Geochemical and fractal analysis of enclaves in the Dehe-Bala intrusion, (Northwestern Iran) : a new concept to the interpretation of crust-mantle interaction process

The Dehe-Bala intrusion is one of the remarkable intrusions of granodiorite rocks with I-type affinity and abundant mafic microgranular enclaves (MMEs) in the Buin Zahra area, Qazvin, Iran. The MMEs, composed of diorite and quartz-monzodiorites, are haphazardly widespread in the granodiorites. The Dehe-Bala Granodiorites (DBG) usually are characterized by high contents of SiO2 (64.2-66.9), Na2O (3-3.23), K2O (3.49-4), Mg# 4.84 and Th/Ta ratio (≈7.9). In comparison to the DBG, the MMEs can be distinguished by their lower value of SiO2 (52.8-58.2), K2O (1.4-3.8) and higher Mg# (0.4-0.46). All these characteristics show a different composition of the DBG and MMEs, more importantly, can argue in favor of a magma mixing/mingling origin in the DBG. The enrichment in total REEs and HFSEs in the MMEs clearly reflects a marked diffusional process from the felsic to mafic magma that could have been achieved by chemical exchange during the magma mixing/mingling process. The fractal dimensions (Dbox) of MMEs differ from 1.14 to 1.29 with the highest frequency at 1.29. The textural heterogeneity and geochemical features combined with high Dbox values in the MMEs compared with the DBG show lower degrees of mixing/mingling between mantle-derived mafic and lower crust-derived felsic magmas

Petrogenesis of Middle-Eocene granitoids and their Mafic microgranular enclaves in central Urmia-Dokhtar Magmatic Arc (Iran): Evidence for interaction between felsic and mafic magmas

Whole rock major and trace element geochemistry together with zircon U-Pb ages and Sr-Nd isotope compositions for the Middle Eocene intrusive rocks in the Haji Abad region are presented. The granitoid hosts, including granodiorite and diorite, yielded zircon U-Pb ages with a weighted mean value of 40.0 ± 0.7 Ma for the granodiorite phase. Mafic microgranular enclaves (MMEs) are common in these plutons, and have relatively low SiO2 contents (53.04–57.08 wt.%) and high Mg# (42.6–60.1), probably reflecting a mantle-derived origin. The host rocks are metaluminous (A/CNK = 0.69–1.03), arc-related calc-alkaline, and I-type in composition, possessing higher SiO2 contents (59.7–66.77 wt.%) and lower Mg# (38.6–52.2); they are considered a product of partial melting of the mafic lower crust. Chondrite-normalized REE patterns of the MMEs and granitoid hosts are characterized by LREE enrichment and show slight negative Eu anomalies (Eu/Eu* = 0.60–0.93). The host granodiorite samples yield (87Sr/86Sr)i ratios ranging from 0.70498 to 0.70591, positive εNd(t) values varying from +0.21 to +2.3, and TDM2 ranging from 760 to 909 Ma, which is consistent with that of associated mafic microgranular enclaves (87Sr/86Sr)i = 0.705111–0.705113, εNd(t) = +2.14 to +2.16, TDM2 = 697–785 Ma). Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves originated by interaction between basaltic lower crust-derived felsic and mantle-derived mafic magmas in an active continental margin arc environment. Keywords: Geochemistry, U-Pb geochronology, Granitoid, Haji Abad, Low angle subduction, Urumieh-Dokhtar Magmatic Ar

Geochemistry and petrogenesis of the Feshark intrusion (NE Isfahan city)

