Mantle Source Characteristics of Alkali Basalts and Basanites in an Extensional Intracontinental Plate Setting, Western Anatolia, Turkey: Implications for Multi-stage Melting

Abstract

The alkaline volcanic province of Western Anatolia is characterized by intra-continental plate alkali olivine basalts and basanites extruded along localized extensional basins during Late Miocene (<11 Ma) to Quaternary (>0.13 Ma) time. The rocks are characterized by low 87Sr/86Sr (0.70311-0.70325) and high 143Nd/144Nd (0.51293-0.51298) ratios; they have OIB-like trace-element patterns characterized by enrichment in LILE, HFSE, and L-MREE, and a slight depletion in HREE relative to N-MORB. Trace-element modeling indicates that the mafic magmas formed by variable degrees (˜2% -10%) of partial melting of an isotopically homogenous garnet-bearing mantle source. The degree of melting decreased progressively from early-formed alkali olivine basalts to later basanites. Major- and trace-element systematics reveal two distinct depth ranges of melt segregation from the source mantle: (1) garnet and spinel + garnet stability zone for the Late Miocene rocks; and (2) spinel and spinel + garnet stability zone for the Quaternary rocks. Source concentrations, calculated using an inverse numerical method, show that the mantle from which the alkaline magmas were generated was enriched in all incompatible elements (e.g., LILE, HFSE, and LMREE) relative to depleted MORB mantle (DMM) and primitive mantle (PM) compositions. The isotopically depleted nature of the alkaline rocks relative to bulk silicate earth (BSE) further indicates that this enrichment is a recent event related to small-degree, multi-stage melting processes that involve local metasomatism of the mantle.