Geochemical and isotopic constraints on the evolution of magma plumbing system at Damavand Volcano, N Iran

摘要

Damavand is a Quaternary stratovolcano in N Iran, associated with small volumes of under-saturated to weakly-saturated mafic lavas (Tephrite-Basanite-Alkali olivine basalt), and large volumes of saturated lavas (trachyandesite-trachyte (TT)) together with minor pyroclastic rocks. TT rocks are characterized by LILE and HFSE enrichments, whereas the mafic rocks have higher REE and other incompatible elements (except Rb, Th) than TT suite rocks, contradicting the assumed derivation of TT magmas from mafic magmas by simple fractional crystallization processes. The similar whole-rock isotope ratios of mafic rocks, Tr suites and their enclaves do suggest a common origin. Magmatic enclaves in TT suites show decoupled major and some trace elements from their host while they have similar geochemistry to the mafic lavas. Trace element modeling including assimilation-fractional crystallization (AFC) and energy-constrained recharge-assimilation fractional crystallization (EC-RAFC) suggest that the Damavand TT rocks were formed by multi-stage fractionation of mafic magma with minor effects of open system processes. Differentiation started at high pressures (6-8 kbar, initial temperature of magma (T-m) = 1180 degrees C) involving anhydrous minerals (mainly clinopyroxene-orthopyroxene), accompanied by recharge and wall-rock assimilation at equilibration temperature (T-eq) of 1050 degrees C as the magma reached the threshold for extraction from crystal mush (M-m = 0.49) (stage-1). These processes resulted in the enrichment of the residual liquid in incompatible elements. The fractionation of oligoclase-andesine, anorthoclase, phlogopite and apatite from intermediate enriched melts occurred at shallow crustal levels (0.5-3 kbar, T-m = 1050 and T-eq = 830 degrees C): here, FC processes and further recharge and assimilation of crustal materials reduced LILE and REE concentration in the residual liquid (stage-2). The role of enriched melts is supported by unusual trace element composition of key minerals, for instance, feldspars have variable concentrations of Ba (132-5495 ppm), Sr (1471-14413 ppm) and unusual Rb, La and Ce contents. Based upon melt extraction modeling, physical removal of residual melts at stage-2 could produce cumulate residues with trace element compositions comparable to enclaves in TT suites (stage 3). Disruption of crystal mush due to recharge, followed by mixing of enclave material with evolved trachytic melts caused buffering of magma at trachyandesitic composition (stage-4). The results affirm the advantages of using a range of compositional and textural records in modeling of petrogenetic processes in all magmatic plumbing systems. (C) 2019 Elsevier B.V. All rights reserved.