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The mafic to felsic intrusive rocks of Nay (IRN) are located in the northeast of the central Iranian block. In this study, we present new major and trace element geochemistry, U-Pb zircon ages, and Sr-Nd isotopic data to discuss the origin of the IRN postcollisional units. The oldest units in the Nay area belong to Paleocene-early Eocene volcanic and pyroclastic series including basalt-andesite, latite, dacite, and tuff. These series are crosscut by subvolcanic and granitoid rocks with lithological composition varying from quartz gabbro to K-feldspar granite. The youngest igneous activity is represented by quartz monzodiorite dikes. Hornblende-biotite quartz monzonite from Nay granitoids was dated at 40 Ma (zircon U-Pb). The IRN rocks are metaluminous to peraluminous with high-K calc-alkaline and shoshonitic affinities. They display enrichment in light REEs [(La/Yb)N = 3.79-8.71] and LILEs (such as Ba, Th, Rb, U, and K), with depletion in HFSEs (such as Nb, Zr, Y, and Ti). All rocks have negative Eu anomalies [(Eu/Eu*)N = 0.17-0.88] and relatively flat heavy REE patterns [(Gd/Yb)N = 1.12-1.69]. Granitoids have initial 87Sr/86Sr values from 0.7053 to 0.7061 and eNd values from -1.65 to -0.02 calculated at 40 Ma. The geochemical composition of IRN rocks along with the low ISr and positive eNd values and mantle model ages of 0.6-0.8 Ga indicate that two end-members, enriched mantle and a continental crust, were involved in the magma generation. We argue that the Eocene IRN magmatism occurred as a postcollisional product by asthenospheric upwelling owing to the convective removal of the lithosphere during an extensional collapse of the central Iranian block. [ABSTRACT FROM AUTHOR]

Abstract It is well established that the Late Jurassic magmatism in the southeastern area of the South China Block (SCB) was associated with an extensional event. However, the triggering mechanism for this event remains unclear, with models commonly invoking either back-arc extension or intra-continental extension in response to slab roll-back of the Pacific plate or asthenospheric upwelling. The key issue about these models revolves around whether the subducted Pacific slab contributed to the mantle source beneath the SCB during the Jurassic, or not. Basalts erupted during the Late Jurassic can provide significant clues for probing the nature of the lithospheric mantle and underlying convective mantle. In this study, we present detailed zircon U-Pb geochronological, whole-rock element and Sr-Nd isotope data for Late Jurassic trachybasalt and trachyandesite from the Mashan Complex, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA-ICP-MS zircon U-Pb dating suggests that these volcanic rocks erupted in the Late Jurassic (~158 Ma). All volcanic samples have shoshonitic geochemical affinities with high K 2 O (2.03–4.89 wt%) and K 2 O/Na 2 O (0.55–2.50). They are strongly enriched in LILE and LREE with positive K anomalies, positive ε Nd (t) values (from +0.8 to +3.0), moderate Dy/Yb (1.98–2.36) and low Ba/La (11.1–30.7), Th/Yb (2.29–5.77), U/Th (0.18–0.35), Th/Nb (0.16–0.27) and Th/Ce (0.08–0.15) ratios. Such geochemical signatures suggest that the Mashan volcanic rocks were derived from low-degree (1–5%) partial melting of a metasomatised lithospheric mantle with little evidence supporting the involvement of the Pacific slab in the genesis of the magmas. The synthesis of the available data shows that the Jurassic shoshonitic and mafic rocks have higher ε Nd (t) values than the Triassic shoshonitic and mafic rocks (ε Nd (t) < 0), reflecting source transformation from a Triassic enriched lithospheric mantle to a Jurassic depleted source. The metasomatism of the lithospheric mantle is likely to be associated with asthenospheric upwelling prior to the Late Jurassic. The asthenosphere-lithosphere interaction might have produced a newly metasomatised zone within the lowermost lithospheric mantle. It is inferred that the Jurassic magmatism in the southeast of the SCB might be associated with asthenospheric upwelling in an intra-continental extension setting. Highlights • The Mashan shoshonitic volcanic rocks erupted in the Late Jurassic. • They were derived from a newly metasomatised lithospheric mantle • This metasomatism was associated with the asthenosphere-derived melts during the Jurassic. • They formed in an intra-continental extension setting. [ABSTRACT FROM AUTHOR]

