الفهرس 
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Abstract
The present work deals with the remotely sensed, geologic, structural and radiometric studies of Gabal Ras Abda – Um Taghir El Tahtani area, which is bounded by latitudes 26º 39΄ 30˝ - 26º 46΄ 00˝ N and longitudes 33º 43΄ 00˝ - 33º 52΄ 00˝ E, covering an area of about 180 km2 at the extreme southern part of the North Eastern Desert of Egypt.
Using spectral signature curves and satellite imagery data, Landsat-8 and Sentinel-2A band ratios composites ((b7+b4)/(b7), (b2)/(b2+b5), (b5) in RGB) and ((b9+b11+b12)/(b1), (b4-b2), (b11/b6)+(b6) in RGB) respectively, were determined to differentiate among all the exposed rock types in the study area.
The geologic and petrographic studies achieved compliance with the remote sensing results through distinctly differentiating the studied rock types from the oldest to the youngest into: older granitoids (quartz-diorites, tonalites and granodiorites), Dokhan volcanics (andesites, rhyodacites and rhyolites), younger gabbros, younger granites (monzogranites, syenogranites and alkali-feldspar granites), post-granite dykes and offshoots (acidic, microgranitic and basic types), and Phanerozoic sedimentary rocks.
The E-W represents the main structural trend controlling the investigated area followed by WNW-ESE trend, while the least common trends are represented by N-S and NNW-SSE. Ras Abda area is subjected to five main structural events, each event begins with compressional (CP) phase and then extensional (EXT) phase of deformation. These events were chronologically arranged basing on field relationships from the oldest to the youngest as follows: N-S to NNE-SSW (CP), E-W to WNW-ESE (EXT), NE-SW to ENE-WSW (CP), NW-SE to NNW-SSE (EXT), NW-SE to NNW-SSE (CP), NE-SW to ENE-WSW (EXT), E-W to WNW-ESE (CP), N-S to NNE-SSW (EXT), NE-SW to ENE-WSW (CP) and E-W to WNW-ESE (EXT).
Airborne and ground gamma-ray spectrometric data indicated that the acidic Dokhan volcanics, younger granites and microgranitic offshoots have the highest radiometric values among all the studied rocks.
The microgranites showed anomalous radiometric contents of eU and eTh (396.81 ppm and 1742.54 ppm, respectively), which are attributed to their mineral composition of uranium and thorium minerals and U-, Th- and REE-bearing minerals as well as post-magmatic processes of structural controls and hydrothermal solutions activity. The tension faults and joints related to NW-SE extensional phase were the pathways for radioelements-bearing solutions, and the expected alteration assemblages and radioactive disequilibrium state reflect solutions activity in the study area.