Coexisting anorogenic and subduction-related metasomatism in mantle xenoliths from the Betic Cordillera (southern Spain)

Abstract

The mantle xenolith suite from Tallante (Betic Cordillera, southern Spain) comprises peridotites equilibrated in the spinel to plagioclase stability field, ranging in composition from fertile lherzolites to harzburgites residual after partial melting events that occurred in pre-Paleozoic times. Evidence of superimposed metasomatic effects can be attributed to at least two metasomatizing agents with distinct chemical and isotopic compositions. The first was a silica-undersaturated alkaline melt, resulting in variable Light Rare Earth Elements (LREE) and Sr–Nd isotopic enrichment ( 87Sr/ 86Sr=0.70213–0.70476; 143Nd/ 144Nd=0.51339–0.51250) from Depleted Mantle (DM) to Enriched Mantle (EMI) end-members. This predominant metasomatism did not form hydrous phases, secondary textures, or significant mineralogical changes, and appears to be compositionally analogous to that recorded in xenoliths and Alpine peridotite massifs of the Southern European lithospheric mantle. The second metasomatic agent was a silica-oversaturated hydrous melt, producing local (<5–10%) mantle hybridization, with the formation of amphibole/phlogopite-bearing opx-rich harzburgites containing olivine–orthopyroxenite and gabbronorite lenses. This metasomatism is characterized by Sr–Nd isotopic values representing the most extreme enriched signature ( 87Sr/ 86Sr=0.70672–0.70856; 143Nd/ 144Nd=0.51213–0.51211) found in uncontaminated anorogenic basic magmas and associated mantle xenoliths throughout Europe. These Sr–Nd isotopic data closely approach those recorded in the Miocene calc-alkaline and potassic volcanism of the Betic Cordillera, suggesting that this signature could have been inherited from subduction-derived metasomatizing melts during the Tertiary orogenic events of the Betic–Alboran region. The incompatible element distribution of these xenoliths and their constituent pyroxenes is also quite unusual, showing chondrite-normalized upward convex Middle (M)REE patterns coupled with remarkable negative Sr and Eu anomalies. Incompatible element modeling indicates that the peculiar composition of clinopyroxene, orthopyroxene and plagioclase can be accounted for by the interaction between the peridotite matrix and potassic silica-oversaturated melts, approaching lamproites found in the Betic Cordillera. A comparison with analogous worldwide occurrences indicates that some distinctive characteristics of subduction-related metasomatism in mantle materials are (1) silica-oversaturation in the form of excess orthopyroxene crystallization, in association with hydrous phases; (2) high 87Sr/ 86Sr ratios, departing from the anorogenic mantle array; (3) particularly high 87Sr/ 86Sr and low 143Nd/ 144Nd ratios, coupled with strong negative Sr and Eu anomalies, indicating a significant contribution of continental terrigenous components in the source of subduction-related metasomatizing agents.