Importance of crustal relamination in origin of the orogenic mantle peridotite–high-pressure granulite association: example from the Náměšť Granulite Massif (Bohemian Massif, Czech Republic)
Vladimír KusbachVojtěch JanoušekPavlína HasalováKarel SchulmannC. Mark FanningVojtěch ErbanStanislav Ulrich
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Abstract:
A characteristic association of crustal and mantle rocks is commonly used to decipher processes at the mantle–crust interface of HP–UHP collisional orogenic systems. Also, in the Variscan orogenic root of the Bohemian Massif (the Moldanubian Zone), high-pressure felsic granulites are often accompanied by spinel or garnet peridotites. This association was investigated using petrography, zircon geochronology and whole-rock geochemical data from the Náměšť Granulite Massif. The geochemical signature of the granulite is the same as for other Moldanubian occurrences, suggesting nearly isochemically metamorphosed felsic metaigneous rocks of Saxothuringian provenance. SHRIMP zircon dating yielded two main age maxima, at 395.2 ± 4.4 and 337.2 ± 1.7 Ma, reflecting an Early Devonian protolith and Visean HP metamorphism. As shown by Sr–Nd isotopic data, the variably refertilized harzburgite or depleted lherzolite was variously contaminated by mature crustal material resembling the studied granulites. To account for the origin of these HT–HP rock associations we suggest a new geotectonic model. An eastward continental subduction of Early Palaeozoic felsic metaigneous material of Saxothuringian origin was followed by its relamination at the bottom of the autochthonous lower crust. Ascending felsic granulites derived from the relaminated lower plate material sampled refertilized harzburgites originally formed in a back-arc. The complete assemblage was subsequently exhumed, forming large, diapir-like bodies. Supplementary material: Sample coordinates from the Náměšť Granulite Massif, analytical techniques, SHRIMP age measurements on zircon grains and whole-rock geochemical data are available at http://www.geolsoc.org.uk/SUP18833 .Keywords:
Massif
Peridotite
A characteristic association of crustal and mantle rocks is commonly used to decipher processes at the mantle–crust interface of HP–UHP collisional orogenic systems. Also, in the Variscan orogenic root of the Bohemian Massif (the Moldanubian Zone), high-pressure felsic granulites are often accompanied by spinel or garnet peridotites. This association was investigated using petrography, zircon geochronology and whole-rock geochemical data from the Náměšť Granulite Massif. The geochemical signature of the granulite is the same as for other Moldanubian occurrences, suggesting nearly isochemically metamorphosed felsic metaigneous rocks of Saxothuringian provenance. SHRIMP zircon dating yielded two main age maxima, at 395.2 ± 4.4 and 337.2 ± 1.7 Ma, reflecting an Early Devonian protolith and Visean HP metamorphism. As shown by Sr–Nd isotopic data, the variably refertilized harzburgite or depleted lherzolite was variously contaminated by mature crustal material resembling the studied granulites. To account for the origin of these HT–HP rock associations we suggest a new geotectonic model. An eastward continental subduction of Early Palaeozoic felsic metaigneous material of Saxothuringian origin was followed by its relamination at the bottom of the autochthonous lower crust. Ascending felsic granulites derived from the relaminated lower plate material sampled refertilized harzburgites originally formed in a back-arc. The complete assemblage was subsequently exhumed, forming large, diapir-like bodies.
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Summary: The distribution of radioactive (Th, U, K), major and selected trace (Rb, Sr, Ba, Y, Zr, V, Cr, Ni) elements of granulites from the Saxonian Granulite Complex was studied. Similarly to the South Bohemian granulites, the Saxonian granulites can be divided according to the contents of their major and trace elements into two main groups, group A containing mostly acid and subaeid #ranulites (K20 > 25%, SiO 2 > 68%), and group B containing mostly intermediate and basic granulites (K20 < 2-5%, SiO z < 68%). Statistically significant d~fferences between groups A and B were found for all major oxides and several t/ace elements (Rb, V, Cr, Ni). The Saxonian granulites follow the same cale-alkaline trend as the South Bohemian, granulites A being placed mostly in the Hzyolite field and granulites B mostly in the dacite, andesite and basalt fields of this trend. The investigated granulites are characterized by a considerable scatter of Th and U contents accompanied by velT variable Th/U ratios; the Th and U concentrations of granulites A are substantially lower than is usual for rocks of corresponding acidity.
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Charnockite
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To quantify the alteration processes of mantle from geophysical data, an understanding of the relationship between alteration and the physical properties of mantle peridotite is essential. In this study, we employed independent component analysis (ICA) to evaluate variations in the physical properties of altered peridotites collected by the Oman Drilling Project, to understand the alteration processes of mantle peridotite in the Samail ophiolite. We analyzed multivariate physical properties (density, porosity, P-wave velocity, electrical resistivity, permeability, magnetic susceptibility, and color reflectance) that had been measured on core samples. The ICA results show that the observed variations in physical properties can be explained broadly by four independent components. Through their relationships with physical properties and comparisons with petrological and geochemical data from previous studies, we infer the four independent components to represent distinct alteration processes: the early and late stages of serpentinization, magnetite formation, and near-surface carbonation. These processes develop differently during the overall process of alteration, and they influenced each physical property in different ways. Our results demonstrate that ICA can separate the effects of multiple processes of alteration on various physical properties of the altered peridotites, which previously had been difficult to quantify.
