F Bakhshizad; Gh Ghorbani
Abstract
The Zanjan-Takab metamorphic complex includes para- and orthogneisses, amphibolites, various types of schists and migmatites along with weakly metamorphosed, young magmatic dikes and lenses. In this study, we are focusing on the Zanjan-Takab metamorphic rocks from three regions including: 1- Almalu-Ghazi ...
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The Zanjan-Takab metamorphic complex includes para- and orthogneisses, amphibolites, various types of schists and migmatites along with weakly metamorphosed, young magmatic dikes and lenses. In this study, we are focusing on the Zanjan-Takab metamorphic rocks from three regions including: 1- Almalu-Ghazi Kandi-Alam Kandi; 2- Qare Naz-Qozlu and 3- Zaki Kandi-Barut Aghasi. Orthogneisses and migmatite leucosomes from these regions show enrichment in light REEs relative to heavy REEs and are characterized by depletion in Nb-Ta. The Almalu-Ghazi Kandi-Alam Kandi orthogneiss zircons show U-Pb ages of 491-516 Ma, but with older inherited cores. Meta-tonalite zircons from this region yield magmatic ages of 24-26 Ma. Migmatite leucosomes, paragneisses and gneissic amphibolite from Qare Naz-Qozlu contains zircons with partial melting evidences at 25-28 Ma. Nd model ages of migmatite leucosomes vary between 466 and 1629, but most of them show Ordovician and late Neoproterozoic Nd model ages (TDM). The epsilon Hf (t) values of zircon rims from migmatites are positive and their Hf model ages vary between 400 and 700 Ma. According to this study, it seems that in addition to the presence of old metamorphic rocks (~500 Ma) in the Zanjan-Takab region, there are weakly (to intensely) metamorphosed, but young (38-24 Ma) magmatic rocks in this area. Furthermore, although the Cadomian magmatism in the Zanjan-Takab region is conspicuous, but this magmatism is younger relative to other Cadomian outcrops of Iran. It seems that the exhumation of gneissic rocks and hence migmatization in this region can be related to core complex formation due to the extensional phases in the Iranian plate, resulted from roll-back of Neotethyan subducted slab beneath Iran.
M. Moazzen; R. Hajialioghli
Abstract
Intrusion of the Kalaybar nepheline syenite and nepheline gabbro into the Cretaceous pelitic and calcareous rocks during Eocene-Oligocene has caused the development of a thermal aureole up to 1 km thick. The contact between igneous and country rocks is sharp. The ...
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Intrusion of the Kalaybar nepheline syenite and nepheline gabbro into the Cretaceous pelitic and calcareous rocks during Eocene-Oligocene has caused the development of a thermal aureole up to 1 km thick. The contact between igneous and country rocks is sharp. The protholite rocks in the study area had not been metamorphsed prior to contact metamorphism. Pelitic and calcareous rocks in the northern and eastern parts of the aureole and basic rocks in the southern part of the aureole were thermally metamorphsed and formed different types of hornfelses. High-grade metamorphic rocks with chemically suitable compositions were melted adjacent to the contact (within 100m from the contact) due to the heat from the pluton. Migmatites with small-scale leucosomes are produced. Scale of partial melting and volume of produced melt are very small. Main minerals in the light coloured parts of the migmatites (leucosomes) include quartz and K-feldspar with an igneous texture specially euhedral to subhedral texture of K-feldspar, graphic texture of quartz-K-feldspar and interstitial texture of quartz. Textural differences between light leucosomes and dark mesosomes, mineralogical composition of the leucosomes, existence of igneous textures within the leucosomes and restriction of the leucosome formation to the pelitic rocks all are distinct evidence for occurrence of partial melting in the Kalaybar aureole. The liable reactions for melting include fluid-present reactions and fluid-absent reactions. High-grade metamorphic assemblages such as orthopyroxene-bearing assemblages have been accomplished by dehydration of hydrous minerals such as biotite via fluid-absent melting reactions. Mineralogical compositions of leucosomes resemble leucogranites to granites. This indicates crystallisation of the leucosomes from a silicate melt.
