Permo-Triassic ultrahigh-temperature metamorphism in the Kontum massif, central Vietnam
Permo-Triassic ultrahigh-temperature metamorphism in the Kontum massif, central Vietnam
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DOI：
10.2465/jmps.99.225
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发表时间：
2004-08
影响因子：
0.7
通讯作者：
Y. Osanai;N. Nakano;M. Owada;T. N. Nam;T. Toyoshima;T. Tsunogae;Pham Binh
中科院分区：
文献类型：
--
作者：
Y. Osanai;N. Nakano;M. Owada;T. N. Nam;T. Toyoshima;T. Tsunogae;Pham Binh
位于越南中部的Kontum Bagraf由低级片岩和角闪岩-麻粒岩相变质岩组成，这些岩石曾被S型和I型花岗岩侵入。该岩群以前被认为主要由太古代麻粒岩（Kannak杂岩）、元古代角闪岩相变质岩（Ngoc Linh杂岩）和低级片岩（Kham Duc杂岩）组成。它们被认为是东南亚的印度支那盆地的基底。Kannak杂岩主要由在高温至超高温（UHT）条件下变质成麻粒岩相的泥质-半泥质片麻岩组成。少量的镁铁质和钙硅酸盐岩石也夹在片麻岩中。超高温泥质变质岩的主要类型为石榴斜方辉石-硅线石-堇青石片麻岩、含斜方辉石的石榴斜方辉石-堇青石-硅线石-黑云母片麻岩和石榴斜方辉石-碳辉岩质片麻岩。最高级变质条件由石榴子石-斜方辉石-硅线石-堇青石片麻岩确定，表明退变质阶段多期合晶岩的形成是从超高温（1000°C<）等温减压开始的。由于石榴子石中镁铝榴石含量高（高达59mol%），斜方辉石中Al_2O_3含量高（高达10wt%），这些矿物在退变质阶段的P-T条件下不稳定。另一方面，新发现的石榴石-单斜辉石-斜方辉石麻粒岩（榴辉岩超高温镁铁质麻粒岩）显示出一系列的变化，从石榴石-单斜辉石-石英角闪石-石英单斜辉石-斜长石-（石榴石）的二变组合。经鉴定，这些岩石是等温减压、近等压冷却变质演化的结果。Kontum山超高温变质岩出露于沿着Dac To Kan剪切带，呈顺时针压力-温度轨迹，峰值变质条件约为1000 ℃。12 kbar和ca。1050°C（M1变质作用）。高压M0变质作用（约17 kbar<，ca. 1000°C）作为俯冲变质作用和低压（但仍然是超高温）M2变质作用（9-10 kbar，约1000 ° C）的一部分。1000°C）是顺时针压力-温度演化过程中退变质作用的一部分。广泛报道的二叠纪-三叠纪变质事件（约。240-260 Ma），表明变质作用经历了从M0阶段到低压低温退变质阶段（M2晚期）的快速变质演化。石榴斜长角闪岩Sm-Nd等时线年龄为670 Ma，独居石CHIME年龄约为100 Ma。Kannak杂岩中泥质麻粒岩的480-500 Ma年龄也表明Kontum Bagraf也经历了泛非变质事件。研究结果表明，Kontum Bagraf超高温变质岩是古冈瓦纳超大陆变质作用的残余，在东亚大陆碰撞过程中，经历了二叠纪-三叠纪变质作用。
The Kontum massif in central Vietnam consists of low-grade schists and amphibolite- to granulite-facies metamorphic rocks, that have been intruded by S-type and I-type granites. This terrane was formerly considered to be composed mainly of Archean granulites (the Kannak Complex), Proterozoic amphibolite-facies metamorphic rocks (the Ngoc Linh Complex) and low-grade schists (the Kham Duc Complex). They were thought to be the basement of the Indochina Craton in south-east Asia. The Kannak Complex is dominated by pelitic-semipelitic gneisses metamorphosed under high- to ultrahigh-temperature (UHT) conditions into granulite-facies. A minor amount of mafic and calc-silicate rocks are also intercalated within the gneisses. The major types of ultrahigh-temperature pelitic metamorphic rocks in this complex are garnet-orthopyroxene-sillimanite-cordierite gneiss, orthopyroxene-bearing garnet-cordierite-silliamnite-biotite gneiss and garnet-orthopyroxene charnockitic gneiss. The highest-grade metamorphic condition is determined from garnet-orthopyroxene-sillimanite-cordierite gneiss, which indicates that multi-stage symplectite formation during retrograde stage started from isothermal decompression in UHT condition (1000°C<). Because of the high amount of pyrope (up to 59 mole%) in garnet and high-Al2O3 in orthopyroxene (up to 10 wt%) , these minerals were unstable in the P-T conditions during the retrograde stage. On the other hand, the newly found garnet-clinopyroxene-orthopyroxene granulites (eclogitic ultrahigh-temperature mafic granulite) from the Ngoc Linh Complex shows a series of changes in divariant assemblages from garnet-clinopyroxene-quartz to hornblende-quartz through clinopyroxene-orthopyroxene-plagioclase-(garnet). It was identified that these rocks were formed as a result of their metamorphic evolution of isothermal decompression followed by nearly isobaric cooling. The ultrahigh-temperature metamorphic rocks in the Kontum massif, which are exposed along the Dac To Kan shear zone, show a clockwise pressure-temperature path with the peak metamorphic condition of ca. 12 kbar and ca. 1050°C (M1 metamorphism). High-pressure M0 metamorphism (ca. 17 kbar<, ca. 1000°C) as part of the prograde metamorphism and low-pressure (but still ultrahigh-temperature) M2 metamorphism (9-10 kbar, ca. 1000°C) as part of the retrograde metamorphism during the clockwise pressure-temperature evolution are also recognized. The widely reported Permo-Triassic metamorphic event (ca. 240-260 Ma) from the ultrahigh-temperature metamorphic rocks would indicate a rapid metamorphic evolution from M0 stage to low-pressure and low-temperature retrograde stage (later M2 stage). The recently determined Sm-Nd internal isochron age of 670 Ma for the garnet amphibolite and monazite-CHIME age of ca. 480-500 Ma for the pelitic granulites from the Kannak Complex would also indicate that the Kontum massif had also undergone the Pan-African metamorphic event. The present results indicate that ultrahigh-temperature metamorphic rocks from the Kontum massif are remnants of previously metamorphosed rocks that might be derived from the Gondwana super continent and then re-metamorphosed by the Permo-Triassic metamorphism during the continents collision in eastern Asia.