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STRUCTURAL STATE OF PLAGIOCLASE FROM ANORTHOSITIC XENOLITHS IN THE BEAVER BAY COMPLEX, NORTHEASTERN MINNESOTA, USA
Anorthosite xenoliths varying in size from cm to 100s of meters dot the landscape of Northeastern Minnesota. Most of these occur near the tops of sills in the hypabyssal Beaver Bay Complex, part of the Middle Proterozoic Midcontinent Rift System. Samples from this area were collected and X-ray diffraction on mineral separates, as well as microprobe analyses, were carried out in an attempt to understand the thermal history and ultimately the source within the crust of the inclusions.
Unit-cell parameters were calculated using UnitCell (Holland and Redfern, 1997) from reflection positions derived with the profile fit program WinPlotr (Roisnel and Rodriguez-Carvajal, 2000). The indices were standard for the space group C1 for plagioclase with c approximately 7 Å. Unit cell dimensions and anorthite (An) content were used to determine Al,Si distributions using the γ method of Kroll and Ribbe (1980).
The An values show a bimodal distribution. One group with An ranging from 75-79, has granoblastic fabric, and plots on the ordered plagioclase structure trend with T10-<T1M> ranging from 0.15-0.16. This group is similar to the tectonites of Morrison et al. (1983). The second group, with more lath-like igneous textures, has lower An (65-69), and intermediate to ordered structures with T10-<T1M> ranging from 0.14 - 0.20.
Anorthosite masses formed at great depth in the crust would be expected to be disordered because of the high temperature environment. The relatively ordered structure of the granoblastic inclusions may be due to an annealing event, perhaps during a metamorphic event as suggested by Morrison et al. (1983). The less ordered structures of the igneous textured inclusions could indicate they were not as affected by such an event. Together with the differences in chemistry, this leads to the conclusion that these two types of xenoliths may have formed in different crustal environments.
References: Holland, T.J.B. and Redfern, S., 1997, Min. Mag. 61: 65-77; Kroll, H. and Ribbe, P.H., 1980, Am. Min. 65: 449-457; Morrison, D.A. et al., 1983, GSA Bull. 94: 206-221; Roisnel T. and Rodriguez-Carvajal J., 2000, Proc. EPDIC 7: 118--123.