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1 Introduction
Le massif ophiolitique de Khoy (Fig. 1A) se situe au nord-ouest de la ville de Khoy, dans l'Azerbaidjan iranien. Ce massif peu connu est daté du Crétacé supérieur sur les cartes géologiques à et publiées par le service géologique d'Iran [1,4,11]. Hassanipak et Ghazi [5] ont donné récemment une première description pétrographique de l'ophiolite de Khoy et des roches métamorphiques associées, et montré que ses laves ont une composition de type MORB. Leur carte géologique est malheureusement très fantaisiste, et leurs échantillons non localisés. Ghazi et al. [3] ont décrit, sous l'ophiolite, une semelle métamorphique montrant un gradient thermique inverse, dont les amphiboles ont fourni un « âge plateau » 40Ar–39Ar de 104 à 110 Ma. Selon ces auteurs, les métagabbros ophiolitiques ont donné des âges plateau à 154–158 Ma ; par ailleurs, les microfaunes récoltées dans les calcaires pélagiques interstratifiés entre les coulées basaltiques à pillow lavas ont livré des âges proches de la limite Albien–Cénomanien, soit autour de 100 Ma. Il y aurait donc plus de 50 Ma de différence d'âge entre les gabbros et les basaltes ophiolitiques, ces derniers étant un peu plus jeunes que la tectonique d'obduction (semelle métamorphique). Ces résultats, peu cohérents, sont difficilement interprétables.
Geological maps and sampling sites of the Khoy area. (A) Distribution of the ophiolite belts in Iran after Emami et al. [2], and location of the Khoy area. Main Iranian ophiolite complexes: BZ, Band e Ziyarat; KM, Kermanshah; NA, Nain; NY, Neyriz; SB, Sabzevar; SHB, Shar Babrak; THL, Torbat Hydariyah; TK: Tchehel Kureh. (B) Schematic map of the main geological units of the Khoy area described in the text. (C) Geological map of the eastern metamorphic complex and associated meta-ophiolites (Unit 2 in the text), and location of samples dated by 40K/40Ar method. The first number refers to Table 1; letters M, B and A refer to muscovite, biotite and amphibole, respectively (mineral separates used for dating), and the last number gives the apparent age obtained (Ma). (D) Geological map of the non-metamorphic, Upper Cretaceous ophiolite of Khoy (Unit 4 in the text) and related formations, with location of the gabbro samples dated by 40K/40Ar method (Table 1). (E) Geological map of the supra-ophiolitic turbidites and volcanic-sedimentary unit (Unit 3 in the text), and related formations.
Nous présentons ici les résultats de levés cartographiques détaillés et de nouvelles études de laboratoire suggérant l'existence de deux complexes ophiolitiques d'âges différents dans la région de Khoy, ce qui permet de résoudre ces apparentes contradictions.
2 Description géologique de la région de Khoy
Cinq grandes unités géologiques ont été reconnues dans ce travail dans la région de Khoy (Fig. 1B) ; elles sont disposées suivant des bandes cartographiques orientées NW–SE, et peuvent être décrites de la manière suivante, en allant du nord-est au sud-ouest.
2.1 Unité 1 : la marge occidentale du bloc central iranien
Cet ensemble de formations affleure au nord-est de la ville de Khoy, et appartient à la Central Iran Zone définie par Stöcklin [16,17]. On y retrouve une série sédimentaire paléozoı̈que allant, avec des lacunes, du Cambrien au Permien, surmontée par des dépôts sédimentaires d'âge Crétacé, puis par des sédiments d'âge Oligo-Miocène et Quaternaire.
2.2 Unité 2 : le complexe métamorphique oriental
Cette bande de roches métamorphiques (Fig. 1C) comprend essentiellement des amphibolites (dont les affinités géochimiques vont des MORB aux basaltes de type « supra-subduction », Fig. 2A), associées à des paragneiss, des micaschistes, des métaquartzites, des calcschistes et des marbres. Ces roches, qui ont parfois atteint les conditions de l'anatexie commençante, sont affectées par de nombreuses zones de cisaillement ductile. Elles comprennent aussi de grandes écailles tectoniques de roches ophiolitiques métamorphisées, essentiellement des péridotites serpentinisées (lherzolites et harzburgites foliées à textures porphyroclastiques mantelliques), et des ortho-amphibolites (métagabbros intrusifs).
New trace element patterns, normalized to primitive mantle compositions [18], obtained on whole rock samples by ICP/AES (Analyst: J. Cotten, UBO). (A) Selected patterns from the eastern metamorphic complex, showing both MORB-like and supra-subduction patterns (Unit 2). (B) Selected patterns from the non-metamorphic Upper Cretaceous ophiolitic lavas showing MORB-like patterns, and one intrusive diabase dike, with typical supra-subduction characteristics (Nb, Zr and Ti negative anomalies).
