Zircon Geochemical and Geochronological Constraints on Contaminated and Enriched Mantle Sources beneath the Arabian Shield, Saudi Arabia

Abstract

A B S T R A C TArabian Shield granitic zircon geochemistry provides insight into the petrogenetic processes involved in generating one of the planet's largest tracts of juvenile Neoproterozoic crust. New zircon geochemistry supports previous U-Pb and whole-rock data that defined four magmatic groups: (1) ∼870-675 Ma island arc and synorogenic I-type granitoids (IA1Syn), (2) ∼640-585 Ma I-and A-type granitoids from the Nabitah and Halaban Suture (NHSG), (3) ∼610-600 Ma postorogenic perthitic (hypersolvus) A-type granitoids (POPG), and (4) !600 Ma anorogenic aegirine-bearing perthitic (hypersolvus) A-type granitoids (AAPG). The low Nb (∼1-300 ppm) and intrasuite rare earth element variation in IA1Syn and NHSG zircons indicates that these suites are derivatives of contaminated mantle followed by fractionation. AAPG suites, however, have higher Nb content (∼10-400 ppm) and are derived from limited crust-enriched mantle interaction. Each of the IA, Syn, and NHSG suites have discrete granite subsuites distinguished using zircon morphology and geochemistry whose U-Pb ages in each case form three groups. The IA subgroups are ∼867, ∼847, and ∼829 Ma; the Syn subgroups are ∼730, 716, and 696 Ma; and the NHSG subgroups are ∼636, ∼610, and ∼594 Ma. This apparent subevent repetition suggests some form of magmatic pulsing in the Arabian Shield. It is suggested that IA1Syn suites reflect typical volcanic arc granite settings and incremental subduction/accretion of eastward-migrating oceanic fragments of the East African Orogen. The appearance of ∼636 Ma A-type magmatism within suture zones (NHSG) is possibly derived from a long-lived (∼50 m.yr.) melting, assimilation, storage, and homogenization (MASH) zone resulting from an ∼640 Ma slab tear. These A-types are distinguished from more-enriched anorogenic (!600 Ma) A-types, possibly associated with lithospheric delamination.

A discussion on the tectonic implications of Ediacaran late- to post-orogenic A-type granite in the northeastern Arabian Shield, Saudi Arabia

“…Whole‐rock major and trace element geochemistry of the northeastern Idah, Malik, and Abanat granitic suites are taken from Robinson et al [], and using discrimination diagrams and REE chemistry in this study is compared to similar age granitic data from the southern Asir terrane (Kawr Suite) and the northern Arabian Shield (Sinai Peninsula). According to Robinson et al [, ], the Kawr Suite illustrates a geochemical transition spanning 636–594 Ma associated with a slab tear following the Nabitah suture closure at ~640 Ma. Similarly, Eyal et al [] documented four stages of magmatism following subduction cessation in Sinai: (1) calc‐alkaline stage 1 = 636–629 Ma, (2) calc‐alkaline stage 2 = 619–592 Ma, (3) alkaline stage 1 = 609–602 Ma, and (4) alkaline stage 2 = 608–578 Ma.…”

“…Geochemical data from Saudi Arabia are taken from Robinson et al [], while those from Sinai are taken from Eyal et al []. The Kawr Suite age range is taken from Robinson et al []. Abbreviations are as follows: H = Ha'il terrane, Ad = Ad Dawadimi terrane, Af = Afif terrane, As = Asir terrane, CA‐1 = calc‐alkaline stage 1, CA‐2 = calc‐alkaline stage 2, A‐1 = alkaline stage 1, A‐2 = alkaline stage 2, (+Grt) = Malik Granite with garnet, (−Grt) = Malik Granite without garnet, CA = calc‐alkaline, A = alkaline, and PA = peralkaline.…”

“…If slab retreat is considered in isolation (asthenosphere already replaced lithosphere [ Collins and Richards , ]), the calc‐alkaline to alkaline and garnet‐bearing geochemistry becomes difficult to explain. The process of slab tear/rollback‐induced lower crustal melting is also interpreted in the southern Asir terrane (636–594 Ma Kawr Suite [ Robinson et al ., ]), which shows a transition from calc‐alkaline stages 1 (636–629 Ma) through to alkaline stage 2 (608–578 Ma), and a gabbro with a similar isotopic signature to the garnet‐bearing Malik Granite.…”

“…A similar ANS model proposed by Stein and Goldstein [] discusses the impact of an enriched mantle plume (with or without crustal overprinting). Although highly debated, alternative models for the apparent source transition from late‐orgenic to post‐orogenic magmatism is attributed to slab tear/rollback [ Gvirtzman and Nur , ; Flowerdew et al ., ; Robinson et al ., ] and/or to lithospheric delamination [ Elkins‐Tanton , ; Avigad and Gvirtzman , ].…”

Variation in rhyolitic magma composition linked with fractionation from a common source: insights from the Rotoiti eruption, Taupo Volcanic Zone, New Zealand

“…One possible scenario may be magmatic pulsing at depth related to incremental subduction. A recent study from Robinson et al (2015) discusses the origin of granites in volcanic arc settings and the possibility of magmatic pulsing from a lower crustal MASH (melting, assimilation, storage, homogenization) zone that may be related to incremental subduction. A comparable model of magmatic pulsing over a period of <3 Myr has been proposed by Gagnevin et al (2011) in regard to the Tuscan Magmatic Province (Italy).…”