پی سنگ پرکامبرین

Takab-Mahneshan's metallogenic area is essential in gold mineralization, and it is a part of the northern Urmia-Dokhtar magmatic belt. The northern Urmia-Dokhtar has two distinctive features. It contains the Precambrian basement, and the young Mio-Pliocene magmatism rocks overlie its Precambrian basement. Precambrian basement has two different compositions, oceanic and continental crusts (gneiss, granite, mica-schist, greenschist, amphibolite, serpentinite, marble). The Young rocks are composed of hyaloclastite, acidic tuff, andesitic-basaltic, acidic to intermediate intrusions (diorite, granite), limestone, and marl of Qom formation. We categorized the Young rocks into four phases based on their age and positions. The mutual effect of the basement and young magmatism --in the latest phase --creates hydrothermal solutions in the form of a heat engine. The heat engine has caused rotation and variety, increasing mineralization in the area. As a result, there are mines, deposits, gold indications, and some associated gold elements, including Tuzlar, Bayche Bagh, Baghcheh at Mahneshan area, Zarshuran, Agh-darreh, and Ay-Qalesi in Takab. The Tuzlar and Baghcheh deposits with gold mineralization are in a Silicic-argillic zone, including pyrite, gold-bearing silica, argillic and ferrous hydroxides minerals. The Ay-Qalesi polymetallic deposits often contain zinc and lead. The Zarshuran and Agh-darreh deposits are the gold types that the first one has a variety of mineralization of sulfides and sulfosalts. The second one has simple gold mineralization, and finally, the Bayche Bagh deposit with polymetallic type includes chalcopyrite, pyrite, rutile, arsenopyrite, sphalerite, bornite, smaltite, chloanthite, azurite, and malachite minerals.

The study area is located in the south of Bardaskan (south west of Khorasan Razavi). Mafic intrusions of Robat-e-Zangiche and Lakh Bargheshi areas from Kooh-e-Sarhangi zone have been injected into granitic basement rocks and Soltanieh dolomite. Plagioclase, amphibole and clinopyroxene are rock forming minerals. According to the petrographic evidences, mineral chemistry and thermobarometry data amphiboles have different generations of igneous and alteration types. The results of amphibole thermobarometry indicate temperature and pressure ranges of 545-911 °C and 2.2 to 5.8 kb respectively. Such wide range of calculated temperature and pressure could be interpreted by various solidification conditions and also secondary altration processes. On the basis of whole rock chemistry, the studied samples are mainly gabbro. The spider diagrams display geochemical signatures of subduction zones magma. Based on tectonic discrimination diagrams, genesis of the studied rocks is related to the both subduction and within plate settings. Furthermore, whole rock chemistry of the studied samples is compatible with spinel lherzolite melting. As the formation of Sabzevar back arc basin was probably triggered by the Neotethys subduction beneath Central Iranian microcontinent, so the Jurassic back-arc magmatism of the Bardakcan area may be affected by the same subduction process and concomitant asthenospheric upwelling in an extensional regime.

Halab-Mianaj metamorphic area is part of Takab Precambrian metamorphic complex that exposed in north-west of Iran. Halab-Mianaj cross section included a group of metamorphic rocks that have been metamorphosed in the greenschist and amphibolite facies. Garnet porphyroblasts chemical composition in garnet schist rocks is in almandine range. Amphibole composition in Amphibole schist and meta-diorite rocks are calcic and are changed from actinolite to magnesio hornblende. Chemical compositions of the Plagioclase crystals are between albite to oligoclase range. The maximum temperature for garnet schist rocks based on Grt-Bt thermometer determined 552°C that indicates epidote-amphibolite facies. Temperature and pressure metamorphic in the amphibole schist and meta-diorite based on amph-plag thermobarometer determined about 420 - 487°C and 3.9 - 6.7 K-bar that shows the beginning amphibolite facies. Therefore, the geothermal gradient bases on Halab-Mianaj metamorphic rocks with different protolith approximately are 30 ± 1° C/km.

Precambrian basement of Central Iran in Lakh Bargheshi area from SW Bardaskan mainly consists of metapelite, metabasite and mylonitic gneiss and has been invaded by Lakh Bargheshi granitic body with the age of Ediacaran-Cambrian. Metapelites mostly are Garnet schist type and encompass mineralogical paragenesis of garnet biotite muscovite quartz feldspar. Metabasites contain of garnet amphibolite and garnet-epidote amphibolite and show mineralogical paragenesis of amphibole feldspar garnet ± quartz ± epidote± whitemica ±rutile ± sphene ±ilmenite. Metamorphic conditions that obtained by Thermocalc software, petrogenetic grids and conventional thermobarometries are compatible for both metapelitic and metabasitic units and include pressure between 7.1 until 12.3 Kb and temperature between 482 until 661°C that nearly are equivalent of high pressure amphibolite and epidote amphibolite facieses that are dominant in the lower continental crust. Epidote and garnet-bearing granitic body of Lakh Bargheshi and host metapelites and metabasites probably representing thickened continental arc induced by Prototethys subduction beneath Iran plate.