Nouveaux spectres multi-élémentaires normalisés au manteau primitif [18] de roches analysées par ICP/AES (J. Cotten, analyste, UBO) provenant (A) du complexe métamorphique oriental (unité 2 dans le texte) et (B) de l'ophiolite non métamorphique du Crétacé supérieur montrant un profil de MORB et celui d'un dyke de diabase présentant les anomalies négatives en Nb, Zr et Ti caractéristiques des laves des domaines supra-subduction.
2.3 Unité 3 : le complexe volcano-sédimentaire à turbidites du Crétacé supérieur
En contact tectonique avec l'unité précédente, cet ensemble repose en transgression sur les laves de l'ophiolite de Khoy s.s., situées au sud-ouest, et comprend (Fig. 1E) des sédiments flyschoı̈des rythmiques riches en fragments de laves et des coulées de laves basaltiques, andésitiques et ankaramitiques, associées à des niveaux sédimentaires calcaires à Globotruncana, datés du Crétacé supérieur au Paléocène inférieur.
2.4 Unité 4 : l'ophiolite non métamorphique de Khoy s.s., d'âge Crétacé supérieur
Ce puissant complexe ophiolitique (Fig. 1D) se compose d'une séquence mantellique lherzolitique serpentinisée, de petits corps intrusifs de gabbros lités dans les péridotites, le tout surmonté par une épaisse séquence de laves basaltiques sous-marines, essentiellement sous forme de pillow lavas accompagnés de coulées massives, restes de lacs fossiles, sills et dykes basaltiques, brèches hyaloclastiques et abondantes brèches épiclastiques de démantèlement. Les pillows sommitaux sont associés à des niveaux de calcaires pélagiques à Globotruncana du Crétacé supérieur. Aucune trace d'un complexe filonien de diabases n'a pu être observé. Les basaltes ont des affinités géochimiques de type T-MORB, et sont recoupés par des dykes isolés de diabases de nature calco-alcaline « supra-subduction » (Fig. 2B).
2.5 Unité 5 : le complexe métamorphique occidental
Il s'agit d'une série de roches épimétamorphiques, d'âge et d'appartenance inconnus, qui affleurent au sud-ouest de la zone cartographiée. Elles sont rétrocharriées sur des calcaires et conglomérats d'âge Paléocène, eux-mêmes charriés sur la bordure méridionale de l'unité des péridotites ophiolitiques.
3 Datations paléontologiques
Les calcaires pélagiques interstratifiés avec les laves basaltiques sous-marines des unités 3 et 4 décrites ci-dessus ont livré des microfaunes caractéristiques dans 54 localités différentes, essentiellement des associations à Globotruncana du Crétacé supérieur.
Les calcaires liés aux pillow lavas ophiolitiques (unité 4) ont livré des microfaunes d'âge Turonien à Campanien (92 à 72 Ma). Les calcaires liés aux turbidites supra-ophiolitiques (unité 3) ont livré des microfaunes d'âge Santonien (87 à 83 Ma), et ces mêmes turbidites remanient des fragments de calcaires pélagiques dont les microfaunes sont d'âge Santonien à Campanien. Les calcaires roses associés aux pillow lavas supra-ophiolitiques ont des microfaunes d'âge Campanien à Maastrichtien (83 à 65 Ma). Enfin, le membre volcano-sédimentaire coiffant la série supra-ophiolitique comporte des calcaires pélagiques et des radiolarites contenant des microfaunes d'âge Maastrichtien à Paléocène inférieur (72 à 59 Ma).
4 Nouvelles datations par la méthode 40K–40Ar
24 échantillons, localisés sur la Fig. 1C et D, ont été sélectionnés et datés par la méthode 40K–40Ar (Tableau 1) sur minéraux séparés (amphiboles, muscovites, biotites et plagioclases). Ces datations concernent, d'une part, le complexe métamorphique oriental et ses écailles méta-ophiolitiques associées (Fig. 1C), d'autre part les roches plutoniques de l'ophiolite de Khoy non métamorphique (Fig. 1D).
