Petrology and geochemistry of the eclogite in Heihe area of Lancang County, western Yunnan: the magmatism of the initial subduction of the oceanic crust
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摘要:
滇西澜沧黑河地区厘定的奥陶纪谦迈蛇绿混杂岩是区域上勐库地区湾河蛇绿混杂岩的南延部分，是原特提斯洋盆关闭后的残迹；其中发育规模不等的榴辉岩构造岩片。榴辉岩经历了多期不同程度的退变质作用，可见丰富的退变质结构，主要榴辉岩相的变质矿物有石榴子石、绿辉石、多硅白云母、金红石等。岩石的SiO2含量为49.76%~52.71%，TiO2为1.12%~1.96%，Al2O3为14.18%~16.92%，CaO为7.91%~10.04%，MgO为5.93%~9.14%，Na2O为2.81%~4.77%，K2O为0.09%~1.15%，Mg#=56~69，表明岩浆的分异演化程度较低，接近原始岩浆，总体特点类似前弧玄武岩。稀土元素配分曲线向右缓倾，轻稀土元素轻度富集，（La/Sm）N=1.13~2.05，（Gd/Yb）N=1.18~1.56，与E-MORB（富集型洋中脊玄武岩）类似。微量元素特征表明，其原岩为一套富Nb玄武岩，来自富集地幔源区。总之，黑河地区榴辉岩的原岩属前弧玄武岩，是洋壳初始俯冲阶段的产物，代表了原特提斯洋盆由扩张向俯冲消减转换的岩石学记录。
Abstract:The Ordovician Qianmai ophiolitic melange in the Heihe area of Lancang, western Yunnan, is the southern extension of the Wanhe ophiolitic melange in Mengku region, which is the remnant of the closure of the original Tethyan oceanic basin, where some eclogite tectonic slices of different scales are also developed. The eclogite experienced many stages of retrograde metamorphism in different degrees, with abundant retrograde metamorphic structures. The metamorphic minerals of eclogite facies are garnet, omphacite, polysilicate muscovite and rutile, etc. It is chemically characterized by SiO2 content of 49.76%~52.71%, TiO2 content of 1.12%~1.96%, Al2O3 content of 14.18%~16.92%, CaO content of 7.91%~10.04%, MgO content of 5.93%~9.14%, Na2O content of 2.81%~4.77%, K2O content of 0.09%~1.15% and Mg# values of 56~69, indicating that the magma has a low degree of differentiation and evolution, close to the primitive magma and similar to front arc basalt(FAB). The distribution curve of rare earth elements inclines gently to the right, and LREE is slightly enriched. The (La/Sm)N content ranges from 1.13 to 2.05, and (Gd/Yb)N content ranges from 1.18 to 1.56, similar to that of enriched mid-ocean ridge basalt(E-MORB). The trace elements indicate that the protolith is a set of Nb-enriched basalts from the source area of enriched mantle. In a word, the protolith of retrograde eclogite in the Heihe area belongs to FAB, which is the product of the initial subduction of the oceanic crust, and represents the petrological records of the transformation from expansion to subduction of the original Tethyan oceanic basin.
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Keywords:
- petrology /
- geochemistry /
- eclogite /
- Proto-Tethys Ocean /
- front arc basalt /
- Lancang County of western Yunnan
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致谢: 野外工作期间云南省地质调查院李静高级工程师、云南省地质勘查基金管理中心段文婷高级工程师多次亲临项目组进行指导，张伟民高级工程师在岩矿鉴定方面提供了较大帮助，岩石地球化学分析得到武汉上谱分析科技责任有限公司的大力支持，在此一并致谢。
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图 1 研究区大地构造位置图(a)及地质简图(b)
Figure 1. Tectonic location map(a) and simplified geological map(b) of the study area
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图 2 黑河地区榴辉岩野外露头
a—连续出露近300 m的榴辉岩露头；b—榴辉岩呈透镜状分布于石榴白云石英片岩中
Figure 2. Field outcrop of retrograde eclogite in the Heihe area
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图版Ⅰ
a、b.多硅白云母(Phe)-石榴子石(Grt)-绿辉石(Omp)的平衡共生关系(－)；c.指纹状微晶角闪石(Hb)中残留细粒状绿辉石(Omp)(－)；d.石榴子石(Grt)具角闪石(Hb)、斜长石(Pl) 后成合晶退变反应边(－)；e.石榴子石(Grt)中残留的绿辉石(Omp)、金红石(Rt)包裹体(－)；f.指纹状微晶角闪石(Hb)、斜长石(Pl)后成合晶中残留石榴子石(Grt)假象(－)
图版Ⅰ.
