Akademik Veri Yönetim Sistemi
LITHOS, cilt.516, 2025 (SCI-Expanded)
The geochemical and isotopic investigation of arc magmas are key tools for understanding active margin tectonic processes. To better address tectonic evolution, we examined basaltic dykes in Robert Island in the South Shetland Islands that were emplaced between 96.6 and 91.6 Ma based on Ar-40/Ar-39 dating. We integrated these temporal constraints with comprehensive bulk-rock geochemistry and Sr-Nd-Hf-Pb isotopic data to elucidate their mantle source characteristics and tectonic setting. These dykes-ranging from basalt to basaltic andesite with calc-alkaline affinities-exhibit low silica (46.93-53.12 wt% SiO2) and high Mg# values (49-67). Trace element distributions suggest that these dykes were originated from melting of an enriched mantle source analogous to that producing enriched mid-ocean ridge basalts (E-MORB); despite their high Th/Yb ratios. NbTa depletion (e.g., low Delta Nb) indicates that these dykes were generated from melting of an enriched, yet metasomatized mantle above a subduction system. Their isotopic signatures (Sr-87/Sr-86(i): 0.70385-0.70411; epsilon Nd-i: +3.8 to +6.1; epsilon Hf-i: +5.9 to +9.7; Pb-206/Pb-204(i): 18.46-18.64, Pb-207/Pb-204(i): 15.58-15.64, Pb-208/Pb-204(i): 38.21-38.38) further indicate derivation from a juvenile, less radiogenic mantle significantly modified by subducted sediment melt and terrigenous sediment components. Geochemical modeling suggests that the observed magma compositions result from 5 to 10 % partial melting of a mantle wedge metasomatized by <3 % melts derived from subducted terrigenous sediments, imparting a subcontinental lithospheric mantle signature. Notably, the isotopic systematics-including elevated Delta 7/4 values and pronounced NdHf decoupling-reflect the influence of sediment-derived melts and terrigenous sediments on the mantle source while also highlighting a strong zircon effect, indicative of zircon retention during partial melting. We propose that the dykes were formed during the subduction of the young Phoenix Plate, which produced the Antarctic frontal arc during the Mesozoic. A transition from a compressional regime to an extensional arc setting-likely triggered by slab retreat-may have disrupted the early arc, forming extensional conduits that facilitated asthenospheric upwelling, melting of the subcontinental lithospheric mantle, and subsequent injection of basaltic magmas to form dyke swarms. Our findings underscore a significant geodynamic shift along the Antarctic Plate's active margin, offering fresh insights into mantle dynamics and magmatic processes in convergent settings.