New 40K/40Ar dating on mineral separates from lavas and metamorphic rocks in Khoy area. Numbers (1 to 24) in column 1 refer to sampling locations indicated in Fig. 1 C, D and E
Nouvelles données 40K/40Ar sur minéraux séparés à partir des laves et des roches métamorphiques du secteur de Khoy. Les numéros 1 à 24 de la colonne 1 renvoient à la localisation des échantillons datés dans la Fig. 1 C, D et E
| Figure | Sample | Location | Rock type | Dated fraction | Average age | Age ± error | K2O | 40ArR | 40ArR | Analysis number |
| (Ma) | (wt%) | (10−7 cm3 g−1) | (%) | |||||||
| Upper Cretaceous non metamorphic ophiolite | ||||||||||
| 1 | 00-3KH56 | S. Todan | Porphyric diabase dike in gabbro | Plagioclase | 64.9 ± 3.8 | 0.13 | 2.77 | 35.0 | 5881 | |
| 2 | 00-3KH190 | S. Todan | Isotropic gabbro | Plagioclase | 72.6 ± 5.0 | 0.046 | 1.10 | 19.5 | 5891 | |
| 3 | 01-2KH211 | Qorshanlo | Plagioclase vein in gabbro | Plagioclase | 100.7 ± 6.0 | 0.25 | 8.34 | 31.6 | 5890 | |
| Meta-ophiolitic unit within the Eastern metamorphic complex | ||||||||||
| 4 | 99-KH92 | North Aghbash | Weakly deformed gabbro | Amphibole | 80.2 ± 4.6 | 82.4 ± 4.6 | 0.106 | 2.88 | 42.5 | 5894 |
| 77.9 ± 4.6 | 0.106 | 2.72 | 31.1 | 6000 | ||||||
| 5 | 99-KH242 | West Ravand | Metagabbro | Amphibole | 112.9 ± 8.6 | 0.175 | 6.57 | 69.2 | 5632 | |
| 6 | 99-KH134 | North Aghbash | Metagabbro | Amphibole | 116.5 ± 6.0 | 0.215 | 8.34 | 67.2 | 5664 | |
| 7 | 00-3-KH14 | South Aghbash | Metagabbro | Brown amphibole | 134.7 ± 7.1 | 0.245 | 11.05 | 53.0 | 5665 | |
| 8 | 00-3-KH9 | East Ajidgah | Metagabbro | Amphibole | 155.6 ± 11.9 | 0.165 | 8.64 | 69.8 | 5630 | |
| 9 | 99-KH102 | North Aghbash | Metagabbro | Amphibole | 160.4 ± 12.7 | 160.8 ± 12.7 | 0.052 | 2.82 | 40.4 | 5631 |
| 160.0 ± 12.4 | 0.052 | 2.81 | 47.4 | 5655 | ||||||
| 10 | 99-KH145 | N. Aghbash | Metagabbro | Amphibole | 160.7 ± 12.9 | 0.057 | 3.09 | 35.9 | 5633 | |
| 11 | 99-KH359a | North Khoy | Amphibole pegmatitic gabbro | Amphibole | 154.9 ± 11.8 | 0.17 | 8.86 | 66.9 | 5676 | |
| 12 | 99-KH291b | North Khoy | Amphibole pegmatitic gabbro | Amphibole | 194.8 ± 10.1 | 192.3 ± 2.9 | 0.492 | 32.2 | 91.0 | 5646 |
| 197.3 ± 10.1 | 0.492 | 33.1 | 90.8 | 5656 | ||||||
| Eastern metamorphic complex | ||||||||||
| 13 | 00-4KH78 | Qorol-Ajai | Gneissic granite | Muscovite and biotite | 67.5 ± 1.6 | 7.06 | 156.6 | 76.8 | 5990 | |
| 14 | 00-4KH69 | Qorol-Ajai | Gneissic dike | Muscovite | 75.3 ± 1.8 | 10.37 | 257.0 | 73.9 | 5874 | |
| 15 | 99-KH357 | Ajidgah | Quartz, feldspar, muscovite vein | Muscovite | 93.5 ± 1.5 | 8.9 | 275.3 | 78.4 | 5303 | |
| 16 | 99-KH314 | North Dizaj | Micaschist | Muscovite | 69.4 ± 1.6 | 6.56 | 149.6 | 82.6 | 5991 | |
| 17 | 00-3KH7 | Ajidgah | Micaschist | Muscovite | 81.2 ± 1.2 | 7.07 | 189.2 | 92.5 | 5644 | |
| 18 | 99-KH-358 | Ajidgah | Amphibolite | Amphibole | 102.1 ± 5.4 | 102.3 ± 5.3 | 0.27 | 9.16 | 71.8 | 5645 |
| 103.9 ± 5.4 | 0.27 | 9.31 | 67.5 | 5675 | ||||||
| 100.1 ± 5.4 | 0.27 | 8.96 | 49.2 | 5323 | ||||||
| 19 | 99-KH353 | Ghekh yashar | Fine-grained amphibolite | Amphibole | 121.2 ± 6.2 | 0.46 | 18.6 | 75.5 | 5648 | |
| 20 | 99-KH191 | Ghekh yashar | Amphibolite | Amphibole | 151.0 ± 11.5 | 0.195 | 9.90 | 63.5 | 5663 | |
| 21 | 00-4KH71 | Hydarabade | Micaschist | Muscovite and biotite | 146.3 ± 3.4 | 8.03 | 394.4 | 93.7 | 5892 | |
| 22 | 01-4KH81 | Hydarabade | Fine-grained amphibolite | Amphibole | 189.3 ± 10.7 | 182.3 ± 4.3 | 0.48 | 29.7 | 74.8 | 5982 |
| 196.3 ± 10.7 | 0.48 | 32.1 | 75.7 | 5998 | ||||||
| 23 | 01-4KH76 | Hydarabade | Gneiss | Muscovite | 160.5 ± 3.7 | 8.7 | 470.7 | 84.6 | 5981 | |
| 24 | 01-4KH97 | Hydarabade | Quartz and muscovite vein | Muscovite | 181.8 ± 4.2 | 9.78 | 603.2 | 93.3 | 5865 |
