Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania
Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania
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DOI：
10.1007/s004100050115
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发表时间：
1995-11
影响因子：
3.5
通讯作者：
T. Furman
中科院分区：
文献类型：
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作者：
T. Furman
本文利用坦桑尼亚西南部Rungwe火山区原始镁铁质熔岩的地球化学特征，推断其源区矿物学和熔融历史。Rungwe的后中新世基性熔岩包括碱性玄武岩、玄武岩、霞石和苦橄岩，MgO含量高达18.9wt%;霞石（>13.5%标准霞石）仅限于该省南部的Kiejo火山。 Rungwe熔岩不同于大多数西部裂谷火山岩，因为它们不是异常的钾质（K2 O/Na 2 O约。0.40）。稀疏的斑状镁铁质熔岩含有斜长石（An 82 -90）、单斜辉石（4.5-9.5 wt% Al 2 O 3）和橄榄石（Fo 79 -88）的斑晶和捕虏晶;一种碧玄岩含有1 mm的磷灰石捕虏晶，该捕虏晶包含在镁质单斜辉石中。 所有样品都具有高丰度的不相容元素（例如，0.7- 2.2wt%P2O5），相对于HFSE（Hf、Zr、Ti、Y）、Cs、Ba和K富集REE。 一些不相容元素比率在Rungwe套件中是恒定的（例如，Zr/Nb，Sr/Ce，K/Rb），但其他比值变化极大，超过了全球洋岛玄武岩（OIB）的测量范围（例如，Ba/Nb、Sm/Zr、La/Nb、Pb/Ce、Nb/U）。硅饱和度的范围，其良好的相关性与P2 O 5/Al 2 O3，表明Rungwe套件记录不同程度的熔融。霞石和碧玄岩样品中个别不相容微量元素丰度的变化表明，源岩中含有交代角闪石、钛铁矿、磷灰石和锆石。Rungwe套件被解释为一系列的低百分比熔体的富CO2橄榄岩的压力，跨越石榴石尖晶石过渡。龙圭样品的熔岩从其他西部裂谷火山中心的地球化学比较要求源矿物学变化沿着裂谷轴，虽然每个省的下面是交代橄榄岩。西部裂谷熔岩的不相容微量元素特征表明，源区在个别火山的规模上通常是均匀的，尽管每个火山的熔岩反映了一定范围的融化程度。值得注意的是，火山与不同的地球化学总是分开的主要裂谷断层，这表明火山和构造表面的功能可能对应于交代省内的次大陆岩石圈地幔。
This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle.