Geochemical and tectonic significance of Arbat alkali gabbro-monzonite-syenite intrusions, Urumieh–Dokhtar Magmatic Arc, Iran

Abstract

As so ci ate ed i tor: Tomasz Bajda The Oligocene Arbat al kali in tru sions of the East ern Miandoab are lo cated in the northwest ern part of Iran and be long to the Urumieh-Dokhtar Mag matic Arc (UDMA). The in tru sions show a ring struc ture with gab bro-monzogabbro-monzodiorite (mafic units) on the edges, with monzonite-monzosyenite-syenite (fel sic units) grad u ally go ing to wards the cen tral parts. The tex tures in dif fer ent rock types are cu mu late, gran u lar and lam i nated. The high val ues of (La/Sm) n and (La/Yb) n, con tents of K, Rb and Cs (pos i tive anom a lies nor mal ized on the ba sis of the prim i tive man tle), low con cen tra tions of Hf, Nb, Zr and Ta (neg a tive anom a lies), and the changes in Th/Nb, Th/Ta, La/Nb and Ce/Pb ra tios along with the geo chem i cal and tec tonic set ting ev i dence ex hibit a subduction-mod i fied man tle or i gin for the for ma tion of these rocks. Ac cord ingly, the in tru sions were formed be tween the Cen tral Iran and the Ara bian plates as a re sult of the par tial melt ing of a man tle wedge at a syn-colli sion or post-col li sion arc-re lated en vi ron ment. Our data sug gested that, af ter the end of the oblique Neotethys subduction and dur ing/af ter the con ti nen tal col li sion, the break-off or roll back of the Neotethys slab be neath west ern Iran, in the Oligocene, might have oc curred. Such a pro cess led to the change in the geo ther mal gra di ent of the man tle wedge be cause of the subduction flu ids, transtension, pres sure re duc tion along the SE-trending lat eral depth strike-slip fault zones in the upper part of the man tle wedge, de com pres sion par tial melt ing at the man tle, and the re sult ing for ma tion of a mafic po tassium-rich melt. The mafic magma was in jected into crustal magma cham bers; prob a bly, the frac tional crys tal li za tion and par tial con tam i na tion oc curred with crustal com po nents, form ing the in ter me di ate and fel sic rocks in the in tru sions. Geochem i cal ev i dence re lated to the vari a tions in the ra tios of Th/Yb, Ta/Yb, Rb/Y, and Nb/Y and Harker vari a tion di a grams along with the spi der di a grams con firmed frac tional crys tal li za tion and par tial FC (frac tional crys tal li za tion) and AFC (as sim ila tion and frac tional crys tal li za tion) in the in tru sions.

Eocene–Oligocene Crustal Thickening-Collapse of the Eastern Tibetan Plateau: Evidence from the Potassic Granitoids in SW China

“…Mafic Melts. Syenites are thought to form as a result of partial melting of metasomatized mantle wedge or basement material in the lower continental crust (e.g., [81]), or by the fractional crystallization of mantle-derived basaltic magmas (e.g., [82]). The Group 2 syenites have lower SiO 2 and higher MgO, Cr, and Ni contents than the Group 1 adakite-like granites.…”