Pressure–temperature evolution of the basement and cover sequences on Ios, Greece: Evidence for subduction of the Hercynian basement

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-06-27 DOI:10.1111/jmg.12738
Oliver M. Wolfe, Frank S. Spear, Jay B. Thomas, Eric M. Hasegawa, Grant T. Libby, John T. Cheney
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引用次数: 1

Abstract

High-pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P–T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)—the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr-in-rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65–2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be ~480–500°C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P–T trajectory of ~11°C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of ~45 km reaching similar peak metamorphic conditions of ~500–525°C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high-pressure prograde mineral assemblages during exhumation. The existence of similar P–T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ.

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希腊伊俄斯地区基底和盖层的压力-温度演化:海西期基底俯冲的证据
本文研究了希腊基克拉迪群岛伊俄斯岛的高压岩石,以解决由南基克拉迪剪切带(SCSZ)分隔的两个不同的岩石构造单元——由海西基底片麻岩、片岩和角闪岩组成的下盘杂岩和由蓝片岩和榴辉岩组成的上盘杂岩在俯冲过程中所达到的P-T条件。采用弹性张量石英包体在石榴石(QuiG)气压测量和Zr - in -金红石(ZiR)微量元素测温相结合的方法来约束石榴石的最小生长条件。上盘(蓝片岩)单元石榴石记录的地层压力范围为1.5 ~ 1.9 GPa,下盘基底杂岩石榴石记录的地层压力范围为1.65 ~ 2.05 GPa。对石榴石中金红石包裹体的ZiR测温确定了石榴石形成的最低温度为~ 480-500℃。即在QuiG和ZiR结果中没有证据表明蓝片岩上盘和基底岩石在俯冲过程中经历了不同的变质史。这是海西期基底杂岩中首次观察到蓝片岩相变质作用。提出了一个模型,在该模型中,初始俯冲发生在~11°C/km的相对较浅的P-T轨迹上,然后在~45 km的深度过渡到更陡峭的近等温轨迹,在2.0 GPa下达到~ 500-525°C的类似峰值变质条件。这种俯冲路径的改变可以通过增加俯冲速率或增加俯冲带的角度来实现。目前对比矿物组合和石榴石生长历史的样品被解释为是由于体积成分的差异和挖掘过程中高压递进矿物组合保存的变化。两个单元具有相似的P-T条件和进动路径,并不要求岩石全部在同一时间变质,也不要求构造段运动少。相反,这两个单元在不同的时期经历了进变质和高峰变质作用,随后沿着南海并置。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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