Ian L. Millar

Basement chronology of the Antarctic Peninsula: recurrent magmatism and anatexis in the Palaeozoic Gondwana Margin

Abstract: A revised pre-Jurassic chronology is presented for the Antarctic Peninsula on the basis of new U–Pb zircon dating, using both conventional (11 samples) and SHRIMP microprobe data (10 samples). The age range for plutonism and high-grade metamorphism is from 435 ± 8 Ma (Silurian) to c. 206 Ma (approximately the Triassic–Jurassic boundary), with peaks of activity in Devonian, Carboniferous, Permian and mid-Triassic times, although some are represented by rocks with limited outcrops. The new data confirm the importance of Mid- and latest Triassic magmatic events, previously identified using Rb–Sr geochronology. The Adie Inlet gneiss, previously thought to be Neoproterozoic, is recognized as a Permian migmatite derived from paragneiss with a provenance dominated by Cambrian granitoids. Granite gneiss from NW Palmer Land, previously dated as Cambrian, is shown to be Triassic. Detrital zircons in metasedimentary rocks, and inherited zircons in granitoids, are dominated by Mesoproterozoic to Cambrian components, with sparse Palaeoproterozoic to Archaean grains, suggesting sources within Gondwana. At times these sources were nearby, as shown by an Archaean cobble-sized granite clast from a Permo-Triassic conglomerate; another clast in the same conglomerate, previously thought to be Devonian in age, has an Ordovician crystallization age.

Magmatism in the South Sandwich arc

Abstract The South Sandwich Islands are one of the world’s classic examples of an intraoceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas-Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by c. 36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.

Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica

U-Pb detrital zircon geochronology from the upper Cambrian to Devonian part of the Ellsworth Mountains succession, Antarctica, yields dominant late Mesoproterozoic and late Neoproterozoic–Cambrian age populations that are onsistent with a provenance from within Gondwana. Hf isotope compositions reveal a source predominantly within west Gondwana and identify a change in provenance up-stratigraphy that coincides with the change of sedimentation setting from active rift to passive margin, which has been independently determined by stratigraphic, structural, and geochemical arguments. For the Late Cambrian Frasier Ridge Formation, late Mesoproterozoic grains have positive eHf values, suggesting derivation from juvenile crust, and late Neoproterozoic–Cambrian grains have eHf values greater than –5, consistent with remelting of similar juvenile late Mesoproterozoic crust during the Pan African–Ross orogenies. Provenance during rifting was from proximal sources from within west Gondwana, most likely, southernmost Africa and basement to the Ellsworth-Whitmore Mountains block. At higher stratigraphic levels where deposition occurred along a passive margin, in the early Ordovician Mount Twiss Member and middle Devonian Mount Wyatt Earp Formation, late Neoproterozoic–Cambrian grains have eHf values less than –5; this means that early Mesoproterozoic–Archean crust was remelted to generate these zircons. Provenance was from a more expansive source region within west Gondwana, and probably included the Kaapvaal and Congo cratons of south and west Africa. Isolated outcrops of sedimentary rock of uncertain age at Mount Woollard and the Whitmore Mountains have detrital zircon signatures similar to the Frasier Ridge Formation, suggesting correlation with these Late Cambrian deposits. Sedimentary rock from the Stewart Hills contains some late Mesoproterozoic grains with lower eHf values than the previously mentioned samples. This suggests that the Stewart Hills sample has a provenance from within east Gondwana and was possibly deposited on the East Antarctic craton prior to the Ross orogeny and is not part of the displaced Ellsworth-Whitmore Mountains crustal block.

Geochemical tracing of Pacific-to-Atlantic upper-mantle flow through the Drake passage

The Earths convecting upper mantle can be viewed as comprising three main reservoirs, beneath the Pacific, Atlantic and Indian oceans. Because of the uneven global distribution and migration of ridges and subduction zones, the surface area of the Pacific reservoir is at present contracting at about 0.6 km2 yr-1, while the Atlantic and Indian reservoirs are growing at about 0.45 km2 yr-1 and 0.15 km2 yr-1, respectively. Garfunkel and others have argued that there must accordingly be net mantle flow from the Pacific to the Atlantic and Indian reservoirs (in order to maintain mass balance), and Alvarez further predicted that this flow should be restricted to the few parts of the Pacific rim (here termed ‘gateways’) where there are no continental roots or subduction zones that might act as barriers to shallow mantle flow. The main Pacific gateways are, according to Alvarez, the southeast Indian Ocean, the Caribbean Sea and the Drake passage. Here we report geochemical data which confirm that there has been some outflow of Pacific mantle into the Drake passage—but probably in response to regional tectonic constraints, rather than global mass-balance requirements. We also show that a mantle domain boundary, equivalent to the Australian–Antarctic discordance, must lie between the Drake passage and the east Scotia Sea.

