Petrology, petrogenesis, and geochronology review of the Cenozoic adakitic rocks of northeast Iran: Implications for evolution of the northern branch of Neo-Tethys

Cenozoic adakitic rocks of the northern part of the Central Iran Structural Zone (CISZ) are among the notable geological features of the terrains in northeast Iran, so a comprehensive comparison of several of these adakitic sequences is presented. This lithogeochemical analysis is constrained to examining adakitic magmatism of the three magmatic belts within the CISZ, which from southeast to northeast and from oldest to youngest are as follows: (a) south of Shahrood-Damghan, (b) north-northwest of Sabzevar-Neyshabour, and (c) south of Qouchan and west of Esfarayen. Radiogenic isotope analysis using Rb–Sr and Sm–Nd methods show that the adakitic rocks associated with Qouchan-Esfarayen magmatism have 0.512581 to 0.51288 initial 143Nd/144Nd and 0.703903 to 0.705627 initial 87Sr/86Sr, with εNd −0.86 to 4.98. Adakitic rocks in south to southeast Shahrood have 0.512775 to 0.512893 initial 143Nd/144Nd and 0.703746 to 0.705314 initial 87Sr/88Sr, with εNd 3.69 to 6.0, and adakites emplaced into the Sabzevar ophiolite have 0.512846 to 0.512911 initial 143Nd/144Nd and 0.70379 to 0.705019 initial 87Sr/86Sr contents with εNd of 5.26 to 6.54. Isotopic initial ratios of Nd and Sr support an origin involving partial melting of the subducting oceanic lithosphere of the northern branch of Neo-Tethys and the associated suprasubduction mantle wedge in producing these adakitic rocks

Geochemical and Nd-Sr Isotopic Compositions of Hypabyssal Adakites in the Torud-Ahmad Abad Magmatic Belt, Northern Central Iran Zone: Analysis of Petrogenesis and Geodynamic Implications

Eocene intermediate to felsic subvolcanic rocks of the Torud-Ahmad Abad magmatic belt (TAMB), in the northern part of the Central Iran zone, are exposed between the Torud and Ahmad Abad regions in South-Southeast Shahrood. These igneous rocks include hypabyssal dacite, trachyte, andesite, trachy-andesite, and basaltic andesite; they are mainly composed of phenocrysts and microcrystalline groundmass of pyroxene, amphibole, and plagioclase, with minor biotite and titanomagnetite; they form domal structures (plugs and stocks), dikes, and sills that intruded into Neoproterozoic to cogenetic Eocene volcano-sedimentary sequences. Based on isotopic analysis of these intermediate to acidic rocks, initial ratios of 143Nd/144Nd range from 0.512 775 to 0.512 893 and initial ratios of 87Sr/86Sr range from 0.703 746 to 0.705 314, with quite positive εNd(i) values of +3.69 to +6.00. They are enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high-field strength elements, the SiO2 content is (52–62) wt.%, and Na2O content >3 wt.%, Al2O3 content >16 wt.%, Yb <1.8 ppm, and Y <18 ppm. These geological, geochemical, and Sr and Nd isotopic data are consistent with adakitic signatures originating by partial melting of the subducted Neo-Tethys oceanic slab (Sabzevar branch) and lithospheric suprasubduction zone mantle. The mantle signatures typifying the rapidly emplaced adakitic rocks (slab (high-silica adakite) and suprasubduction zone (low-silica adakite) melts) together with their locally voluminous extent are evidences that support a locally extensional geodynamic setting; and the evidence is consistent with an evolution to local transpression in the Late Eocene in this convergent margin arc environment to rifting (basalts to adakites) towards submarine conditions in the Neogene

Timing of magmatic crystallization and Sn-W-Mo greisen vein formation within the Mount Douglas Granite, NB, Canada

U-Pb geochronology was applied to a combination of magmatic and hydrothermal minerals to help constrain the timing of emplacement of three units in the Mount Douglas Granite (MDG) and reveal their association with a complex mineralized hydrothermal system containing endogranitic Sn-W-Mo-Zn-Bi-U-bearing greisen/sheeted veins within the pluton. Magmatic monazite and zircon U-Pb ages obtained by LA ICP-MS overlap at 368 Ma, recording a Late Devonian crystallization age for the MDG. Although discrimination, outside analytical error, of sequential pulses of magmatism is beyond the resolution of LA ICP-MS U-Pb geochronology, geochemical variations of monazite accompanied by previous whole-rock geochemical analyses support a progressive fractional crystallization process starting from a parental magma (Dmd1), leading to the generation of Dmd2, and finally Dmd3 as the most fractionated unit. Hydrothermal uraninite, cassiterite, and monazite, collected from endogranitic greisen/sheeted veins, reveal evidence for syn-magmatic-related mineralization and a longer-lived post-magmatic hydrothermal system. The first stage is recorded by concordant uraninite dates at 367 3 Ma and by an inverse isochron lower intercept of 362 8 Ma for cassiterite. In contrast, hydrothermal monazite crystallized over a wider range of ages from 368 to 344 Ma, demonstrating post-magmatic hydrothermal activity within the MDG. These magmatic and hydrothermal ages combined with the geochemical signature of the MDG are similar to those documented for the nearby Mount Pleasant Sn-W-Mo-Bi-In granite-related deposit, which suggests that the two mineralizing systems occur at different levels of the same magmatic system.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Geochemistry of the highly evolved Sn-W-Mo-bearing Mount Douglas Granite, New Brunswick, Canada: Implications for origin and mineralization

The Late Devonian (368 ± 1 Ma) post-orogenic peraluminous Mount Douglas Granite, located in southwestern New Brunswick, Canada, forms the eastern part of the Saint George Batholith. The batholith was emplaced following the accretion of the Gander and Avalon zones of the northern Appalachians during the Neoacadian Orogeny. The Mount Douglas Granite is divided into three units, Dmd1, Dmd2, and Dmd3, that formed by progressive high degrees of fractional crystallization. The two most fractionated units, i.e., Dmd2 and Dmd3, are locally associated with greisen/sheeted veins-related endogranitic Sn-W-Mo mineralization. Whole-rock δ18O values of +6.0 to +7.3‰, high initial 87Sr/86Sr ratios of 0.70550 to 0.71665, slightly positive εNd(368 Ma) values (+0.3 to +1.1), and Pb isotopic data indicate that the granite was derived dominantly from partial melting of juvenile Avalonian crust, contaminated by supracrustal rocks. Petrochemical features support the idea of multiple periods of fractional crystallization at lower temperatures producing compositionally more evolved magmas. This is supported by estimated zircon saturation temperatures that decrease from Dmd1 (747–826 °C) → Dmd2 (733–817 °C) → Dmd3 (729–816 °C). The estimated crystallization pressure based on normative quartz and feldspar (albite + orthoclase) contents also decreases with increasing silica content, from 2.3 kbar (~7.7 km) in Dmd1 to 0.3 kbar (~1 km) in Dmd3. Whole-rock geochemical characteristics of this system show that unit Dmd3 is the most highly evolved unit; it has the highest SiO2 (avg. 76.4 wt.%.), LILE contents (e.g., Li, Rb, Cs), Rb/Sr (mean = 42), certain HFSE (Ta, Th, U) including Y (≤138 ppm), and REE, and has the most pronounced negative Eu anomalies (avg. Eu/Eu* = 0.08) and the lowest K/Rb (70–127), Nb/Ta (mean = 4.9), and Zr/Hf (mean = 23.5). Therefore, unit Dmd3 with the highest content of incompatible elements is the most prospective for Sn, W, Zn, Bi, and U mineralization, and then Dmd2