Introduction Granitic rocks are the most abundant rock types in various tectonic settings and they have originated from mantle-derived magmas and/or partial melting of crustal rocks. The Oligo-Miocene Feshark intrusion is situated in the northeast of the city of Isfahan, and a small part of Urumieh–Dokhtar Magmatic Arc is between 52º21' E to 52º26'E and 32º50' N to - 32º53' N. The pluton has intruded into lower Eocene volcanic rocks such as rhyolite, andesite, and dacite and limestone. Analytical methods Fifteen representative samples from the Feshark intrusion were selected on the basis of their freshness. The major elements and some trace elements were analyzed by X-ray fluorescence (XRF) at Naruto University in Japan and the trace-element compositions were determined at the ALS Chemex lab. Results The Feshark intrusion can be divided into two phases, namely granodiorite with slightly granite and tonalite composition and quartz diorite with various quartz diorite and quartz monzodiorite abundant enclaves according to Middlemost (1994) classification. The quartz diorite show dark grey and are abundant at the western part of the intrusive rocks. Granodiorite are typically of white-light grey in color and change gradually into granite and tonalite. The granodiorite and granite rocks consist of quartz, K-feldspar, plagioclase, biotite, and amphibole, whereas in the quartz diorites the mineral assemblages between different minerals are very similar to those observed in the granodiorite. However, amphibole and plagioclase are more abundant and quartz and K-feldspar modal contents are lower than in the granodiorite whereas pyroxene occurs as rare grains. They are characterized as metaluminous to mildly peraluminous based on alumina saturation index (e.g. Shand, 1943) and are mostly medium-K calc-alkaline in nature (Rickwood, 1989). Discussion In the Yb vs. La/Yb and Tb/Yb variation diagrams (He et al., 2009), the studied samples show small variations in La/Yb and Tb/Yb ratios, suggesting fractional crystallization. Chondrite-normalized REE patterns (Sun and McDonough, 1989) of all the samples essentially have the same shape with light REE (LREE) enrichment, flat high REE (HREE) and significant negative Eu anomalies. All of the samples exhibit similar trace element abundance patterns, with enrichment in large ion lithophile elements (LILE) and negative anomalies in high field strength elements (HFSE; e.g. Ba, Nb, Ta, P, and Ti) compared to primitive mantle (Sun and McDonough, 1989). The enrichment of LILE and LREE relative to the HFSE and HREE along with Nb, Ta, and Ti anomalies display close similarities to those of magmatic arc granites (Pearce et al., 1984) and also negative Nb–Ti anomalies are thought to be related to the fractionation of Ti-bearing phases (titanite, etc.). Moreover, these are the typical features of arc and / or crustal contamination (Kuster and Harms, 1998), while the negative P anomalies should result from apatite fractionation. The increasing of Ba and slightly decreasing Sr with increasing Rb, indicate that plagioclase fractionation plays an important role in the evolution of the studied intrusion. Tectonic environment discrimination diagrams such as Nb vs. Y, Nb vs. Yb+Ta (Pearce et al., 1984) and Th/Yb vs. Ta/Yb (Pearce, 1983) with enrichment in the LILE and LREE relative to HFSE and HREE and negative anomaly in the Nb, Ti and Eu indicate that their initial magma is generated in the subduction zone related to an active continental margin setting. ‏The rocks genesis determining diagrams such as Nb vs. Nb/U (Taylor and McLennan, 1985), Ti vs. Ti/Zr (Rudnick et al. 2000), (La/Sm)cn vs. Nb/U (Hofmann et al., 1986), and Sr/Y vs. Y (Sun and McDonough, 1989) show that the magma was probably generated by partial melting of amphibolitic continental crust. References He, Y., Zhao, G., Sun, M. and Han, Y., 2009. Petrogenesis and tectonic setting of volcanic rocks in the Xiaoshan and Waifangshan areas along the southern margin of the North China Craton: Constraints from bulk-rock geochemistry and Sr-Nd isotopic composition. Lithos, 114(1-2): 186-199. Hofmann, A.W., Jochum, K.P., Seufert, M. and White, W.M., 1986. Nb and Pb in oceanic basalts: new constraints on mantle evolution. Earth and Planetary Science Letters ,79(1-2): 33-45. Kuster, D. and Harms, U., 1998. post – collisional potassic granitoids from the southern and northwestern parts of the late neoporterozoic East African Orogen: a review. Lithos. 45(1-4):177-195. Pearce, J.A., 1983. The role of sub-continental lithosphere in magma genesis at destructive plate margins. In: C.J. Hawkesworth and M.j. Norry (Editors), continental basalts and mantle xenoliths. Shiva Publications, Nantwhich, pp. 230-249. Middlemost, E.A.A. 1994. Naming materials in the magma/igneous rock system. Earth- Science Review. 37(3-4): 215–224. Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25(4): 956 – 983. Rickwood, P.C., 1989. Boundary lines within petrologic diagrams which use of major and minor element. Lithos, 22(4): 247-263. Rudnick, R.L., Barth, M., Horn, I. and McDonough, W. F., 2000. Rutile-Bearing Refractory Eclogites: Missing Link Between Continents and Depleted Mantle. Science, 287(5451): 278-281. Shand, S.J., 1943. The Eruptive Rocks. 2nd edition. John Wiley, New York, 444 pp. Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications, 42, pp. 313-345. Taylor, S.R. and McLennan, S.M., 1985. The continental crust: its compositions and evolution. Blackwell, Oxford, 312 pp