Les nombreux résultats sur le complexe métamorphique oriental montrent un large étalement des âges apparents obtenus sur lots de minéraux séparés et suggèrent une longue histoire métamorphique polyphasée, marquée par quatre périodes majeures. (1) Jurassique inférieur (197–181 Ma) : les âges apparents les plus anciens ont été trouvés à la fois dans les métagabbros ophiolitiques et dans les amphibolites à grains fins de la sous-unité m1 (Fig. 1C). Les pegmatites à quartz-muscovite recoupant ces amphibolites ont un âge apparent de 181±4,2 Ma. L'âge magmatique primaire des métagabbros ophiolitiques devrait donc être antérieur à ce métamorphisme (Trias supérieur ?). (2) Jurassique moyen (160–155 Ma) : ces âges ont été trouvés également dans les métagabbros ophiolitiques et dans les micaschistes à deux micas. (3) Crétacé inférieur (121–112 Ma) : on trouve ces âges essentiellement dans des zones de cisaillement ductile affectant les roches métamorphiques, et les métagabbros ophiolitiques. (4) Crétacé supérieur (93 à 69 Ma) : ces âges sont montrés surtout par les micaschistes et les intrusions granitiques à pegmatitiques qui les recoupent et un gabbro à amphibole faiblement métamorphique (à 82,4±4,6 Ma).
Deux datations effectuées sur les plagioclases des gabbros lités de l'ophiolite non métamorphique de Khoy ont confirmé leur âge Crétacé supérieur (100 à 72 Ma), âge cohérent avec celui des pillow lavas susjacents (Turonien à Campanien, soit 92 à 72 Ma). Une diabase porphyrique, recoupant ces gabbros, a donné un âge apparent de 65 Ma.
5 Conclusions
Ces nouvelles données nous permettent de conclure sur trois points.
- (1) Il y a de toute évidence deux complexes ophiolitiques d'âges différents dans la région de Khoy.
- (2) Le complexe métamorphique oriental, dont l'âge du métamorphisme va du Jurassique inférieur au Crétacé supérieur, n'est pas une semelle métamorphique infra-ophiolitique, comme l'ont suggéré Hassanipak et Ghazi [5], mais représente très vraisemblablement un complexe métamorphique de subduction, affectant essentiellement une lithosphère océanique néo-téthysienne, subductée sous l'Iran central pendant tout le Mésozoı̈que [13], et les sédiments associés.
- (3) L'ophiolite non métamorphique de Khoy, d'âge Crétacé supérieur, présente toutes les caractéristiques d'une dorsale océanique « lente », de type dorsale médio-Atlantique (manteau résiduel lherzolitique indiquant un faible taux de fusion partielle, plutons gabbroı̈ques intrusifs dans le manteau supérieur, absence de complexe filonien de diabases, laves très porphyriques) [6].
1 Introduction
The ophiolite of Khoy is located in the northwestern corner of the Iranian Azerbaijan province, extending till the Turkish border (Fig. 1A). The geology of the Khoy area is still poorly known, and there are very few reports about its geological features. After a very general description by Khamineni and Mortimer [7], this area was mapped at various scales by the Geological Survey of Iran (GSI): the sheet of Khoy at 1:250 000 was published first [4], followed by two sheets at 1:100 000 scale, the sheet of Khoy [11], and the adjacent sheet of Dizaj [1], with many useful geological information. On these sheets, the GSI geologists identified clearly a complete ophiolite complex of Upper Cretaceous age, including serpentinites and peridotites, gabbros, diabases and basaltic pillow lavas.
More recently, Hassanipak and Ghazi [5] gave a first dataset on the petrology and geochemistry of the Khoy ophiolite. In this paper, the authors distinguished, in the ophiolitic volcanic sequence, a lower pillow basalt unit, displaying REE patterns intermediary between E-MORB and N-MORB profiles, and an upper massive basalt unit with E-MORB-type REE patterns. According to them, the REE patterns for the gabbros and diorites indicate that the crustal rock suite was derived by fractional crystallization from a common basaltic melt, generated by 20–25% partial melting of a simple lherzolite source. In their conclusion, the authors suggest that the “Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g., Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band e Zyarat, Fig. 1A), and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent”. Unfortunately, their description of the geology of the Khoy area is extremely schematic and often erroneous, and the analysed samples are not located.
