Geodynamic and source characterization of intrusive rocks from Nusha: a key to Neothetyan arc related tectonomagmatic evolution of Central Alborz

Nusha intrusive rocks south of Ramsar (N Iran) include gabbronorites, gabbros, monzogabbros, diorites, syenites, monzonites, monzodiorites, granodiorites, and quartz monzonite with Eocene age (56±2 Ma). Electron probe micro analyzing (EPMA) on clinopyroxene, orthopyroxene and biotite of these rocks reveals that they are compositionally augite to salite, hypersthene, and Mg biotite to phlogopite, respectively. Geothermometry and Geobarometry calculations based on CPX compositions correspond to temperatures of approximately 1170 to 1210 °C and mostly 6 to 7 kb pressure. Whole-rock geochemistry indicates that nearly all rocks are high-K calc-alkaline to shoshonitic in nature and display continental arc-related geochemical characteristics, with enrichment in LILE and LREE and low concentration in HFSE and HREE, along with Nb, Ta and Ti negative anomalies. The positive anomaly of Pb, and K, and the relatively low Ce/Yb and Nb/U ratios suggest crustal contamination. Nb/La ratios mostly between 0.4 and 1.34, as well as Nb/Zr ratio (0.76) indicate a lithospheric mantle origin. Trace-element ratios, when compared with experimental studies, point to an enriched lithospheric mantle source composed of spinel-garnet lherzolite, which underwent a relatively low degree of partial melting (3–20%) to produce the Nusha intrusions.