Year

2000

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

The Taknar and Sabzevar Zones in the northeastern Central Iran Plate (CIP) encompass a large variety of volcanic and plutonic rocks. The plutonic rocks are mainly I-type granitoids, ranging in age from Late Jurassic to Middle-Late Eocene. Among the plutonic rocks, granodiorite and granite are the most abundant rock types. This dissertation summarises the results of a detailed petrographic, geochemical and isotopic study of granitoid rocks from three areas of northeastern CIP, comprising the Kashmar and Bornavard granitoids in the Taknar Zone, and the Kuh Mish intrusions in the Sabzevar Zone. The Kashmar and Bornavard granitoids are generally high in Na20, total Fe as Fe203, Mn, Ba, Zr and Sr, and low in Ti02, P20 S, Rb, Nb, Cr, Ni and Sn contents. On Harker plots, they show regular trends for most major and trace element concentrations. They are characterised by steep negative slopes for LREE, flat to slightly negative gradients for HREE and moderate to strongly negative anomalies for Eu, features attributed to fractional crystallisation. However, in the Kashmar granitoid, restite separation and fractional crystallisation may be responsible for compositional variations. Mineralogical and chemical data suggest that the Kashmar and Bomavard granitoids have formed from low temperature I-type magmas and can be assigned to a 'simple suite' of White et al. (2000). In the Kashmar granitoid, initial 87Sr/86Sr (0.70471-0.70569) and ENd (-0.70 to -1.86) values are low and exhibit a restricted range, indicating a homogeneous lower crustal protolith. In the Bornavard granitoid, however, initial 87Sr/86Sr (0.70757-0.75008) and ENd values (-1.41 to -5.20) exhibit a large range, suggesting that magmas were extensively contaminated with older continental crust or they were derived from partial melting of older felsic rocks of the continental crust.

The Kuh Mish intrusions are compositionally diverse, ranging from gabbro to quartz monzodiorite, but are dominated by granodiorite. They have low abundances of alkalis, LFSE, HFSE and LREE relative to the Kashmar and Bornavard granitoids. They are also the most isotopically primitive plutonic rocks in northe:astem CIP, typically having initial (at 42.8 Ma) 87Sr/86Sr of 0.70386-0.70475 and ENd values of +8.02 to +6.30 that indicate a mantle source. In these aspects, they are similar to the tonalitic association in the American Cordillera and, in particular, to the western Peninsular Ranges Batholith.

The granitoid rocks of the northeastern CIP show characteristics of magmas that originated in a subduction-related environment. The Rb/Sr ages of biotite-whole rock pairs from granitoids of northeastern CIP are consistent with the timing of subduction of the Neo­Tethys Oceanic crust beneath the CIP. In particular, Sr-Nd isotopic data show that in the northeastern CIP, Middle-Late Eocene granitoids are isotopically less evolved or have primitive features compared with Late Jurassic/Early Cretaceous granitoids. It seems that voluminous injection of basaltic and andesitic magmas derived from subduction of the oceanic crust resulted in a complete change in the genesis of magmas in the northeastern CIP. Using tectonic discrimination diagrams, the Kashmar and Bornavard granitoids typically plot in the 'volcanic arc and syn-collisional' granite field. However, the Kuh Mish intrusions are strongly depleted in Rb, Nb and Y contents suggesting that they may have emplaced in an island arc environment.

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