A combined zircon SHRIMP and Sm-Nd isotope study of high-grade paragneisses from the Mid-German Crystalline Rise: evidence for northern Gondwanan and Grenvillian provenance

SHRIMP analyses of detrital zircon cores from high-grade metasediments from the Mid-German Crystalline Rise, which is situated between Eastern Avalonia in the NW and Saxo-Thuringia in the SE, yield mostly ages of c. 550 Ma and c. 2.06 Ga, with minor c. 1.0 and 2.4–2.9 Ga components. The sedimentary protoliths were deposited during the Late Proterozoic to Early Cambrian, probably prior to the break up of the northern Gondwana margin at 460–500 Ma. These data are consistent with the sediments’ high ϵNd values (0.9 to −3.0), which are comparable to those of well-documented Late Proterozoic sediments from other parts of Europe. The combined isotopic data suggest derivation of the sediments from at least three distinct crustal source regions. Dominant sources were the Avalonian–Cadomian orogenic belt (c. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West African and/or eastern Amazonian cratons (c. 45%). The Grenvillian belt was a minor source (c. 10%).

Geodynamic evolution of the Antarctic Peninsula during Mesozoic times and its bearing on Weddell Sea history

Abstract This review of the tectonic evolution of the Antarctic Peninsula during Mesozoic times highlights four main events; (1) Late Triassic-Late Jurassic extension, (2) Late Jurassic-Early Cretaceous dextral transpression, (3) Early Cretaceous extension and (4) mid-Cretaceous compression. Magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in Early Jurassic and Late Jurassic-Early Cretaceous times. The second of these breaks corresponded to the first compressional event. There was no apparent hiatus in the magmatic record during the mid-Cretaceous compressional event, although there was a significant change in the pattern of sedimentation in the Larsen basin on the eastern margin of the Weddell Sea at about that time. The tectonic evolution of the peninsula is compared to, and puts some constraints on, existing Weddell Sea models. The Late Triassic-Late Jurassic arc extension correlates with initial rifting in the Weddell Sea region during sinistral motion between East and West Gondwana. However, there is no known record of large-scale pre-Mid-Jurassic transcurrent deformation in the Antarctic Peninsula that would have been consistent with rotation of West Antarctic crustal blocks in the initial rifting period. The peninsula-wide Late Jurassic-Early Cretaceous compressional event may correlate with the major change in Gondwana plate motions from E-W to N-S (African reference frame) separation of East and West Gondwana. This change probably resulted in formation of an ocean-continent boundary along the northern margin of the Weddell Sea embayment and initial seafloor spreading. Geological data do not seem to support subduction of southwestern proto-Weddell Sea oceanic lithosphere beneath the eastern margin of the peninsula at that time. Cretaceous arc extension was coeval with the initial seafloor spreading phase in the Weddell Sea. Mid-Cretaceous arc compression, linked to a global increase in ocean floor spreading rates and a superplume event, correlates with a change from NE-SW to NW-SE spreading in the Weddell Sea.

The Pre‐Cenozoic magmatic history of the Thurston Island Crustal Block, west Antarctica

New Rb-Sr and K-Ar geochronological data are presented for the majority of known pre-Cenozoic outcrops in Thurston Island, the Jones Mountains, and the western Eights Coast, which collectively represent the basement geology of the Thurston Island crustal block of West Antarctica. Almost all are of calc-alkaline igneous or metaigneous rocks, and indicate long-standing proximity to a magmatic arc. The observable history began with Late Carboniferous (309±5 Ma) emplacement of mantle-derived orthogneiss precursors in eastern Thurston Island. Nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200–1400 Ma throughout the area. A variety of cumulate gabbros was emplaced soon after gneiss formation, followed by crust-contaminated diorites that have Triassic mineral cooling dates of 240–220 Ma. In the nearby Jones Mountains, the oldest exposed rock is a muscovite-bearing granite with an Early Jurassic age of 198±2 Ma; its initial 87Sr/86Sr ratio of 0.710 and ϵNdt values of −5 to −7 indicate either anatexis or, at least, a high degree of crustal input during magma genesis. This belongs to a suite of such granites known throughout the Antarctic Peninsula and related to earliest rifting of the Gondwana supercontinent. The subsequent evolution of the Thurston Island area was dominated by I-type magmatism, apparently in two major episodes at 152–142 Ma (Late Jurassic granites) and 125–110 Ma (Early Cretaceous bimodal suite). Most of these magmas had initial 87Sr/86Sr ratios of 0.705–0.706 and ϵNdt values of +2 to −4 and were derived from slightly enriched mantle or from juvenile lower crust. They are thought to signify subduction of Pacific Ocean floor as in the adjacent parts of West Antarctica, although the Late Jurassic episode was of greater intensity in Thurston Island than elsewhere. The Cretaceous magmatism was intense and of Andean-type. Between 100 and 90 Ma, volcanism in the Jones Mountains became predominantly silicic, with increasing incorporation of crustal components (initial 87Sr/86Sr ratios of 0.706–0.709 and ϵNdt values of −3 to −6), as subduction-related magmatism ceased in this part of the margin.

