West Central Arabian Shield Research Articles

Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and $1250 Ma. The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the nothern end of the 900 km-long Nabitah mobile belt was dated by the zircon UPb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

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

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.

Geochronology and geochemistry of Wadi Shuqub granitic rocks, west-central Arabian shield

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 (2σ) 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 (2σ) with an initial 87Sr/86Sr ratio of 0.7029 ± 0.00005 (2σ) 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 (2σ) 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, TiO2, and FeOt (total Fe as FeO is denoted as FeOt. 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 SiO2, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeOt 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.