Contrasting P–T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt
Contrasting P–T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt
复制标题
DOI：
10.1093/petrology/41.12.1673
复制
发表时间：
2000-12
影响因子：
3.9
通讯作者：
B. Goscombe;M. Hand
中科院分区：
文献类型：
--
作者：
B. Goscombe;M. Hand
D-F东部两个剖面岩石学和相平衡。基质组合被减压反应结构所叠加，在F区和G区进展到硅线石领域。尼泊尔从小喜马拉雅层序开始，穿过主中央冲断带进入大喜马拉雅层序。在硅线石-黑云母±方晶面理层（S3）的形成过程中，这两个岩块并置，揭示了一个成对的变质山带（PMMB），由两个变质条件截然不同的冲断约束变质岩块组成，其变质条件为674 ± 33°C和5.7 ± 1.1 kbar。对比两个单元岩的平均地温梯度和P-T轨迹。在较高的构造层次上，大喜马拉雅层序经历了高T/中P变质作用，变质程度不一，表明它们组成了一个PMMB。大喜马拉雅层序的上板位置产生了一个逆冲P-T轨迹，石英+铁尖晶石+硅线石的低磷包裹体组合被逆冲峰值P-T轨迹所覆盖，高的平均地温梯度可能是由于高的放射性成因元素含量。变质石榴子石+堇青石+硅线石组合的平衡温度为837 ± 59°C，压力为6.7 ± 1.0 kbar。与此相反，下板小喜马拉雅层序被深埋，变质在顺时针P-T路径和显示矿物被众多的变质反应纹理，记录等压冷却和再平衡的样品保存倒置的等梯度线，作为一个渐进的韧性逆冲推覆的热大喜马拉雅层序在逆冲变质作用。在5.7 ± 1.1 kbar下冷却至600 ± 45°C的证据。在主中央冲断层之下，小喜马拉雅层序是一个连续的（尽管是倒置的）高磷/中温变质岩的巴罗层序。变质带从
Petrology and phase equilibria of rocks from two profiles in Eastern D–F. Matrix assemblages are overprinted by decompression reaction textures, and in Zones F and G progress into the sillimanite field. Nepal from the Lesser Himalayan Sequences, across the Main Central Thrust Zone and into the Greater Himalayan Sequences The two terranes were brought into juxtaposition during formation of sillimanite–biotite ± gedrite foliation seams (S3) formed at reveal a Paired Metamorphic Mountain Belt (PMMB) composed of two thrust-bound metamorphic terranes of contrasting metamorphic conditions of 674 ± 33°C and 5·7 ± 1·1 kbar. The contrasting average geothermal gradients and P–T paths of these two metastyle. At the higher structural level, the Greater Himalayan Sequences experienced high-T/moderate-P metamorphism, with an anmorphic terranes suggest they make up a PMMB. The upperplate position of the Greater Himalayan Sequences produced an ticlockwise P–T path. Low-P inclusion assemblages of quartz + hercynitic spinel + sillimanite have been overgrown by peak anticlockwise P–T path, with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane. metamorphic garnet + cordierite + sillimanite assemblages that equilibrated at 837 ± 59°C and 6·7 ± 1·0 kbar. Matrix In contrast, the lower-plate Lesser Himalayan Sequences were deeply buried, metamorphosed in a clockwise P–T path and display minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re-equilibrated samples preserve inverted isograds as a result of progressive ductile overthrusting of the hot Greater Himalayan Sequences during prograde metamorphism. evidence of cooling to 600 ± 45°C at 5·7 ±1·1 kbar. Below the Main Central Thrust, the Lesser Himalayan Sequences are a continuous (though inverted) Barrovian sequence of high-P/moderate-T metamorphic rocks. Metamorphic zones upwards from the