Trace element variations and isotopic composition of charnockitic acidic rocks related to anorthosites (Rogaland, S.W. Norway)

Abstract

Charnockitic acidic rocks related to anorthosites are well represented in the Rogaland anorthositic province. Quartz mangerites constitute the upper level of the Bjerkrem-Sogndal layered lopolith and are related to anorthosites through a complete sequence of differentiated products which includes leuconorites, norites and mangerites. In the Hidra body, charnockitic rocks occur as a stockwork of dykes cutting across anorthosite and leuconorite. The southwest end of the magmatic province consists of two distinct bodies of acidic rocks: the Farsund charnockite and the Lyngdal hornblende granodiorite. They are considered to be related to the neighboring anorthosite massifs. Some trace and major elements (REE, Rb, Sr, U, Th, Hf, Zr, K 2O, SiO 2) of these rocks as well as the strontium and oxygen isotopic ratios are investigated. Quantitative modeling of the behavior of trace elements using the Rayleigh equation indicates in agreement with the isotopic data that direct derivation of the acidic rocks by fractional crystallization of a parental magma is a possible mechanism in Hidra and in the Lyngdal granodiorite. In Bjerkrem-Sogndal, however, the REE evolution in the acidic rocks and the high value of the 87 Sr 86 Sr initial ratio (0.7105) compared to that of the anorthosites (0.705–0.706) cannot be explained by a simple fractional crystallization process. Contamination by supracrustal material relatively poor in REE and with a high Sr isotopic ratio must also be invoked. In the Farsund charnockite REE distribution and Sr and O isotopic ratios suggest an origin by anatexis of gneisses of sedimentary origin.