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In this paper, a unique set of samples from the uppermost crust down to the lithospheric mantle of Jordan is analyzed for composition and petrophysical properties (density. thermal conductivity, radiogenic heat production) These data, covering a vertical section of almost 65 km. are used in conjunction with surface heat flow to generate a detailed and comprehensive lithospheric thermal model that reflects the conditions of the Arabian Shield (AS) prior to the post- Oligocene onset of lithosphere thinning and Voluminous basaltic volcanism. The pre-Miocene model geotherms, based on conductive surface heat flows of 55 and 60 mW m(-2). (a) meet the range of lithosphere-asthenosphere boundary depths of 110-160 km known from seismology, (b) conform to results of thermomechanical models on the on.-in of the Dead Sea basin that started in Miocene time. and (c) are consistent with typical xenolith-derived geotherms for terranes of similar age and lithospheric thickness. Moho temperatures (at depths between 35 and 40 km) of the AS in pre-Miocene times were most likely in the order of 530-650 degrees C, with mantle heat flows averaging between 24 and 29 mW m(-2) Results contradict former views of the late Proterozoic/early Cambrian-stabilized AS being an anomalously cold terrane A "cold" thermal structure inferred from previously measured low surface heat flows (generally <= 45 mW m(-2)) is inconsistent with the thickness, composition, and petrophysical properties of the stable lithosphere of the shield.
The Takab complex is composed of a variety of metamorphic rocks including amphibolites, metapelites, mafic granulites, migmatites and meta-ultramafics, which are intruded by the granitoid. The granitoid magmatic activity occurred in relation to the subduction of the Neo-Tethys oceanic crust beneath the Iranian crust during Tertiary times. The granitoids are mainly granodiorite, quartz monzodiorite, monzonite and quartz diorite. Chemically, the magmatic rocks are characterized by ASI < 1.04, AI < 0.87 and high contents of CaO (up to ∼ 14.5 wt %), which are consistent with the I-type magmatic series. Low FeO t /(FeO t +MgO) values (< 0.75) as well as low Nb, Y and K 2 O contents of the investigated rocks resemble the calc-alkaline series. Low SiO 2 , K 2 O/Na 2 O and Al 2 O 3 accompanied by high CaO and FeO contents indicate melting of metabasites as an appropriate source for the intrusions. Negative Ti and Nb anomalies verify a metaluminous crustal origin for the protoliths of the investigated igneous rocks. These are comparable with compositions of the associated mafic migmatites, in the Takab metamorphic complex, which originated from the partial melting of amphibolites. Therefore, crustal melting and a collision-related origin for the Takab calc-alkaline intrusions are proposed here on the basis of mineralogy and geochemical characteristics. The P–T evolution during magmatic crystallization and subsolidus cooling stages is determined by the study of mineral chemistry of the granodiorite and the quartz diorite. Magmatic crystallization pressure and temperature for the quartz-diorite and the granodiorite are estimated to be P ∼ 7.8 ± 2.5 kbar, T ∼ 760 ± 75 ◦C and P ∼ 5 ± 1 kbar, T ∼ 700 ◦C, respectively. Subsolidus conditions are consistent with temperatures of ∼ 620 ◦C and ∼ 600 ◦C, and pressures of ∼ 5 kbar and ∼ 3.5 kbar for the quartz-diorite and the granodiorite, respectively.
Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the similar to 300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2 degrees, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.