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

Proterozoic massif anorthosites are usually associated with variable INTRODUCTION amounts of a characteristic suite of rocks ranging from a melanocratic Proterozoic massif anorthosites are usually associated facies highly enriched in Fe, Ti and P (FTP rocks) to mafic and with variable amounts of a characteristic suite of mafic granitic rocks (the jotunite–charnockite suite). Here experimental to granitic rocks. The least evolved rocks of this suite are and geochemical data on fine-grained (chilled) samples from several enriched in mafic minerals (lowand high-Ca pyroxenes, intrusions of the Rogaland Province are used to decipher their Fe–Ti oxides, apatite), and in some cases very high petrogenesis. Modeling of these data supports the hypothesis that concentrations of these phases give rise to melanocratic extensive fractionation of primitive jotunites can produce quartz rocks. Various names including ferrodiorite, monmangerites with REE concentrations in the range of jotunites, strong zonorite, jotunite, Fe–Ti–P-rich rocks (FTP) and oxide– depletions in U, Th, Sr and Ti, and smaller to no relative depletions apatite gabbronorite have been used; however, in this in Hf and Zr. Experimental and petrographic data indicate that study, we will refer to them by the collective term of the FTP rocks represent accumulations of a dense oxide– jotunite (hypersthene monzodiorite). Evolved rocks of the apatite–pigeonite assemblage from coexisting multisaturated jotunitic suite include mangerites (hypersthene monzonite), quartz to mangeritic liquids. The Rogaland jotunitic–charnockitic trend mangerites and charnockites (hypersthene granite). We corresponds to a multi-stage process of polybaric fractional cryswill refer to the suite as a whole as the jotunite suite. tallization and crystal accumulation. The early stage, in which a The origin of jotunites remains the subject of conprimitive jotunitic magma fractionates to produce an evolved jotunite, siderable debate, despite their similar textural and geoprobably took place several kilometers below the intrusion level of chemical characteristics from one anorthosite complex dikes, either in mafic chambers similar to that of the Bjerkreim– to another. Several hypotheses, not mutually exclusive, Sokndal layered intrusion or in masses of crystallizing andesine have been proposed: (1) jotunites are residual liquids anorthosite. The later stage of fractionation, which may have after anorthosite crystallization (Ashwal, 1982; Morse, involved flow differentiation, took place within the dikes themselves 1982; Wiebe, 1990; Emslie et al., 1994); (2) they are and produced compositions ranging from evolved jotunite to mangerite the parental magmas of the andesine anorthosite suite to quartz mangerite and charnockite. (Duchesne et al., 1974; Duchesne & Demaiffe, 1978; Demaiffe & Hertogen, 1981); (3) they are products of partial melting of the lower crust (Duchesne et al., 1985, 1989; Duchesne, 1990); (4) they are transitional rocks in a comagmatic sequence from anorthosite to mangerite (Wilmart et al., 1989; Owens et al., 1993; Duchesne & Wilmart, 1997); (5) they are derived by fractionation of