Geochronology of the Elat Terrain, metamorphic

Isr.

J. Earth Sci.; 40: 5-16

Geochronology of the Elat Terrain, metamorphic basement, and its implication for crustal evolution of the NE part of the Arabian-Nubian Shield Yehuda Eyal,a Moshe Eyal,a and Alfred Kronerb "Department of Geology and Mineralogy, Ben-Gurion University of the Negev, P.D.B. 653, Beer Sheva 84105, Israel bIllstitut fiir Geowissenschaften, Universitat Mainz, Postfach 3980, 6500 Mainz, Germany (Received 19 April 1991)

ABSTRACT Eyal, Y., Eyal, M., Kroner, A.1991. Geochronology of the Elat Terrain, metamorphic basement, and its implication for crustal evolution of the NE part of the Arabian-Nubian Shield. Isr. J. Earth Sci. 40: 5-16. The isotopic age of certain metamorphic rock units in the Elat Terrain was determined by using the 207Pbpo6Pb single zircon evaporation method. No evidence was found for prePan-African (>950 Ma) continental crust. The results enable us to distinguish the following evolutionary stages: An island arc stage, ca. 800-820 Ma, is represented by detrital zircons in the Elat Schist. The source material of the Elat Schist was derived as detritus from an emerging island arc and its first deformation and metamorphism occurred prior to the intrusion of the Taba Gneiss (ca. 780 Ma). A plutonic cycle, ca. 78G-740 Ma, began with quartz dioritic and tonalitic intrusions (Taba Gneiss and Fjord Gneiss respecti vely) and terminated with a granitic phase (Elat Granitic Gneiss). The age of deformation of these plutonic rocks is yet unclear. A second plutonic stage, ca. 64G-660 Ma. includes the Shahmon Metabasites and granitic dikes. The rocks of the last stage were deformed and metamorphosed, together with the older rock units, shortly after their intrusion. The last deformation stage is attributed to the accretion of the island arc, comprising the Arabian Shield at that time, to the African craton.

INTRODUCTION

The Precambrian rocks of the Elat Terrain (Fig. 1) represent the NE extension of the Precambrian basement of the Sinai Peninsula. This terrain, together with the basement outcrops of southern Jordan, comprises the northernmost part of the Arabian-Nubian Shield (ANS).

The evolution of the ANS took place in four main stages (Bentor, 1985; Gass, 1977). These stages have been interpreted in plate tectonic tenns by many authors (e.g" Brown, 1980; Engel et al., 1980; Fleck et al., 1980a; Gass, 1982; Roobol et al" 1983; Kroner et aI., 1987). During the first stage (-950-850 Ma), oceanic crust composed mainly oftholeitic pillow basalts, gab© 1992 The Welzmann Science Press of Israel

bros, and some trondhjemites was fonned. The second stage (-850-650 Ma) was characterized by island arc volcanism producing mainly andesitic to dacitic rocks, their sedimentary derivatives, and plutons with similar composition. The rocks of the first two stages underwent defonnation and metamorphism to various degrees and, at the end of the second stage, were accreted to fonn the ANS. The third stage (-650-580 Ma) represents cratonization of the shield and is characterized by widespread calc-alkaline magmatism, mainly of intennediate to acid composition. The last stage (-600-550 Ma) comprises mainly alkaline to peralkaline granites and rhyolites, representing an intracratonic-taphrogenic magmatism. Within the Elat Terrain, only rocks of the last three stages are represented. Our

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Israel Journal of Earth Sciences

study is related to the second island arc stage. The aims of this study were: I. To date the oldest rock units of the Elat Terrain. 2. To detennine whether the Elat Terrain developed in one or two orogenic cycles. 3. To detennine the tectonic environment in which the protoliths of the schists formed. Previous studies have suggested two alternative environments: (a) a sedimentary prism along a continental passive margin (e.g., Shirnron, 1980; Eyal et aI., 1985) and (b) an island arc (Bielski, 1982 and Kroner et aI., 1990). 4. To place the late Precambrian rock units of the Elat Terrain within the general framework of the ANS.

1. The Elat Schist (Bentor and Vroman, 1955) is composed of subequal amounts of quartz, oligoclase, and biotite. Porphyroblasts of garnet, staurolite, cordierite, and andalusite are present and testify to the approOuaternary deposits ~ Paleozoic-Mesozoic: sedimentary rOCk;.

~ Etal conglomerate

\,_

~ ACid volcaniC rocks ~ Elo.l granIte

G:::!J

Elal granitic gneiss

E:::J

Roded quartz diorite

[[[[I

lltclo""

arm

schist

lobo gneiss

E=:1

FIord gneIss

_

Metobaslte

III]

Elat

Schist

--...... Fault

GEOLOGICAL SETTING

.... International -

The metamorphic rocks of the Elat Terrain consist of schi3ts, migmatites, metabasites, and various gneisses (Bemor, 1961; Garfunkel, 1970, 1980; Page, 1972; Shimron, 1972; Eyal, 1976, 1980; Fig. I). The following description of the rock units from the Elat Terrain, from the oldest to the youngest (Kroner, et aI., 1990), is restricted to those units from which samples were dated (Table 1).

bOlder

o

Skm

~

Fig. I. Geological map ofElat Terrain (modified after Kroner et al., 1990).

Table I a. Rock units, defonnation phases, magmatic events, and their relative age based on field evidence and structural analysis (after Shimron. 1972 and EyaJ. 1980) Rock unit

Elat Schist

First magmatic event

First deformation pbase, DI

Second magmatic event

FI folds, SI Slaty cleavage

Second defonnation phase. D2

Third magmatic event

F2 folds, S2 Spaced cleavage

Fourth deformation phase, D4

F, folds

F" kink folds

Weak cleavage

Shahmon Met. Intmson Weak cleavage into Elat Scillst Fjord Gneiss

Third deformation phase, D3

Weak cleavage parallel to Elat Schist Intmion into Hat Schist

F, folds

S, folds, and planar gneissosity

ditto

Weak foliation

Taba Gneiss

Intmsion into Elat Schist

Elat Gr. Gn.

Intmsioll into ditto Elat Schist & Taha Gneiss

Dikes Gr. Gn.

Intmsion into Mainly mineral Shahmon Met.lineatioll

Mainly mineral F4 kink folds and lineation shear zones parallel to F, fold axes . ditto

Y. Eyal. M. Eyal. and A. Kroner. Geochronology of the Elat Terrain metamorphic basement

7

Table 1b. Rock units, deformation phases, magmatic events, and their age based on single-zircon evaporation dating (after Kreiner