Journal of Economic Geology (Jul 2019)
Geochemistry and tectonomagmatic setting of protolite rocks of meta-volcanics in the Halab metamorphic complex (SW Dandy, Zanjan Province)
Abstract
Introduction Three fundamental goals will be followed in the study of metamorphic terrains including: 1- study of fabric in metamorphic rocks in order to recognize the relationship between metamorphism and deformation, 2- the identification of thermodynamic conditions of metamorphism for evaluating the geothermal gradient, and 3- study of protolites of metamorphic rocks and the recognition of Paleo-tectonomagmatic setting of igneous rocks. Takab-Takht-e-Soleyman-Angouran metallogenic –metamorphic zone located parallel to the Zagros suture zone within the Alpine–Himalayan orogenic belt. Halab metamorphic sequence is located in the Eastern part of this zone. This metamorphic sequence is composed of pelitic, mafic and felsic schists intercalated with marble, mylonitic rhyolite and quartzite which are metamorphosed in amphibolite and green schist facieses. Takab-Takht-e-Soleyman-Angouran metallogenic –metamorphic zone is one of the most important metallogenic zones in Iran. The Zarshouran As–Au deposit, Aghdareh Sb–Au deposit and Angouran Zn–Pb deposit along with some Fe, Pb–Zn, Au, Cu and Mn mineralization were formed within this zone. Most of this mineralization was studied during the past years and valuable information is present about their geological and mineralization characteristics. However, geochemistry and tectonomagmatic settings of metamorphosed volcanic rocks (felsic and mafic schists) were not studied. Materials and methods This research can be divided into two parts including field and laboratory studies. Field studies include the recognition of different metamorphic rocks along with sampling from metamorphic rocks for laboratory studies. In this base, 40 samples were chosen for petrographic and analytical studies. Twenty thin sections were used for petrographic studies and recognition of metamorphic fabrics. For geochemical studies, thirteen samples from felsic and mafic schists were analyzed by XRF and ICP–MS methods in GSI and Zarazma laboratories. Results Mafic schists are one of the most important metamorphic rocks in the Halab area. Compositionally, these rocks include actinolite schist, hornblende schist and amphibole schist. Felsic schists are the other important rocks in the Halab metamorphic sequence. These rocks include albite-quartz schist, biotite-quartz schist, amphibole-biotite-quartz schist and mylonitic rhyolite. Geochemically, mafic schists show a similar composition to basalt, trachy-basalt, basaltic andesite and basaltic trachy-andesite while felsic schist show rhyolitic composition. All of these rocks have calc-alkaline to high-K calc-alkaline affinity. Trace elements normalized by primitive mantle (McDonough and Sun, 1995) and NMORB (Gale et. al., 2013) for felsic schists indicate LILE enrichment along with negative HFSE anomaly and distinctive positive Pb anomaly. A similar pattern is observed for most of the mafic schists. Amphibole schists do not show LILE enrichment, as well as positive Pb and negative HFSE anomalies. Chondrite-normalized (McDonough and Sun, 1995) REE patterns for felsic schists demonstrate LREE enrichment along with negative Eu anomaly and flat HREE patterns. Most of the mafic schists have similar patterns without negative Eu anomaly. Amphibole schists indicate a flat REE pattern with less LREE enrichment and relative enrichment in HREE compared with other mafic schists. Comparison of Chondrite-normalized REE patterns of mafic schists with NMORB and EMORB patterns and island arc basalts (Gale et al., 2013) indicate that basic schists of Halab area have similar patterns to EMORB. Based on Ta/Yb vs. Th/Yb and Yb vs. Th/Ta discrimination diagrams, protolites of mafic schists were formed in within plate volcanic zone and active continental margin while protolites of felsic schists were formed within active continental margin. On the Nb/Yb vs. Th/Yb diagram, mafic schists belongs to subduction-unrelated setting and originated from mantle similar to OIB source. Discussion Takab-Takht-e-Soleyman-Angouran metallogenic –metamorphic zone is considered as a micro-continent with similar features to Gondwana (Hajialioghli et al., 2007). The oldest outcrops of metamorphic rocks in this zone are the result of metamorphism of magmatic arc rocks with Neo-Protrozoic–Early Cambrian age (Saki, 2010). As it was mentioned before, mafic and felsic schists of the Halab area demonstrate calc-alkaline to high-K calc-alkaline affinity. High-K calc-alkaline rocks are usually formed in magmatic arcs and post collision setting and are less seen within plate setting (Bonin, 2004). Enrichment in LILE and LREE along with Nb and Ti negative anomalies in spider diagrams are indicators of subduction related magmas which are originated from enriched mantle by metasomatic fluids released from subducted slab (Wang and Chung, 2004). Geochemical characteristics of mafic and felsic schists of the Halab area indicate that the protolites of mafic schists originated from partial melting of metasomatized mantle by past subduction in an extensional setting within a magmatic arc. Felsic schists are the result of crustal partial melting by mentioned basaltic magma. Acknowledgment This research was made possible by the grant of the office of vice-chancellor for research and technology, the University of Zanjan. We acknowledge their support. Journal of Economic geology reviewers and editor are also thanked for their constructive suggestions on alterations to the manuscript. References Bonin, B., 2004. Do coeval mafic and felsic magmas in post-collisional to within plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review. Lithos, 78(1–2): 1–24. Gale, A., Dalton, C.A., Langmuir, C.H., Su, H.J. and Schilling, J.G., 2013. The mean composition of ocean ridge basalts. 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