Accretionary wedge harzburgite serpentinization and rodingitization constrained by perovskite U/Pb SIMS age, trace elements and Sm/Nd isotopes: Case study from the Western Carpathians, Slovakia
Accretionary wedge harzburgite serpentinization and rodingitization constrained by perovskite U/Pb SIMS age, trace elements and Sm/Nd isotopes: Case study from the Western Carpathians, Slovakia
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钙钛矿 U/Pb SIMS 年龄、微量元素和 Sm/Nd 同位素限制的增生楔方方辉石蛇纹石化和罗金石化：来自斯洛伐克西喀尔巴阡山脉的案例研究
DOI：
10.1016/j.lithos.2014.06.001
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发表时间：
2014-09
期刊：
影响因子：
3.5
通讯作者：
Dyda, Marian
中科院分区：
文献类型：
--
作者：
Putis, Marian;Yang, Yue-Heng;Koppa, Matus;Dyda, Marian
含钙钛矿的方辉橄榄岩产于斯洛伐克内西喀尔巴底山脉Meliata单元的“混杂”型蓝片岩增生楔状杂岩中。虽然方辉橄榄石的黑色圆形、略有水合作用的残余“核”不含钙钛矿，但周围蛇纹岩和钙钛矿中存在的钙钛矿(PRV)使水合作用的年龄得以确定，导致了两代变质交代的PRV。钙钛矿(1)在蛇纹化斜方辉石(Opx1)碎斑岩中平行于残留的单斜辉石出溶片层生长或形成随机取向的颗粒团，常伴有微小的钙钛矿片层，或部分被钛辉石取代。钙钛矿结晶表明，在方辉橄榄岩“核”和无PRV蛇纹岩之间的边界上，不断演化的辉石化流体。PRV(1)在1-20厘米范围内的严格限制赋存意味着PRV结晶被LREE(Ce，La)、Ca~(2+)、Ti/Fe~(3+)富集水流体略微推迟到蛇纹岩作用。钙钛从寄主斜方辉石碎斑岩、尖晶石(钛)和晶界渗透流体中分离出来的颗粒尺度交代机制是将钙和钛分凝到PrV。相反，PrV(2)出现在赋存蛇纹岩和橄榄岩脉之间的富绿泥石黑墙带1~3厘米处，表明有通道的橄榄岩化流体流动和伴生的水力压裂。这里，PrV(2)是由绿泥石和磷灰石嵌入的。部分PRV(2)形成于由透辉石、透辉石、绿帘石、绿帘石/橄榄石、磷灰石和绿泥石组成的橄榄石脉状矿物组合中。钙钛矿1和2都被沿颗粒边缘和内部的钙钛矿取代；最可能是通过流体辅助的溶解-再沉淀耦合作用，增加了Si-Fe-Mn-Al元素在蛇纹岩化方辉橄榄岩中的溶解度。两代PRV，特别是PRV(2)可部分或几乎全部被(Ti-)ADR取代。在蛇纹岩和橄榄岩之间的接触带中，偶尔可以观察到安山岩的尖晶石过度生长。LA-ICP-MS研究显示，从PRV(1)到PRV(2)的LREE强烈亏损，而PRV(2)的Eu异常更为典型。我们的蜘蛛图描绘了两代PRV中U、Nb、La、Ce、Pr、Nd的相对富集度，而Rb、Ba、Th、Ta、Pb、Sr、Zr的相对富集度。3个样品中PRV(1)的U/PbSIMS协和年龄范围为137.00±1.31~135.41 Ma，平均为135.6±0.58Ma，PRV(2)的U/PbSIMS协和年龄为133.7±15.4Ma。这种可以忽略的年龄差异暗示了一种相对短暂的罗地尼化事件，这是导致两代PRV结晶的原因。LA-MC-ICP-MS测得PRV(1)的143ND/144ND平均值为0.512153±0.000017，相当于初始ɛND(t=1135)=−=8.2N±100.4(数学平均值)。这表明，俯冲脱水的陆壳是在Meliata-Hallstatt三叠纪至侏罗纪大洋弧后盆地闭合和40Ar/39Ar年约160-150 Ma的高压变质作用之后，在新特提斯海相增生楔形体中启动蛇纹岩化和罗丁岩化作用的相互作用流体的主要来源。
Perovskite-bearing harzburgites occur in a “mélange” type blueschist-bearing accretionary wedge complex of the Inner Western Carpathians Meliata Unit in Slovakia. Although dark rounded, slightly hydrated relic “cores” of harzburgite boulders are perovskite-free, perovskite (Prv) occurrence in the surrounding serpentinites and rodingites enabled dating of hydration, resulting in two metamorphic–metasomatic Prv generations. Perovskite (1) grows parallel to relic clinopyroxene exsolution lamellae or forms randomly oriented grain clusters in serpentinized orthopyroxene (Opx1) porphyroclasts, often accompanied by tiny andradite lamellae clusters, or it is partly replaced by Ti-andradite. Perovskite crystallization indicates evolving rodingitization fluids pervading the boundary between the harzburgite “cores” and Prv-free serpentinite. This strictly limited occurrence of Prv (1) within a 1 to 20-cm across-zone implies slightly postponed Prv crystallization to serpentinization by