Igneous Charnockites and Related Rocks from the Bjerkreim-Sokndal Layered Intrusion (Southwest Norway): a Jotunite (Hypersthene Monzodiorite)-Derived A-type Granitoid Suite

Abstract

The upper part of the Bjerkreim–Sokndal layered intrusion (BKSK) INTRODUCTION comprises a suite of charnockitic granitoids (mangerite, quartz Charnockitic granitoids are a major component of mangerite and charnockite) with some jotunites. The petrology and anorthosite–mangerite–charnockite–(rapakivi) granite geochemistry (major and trace elements) of eight main rock types (AMCG) complexes and therefore play a crucial role in (88 samples) are considered. The rocks form a suite of K2O-rich the formation and evolution of the Proterozoic crust. alkali–calcic granitoids of A-type, deriving from jotunitic magma. Their origin has been long debated. Though intimately Two trends of inferred liquids are distinguished: the main liquid associated with massif anorthosites, they are generally line of descent which passes from jotunites through two-pyroxene considered to be derived from distinct parental magmas, quartz mangerites to amphibole charnockites, the second one which generated in different sources (Emslie, 1978, 1991; Morse, encompasses olivine-bearing quartz mangerites and charnockites and 1982; Duchesne, 1984; Wiebe, 1992; Ashwal, 1993). It is rooted into mangeritic liquids. Comparison with the Tellnes rock has, however, been recognized that some charnockitic series reveals that contamination by leucogranitic material has taken rocks could also be derived from an intermediate monplace simultaneously with fractional crystallization in both BKSK zodioritic to ferrodioritic ( jotunitic) magma (Duchesne trends. Zr, Rb, Cs and Th are particularly sensitive to assimilation et al., 1989; Wilmart et al., 1989; Owens et al., 1993) and the higher water content in BKSK. Zircon and apatite have whose origin is also controversial. The jotunitic magma saturated at the liquidus of quartz mangerite around 900°C, close could either be derived from a crustal source by the to the fayalite–magnetite–quartz (FMQ) buffer. The two-pyroxene partial melting of basic rocks (Duchesne et al., 1989; quartz mangerites cannot have been equilibrated with the mangerite Duchesne, 1990), or be residual magmas after the forcumulates on which they rest. A major influx of magma took place mation of massif anorthosites which have been conwhen mangeritic cumulates were being deposited. It mingled and partly mixed with the resident contaminated magma. taminated to various degrees by crustal material (Emslie et al., 1994). The latter hypothesis seems to explain a large number of AMCG complexes in the Grenville and Nain provinces and elsewhere. Accepting that some charnockites are derived from jotunites, there is a logical