PETROGENESIS, PALEOTECTONIC SETTING, METALLOGENY OF MASHHAD PALEO-TETHYS GRANITOIDS, IRAN

Our studies in northeastern Iran indicate that Paleo-Tethys opened in Silurian time and subduction under the Turan plate started in late Devonian. The Turan plate was later obducted over the Iran Plate as allochthonous ophiolite sheets, dated by Ar-Ar at 281.4 and 277.4 Ma. Meta-ophiolite and meta-flysch remnants were intruded by granitoids during three episodes, Triassic to Cretaceous. The Vakilabad early Triassic tonalite-granodiorite is meta- to moderately peraluminous, S- to I-type sub-alkaline calcic. The Sangbast late Triassic feldspar monzogranite porphyry is K-rich calc-alkaline. The Khajehmourad Jurassic biotite-muscovite leucogranite and late pegmatite dikes are highly felsic peraluminous S-type granitoids, believed to be syn-collisional granite.

The total REE content of the tonalite is 101 -136 ppm, feldspar monzogranite 221-238, and leucogranite 130-170. (La/Yb)N = 37-124 for leucogranite, and 7- 22 for tonalite-granodiorite. The (Eu/Eu* = 0.55 to 1.1).

Tonalite magma originated from a mafic source, feldspar monzogranite from a deeper source, was contaminated in the crust, and leucogranite from a psammite source. High concentrations of Ba (850 - 1100 ppm) and MgO, and higher K2O/(CaO+Na2O) ratios in leucogranite indicate that the source rock was enriched in K-feldspar, muscovite, and biotite and that all K-feldspar, muscovite and some biotite were melted. The Sn, W, and Au contents of all intrusive rocks are low. For all intrusions, Au = 10 - 15 ppb, Ag < 40 ppb, W = 1.5 - 5.4 ppm, Mo = 1 - 2 ppm, Sn 1 - 15 ppm, and F = 700 - 800 ppm.

Since the Mashhad granitoids are S-type, but contain no Sn, W or Au, a comparison was made of major, trace and rare earth data for granitoids associated with Sn worldwide, including Australia, China, Brazil and Thailand. The latter were found to have Sm/Eu 10 - 100 ppm, whereas this ratio is <10 for all the Mashhad granitoids. This ratio should be useful for exploration.