ژئوشیمی ایزوتوپ های Rb-Sr و Nd-Sm و پتروژنز دایک های بازیک کوههای میشو (شمال غرب ایران)

Abstract

Introduction There are some theories about the Paleotethys event during the Paleozoic that have been proposed by geologists (Metcalfe, 2006). Some scientist offered some pieces of evidence about the northern margin of Gondwana (Zhu et al., 2010). The Paleotethys Ocean and Hercynian orogenic report first in Iran, have been Offered from the Morrow and Misho Mountain (Eftekharnejad, 1981). Misho Mountains is located between the north and south Misho faults and cause the formation of a positive flower structure (Moayyed and Hossainzade, 2011). There is theory about Misho southern fault as the best candidate of the Paleotethys suture zone (Moayyed and Hossainzade, 2011). Geochemistry and Sr –Nd isotopic data of the A2 granitic and Synite rocks of the East Misho, indicate that the magmatism post collision has occurred in the active continental margin by extensional zones of the following the closure of the Paleotethys (Ahankoub, 2012). Granite and syenite rocks have been cut by mafic dikes. Mafic dikes are often formed in extensional tectonic settings related to mantle plume or continental break –up (Zhu et al., 2009). In this paper, we use the geochemistry and Nd-Sr isotope data to determined petrogenesis, tectono-magmatic setting and age of Misho mafic dikes. Materials and methods After petrography study of 30 thin sections of mafic dike rocks, 8 samples were selected for whole-rock chemical analyses using ICP-MS and ICP-AES instruments by ACME Company in Vancouver, Canada. We prepared 6 sample For Sm-Nd and Rb-Sr analysis. Sr and Nd isotope ratios were measured with a thermal ionization mass spectrometer, VG Sector 54–30 at the Nagoya University. The isotope abundances of Rb, Sr, Nd, and Sm were measured by the ID method with a Finnigan MAT Thermoquad THQ thermal ionization quadrupole mass spectrometer at the Nagoya University. NBS987 and JNdi-1 were measured as natural Sr and Nd isotope ratio standards (Tanaka et al., 2000). Averages and 2σ errors for the repeated analyses of the standards during this study were as follows: NBS987 87Sr/86Sr=0.710264± 0.00001 (1 σ, n=9) and JNdi-1 143Nd/144Nd= 0.512097± 0.00001 (1σ, n=9). Results Results of the ICP-AES and ICP-MS analysis present that dikes chemical compounds contain SiO2 = 50.94 – 48.3%, TiO2 = 1.53 -1.43%, Al2O3= 16.37 -15. 64 and MgO = 6.61 -5. 54. Major and trace elements display the natural of the with in plate Calc-Alkalin basalts of the metaluminous. Amounts of the Mg # indicate the variety of the fractional crystallization processes (ol and Cpx) in these rocks. Also, the low Nb / La refers to crustal assimilation during fractional crystallization processes. Chondrite-normalized REE patterns of the samples (Sun and McDonough, 1989), indicate an enrichment LREE / HREE because of low partial melting of garnet in the source (Martin, 1999). The low degree of partial melting of the mantle caused LREE enriched to HREE (Wass and Rogers, 1980). There are Eu Positive anomalies that are due to the accumulation of plagioclase. REE normalized patterns to Chondrite point out the enrichment REE and Tb samples by separation amphibole, pyroxene, Hornblende, titanite and rutile (Thirlwall et al., 1994). Spider diagram (Sun and McDonough, 1989) displays enrichment Rb, Th and U elements and depletion in Nb, Ti and p because of source depletion or Nb minerals existence (such as rutile, ilmenite and spinel). Enrichment Cs, Th, U, Nb and Ti, p negative anomalies of the mafic dike are similar to geochemical characteristics of continental margin rocks. Nb, Ti negative anomalies and Pb positive anomalies demonstrate the interference of the crust in magmatic source (Martin, 1999). The TDM model ages of mafic dikes are 1.2 -1.8 milliard years that show time of the separation of the source of mafic rocks of the Proterozoic crust. Also Sr-Rb data indicate the formation of Misho mountain mafic dikes at 232 ma years age. The eNd (T) is -1 to -4 that indicates the array mantel component of the mafic dike. Discussion Geochemistry data indicate that Misho mafic dikes are similar to calc-alkaline basalts of the oceanic island basalts (OIB) whereas Nb and Ti negative anomalies of the trace elements patterns are similar to crustal contamination. Negative amount of the eNd(T) indicated depleted mantel source (array mantel) with some continental crust contamination during AFC processes . Base on the results of analysis, the upper crust is the best candidates for magma contamination of the mafic dikes in Misho. Isotopic data indicated to replace mafic dike 232ma years ago by closing of paleotethys and forming the extension zone (break up) in active continental margin. 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