フィッション・トラック

ニュースレター

第22号

31-32

2009年

Apatite Fission-Track and Zircon U-Pb LA-ICP-MS Dating of
Abu Zenima area West-Central Sinai, Egypt
Sherif Mansour*, Noriko Hasebe** and Shoji Arai** Graduate School of Natural Science and Technology, Kanazawa University, Japan** Institute of Nature and Environmental Technology, Kanazawa Univ.

Abu Zenima area occupy the northwestern corner of the basement complex ofSinai which mainly formed during theNeoproterozoic and early Phanerozoic
during the Pan-African orogeny, locateddirectly on the eastern border of the OligoMiocene Gulf of Suez. The studied area
(Fig. 1) consists from (1) metasediments andmetavolcanics formed in the earliest stage of
the Pan-African orogeny, (2) older grey
granites (mainly diorite and trondhjemite)formed during compressional environment
during the next stage of the Pan-African
orogeny, (3) limited extent of younger pinkgranites intruding in older granites formed at
the last stage of the Pan-African orogeny
which may be interpreted as an extensional
collapse stage (e.g. Dewey, 1988; Platt andEngland, 1993), (4) and successive dike setsof bimodal chemistry (rhyolitic andbasaltic).
The U-Pb system is resistant to thermal
overprinting and is thus unaffected by later
thermal events when suitable minerals suchas zircon are analyzed (>900 °C). Therefore,
U–Pb analysis of zircon, is generally thebest way to date the crystallization age ofold igneous rocks. While, fission track (FT)
dating observe a damage zone formed by the
spontaneous fission of heavy elements suchas uranium, and useful to reconstruct the latethermal event of the region.
The laser ablation-inductively coupledplasma-mass spectrometry (LA-ICP-MS) isa rapidly developing method that is taking
its place among other in situ trace element
and isotope measurement methods such as
SIMS (Secondary Ion Mass Spectrometry)or SHRIMP (Super High Resolution IonMicro Probe). The advantages are (1) in situ
dating of the accessory phases, (2) spatial
resolution of several spot analyses within asingle grain of an accessory mineral that isoften less than 200 - 300 microns in size, (3)small or zero Pb blank introduced to thesample during the analysis, and (4) quick

and simple sample preparation procedures.
These advantages in the technology of LAICP-MS make it a rapid, precise andaccurate method for FT and U–Pb dates
obtained simultaneously. The U and Th
concentrations as well as concentrations ofPb isotopes were measured by the LA-ICPMS unit at Kanazawa University.
Eighteen samples were collected torepresent the main rock units exist in thestudied area; eight were treated with U-Pb
technique in order to investigate theircrystallization ages and ten with FTtechnique to calculate their cooling ages.
Seven igneous and metamorphic samples
dated by U-Pb technique range from diorite(581±15 Ma, 565±28 Ma), red granite(592±11 Ma, 580±10 Ma), gneissose granite(595±12 Ma) to rhyolitic and granitic dykes(566±9 Ma, 563±9 Ma). These data indicate
the coeval occurrence of both alkaline andcalc-alkaline granitoids in the time span
extended from 595 Ma to 565 Ma followed
directly by dike injection stage. One detrital
sample collected from the sandy
Carboniferous Ataqa Formation represented
four provenance sources; Pre-Pan-African
suite (1831±31 Ma, 1009±46 Ma), older
granitoids suite (8 grains ranges from736±33 Ma to 616±28 Ma), younger
granitoids suite (7 grains ranges from607±28 Ma to 566±26 Ma), post
cratonization suite (501±23 Ma, 477±42 Ma,460±21 Ma, 312±14 Ma). Indicate the prePan-African probability, these data indicate
the presence of older granite suite in Sinai
which may be metamorphosed, eroded orremelted, and probable magmatic event in
the Devonian-Carboniferous time
accompanied by the Hercynian tectonic
event. FT apatite data were obtained fromten igneous and metamorphic samples range
in composition between red granite, diorite,
rhyolitic dikes, granitoid dikes, schist andgneiss. These samples show two age groups,Carboniferous and Cretaceous, separated

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spatially (Fig. 1). Considering thesimultaneous crystallization age of the mainrock units in the studied area and the small
distance between distribution of the two age
groups, different thermo- tectonic history

between the two age zones indicates thepreferential uplift in the studied area which
had occurred during the Carboniferous
Hercynian tectonic event.

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