LREE(Ce,La), Ca2+, Ti/Fe3+-enriched aqueous fluids. A grain scale metasomatic mechanism partitioned Ca and Ti from the host orthopyroxene porphyroclasts, spinel (Ti) and grain-boundary pervasive fluids to Prv. In contrast, Prv (2) occurs in a 1 to 3 cm across chlorite-rich blackwall zone between hosting serpentinite and rodingite veins, thus indicating channelled rodingitization fluid flow and accompanying hydraulic fracturing. Here, Prv (2) is ingrown by chlorite and apatite. Part of this Prv (2) formed in a rodingite vein mineral assemblage composed of diopside, andradite, vesuvianite, epidote/zoisite, apatite and chlorite. Both perovskite 1 and 2 are replaced by pyrophanite along the grain rims and interiors; most likely via fluid-aided coupled dissolution–reprecipitation at increased Si–Fe–Mn–Al element solubility in rodingitization fluids pervading serpentinized harzburgite. Both Prv generations, especially Prv (2), can be partly to almost totally replaced by (Ti-) Adr. Overgrowths of spinel by andradite are occasionally observed in contact zones between the serpentinites and rodingites. LA-ICP-MS study revealed strong depletion in LREE from Prv (1) to Prv (2), and a more typically positive Eu anomaly for Prv (2). Our spider diagram depicts relative enrichment in U, Nb, La, Ce, Pr, Nd, and decreased Rb, Ba, Th, Ta, Pb, Sr, Zr in both Prv generations. The U/Pb SIMS concordia ages of Prv (1) from 3 samples range from 137 ± 1 Ma to 135 ± 1 Ma, with a mean of 135.6 ± 0.58 Ma, while Prv (2) was dated at 133.7 ± 5.4 Ma. Such negligible age differences imply a relatively short-lived rodingitization event responsible for crystallization of both Prv generations. The143Nd/144Nd mean value of Prv (1) is 0.512153 ± 0.000017 by LA-MC-ICP-MS, thus corresponding to the initial ɛNd(t = 135) = − 8.2 ± 0.4 (math's mean). This suggests that the subducted and dehydrated continental crust was the main source of the interactive fluids which initiated serpentinization and rodingitization in the Neotethyan Meliatic accretionary wedge following closure of the Meliata–Hallstatt Triassic to Jurassic oceanic back-arc basin and the high-pressure metamorphism dated at ca. 160–150 Ma by40Ar/39Ar.
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影响因子：
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作者：
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通讯作者：
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影响因子：
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影响因子：
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影响因子：
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作者：
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通讯作者：
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作者：
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通讯作者：
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