Ezz El Din Abdel Hakim Khalaf

Origin and evolution of post-collisional volcanism: an example from Neoproterozoic Dokhan volcanics at Gabal Nugara area, Northeastern Desert, Egypt

Nugara volcanics are one of the northernmost outcrops of the Arabian–Nubian Shield. Two distinct volcanic successions are found in the Nugara basin: (1) old volcanic sequence composed of voluminous medium- to high-K calc-alkaline lavas and minor alkali basalt and (2) young volcanic sequence composed of subordinate tholeiitic mafic lavas. Their eruptions were punctuated by occasional volcaniclastic deposits that generated fall, flow, or reworked suites compositionally identical to the lava flows. These volcanics are a part of a post-subduction and extensional-related magmatic event in Northeastern Desert of Egypt. The volcanic rocks of the Nugara basin are characterized by strong enrichment in LILE relative to HESF, high LILE/HFSE ratios, and depletions of Nb on MORB-normalized multi-element diagrams. The geochemical features of the volcanic rocks suggest that they experienced fractional crystallization, along with mixing processes. Crustal contributions to the magma sources may also have occurred during magmatic evolution. These processes have resulted in scattered major and trace element variations with respect to increasing silica contents. The model proposed for their origin involves contrasting ascent paths and differentiation histories through crustal columns with different thermal and density gradients. The geochemical features of the most mafic samples suggest that the volcanic rocks in the region were derived from a mainly lithospheric mantle source that had been heterogeneously metasomatized by previous subduction events during convergence between the East and West Gondwanaland. The volcanic activity in the region is best explained by the delamination of lithospheric mantle slices that were heterogeneously enriched by previous subduction-related processes.Abstractتعتبر بركانبات جبل نقارة جزء من القشرة العربية النوبية – وفد اثبيت الدراسة ان هناك نوعين من البركانيات فى منطقة الدراسة :-(1) التتايع الاقدم ويشمل على بركانيات الدخان الكلسية – القلوية مع قليل من صخور بازليتة فلوية .(2) التتابع الاحدث ويشمل على صخور مافية ثوليايتة . ويتخلل هدة البركانيات رواسب بركانية فتتاتية . حيث ان هدة الصخور تكونت فى منطقة ما بعد نطاق الغوران وبداية الشد الجانيى فى شمال الصحراء الشرقية – مصر .وتتميز هدة البركانيات بعلو ملحظوز فى العناصر القلوية الارضية وفقر فى عنصر النليبون . وقد اوضحت الصفات الكيمائبة ان هدة البركانبات قد تكونت بعمليات التجزئة الصهيربة مع تداخل من عمليات الخلط والامتزاج . وان هدة العمليات قد تسبيت فى تشتت العناصر الاسياسية والشحيحة على مخططات التغير . وان الموديل المقترح يدل ان هدان التتابعان لهما مسارات متبلورة منفصلة وفد نكونا من صهيرين منفصلين .وتدل الصفات الكيمائية ان مصدر هدة البركانيات هى جبة الليثوسفير النى تاثرت باحداث الغوران السابقة خلال عملية التصادم بين شرق وغرب ارض الجدروانا .

Syn-eruptive/inter-eruptive relationships in Late Neoproterozoic volcano-sedimentary deposits of the Hamid area, North Eastern Desert, Egypt

Nonmarine volcano-sedimentary successions in the Late Neoproterozoic Hamid Basin were studied in order to examine the distinctive characteristics of accumulation during syn-eruptive and inter-eruptive periods in a depocenter associated with active volcanism and both extensional and dextral strike-slip tectonics. In particular, the syn-rift fill in this area comprises a wide range of compositions and transport and depositional processes in which lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space. Seven different accumulation units were identified in the syn-rift fill: (1) polymictic alluvial fan units, (2) fluvial braid plain units, (3) lacustrine units, (4) coherent volcanic bodies/shallow intrusion units, (5) pyroclastic fall units, (6) phreatomagmatic volcanic units, and (7) pyroclastic density current units. These deposits are organized into several stratal packages with contrasting geometries. Analysis of these units and the relationships between them provided insights into the evolution of the syn-rift sedimentary environments and permitted identification of different stages of effusive activity, explosive activity, and relative quiescence, determining syn-eruptive and inter-eruptive rock units. These units provide important clues to the distribution of, and temporal changes in, accommodation space, and hence the configuration and structural evolution of the Hamid Basin. Two accumulation stages were defined. The underfilled stage occurs when the material supplied to the depocenter during the eruptive events is not enough to level the existing topography, allowing the development of high-gradient alluvial systems during the next inter-eruptive period. The overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systems to develop during the subsequent inter-eruptive periods. The Hamid Basin is thus interpreted to have been hybrid in nature, influenced by the dynamic changes of the basin–margin faults, which were either normal or strike-slip.

Diagenetic evolution of the volcaniclastic deposits: an example from Neoproterozoic Dokhan Volcanics in Wadi Queih basin, central Eastern Desert, Egypt

Wadi Queih basin hosts a ∼2,500-m thick Neoproterozoic volcanoclastic successions that unconformably lie over the oldest Precambrian basement. These successions were deposited in alluvial fan, fluviatile, lacustrine, and aeolian depositional environments. Diagenetic minerals from these volcaniclastic successions were studied by X-ray diffractometry, scanning electron microscopy, and analytical electron microscopy. The diagenetic processes recognized include mechanical compaction, cementation, and dissolution. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore-lining and pore-filling clay cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains. In addition to calcite, several different clay minerals including kaolinite, illite, and chlorite with minor smectite occur as pore-filling and pore-lining cements. Chlorite coating grains helps to retain primary porosity by retarding the envelopment of quartz overgrowths. Clay minerals and their diagenetic assemblages has been distinguished between primary volcaniclastics directly produced by pyroclastic eruptions and epiclastic volcaniclastics derived from erosion of the pre-existing volcanic rocks. Phyllosilicates of the epiclastic rocks display wider compositional variations owing to wide variations in the mineralogical and chemical compositions of the parent material. Most of the phyllosilicates (kaolinite, illite, chlorite, mica, and smectite) are inherited minerals derived from the erosion of the volcanic basement complex, which had undergone hydrothermal alteration. Smectites of the epiclastic rocks are beidellite–montmorillonite derived from the altered volcanic materials of the sedimentary environment. Conversely, phyllosilicate minerals of the pyroclastic rocks are dominated by kaolinite, illite, and mica, which were formed by pedogenetic processes through the hydrothermal influence.