TECTONICS, VOL. 15, NO. 6, PAGES 1309-1324, DECEMBER 1996 Zircon U-Pb geochronology of plutonic rocks from the Antarctic Peninsula: Confirmation of the presence of unexposed Paleozoic crust Jean A. Tangeman and Samuel B. Mukasa Department of Geological Sciences, University of Michigan, AnnArbor Anne M. Grunow Byrd Polar Research Center, OhioState University, Columbus Abstract. The Antarctic Peninsula represents the southernmost segmentof a magmaticarc once present along the western margin of Gondwanaland. Zircon U-Pb geochronology of a suite of calc-alkalinegranitoids from the west coast of Graham Land, northern Antarctic Peninsula, resulted in ages for the rocksranging from 117.0 _+ 0.8 to 7 3.6 _+ 0.4 Ma. Discordant zircon populations from peraluminous granites reveal inheritance characteristicsresulting from entrainment of older crustalmaterials. Upper intercept ages of discordant zirconpopulations in four samples indicate that the age of assimilated material is approximately late Paleozoic. This inherited component is present in four samples from Bone Bay, Charlotte Bay, and Stonington Island and thus may extendfor ~600 km along the west coastof Graham Land. A possible source for the inherited components is the late Paleozoic(?) sediments of the Trinity Peninsula Group. However, a late Proterozoic upper intercept age for the Charlotte Bay granodiorite in western central Graham Land indicates that the age of the rocks assimilated by this suite of plutons is not uniformly late Paleozoic and might include unexposed basement rocks. Finally, an upperintercept age of 431 _+ 12 Ma from a granite clast in metaconglomerate from Horseshoe Island provides further evidence for thepresen ce of mid-Paleozoic basement in Graham Land. These results confirm the already postulatedminimum early Paleozoic age for Antarctic Peninsula basement. Introduction and Geologic Setting We report age data on a suite of plutonic rocks collected from five Antarctic Peninsula locations along a 1,000-km stretch of the westcoastof Graham Land, between Stonington Island and Bone Bay (Table 1). The data lend support to theories of the tectonic and magmatic evolution of the Antarctic Peninsula which have developed in large part in response to the extensivemapping and research carried out by the British Antarctic Survey over the past 40 years. Prior to this work, however, zircon U-Pb determinations had been limited to those of Miller et al. [1987] who focused on provenances of detrital zircons from the Trinity Peninsula Copyright 1996 by theAmerican Geophysical Union. Paper number 96TC00840. 0278-7407/96/96TC-00840512.00 Formation, northern Antarctic Peninsula, and Harrison and Loske [1988] who reported a Paleozoicage for an orthogneiss in Palmer Land. The Antarctic Peninsula is a long narrow arcuate sliver of continental crust, borderedby oceanic crust of the South Pacific Ocean to the west, Weddell Sea to the east, and Scotia Sea to the north [Barker, 1982]. The Antarctic Peninsula is one of four crustal blockswhich make up West Antarctica: (1) Antarctic Peninsula,(2) Thurston Island, (3) Marie Byrd Land, and (4)Ellsworth-Whitmore Mountains (Figure 1; inset A) [Dalziel and Elliot, 1982], eachhaving a distinct Phanerozoic history. Of theseblocks,the Antarctic Peninsula is the largest and is divided into a southern sector, Palmer Land, and a northern sector, Graham Land (Figure 1). The Antarctic Peninsulawas near its present-day position with respect to East Antarcticaby about 130 to 100 Ma, as indicatedby early andmid-Cretaceous palinspasticpoles [Grunow,1993]. Prior to the initiationof Gondwana breakup at 175 Ma, however,the relative positionof the Antarctic Peninsula is unknown. The exposed geology of the northern Antarctic Peninsula is dominated by mid-Mesozoicto Cenozoicplutonicand volcanic rocks[Pankhurst,1990]. These rockswere formed in response to the subduction of Pacific ocean crust, specifically the Phoenix (Aluk)plate, beneathGrahamLand [Saunders et al., 1980; Barker, 1982]. This magmatic arc was the southerly continuation of the Andean Cordillera during the Mesozoic until fragmentation of the intervening Scotia arc and the concomitant opening of the Drake Passage between southern Chile and the Antarctic Peninsula approximately 30 Ma ago [Barker and Burrell, 1977; Pankhurst, 1990]. Evidencethat the Antarctic Peninsulawas contiguouswith southern South America,andwaspartof the Pacific convergent margin of Gondwanaland beginning in the mid-Paleozoic, is found in pre-breakupmagmatic arc rocks. For example, graniticsheets in a banded migmatite from TargetHill (Figure 1), centraleastern GrahamLand, give an apparent Rb-Sr whole rock age of 336 +_ 34 Ma [Pankhurst,1983]. Milne and Millar [1991] analyzedorthogneiss from Target Hill and reported three Rb-Sr wholerock isochronagesof 410 +_ 15, 426 + 12, and 375 + 68 Ma. A suite of granophyric cobbles in a conglomerate from the View Point Formation, Trinity Peninsula Group at the northeastern tip of Graham Land yielded a 386 _+39 Ma Rb-Sr errorchron (MSWD=80.3) [Pankhurst, 1983]. Harrison and Piercy [1991] described orthogneiss from Campbell Ridges (northwestern Palmer Land) which yielded 1309