Journal of Petrology, Volume 39, Issue 3: March 1 1998.

A Liquid Line of Descent of the Jotunite (Hypersthene Monzodiorite) Suite

JACQUELINE VANDER AUWERA¹, JOHN LONGHI² AND JEAN-CLAIR DUCHESNE¹

¹L.A. GÉOLOGIE, PÉTROLOGIE ET GÉOCHIMIE, UNIVERSITÉ DE LIÈGE, B-4000 LIÈGE, BELGIUM ²LAMONT-DOHERTY EARTH OBSERVATORY, PALISADES, NY 10964, USA

Proterozoic massif anorthosites are usually associated with variable amounts of a characteristic suite of rocks ranging from a melanocratic facies highly enriched in Fe, Ti and P (FTP rocks) to mafic and granitic rocks (the jotunite-charnockite suite). Here experimental and geochemical data on fine-grained (chilled) samples from several intrusions of the Rogaland Province are used to decipher their petrogenesis. Modeling of these data supports the hypothesis that extensive fractionation of primitive jotunites can produce quartz mangerites with REE concentrations in the range of jotunites, strong depletions in U, Th, Sr and Ti, and smaller to no relative depletions in Hf and Zr. Experimental and petrographic data indicate that the FTP rocks represent accumulations of a dense oxide-apatite-pigeonite assemblage from coexisting multisaturated jotunitic to mangeritic liquids. The Rogaland jotunitic-charnockitic trend corresponds to a multi-stage process of polybaric fractional crystallization and crystal accumulation. The early stage, in which a primitive jotunitic magma fractionates to produce an evolved jotunite, probably took place several kilometers below the intrusion level of dikes, either in mafic chambers similar to that of the Bjerkreim-Sokndal layered intrusion or in masses of crystallizing andesine anorthosite. The later stage of fractionation, which may have involved flow differentiation, took place within the dikes themselves and produced compositions ranging from evolved jotunite to mangerite to quartz mangerite and charnockite.

Keywords: anorthosite;experimental petrology; geochemistry; monzodiorite; Rogaland anorthosite complex

Pages 439-468