Melt percolation beneath a spreading ridge: evidence from the Semail peridotite, Oman
Melt percolation beneath a spreading ridge: evidence from the Semail peridotite, Oman
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扩张山脊下的融化渗透：来自阿曼 Semail 橄榄岩的证据
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发表时间：
1984
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通讯作者：
R. Gregory
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作者：
R. Gregory
摘要 对阿曼山脉东南部 Semail 蛇绿岩杂岩进行的野外测绘揭示了厚厚的海洋地壳部分（平均厚度 >6 公里），其下方是 9-12 公里厚的上地幔层序，该层序由基底方辉橄榄岩-纯橄榄岩带组成，上面覆盖着包含纯橄榄石（厘米级）和橄榄石正辉石岩（厘米级）一致层的叶状方辉橄榄岩。方辉橄榄岩和协调层被不和谐的纯橄榄岩体（cm-km 尺度）和纯橄榄岩、韦氏辉长岩、橄榄石斜辉长岩和辉长岩岩脉（cm-m 尺度）横切。这些特征的序列和矿物学都与一致层和不一致层代表上升熔体和宿主橄榄岩之间的结晶或反应产物的解释一致。纯橄榄岩是横切橄榄岩的唯一具有体积意义的不和谐或和谐岩石类型，占橄榄岩部分基底 3-4 公里的约 50%，而其他地方则小于 15%。在 Semail 平均地壳成分中添加约 2 km 的地幔纯橄榄岩表明，向上渗透穿过 Semail 地幔的平均熔体具有苦苦精的亲和力（PLAG 32、DIOP 16、OL 44、SI 08）。现场证据并不要求方辉橄榄岩与产生上覆洋壳部分的上升熔体同生。 Semail 方辉橄榄岩的极度贫化特征是由于在化石晚白垩世山脊下发现的整体高熔体/岩石比（0.5-0.8）区域内与瞬时熔体的相互作用造成的。熔体/岩石比的计算假设基底方辉橄榄岩-纯橄榄岩代表山脊下方岩浆向上迁移区域的浅层“离轴”边界。明显比正常地壳截面厚以及塞梅尔橄榄岩的贫化特征可能表明，塞梅尔山脊系统的熔体供应量大于在封闭的哈瓦西纳洋盆中扩散所能容纳的量。
Summary Field mapping in the Semail ophiolite complex, southeastern Oman Mountains reveals a thick oceanic crustal section (on average >6 km thick) underlain by a 9–12 km thick upper-mantle sequence consisting of a basal harzburgite-dunite zone overlain by a foliated harzburgite containing concordant layers of dunite (cm-m scale) and olivine orthopyroxenite (cm scale). The harzburgite and the concordant layers are crosscut by discordant dunite bodies (cm-km scale) and by dykes of dunite, websterite, olivine clinopyroxenite, and gabbro (cm-m scale). The sequence and mineralogy of these features are all consistent with the interpretation that both the concordant and discordant layers represent crystallization or reaction products between ascending melts and host peridotite. Dunite is the only volumetrically significant discordant or concordant rock type crosscutting the peridotite representing ∼50% of the basal 3–4 km of the peridotite section and <15% elsewhere. Addition of ∼2 km of mantle dunite to the Semail average crustal composition suggests that the average melt percolating up through the Semail mantle had picritic affinities (PLAG 32, DIOP 16, OL 44, SI 08). Field evidence does not require that the harzburgite be cogenetic with the ascending melts that produced the overlying oceanic crustal section. The extremely depleted character of the Semail harzburgite results from interaction with transient melts in the regime of overall high melt/rock ratio (0.5–0.8) found beneath the fossil late-Cretaceous ridge. The melt/rock ratio is calculated assuming that the basal harzburgite-dunite represented the shallow-level, ‘off-axis’, boundary of the zone of upward magma migration beneath the ridge. The apparently thicker than normal crustal section and the depleted character of the Semail peridotites may suggest that the supply of melt to the Semail ridge system was greater than could be accommodated by spreading in the closing Hawasina ocean basin.