Discovery of a Jurassic SSZ ophiolite in the Myitkyina region of Myanmar
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摘要:
本文报道了在缅甸的密支那地区发现的一套侏罗纪SSZ型蛇绿岩和有关的岩石单元。蛇绿岩由地幔橄榄岩、安山玄武岩、淡色辉长岩和辉长岩类、橄榄辉石岩和含长辉石岩类、以及斜长花岗岩组成。锆石的U-Pb定年获得安山玄武岩的形成年龄为166±3Ma、淡色辉长岩177±1Ma、橄榄辉石岩171±2Ma，和斜长花岗岩176±1Ma。安山玄武岩以低K2O(平均0.21％)和中TiO2(0.8%~1.2％)为特征，熔岩的球粒陨石标准化属于近平坦和LREE轻微富集型〔(La/Yb)N变化于1~1.3之间〕。微量元素MORB标准化蛛网图显示，熔岩和其他基性岩的大离子亲石元素Sr，K，Rb和Ba含量明显富集，而高场强元素Nb、Th、Ta、Zr、Ti明显亏损，为典型的SSZ构造背景成因的熔岩特征。87Sr/86Sr(i)比值为0.70367~0.70397，以及εNd(t)为正值(4~5.3)，均表明它们的岩浆源区为亏损地幔源区。从空间展布看，密支那蛇绿岩与缅甸东带蛇绿岩应属同一条蛇绿岩带，与我国西藏的雅鲁藏布江缝合带相连。但不同的是，雅鲁藏布江缝合带侏罗纪时(约170Ma)为MOR型蛇绿岩，但密支那侏罗纪出现的是SSZ型蛇绿岩。两者时间如此相近，我们认为用洋内俯冲来解释较为合适。但值得注意的是，也有人将缅甸东带蛇绿岩与西藏北带的班公湖-怒江缝合带相连，并且后者在侏罗纪时也已经出现SSZ型蛇绿岩。因此，有必要进一步开展对密支那蛇绿岩构造背景的研究。
Abstract:A Jurassic SSZ-type ophiolite and related rock units have been recognized in the Myitkyina region of Myanmar. The ophiolite consists of mantle peridotite, andesite-basalt, leucogabbro and gabbro, olivine pyroxenite and plagioclase-bearing pyroxenite, and plagiogranite. U-Pb dates on zircon yield ages of 166±3Ma for the andesite-basalt, 177±1Ma for the leucogabbro, 171±2Ma for the olivine pyroxenite, and 176±1Ma for the plagiogranite. The andesite-basalts are characterized by low K2O (average 0.21%) and medium TiO2 (0.8%~1.2%), and have flat to slightly LREE-enriched chondrite-normalized REE patterns [(La/Yb)N=1~1.3]. In MORB-normalized spider diagrams, the lavas are enriched in Sr, K, Rb and Ba, and depleted in Nb, Th, Ta, Zr and Ti. They have low 87Sr/86Sr(i), ranging from 0.70367 to 0.70397 and εNd(t) of +4 to +5.3, indicating a depleted mantle source. However, the enrichment in LILE and depletion in Nb and Ta suggest formation in a subduction-related (SSZ) tectonic setting, such as a forearc or backarc basin. Based on the regional geology, the Myitkyina ophiolite can be part of the East Ophiolite of Myanmar, and lies in an eastward extension of the southern Tibet Yarlung Zangbo suture zone. However, during the Jurassic the Neotethyan ophiolites along this suture were being formed in a MOR setting, and therefore, an intra-oceanic subduction model is probably suited for the Myitkyina ophiolite. We also notice that the East Ophiolite Belt of Myanmar has been connected to the Jurassic Mesotethyan ophiolites along the Bangong-Nujiang suture of central Tibet, which were being formed in a SSZ setting. Thus, further study is needed to determine its specific tectonic setting.
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图 1
缅甸及周缘地质构造概图(据 Mitchell, 1993；Mitchell et al., 2004)
Figure 1.
Geological and tectonic sketch map of Myanmar and surround region (after Mitchell, 1993; Mitchell et al., 2004)
图 2
缅甸密支那蛇绿岩的野外产出照片
Figure 2.
Field occurrences of the Myitkyina ophiolitic units, Myanmar
图 3
缅甸密支那蛇绿岩有关岩石的显微镜下特征
Figure 3.
Photomicrgraphs of different rock types of the Myitkyina ophiolite, Myanmar
图 4
缅甸密支那安山玄武岩(样品09Y-40)锆石的阴极发光图像(a)和SHRIMP U-Pb谐和图(b)以及加权年龄平均值(c)
Figure 4.
Cathodoluminescence images of zircons (a), concordia curves (b) and weighed mean age (c) of SHRIMP U-Pb data from the lava (sample 09Y-40) of the Myitkyina ophiolite, Myanmar
图 5
缅甸密支那蛇绿岩辉长岩(样品09Y-51)锆石的阴极发光图像(a)和LA-ICP-MS U-Pb年龄谐和图(b)以及加权年龄平均值(c)
Figure 5.
Cathodoluminescence images of zircons (a) and concordia curves (b) and weighed mean age (c) of LA-ICP-MS U-Pb data from the gabbro (sample 09Y-51) of the Myitkyina ophiolite, Myanmar
图 6
缅甸密支那蛇绿岩的斜长花岗岩样品(09Y-55)锆石阴极发光图像(a)和LA-ICP-MS U-Pb谐和图(b)以及加权年龄平均值(c)
Figure 6.
Cathodoluminescence images of zircons (a), concordia curves (b) and weighed mean age (c) of LA-ICP-MS U-Pb data from the plagioranite (sample 09Y-55) of the Myitkyina ophiolite, Myanmar
图 7
缅甸密支那蛇绿岩橄榄辉石岩(样品09Y-76)锆石阴极发光图像(a)和SHRIMP U-Pb谐和图(b)以及加权年龄平均值(c)
Figure 7.
Cathodoluminescence images of zircons (a), concordia curves (b) and weighed mean age (c) of SHRIMP U-Pb data from the olivine-bearing pyroxenite (sample 09Y-76) of the Myitkyina ophiolite, Myanmar
图 8
不同岩石类型的MgO与SiO2、Al2O3、CaO、Na2O、K2O和TiO2等其他主元素的成分变异图
Figure 8.
MgO vs. SiO2, Al2O3, CaO, Na2O, K2O and TiO2 diagrams of different rock types from the Myitkyina, Myanmar
图 9
密支那地区4类岩石的稀土元素球粒陨石标准化图(标准化值据Taylor and Mclennan, 1985)
Figure 9.
Chrondrited REE patterns of the rocks form the Myitkyina ophiollite, Myanmar (normalization values after Taylor and Mclennan, 1985)
图 10
密支那地区4类岩石的原始地幔标准化蛛网图(标准化值据Sum and McDonough, 1989)
Figure 10.
Distribution patterns of trace elements of the rocks from the Myitkyinan ophiolite, Myanmar(normalization values after Sum and McDonough, 1989)
图 11
缅甸密支那蛇绿岩熔岩的构造环境判别图
Figure 11.
Discrimination diagrams of tectonic setting for the lavas from the Myitkyina ophiolite, Myanmar
图 12
不同岩石类型的143Nd/144Nd(i)vs. 87Sr/86Sr(i) (据Zindler and Hart, 1986)
Figure 12.
Variation of 143Nd/144Nd(i)vs. 87Sr/86Sr(i) different rock types from the Myitkyina, Myanmar (after Zindler and Hart, 1986)
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