Journal of Economic Geology (Jun 2019)
Petrology of porphyritic quartz monzodiorite stock and Eocene dykes with adakitic nature from SW of Jandaq (NE of Isfahan province); Evidence of oceanic crust subduction around the Central-East Iranian Microcontinent
Abstract
The “adakite” term was used for the first time by Defant and Drummond (1990) to display Cenozoic arcs igneous rocks with intermediate composition (SiO2> 56 wt.%), which were produced by partial melting of subducted oceanic crust. The adakites are series of intermediate to acidic rocks, with composition range from hornblende-andesite to dacite and rhyolite; and basaltic composition are lacking. In adakitic magmas, phenocrysts are mainly plagioclase, hornblende and biotite; while orthopyroxene and clinopyroxene phenocrysts are known only in mafic andesites (Calmus et al., 2003). Geochemically, adakites are identified with SiO2> 56 wt.%, Al2O3> 15 wt.%, MgO 400 ppm and enriched LILE and LREE and depleted Y and HREE (Y 40 and La/Yb> 20 (Castillo, 2006 and Castillo, 2012). By using geochemical data, adakites were classified into high silica adakites (HSA, SiO2> 60 wt.%) and low silica adakites (LSA, SiO2< 60 wt.%) main groups. The high silica adakites were produced by partial melting of subducted oceanic crust basalts and the resulting melts also interact with peridotite during their ascent through the mantle wedge. While, low silica adakites were produced by melting of mantle peridotite that were metasomatized by melts resulting from slab (Martin and Moyen, 2002). The intrusion bodies with porphyritic texture has been studied and reported in different areas (e.g. Lan et al., 2012; Zhang et al., 2015). This intrusion bodies are often in a stock shape and the texture is porphyritic due to fast crystallization. The study area (Kuh-e- Godar-e Siah) is located in southwest of Jandaq (northeast of Isfahan province) and northwest of Central-East Iranian Microcontinent. The quartz monzodiorite intrusion with stock shape cross cutting by Eocene dykes swarm with trachy andesitic composition. In this paper, the petrology and chemical characteristics of quartz monzodiorites and trachy andesitic dykes are discussed. Material and methods The chemical compositions of minerals from quartz monzodiorites and dykes were conducted by a JEOL JXA-8600 (WDS) electron probe microanalyzer (EPMA) at the Kanazawa University, Japan. Analyses were performed by an accelerating voltage of 20 kV and a beam current of 20 nA. The Fe2+ and Fe3+ contents of minerals were calculated by assuming mineral stoichiometry. The Fe2+# and Mg# parameters of minerals are Fe2+/(Fe2++Mg) and Mg/(Mg+Fe2+) atomic ratios, respectively. Representative chemical analyses of the minerals are listed in Table 1 and 2. To obtain whole rock chemical data, eighteen samples of the studied rocks were analyzed at the ALS-Mineral Company of Canada, by a combination of inductively coupled plasma spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) methods. The whole rocks geochemical data are presented in Table 3 and 4. Also, X-ray diffraction analyses were carried out in order to typify the K-feldspar mineral using an XRD D8 ADVANCE, Bruker machine, at the Central Laboratory of the University of Isfahan. The FeO and Fe2O3 concentrations are recalculated from Fe2O3*, using recommended ratios of Middlemost (1989). Mineral abbreviations are from Whitney and Evans (2010). Results and discussion The main texture in quartz monzodiorites is porphyritic; and Eocene dykes are granular, intergranular and porphyritic in texture. The quartz monzodiorites consist of plagioclase (albite), sanidine, quartz, biotite, muscovite, chlorite, magnetite, calcite and apatite. The minerals in trachy andesitic dykes are plagioclase (andesine and labradorite), clinopyroxene (diopside and augite), sanidine, phlogopite, quartz, amphibole, magnetite, calcite and apatite. The chondrite-normalized REE patterns and primitive mantle-normalized multi-elemental diagram of the quartz monzodiorites and trachy andesitic dykes show enrichment in LREE and LILEs and depletion in HFSEs such as Ta, Nb and Ti. There is no evident positive or negative anomaly of Eu. Petrographical and geochemical characteristics of quartz monzodiorites and trachy andesitic dykes show that these rocks have been derived from different sources. The quartz monzodiorites have high content of La/Yb= 17.49-41.89, SiO2= 64.60-68.80 wt.%, Sr= 434-1855 ppm, Sr/Y= 53.58-168.63 and low content of MgO= 0.16-1.10 wt.%, Y< 11 ppm and Yb< 0.95 ppm that show characteristics of high silica adakites which have been produced by melting of subducted oceanic crust. The trachy andesitic dykes have La/Yb= 33.45-59.76, SiO2= 53.40-57.60 wt.%, Sr= 859-2050 ppm, Sr/Y= 50.82-125, MgO= 1.93-4.53 wt.%, Y< 13.8 ppm and Yb< 1.14 ppm, which display characteristics related to low silica adakites, produced by melting of metasomatized mantle peridotite. Acknowledgments The authors thank the University of Isfahan for financial supports. References Calmus, T., Aguillon-Robles, A., Maury, R.C., Bellon, H., Benoit, M., Cotten, J., Bourgois, J. and Michaud, F., 2003. Spatial and temporal evolution of basalts and magnesian andesites (“bajaites”) from Baja California, Mexico: the role of slab melts. Lithos, 66(1): 77–105. Castillo, P.R., 2006. An overview of adakite petrogenesis. Chinese Science Bulletin, 51(3): 257–268. Castillo, P.R., 2012. Adakite petrogenesis. Lithos, 134(5): 304–316. Defant, M.J. and Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature, 347(6294): 662–665. 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