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到 granulite 外形的角闪岩外形岩石的高级脱水是能包含部分融化的一个过程，帮助液体固态两个的脱水，或变化的度。在局部性的米规模上，固态由于沿着包围岩石的矿物质谷物边界向外沿着一些裂缝或快克并且随后旅行的 CO2 富有的液体，脱水通常是由的工具到 orthopyroxene 和 clinopyroxene 的黑云母和闪石的故障发生。在在这些岩石中看见的矿物质化学的各种各样的矿物质质地和变化也在与伴随角闪岩外形岩石一起，形式更低的外壳穿越的更多的地区性的 orthopyroxene-clinopyroxene-bearing 岩石中被看见。这建议那固态脱水能在高级变态期间发生在更多的地区性的规模上。在 traverse 的 granulite 外形部分的这些导致液体的质地突出的更多拿形式沿着石英谷物边界和在 fluorapatite 的独居石包括的形成的 K 长石微静脉。相信负责的液体从沿着谷物边界向上旅行的地下室 ultramafic 岩浆热来源拿集中的 NaCl- 和 KCl- 盐水的形式。在 NaCl 盐水的另外的试验性的工作包含 CaSO4 溶解，在 granulite 外形岩石中结合了氧化物和硫化物矿物质协会的自然观察，表明了与一个 CaSO4 部件， NaCl 盐水能与在他们的 protoliths 是固有的氧易逃逸对比在这些岩石上强加氧易逃逸的可能性。这些结果，一起拿，使更低的外壳的地区性的化学修正是沿着谷物边界从 lithospheric 披风向上移居进并且通过他们两个都修改岩石的更低的外壳的液体控制的一个进化过程并且被它修改的想法变得更加可信。他们的存在允许在他们通过通过的岩石中的快速的质量和热运输和随后的矿物质开始和矿物质重新使平衡。
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Two-pyroxene-bearing granulites 在南部的 Qinling 山的 Hercynian-lndosinian 变形的带第一次被发现。granulites 发生在 Fuping 建筑群的更低的部分并且被中介酸的岩石统治与乐队的小数量和基本 granulites 的透镜设置了。主要变形矿物质包括 orthopyroxene， clinopyroxene，黑云母，斜长石和石英，并且 orthopyroxenes 倒退地经常被转变成闪石。变形条件被估计了赌 = 720 鈥 ? 80 掳 C andP = 0。6 GPa。忍受 granulite Fuping 建筑群可能在年龄原生地早属于。它是否在 collisional 过程作为南部的 Qinling Hercynian-lndosinian orogenic 带的水晶的地下室或作为插入的片发生，需要进一步的学习。
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Previous reports on Nové Dvory garnet peridotites indicated that they lacked primary spinel, and prograde metamorphism evidence was uncommon. This study revealed the presence of Al– and Cr–rich spinels, clinopyroxenes with a lower content of Na and Fe than what has been previously reported, and chemical heterogeneity in garnet in Nové Dvory garnet peridotites. Cr–poor (0.06–0.12 a.p.f.u.) and Cr–rich (0.10–0.27 a.p.f.u.) garnet populations were identified, and they had contrasting coronas around garnet. The garnet peridotite samples were classified into three types on the basis of the chemical composition of garnet and constituent minerals: type A that includes Cr–rich spinel and Cr–rich garnet; type B that includes Cr–poor garnet and no spinel; and type C that includes both Al– and Cr–rich spinel and both Cr–rich and Cr–poor garnet. The finding of spinel relics {Cr# = [100 Cr/(Cr + Al)] ~ 60–70} in garnet from type A peridotite suggests that the Nové Dvory peridotite body may have been located at relatively shallow depths prior to the ultrahigh–pressure metamorphic stage of >4 GPa. Suitable geothermobarometers were used to establish the P–T history of the rocks. The core compositions of clinopyroxene inclusions in garnet and host garnet yielded 978–1002 °C, 4.87–5.12 GPa (type B) and 1034 °C, 4.93 GPa (type C); stage I. Garnet porphyroblasts in type A peridotite lack clinopyroxene inclusions. The core compositions of garnet and pyroxenes yielded 1005–1072 °C, 4.42–4.46 GPa (type A); stage II. The innermost garnet rims and cores of matrix pyroxenes surrounding garnet yielded 1222–1325 °C, 5.03–5.67 GPa (type B) and 1189–1267 °C, 5.59–6.97 GPa (type C); stage II. Thus, Nové Dvory peridotite has experienced an increase in the P–T conditions, and its chemical heterogeneity (especially in the Cr content of garnet) in type C peridotites may have been created by the mechanical mixing of different rock types (i.e., Cr–rich and Cr–poor types) during the compression and/or decompression stage(s).
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