Structural and metamorphic evolution of the southern Sanandaj-Sirjan zone, southern Iran

International audienceThe southern Sanandaj-Sirjan Zone (SSZ) forms the core of the Zagros orogen and consists of a stack of four major tectonic units, all of which were thrust southwestward over the Neyriz ophiolites. Precambrian and early Paleozoic rocks were affected by high-grade metamorphism, whilst the late Paleozoic (Carboniferous and Permian) and Mesozoic rocks are of low grade to unmetamorphosed. Geothermobarometric results indicate that the investigated area experienced peak temperature-pressure conditions of ~ 650 °C and 8-9 kbar, before being partly overprinted by greenschist facies conditions. We identified two main deformational phases (D2, D3) and some relics of an older one (D1) in the southern SSZ. D1 is inferred based on local evidence of tight D2 folds that fold a pre-existing schistosity (S1), associated with prograde- and peak metamorphism. D2 shows large-scale, tight-to isoclinal F2 folds and a penetrative S2 foliation, forming the dominant foliation in our study area. The P-T path is clockwise and associated with a geothermal gradient of ~ 20 °C/km, suggesting a collision-related geodynamic setting, preceding D2, and possibly linked to an early stage of D1. D3 is characterized by open folds (F3) and thrusts developed after greenschist facies metamorphism. Based on the metamorphic grade and stratigraphic age of folded units, D2 is inferred to be Eo-Cimmerian, and D3 post-Cretaceous, coeval to the Zagros orogeny. Concerning the oldest phase (D1), for which only circumstantial structural evidence exists, we discuss whether its age is pre-Cimmerian, possibly Variscan, or Eo-Cimmerian as suggested in previous literature. This study uses metamorphic assemblages and structures of the southern SSZ, to retrace its geodynamic evolution, which started with burial associated with high-temperature metamorphism, followed by Eo-Cimmerian shortening, and finally terminated with additional shortening related to the Zagros collisional event

Fossil thermal structure of the southern Sanandaj-Sirjan zone (SW Iran): Implications for regional-scale tectonics

International audienceThe Sanandaj-Sirjan Zone (SSZ) is one of the Cimmerian blocks accreted to Eurasia during the Late Triassic and the only tectonic unit of the Zagros orogen affected by Barrovian metamorphism. Both the age and absolute temperature of this metamorphic event are still poorly defined. In the present study, we use the Raman Spectrometry of Carbonaceous Material (RSCM) to estimate maximum temperatures attained in the southern SSZ, along two SW-NE cross-sections. While maximum obtained temperature for Precambrian to Jurassic rocks ranges from ~200 to ~600 °C, all Precambrian and Early Paleozoic samples record temperatures >500 °C. Results thus reveal the existence of a major temperature contrast between Precambrian to Devonian samples (showing Tmax > 500 °C) and Late Devonian to Cretaceous ones (Tmax < 400 °C). This temperature contrast can be assigned to two distinct end-member scenarios, either to a so far unreported Variscan thermal event or to differential exhumation along later thrusts. The stepwise decrease of temperatures with age, and the tectonic style common to rocks older than the Early Paleozoic, strongly support the existence of a thermal (and likely) deformation event at ~380-360 Ma. The SSZ, which was part of the northern edge of Gondwana at that time, was thus affected by Variscan tectonics, similar to what is known in western Europe or NW Africa

Geochronology and petrogenesis of granitoids and associated mafic enclaves from Ghohroud in the Urumieh–Dokhtar Magmatic Arc (Iran): Evidence for magma mixing during the closure of the Neotethyan Ocean

