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

1997; Oxford University Press; Volume: 38; Issue: 3 Linguagem: Inglês

10.1093/petrology/38.3.337

1460-2415

Duskin Douglas J.,

earthquake and tectonic studies

The upper part of the Bjerkreim–Sokndal layered intrusion (BKSK) comprises a suite of charnockitic granitoids (mangerite, quartz mangerite and charnockite) with some jotunites. The petrology and geochemistry (major and trace elements) of eight main rock types (88 samples) are considered. The rocks form a suite of K20-rich alkali–calcic granitoids of A-type, deriving from jotunitic magma. Two trends of inferred liquids are distinguished: the main liquid line of descent which passes from jotunites through two-pyroxene quartz mangerites to amphibole charnockites, the second one which encompasses olivine-bearing quartz mangerites and charnockites and is rooted into mangeritic liquids. Comparison with the Tellnes rock series reveals that contamination by leucogranitic material has taken place simultaneously with fractional crystallization in both BKSK trends. Zr, Rb, Cs and Th are particularly sensitive to assimilation and the higher water content in BKSK. Zircon and apatite have saturated at the liquidus of quartz mangerite around 900°C, close to the fayalite-magnetite-quartz (FMQ) buffer. The two-pyroxene quartz mangerites cannot have been equilibrated with the mangerite cumulates on which they rest. A major influx of magma took place when mangeritic cumulates were being deposited. It mingled and partly mixed with the resident contaminated magma.