Christopher D. Wareham

Pb, Nd, and Sr isotope mapping of Grenville‐age crustal provinces in Rodinia

New Pb, Nd, and Sr isotope data are presented for geochemically similar, ∼1.1–1.2 Ga, granitoids and tonalitic‐granitic orthogneisses from Antarctica, southern Africa, and the Falkland Islands and adjoining plateau, areas originally within the supercontinents of Rodinia and Gondwana. These data support contentions for the presence of a Mesoproterozoic (∼1.2 Ga) destructive plate margin running from Namaqua‐Natal (southern Africa), through the displaced microplates of the Falkland Plateau and Falkland Islands, the Haag Nunatak crustal block (West Antarctica) and into western Dronning Maud Land (East Antarctica). The bulk of these granitoids represent juvenile Mesoproterozoic additions to the crust, except for in parts of East Antarctica (i.e., the Sverdrupfjella) where older (Paleoproterozoic or Archean) crust was involved in granitoid generation. Our isotope data permit plate reconstructions in which southern Africa, East Antarctica, and the Falkland Islands and plateau were adjacent within Rodinia.

Tectonic setting of primitive magmas in volcanic arcs: an example from the Antarctic Peninsula

Abstract: Primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment. Most examples are either associated with high rates of arc-parallel extension, or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression. In two cases, the Vanuatu and Solomon Islands arcs, such arc compression is being caused by collision of seamounts. In the Antarctic Peninsula, primitive mafic dykes were emplaced perpendicular to the continental arc. Ar–Ar and K–Ar data suggest intrusion of the dykes atc. 126–106 Ma, possibly during mid-Cretaceous regional compression of the arc. The dykes form two compositional groups. One group has low LaN/YbN ratios (0.31–0.49), lower Nb/Yb and higher Th/Nb than N-MORB, age-corrected ϵNd values of +7.3 to +7.9, and are interpreted as melts of subduction modified sub-arc asthenosphere. The other has high LaN/YbN ratios (3.86–8.92), higher Nb/Yb and Th/Nb than N-MORB, age-corrected ϵNd values of −2.8 to +3.4, and are interpreted as melts of sub-arc lithosphere. The absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers.

The role of crustal and mantle sources in the genesis of granitoids of the Antarctic Peninsula and adjacent crustal blocks

Abstract: Magmatic rocks from the Antarctic Peninsula show marked variations in isotope composition, which reflect changes in the geodynamic evolution of the peninsula through time. Most Antarctic Peninsula granitoids formed as a result of subduction: they fall on well‐defined trends on plots of ϵNd, 207Pb/204Pb and δ18O against 87Sr/86Sri, between a component derived from subduction‐modified mantle or juvenile basaltic underplate (ϵNdi>6, 207Pb/204Pb=15.61, δ18O=5.5‰, 87Sr/86Sr<0.704) and an end‐member interpreted as a melt of Proterozoic lower crust ( ϵNd=−7, 207Pb/204Pb=15.67, δ18O=10‰,87Sr/86Sr=0.709). A small group of granitoids, emplaced before or during Gondwana break‐up, plot on distinct trends towards high 87Sr/86Sri compositions, reflecting mixing between melts derived from Proterozoic lower crust and melts of middle–upper crustal rocks (ϵNdi=−9, 207Pb/204Pb=15.64, δ18O=10‰, 87Sr/86Sr=0.726), with little or no input of new material derived from the mantle or from juvenile basaltic underplate. These granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of Gondwana break‐up. Similar trends are shown by data from granitoids of the adjacent crustal blocks of West Antarctica. The isotope data suggest that an enriched Ferrar/Karoo‐type lithosphere was not involved in the genesis of granitoids of the Antarctic Peninsula or of the Ellsworth–Whitmore Mountains crustal block.

Petrogenesis of the Cambrian Liv Group, a bimodal volcanic rock suite from the Ross orogen, Transantarctic Mountains

Cambrian volcanic rocks of the Liv Group, defined here as including the Wyatt, Ackerman, Taylor, Fairweather, and Leverett Formations, occur along the paleo-Pacific margin of Gondwana, in the Queen Maud Mountains, Transantarctic Mountains. The Ackerman and Wyatt Formations are dominated by massive dacite lava flows and were erupted ca. 525 Ma. The Taylor, Fairweather, and Leverett Formations form a bimodal assemblage of basalts and rhyolites and were erupted ca. 515 Ma. The dacites of the Ackerman and Wyatt Formations are the most light rare earth element (REE) and large ion lithophile element (LILE) enriched rocks (LaN/YbN = 6.6−10.2; Th/Nb = 0.9−1.8) of the Liv Group. They have the lowest ϵNdi (−1.8 to −3.1) of all the Liv Group volcanic rocks and are interpreted to be partial melts of continental crust. Sm-Nd model ages suggest that some of this crust may be as old as 1.5 Ga. The volumetrically minor basalts and basaltic andesites of the Taylor, Fairweather, and Leverett Formations are variably light REE and LILE enriched (varying from LaN/YbN = 1.5 to 6.0) and have ϵNdi between 5.7 and −1.1. The most depleted of these basalts are transitional between normal midocean ridge basalt (MORB) and enriched MORB and are interpreted as melts of asthenospheric mantle that were variably enriched in light REE and LILE by melts from lithospheric mantle and/or continental crust. The rhyolites of the Taylor, Fairweather, and Leverett Formations have LaN/YbN = 2.8−6.0, Th/Nb = 1.0−1.6, and ϵNdi between 2.1 and −2.8 and are interpreted as mixtures of fractionated mafic magma and crustal partial melt. The Liv Group rhyolites were probably generated in response to the mafic magmatism. The most likely tectonic setting for the Liv Group was in an extensional rift environment within or behind an active volcanic arc.