A petrological, geochemical and Sr–Nd–Pb isotopic study was carried out on the Tarom-Olya pluton, Iran, in the central part of the Alpine–Himalayan orogenic belt. The pluton is composed of diorite, monzonite, quartz-monzonite and monzogranite, which form part of the Western Alborz magmatic belt. LA–ICP–MS analyses of zircons yield ages from 35.7 ± 0.8 Ma to 37.7 ± 0.5 Ma, interpreted as the ages of crystallization of magmas. Rocks from the pluton have SiO 2 contents ranging from 57.0 to 69.9 wt.%, high K 2 O + Na 2 O (5.5 to 10.3 wt.%) and K 2 O/Na 2 O ratio of 0.9 to 2.0. Geochemical discrimination criteria show I-type and shoshonitic features for the studied rocks. All investigated rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), depleted in high-field strength elements (HFSEs), and show weak or insignificant Eu anomalies (Eu/Eu* = 0.57–1.02) in chondrite-normalized trace element patterns. The Tarom-Olya pluton samples also show depletions in Nb, Ta and Ti typical of subduction-related arc magmatic signatures. The samples have relatively low ISr (0.7047–0.7051) and positive εNd (36 Ma) (+ 0.39 to + 2.10) values. The Pb isotopic ratios show a ( 206 Pb/ 204 Pb) i ratio of 18.49–18.67, ( 207 Pb/ 204 Pb) i ratio of 15.58–15.61 and ( 208 Pb/ 204 Pb) i ratio of 38.33–38.77. The εHf (t) values of the Tarom-Olya pluton zircons vary from − 5.9 to + 8.4, with a peak at + 2 to + 4. The depleted mantle Hf model ages for the Tarom-Olya samples are close to 600 Ma. These isotope evidences indicate contribution of juvenile sources in petrogenesis of the Tarom-Olya pluton. Geochemical and isotopic data suggest that the parental magma of the Tarom-Olya pluton was mainly derived from a sub-continental lithospheric mantle source, which was metasomatized by fluids and melts from the subducted Neotethyan slab with a minor crustal contribution. Subsequent hot asthenospheric upwelling and lithospheric extension caused decompression melting in the final stage of Neotethyan subduction through slab rollback or shortly after the cessation of arc magmatism. [ABSTRACT FROM AUTHOR]

To investigate the nature and origin of across-arc geochemical variations over time in mantle wedge derived magmas, we have carried out a geochemical study of basalts in the NE Japan arc spanning an age range from 35Ma to the present. Back-arc basalts erupted at 24-18 Ma, 10-8 Ma, 6-3Ma and 2-5-0Ma have higher concentrations of both high field strength elements (HFSE) and rare earth elements (REE) [particularly light REE (LREE) and middle REE (MREE)], and higher incompatible trace element ratios compared with frontal-arc basalts at any given time. Geochemical modeling of Nb/Yb versus Nb shows that the frontal-arc and back-arc compositional differences are independent of subduction modification and can, in many cases, be explained by different degrees of melting (higher degrees of melting for frontal-arc magmas and lower degrees of melting for back-arc magmas) of a nearly homogeneous depleted mid-ocean ridge basalt (MORB) mantle (DMM)-like source, although there are several exceptions. These include some Pliocene frontal-arc basalts that may originate from a source that is slightly more depleted than DMM, several 35-32Ma and 24-18Ma back-arc basalts derived from a lithospheric mantle source that is enriched in HFSE compared with DMM, and a rare 16-12Ma basalt that was erupted in the back-arc but was produced by a similar degree of melting to frontal-arc basalts erupted at the same time. Variations in ratios of fluid-mobile and -immobile elements and those of melt-mobile and -immobile elements for the 35-0Ma NE Japan basalts indicate that the principal subduction component added to the source mantle prior to generation of these basalt magmas is a sediment-derived melt. Comparison of Sr and Nd isotopic compositions for Pacific Ocean MORB, the NE Japan basalts and subducting sediments suggests that the isotopic compositions of most post-16Ma more depleted back-arc basalts can be explained by the addition of <2% bulk sediment; the most enriched isotope compositions of the subcontinental lithosphere-derived magmas can be accounted for by addition of a maximum 5-7% Japan Trench Sediment (JTS), if the original Sr and Nd compositions of the lithosphere approximated that of DMM. The Sr and Nd isotope composition of the frontal-arc basalts can be accounted for by the addition of 1-5% JTS. A depleted asthenospheric mantle (DMM-like) upwelling model with interaction between asthenospheric mantle-derived magmas and overlying lithospheric mantle can account for the geochemical characteristics of the 35-0Ma NE Japan basalts. The frontal-arc magmas were generally generated by higher degrees of melting of the shallower part of the asthenospheric mantle, whereas the back-arc magmas resulted from lower degrees of melting of the deeper part of asthenospheric mantle. These latter magmas underwent interaction with the lithospheric mantle, resulting in more enriched Sr and Nd isotopic signatures for the pre-18Ma back-arc basalts and post-22Ma frontal-arc basalts, but less interaction, resulting in more depleted Sr and Nd isotopic signatures, for most of the back-arc basalts younger than 16 Ma. [ABSTRACT FROM AUTHOR]

