Serpentine polymorphs andP-Tevolution of metaperidotites and serpentinites in the Takab area, NW Iran

Abstract

The Takab meta-ultramafic rocks of northwestern Iran crop out in association with a variety of metamorphic rocks and they can be divided into three types on the basis of dominant mineral assemblage and degree of serpentinization: metaperidotites, talc-serpentine rocks and serpentinites. The peridotite protoliths were mainly spinel-harzburgites and -dunites. The talc-serpentine rocks formed under low-grade conditions and their dominant mineral assemblage is lizardite/chrysotile + talc + calcite/dolomite. Pseudomorphic textures after olivine and pyroxene are typical. Later, lower amphibolite-facies metamorphism produced antigorite, clinochlore and tremolite. Late alteration, assisted by local shearing, produced more antigorite and late lizardite in the metaperidotites. The mineral chemistry of the serpentine polymorphs is variable. Compositions of coexisting spinel and olivine point to an ophiolitic origin. Thermometric estimates from co-existing relict olivine, pyroxenes and spinel are in the range 1000–1200ºC; clinopyroxene barometry yields a pressure of 24± 2.7 kbar for the mantle conditions. Exhumation and hydration followed, and the stability of lizardite/chrysotile constrains the T and XCO2 to <280ºC and <0.002, respectively. Subsequent prograde regional metamorphism took place in an orogenic setting, reaching peak temperatures in the range 410–540ºC at XCO2 >0.1. A Neoproterozoic–Early Cambrian age for the Takab protoliths is suggested on the basis of correlations with the Central Iran Zone. The strips of ultramafic rock in the Takab complex can be attributed to lithospheric remnants of the Proto-Tethyan ocean which closed during the Pan-African orogeny.

Mineralogy, geochemistry and geodynamic setting of the granitoids from NW Iran

“…Figure 2 illustrates sample localities and rock types. The Mahneshan area is composed mainly by metamorphic rocks including granulites, amphibolites, meta-ultramafites, gneisses, granitic gneisses and metapelitic rocks (Hajialioghli et al, 2007;Saki et al, 2008), which were intruded by granitoid intrusions. Almost the rocks in the area have been influenced by intense deformation and metamorphism.…”

“…The presence of palaeo-suture zones and ophiolitic rocks (i.e. serpentinite, meta-mafic and meta-ultramafic rocks, Hajialioghli et al, 2007) around the high-grade metamorphic rocks of the Mahneshan Complex show that an island-arc type cratonization formed the Iranian Precambrian basement. After formation of the basement complex and metamorphism, the Arabo-Iranian coherent platform was established at the end of the Pan African Orogeny.…”

“…The study area is located to the south of Mahneshan town, Western Zanjan province of Iran (Figures 1 and 2). The area is limited to geographical latitude 478 07 0 -478 45 0 E and longitude 368 30 0 -378 00 0 N. The unconformity between the metamorphosed Neoproterozoic-Early Cambrian rocks (Babakhani and Ghalamghash, 1990) and the existence of sedimentary facies that include large thicknesses of dolomitic marble interlayered with volcanoclastic and volcanic rocks, corresponding to deepwater, turbiditic environments, can be interpreted as evidence for a former arc-trench system along the Proto-Tehran margin in NW Iran (Hajialioghli et al, 2007). Evolution of the Mahneshan…”

“…Complex from greenschist to alkali granites show evidences for progressive metamorphism which ended by anatectic phenomena and formed leucogranites (Hajialioghli et al, 2007). The metamorphic rocks have been thrust over Oligocene to Miocene volcano-sedimentary rocks along the NW-SE Qeynarj-e-Chartagh thrust fault (Figure 2), but they are overlained unconformably by Tertiary rocks in the east of the area (Babakhani and Ghalamghash, 1990).…”

“…This is the first study of the petrology and petrogenesis of these rocks. According to mineralogy, these granitoid rocks are classified into two subgroups including, I-type (containing hornblende) and S-type (containing garnet and muscovite) granitoids (Haghipour and Pelissier, 1967;Chappell and White, 1974;Berberian and King, 1981;Saki, 2007;Hajialioghli et al, 2007). The S-type granitoids are the subject of this study.…”