Ghazi et al. [3] proposed the existence beneath the ophiolite of a basal metamorphic zone, displaying an inverse thermal gradient, ranging from amphibolite to the greenschist facies. Based on 40Ar–39Ar dating, these authors present two 40Ar–39Ar plateau ages of 158.6±1.4 Ma and 154.9±1.0 Ma for hornblende gabbros, and conclude that the plutonic rocks of the Khoy ophiolite were formed during Late Jurassic. They present also four 40Ar–39Ar plateau ages of about 104 to 110 Ma for hornblendes from amphibolites within the basal metamorphic zone, concluding to a tectonic emplacement of Mid-Albian age for the ophiolite complex. As the pelagic limestones interbedded with the ophiolitic pillow lavas give microfaunas of somewhat younger ages (Upper Albian to Lower Cenomanian, around 100 Ma), the authors have some difficulties to explain how the plutonic gabbros and the volcanic pillow lavas in the same ophiolite complex show a difference in age of more than 50 Ma, and how the pillow lavas can be younger than the metamorphic sole, supposed to mark the beginning of the detachment and obduction process.
We present here the results of new field and laboratory studies, leading to the distinction of two ophiolitic complexes in the Khoy area, which resolves many apparent contradictions.
2 Geological description of the Khoy area
The Khoy area exposes five main geological units, grossly forming NW–SE stripes (Fig. 1B). From northeast to southwest: (1) the southern continental margin of the Central Iranian Block; (2) the eastern metamorphic unit, including an old meta-ophiolitic complex; (3) the supra-ophiolitic turbiditic and volcanic-sedimentary zone; (4) the non-metamorphic Upper Cretaceous Khoy ophiolite; (5) the western metamorphic unit.
2.1 The southern continental margin
The southern continental margin of the Central Iranian Block is exposed to the NNE of Khoy, with basal Precambrian to Cambrian formations, overlain in disconformity by Permian sediments. These Palaeozoic formations are overlain tectonically by Jurassic and Lower Cretaceous sediments, and finally by Miocene to Pliocene–Quaternary deposits. These formations belong to the Central Iran Zone units, as defined by Stöcklin [16,17].
2.2 The eastern metamorphic unit
The eastern metamorphic unit contains four main sub-units (labelled m1, m2, m3 and m4). Sub-unit m1 consists of micaschists, amphibolites (with typical supra-subduction trace element patterns, Fig. 2A) and gneisses, crosscut by quartz-muscovite veins at the base (Fig. 1C). Gneisses containing rotated quartz and feldspar megacrysts are more and more abundant going up in the section.
Sub-unit m2 consists of fine-grained amphibolites (showing MORB-like trace element patterns, Fig. 2A), greenschists, metavolcanics, locally interbedded with metaquartzites and leptynitic gneisses. It shows tight isoclinal folds, corresponding to three successive deformations marked by foliations S1, S2, and locally S3. This sub-unit includes huge tectonic slices of meta-ophiolitic plutonic rocks, composed of well-foliated lherzolitic and harzburgitic tectonites, as well as associated ultramafic meta-cumulates. These ultramafic rocks are crosscut by abundant sills and dikes of meta-gabbros (amphibolites), whose flat REE patterns suggest a MORB-like oceanic origin (Fig. 2A).
Both sub-unit m3 (greenschists, metavolcanic schists, meta-ankaramites), and sub-unit m4 (greenschists), show low-grade metamorphism and were tectonically emplaced with the other sub-units (Fig. 1C).
The m2 sub-unit is intruded by the gneissic-granitic intrusion of Qorol-Ajai, and its related quartz–feldspar–muscovite-bearing dikes and veins, which crosscut both the m2 sub-unit and the gneisses of the m1 sub-unit.
2.3 The supra-ophiolitic turbiditic and volcanic-sedimentary unit
The supra-ophiolitic turbiditic and volcanic-sedimentary unit is exposed along a wide strip developed to the southwest of the eastern metamorphic complex, with systematic tectonic contacts (Fig. 1E). The meta-ophiolite is thrust over this unit in the north of the studied area. This unit includes turbidites at the base, with coarse-grained breccias lenses containing reworked limestone pebbles, and also angular to rounded basaltic fragments of various origins. Some of these show flat REE patterns, whereas others show enrichment in light REE, with a negative anomaly of Nb typical of supra-subduction lavas. Higher in the sequence, volcanic breccias and pillow basalts show also flat REE patterns, with sometimes a weak Nb negative anomaly (Fig. 2B).
