Revisiting the high temperature metamorphic field gradient of the Ryoke Belt (SW Japan): new constraints from the Iwakuni-Yanai area.
Revisiting the high temperature metamorphic field gradient of the Ryoke Belt (SW Japan): new constraints from the Iwakuni-Yanai area.
复制标题
重新审视 Ryoke 带（日本西南部）的高温变质场梯度：来自岩国-柳井地区的新约束。
DOI：
10.1016/j.lithos.2016.04.025
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发表时间：
2016
期刊：
影响因子：
3.5
通讯作者：
Ikeda T.,
中科院分区：
文献类型：
--
作者：
Skrzypek E.;Kawakami T.;Hirajima T.;Sakata S.;Hirata T.;Ikeda T.,
我们重新审视起源的变质场梯度暴露在Ryoke带（日本西南部）-低压/高温（LP/HT）条件下的典型位置-根据岩国-柳井地区（W Ryoke）的组合结构，岩石学和地质年代学数据。该区变质沉积岩从北部的片岩过渡到南部的部分混合岩副片麻岩，并被众多的花岗岩类岩体所包围。强调三个变形阶段：增生阶段或埋藏变形强度弱（D 0），垂直缩短（D1），产生了近水平，局部拉张面理在南部片麻岩区，和E-W直立褶皱（D2）改造的北方片岩和南部片麻岩区。结晶-变形关系表明，在片岩带中，大多数红柱石、堇青石和云母变斑晶形成于D 0之后和D1之前。Syn-D1矿物组合为东西向变质带，变质程度由北向南递增，罕见的Syn-D2共生体也表现出类似的特征。持续的岩浆活动从约。105至94 Ma。最古老的花岗岩类（Shimokuhara，Soo，Namera），发现于该带的西部，在105-100 Ma时侵位在片状岩石之下。随后，从100到94 Ma，同D1后花岗闪长岩（Gamano）与南部高级片麻状岩的叶理一致侵入。一个大的，syn-D1岩体（Kibe）侵入带的中心在98马，而一个年轻的花岗岩（岩国）和它的卫星岩墙被侵位到东北96-94马。对D1（103-99 Ma）和可能的穿时D2（100-94 Ma）的时间限制表明，在岩浆侵入和变质过程中存在一个变形连续体，我们认为，出露变质场梯度是一个明显的，但偶然的，由时空上不同的热事件产生的P-T数据的对齐。相对较浅的片岩带的峰值温度条件是在第一次构造前接触变质事件中获得的。第二种是区域变质叠加作用，它是由花岗岩类在较深片麻岩带中的同构造侵入作用引起的。一次较年轻的接触变质事件局部影响了片岩带的东部。修正后的构造-热史突出了Ryoke变质场梯度的复合性和穿时性。
We revisit the origin of the metamorphic field gradient exposed in the Ryoke Belt (SW Japan) — the type locality for low pressure/high temperature (LP/HT) conditions — based on combined structural, petrological and geochronological data from the Iwakuni–Yanai area (W Ryoke). In this area metasedimentary rocks pass from schist in the north to partly migmatitic paragneiss in the south, and are surrounded by numerous granitoid bodies. Three deformation phases are emphasized; an accretionary-stage or burial deformation of weak intensity (D0), a vertical shortening (D1) which generated a subhorizontal, locally extensional foliation in the southern gneiss zone, and E–W upright folding (D2) which reworked both the northern schist and southern gneiss zones. Crystallization–deformation relationships indicate that, in the schist zone, most andalusite, cordierite and mica porphyroblasts formed after D0 and before D1. Syn-D1 mineral assemblages define E–W trending metamorphic zones with increasing grade from north to south, and rare syn-D2 parageneses reveal a similar pattern. Continuous magmatic activity lasted from ca. 105 to 94 Ma. The oldest granitoids (Shimokuhara, Soo, Namera), found in the western part of the belt, were emplaced below schistose rocks at 105–100 Ma. Subsequently, syn- to post-D1 granodiorite (Gamano) intruded concordantly with the foliation of southern, high-grade gneissic rocks from 100 to 94 Ma. A large, syn-D1 pluton (Kibe) intruded the center of the belt at 98 Ma, whereas a younger granite (Iwakuni) and its satellite dykes were emplaced to the northeast at 96–94 Ma. Constraints on the timing of D1 (103–99 Ma) and the likely diachronous D2 phase (100–94 Ma) suggest a deformation continuum during magma intrusion and metamorphism.We conclude that the exposed metamorphic field gradient is an apparent, but fortuitous, alignment ofP–Tdata resulting from spatially and temporally distinct thermal events. Peak temperature conditions in the relatively shallow schist zone were attained during a first, pre-tectonic, contact metamorphic event. A second, regional metamorphic overprint arose from the syn-tectonic intrusion of granitoids in the deeper gneiss zone. A younger contact metamorphic event locally affected the eastern part of the schist zone. The revised tectono-thermal history highlights the composite and diachronous character of the Ryoke metamorphic field gradient.