Geochemistry and apatite U–Pb geochronology of alkaline gabbros from the Nodoushan plutonic complex, Sanandaj–Sirjan Zone, Central Iran: Evidence for Early Palaeozoic rifting of northern Gondwana

Abstract

The Zagros Orogen formed during the Cenozoic collision of Arabia with Eurasia and resulted in the closure of the Neo‐Tethys Ocean. Collision was preceded by a complicated tectonic history involving Pan‐African orogenesis, Late Palaeozoic rifting and the formation of Neo‐Tethys, and subsequent Mesozoic convergence on the northern margin of the ocean contemporaneous with ophiolite obduction on its southern margin. The Sanandaj–Sirjan Zone (SaSZ) is a metamorphic belt within the Zagros Orogen of Gondwanan provenance. U–Pb zircon geochronology has revealed the presence of Pan‐African igneous and metamorphic basement complexes, in addition to minor late Palaeozoic plutons and abundant Jurassic plutonic rocks. This study presents a LA‐ICP‐MS U–Pb apatite age of 440.7 ± 3.7 Ma from a gabbroic body on the central north‐eastern margin of the SaSZ (Nodoushan region). Based on petrography and geochemistry, en route fractional crystallization of ascending magma was an important process in the evolution of the intrusive rocks. Geochemical characteristics imply that the south‐west Nodoushan alkaline gabbroic magma was originated from the asthenospheric mantle source, whereas the high ratios of (La/Yb)N and (Dy/Yb)N are related to the low degree of partial melting from the garnet‐bearing mantle source. Enrichment pattern of Nb, Ta, and Ti and depletion of Rb, Th, and Y are similar to the OIB pattern and intraplate alkaline magmatic rocks. High εNd(t) values (+1.29 to +1.50), low initial Sr isotopic ratios (0.703679–0.704098), and trace element ratios (e.g., Nb/La, Ce/Pb, Ba/Nb, and Th/Nb) indicate that crustal contamination was insignificant in its petrogenesis. The petrogenesis of the Nodoushan alkaline gabbroic rocks could be related to the presence of extensional phase, upwelling and decompressional melting of asthenospheric mantle in the rift basin which was associated with the opening of the Paleo‐Tethys Ocean in Late Ordovician to Silurian times.

Coeval formation and exhumation of metamorphic sole and ophiolite in the Saga ophiolitic mélange: Insights into subduction initiation of the Neo‐Tethys

“…Tera–Wasserburg U–Pb lower intercept ages and weighted average 207 Pb‐corrected data (a,d), chondrite‐normalized REE patterns (b,e) and ΣLREE versus Sr/Y diagrams (c,f) for the apatite of Cpx amphibolite and hornblende gabbro from the Saga ophiolitic mélange. Data of trace elements of apatite grains in gabbro are from Shahsavari Alavijeh et al (2019) and granulite are from Duan et al (2022). ALK, alkali‐rich igneous; HM, high‐grade metamorphic; IM, mafic I‐type granitoid and mafic igneous; LM, low‐ and medium‐grade metamorphic and metasomatic; S, S‐type granitoid and high aluminium saturation index ‘felsic’ I‐types; UM, ultramafic.…”

Apatite Textures, Elemental and Isotopic Compositions Unmask the Homogenizing Process in Silicic Magma Chambers

“…Therefore, we cannot directly calculate the magma SiO 2 content either by using fixed $${\text{Kd}}_{\mathrm{I}}^{\text{Ap}/\mathrm{L}}$$ value in mafic or felsic silicate melts, or by using a linear fit through the logarithmic $${\text{Kd}}_{\mathrm{I}}^{\text{Ap}/\mathrm{L}}$$ values for intermediate melt without knowing the melt REE concentrations. Instead, we have compiled apatite REE compositions from mafic to felsic silicate melts with known SiO 2 contents (Alavijeh et al., 2019; Belousova et al., 2001; Chu et al., 2009; Jiang et al., 2018; Laurent et al., 2017; Mao et al., 2016; Van Hoose et al., 2013; Xing & Wang, 2017; Zhang et al., 2020). Our results show that the REE (e.g., La, Ce, Sm, Gd, and Dy) concentrations of C‐type, R2‐type, and R1‐type apatite domains generally match well with the apatite from melts with SiO 2 of 45–55 wt.%, 55–65 wt.%, and 65–75 wt.%, respectively (Figure 3f).…”

“…Apatite crystallized from primitive mafic magma thus contains lower REE and Si concentrations than those from more evolved intermediate-felsic magma (Chu et al, 2009;Laurent et al, 2017;Prowatke & Klemme, 2006). Laurent et al (2017) (Alavijeh et al, 2019;Belousova et al, 2001;Chu et al, 2009;Jiang et al, 2018;Laurent et al, 2017;Mao et al, 2016;Van Hoose et al, 2013;Xing & Wang, 2017;Zhang et al, 2020). Our results show that the REE (e.g., La, Ce, Sm, Gd, and Dy) concentrations of C-type, R2-type, and R1-type apatite domains generally match well with the apatite from melts with SiO 2 of 45-55 wt.%, 55-65 wt.%, and 65-75 wt.%, respectively (Figure 3f).…”

“…Seawater Nd isotope data are fromRickli et al (2009). (f) Plot of Ce versus REE (La + Ce + Sm + Gd + Dy) concentrations in apatites from the YAGs compared to apatites crystallized from mafic to felsic rocks with SiO 2 contents ranging from 45 wt.% to 75 wt.% (data fromAlavijeh et al, 2019;Belousova et al, 2001;Chu et al, 2009;Jiang et al, 2018;Laurent et al, 2017;Mao et al, 2016;Quan et al, 2023;Van Hoose et al, 2013;Xing & Wang, 2017;Zhang et al, 2020).…”

Geochronology and geochemistry of the Ediacaran orthogneisses from the north Shahrekord (Sadegh‐Abad), Sanandaj‐Sirjan Zone: Insights into magmatic evolution of the Iranian basement

“…The SaSZ is a part of NW‐SE trending Zagros orogen and extends parallel to the Zagros fold‐thrust belt in southwestern Iran (e.g., Babazadeh, Ghorbani, Cottle, & Bröcker, 2019). The SaSZ is over 1,500 km in length and 150–200 Km in width and separates the Central Iranian Terrane, consisting of the Urumieh‐Dokhtar magmatic belt and the Lut, Tabas, and the Yazd blocks, from the Zagros orogen and the Arabian Plate (Babaahmadi et al, 2021; Sarjoughian, Kananian, Haschke, & Ahmadian, 2016; Shahsavari Alavijeh et al, 2019). The rocks are metamorphosed, deformed, and exhumed during the subduction of the Neo‐Tethys Ocean under SaSZ during the Jurassic and during the collision between the Afro‐Arabian continent and the Central Iranian Terrane in Cretaceous–Paleogene times (e.g., Davoudian et al, 2016; Mohajjel, Fergusson, & Sahandi, 2003).…”