Other| February 01, 1995 Significance of pyroxene megacrysts for massif anorthosite petrogenesis: Constraints from the Labrieville, Quebec, pluton Brent E. Owens; Brent E. Owens Washington University, Department of Earth and Planetary Sciences, Saint Louis, MO, United States Search for other works by this author on: GSW Google Scholar Robert F. Dymek Robert F. Dymek Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (1-2): 144–161. https://doi.org/10.2138/am-1995-1-214 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Brent E. Owens, Robert F. Dymek; Significance of pyroxene megacrysts for massif anorthosite petrogenesis: Constraints from the Labrieville, Quebec, pluton. American Mineralogist 1995;; 80 (1-2): 144–161. doi: https://doi.org/10.2138/am-1995-1-214 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Newly discovered orthopyroxene and clinopyroxene megacrysts (OPMs and CPMs) in the Labrieville massif provide additional constraints on megacryst origins and their significance in massif anorthosite petrogenesis. Pyroxene megacrysts (~ 1–15 cm across) occur throughout the anorthosite, some in layered zones (as cumulates?) and others as scattered crystals that form subophitic intergrowths with plagioclase; some also contain tabular plagioclase inclusions. In many cases, pyroxene megacrysts are intergrown with ilmenite and locally occur within meter-sized masses of ilmenite. Plagioclase and ilmenite are the dominant exsolution products in OPMs, whereas CPMs contain additional lamellae of orthopyroxene. Biotite consistently occurs with megacrysts, locally as an integral portion of exsolution lamellae.Microprobe spot compositions of the megacrystic pyroxenes (OPM ~ En58–66, CPM ~ Wo45En40Fs15) are essentially the same as those of smaller matrix pyroxenes in the same outcrops. In contrast, three bulk OPMs (4.4–5.9 wt% Al2O3) and one CPM (6.4 wt% Al2O3) are enriched in Ca, Na, Al, and Ti, compatible with observed exsolution products. Plagioclase lamellae (An34–53) are more calcic than matrix plagioclase (An32–38) but are the most sodic recognized in any massif anorthosite thus far.Field characteristics of megacrysts indicate that they formed contemporaneously with the other minerals in the same outcrops. Furthermore, bulk OPMs have XMg similar to matrix pyroxene (or are slightly more Fe-rich). In comparison, OPMs from the Saint Urbain massif are more magnesian (~En66–72), whereas lamellae and matrix plagioclase are more anorthitic (~An55–75 and ~An40, respectively). These systematic differences between the two massifs suggest that the En content of OPMs and the An content of plagioclase (matrix and lamellae) are strongly controlled by local magma composition. Collectively, these results can be explained best by the in-situ growth of megacrysts, together with or after adjacent plagioclase. The megacrysts bulk compositions can be adequately accounted for by a rapid growth mechanism. The common association of OPMs with ilmenite (and biotite) is noteworthy and suggests that oxidizing conditions played an important role in controlling the properties and compositions of the megacrysts. The implication is that these megacrysts are not relics of prior high-pressure pyroxene fractionation and provide no demonstrable link to a basaltic parent magma for this massif. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.