عنوان

Analysis of the Petrology and Geochemistry of the Magmas of the Galema Range in the Somalian Plateau, Ethiopia:

شماره

TLpq2388717624

زبان متن نوشتاري يا گفتاري و مانند آن

انگلیسی

عنوان اصلي

Analysis of the Petrology and Geochemistry of the Magmas of the Galema Range in the Somalian Plateau, Ethiopia:

نام عام مواد

[Thesis]

نام نخستين پديدآور

Chiasera, Brandon

عنوان اصلي به قلم نويسنده ديگر

A Probe of Lithospheric Processes of the Northern East African Rift System

نام ساير پديدآوران

Rooney, Tyrone O.

نام ناشر، پخش کننده و غيره

Michigan State University

تاریخ نشرو بخش و غیره

2020

نام خاص و کميت اثر

203

جزئيات پايان نامه و نوع درجه آن

Ph.D.

کسي که مدرک را اعطا کرده

Michigan State University

امتياز متن

2020

متن يادداشت

As a continental rift evolves towards a mid ocean ridge, the continental lithosphere must rupture and extension must become accommodated exclusively by seafloor spreading. During this process, extensional strain is initially accommodated along the nascent rift margins, subsequently localizing to zones of focused magmatic intrusion. Asthenospheric conditions such as increased mantle potential temperature or presence of an upwelling, deep seated mantle anomaly may influence, or be responsible for, these extensional processes. While these processes are integral in the process of continental rifting, how they relate and influence each other remains an area of active scientific inquiry and debate. The Main Ethiopian Rift (MER) in East Africa is the ideal location to study extensional processes. We examine the Galema range, an area of focused magmatic activity along the eastern margin of the Central Main Ethiopian Rift, which is morphologically similar to areas of focused magmatism within the rift. To better understand the complex processes involved in continental rifting and how they are related, we present whole-rock, major and trace element data on 77 samples of the Galema range magmas, including isotopic geochemistry on 22 of these samples. We find that whole rock thermodynamic modeling and thermobarometric calculations on mineral-liquid pairs suggest that fractionation (and hence magma stalling depths) within the Galema range is polybaric (~7 and ~3 kbar). These results, when compared to zones of focused intrusion within the rift, indicate an incipient magmatic plumbing system. Trace element models of melt generation reveal melting conditions of 1418-1450°C at 2.9-3.2 GPa. These conditions reveal that Ethiopian mantle TP is elevated by ~68-100°C over ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re-equilibrated with the mantle, yielded broadly similar temperatures (1435-1474°C) but at lower pressures (2.1-2.6 ± 0.2 GPa). We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent, which we contend is the lithosphere-asthenosphere boundary (LAB). Isotopic analyses of the magmas of the Galema range indicate the presence of signatures of the Afar plume, Pan-African lithosphere, and depleted mantle. A subset of these isotopic analyses indicates the presence of a previously unknown, 4th mantle end member, which we attribute to be a contribution from a recently created, and destroyed, sub-continental lithosphere beneath the eastern Ethiopian plateau. We contend that diking associated with the Galema range, which pre-dates magmatic belts within the rift; thermomechanically modified the lithosphere along this margin. The thermomechanical modification of the lithosphere mantle along this margin facilitated the subsequent development of within-rift magmatic chains. The implications of this are that off-rift magmatic activity may play an integral role in facilitating the development of rift architecture. We interpret contrasting results between two trace element modeling approaches as evidence for magma ponding subsequent to melt generation. While the continental lithosphere has thinned during extension, the lithosphere remains relatively thick late into the rifting process, despite the development of magmatic extension at crustal depths. The presence of an isotopic signature of a sub-continental lithospheric mantle indicates rapid creation and destruction of lithospheric material in processes attributed to the upwelling Afar plume. This process may be one way in which the lithosphere is thinned in the EARS, along with foundering and assimilation of material into the depleted asthenosphere.

موضوع مستند نشده

Geology

موضوع مستند نشده

Petrology

مستند نام اشخاص تاييد نشده

Chiasera, Brandon

مستند نام اشخاص تاييد نشده

Rooney, Tyrone O.

نام الکترونيکي

فرمت انتشار

p

نوع ماده

[Thesis]

کد کاربرگه

276903

سطح دسترسي

a

تكميل شده

Y