Mineralization and Alteration Processes in Anatectic Granitoids in Mah-Neshan Area and their Geodynamic Problems (Zanjan Province-NW Iran)

The Mahneshan sheet area lies in Zanjan Province in the northwest of Iran. From the morphotectonic point of view, it is generally zoned in a number of parallel northwest – southeast trending mountain ranges and their intervening valley plains. From the northeast to southwest, the subzones are as follows: - The Andabad - Moshampa basin of thick sequence of Lower Oligocene detrital sediments and hummocky Neogene red-green beds of shallow and evaporating environments (subzone No.1). - The Habash - Moghanlu mountains of longitudinal imbricate subzone resulted from the stacking of many thrust sheets verging to northeast and subsequently exposed the Precambrian (only Kahar FM.) and Phanerozoic rock units (subzone No.2).- The Duzkand - Mahneshan- Yengejeh plain of relatively steppe characters diagonally placed across the sheet area (subzone No.3).- The Ebrahimabad - Inche-Almalu mountain that on the northwest side formed exclusively by Precambrian high- grade metamorphic rock-units and few outcrops of Kahar formation. (Subzone No.4). - The Dandi - Anguran plain of QPl fluvial - colluvial sediments dissected intensely by affluence (subzone No.5).The lithostratigraphic settings and petrological characteristics of rock sequences in two main upland subzones (No.2 & No.4) reflect two sedimentary rift basins and platform regimes for their initial protolith creations.The initial sediments and related volcanic rocks in subzone No. 4 have been intensely transformed by metamorphism to produce gneisses, amphibolites, green schists and marbles, whereas in subzone No. 2 slates, phyllites and semischists developed. It should be noted that the geothermal gradient and metamorphic intensity from rift basin (subzone No. 4) towards platform environment (subzone No. 2) are gradually decreased. Furthermore, ultrametamorphism had occurred and generated the anatectic S-type granitoids throughout the rift basin (subzone No. 4).Some of these anatectic granitoids (such as Moghanlu graintoid) were emplaced as powerful diapiric intrusives very later than their magmatic crystallization and produced brecciated halos instead of metamorphic aureoles around the granitoid masses. Such brecciated halos play the role of suitable channel ways for hydrothermal ore-fluid migrations and development of pervasive alterations alongside the related ore-mineralization. For example, potassic, moderate to advanced argillic alterations and their associated epithermal Sb-mineralization around the Moghanlu S-type granitoid are described in text.