2.4 The non-metamorphic ophiolite complex of Khoy
The non-metamorphic ophiolite complex of Khoy is composed of sepentinized peridotites, layered gabbros, isolated diabasic dikes, and a huge volcanic pile made of alternatively phyric and aphyric pillow basalts, massive sheet flows and interbedded hyaloclastic breccias and tuffs (Fig. 1D). Huge epiclastic slope breccias with abundant carbonate matrix are developed on top of this volcanic pile. Diabase sills and dikes, and some picrite/wehrlite dikes crosscut the volcanics. The REE patterns of the pillow basalts and picrites are flat, without any visible negative Nb or Ti anomalies, and show a slightly positive slope toward LREE (Fig. 2B). These basalts are typical E-MORB type basalts, as already recognized by Hassanipak and Ghazi [5]. On the other hand, isolated diabase dikes are well differentiated and show clear Nb and Ti negative anomalies, analogous to those observed in pillow basalt reworked fragments from the volcanic-sedimentary unit.
We did not find any trace of a diabase sheeted dike complex, contrarily to previous descriptions [5], and we did not observe the effects of any regional metamorphism, except those due to early oceanic hydrothermal circulations. We refute the idea of a ‘massive lava unit’ lying over a ‘pillow lava unit’, as claimed by these authors [5], but found that the massive basaltic flows (less than 10% of the whole extrusive sequence) are interbedded within the pillow lava pile at all levels, as is usual on modern oceanic ridges [6].
2.5 The western metamorphic unit
The western metamorphic unit consists of low-grade metamorphic rocks (greenschists), whose age and origin are presently not known. They are thrust back over limestones and conglomerates of Palaeocene age, themselves thrust over the southern margin of the Upper Cretaceous ophiolitic unit.
This western metamorphic unit may represent an eastern extension of the Pütürge–Bitlis metamorphic zone of eastern Turkey [9,14], but this remains very speculative.
3 Palaeontological dating
The sediments interbedded with the submarine lavas in several non-metamorphic units described here contain by places abundant microfauna. These microfossils, collected in 54 different localities by one of us (MKJ), were determined by the Palaeontological Group of the GSI, and permitted the dating of their host-sediments. The units concerned by such palaeontological dating are detailed in Fig. 1 D and E. Hereafter is the list of the diagnostic microfossils found in these units, with the corresponding inferred ages.
3.1 Ophiolitic pillow basalt sequence
The thin limestone beds and lenses between the pillow basalts gave abundant microfauna of Turonian–Santonian–Campanian age: Globotruncana lapparenti tricarinata, Globotruncana lapparenti lapparenti, Globotruncana renzi, Globotruncana concarata, Globotruncana arca, Globotruncana gansseri, Globotruncana falsostuarti, Globotruncana lapparenti, Globotruncana ventricosa, Globotruncana conica, Globotruncana helvetica, Globotruncana fornicata, Globotruncana sp., Heterohelix sp., Radiolarians. These data confirm the Upper Cretaceous age of the pillow lava pile.
3.2 The supra-ophiolitic avalanche debris and slope breccias unit
The supra-ophiolitic avalanche debris and slope breccias unit includes pelagic limestones in the matrix of the breccias, which contain Globotruncana sp. and Radiolarians of Upper Cretaceous age.
3.3 The supra-ophiolitic turbiditic unit
The supra-ophiolitic turbiditic unit contains reworked and autochthonous limestones. The reworked limestones contain microfauna of Santonian–Campanian age (and even Campanian–Maastrichtian in one sample), with the following fossils: Globotruncana lapparenti tricarinata, Globotruncana lapparenti lapparenti, Globotruncana arca, Globotruncana lapparenti, Globotruncana bulloides, Globotruncana gansseri, Globotruncana confusca, Globotruncana stratiformis, Hedbergella sp., Heterohelix sp., Radiolarians. The autochthonous and grey limestone beds in the turbidites, however, contain faunas of Santonian age with the following fossils: Globotruncana sp., Heterohelix sp., Heterohelix reossi.
3.4 Supra-ophiolitic pillow basalt unit
This unit contains some interbedded cherts, radiolarites and pink limestones with the following microfaunas, supposed to be of Campanian–Maastrichtian age: Globotruncana calcarata, Globotruncana aff. falsostuarti, Globotruncana sp., Hedbergella sp., Lenticulina, Radiolarians.
3.5 Supra-ophiolitic volcanic-sedimentary sequence
This upper member of the supra-ophiolite formations contains some pinkish limestones and radiolarites with microfauna of Upper Cretaceous–Lower Palaeocene age, specially in the pinkish limestones, in particular: Globotruncana cf. stuarti, Globotruncana catarata, Globotruncana stuartiformis, Globotruncana arca, Globigerina sp., Heterohelix sp., Cibicides sp., Lagena sp., Rotalia sp., Miliolids, Radiolarians.
