Geochronology and petrogenesis of the Katerina ring complex, Southern Sinai, Egypt

Resumen

This PhD thesis deals with the petrogenesis of the Katerina Ring Complex. It is part of the northernmost sector of the Arabian-Nubian Shield (ANS) that consists of Neoproterozoic juvenile crust formed by protracted accretion of island-arc terrains, although inherited pre-Neoproterozoic zircons have been reported. The post-collisional magmatism from the ANS comprises two suites with an important time overlap: a calc-alkaline suite formed at 635-590 Ma followed by a second one with A-type affinity at 608-580 Ma. The Katerina Ring Complex consists of a large diversity of roughly coeval felsic igneous rocks with A-type affinity that crop out over an area of about 400 Km2. Country rocks to the Katerina Ring Complex comprise the El-Sheikh granodioritic pluton (610 ± 5 Ma), the Moneiga quartz diorites (844 ± 4 Ma), which are the oldest rocks in the area, and volcano-sedimentary material from the Rutig Formation. This latter consists of volcanic rocks (622 ± 3 Ma) and conglomerates containing granite boulders with ages of 623 ± 3 Ma, 735 ± 6 Ma and 748 ± 11 Ma. The complex consists of two ring dikes composed of quartz syenites locally crosscut by dikes of microsyenites. The ring dikes are intruded by: 1) a quartz monzonitic pluton in the eastern zone of the complex (Wadi Isbaiya); 2) a pluton of fluorite-bearing aluminous syenogranites and alkali feldspar granites with aplite apophyses (Katerina pluton); 3) a small pluton of peralkaline granites at the top of Gebel Musa (Mount Sinai). SHRIMP dating reveals that most rocks in the Katerina Ring Complex were formed in a narrow time slot of ~9 Ma, from 602 Ma to 593 Ma, called the Katerina magmatic cycle that began with the intrusion of quartz syenites and microsyenites from the ring dikes at ca. 602 Ma, being closely followed by intrusions of the Isbaiya quartz monzonites at 599 ± 3 Ma and the main Katerina pluton at 596 ± 3 Ma, and ended with the intrusion of the Gebel Musa peralkaline granite pluton at 593 ± 2 Ma. Aluminous alkali feldspar granites from the Katerina pluton contain inherited Archean zircons (~3.2 Ga), which may suggest mantle recycling of Archean materials. Quartz syenites are weakly metaluminous; whereas microsyenites are metaluminous, but tending to peralkaline. Peralkaline granites are also marginally peralkaline, in spite of the presence of arfvedsonite, with atomic Al/(Na+K) ratios close to one (range: 0.95-1.01). Both quartz monzonites and aluminous granites vary from metaluminous to peraluminous. All rock types show a ferroan character, excepting quartz monzonites that are weakly magnesian. Quartz syenites and microsyenites show alkalic compositions; whereas quartz monzonites, aluminous granites and peralkaline granites are alkali-calcic. Chondrite-normalized REE patterns have, in general, a marked enrichment in LREE compared to HREE in all rock types. Aluminous granites and peralkaline granites show a strong negative Eu anomaly (Eu/Eu* range: 0.30-0.38 in syenogranites; 0.03-0.26 in alkali feldspar granites; 0.01-0.18 in aplites; and 0.02-0.08 in peralkaline granites). In microsyenites and quartz monzonites, negative Eu anomalies are less pronounced with Eu/Eu* values ranging 0.26-0.83 in microsyenites and 0.73-0.85 in quartz monzonites; whereas in quartz syenites, Eu anomalies can be positive (Eu/Eu* = 1.22-1.52) or weak negative (Eu/Eu* = 0.73-0.96). Silicate Earth-normalized trace element patterns are enriched in incompatible elements with a negative Nb-Ta anomaly in all rock types, but in aluminous granites. Peralkaline granites and quartz syenites present mantle-like Nb/Ta ratios; whereas in quartz monzonites and aluminous granites, they are more moderate with continental-crust like values. Slight negative to weak positive or flat to weak positive Pb anomalies are present in quartz syenites, microsyenites and peralkaline granites; whereas in aluminous granites and quartz monzonites, there are positive Pb anomalies that are more marked in aluminous granites that experienced substantial LREE accessory fractionation. The aluminous magmas experienced a complex internal evolution involving fractionation of feldspar, mafic minerals, zircon, and LREE accessories allanite and monazite from a heterogeneous (or from more than one) magma batch with high-Fe syenogranitic composition to produce low-Fe alkali feldspar granites. In peralkaline granites, liquids experienced important compositional variations caused by uncoupled (1) feldspar fractionation, which caused depletion in Ba and Sr and decreasing in Eu/Eu* ratio, and (2) HFSE and ferromagmensian component enrichment, most probably related to a fluorine-rich mobile phase. All rock types from the Katerina Ring Complex show positive !Ndi values (range: 3.1 to 6.4) and relatively young Nd model ages (621-855 Ma, except one sample of aluminous granite that yields 985 Ma) that are in accordance with the juvenile character of the ANS. As a consequence of this latter, there is not enough isotopic contrast for discriminating between mantle-derived and continental crust-derived magmas. Therefore, the nature of magma sources for the A-type magmatism has not been firmly established. Trace-element relationships such as Y/Nb versus Th/Nb and Th/Ta provide new constraints for unraveling the origin of A-type granitoids, as roughly conserved during internal fractionation of aluminous and peralkaline magmas. By contrast, Ce/Pb may experience significant variation in aluminous granites due to fractionation of allanite and monazite; however, less evolved high-Fe syenogranites as well as quartz monzonites, which did not experience significant LREE accessory phase fractionation, have a continental crust- like Ce/Pb ratio, which may be signature for an important continental crust component. Alkalic metaluminous quartz syenites and alkali-calcic peralkaline granites were generated by high-pressure fractionation and low-pressure fractionation, respectively, of magmas derived from mantle sources with minor, if any, involvement of the continental crust. By contrast, alkali-calcic, metaluminous to peraluminous quartz monzonites and alkali-calcic, essentially peraluminous, syenogranites were formed from a source with an important continental crustal component.