What impact does the fission of magma in the Afar narrow section have on the activity of the Ma'Alalta volcanic region?

Tech 2023-06-08 01:56:01 Source: Network

Article: Xiaoxin Love InventoryEditor: Xiaoxin Ai InventoryDuring the fission process, strain and Magmatism are considered to be concentrated in a narrow magmatic segment, while the fission edge is gradually abandoned. The Ma'Alalta Volcanic field near the western edge of Afar is an active magmatic segment

Article: Xiaoxin Love Inventory

Editor: Xiaoxin Ai Inventory

During the fission process, strain and Magmatism are considered to be concentrated in a narrow magmatic segment, while the fission edge is gradually abandoned. The Ma'Alalta Volcanic field near the western edge of Afar is an active magmatic segment.

Magmatism started from the lava flow of the central building and the igneous rock formed by the massive Caldera. However, the recent magmatic activity turned to the black lava field, the cinder cone and the silica dome rich in obsidian, and erupted from the NNW-SSE direction vent.

The geochemical properties of magmatic rocks are similar to those of nearby axial flow volcanoes. The depth of magma storage inferred from the mineral geological survey method shows that there is a shallow siliceous chamber about 5km deep below the formation volcano, while there is an accumulated pipeline system at 9 to 24km deep, with at least three magma storage layers, providing water for the nearest basalt.

Today, Xiaoxin will talk to you about the impact of magma fission in the Afar narrow section on the activity of the Ma'Altata volcanic area.

Ma'Alalta Volcano

Ma'Alalta VolcanoMVFDabbahu50MVF

Due to its location at the edge of the Afar depression, MVF was initially classified as an edge unit. However, little is known about MVF's Petrology, geochemistry and tectonics, as well as its relationship with the tectonics of the Afar continental fault.

Afar Rift Valley is the triple junction of the main Ethiopian rift, the Red Sea and the Gulf of Aden rift arm. The volcanic activity in Afar began about 30 Ma ago, and was initially dominated by proluvial basalt.

Starting from at least about 1 Ma, active rift and volcanic activity has been concentrated in the central axis magma segment that marks the current rift plate boundary, approximately 70 kilometers long and about 20 kilometers wide, such as the Dabbahu segment, MandaHararo segment, and ErtaAle segment.

The Holocene rift margin volcanic activity also produced a large number of lavas, such as the siliceous stratum volcanoes (marginal volcanoes) arranged by Nabro Mallahle, or the lavas erupted by basalt shield volcanoes (transverse volcanoes) such as Dabbayra.

However, despite the unique nature of edge volcanoes, the current understanding of edge volcanoes, their magma sources and pipeline systems, and their relationship with nearby magma segments is one-sided. Due to their remote location, observations mainly focus on axial volcanoes.

Afar depression is the triple junction between the Red Sea, the Gulf of Aden and the main Ethiopian rift, which is caused by the divergent movement of the Nubia plate, the Arabian Plate and the Somali Plate.

To the west of Afar is the Ethiopian Plateau, to the southeast is the Somali Plateau, and to the east are the Danakir and Ali Sabih blocks.

The volcanic activity in Ethiopia is believed to have been caused by mantle plumes, which produced Ethiopian pluvial basalts, with most of the basaltic magma erupting only about 1 Myr at approximately 30 Ma.

From the Oligocene, magmatic activity occurred at the bottom of the newly formed Afar Rift, and gradually migrated to the current magmatic segment in the Miocene.

The bottom of the Afar Rift is mainly covered by the stratospheric series (4-1Ma), followed by the bay basalt (1.1-0.6Ma), while Magmatism and extension are currently mainly concentrated in the axial magmatic segment within the rift. In the central and northern parts of Afar, the direction of these magma segments is NNW-SSE, which is also the direction of the Red Sea

These magmatic segments include fissure eruption related to extension faults, and sometimes related to Shield volcano. There have also been reports of volcanic activity near the edge of the Afar ground. Describe the horizontal arrangement of volcanoes located on the east and west sides of the Afar magma segment, with directions almost orthogonal to the Red Sea trend.

In addition, they also classified the stratigraphic volcanoes on both sides of the rift valley, such as Nabro and Ma'Altata, as marginal volcanoes. These are large stratigraphic volcanoes, resulting in explosive eruptions that form Caldera. The lava flow is generally more alkaline than the cavern transitional axial volcanoes.

The most recent eruption of the marginal volcano was in Nabro, producing basaltic to basaltic trapezoidal head rocks and lava. The MVF is located in the northern region of Afar, with an extension rate of~20 mm/year and a direction of~NE-SW.

Maalarta Volcanic field

The MVF is surrounded by Jurassic limestone, meta sediments from the Neogene basement, and outcrops of meta volcanoes. It is mainly composed of a 1745 meter high stratigraphic volcano, with its crater extending in a northeast southwest direction, approximately perpendicular to the Red Sea trend.

There are three burnt rock slices below, alternating with the falling layer of pumice, and before the formation of the third burnt rock unit, there were also basalt coke soil deposits and related small coke soil cones.

On the basis of this sequence, they recognized that obsidian flow, igneous rock and porphyry Anorthosite flow and the dome of formation volcanoes, the latest products are feldspathic lava dome and obsidian flow, which erupted along the NNW-SSE fracture in the south of formation volcanoes.

