Prolonged slab-derived silicate and carbonate metasomatism of a cratonic mantle wedge (Maowu ultramafic body, China)
Prolonged slab-derived silicate and carbonate metasomatism of a cratonic mantle wedge (Maowu ultramafic body, China)
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克拉通地幔楔的长期板状硅酸盐和碳酸盐交代作用（毛屋超镁铁体，中国）
DOI：
10.1093/petrology/egab081
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发表时间：
2021-09
影响因子：
3.9
通讯作者：
Qing Xiong
中科院分区：
文献类型：
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作者：
Yi Zhao;Jian-Ping Zheng;Qing Xiong
大陆边缘壳幔物质交换是形成地幔不均匀性和地球深部物质循环的重要过程。物质传递通常是由板片和/或软流圈源组分完成的，它们交代了地幔楔。澄清这些记录的起源和时间顺序是基本的目标，但解决得很差。通过对大别山毛屋超镁铁质岩体（包括方辉橄榄岩和石榴斜方辉石岩）的岩石学、全岩和矿物成分（包括铂族元素）、锆石U-Pb年龄、微量元素和Hf同位素等综合研究，揭示了南华北造山带复杂的交代历史。该超镁铁质岩体是典型的地幔楔碎片，其平衡压力为4.0 ± 1.0 GPa，温度为750 °C ± 50 °C。方辉橄榄岩具有高Mg#值（高达92）和Ni含量（2537-2892 ppm），低Al_2 O_3（0.26-0.76 wt%）和CaO（0.05-0.32 wt%），高橄榄石Fo值（91-93），支持其部分熔融程度为~20- 25%的地幔楔的成因。方辉橄榄岩中石榴斜方辉橄榄岩呈脉状产出，Mg#值范围宽（83-91），Ni含量高（963-2353 ppm），轻稀土元素和大离子亲石元素明显富集。它们的PGE含量和模式与寄主方辉橄榄岩相似。结合地球化学模拟，这些特征表明，石榴斜方辉石是通过方辉橄榄岩与来自大陆地壳的硅酸盐熔体反应而产生的。
毛屋超镁铁质岩体中的锆石有三种类型。从方辉橄榄岩中分离出U-Pb年龄为1927 ~465 Ma的I组锆石。它们的特征是振荡带、高的重稀土元素（HREE）含量、宽范围的εHf（t）值（从−24.9到+21.0）和长英质包裹体（例如，石英和斜长石）。它们被解释为强烈的硅酸盐熔体交代作用的记录触发的附加壳源硅酸盐沉积物（含元古代碎屑锆石）到NCC地幔楔。交代作用形成了NCC地幔楔南部的石榴正辉石岩脉。第II组锆石具有暗核和亮边，在方辉橄榄岩和石榴斜方辉石中的U-Pb年龄范围为421 ± 8 ~ 256 ± 6 Ma。它们的Th、U含量高（高达5000 ppm），HREE配分模式陡，Eu负异常，εHf（t）值为正（+0.6 ~+8.2）。碳酸盐被确定为锆石中的包裹体和方辉橄榄岩基质中的豆荚或细脉。这些功能可能反映了一个长期的和普遍的交代作用的氧化碳酸盐流体在石榴石斜方辉石脉地幔楔，同岁的长期俯冲的特提斯板块，主要进行沉积碳酸盐。方辉橄榄岩和石榴斜方辉石的III族锆石U-Pb年龄一致，为227 ± 6 Ma。它们记录了三叠纪扬子板块与华北克拉通板块的碰撞过程。总的来说，这项研究提供了地球化学，矿物学和岩石学的证据，一个长期熔融和各种交代作用的地幔边缘可以存档复杂的历史沿着会聚边缘，从大洋俯冲到大陆碰撞。
Mass transfer between crust and mantle at continental margins is a crucial process in shaping mantle heterogeneity and material cycling in deep Earth. Mass transfer is usually archived by slab-derived and/or asthenosphere-derived component, which metasomatize a cratonic mantle wedge. Clarifying the origin and temporal sequences of these records is fundamental aim yet poorly resolved. Comprehensive petrography, whole-rock as well as mineral compositions (including platinum-group elements (PGE)), zircon U–Pb ages, trace elements, and Hf isotopes of the Maowu ultramafic body (comprising harzburgites and garnet orthopyroxenites) in the Dabie orogen (Central China) are presented in this study to decode the complex metasomatic histories beneath the southern North China Craton (NCC). The ultramafic body represents a typical cratonic mantle-wedge fragment with an equilibration pressure of 4.0 ± 1.0 GPa and temperature of 750 °C ± 50 °C. The harzburgites have high Mg# values (up to 92) as well as Ni contents (2537–2892 ppm), low Al2O3 (0.26–0.76 wt%) as well as CaO (0.05–0.32 wt%) contents, and high olivine Fo values (91–93), supporting the origin from cratonic mantle wedge with partial melting extents of ~20–25%. Garnet orthopyroxenites occurring as veins in the harzburgites have a wide range of Mg# values (83–91), high-Ni contents (963–2353 ppm), and significant enrichment in light rare earth elements and large-ion lithophile elements. They show PGE contents and patterns similar to those of the host harzburgites. These characteristics, combined with geochemical modeling, suggest that the garnet orthopyroxenites were produced through the reaction of the harzburgites with silicate melts derived from the continental crust.
Three types of zircons were identified in the Maowu ultramafic body. The group-I zircons with U–Pb ages ranging from ~1927 to ~465 Ma were separated from the harzburgites. They are characterized by oscillatory zones, high heavy rare-earth element (HREE) contents, a wide range of εHf(t) values (from −24.9 to +21.0), and felsic inclusions (e.g., quartz and plagioclase). They are interpreted as records of the intense silicate-melt metasomatism triggered by the addition of crust-derived silicate sediments (containing Proterozoic detrital zircons) into the NCC mantle wedge. This metasomatism formed the garnet orthopyroxenite veins in the southern NCC mantle wedge. The group-II zircons have dark cores and bright rims and yield a range of U−Pb ages from 421 ± 8 to 256 ± 6 Ma in both the harzburgites and garnet orthopyroxenites. They contain high Th and U contents (up to 5000 ppm) and display steep HREE patterns, negative Eu anomalies, and positive εHf(t) values (+0.6 to +8.2). Carbonates were identified as inclusions in the zircons and as pods or veinlets in the matrix of the harzburgites. These features may reflect a long-term and pervasive metasomatism by oxidized carbonate fluids in the garnet orthopyroxenite-veined mantle wedge, coeval with the secular subduction of the Tethyan slab that carried mainly sedimentary carbonates. The group-III zircons from both the harzburgites and garnet orthopyroxenites show concordant U−Pb ages of 227 ± 6 Ma. They are interpreted to record the Triassic continental collision between the Yangtze Craton and NCC. Collectively, this study provides geochemical, mineralogical, and petrological evidence that the periphery of a cratonic mantle with prolonged fluxing and various metasomatism can archive the complex history along a convergent margin that evolved from oceanic subduction to continental collision.