The Ghohroud granitoids (GG), containing mafic microgranular enclaves (MMEs) are located in the central part of the Urumieh‐Dokhtar Magmatic Arc (UDMA) in central Iran. They are associated with the subduction‐related magmatism in the Alpine‐Himalayan orogenic belt. The GG are comprised of a variety of intermediate and felsic rocks, including tonalite, granodiorite, granite, diorite porphyry and monzodiorite. The MMEs are gabbroic diorite and tonalite in composition and characterized by a fine‐grained hypidiomorphic microgranular texture with occasional chilled margins. They show rounded, sharp or irregular contact with the host granitoids. The occurrences of quartz, K‐feldspar and corroded plagioclase indicate that MMEs are the products of mixing between mantle and crust‐derived magmas. New ages of zircon U–Pb dating reveal that the GG in the Kashan area emplaced at ca. 19–17 Ma (Burdigalian). All the samples of MMEs and granitoid host rocks in this study are metaluminous and calc‐alkaline with I‐type affinities. They are enriched in light rare earth elements (LREEs) and show slight negative Eu anomalies (Eu/Eu* = 0.36–0.95). These features in a combination with the relative depletion in Nb, Ta, Ti and P, indicate the granitoids and MMEs are closely associated with subduction‐related magmas at an active continental margin. The host rocks yield relatively homogeneous isotopic compositions of initial 87Sr/86Sr ratios ranging from 0.706036 to 0.707055, εNd(t) values varying from −2.25 to 0.8, and the Nd model ages (TDM) vary in a limited range of 0.70–0.96 Ga. The MMEs show similar initial 87Sr/86Sr ratios (0.706420–0.707366), εNd(t) values (−1.32 to −0.27), TDM (0.68–1.09 Ga) and Pb isotopic compositions with host granitoids, which imply they attained isotopic equilibration during magma mingling and mixing. In combination with the petrographic, chemical and isotopic results, we suggest that the origin of MMEs and their host rocks were related to the interaction between crust‐derived melts and mantle‐derived mafic magmas. The magma‐mixing event possibly occurred during the transition from subduction to collision in the UDMA along with the closure of the Neotethyan ocean.A comprehensive dataset from petrographic characteristics to geochemical compositions of the mafic microgranular enclaves and granitoid host rocks from the Urumieh–Dokhtar Magmatic Arc (Iran) was presented. The new data provide significant insight into the evolution of magmatism in this area, which was tightly related to the Neotethyan closure. imageNational Nature Science Foundation of ChinaTMU Research Grant Counci

Zircon U–Pb geochronology, major-trace elements and Sr–Nd isotope geochemistry of Mashhad granodiorites (NE Iran) and their mafic microgranular enclaves: evidence for magma mixing and mingling

International audienceMashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO2 = 67.5–69.4 wt%, high K2O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd(t) (−3 to −2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO2 (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd(t) (−2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs

Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit

International audienceSerpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern Iran. They show irregular and discontinuous shapes with variable sizes ranging from 30 to 60 cm. Chromian spinel grains are observed within both magnetite ores and host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, with compositions close to (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+0.05)O4 are preserved in the host serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite and three different spinel-structure minerals: Cr-spinel (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe3+0.2)O4, named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr2O4. In the magnetite ore body, no (Cr,Al)-spinel I is found and Cr-spinel II occurs as relict cores surrounded by ferritchromite and magnetite. Detailed X-ray elemental mapping revealed that the 200 μm-thick magnetite rim is composed of two magnetite types with different minor element compositions: the first rim found at the contact with ferritchromite is thin (20 μm; magnetite-I); the thicker outer rim contains numerous Fe-poor and Mg- and Si-rich silicate inclusions (magnetite-II). Observations at the TEM scale allows to identify ferritchromite which occurs as a micrometer-sized rim between Cr-spinel II and magnetite I. Thermodynamic modelling of the phase relationships in the studied Sabzevar serpentinite suggests that Cr-spinel II is produced along with chlorite during a first alteration stage at temperatures between 725 and 575 °C in the course of peridotite-water interactions. A second hydrothermal alteration stage producing ferritchromite and magnetite is inferred from the thermochemical modelling at temperatures  10 m during serpentinization