U–Pb zircon (SHRIMP) ages for the Lebombo rhyolites, South Africa: refining the duration of Karoo volcanism

U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded rhyolites yield ages of 182.0 ± 2.1 and 179.9 ± 1.8 Ma, whilst the overlying Jozini Formation rhyolite yields an age of 182.1 ± 2.9 Ma. Combined with existing 40Ar/39Ar geochronology, the new SHRIMP data fine-tunes the chronology of the Karoo volcanic province and indicates the 12 km succession of volcanic rocks in the Lebombo rift were erupted in 1–2 million years and lends considerable support to the links between the Pleinsbachian–Toarcian extinction event and the global environmental impact of Karoo volcanism.

Neoproterozoic extensional basic magmatism associated with the West Highland granite gneiss in the Moine Supergroup of NW Scotland

Pre-tectonic metagabbroic rocks emplaced into the Glenfinnan and Loch Eil groups of the Moine Supergroup give a U–-Pb zircon age of 873 ± 6 Ma. This new age for the metagabbros confirms the absence of Grenvillian (c. 1.0 Ga) tectonic events in the Moine assemblage. The metagabbros are spatially associated with the Glen Doe body of the West Highland granite gneiss, and were emplaced soon after the granite gneiss protolith. The metagabbros have chemical characteristics indicating contamination with local country rocks. A regionally developed suite of tholeiitic metadolerite dykes post-dates the metagabbros. These dykes are geochemically similar to modern mid-ocean ridge basalt, albeit modified by interaction with metamorphic fluids. The presence of abundant MORB-like basaltic dykes, coupled with the lack of major compressional structures associated with the intrusive events, suggests that the c. 873 Ma event may have been dominated by extensional tectonics.

Trace-element and isotopic characteristics of small-degree melts of the asthenosphere: Evidence from the alkalic basalts of the Antarctic Peninsula

Abstract Miocene-Recent continental alkalic basalts were erupted along the Antarctic Peninsula as a result of decompressional melting of the asthenosphere caused by the formation of slab-windows beneath the continental margin following the cessation of subduction. The basalts appear not to be related to a period of major lithospheric attenuation, nor were they formed as a result of the influence of a mantle plume. They exhibit strong trace-element and isotopic affinities with OIB, Sr- and Nd-isotope compositions ranging from 0.70269 to 0.70343 and 0.512863 to 0.51300, respectively, similar to the composition of HIMU OIB. However, new Pb-isotope analyses show that 206 Pb 204 Pb ratios (18.79–19.28) fall within the range for E-type MORB with Δ8 4 and Δ7 4 varying from −28 to +26 and from +1 to +10, respectively. Δ8 4- values , Sr-isotope ratios and some LILE/HFSE ratios exhibit negative covariations with La n Yb n and Nb/Y ratios implying some control of degree of partial melting on geochemical composition. Nb/U ratios (14–40) are considerably lower than most OIB and MORB. The basalts also have unusually low absolute abundances of Rb and Ba and high K/Ba and K/Rb ratios (50–140 and 400–1500, respectively). Correlated PbSrNd isotope and trace-element behaviour suggests that the asthenosphere from which these basalts were derived was subjected to multiple melt extraction/depletion events. One period of melt extraction was ancient (∼ 1.7 Ga) and similar to that affecting MORB source mantle, and was followed by a more recent (?Mesozoic) event. This more recent event resulted in increased U/Pb, U/Nb and U/Th ratios and further depletion in ultra-incompatible element such as Rb and Ba, causing high K/Rb and K/Ba ratios in the erupted lavas. This implies that the asthenosphere beneath the Antarctic Peninsula is heterogeneous on a small scale. Small-degree melts are capable of sampling geochemically, and possibly mineralogically, distinct mantle domains from larger-degree melts. During larger degrees of partial melting, the scale of melting approaches the scale of heterogeneity and integration of melts from different geochemical domains occurs.