Interpretaion of 40K/40Ar datings of the Eastern metamorphic rocks and ophiolitic complexs of the region of Khoy.
The ophiolites of Khoy are exposed in an area located to the north-west of the city of Khoy in the north-western part of the Iranien Azerbaidjan province, extending practically till the Turkish border.This article gives a detailed 40K-40Ar geochronological studies were done on selected rocks samples, all carefully located on the map. The following units are described successively, from NE to SW: (1) the south-western margin of the Central Iranien Block, (2) the eastern metamorphic complex, contain meta-ophiolite complex (3) a turbiditic and volcanic-sedimentary unit of Late Cretaceous age-Early Cretaceous, (4) the Late Cretaceous, non metamorphic ophiolite complex of Khoy s.s., (5) the western metamorphic complex.
We propose to distinguish four groups of chronological events, based on the measurements done on separated amphiboles and micas on the eastern metamorphic ophiolite complex, contain meta-ophiolite complex:
(1) The Lower Jurassic group (195-181 Ma). The oldest apparent ages were found in two rock types: (a) in amphibole-rich pegmatitic metagabbros from the meta-ophiolite association. In these metagabbros, there are locally small blocks of weakly deformed gabbros with Lower Jurassic apparent average ages (194.8 ? 10.1 Ma). These ages suggest that the primary cooling age of these gabbros was somewhat older, perhaps Upper Triassic. (b) in fine-grained amphibolites exposed in the base of the ( m1) metamorphic group, with a range between 196.3 ? 10.7 and 182.3 ? 4.3 Ma. The quartz-muscovite pegmatite veins crosscutting these amphibolites have an apparent age of 181.8 ? 4.2 Ma.
(2 ) The Middle Jurassic group (160-155 Ma). The second group of apparent ages is found in the following metamorphic facies:
(a) in amphiboles of metagabbros and ortho-amphibolites from the meta-ophiolitic complex, at 160.8 ? 12.7 and 160.7 ? 12.9 Ma (north of Aghbash village) , and at 155.6 ? 11.9 Ma (east of Ajidgah village).
(b) in the (m1) metamorphic group, the muscovites of the gneisses of Hydarabad village (160.5 ? 3.7 Ma) and the amphiboles from the amphibolites of Gheh Yashar village (151.0 ? 11.5 Ma). Also, well crystallized micaschists gave both muscovites and biotites with an apparent age of 146.3 ? 3.4 Ma.
(3) The Lower Cretaceous group. The third group of isotopic ages was found in the metamorphic complex (m1, m2) and in the meta-ophiolitic gabbros. They include: (a) two 40K/40Ar datings of amphiboles at 121.2 ? 6.2 Ma from the fine-grained and recrystallized amphibolites (north Ghekh Yashar village), and 102.1 ? 5.4 Ma from the amphibolites of Ajidgah village; (b) two 40K/40Ar datings obtained from the amphiboles of meta-ophiolitic gabbros, at 116.5 ? 6.0 Ma (north Ajidgah) and 112.9 ? 8.6 Ma (west Ravand).
(4) The Upper Cretaceous group. The muscovites of the micaschists from Ajidgah , gave an age of 81.2 ? 1.2 Ma. The muscovites separated from micaschists north of Dizaj gave 69.4 ? 1.6 Ma (Maestrichtian); this young age can be related to tectonic element that caused the local S3 deformation.
In the vicinity of Qorol-Ajai village, a gneissic granitic intrusion crosscuts the metamorphic rocks, extending to the north of the studied area. Many quartz-feldspar-bearing dikes and veins, probably related to this granite, crosscut the metamorphic rocks. The separated pegmatitic muscovites from granitic veins from Ajidgah give an age of 105.8 ? Ma, coarse-grained muscovites of other granitic dikes in the vicinity of Qorol-Ajai give an age of 75.3 ? 1.8 Ma, and the separated muscovites and biotites from the Qorol-Ajai granite-gneisses give an age of 67.5 ?1.6 Ma.
Finally, the separated fine-grained amphiboles from a weakly deformed, unmetamorphosed gabbro intruding the meta-ophiolites gave an Upper Cretaceous average age of 80.2 ? 4.6 Ma (Table 1) .
The main conclusion is that there are not one, but two ophiolitic complexs in the Khoy area: (1) an old, poly-metamorphic ophiolite, tectonically included within a metamorphic subduction complex, whose oldest metamorphic amphibolites yield a Early Jurassic apparent 40K/40Ar age, and whose primary magmatic age should logically be pre-Jurassic; (2) a younger non metamorphic ophiolite of Late Cretaceousage, overlaine by a turbiditic, flysh-like volcanogenic series, of Late Cretaceous-Early Paleocene age which their detailes will discus t un other article .
This article is the result of a PhD work on the geological, pertro-geochemical and geochronological of Khoy ophiolites (Khalatbari, december 2002), carried out in the frame of a French-Iranian cooperative programme, supported by the French Ministry of Foreign Affairs, the Cultural Service of the French Embassy at Tehran, and the Geological Survey of Iran (GSI).