Geological position and isotopic age of intrusive charnockitic rocks in the Jennings Promontory (Eastern Wall of the Amery Glacier, East Antarctica)

Abstract

Intrusive hypersthene granitoids (charnockitic rocks) are widespread in the mountainous system bordering the Lambert Glacier with the world’s largest outlet and Amery Ice Shelf continuing in East Antarctica. In the eastern margin of the Amery Ice Shelf, they constitute large bedrock outcrops in the Jennings Promontory (approximately 6 km 2 ) and Branstetter Rocks (approximately 0.7 km 2 ) and several outcrops approximately 9.5 km 2 in total in Gillock Island (Fig. 1). These outcrops of charnockitic rocks most likely characterize a relatively large massif, which extends in the northeasterly direction and occupies an area of at least 140 km 2 in size. This massif intrudes metamorphic granulitic rocks represented by the Reinbolt orthogneiss and Larsemann paragneiss groups (Fig. 1). According to [1], the protolith of the Reinbolt Group is 1174‐1019 Ma old, while the sedimentary Larsemann Group accumulated 932‐916 Ma ago. Granulitic metamorphism superimposed on igneous and sedimentary rocks of these groups occurred in the period of 533 to 529 Ma ago [2]. The charnockites of the Jennings Promontory are considered now as age analogues of similar rocks in the Loewe Massif (western margin of the Amery Ice Shelf), the crystallization age of which is estimated by the SHRIMP method to be 980 ± 21 Ma [2]. In addition, some researchers [3] correlate these charnockitic rocks with their metamorphosed porphyroblasrtic counterparts from the Reinbolt Hills dated by the U‐Pb method on zircons back to approximately 896 Ma [4]. These views are, however, inconsistent with the data on the age of granulitic metamorphism of the Reinbolt and