Gabbroic-dioritic dykes from the Sanadaj-Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

The Sanandaj-Sirjan Zone (SaSiZ) is a magmatic terrane within the Zagros Orogen, western Iran, marking the Tethyan suture zone between the Afro-Arabian Plate and the Central Iran Micro-Continent. Mafic-intermediate dyke swarms with Middle Jurassic (Group-1: hornblende gabbro and diorite) and Late Eocene (Group-2: hornblende-pyroxene gabbro) ages are recognized in the Malayer-Boroujerd Plutonic Complex of the northern SaSiZ. Group-1 dykes have elemental and isotopic signatures consistent with melting of a mantle source modified during Neo-Tethyan subduction. Some Group-1 magmas evolved to intermediate compositions through assimilation and fractional crystallization. Group-2 dykes have within-plate trace element geochemical signatures, modelled as deriving from low-degree melting of asthenospheric mantle without a subduction influence. Published models postulate either a Cretaceous-Eocene Neo-Tethyan flat-slab scenario, or a Latest Cretaceous-Palaeogene Neo-Tethyan break-off event beneath the SaSiZ. Such models do not reconcile with the Late Eocene presence of within-plate magmatism in westernmost Iran, very close to the Zagros Suture. We argue that a period of flat-slab subduction concluded with sub-parallel subduction of a Neo-Tethyan ridge to the trench. The resulting slab break-off event in the Late Eocene is responsible for generation of the distinct Mesopotamia and Zagros slabs in mantle tomography models. Break-off was followed by small volume within-plate type magmatism before short-lived re-establishment of Tethyan subduction prior to the final Arabia-Eurasia collision

Mesozoic-Cenozoic mafic magmatism in Sanandaj-Sirjan Zone, Zagros Orogen (Western Iran): geochemical and isotopic inferences from Middle Jurassic and Late Eocene gabbros

One of the consequences of Neo-Tethys ocean subduction beneath the Central Iranian Micro-continent (CIMC) is the development of rare gabbroic intrusions in the Malayer-Boroujerd Plutonic Complex (MBPC) located in the Sanandaj-Sirjan Zone (SaSZ) of the Zagros Orogenic belt. The MBPC is a suite of extensive felsic and lesser mafic magmatic products in the northern SaSZ with geochemical signatures of arc-like magmatism during the Middle Jurassic (Ghorveh-Aligudarz arc) and intraplate type in the Late Eocene. Middle Jurassic gabbros (non-cumulate and cumulate) have low-Ti concentrations (< 1 wt. %) and quite uniform isotopic compositions (initial 87Sr/86Sr: 0.7035‒0.70593 and εNd(t): - 6.18‒-0.7), enriched LILE relative to HFSE, variable fractionation between the LREE and HREE ((La/Yb)cn: 2.27‒7.45) and both negative to positive Eu anomalies. These distinctive features of arc-type magmatism are consistent with a subduction-modified mantle source for these rocks. Trace element and REE models indicate ~ 15% melting of a metasomatized amphibole-bearing garnet-spinel lherzolite (garnet:spinel ~ 7:3) in the sub-arc mantle wedge. The cumulate gabbros and non-cumulates belong to common liquid line of descent, with complementary trace element patterns. Much of the variation between samples can be modeled by fractional crystallization (FC) of a common parent; only one cumulate gabbro from this suite exhibits isotopic evidence of contamination, probably by Rb-depleted crustal materials. The Late Eocene gabbros have relatively high Ti (>1 wt. %) and display isotopically depleted Sr-Nd values (initial 87Sr/86Sr: 0.7044-0.7087, εNd(t): 1.9-+3.2, barring one crustally contaminated sample). OIB-like trace element characteristics such as enriched HFSE, and only minor enrichment of LILE and LREE, reflect a within-plate character and asthenospheric source. Trace element modeling indicates small degree melting (fmelting: 0.05) of upper mantle lherzolite (garnet:spinel ~ 3:1) followed by higher degree melting (fmelting: 0.15) at shallower depths (garnet:spinel ~4.5:2). The Eocene parental magma underwent FC of olivine and clinopyroxene. We propose that Eocene asthenospheric upwelling was triggered by slab tearing in response to slab-rollback, which is elsewhere reported to have triggered a 'flareup' of extension-related magmatism across Iran. Three stages of tectonomagmatic evolution in the Ghorveh-Aligudarz arc segment of the N-SaSZ are represented by: 1) arc-like magmatism during active subduction of the Neo-Tethys seaway at Middle Jurassic, 2) magmatic quiescence during an interval of shallow-angle or highly oblique subduction during the Cretaceous‒Paleocene, and 3) asthenosphere melting during slab tearing shortly before the onset of the Arabia-Eurasia collision

