Nature and timing of anatectic event of the Hida Belt (Japan): Constraints from titanite geochemistry and U-Pb age of clinopyroxene-bearing leucogranite
Nature and timing of anatectic event of the Hida Belt (Japan): Constraints from titanite geochemistry and U-Pb age of clinopyroxene-bearing leucogranite
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DOI：
10.1016/j.lithos.2021.106256
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发表时间：
2021-05
期刊：
影响因子：
3.5
通讯作者：
Hironobu Harada;T. Tsujimori;Y. Kon;S. Aoki;K. Aoki
中科院分区：
文献类型：
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作者：
Hironobu Harada;T. Tsujimori;Y. Kon;S. Aoki;K. Aoki
日本中部的希达带是一个大陆碎片，曾经是东亚大陆边缘地壳基底的一部分。它主要由二叠-三叠纪花岗片麻岩杂岩组成，其中既有同-晚变质混合岩，也有花岗岩体。含单斜辉石的浅色花岗岩产于与角闪岩和大理岩共生的混合岩带中，当地称之为“石氏”型混合岩。浅色花岗岩的特征是在斜长石为主的基质中存在粗粒透辉石-铁辉石系列单斜辉石和钛铁矿。单斜辉石中含有丰富的方解石包裹体。自形钛铁矿具有较高的Th/U比值（~2.8-7.8）和REE含量（~4514- 14，069 μg/g），含有多晶“花岗岩”包裹体。这些矿物学特征表明在深熔作用中有碳酸盐的参与。考虑到希达片麻岩的标称压力为0.4-0.7 GPa，在aTiO 2 = 0.5和1时，钛酸锆测温法分别得出730-810 °C和770-850 °C的温度。结果表明，钛铁矿的U/Pb（~（238）U/~（206）Pb = 15.0-24.0）和Pb（~（207）Pb/~（206）Pb = 0.172-0.419）同位素比值变化很大，呈不一致线分布，截距较低，为225.4 ± 1.9Ma。这一钛铁矿年龄代表了希达带区域变质作用后期发生的区域深熔作用的时间。中地壳水平的高视热梯度可能是区域伸展作用的结果。
The Hida Belt, central Japan, is a continental fragment, which was once a part of the crustal basement of the East Asian continental margin. It consists mainly of Permo-Triassic granite-gneiss complexes with both syn-to-late-metamorphic migmatite or granite bodies. Clinopyroxene-bearing leucogranite, locally called as ‘Inishi’-type migmatite, occurs characteristically in the migmatite zone associated with amphibolite and marble. The leucogranite is characterized by the presence of coarse-grained diopside–hedenbergite series clinopyroxene and titanite in plagioclase-dominated matrix. Clinopyroxene contains abundant calcite inclusions. Euhedral titanite with high Th/U ratios (~2.8–7.8) and REE contents (~4514–14,069 μg/g) contains polycrystalline ‘granitic’ inclusions. Those mineralogical features indicate the involvement of carbonate during anatexis. Considering a nominal pressure of 0.4–0.7 GPa of the Hida gneiss, Zr-in-titanite thermometry yields a temperature of 730–810 °C and 770–850 °C at aTiO2= 0.5 and 1, respectively. The titanites show highly variable U/Pb (238U/206Pb = 15.0–24.0) and Pb (207Pb/206Pb = 0.172–0.419) isotope ratios, and the scattered trend follows a discordia line with a lower intercept at 225.4 ± 1.9 Ma. This titanite age would represent the timing of regional anatexis that have occurred in a later stage of the regional metamorphism of the Hida Belt. A high apparent thermal gradient at middle crustal levels might have been caused by regional extension.