Magnetic petrology of lower crust and upper mantle xenoliths from McMurdo Sound, Antarctica

Abstract

A suite of xenoliths was collected from Ross Island in the McMurdo Sound area, Antarctica. The purpose of the study of these xenoliths was to determine the level of magnetization in lower crust and upper mantle lithologies and to discover the mineralogical basis for the magnetization records. The majority of xenoliths are pyroxene granulites of mafic composition; a few hornblendites occur, and upper mantle dunites are fairly common. The primary mineral assemblage of the pyroxene granulites is plagioclase + calcic pyroxene ± orthopyroxene ± olivine ± ilmenite; mineral stability relations together with two-pyroxene geothermometry indicate derivation from depths of 12 to 20 km. The pyroxene granulites show evidence of partial melting, presumably related to incorporation of the xenoliths in the host basaltic magmas. Prior to their ascent to the surface, many underwent metamorphic reactions producing secondary hornblende, biotite and titanomagnetite. On the whole, the Antarctic xenoliths are weakly magnetic, but a few ilmenite-rich pyroxene granulites have NRM > 40 × 10 −4 A m 2/kg and X o > 0.005 SI units. The latter contain 1–3% titanomagnetite, which is largely of secondary origin and occurs chiefly as tiny quench crystals concentrated in fine-grained melt areas and as coronas about primary ilmenite crystals. Oxidation of Fe-Ti oxides is pervasive as revealed by the frequent occurrence of wispy ferrian rutile lamellae in ilmenite and trellis-type ilmenite oxyexsolution lamellae in titanomagnetite. The oxidized nature of the samples is further indicated by the presence of hematite and pseudobrookite-titanohematite intergrowths. The oxidation has caused a shift in titanomagnetite toward Fe 3O 4-rich compositions and in ilmenite toward higher Fe 2O 3. As a consequence, the pyroxene granulites exhibit thermal unblocking in the vicinity of 500–570°C. The high geothermal gradient, characteristic of a continental rift setting, argues for about 12 km of magnetic crust before the Curie point is exceeded. The primary oxide of consequence in this rift environment appears to be ilmenite, which may be a common lower crust oxide mineral in this type of tectonic setting. No evidence was found in thermomagnetic experiments or in petrographic studies to suggest the presence of native iron. The secondary origin of much of the titanomagnetite now present suggests that unaltered, in situ granulite lower crust in this area would be weakly magnetic even in a situation where the geothermal gradient was lower or where a tectonic reconfiguration would render the lower crust above the Curie geotherm. The strongest magnetizations reside in Cenozoic volcanic cap rocks, and these are expected to be the major source of local magnetic anomalies in the area.