Subduction-related mafic to felsic magmatism in the Malayer-Boroujerd plutonic complex, western Iran

The Malayer–Boroujerd plutonic complex (MBPC) in western Iran, consists of a portion of a magmatic arc built by the northeast verging subduction of the Neo-Tethys plate beneath the Central Iranian Microcontinent (CIMC). Middle Jurassic-aged felsic magmatic activity in MBPC is manifested by I-type and S-type granites. The mafic rocks include gabbroic intrusions and dykes and intermediate rocks are dioritic dykes and minor intrusions, as well as mafic microgranular enclaves (MMEs). MBPC Jurassic-aged rocks exhibit arc-like geochemical signatures, as they are LILE- and LREE-enriched and HFSE- and HREE-depleted and display negative Nb–Ta anomalies. The gabbro dykes and intrusions originated from metasomatically enriched garnet-spinel lherzolite [Degree of melting (fmel) ~ 15%] and exhibit negative Nd and positive to slightly negative εHf(T) (+ 3.0 to − 1.6). The data reveal that evolution of Middle Jurassic magmatism occurred in two stages: (1) deep mantle-crust interplay zone and (2) the shallow level upper crustal magma chamber. The geochemical and isotopic data, as well as trace element modeling, indicate the parent magma for the MBPC S-type granites are products of upper crustal greywacke (fmel: 0.2), while I-type granites formed by partial melting of amphibolitic lower crust (fmel: 0.25) and mixing with upper crustal greywacke melt in a shallow level magma chamber [Degree of mixing (fmix): 0.3]. Mixing between andesitic melt leaving behind a refractory dense cumulates during partial crystallization of mantle-derived magma and lower crustal partial melt most likely produced MMEs (fmix: 0.2). However, enriched and moderately variable εNd(T) (− 3.21 to − 4.33) and high (87Sr/86Sr)i (0.7085–0.7092) in dioritic intrusions indicate that these magmas are likely experienced assimilation of upper crustal materials. The interpretations of magmatic activity in the MBPC is consistent with the role considered for mantle-derived magma as heat and mass supplier for initiation and evolution of magmatism in continental arc setting, elsewhere

Biotite compositional variations – a physicochemical approach to investigate crustal involvement and ore potential in Middle Jurassic plutonic rocks from the Malayer-Boroujerd Plutonic complex, W Iran

