Abstract
In the Reeves Bluffs area, southern Victoria Land, Antarctica, metasediments and metabasalts belonging to the Skelton Group are the oldest rock types. Lithologies include pelite/psammite pairs, calc-silicate skarns and amphibolite bands. The mineralogy of these lithologies is consistent with amphibolite facies metamorphism, which occurred during the early stages of the late Neoproterozoic-early Paleozoic Ross Orogeny. Detrital zircon U-Pb ages from a psammite sample reveal a significant Grenville-aged (c. 1.1-1.3 Ga) sedimentary source component. Potential Grenville-aged sources for the Skelton Group sediments in the area include the East Antarctic Craton and Laurentia (which was possibly in close proximity to East Antarctica during the Neoproterozoic).
Based on geochemistry and petrography, three distinct granitoid types intruded the Skelton Group in the Reeves Bluffs area during the Ross Orogeny: The Reeves Bluffs calc-alkaline suite, which has I-type calc-alkaline affinities, is the most voluminous of the granitoid types. This suite has been subdivided into at least four plutons of which the Reeves Bluffs granite is the largest, occurring throughout the area. The Northface and Rhino granites and the Camp Ridge granodiorite are smaller plutons that are less than 1.5 km across. A U-Pb age of 540 ± 3.8 Ma was obtained from the Camp Ridge granodiorite, which is similar to a previously obtained age of 548 Ma from the calc-alkaline Cape Murray granodiorite from the nearby Fontaine Bluff. These are the oldest granitoid ages from the Carlyon-Darwin area, suggesting magmatism associated with the Ross Orogeny was initiated along this segment of the East Antarctic Craton margin at 548-540 Ma. Field relations show that calc-alkaline magmatism continued in the Carlyon-Darwin area until at least 536 Ma.
The A-type West Carlyon leucogranite occurs as small intrusions and dikes throughout the northern half of the Reeves Bluffs area and is the second oldest granite type in the Reeves Bluffs area based on a previous U-Pb age of 536 Ma from the nearby 'A-type' Foggy Dog Granite suite in the Brown Hills. These granites are characterized by high SiO₂, alkalis, Ga/Al, HFSE and low CaO, Ba and Sr relative to other granitoid types. A-type granitoids typically occur in extensional-anorogenic tectonic settings and have been inferred to form by high-temperature melting of depleted crust from which a previous melt has been extracted. In the Reeves Bluffs area, earlier calc-alkaline magmatism would have depleted the local crust. The gabbroic Fontaine Pluton in the Carlyon Glacier area, which is broadly synchronous with A-type magmatism in the area, is perhaps representative of a high temperature magmatic episode capable of melting depleted crust.
The Reeves Bluff adakite represents the youngest and second most voluminous granitoid type in the Reeves Bluff area. Based on mapping and previous work, a large adakite pluton or series of plutons are interpreted to exist to the east of Reeves Bluffs. Relative to other granitoid types these rocks contain high Sr/Y ratio and low Y and HREE, which can be attributed to a melt derived from below the garnet-in transition (>40 km depth), in the absence of residual or fractionating plagioclase. The partial melting of basaltic underplate at a depth exceeding 40 km, which may have developed during voluminous calc-alkaline magmatism in the area, is the inferred model for adakitic magmatism in the area. Based on two previous U-Pb ages on adakites in the Carlyon-Darwin area, the Reeves Bluffs adakite is probably aged between 515 and 491 Ma.
The broadly synchronous relationship between convergent-related calc-alkaline and extensional-related A-type magmatism in the Carlyon-Darwin area also occurs c. 60 km to the northeast in the Mulock Glacier area. The Mulock-Darwin area may therefore represent a transitional zone between the extensional Koettlitz Glacier Alkaline Province (c. 70 km to the north of Reeves Bluffs, which is dominated by alkaline magmatism) and a more convergent margin to the south.