Rb Sr geochronology of some gneisses and felsic intrusions from Afif-Halaban-Ad-Dawadiml-Ar-Rayn Areas, Saudi Arabia

Abstract EN

The ea rliest recognized older ba sement ro ck unit in the ea stern region of the Arabi an Shield is the Ajal group.

It comp rise s interbedded amphibolite.

b iotite-schist and para-gnei ss .

A high grade amphibolite so uth-so uthwe st o f Halaban villa ge gave a Rb /Sr age of 845 :!: 19 Ma.

It is th e oldest Rb /Sr age reported from the area , SO fa r.

It is int erpreted as the age of metamorphism during the Early Najd Cycle .

[t pr eceded th e Hulayfah group deposit ion .

The west Al-Quwuy'iy ah (Ad-Dawadiml Province) and Jabal AI Humayy (Afif Province).

a re foli at ed large se micircula r granodiorite-granitic complexes.

They form ucgativc geomorphologic feature s surrounded by high circular ridges of slightl y metamorphosed Hulayfah and Murdama group rocks.

The Rb /Sr age s o btained for Al-Quwayiyah and Jabal AIHumayy granodiorite-granitic complexes are 725 :!: 10 Ma and 675 :!: 30 Ma , re spectively.

These ag es a rc interpreted as post -Hulayfuh as well as post-Abt Formation tecton ism (Tuluhah or ogeny).

The cast J abal Bitran gne iss belt (Ar-Rayn Province) extends in N-S trend for a dist ance of 100km by 40 km, and probably eastwards below the Phanerozoic cover.

The gneiss is granodioritic and in places quartz-diorit ic and umphibolitic .

It wa s formerl y mapped as old fundamental gneiss, but the Rb/Sr age obt ained is 5R4 :!: 14Ma suggest ing that the gneisses sho uld be co nside red as post-Murdama .

Furthermore, a small pink granite stock cutt ing th e gnei ss gave a Rb /Sr age o f 492 :!: 12 Ma , indicating th at th e te cton o-thermal event a t 510 Ma recognized in the western parts of th e Arabian Shield had a lso affect ed th is a re a .-