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首页> 外文期刊>Contributions to Mineralogy and Petrology >Tectonic implications of garnet-bearing mantle xenoliths exhumed by Quaternary magmatism in the Hangay dome, central Mongolia
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Tectonic implications of garnet-bearing mantle xenoliths exhumed by Quaternary magmatism in the Hangay dome, central Mongolia

机译：第四纪岩浆作用带出石榴石地幔异岩的构造意义

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摘要

Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pres-sures of 17-18 kbars and temperatures of 1,070-1,090°C, whereas garnet lherzolites were derived from greater depths (18-20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18-22 kbars under significantly cooler conditions (960-1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abun-dances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermo-barometric and textural data suggest that lherzolite xeno-liths from both sites were derived from depths of 60-70 km and entrained in magma at 1,200-1,300°C. The average rate of ascent, as determined by olivine zoning, lies in therange 0.2-0.3 m s~(-1). The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift.
机译：含石榴石的地幔异岩是从蒙古中部汉盖穹顶内和外围的第四纪碱性玄武岩中回收的。微结构分析和热风压法，结合经验热压计和THERMOCALC的自洽数据集，表明在17-18 kbars的压力下，尖晶石-锂铁矿上地幔中形成了Shavaryn-Tsaram火山中心的石榴石韦氏石。温度为1,070-1,090°C，而石榴石绿铁矿则来自更大的深度（18-20 kbars）。来自Baga Togo Uul通风孔的靠近圆顶边缘的石榴石绿沸石在18-22 kbars的温度下（960-1,000°C）形成。这些异种石揭示了（1）邻苯并rox，斜靠rox，斜长石和尖晶石幔石榴石的反应日冕; （2）海绵状的轮缘代替邻苯二甲酚，（3）低钠，低铝的次氯环己烯代替主要的次氯环己烯。痕量元素丰度表明，这些日冕中的次氯环己烯与宿主岩浆处于化学平衡状态。热压和构造数据表明，这两个地点的锂铁矿异岩均来自60-70 km的深度，并夹带在1200-1,300°C的岩浆中。通过橄榄石带划分确定的平均上升速度在0.2-0.3 m s〜（-1）范围内。这两个位置之间的上地幔热剖面的对比与Hangay穹顶下方的地幔柱相吻合，穹顶核心下方的热状况升高，可与贝加尔湖裂谷下的更新世地热估算相提并论。

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