Occurrence of Ti-minerals in Alvand Leucogranitoids
Leucogranitoids occur in the form of veins and thin scattered masses with mafic to intermediate rocks along faults present in south and south-west Hamedan within the main granitoids of the Alvand batholith.
The Alvand leucogranitoids include Leuco-quartzdiorites, trondhjemite, leuco-granodiorites and leuco-monzogranite. They do not show any genetic and relationship with the main Alvand batholith. On view of the geochemical data, these rocks could not have been formed by partial melting of the pelitic rock.
Ti-minerals are the dominant coloured minerals occurring mostly as sphene or rutile or sphene+rutile.
In the primary melt of trondhjmitic composition, sphene floats upward and its ascent is facilitated by hydraulic fractures. Along with the injection of trondhjemite melt in a shear stress zone, oriented lenses of sphene crystals are developed. Along with the progress of crystallization and with the release of pressure sphene decomposition and due to the lack of conditions necessary for the development of minerals which concentrate Ti (e.g. biotite, hornblende and amphiboles) as well as due to the high content of calcium in the leucogranitoids, large crystals of sphene (sometimes along with rutile) persist up to the pegmatitic stage. The presence of quartz veins containing sphene and rutile support this sequence of events.