Petrology and Tectonic Setting of Lakhshak Granodiorite, NW of Zahedan, Iran

The Lakhshak granitoid pluton which is located at 10 km northwest of Zahedan, has intruded into the Eocene flysch sediments with an elliptical shape and NW-SE direction. This pluton after emplacement has been cut by numerous dykes with NE-SW trend. These dykes comprised about 20-30% of the Pluton with various compositions, ranging from granodiorite to monzodiorite in composition. The Lakhshak plutonic rocks are mainly metaluminous, calc-alkaline and belong to I type granites based on the P2O5 and Th content versus SiO2. Regarding TiO2 content these rocks resemble the continental margin granites. The MgO, Na2O, Ni, Cr content as well as Mg# and depletion in Y, these plutonic rocks are similar to the adakite, a rock type produced by partial melting of young oceanic crust. The low Ba/La content of the studied samples may suggest that subducted slab suffered dehydration prior to partial melting. These rocks are enriched in LIL, LREE, however, they are depleted in HREE and Y. In addition, they show negative anomalies of Nb, Ta, P and Ti, and positive anomaly of Pb. The negative anomalies of Nb and Ta may indicate the effect of mantle wedge metasomatism by oceanic crust. The positive anomaly of Pb may demonstrate continental crust assimilation by magma associated with mantle metasomatism. It seems that Lakshak pluton has been formed by subduction of Sistan young oceanic crust under the Afghan Block. Moreover, the low content of HREE and Y besides a decreased ratio of Yb versus SiO2, Y<15.13, Yb<1.2 and existence of amphibolite enxenoliths in these rocks may suggest partial melting of amphibolites. The latter is formed during the oceanic crust subduction in depth more than 35 km.