TWO-STAGE DIFFERENTIATION IN A MANGERITE BATHOLITH, CHARLEVOIX REGION, GRENVILLE PROVINCE, QUEBEC

Granitoid plutons consisting of green-colored, pyroxene-bearing, ultramafic to felsic rocks (~35-68 wt% SiO₂), referred to herein as ″mangerite,″ are found in spatial and temporal association with Proterozoic massif anorthosite worldwide. A vast literature has discussed the relationship between mangerite and anorthosite, yet little attention has been paid to the differentiation of the mangerite itself. The Lac des Martres mangerite batholith ranges in composition from pyroxenite to monzodiorite to qtz monzodiorite/monzonite, and is noteworthy for high concentrations of Fe, Ti, and P (reflected in high modal apatite, ilmenite, and magnetite), which distinguishes it from other igneous rocks at any level of SiO₂.

Monzodiorite (also termed ″jotunite″) forms a wide-ranging but narrow border zone to the batholith and grades into a core zone of coarse- to very coarse-grained qtz monzodiorite/monzonite. The monzodiorite border contains layers and lenses of pyroxenite, which are interpreted as cumulate-like segregations, possibly of an immiscible FTP-liquid. In a few cases, these pyroxenites have segregated on a large scale and ponded locally against the contact with adjacent anorthosite. Where in contact with older plutons or country-rock gneisses, the monzodiorite border tends to be coarse-grained and generally massive, but where in contact with anorthosite, the monzodiorite is foliated and uniformly finer-grained, perhaps indicative of quenching. If so, then monzodiorite (=jotunite) should be regarded as parental to mangerite but not anorthosite. Indeed, the widespread presence of monzodiorite dikes in the region attests to the existence of liquids having such compositions.

Trends in whole-rock, major- and trace-element compositions suggest an overall differentiation sequence from pyroxenite through to qtz monzodiorite/monzonite. Normative feldspar compositions decrease from An_40-35 (pyroxenite) to An_35-25 (monzodiorite) to An_25-15 (qtz monzodiorite/monzonite), with Or increasing in this sequence. On SiO₂ variation diagrams, the pyroxenites and monzodiorites form linear arrays indicative of mixing between plagioclase and an FTP-component, whereas differentiation in the qtz monzodioritic/monzonitic portion of the batholith seems to be dominated by separation of plag + Kf.