Biotite is the most frequent ferromagnesian phase in granitoids from the Malayer-Boroujerd Plutonic complex (MBPC), Zagros Orogen. The abundance of biotite decreases from granitoids towards dioritic intrusions, dioritic and gabbroic dykes to gabbroic intrusions coincident with increase in hornblende/biotite ratio. Primary magmatic biotite composition in the MBPC granitoids (Fe-rich type) changes from annite to siderophyllite, while Fe-depleted secondary biotites from the altered gabbros and gabbroic-dioritic dykes approximate to the phlogopite-eastonite series, and those from least-altered gabbros display average composition of these groups, however, extend towards high-Al end-members. Using Ti-in-biotite geothermometer, the crystallization temperature varies between 550 °C to 750 °C in MBPC granitoids (P < 5 kb), consistent with the results for the well-calibrated garnet-biotite thermometer applied in S-type granites (~721 °C). The temperature decreases from I-type toward S-type granitoids, as the Al2O3/TiO2ratios increase and Mg # decrease, indicating Ti-deficiency in magmas originating at lower temperatures. Coupled with existing data, new geochemical and P-T determinationsimply three modes of oxygen fugacity f (O2) and crustal involvement during the Middle Jurassic magmatic activities. Mode-I: primary biotite plotting below the NNO buffer, with high alumina and iron content (low-Mg # ; high Fe # and AlTvalues), at low f (O2) (10–18 to 10–16 bars) found in S-type granites. Mode-II: biotites crystallized at higher f (O 2) in I-type granitoids (f (O2) > 10–15 bars). Mode-III: biotite crystallized at higher oxidizing conditions (f(O2) > 10–10 bars) in gabbroic and dioritic dykes as well as gabbroic intrusions. The progressive decreases in f (O 2)–values and oxidizing conditions from I-type to S-type granites (above HM-to below QFM-buffer) reflect increasing incorporation of less-oxidized upper crustal materials during magmagenesis. In addition, the highest f (O2) in MBPC mafic intrusive rocks is consistent with a modest crustal contamination, if any. There is lack of evidence for porphyry type alteration zoning and subvolcanic porphyritic intrusions associated with porphyry copper system. The small patches of calc-silicate rocks from the NW-MBPC formed in contact with less oxidized and highly fractionated, crustal-derived granular granitoids and are less predisposed to develop Cu-Au mineraliza- tion. However, given the relatively higher f (O2)-values, the Middle Jurassic mafic intrusive rocks originated from the meta- somatised mantle-wedge beneath SaSiZ, appears to be the best candidate for further ore exploration programs in the area

Geochemical and isotopic constraints on the evolution of magma plumbing system at Damavand Volcano, N Iran

Damavand is a Quaternary stratovolcano in N Iran, associated with small volumes of under-saturated to weakly-saturated mafic lavas (Tephrite-Basanite-Alkali olivine basalt), and large volumes of saturated lavas (trachyandesite-trachyte (TT)) together with minor pyroclastic rocks. TT rocks are characterized by LILE and HFSE enrichments, whereas the mafic rocks have higher REE and other incompatible elements (except Rb, Th) than TT suite rocks, contradicting the assumed derivation of TT magmas from mafic magmas by simple fractional crystallization processes. The similar whole-rock isotope ratios of mafic rocks, TT suites and their enclaves do suggest a common origin. Magmatic enclaves in TT suites show decoupled major and some trace elements from their host while they have similar geochemistry to the mafic lavas. Trace element modeling including assimilation-fractional crystallization (AFC) and energy-constrained recharge-assimilation fractional crystallization (EC-RAFC) suggest that the Damavand TT rocks were formed by multi-stage fractionation of mafic magma with minor effects of open system processes. Differentiation started at high pressures (6–8 kbar, initial temperature of magma (Tm) = 1180 °C) involving anhydrous minerals (mainly clinopyroxene-orthopyroxene), accompanied by recharge and wall-rock assimilation at equilibration temperature (Teq) of 1050 °C as the magma reached the threshold for extraction from crystal mush (Mm = 0.49) (stage-1). These processes resulted in the enrichment of the residual liquid in incompatible elements. The fractionation of oligoclase-andesine, anorthoclase, phlogopite and apatite from intermediate enriched melts occurred at shallow crustal levels (0.5–3 kbar, Tm = 1050 and Teq = 830 °C): here, FC processes and further recharge and assimilation of crustal materials reduced LILE and REE concentration in the residual liquid (stage-2). The role of enriched melts is supported by unusual trace element composition of key minerals, for instance, feldspars have variable concentrations of Ba (132–5495 ppm), Sr (1471-14413 ppm) and unusual Rb, La and Ce contents. Based upon melt extraction modeling, physical removal of residual melts at stage-2 could produce cumulate residues with trace element compositions comparable to enclaves in TT suites (stage-3). Disruption of crystal mush due to recharge, followed by mixing of enclave material with evolved trachytic melts caused buffering of magma at trachyandesitic composition (stage-4). The results affirm the advantages of using a range of compositional and textural records in modeling of petrogenetic processes in all magmatic plumbing systems

