Petrological and Chemical Evolution of the Northeastern Troodos Extrusive Series, Cyprus

Abstract

Four magma series are distinguished in the northeastern Troodos Extrusive Series: (A) a Lower Low-Ti Series (Lo-LTS) of basaltic andesites, (B) a High-Ti Series (HTS) of basaltic andesites to rhyodacites, (C) a Low-Ti Series(DLTS), the last two being of basaltic andesite. Trace-element characteristics vary systematically from Series A to D and are interpreted in terms of a variable contribution of three major source components (SCs). LILE-enriched water-rich fluids (SCI) derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge. Increasing LILE/HFSE and LILE/REE ratios and decreasing absolute HFSE and REE concentrations from Series A to Dindicate progressive depletion of the actual mantle source (SCII). A negative Ta anomaly in the lavas decreases from Series A to D and is interpreted to have resulted from partial melting of the lower crust (SCII) where Ta-Nb-Ti may be fractionated by Ti-rich accessory phases. The contribution of SCIII decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases, preventing partial melting of the lower crust. Chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series A to D. Lo-LTS and IITS lavas are intrepreted to be directly related by open-system fractional crystallization in crustal magma chambers. Removal of observed phenocryst phases clinopyroxene, orthopyroxene, plagioclase, and magnetite, and repeated subsequent mixing of dacitc to rhyodacitic magmas with batches of replenshing basaltic andesites are the major processes, possibly induced by vesicle formation in the mafic layer after a period of some crystallization. LTS and DLTS magmas were directly fed to the surface without stagnating at crustal levels, with feeder dykes positioned marginal to the central rift zone and thus by-passing the central magma chambers. These magmas apparently experienced only limited fractional crystallization of 10–15 wt.% olivine+clinopyroxene+chromite, probably at the mantle-crust boundary.