The al Ays batholiths : a late proterozoic granitoid batholith associated with the al Ays Volcanic Arc Complex, Hijaz Terrane, Arabian Shield

Abstract EN

-The AI Ays batholith is located in the Hijaz microplate of the Arabian Shield.

It intrudes the volcanic fac ies (andesitic and basaltic lavas and pyroclastic material) of the mature arc-derived AI Ays volcanosedimentary sequence.

The batholith is composite, 3-30 km wide by about 50 km long, and ranges from diorite to granite .

A 5-point Rb/Sr isochron gave an age of706 ± 2 Ma and an initial K7Srr"Sr ratio ofO .70319 ± 0.00002.

The granitoid rocks of the batholith are calc-alkaline.

metaluminous to subaluminous, with moderate LREE enrichment and a negative Eu anomaly , and show a wide compositional range (Si0254.40-72.30% , CaO 1.166.29% , Kp 1.8-3 .94%.

Rb 7-85 ppm, Sr 89-397 ppm, Y 7-40 ppm, Zr 40337 ppm, Rb/Sr ratios 0.06-0.95) .

The young age.

calc-alkaline association, compositionally expanded nature with a spectrum of rock types from dioritic to granitic, short crustal residence time, and the syn- to late-collisional tectonic 'enviro nme nt all suggest a model that the Al Ays volcano-plutonic complex (743 ± 12·to 725 ± 12 Ma) formed as a result of subduction and melting of oceanic lithosphere beneath the pre-existing arc-