Oligocene calc-alkaline lamprophyres and K-rich association in the eastern Iranian ranges: Products of low-degree melting of subduction-modified lithospheric mantle in post-orogenic setting

Abstract

Late- to post-orogenic lamprophyric and K-rich associations in the eastern Iranian ranges may be able to reveal some obscure issues about the petrogenesis and the role of tectonics in the formation of this rare group of igneous rocks. In this paper, we investigate new zircon U–Pb ages and elemental and Sr–Nd isotope characteristics of coexisting various rock suites as the post-tectonic products of the Paleogene orogeny . Lamprophyres are found in three occurrence modes in the eastern Iranian ranges. The first group is composed of olivine minette/shonkinite bodies in the Lar Complex, which is expressed as a post-tectonic volcano with Calderon subsidence that occurs currently as a ring-dike association and related explosion breccia . The second group consists of lamprophyric lavas of mainly mafic phonolite composition in Hormak. The last group is appinite/porphyrite occurring as stock/sub-volcanic bodies in Malek Siah Kuh. These three magmatic suites including lamprophyre and associated magmatic rocks are genetically, spatially and temporally related to one common K-rich mantle-derived magma . Zircon U–Pb dating of representative monzonite/syenite samples of the Lar Complex yielded ages of 31.43 ± 0.3 and 31.60 ± 0.40 Ma, respectively. Whole-rock geochemical and Sr–Nd isotope investigations demonstrated similar rare earth element (REE) patterns, with distinct enrichment in large lithophile elements (LILEs) and depletion in Ta–Nb–Ti, and limited variations in εNd (t) (+0.50 − +4.80) and 87 Sr/ 86 Sr (0.7043–0.7066) values. These characteristics, together with modeling of batch melting suggest that they were comagmatic and derived from a metasomatized lithospheric mantle source modified by a precursory subduction episode, through a low degree of partial melting (2–5%) at high-pressure conditions at depths >70 km with garnet in the residue. The Harirud fault activity possibly controlled the path, station and rapidity of magma ascent to the crustal levels, and, consequently, the water pressure of crystallization of the K-rich magmas and diversity of magmatic products. • The lithospheric mantle in the eastern Iranian ranges is highly heterogeneous. • Lamprophyres and K-rich association were products of post-orogenic magmatism. • The Harirud fault has crucial role in generation, transport and evolution of K-rich magmas.