Petrology and geochemistry of Late Proterozoic hornblende gabbros from southeast of Fariman, Khorasan Razavi province, Iran

Hornblende-bearing gabbroic rocks are quite common in subduction-related magmatic suites and considered to represent magmatic differentiation process in arc magmas (Heliker، 1995; Hickey-Vargas et al.، 1995; Mandal and Ray، 2012). The presence of hornblende as an important mineral phase in gabbroic rocks of subduction zone has been considered either as an early crystallizing mineral from water-bearing mafic magmas (Beard and Borgia 1989; Mandal and Ray، 2012) or as a product of reaction of early crystallized minerals (olivine، pyroxene and plagioclase) and water-rich evolved melt/aqueous fluid (Costa et al.، 2002; Mandal and Ray، 2012). The careful study of petrology and geochemistry of hornblende-bearing gabbroic rocks from Chahak area، of Neoproterozoic age، can provide important information about their petrogenesis. Because of the special characteristics of Chahak hornblende gabbros according to their age and their situation in the main structural units of Iran، their study can present critical keys for the knowledge of geological history of Iran specially central Iran zone. This study carried out in two parts including field and laboratory works. Sampling and structural studies were carried out during field work. Geological map for the study area was also prepared. 65 thin and polished thin sections for petrographical purpose were studied. Major oxides، rare earth elements and trace elements were analyzed for 4 samples (92P-1، 92P-3، B1and B6) from hornblende gabbros on the basis of 4AB1 method using ICP-MS of ACME Laboratory from Canada. In addition، major oxides of three hornblende gabbro samples (89P-62، 89P-59 and 89P-46) were used from Partovifar (Partovifar، 2012). Fariman metamorphic terrains، of Proterozoic age، consist of metamorphosed sedimentary and igneous (plutonic and volcanic) rocks. Hornblende gabbros of the study area include plagioclase، hornblende، biotite pyroxene and olivine as major minerals and apatite، ilmenite and magnetite as minor minerals. In many examples، hornblende and biotite can be seen as corona textures around plagioclase، pyroxene and olivine، while plagioclase، pyroxene and olivine show obviously corrosion features. This can be considered to be formed by the reaction of early formed crystals with aqueous fluid/evolved melt. In some cases، amphiboles show rhythmic overgrowths. The rhythmic amphibole overgrowths represent deep-seated crystallization in a volatile-rich magma under conditions of high but varying gas pressure. In the study area، the most dominant texture of the hornblende gabbros is hypidiomorphic granular، but intergranular and porphyric textures are common too. Based on geochemical data from major and minor elements، studied rocks belong to tholeiite series with meta–aluminous nature. The geochemical behavior of main elements of the studied rocks reveals the normal trend of differentiation in their magma. Chondrite-normalized REE diagram of hornblende gabbros indicates an obvious enrichment of LREE in compare with HREE. MORB-normalized spider diagrams indicate variable enrichment in LILE and depletion in high field strength elements (HFSE). Primitive mantle-normalized spider diagram show negative anomaly for Nb and Zr. Gabbros from southeast of Fariman have an island arc tholeiite nature and based on trace element diagrams، they formed as a result of 3 to 10% partial melting of a garnet lherzolite source. The mineralogy، texture and geochemistry of the studied rocks show striking similarities with gabbroic rocks of subduction zone developed in supra subduction zone of arc-marginal basin setting.