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图 3 黑河地区榴辉岩稀土元素球粒陨石标准化配分模式图(a)和原始地幔标准化微量元素蛛网图(b)(N-MORB、E-MORB、OIB数据及标准化值均据参考文献[18])
Figure 3. Chondrite-normalized REE distribution patterns(a)and primitive mantle normalized spider-grams(b) of eclogite in the Heihe area
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图 4 黑河地区榴辉岩Nb/Y-Zr/TiO2图解(a)及Co-Th图解(b)
Figure 4. Nb/Y-Zr/TiO2(a)and Co-Th(b)diagrams of the eclogite in the Heihe area
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图 5 黑河地区榴辉岩Nb/Yb-Th/Yb(a)、La/Yb-Th/Ta(b)、Zr/Y-Nb/Y(c)和Nb标准化(d)图解
N-MORB—正常洋中脊玄武岩；E-MORB—富集型洋中脊玄武岩；OIB—洋岛玄武岩
Figure 5. Nb/Yb-Th/Yb(a), La/Yb-Th/Ta(b), Zr/Y-Nb/Y(c)and Nb normalized(d)diagrams of the eclogite in the Heihe area
表 1 黑河地区榴辉岩主量、微量和稀土元素分析成果
Table 1 Major, trace element and REE contents of the eclogite in the Heihe area
编号 D2540-1 PM008 -2-1 PM008 -37-1 PM008 -50-1 PM008 -31-2 PM00 8-47-2 PM008 -48-1 PM008 -51-3 PM008 -53-1 PM008 83-2 PM021 -13-1 PM021 -21-2 SiO2 49.94 52.27 51.04 52.71 49.76 51.87 51.10 50.01 51.23 50.33 49.77 50.23 TiO2 1.16 1.67 1.72 1.23 1.56 1.47 1.12 1.63 1.64 1.56 1.55 1.96 Al2O3 16.92 14.62 14.46 14.91 14.71 15.17 15.15 15.11 14.18 14.97 15.88 15.04 Fe2O3 3.10 3.57 3.41 2.75 3.69 3.40 3.35 3.47 3.71 3.22 3.77 3.92 FeO 6.97 7.55 7.37 6.02 8.11 7.12 6.50 7.93 8.42 7.43 8.04 8.48 MnO 0.23 0.21 0.23 0.21 0.19 0.21 0.19 0.25 0.23 0.21 0.21 0.22 MgO 8.47 6.59 6.71 7.29 6.90 5.93 7.60 8.19 6.04 9.14 7.39 6.67 CaO 8.30 8.12 9.76 10.04 9.74 9.37 8.99 8.41 9.56 8.37 7.91 8.01 Na2O 3.25 3.20 4.12 3.57 3.84 4.26 4.77 3.78 3.36 2.81 3.57 3.75 K2O 0.75 1.15 0.14 0.47 0.36 0.20 0.34 0.09 0.49 0.96 0.87 0.54 P2O5 0.12 0.21 0.22 0.14 0.23 0.21 0.17 0.25 0.20 0.17 0.15 0.23 Mg# 69 61 62 69 60 60 68 65 56 69 62 59 Zr 76.01 102.17 97.17 80.32 114.43 154.15 88.57 108.34 112.70 109.02 86.75 121.92 Zn 126.98 101.34 105.73 126.37 98.03 113.53 99.22 89.26 112.93 76.16 93.54 108.09 V 258.90 338.88 316.15 241.96 316.11 296.79 240.20 337.85 340.66 221.21 291.94 298.96 Th 1.76 2.44 1.13 1.48 1.76 2.10 1.10 1.43 1.92 2.66 1.20 1.83 Sc 39.62 43.18 38.39 33.68 37.38 35.55 35.03 36.39 38.88 32.16 38.10 37.95 Sr 236.97 116.74 95.07 176.55 79.16 172.81 106.41 105.08 96.96 187.61 99.69 81.94 Rb 43.26 74.01 4.05 13.03 12.18 8.51 10.68 4.99 14.17 43.94 40.26 28.47 Ni 