Garnet Peridotite Xenoliths from the Vitim Volcanic Field, Baikal Region: the Nature of the Garnet—Spinel Peridotite Transition Zone in the Continental Mantle
Garnet Peridotite Xenoliths from the Vitim Volcanic Field, Baikal Region: the Nature of the Garnet—Spinel Peridotite Transition Zone in the Continental Mantle
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DOI：
10.1093/petrology/34.6.1141
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发表时间：
1993-12
影响因子：
3.9
通讯作者：
D. Ionov;I. Ashchepkov;H. Stosch;G. Witt-Eickschen;H. Seck
中科院分区：
文献类型：
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作者：
D. Ionov;I. Ashchepkov;H. Stosch;G. Witt-Eickschen;H. Seck
贝加尔湖以东约 200 公里处的新生代 Vitim 火山田中的一组大而新鲜的橄榄岩捕虏体出自苦橄榄凝灰岩矿床，显示出从原粒尖晶石到石榴石尖晶石再到非常丰富的石榴石橄榄岩的连续渐变。这包括所有这些岩性以几厘米的规模共存的复合结核。石榴石和许多尖晶石二橄榄岩的“玄武质”主要元素含量非常丰富（CaO 3·0–3·7%，MgO 37–40%，Ca/Al=1·1，Cr/Al<0·13），而一些石榴石-尖晶石和尖晶石橄榄岩则中度贫化（Cr/Al 0·14–0·45）。 对于显然是从 40-50 公里浅层中生长出来的肥沃尖晶石二辉橄榄岩来说，其温度为 850-880°C。这与贫化尖晶石橄榄岩的 980–1030°C 和含石榴石橄榄岩的 1000–1150°C 形成鲜明对比，推断其平衡压力在 16 到 23 kbar 之间。数据表明，石榴石和尖晶石橄榄岩在约 18 kbar 的压力下以约 2 kbar 的间隔共存于 Vitim 下地幔中，并出现石榴石，并且除了 P-T 条件外，该过渡带中的石榴石与尖晶石的比例主要受 Ca、Al、Cr 和 Cr/Al 比值的大块岩石含量控制。 Vitim 橄榄岩显示 交代富集的证据很少：与中间稀土元素相比，它们通常表现出轻稀土元素的消耗；这还包括罕见的含角闪石矿脉。肥沃的尖晶石和石榴石二辉橄榄岩具有非常相似的块岩主要氧化物含量和稀土元素分布模式；这些特征表明该地区缺乏显着的化学垂直地幔分层。维蒂姆的石榴石橄榄岩与雅库特和南非金伯利岩的石榴石橄榄岩捕虏体在形态和化学成分上存在很大差异，表明太古代克拉通和后太古代移动带具有不同的岩石圈地幔结构和成分。
A suite of large and fresh peridotite xenoliths from a picrite tuff deposit in the Cenozoic Vitim volcanic field, ˜200 km east of Lake Baikal, shows a continuous gradation from protogranular spinel through garnet–spinel to very abundant garnet peridotites. This includes composite nodules in which all these lithologies coexist on the scale of a few centimeters. Garnet and many spinel lherzolites are remarkably fertile in terms of their ‘basaltic’ major element contents (CaO 3·0–3·7%, MgO 37–40%, Ca/Al=1·1, Cr/Al<0·13), whereas some garnet–spinel and spinel peridotites are moderately depleted (Cr/Al 0·14–0·45).Testimates are 850–880°C for the fertile spinel lherzolites apparently brought up from shallow depths of 40–50 km. This contrasts with 980–1030°C for depleted spinel peridotites and 1000–1150°C for the garnet-bearing peridotites for which equilibration pressures between 16 and 23 kbar are inferred. The data suggest that garnet and spinel peridotites coexist in the sub-Vitim mantle at a pressure of ˜18 kbar over an interval of ˜2 kbar, with the appearance of garnet, and with the garnetto-spinel ratio in this transitional zone primarily being controlled by bulk rock contents of Ca, Al, Cr, and Cr/Al ratios, in addition toP–Tconditions.The Vitim peridotites show little evidence for metasomatic enrichment: they commonly show depletion of LREE compared with intermediate REE; this includes also rare amphibole-bearing veins. The fertile spinel and garnet lherzolites have very similar bulk rock major oxide contents and REE distribution patterns; these features indicate a lack of significant chemical vertical mantle stratification in that region. Garnet peridotites from Vitim show large differences in modal and chemical composition from garnet peridotite xenoliths from Yakutian and South African kimberlites, suggesting distinct lithospheric mantle structure and composition in Archean cratons and post-Archean mobile belts.