There are also well exposed Upper Palaeocene to Oligocene limestones, resting tectonically over the ophiolitic pillow basalts, and also as many tectonic ‘exotic’ blocks over the supra-ophiolitic volcanic-sedimentary unit. The following microfauna were found in the southern part of the Khoy area: Assilina sp., Discocyclina sp., Operculina sp., Flosculina patticilat., Alveolina sp., Alveolina (Floculina) sp., Opertorbitolites sp., and other microfouna found in the northern area: Volvulina sp., Cympolia cf. hetrachi, Broechella sp., Rotalia viennotti sp., Heterostegina sp.,Operculina sp., Asterigerina sp., Rotalia sp., Amphistegina sp., Victoriella sp., Peneroplis sp., Miliolids, Bryozoans, Subterranophyllum thomas., Lithothamnium sp.
4 New isotopic 40K/40Ar ages and interpretations
Twenty-four rock samples were selected from different geological units, and mineral separates of amphiboles, muscovites, biotites, and feldspars were dated by 40K/40Ar method (Table 1). Ages are calculated using the recommended constants in [15], and errors are quoted after [8].
4.1 40K/40Ar dating of the eastern metamorphic unit and its associated meta-ophiolite complex
As shown in Table 1, the separated amphiboles, biotites and muscovites in this unit display a wide range of apparent ages, suggesting a long polyphased metamorphic history. We propose to distinguish four groups of chronological events. These ages are interpreted as reflecting the time when the different minerals passed below their respective isotopic temperature closures [19].
4.1.1 Lower Jurassic group (197–181 Ma)
The oldest isotopic ages are found in two rock types: (a) amphibole-rich pegmatitic metagabbros from the meta-ophiolite association (Fig. 1B and C). In these metagabbros, showing spectacular ductile deformations, there are locally (site 12, Fig. 1C) small blocks of weakly deformed gabbros with Lower Jurassic apparent ages (197.3±10.1 to 192.3±2.9 Ma). These ages suggest that the primary cooling age of these gabbros was somewhat older, perhaps Upper Triassic. (b) 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.
4.1.2 Middle Jurassic group (160–155 Ma)
The second group of isotopic ages is found in the following metamorphic facies.
- (a) For 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). Under microscope, they show amphiboles that replaced the frame of pyroxenes; there are also abundant recrystallized plagioclases with abundant triple junctions, indicating ductile deformations in shear fault zones [10].
- (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. They are exposed with the fine-grained amphibolites [1]. Under microscope, the gneisses of Hydarabad show rotated feldspar porphyroclasts and twinned coarse grained quartz crystals with mortar textures, embedded by the foliation. These macroscopic and microscopic features indicate an older metamorphic event, eventually older than Middle Jurassic.
4.1.3 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 ages 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 ages obtained from the amphiboles of meta-ophiolitic gabbros, at 116.5±6.0 Ma (north Ajidgah) and 112.9±8.6 Ma (west Ravand). This metamorphism of Lower Cretaceous age is associated with thick ductile shear zones oriented NW–SE to north–south, affecting both the metamorphic complex and the meta-ophiolites, with evidences of incipient partial melting.
4.1.4 Upper Cretaceous group
The muscovites of the micaschists from Ajidgah gave an age of 81.2±1.2 Ma. The pure muscovites separated from micaschists north of Dizaj give an age of 69.4±1.6 Ma (Maastrichtian), which can be related to tectonic events that caused the S3 deformation appearing locally in the metamorphic complex. In the vicinity of Qorol-Ajai village, a gneissic granite intrusion crosscuts the metamorphic rocks, extending to the north, out 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 93.5±1.5 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–gneiss yield an age of 67.5±1.6 Ma.
Finally, the separated fine-grained amphiboles from a weakly deformed gabbro in the meta-ophiolites gave a Late Cretaceous age of 80.2±4.6 Ma (average of two measurements, Table 1).
4.2 40K/40Ar dating of the non-metamorphic ophiolite of Khoy
The non-metamorphic ophiolite of Khoy is much more difficult to date by the K/Ar method, because of the absence of amphiboles and the very low primary potassium contents of the separated plagioclases. Two isotopic ages were obtained on plagioclases from the layered gabbros (location in Fig. 1D). The first one is at 100.7±6.0 Ma, from the feldspars of plagioclase-rich veins in the layered gabbros close to Qorshanlo village. These veins formed parallel to the magmatic layering, or cutting it at low angle. This value may indicate the probable cooling age of the layered gabbros. The second value obtained is 72.6±5.0 Ma from an isotropic gabbro vein, south of Todan village, crosscutting the layered gabbros. These isotopic ages are compatible with the palaeontological data obtained on the ophiolitic pillow basalts (Campanian to Santonian, that is, 92 to 72 Ma). A third isotopic age close to the Upper Cretaceous–Lower Palaeocene boundary, at 64.9±3.8 Ma, was obtained from the plagioclase phenocrysts of a porphyritic diabasic dike crosscutting the gabbros.
