区构造单元属北拉萨地体，到目前为止，该地体尚未有寒武纪岩浆活动的报道.本文就班戈地区首次发现的辉长闪长岩体，在野外地质调查、元素地球化学及同位素年代学研究的基础上，得出以下结论：该岩体属高钾钙碱性系列，具有富集大离子亲石元素（如Rb、Ba）和轻稀土元素（La、Ce），亏损Nb、Ta、Zr、Hf和Ti等高场强元素，呈现出安第斯型岛弧岩浆特征，显示该岩体是俯冲沉积物部分熔融形成的熔体交代上覆地幔楔的产物.锆石U-Pb年代学指示其侵入年龄为512 ±3 Ma，形成于寒武纪，为北拉萨地体存在早古生代岩浆活动提供了可靠的年代学证据.综合证实其属于冈瓦纳大陆北缘的一部分，该成果为进一步探讨青藏高原的构造演化提供了新的约束.Abstract: The Bange area is part of North Lhasa terrane. Cambrian magmatism has not been reported in the north Lhasa terrane. In this study, field geological survey and geochemical studies, as well as geochronology, were used to study the firstly discovered gabbro-diorite in Bange and the following conclusions are drawn. The gabbro-diorites belong to the high-K calc-alkaline series. They are rich in large ion lithophilic elements (e.g., Rb and Ba) and light rare earth elements (e.g., La and Ce), and are depleted in high-field strength elements (e.g., Nb, Ta, Zr, Hf and Ti), similar to the Andean island arc magmatic rocks. The rock is interpreted as resulting from partial melting of subduction sediments metasomatized with the overlying mantle wedge. Zircon U-Pb dating demonstrates that the gabbro-diorite formed at 512±3 Ma, which indicates it formed at Cambrian and provides reliable chronological evidence for the existence of Early Paleozoic magmatic events in the North Lhasa terrane. It is confirmed that the North Lhasa terrane is part of northern margin of the Gondwana land. The results provide a new window for further study of the evolution of the tectonic of Tibetan Plateau.
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Key words:
- gabbro-diorite /
- Cambrian /
- tectonic implication /
- Bange district /
- Tibet /
- petrology
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图 1 (a) 亚欧大陆遥感地质简图及部分早古生代岩浆岩，(b)青藏高原地质简图，(c)班戈研究区地质简图和样品位置
图a中, BT.宝山地体，TB.腾冲地体，GT.贡山地体.图b中, SL.南拉萨地体，CL.中拉萨地体，NL.北拉萨地体.图a, b据Zhu et al.(2012)修改
Fig. 1. (a) Geographic map of Eurasia showing the distributions of Early Paleozoic magmatic rocks, (b) geological map of Qinghai-Tibetan plateau, (c) geological map of Bangge district and sample location
图 2 (a) SiO2-Na2O+K2O图解, (b) SiO2-K2O图解
a.据Middlemost(1994)；b据Peccerillo and Taylor(1976)；Ir.Irvine分界线，上方为碱性，下方为亚碱性.深成岩：1.橄榄辉长岩；2a.碱性辉长岩；2b.亚碱性辉长岩；3.辉长闪长岩；4.闪长岩；5.花岗闪长岩；6.花岗岩；7.硅英岩；8.二长辉长岩；9.二长闪长岩；10.二长岩；11.石英二长岩；12.正长岩；13.副长石辉长岩；14.副长石二长闪长岩；15.副长石二长正长岩；16.副长正长岩；17.副长深成岩；18.霓方钠岩/磷霞岩/粗白榴岩；火山岩：1.苦橄玄武岩；2.玄武岩；3.玄武安山岩；4.安山岩；5.英安岩；6.流纹岩；7.英石岩；8.粗面玄武岩；9.玄武岩质粗面安山岩；10.粗面安山岩；11.粗面英安岩；12.粗面岩；13.碱玄岩；14.响质碱玄岩；15.碱玄质响岩；16.响岩；17.副长火山岩；18.方钠岩/霞石岩/纯白榴岩
