PETROLOGICAL AND CHEMICAL EVOLUTION OF THE NORTHEASTERN TROODOS EXTRUSIVE SERIES, CYPRUS

Four magma series are distinguished in the northeastern Troodos Extrus ive 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 (LTS) and (D) a Depleted Low-Ti Series (DLTS), the last two being of basaltic andesite. Trace-element characteristics vary sy stematically from Series A to D and are interpreted in terms of a vari able contribution of three major source components (SCs). LILE-enriche d water-rich fluids (SCI) derived from dehydration of a subducted lith osphere slab were continuously added to the overlying mantle wedge. In creasing LILE/HFSE and LILE/REE ratios and decreasing absolute HFSE an d REE concentrations from Series A to D indicate progressive depletion of the actual mantle source (SCII). A negative Ta anomaly in the lava s decreases from Series A to D and is interpreted to have resulted fro m partial melting of the lower crust (SCII) where Ta-Nb-Ti may be frac tionated by Ti-rich accessory phases. The contribution of SCIII decrea ses when the eruptive sites successively move away from the central ax ial zone and the temperature of the lower crust decreases, preventing partial melting of the lower crust. Chemical compositions of fresh gla ss separates and phenocrysts indicate a change of major petrogenetic p rocesses from series A to D. Lo-LTS and HTS lavas are intrepreted to b e directly related by open-system fractional crystallization in crusta l magma chambers. Removal of observed phenocryst phases clinopyroxene, orthopyroxene, plagioclase, and magnetite, and repeated subsequent mi xing of dacitic to rhyodacitic magmas with batches of replenishing bas altic andesites are the major processes, possibly induced by vesicle f ormation in the mafic layer after a period of some crystallization. LT S and DLTS magmas were directly fed to the surface without stagnating at crustal levels, with feeder dykes positioned marginal to the centra l rift zone and thus by-passing the central magma chambers. These magm as apparently experienced only limited fractional crystallization of 1 0-15 wt. % olivine + clinopyroxene + chromite, probably at the mantle- crust boundary.