Beryllium in Antarctic Ultrahigh-Temperature Granulite-Facies Rocks and its Role in Partial Melting of the Lower Continental Crust

Text: Geologic evolution of the Napier Complex began in the Early Archean (3800
Ma) prior to a regional ultrahigh temperature metamorphic event (up to 7-11
kbar, 1050-1120 degrees C) sometime between 2550 and 2480 Ma, after which
metamorphic
and igneous activity was largely confined to mafic dike emplacement and
amphibolite to granulite-facies events at 1600 Ma, ~1000-900 Ma and 540-520 Ma
along the periphery and in highly localized shear zones. U-Pb isotopic data
reported in the literature for zircon from a pegmatite (sample #2233) and for
monazite from a paragneiss at "Zircon Point", Khmara Bay, gave
upper and lower intercept ages of 2400-2500 Ma and ~1100 Ma, respectively.
Electron microprobe age determinations from monazite-(Ce) in pegmatites and
quartz granulites show considerable micrometer-scale age heterogeneity.
Monazite-(Ce) grains in the pegmatites are coarse (1 to 5 cm) and exhibit
considerable chemical variation (up to 28 wt% ThO2). Th, U, and Pb
compositional maps of these coarse monazites show sector and growth zoning, but
apparent age maps exhibit no consistent age domains. Apparent age variations
are continuous with a few scattered, slightly older domains. Chemical ages from
5 micrometers spots vary from 2352 +/-85 Ma to 800 +/-40 Ma and are observed
to
vary as much as 400 Ma in apparent age over a 30 micrometer distance. Alkali
feldspars in contact with monazite-(Ce) grains are strongly enriched in Pb. An
aureole 2 to 4 mm thick surrounding the monazite-(Ce) contains 0.2 to 2.0 wt%
PbO, and K-feldspar included in monazite-(Ce) contains up to 9.9 wt% PbO.
Monazite-(Ce) grains from quartz granulites range from 20 to 100 micrometers
and
tend to be more uniform in composition (6 - 8 wt% ThO2 and 0.6 to 1 wt%
PbO). They have higher Y and have measurable HREE contents. Apparent age maps
exhibit scattered, uniformly old apparent age domains (~2400 +/-100Ma). They
also have both linear and irregular, variable apparent age domains, varying
from 1650 to 2010 Ma. We attribute the chronological heterogeneity to partial
lead loss during a late-Proterozoic event at ~1100 Ma that resulted in partial
lead loss. Lead from the pegmatitic monazite-(Ce) diffused into contiguous
alkali feldspar. Where there are linear zones of low apparent age, lead loss
may have been controlled by fractures, whereas irregular micron-scale
heterogeneity suggests recrystallization. TEM investigations of monazite-(Ce)
at the sub-micron scale are underway to evaluate these two alternatives.

--- Makarochkinite, a new beryllosilicate mineral of the aenigmatite group.
Together with J. Barbier, E.P. Shcherbakova and others I have succeeded in
obtaining approval of makarochkinite, Ca2Fe2+4Fe3+TiSi4BeAlO20, as new mineral
species from the Commission on New Minerals and Mineral Names, International
Mineralogical Association (number 2002-009). Validation of makarochkinite as a
mineral distinct from i is part of a larger crystallographic study of
beryllosilicate members of the aenigmatite group and related minerals,
including beryllian sapphirine, khmaralite, surinamite and welshite. A
manuscript is currently presently being written up; a portion of this study has
been accepted for oral presentation at the 32nd International Geological
Congress in Florence, Italy, August, 2004. In both makarochkinite and
hogtuvaite, the one Be per formula unit is evenly divided between the T1 and T4
(Q3) sites so that Be-O-Be bridges are minimized. Ca and Na occupy the
7-coordinated M8 and M9 sites, and Mg together with Fe occupy the M1-6
octahedral sites without evidence of strong ordering in either case. The
shorter average M7-O distances (2.03 angstroms) and the associated Ueq
displacement parameters led to assign all Ti to the M7 octahedral sites in both
structures, together with Fe3+ and other minor high-valent elements. As a
result, the M7 site occupancy in makarochkinite (59% Ti + 37% Fe) establishes
it as a distinct mineral and also distinguishes it from hogtuvaite (70% Fe +
24% Ti). In contrast to makarochkinite and hogtuvaite, surinamite has only one
T site joined to three others (so-called Q3 site), and this T site is fully
occupied by Be, with negligible Be on other T sites. Welshite contains more Be
than either makarochkinite or hogtuvaite, and we are currently trying to refine
Be
occupancy of this complex mineral.

--- Beryllium and boron minerals in Indian granulite-facies rocks. The Eastern
Ghats belt of southeastern India is an ultrahigh-temperature granulite-facies
terrain having many similarities to the Napier Complex; moreover, these two
terrains are believed to have been juxtaposed in Gondwana reassemblies. For
these reasons, a report of surinamite, a beryllosilicate found in Napier
Complex anatectic pegmatites, attracted my interest. A study of the Eastern
Ghats surinamite paragenesis would be highly relevant to my project on
beryllium and its minerals in the Napier Complex. However, no surinamite was
found either by me or by one co-author of the original report. My conclusion
that the reported surinamite was misidentified hypersthene was published in
Current Science. In the course of studying rocks from the locality where
surinamite was reported to occur, I discovered prismatine, a new locality for
India, and this find was published in Mineralogical Magazine.