Origin of mafic xenoliths in granitoids of the central transantarctic Mountains, South Victoria Land (SVL), Antarctica

It is seldom possible to pose an unequivocal origin for mafic xenoliths in granitoids of a given locality. However, in several SVL localities, certain constraints on origin can be placed. Here, both angular and rounded mafic xenoliths occur in elongate bands in the mesozonal Larsen granodiorite, which concordantly intrudes adjacent Cambro-Ordovician metasediments of the Koettlitz group. The angular xenoliths (1) exhibit foliation, (2) contain the assemblage clinopyroxene + biotite +/- hornblende + plagioclase + quartz, and (3) represent nearly in situ stopping of adjacent Koettlitz amphibolite of identical texture and mineralogy. In contrast, the rounded xenoliths are unfoliated and contain the above assemblage, but without quartz. The authors regard the rounded xenoliths as stopped amphibolite from which quartz has been melted and mobilized out. Minimum temperatures for melting of quartz in an amphibolite are 700/sup 0/ - 750/sup 0/C at mid-crustal levels, yet two-feldspar geothermometry and mineral equilibria in Koettlitz gneisses and marbles, respectively, yield peak metamorphic P/sub L/-T conditions of 4 kbar and only 640/sup 0/C. Assuming thermal equilibrium between magma and Koettlitz metasediments, it follows that the rounded xenoliths must have originated several km deeper than the angular ones, where temperatures were sufficiently high to melt quartz. Presencemore » in the granitoid of flow-banding, normally-zoned plagioclase, and interstitial quartz and K feldspar suggest that the granitoid was intruded as a liquid-crystal mush, whose liquid was of near-minimum-melt composition (ca. 650/sup 0/C), and whose extensive crystal network supported dense mafic xenoliths during slow upward convection.« less