64.94 58.37 88.87 108.33 92.99 59.29 130.10 97.63 29.11 205.33 55.36 90.37 Nb 6.79 7.41 10.51 6.32 10.25 11.61 7.80 9.48 10.34 5.43 6.56 7.45 Cu 48.55 55.38 60.10 50.86 44.03 52.96 59.25 197.07 38.84 45.34 39.45 48.00 Cr 189.90 191.79 187.20 262.60 230.04 146.07 305.86 247.55 30.70 547.44 75.94 231.75 Co 41.34 40.63 42.96 38.45 36.62 35.03 32.00 46.25 36.82 47.90 42.19 40.77 Ba 129.66 326.74 66.27 148.90 118.98 49.51 109.58 10.87 167.09 305.14 447.97 101.62 Hf 1.98 3.21 2.80 2.33 2.98 4.04 2.23 2.93 3.14 2.84 2.45 3.42 Ta 0.43 0.54 0.69 0.51 0.62 0.70 0.43 0.60 0.64 0.37 0.40 0.46 U 0.44 0.59 0.32 0.29 0.55 0.86 0.22 0.36 0.52 0.60 0.34 0.41 Pb 34.95 7.73 5.59 18.33 3.67 3.10 2.48 3.95 12.71 7.37 17.68 4.99 Be 3.49 1.08 1.20 1.04 0.91 1.05 1.01 1.17 1.06 1.66 0.87 0.76 La 8.62 11.36 9.62 9.56 11.38 13.56 8.89 8.47 10.38 11.63 6.36 8.32 Ce 17.45 25.50 22.19 21.37 25.68 30.84 20.34 19.65 23.22 25.28 15.50 20.35 Pr 2.12 3.55 3.05 2.79 3.53 4.10 2.73 2.71 3.23 3.41 2.22 2.97 Nd 9.65 16.24 14.36 12.88 16.11 18.49 13.05 12.69 15.24 15.27 11.24 15.10 Sm 2.64 4.62 4.19 3.66 4.38 5.09 3.34 3.93 4.23 3.73 3.53 4.40 Eu 1.36 1.46 1.40 1.32 1.54 1.61 1.23 1.35 1.55 1.41 1.32 1.72 Gd 3.53 5.61 5.37 4.32 5.81 6.08 4.11 4.82 5.54 4.48 4.92 6.01 Tb 0.61 0.95 0.90 0.71 0.98 1.08 0.75 0.86 0.95 0.76 0.84 1.04 Dy 3.80 6.53 6.26 4.60 6.45 6.76 4.94 5.66 6.50 4.63 5.78 7.09 Ho 0.80 1.32 1.24 0.95 1.34 1.36 0.92 1.17 1.30 0.92 1.17 1.42 Er 2.43 3.94 3.65 2.68 3.98 4.10 2.96 3.50 4.00 2.66 3.33 4.07 Tm 0.38 0.60 0.55 0.41 0.55 0.60 0.43 0.49 0.57 0.41 0.53 0.64 Yb 2.42 3.81 3.45 2.55 3.77 3.87 2.70 3.10 3.70 2.32 2.92 3.98 Lu 0.35 0.56 0.52 0.37 0.56 0.59 0.43 0.48 0.58 0.39 0.53 0.63 Y 24.18 38.22 36.23 26.58 38.14 39.24 28.04 32.95 37.86 28.22 33.24 40.31 ∑REE 56.17 86.04 76.76 68.17 86.07 98.12 66.83 68.88 80.97 77.29 60.17 77.74 δEu 1.36 0.88 0.91 1.01 0.93 0.88 1.02 0.95 0.98 1.06 0.97 1.02 δCe 0.97 0.98 1.00 1.00 0.99 1.00 1.00 1.00 0.98 0.97 1.01 1.00 (La/Yb)N 2.55 2.14 2.00 2.69 2.16 2.51 2.36 1.96 2.01 3.60 1.56 1.50 (La/Sm)N 2.10 1.59 1.48 1.69 1.68 1.72 1.72 1.39 1.58 2.01 1.16 1.22 (Gd/Yb)N 1.20 1.22 1.29 1.40 1.27 1.30 1.26 1.29 1.24 1.60 1.39 1.25 注：原始分析数据按11项氧化物进行标准化，然后按里特曼法进行全铁调整；主量元素含量单位为%，微量和稀土元素含量单位为10-6 
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