BH250-221
Title
Subject
Description
Minor Minerals: zircon, apatite
Mineralogy Comments: good example of S-type of igneous rock
BH250-221 from Moody Nunatak is an excellent example of a peraluminous, garnet-bearing S-type granite, an igneous rock derived primarily from the partial melting of sedimentary source rocks. In hand sample, the rock is light colored and coarse grained, with interlocking crystals of Quartz and feldspar forming the dominant framework. Scattered reddish crystals of Garnet create a striking visual contrast and immediately distinguish this specimen from more typical granitoids. The presence of both Muscovite and Biotite further supports its strongly peraluminous composition and sediment-derived origin. Orthoclase and Plagioclase form the dominant feldspar assemblage, while accessory Zircon and Apatite preserve important trace element and geochronologic information.
This specimen serves as an excellent teaching example of S-type granite, where the “S” refers to a sedimentary protolith. These granites typically form when aluminum-rich sedimentary rocks such as shale or pelitic sediments undergo partial melting during continental collision, crustal thickening, or regional metamorphism. The resulting melts become enriched in aluminum relative to sodium, calcium, and potassium, allowing minerals such as garnet and muscovite to crystallize. This rock provides an excellent opportunity to discuss crustal recycling and the generation of felsic magmas from pre-existing continental materials. The occurrence at Moody Nunatak is especially significant because nunataks are isolated mountain peaks that protrude above surrounding ice sheets and expose rare bedrock otherwise hidden beneath Antarctic ice. These exposures help geologists reconstruct the tectonic evolution of East Antarctica and the assembly and breakup of Gondwana.
This sample also carries important historical significance because it was collected and donated by John Goodge (class of 1990 and professor of geology, University of Minnesota Duluth), whose Antarctic fieldwork contributed directly to expanding teaching collections and connecting students to real field-based geological exploration. That provenance adds an important educational dimension, students are not only studying an unusual granite, but also engaging with a specimen tied directly to polar field research.
The geochemistry of BH250-221 adds another important layer to its interpretation. Although the rock is mineralogically unmistakably granitic, its bulk composition is somewhat unusual compared with many typical granites. The sample contains 65.74 wt.% SiO₂, which is slightly lower than the common granite range of approximately 68–77 wt.% SiO₂ and places it near the boundary between granite and granodioritic compositions based solely on chemistry. This lower silica content likely reflects the relatively high abundance of ferromagnesian minerals such as biotite and garnet, both of which incorporate significant Fe, Mg, and Al. The rock also contains 8.06 wt.% Fe₂O₃(T), which is relatively high for an evolved granite and contributes to the lower overall silica percentage.
The relatively high Al₂O₃ content (15.82 wt.%) strongly supports its classification as a peraluminous S-type granite. Aluminum-rich sedimentary source rocks commonly produce melts enriched in aluminum, allowing minerals such as muscovite and garnet to crystallize. Moderate K₂O (2.82 wt.%) and Na₂O (2.77 wt.%) indicate a relatively balanced feldspar assemblage, while CaO (3.26 wt.%) suggests a modest contribution from plagioclase or melting of source rocks that were not entirely pelitic. Trace element enrichment in Rb (137 ppm), Th (26 ppm), U (4 ppm), Ce (165 ppm), La (84 ppm), and Zr (430 ppm) further supports an evolved crustal origin, with elevated zirconium likely reflecting abundant zircon and elevated phosphorus likely associated with apatite.
This specimen provides an important teaching reminder that igneous rock classification is based on modal mineralogy (QAPF classification) rather than chemistry alone. Even though the SiO₂ content is slightly lower than what many students expect for granite, the abundance of quartz and alkali feldspar firmly places this rock in the granite field. Its unusual chemistry reflects its garnet-rich mineral assemblage, elevated iron content, and derivation from a complex sedimentary source during crustal melting in Antarctica. BH250-221 is therefore an outstanding specimen for teaching igneous petrology, geochemistry, crustal recycling, and Antarctic tectonic history.
Coverage
Moody Nunatak