Supplementary Item 2. Gabbroic-dioritic dykes from the Sanandaj–Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

Supplementary Item 2: Material and methods

Supplementary Item 3. Gabbroic-dioritic dykes from the Sanandaj–Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

Supplementary Item 3: Tables of geochemical modelling parameters

Redox evolution of differentiating hydrous basaltic magmas recorded by zircon and apatites in mafic cumulates: The case of the Malayer Plutonic Complex, Western Iran

Mafic-ultramafic cumulates can provide records of basaltic magma chambers' conditions and processes, which are often difficult to determine in areas dominated by crustal-derived felsic intrusions, such as the Malayer Plutonic Complex (MPC), Western Iran. New U-Pb zircon ages for mafic cumulates in the MPC confirm the presence of isolated magma chambers of contrasting compositions during Middle Jurassic. Mafic cumulates found in seven separate zones across the MPC vary from olivine gabbro to anorthosite. While the mineralogical, textural, and geochemical lines of evidence recorded in mafic cumulates indicate pH2O controls on the liquidus phases, the estimated oxygen fugacity (logfO2) using zircon and apatite chemistry suggests a smoothly rising redox state during the fractionation process, consistent with the trend expected for late-stages differentiation of hydrous arc magmas. This trend is further confirmed by sulfur speciation in apatites determined from microbeam sulfur K-edge X-ray absorption near edge structure (μ-XANES) spectra (S6+/ ∑S = 0.93–0.98 ~ FMQ + 2 to 0.99 ~ FMQ + 3, where ∑S = S6++S4++S2- ). The low S content and increasing redox state of the fractionating basaltic melts most likely resulted from preferential removal of sulfur en-route to the magma chambers along with effective assimilation of oxidizing crustal components. The reduced condition in the early basaltic melt is also evidenced by the presence of pyrite and magnetite inclusions in olivines in mafic cumulates. The shift in the prevailing fO2 from sulfide-saturated to sulfate-bearing recorded by MPC mafic cumulates, similar to that in other magmatic arcs, is accompanied by changes in the differentiation path from transitional tholeiitic to calcalkaline

Zircon U–Pb ages and Sr–Nd–Pb–Hf isotopic compositions constrain the tectono-magmatic evolution of the Anomaly 21-A iron ore region, Bafq metallogenic province, Central Iran

The late Proterozoic - early Paleozoic tectono-magmatic evolution of Central Iran is considered to be a result of subduction of the Proto-Tethys Ocean and the amalgamation of Gondwana continental fragments. Here, we present whole-rock geochemistry, Sr-Nd-Pb isotopes and zircon U-Pb-Hf data for monzonite to quartz monzonite from deep drill core in the area of the Iron Anomaly 21-A, Bafq metallogenic province of Central Iran, to decipher the petrogenetic evolution of the mantle during late Proterozoic–early Paleozoic times. The plutonic rocks display enrichment in large-ion lithophile elements (Rb, Ba, K, and Cs), and depletion in high-field-strength elements (Nb, Ta, Ti), typical of continental arcs. Zircon U-Pb ages of the studied rocks are in the range of 474–512 Ma, which is consistent with the general consensus on the age of the Cadomian basement of Central Iran, and the culmination of subduction along northern Gondwana in the early Paleozoic. The isotopic signatures of the samples, e.g., (87Sr/86Sr)i = 0.706 to 0.718, εNd(t) = -3.3 to +1.8, (206Pb/204Pb)i = 18.87 to 20.32, (207Pb/204Pb)i = 15.72 to 15.84, (208Pb/204Pb)i = 40.74 to 42.32, and εHf(t) = -4.7 to +11.6, cover a compositional range of mantle-derived melts with variable degree of contamination by Neoproterozoic continental crust. A setting of back-arc continental rifting is envisaged for the late Neoproterozoic to early Paleozoic magmatism in the Bafq province