Abstract: Large-scale late Mesozoic to early Cenozoic plutons in the Eastern Pontide orogenic belt mostly show calc-alkaline and I-type geochemical features. However, we identify the Sisdaği pluton that has shoshonitic affinity and I-type character in the region. The pluton was emplaced at shallow depths (<5km), revealed by Al-in hornblende barometry, in the northern margin of the orogenic belt, with SHRIMP zircon U–Pb age of 41.55±0.31Ma, interpreted as dating magma crystallization. It is composed of monzonite, monzodiorite and monzogabbro. The samples show low zircon saturation temperature (T Zr) ranging from 603 to 769°C compared to A-type rocks. They exhibit SiO2 contents of 47.6–61.4wt.%, and high K2O+Na2O (5.0–8.8wt.%) and K2O/Na2O (0.8–1.8). All the samples are characterized by low Mg# (<47) and relatively high but variable Al2O3 content (16.9–22.2wt.%). They are also enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), with a weak negative Eu anomaly (Eu/Eu* =0.58–1.26) in mantle-normalized trace element patterns. The samples possess homogeneous initial Sr and Nd isotopic compositions, marked with low I Sr =0.70376–0.70408 and ε Nd (42Ma)=+1.3 to +2.4. T DM ages of the samples range from 0.70 to 0.85Ga. The Pb isotopic ratios are (206Pb/204Pb)=18.64–18.72, (207Pb/204Pb)=15.51–15.58 and (208Pb/204Pb)=38.31–38.65. These geochemical features imply that the parental magma resulted from melting of chemically enriched lithospheric mantle source. In such a case, a hot upwelling asthenosphere is necessary to partially melt the lithospheric mantle in order to form the parental magma. The pluton is considered to be a post-orogenic intrusion that was emplaced in an environment of lithospheric extension, triggering asthenospheric upwelling. The thermal anomaly induced by asthenospheric upwelling resulted in partial fusion of chemically enriched subcontinental lithospheric mantle beneath the region. Then, the shoshonitic melt, which subsequently underwent fractional crystallization with minor crustal contamination, ascent to shallower crustal levels to generate a monzonitic rock series ranging from monzogabbro to monzonite. All these data combined with the regional geology suggest that the crustal thickening as a consequence of regional compression during the Paleocene changed into a lithospheric extension and thinning throughout the early Cenozoic (at ∼42Ma) in the Eastern Pontides. Hence, the middle Eocene shoshonitic I-type magmatism is a unique pluton, signifying initiation of lithospheric thinning and thus of hot asthenospheric upwelling in the region. These interpretations also argue against the presence of an early Cenozoic subduction of oceanic slab in the Eastern Pontides. [Copyright &y& Elsevier]

Abstract: In the Niigata region of the NE Japan arc, basaltic and intermediate to felsic volcanic rocks (silicic andesite, dacite and rhyolite) have been produced during the Early Miocene, Middle Miocene and Pliocene ages, spanning the pre-Japan Sea opening to post-opening stages. Early Miocene basaltic rocks are characterized by enriched Sr and Nd isotopic signatures (initial 87Sr/ 86Sr (SrI)=0.70557 to 0.70592 and initial 143Nd/ 144Nd (NdI)=0.51255 to 0.51262), whereas Middle Miocene and Pliocene basaltic rocks are slightly enriched than MORB in terms of SrI (0.70314–0.70416) and NdI (0.51286–0.51310). Early Miocene basaltic rocks are also characterized by higher abundances of HFSE and LREE, and higher Zr/Y compared to Middle Miocene and Pliocene basaltic rocks. The geochemical features of Early Miocene basaltic rocks are almost identical to those of basaltic rocks found in continental rift zones, such as the Rio Grande rift. The different geochemical signatures of Early and Middle Miocene basaltic rocks can be ascribed to the geochemical differences in the mantle source. Apart from andesitic rocks of the Kakuda area, most of Early and Middle Miocene intermediate to felsic volcanic rocks from the Niigata region show significantly higher SrI values (0.70673–0.70773) and lower NdI values (0.51234–0.51254) than Early Miocene basaltic rocks, indicating a lower crustal origin for these more felsic volcanic rocks. A possible tectono-magmatic model for the production of post-Early Miocene volcanic rocks from the Niigata region includes: (1) at the Early Miocene (22–20 Ma); commencement of asthenospheric mantle upwelling followed by both partial melting of the overlying lithospheric mantle and crustal rifting, resulting in the production of continental rift zone-type basaltic magmas which have either erupted on the surface or initiated partial melting of the lower crust to produce more felsic magmas, and (2) at the Middle Miocene (after 15 Ma); thinning of the overlying lithosphere due to the opening of the Japan Sea. This was associated with asthenospheric upwelling followed by partial melting of the asthenosphere to generate extensive basaltic magmas. These were either erupted or melted the lower crust to generate more felsic magmas. Such tectono-magmatic processes may also be applicable to the generation of Early to Middle Miocene basaltic to felsic volcanic rocks in many other parts of the back-arc margin of the NE Japan arc. [Copyright &y& Elsevier]