摘要
Polymetamorphic units are important constituents of continent-continent collisional orogens,and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision.This study reports the metamorphic conditions and evolution of the Dorud-Azna metamorphic units in the central part of the Sanandaj-Sirjan zone(SSZ),Iran.Here,new geothermobarometry results are integrated with ^(40)Ar/^(39)Ar mineral and Th-U-Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ,the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units.In the Amphibolite-Metagabbro unit,staurolite micaschist underwent a prograde P-T evolution from 640±20℃/6.2±0.8 kbar in garnet cores(M1)to 680±20℃/7.2±1.0 kbar in garnet rims(M2).Three Th-U-Pb monazite ages of 306±5 Ma,322±28 Ma and 336±39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism.In the same unit,the metagabbro records P-T conditions of 4.0±0.8 kbar and 580±50℃ in the(magmatic)amphibole core(Late Carboniferous intrusion)to 7.5±0.7 kbar and 700±20℃ in the amphibole rim indicating a prograde P-T path during subsequent burial(M1).New ^(40)Ar/^(39)Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36±12 Ma to 170±2 Ma.This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca.425±25℃ after M2 metamorphism and a Paleogene low-grade metamorphic overprint(M3),while ^(40)Ar/^(39)Ar white mica dating of garnet micaschist yielded a plateau age of 137.84±0.65 Ma.We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma.Furthermore,it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism(at temperatures of~350-240℃)during final shortening and exhumation.In the underlying Galeh-Doz unit,the Panafrican granitic orthogneiss intruded at P-T conditions of 3.2±4 kbar and 700±20℃,then it was metamorphosed and deformed at 600±50℃ and 2.0±0.8 kbar(metamorphic stage M1)prior to Late Carboniferous intrusion of mafic dikes.^(40)Ar/^(39)Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma,representing the age of cooling through ca.500℃ after the M1 metamorphic event.Interestingly,the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P-T conditions and final thickskinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex.Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism,as result of rifting.We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean.Eventually,the investigated units reflect various stages of a Wilson cycle,from rifting to initiation of the subduction in final plate collision.
基金
support by the Afro-Asiatisches Institut,Salzburg for her Ph D thesis at the Salzburg University。