5 Discussion and conclusions
The new data presented in this paper concerning the ophiolites of the Khoy area may be discussed through the following points:
5.1 There are not one, but two ophiolite complexes outcropping in the Khoy area
Several contradictions, particularly in the 40K–40Ar dating compared to the palaeontological dating, disappear if we consider that there are two distinct ophiolite complexes in the Khoy area:
5.1.1 An older meta-ophiolitic complex
The oldest ophiolites form huge tectonic slices within the eastern metamorphic complex. They are themselves totally recrystallized in the amphibolite facies, and their oldest metamorphic amphiboles give 40K–40Ar apparent ages ranging from Lower to Middle Jurassic (from about 197 Ma to about 160 Ma).
Ghazi et al. [3] obtained two 40Ar–39Ar plateau ages on the amphiboles of two metagabbros belonging obviously to this complex, with apparent ages of 158.6±1.4 and 154.9±1.0 Ma, respectively. They deduced erroneously that “rocks from this ophiolite were formed during the Late Jurassic”. Clearly these ages are dating a metamorphic event, not a magmatic accretion stage. Besides, our data show that there is not one metamorphic event, but at least four metamorphic episodes: Lower Jurassic, Middle Jurassic, Lower Cretaceous and Upper Cretaceous, respectively. The first three episodes affect both the meta-ophiolites and their metamorphic host-rocks, suggesting a common metamorphic history. The youngest episode (Upper Cretaceous) affects mainly weakly metamorphosed granitic and gabbroic plutons and dykes, intrusive in the metamorphic series. We suggest therefore that the eastern metamorphic complex consists of several slab fragments of various Mesozoic ages, piled up and tectonically stacked in a subduction accretion complex, developed beneath the Central Iran Block margin. The primary magmatic age of the oldest meta-ophiolites should be somewhat older than our oldest metamorphic age (197 Ma), that is, at least of Upper Triassic age.
Subduction began after the collision of the Central Iran Block with Eurasia during Middle Upper Triassic [13], trapping and stacking the early Tethyan oceanic lithosphere. Several slab fragments of various Mesozoic ages are probably piled up as tectonic slices in the metamorphic complex. We refute the idea that this metamorphic complex may represent an infra-ophiolitic metamorphic sole, as suggested in [5]. As attested by our geochronological data, the ophiolites and associated sedimentary series were metamorphized together during various metamorphic episodes.
5.1.2 A younger non-metamorphic ophiolite
The youngest ophiolites outcrop in the western part of the studied area. They do not show any trace of regional metamorphism. The pillows still have their delicate glassy crust, and the layered gabbros are amphibole-free, displaying numerous cumulate structures and textures. Their Upper Cretaceous age is attested (1) by the abundant microfauna found in the pelagic limestones interbedded within the extrusive sequence; (2) by 40K–40Ar data carried on the plagioclases of some leucogabbroic veins in the layered gabbros. This complex has thus the same age as other well-known ophiolites of western Iran, Turkey and Oman, belonging to the peri-arabic ‘ophiolitic crescent’ [12]. All these ophiolites, devoid of regional metamorphism, were obducted during Late Cretaceous over the southern continental margin of the Neo-Tethys ocean (Arabian-African platform), or over ‘Gondwanian’ continental fragments, detached from the Gondwana block during Permian–Triassic times.
5.2 The non-metamorphic ophiolite of Khoy represents a piece of a slow-spreading oceanic ridge from the Neo-Tethys Ocean
The stratigraphy and structural organization of the non-metamorphic ophiolite of Khoy is typical of a slow-spreading ridge environment. (1) The lower part of the ophiolite, exposed to the southwest, consists of a lherzolitic residual mantle sequence, containing typically small intrusive bodies of layered gabbros. (2) The extrusive sequence rests directly over the serpentinized lherzolites and associated gabbros. No trace of any sheeted diabase dike complex was observed. This is an important divergence we have with Hassanipak and Ghazi [5], who described a diabase sheeted dike complex “developed in several locations”, and show important surfaces covered by “diabases” on their very schematic map. (3) The extrusive sequence is mainly composed of pillow lava flows (more than 90%), and a few massive lava flows. These oceanic basalts are frequently highly phyric, with concentration of plagioclase phenocrysts.
These features indicate rather low partial melting rates (lherzolitic residue) typical of slow-spreading oceanic ridges (Mid-Atlantic Ridge, Southwest Indian Ridge). The absence of a diabase sheeted dike complex, the scarcity of fluid sheet flows or lava lakes, and the frequency of phyric pillow lavas are also well-known characteristics of slow-spreading oceanic ridges [6].
Acknowledgements
These studies were 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). J.-C. Philippet and J. Cotten, in UBO, have greatly contributed with rock and mineral dating and ICP geochemical analyses. We thank M. Partoazar, Dr. F. Mohtat, Q. Asgari, Mrs. Allahmadadi, and F. Vakili, members of the Palaeontological Group of the Geological Survey of Iran who made the numerous palaeontological determinations for more than 50 sites.