RbSr geochronology and geochemistry of plutonic rocks from the Wadi Shuqub quadrangle, west-central Arabian Shield

Abstract

New geochemical and RbSr isotopic age data are presented for three suites of plutonic rocks from the central and western parts of the Wadi Shuqub quadrangle. The syn-tectonic diorite-tonalite are the products of early arc magmatism and the post-tectonic biotite-granite is the result of intra-arc plutonism. These plutonic suites have an intrunsive relationship with the layered metavolcanic and metasedimentary rocks of the Baish and Bahah groups, which have contributed to the development of an oceanic crust. The syn-tectonic tonalite, which forms a large batholith of tonalite-quartz diorite in the western part of the Wadi Shuqub quadrangle, yields an age of 854 ± 10 Ma (2 σ) with an initial 87Sr/ 86Sr ratio of 0.70159 ± 0.00004 and an MSWD value of 1.97. The late-tectonic diorite-tonalite stock yields an age of 815 ± 13 Ma (2 σ) with an initial 87Sr/ 86Sr ratio of 0.7029 ± 0.00005 (2 σ) and an MSWD value of 1.81. The post-tectonic suite of biotite-granite from the Gabalat area defines a poorly fitted Rb-Sr isochron age of 552 ± 20 Ma with an initial 87Sr/ 86Sr ratio of 0.71327 ± 0.003 (2 σ) and an MSWD value of 4.47. The syn- to late-tectonic tonalite and diorite-tonalite are of low-K calc-alkaline affinity and are characterized by low abundances of K, Rb/Sr, and high Sr, CaO, MgO, TiO 2, and FeO t (total Fe as FeO is denoted as FeO t. These two suites with low initial 87Sr/ 86Sr ratios are likely to be derived from a mantle differentiation or partial melting of the basic rocks above the subduction zone. The post-tectonic biotite-granite is enriched in SiO 2, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeO t and is of dominantly high-K calc-alkaline affinity. This suite of biotite-granite with high initial 87Sr/ 86Sr ratio appears to have been derived from lower crustal material. Limited trace element data on diorite-tonalite and biotite-granite show that the biotite-granite is enriched in Rb, Pb, Zn, Sn, U, Th, Zr, and Y and depleted in Sr, Ba, Cu and Ni compared with diorite-tonalite.