Geochemical Constraints on the Origin of Primitive Potassic Lavas in the Eastern Virunga Volcanic Province
Young mafic lavas from the East African Western Rift record melting of subcontinental lithospheric mantle that was metasomatically modified by multiple tectonic events. We report new isotope data from monogenetic cinder cones near Bufumbira, Uganda, in the Virunga Volcanic Field: 87Sr/86Sr = 0.7059–...
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| Published in: | Geochemistry, geophysics, geosystems : G3 Vol. 24; no. 6 |
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| Main Authors: | , , , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Washington
John Wiley & Sons, Inc
01-06-2023
Wiley |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Young mafic lavas from the East African Western Rift record melting of subcontinental lithospheric mantle that was metasomatically modified by multiple tectonic events. We report new isotope data from monogenetic cinder cones near Bufumbira, Uganda, in the Virunga Volcanic Field: 87Sr/86Sr = 0.7059–0.7079, εNd = −6.5 to −1.3, εHf = −6.3 to +0.9, 208Pb/204Pb = 40.1–40.7, 207Pb/204Pb = 15.68–15.75, and 206Pb/204Pb = 19.27–19.45. Olivine phenocrysts from the Bufumbira lavas have 3He/4He = 6.0–7.4 RA. The isotopic data, in conjunction with major and trace element systematics, indicate that primitive Bufumbira magmas are derived from two different metasomatized lithospheric source domains. Melts generated by lower degrees of melting record greater contributions from ∼1 to 2 Ga isotopically enriched garnet‐amphibole‐phlogopite pyroxenite veins within the lithosphere. As melting progresses, these vein melts become increasingly diluted by melts that originate near the lithosphere/asthenosphere boundary, shifting the isotopic compositions toward the common lithospheric mantle (CLM) proposed by Furman and Graham (1999, https://doi.org/10.1016/s0024-4937(99)00031-6). This ∼450–500 Ma source domain appears to underlie all Western Rift volcanic provinces and is characterized by 87Sr/86Sr ∼ 0.705, εNd ∼ 0, εHf ∼ +1 to +3, 206Pb/204Pb ∼ 19.0–19.2, 208Pb/204Pb ∼ 39.7, and 3He/4He ∼ 7 RA. Basal portions of the dense subcontinental lithospheric mantle may become gravitationally unstable and founder into underlying warmer asthenosphere, exposing surfaces where melting of locally heterogeneous veins produces small‐volume, alkaline mafic melts. Mafic lavas from all Western Rift volcanic provinces record mixing between the CLM and locally variable metasomatized source domains, suggesting this style of melt generation is fundamental to the development of magma‐poor rifts.
Plain Language Summary
Volcanic rocks in continental rift zones are critically important to igneous geochemistry and geodynamics. Lavas in the western branch of the East African Rift have some of the most extreme chemical and isotopic compositions on Earth. Basalts from the Bufumbira volcanic field (Virunga Volcanic Province, southwestern Uganda) have geochemical compositions that reflect the complex tectonic history of the underlying continental lithosphere. Their Sr‐Nd‐Pb‐Hf‐He isotope variations reveal contributions from two different lithospheric domains with distinct ages and histories of interaction with melts and fluids. One source domain is over 1 billion years old and is locally heterogeneous, with readily melted veins of garnet, amphibole, and phlogopite embedded within a pyroxenite‐rich mineralogy. Over time, vein melts become progressively diluted by melts from a deeper lithospheric source that appears to be present beneath the entire Western Rift; this region was metasomatized ∼450–500 million years ago. The deep source material is gravitationally unstable and founders off the base of the continental lithosphere. Basaltic lavas that record mixing between the deep widespread source region and a heterogeneous shallower source domain are found in each Western Rift volcanic province, suggesting this style of melt generation is fundamental to continental rifting in areas of limited magmatism and extension.
Key Points
Mafic lava from Bufumbira, Uganda record the melting of subcontinental lithospheric mantle modified by multiple metasomatic/tectonic events
We constrain the Sr‐Nd‐Pb‐Hf isotope composition of the Common Lithospheric Mantle beneath large portions of the East African Rift System
Melting associated with lithospheric foundering may be fundamental to the development of magma‐poor continental rifts |
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| ISSN: | 1525-2027 1525-2027 |
| DOI: | 10.1029/2023GC010950 |