Fig. 2. (a) SiO2 vs. Na2O+K2O diagram for classification, (b) SiO2 vs. K2O diagram
图 3 (a) 稀土元素球粒陨石标准化图解, (b)微量元素原始地幔标准化蛛网
图中洋岛玄武岩(OIB), 异常洋脊玄武岩(E-MORB)和正常洋脊玄武岩(N-MORB).据Sun and McDonough(1989)
Fig. 3. Chondrite-normalized REE (a), primitive mantle-normalized trace element spider diagram (b)
图 4 班戈地区辉长闪长岩锆石CL图像
Fig. 4. Zircon CL images of gabbro-diorites from the Bange district
图 5 (a) 班戈地区辉长闪长岩(BG12-1)中锆石U-Pb谐和图, (b)班戈地区辉长闪长岩(BG12-1)中稀土元素球粒陨石标准化图解
球粒陨石标准化值据Sun and McDonough(1989)
Fig. 5. Zircon U-Pb concordia diagram of gabbro-diorite (BG12-1) from the Bange in Tibet (a), Chondrite-normalized REE of gabbro-diorite (BG12-1) from the Bange in Tibet (b)
图 6 (a) Zr/Y-Zr图解，(b) Th/Yb-Nb/Yb图解，(c, d)IAB-MORB-CRB+OIB and TiO2/10-MnO-P2O5图解
1.中拉萨地体变玄武岩(~492 Ma)；2.高喜马拉雅Mandi玄武岩(~496 Ma)；3.SW Turkey弧后盆地玄武岩(~535 Ma)；4.Siberian Traps; 5.Kalkarindji玄武岩(~506 Ma)；6.Middle Okinawa Trough弧后盆地玄武岩；7.峨眉山大陆溢流玄武岩；8.安第斯型岛弧玄武岩；9.辉长闪长岩；OIT.洋岛拉斑玄武岩；OIA.洋岛碱性玄武岩；MORB.洋中脊玄武岩；IAT.岛弧拉斑玄武岩；CAB.钙碱性玄武岩; IAB.岛弧玄武岩；CRB.大陆裂谷玄武岩.数据来源: Mandi玄武岩(Miller et al., 2001)，Kalkarindji玄武岩(Glass and Phillips, 2006；Evins et al., 2009)，SW Turkey弧后盆地玄武岩(Gürsu and Göncüoglu，2005)，Middle Okinawa Trough弧后盆地玄武岩(Shinjo et al., 1999)，安第斯型岛弧玄武岩(Hickey et al., 1986)，峨眉山大陆溢流玄武岩(Xiao et al., 2004)，and Siberian Traps(http://georoc.mpch-mainz.gwdg.de/georoc/Entry.html).a据Pearce and Norry(1979), 弧后盆地玄武岩范围来自Floyd et al.(1991)；b据Pearce and Peate(1995)；c据Agrawal et al.(2008)
Fig. 6. Zr/Y-Zr diagram (a), Th/Yb-Nb/Yb diagrams showing the tectonic setting of the gabbro-diorite (b), IAB-MORB-CRB+OIB and TiO2/10-MnO-P2O5 diagrams (c, d)
图 7 锆石微量元素U/Yb-Y图解(据Grimes et al., 2007)和U/Yb-Nb/Yb图解(据Grimes et al., 2015)
Fig. 7. Zircon trace-element of U/Yb-Y (a) (after Grimes et al., 2007) and U/Yb-Nb/Yb (b) (after Grimes et al., 2015) diagrams for the gabbro-diorite
图 8 (a) 辉长闪长岩的La/Yb-Sm/Yb(据Johnson et al., 1990)，(b)(Sm/Yb)N-(La/Sm)N (据D’orazio et al., 2001)，(c)Ba/La-Th/Nd；(d)Ba/Th-(La/Sm)N图解
Fig. 8. Plots of (a) La/Yb vs. Sm/Yb (after Johnson et al., 1990), (b) (Sm/Yb)N vs. (La/Sm)N (after D'orazio et al., 2001), (c) Ba/La vs. Th/Nd, (d) Ba/Th vs. (La/Sm)N for representative gabbro-diorite samples
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