It distinguishes the early stage of formation volcanic construction from the later stage. The former includes feldspar type non high alkaline lava, and the latter produces large volume of igneous rock, dome in Caldera, and high alkaline lava flow, especially rich in amphibole surface crystals.

The feldspar dome and lava flow south of the stratigraphic volcano are more alkaline and siliceous compared to the products of the stratigraphic volcano, and they are almost distant.

Stratigraphic volcanoes have feldspathic domes and lava flows in the west and southwest, but there are also recent black cloud coal cones and NNW-SSE fissure lava flows. In the east, there are more weathered and eroded lava known as "old lava".

The rock is mainly composed of plagioclase and rarely rhyolite. The samples from the top of the building are mainly highly alkaline, while the rocks at the bottom are trace rocks containing metals, although very close to high alkalinity, there is also a pantothenic acid rock.

Source of MVF magma and comparison with other rift volcanoes

Overall, apart from the lava at the top of the stratigraphic volcano reaching a very high content of incompatible elements, MVF products exhibit a distribution of major and trace elements similar to that of the Dabau volcano.

Especially the volcanic activity on the axis of the Dabbahu volcano in MVF is characterized by the eruption of a large amount of transitional black cloud magma. The recent black cloud flows, like those at Dabbahu volcano, are related to cometary rocks rich in obsidian that erupt from several scattered vents and produce small volume domes or lava flows.

The thinning of the Lithosphere is an important process that affects the partial melting depth of the astrosphere rising under the rift system. The degree of Lithosphere stretching and thinning changes across and along the cross section, leading to partial melting at different depths of decompression, resulting in different geochemical compositions of basalt melts.

By comparing the chemical composition (MgOwt%> 4) of MVF black rock products with similar black rocks from other Afar volcanoes using rare earth element ratios, the ratios were used to evaluate the depth of partial melting and melting columns.

In fact, magma generated at a depth sufficient to contain residual garnet at the source exhibits significant fractionation in terms of medium to heavy rare earth elements. The high-pressure behavior of spinel sources is somewhat similar to that of garnet, but the effect is much lower.

So when considering the fracture products of the Nabro volcano on the edge and the Dabbahu volcano on the horizontal axis of the Assab mountain range, it was found that as a whole, the LaN/SmN range of the magma products in these volcanic centers is 0.8 to 3.8.

Due to the low thickness of the crust, the contribution of melting the lower crust containing garnet can be ruled out, ranging from 15 kilometers below the Danakir Depression to 25 to 30 kilometers below the Nanakir Plate.

Given the lack of water phase in Afar magmatic volcanic activity, it can also be ruled out that garnet fractionation under water saturation conditions observed in some arc-shaped environments. Therefore, we will consider the results of melting column depth changes caused by Lithosphere thickness changes.

Marginal volcanoes are related to deeper, garnet bearing sources, while MVF is related to axial volcanoes, and spinel Diabase is still in the garnet stable domain in the basin and mountain model related to shallow sources.

This difference is consistent with the fission stage of the axial magma segment and the thinner Lithosphere, strengthening the analogy between the recent volcanic activity of MVF and the axial volcanic activity.

The geochemical composition of the existing MVF magma samples, consistent with other observations, can be taken as a sign that the recent stage of MVF is a relatively typical axial volcanic activity.

In order to fully determine the source and melting conditions of the mantle, it is necessary to conduct additional work specifically on the magmatic rocks of MVF. However, according to several different evidences from Volcanology, geochemistry, Seismology and Petrology, MVF evolved from the original feldspathic central stratum volcano to an active magmatic segment.

Strain and magma are generally believed to localize into narrow magma segments during the magma rich fission process, and the fission edge is gradually abandoned.

However, here we present evidence of a young axial segment near the edge of the Afar Rift Valley, indicating that the process of strain localization may be more complex, just as some Fracture zone have not fully evolved, such as the Fracture zone on the flank of the volcano, which initially developed on the pre-existing cross fault structure.

On the other hand, sub parallel overlapping fractures can be active at the same time to cope with plate migration relative to stable plumes, such as the eastern and western Volcanic belt.

Using this analogy, the origin of MVF may be related to the pre-existing Lithosphere weakness guidance, where the strain will be related to the interaction between plate boundaries and hot spots.

conclusion

The volcanic structure indicates a tectonic system with a cross fault trend during this first stage. In contrast, MVF's recent volcanic activity has shifted towards black cloud lava fields, coal cinder cones, and siliceous domes rich in peralkaline obsidian, erupting from several vents parallel to the Red Sea Rift in a northwest southeast direction.

The inferred depth of magma storage from mineral geological surveys indicates that there is a shallow chamber approximately 5 kilometers below the siliceous volcanic formation, and a multi magma storage level accumulation pipeline system at depths of approximately 9 to 24 kilometers, providing water for the nearest basalt.

Therefore, there are different pipeline systems and differentiation paths of feldspar magma between the early stratigraphic volcano and the recent volcanic stage. Therefore, the recent geochemical characteristics of the biotite are similar to the nearby Dabbahu volcano, but different from the marginal activities.

The evidence of the evolution of MVF activity from feldspathic central volcanic formations to active rift segments suggests that local axial extension can significantly shift towards the edge of the rift.

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