TY  - JOUR
A1  - Ferrero, Silvio
A1  - Godard, Gaston
A1  - Palmeri, Rosaria
A1  - Wunder, Bernd
A1  - Cesare, Bernardo
T1  - Partial melting of ultramafic granulites from Dronning Maud Land, Antarctica
BT  - constraints from melt inclusions and thermodynamic modeling
JF  - American mineralogist : an international journal of earth and planetary materials
N2  - In the Pan-African belt of the Dronning Maud Land, Antarctica, crystallized melt inclusions (nanogranitoids) occur in garnet from ultramafic granulites. The granulites contain the peak assemblage pargasite+garnet+clinopyroxene with rare relict orthopyroxene and biotite, and retrograde symplectites at contacts between garnet and amphibole. Garnet contains two generations of melt inclusions. Type 1 inclusions, interpreted as primary, are isolated, < 10 mu m in size, and generally have negative crystal shapes. They contain kokchetavite, kumdykolite, and phlogopite, with quartz and zoisite as minor phases, and undevitrified glass was identified in one inclusion. Type 2 inclusions are < 30 mu m in size, secondary, and contain amphibole, feldspars, and zoisite. Type 2 inclusions appear to be the crystallization products of a melt that coexisted with an immiscible CO2-rich fluid. The nanogranitoids were re-homogenized after heating in a piston-cylinder in a series of four experiments to investigate their composition. The conditions ranged between 900 and 950 degrees C at 1.5-2.4 GPa. Type 1 inclusions are trachytic and ultrapotassic, whereas type 2 melts are dacitic to rhyolitic. Thermodynamic modeling of the ultramafic composition in the MnNCKFMASHTO system shows that anatexis occurred at the end of the prograde P-T path, between the solidus (at ca. 860 degrees C-1.4 GPa) and the peak conditions (at ca. 960 degrees C-1.7 GPa). The model melt composition is felsic and similar to that of type 1 inclusions, particularly when the melting degree is low (< 1 mol%), close to the solidus. However the modeling fails to reproduce the highly potassic signature of the melt and its low H2O content. The combination of petrology, melt inclusion study, and thermodynamic modeling supports the interpretation that melt was produced by anatexis of the ultramafic boudins near peak P-T conditions, and that type 1 inclusions contain the anatectic melt that was present during garnet growth. The felsic, ultrapotassic composition of the primary anatectic melts is compatible with low melting degrees in the presence of biotite and amphibole as reactants.
KW  - Nanogranitoids
KW  - partial melting
KW  - thermodynamic modeling
KW  - Antarctica
KW  - ultramafic granulites
KW  - kumdykolite
KW  - kokchetavite
KW  - High-Grade Metamorphism
KW  - Anatexis
KW  - and Granite Magmatism
Y1  - 2018
U6  - https://doi.org/10.2138/am-2018-6214
SN  - 0003-004X
SN  - 1945-3027
VL  - 103
IS  - 4
SP  - 610
EP  - 622
PB  - Mineralogical Society of America
CY  - Chantilly
ER  -