Abstract The break-up of Gondwana during the Early–Middle Jurassic was associated with flood basalt volcanism in southern Africa and Antarctica (Karoo–Ferrar provinces), and formed one of the most extensive episodes of continental magmatism of the Phanerozoic. Contemporaneous felsic magmatism along the proto-Pacific margin of Gondwana has been referred to as a silicic large igneous province, and is exposed extensively in Patagonian South America, the Antarctic Peninsula and elsewhere in West Antarctica. Jurassic-age silicic volcanism in Patagonia is defined as the Chon Aike province and forms one of the most voluminous silicic provinces globally. The Chon Aike province is predominantly pyroclastic in origin, and is characterized by crystal tuffs and ignimbrite units of rhyolite composition. Silicic volcanic rocks of the once contiguous Antarctic Peninsula form a southward extension of the Chon Aike province and are also dominated by silicic ignimbrite units, with a total thickness exceeding 1 km. The ignimbrites include high-grade rheomorphic ignimbrites, as well as unwelded, lithic-rich ignimbrites. Rhyolite lava flows, air-fall horizons, debris-flow deposits and epiclastic deposits are volumetrically minor, occurring as interbedded units within the ignimbrite succession.

Abstract Large-volume rhyolitic volcanism along the proto-Pacific margin of Gondwana consists of three major episodes of magmatism or ‘flare-ups’. The initial episode (V1) overlaps with the Karoo–Ferrar large igneous provinces at c. 183 Ma. A second (V2) episode was erupted in the interval 171–167 Ma, and a third episode (V3) was emplaced in the interval 157–153 Ma. The magmatic events of the V1 and V2 episodes of the Antarctic Peninsula are reviewed here describing major and trace elements, and isotopic (Sr, Nd, O) data from rhyolitic volcanic rocks and more minor basaltic magmatism. An isotopically uniform intermediate magma developed as a result of anatexis of hydrous mafic lower crust, which can be linked to earlier, arc-related underplating. The subsequent lower-crust partial melts mixed with fractionated mafic underplate, followed by mid-crust storage and homogenization. Early Jurassic (V1) volcanic rocks of the southern Antarctic Peninsula are derived from the isotopically uniform magma, but they have mixed with melts of upper-crustal paragneiss in high-level magma chambers. The V2 rhyolites from the northern Antarctic Peninsula are the result of assimilation and fractional crystallization of the isotopically uniform magma. This process took place in upper-crust magma reservoirs involving crustal assimilants with an isotopic composition akin to that of the magma. A continental margin-arc setting was critical in allowing the development of an hydrous, fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the mantle plume thought to be responsible for the contemporaneous Karoo magmatic province and rifting associated with the initial break-up of Gondwana.

Northwestern Palmer Land is situated within the Mesozoic magmatic arc of the Antarctic Peninsula. Three temporally distinct granitoid groups, of lower Paleozoic, Jurassic, and Cretaceous ages, have been identified in this region. They are all calc-alkaline, I-type granitoids, ranging from quartz-diorites to granites, have high LIL/HFS element ratios, and their analyses all fall in the volcanic arc granite field when plotted on granite discrimination diagrams. The Paleozoic group comprises orthogneisses that have undergone high-grade metamorphism. The Jurassic group is variably foliated and partially recrystallized. It is subdivided into high-K metagranitoids and a smaller subgroup of low-K metatonalites, which are restricted to the western margin. The Cretaceous granitoids, which are undeformed, are subdivided on the basis of petrography into granophyric granites and nongranophyric granitoids. The orthogneisses, high-K metagranitoids, and nongranophyric granitoids are characterized by increasing LIL elements (particularly K and Rb) with increasing SiO 2 and an initial increase followed by a decrease in the HFS elements (Zr, Nb, and, to a lesser extent, Y). Variable degrees of partial melting or fractional crystallization could have produced the mafic to intermediate compositions, but plagioclase, K-feldspar, and minor phase fractionation are most likely responsible for the geochemical trends in the intermediate to acid compositions. In contrast the metatonalites show no enrichment in either the LIL or the HFS elements but still have high LIL/HFS element ratios. Their origin is uncertain, although they appear to have been derived from the same source as the high-K metagranitoids, but probably evolved under higher P H 2 O and f O 2 conditions. The Cretaceous granophyric granites are more enriched in both the LIL elements and the HFS elements at high SiO 2 levels than the nongranophyric granitoids. These are interpreted to have differentiated from the nongranophyric granitoids by the process of filter-pressing. Rb-Sr isotope data for all the groups suggest there is no correlation between initial 87 Sr/ 86 Sr ratios and time. The values lie in the range 0.705 to 0.707, suggesting that all the groups are predominantly mantle-derived but may contain a significant crustal component. The orthogneisses are part of the extensive Paleozoic plutonism that occurred along the length of the Pacific margin of Gondwanaland. Within the context of the Antarctic Peninsula as a whole, the Jurassic granitoids are geographically more restricted and geochemically more variable than the Cretaceous granitoids. The latter form a geochemically uniform group of plutons that occur throughout the Antarctic Peninsula.