Origin and evolution of the early magmatism in the Oslo Rift (Southeast Norway): Evidence from multiple generations of clinopyroxene

Abstract

This study presents in-situ major and trace element data on zoned clinopyroxenes in the oldest lavas (B1, ca. 300 Ma) in the Vestfold-Jeløya and Skien areas in the southern part of the Oslo Rift (southeast Norway), in addition to Sr-Nd data on clinopyroxene separates. The aim of the study is to throw light on the causal mechanism for the formation of the Oslo Rift, and the alkaline nature of the magmatism. Two different models have been proposed: (i) passive rifting caused by extension north of the Variscan Front, in which MORB-type melts became enriched through interaction with enriched sub-continental lithospheric mantle (SCLM), and (ii) active rifting in which the magmatic products were produced by partial melting of enriched sub-lithospheric plume material. Clinopyroxene in B1 basalts commonly consist of three types: (1) a resorbed, homogeneous Cr-Mg-rich diopside core (CrMgDi) strongly depleted in Zr-Hf and Yb-Lu, but enriched in MREE relative to LREE and HREE; (2) Cr-poor diopside (MgDi) that forms concentrically zoned overgrowth on the CrMgDi cores, and may form reaction rims on the CrMgDi; and (3) outer zones of augite. The concentrations of incompatible trace elements show a general increase from the CrMgDi, through the MgDi to the outer rims of augite (although reverse zoning is common). In the Vestfold-Jeløya area some CrMgDi cores have ɛNd300 that are significantly higher (≤+6.0) than found in any magmatic rock in the Oslo Rift (≤4.4), implying that the resorbed CrMgDi cores represent xenocrysts from a mantle source that was depleted up to the initiation of the Oslo rifting event: the subcontinental lithospheric mantle (SCLM). High LREE and MREE contents in the CrMgDi are caused by interaction with enriched host magma. The enriched trace element signatures of the B1 magmatism in the Vestfold-Jeløya area is thus a primary feature, as implied by the plume model (ii). In the Skien area, all clinopyroxene types, as well as the host B1 lavas, show HIMU Sr-Nd signatures, supporting former suggestions that the underlying SCLM has suffered local metasomatism during emplacement of the 580 Ma old Fen carbonatite complex. The MgDi in both areas formed by reaction between B1 magmas and CrMgDi xenocrysts, followed by crystallization from magnesian B1 melts undergoing fractional crystallization, magma mixing and assimilation of crustal melts. Progressive evolution of the B1 melts by these processes led to the crystallization of augite.