A Lower Cretaceous, syn-extensional magmatic source for a linear belt of positive magnetic anomalies: the Pacific Margin Anomaly (PMA), western Palmer Land, Antarctica

Ar–Ar laserprobe dating suggests that in western Palmer Land, plutons associated with a curvilinear belt of positive magnetic anomalies along the Pacific margin of the Antarctic Peninsula, the Pacific Margin Anomaly (PMA), are Early Cretaceous in age. The new ages, combined with published structural and geochemical studies, suggest that highly magnetically susceptible gabbroic to tonalitic–granodioritic rocks, the probable source of the Palmer Land segment of the PMA, were generated during Early Cretaceous extension when mantle-derived basaltic magma intruded mafic lower to middle crust. Continued extension uplifted newly generated, lower to middle crust through the Curie Isotherm (ca. 600°C) forming the magnetic anomaly. The PMA broadly tracks an arc-parallel band in western Palmer Land where crustal extension and uplift of lower crust were greatest. The close spatial relationship between the PMA and Early Cretaceous, syn-extensional plutons suggests that anomaly area can be used as a crude proxy for the volume of a related plutonic complex; the areal extent of the PMA indicates that a significant proportion of the arc crust was newly generated during the Early Cretaceous in western Palmer Land.

GRANITOID PLUTON FORMATION BY SPREADING OF CONTINENTAL CRUST : THE WILEY GLACIER COMPLEX, NORTHWEST PALMER LAND, ANTARCTICA

Abstract The emplacement mechanism, geometry, and isotope geochemistry of plutons of the Wiley Glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the Antarctic Peninsula magmatic arc in Early Cretaceous times. The suggested mechanism is analogous to basalt dyke injection during sea-floor spreading. During normal-dextral shear, the Burns Bluff pluton, a sheeted, moderately east-dipping, syn-magmatically sheared tonalite-granodiorite intruded syn-magmatically sheared quartz diorite of the Creswick Gap pluton and 140 ± 5 Ma hornblende gabbro. UPb dating of zircon and ArAr dating of hornblende and biotite suggest that both granite s.l. plutons were emplaced between 145 and 140 Ma, but that extensional shearing was active from the time of emplacement until ca. 127 Ma. The Burns Bluff pluton is chilled at its margin, and grades through mylonitised, porphyritic tonalite-granodiorite sheets and tonalite-granodiorite sheets with minor chilling, to a kilometre-scale body of coarse-grained, hypidiomorphic tonalite-granodiorite. Co-magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves. These dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal-dextral shear. Pluton-marginal porphyritic and hypidiomorphic tonalite-granodiorite forms large, fault-hosted sheets emplaced progressively under extension with minor dextral shear. Kinematic indicators from pluton-marginal granite s.l. dykes suggest that early in pluton accretion, intrusive sheets cooled rapidly, with simple shear prior to full crystallisation changing to ductile simple shear during cooling. Kinematic indicators towards the pluton core suggest that as the pluton grew, and cooled more slowly, emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma. Cross-cutting relationships with the coeval, syn-extensional, Creswick Gap pluton suggest that the Burns Bluff pluton was emplaced in a steeper, second generation shear structure, like those in normal fault systems. This suggests that the Wiley Glacier complex was emplaced above the base of the brittle-ductile transition zone (15–18 km depth). The Burns Bluff pluton has Nd and Sr isotope values that range from mantle dominated (ϵNd141 = +3.8, 87 Sr 86 Sr 141 = 0.70468 ) to more crustally influenced (ϵNd141 = −1.7, 87 Sr 86 Sr 141 = 0.70652 ). This range probably represents different degrees of mixing between mantle-derived magma and lower crustal partial melts generated in the garnet-stability zone (40+ km depth). Addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s.l. and mafic, modified, underplated magma to mid-crustal levels along extensional shear zones as the arc ‘spread’ were the primary mechanisms of crustal growth.

The Wiley Glacier complex, Antarctic Peninsula: pluton growth by pulsing of granitoid magmas

Early Cretaceous gabbro, quartz-diorite-granodiorite, and tonalite-granodiorite plutons of the Wiley Glacier complex, in the Antarctic Peninsula magmatic arc, were emplaced in a zone of syn-magmatic extensional shearing. The oldest component pluton is the Creswick Gap quartz-diorite-granodiorite pluton and this is cut by slightly younger hornblende gabbro of the Moore Point pluton. These plutons form the wall rock to the Burns Bluff tonalite-granodiorite pluton, the youngest component of the complex. The Burns Bluff pluton comprises tonalite-granodiorite sheets which, at the plutons margins, are interleaved with screens of mylonite wall rock. The Moore Point hornblende gabbro has ϵNd141 of ca. +4 and ϵSr141 of ca. +1. It is predominantly mantle in origin, although its parent magma assimilated some continental crust during emplacement. The Creswick Gap pluton has ϵNd141 between ca. +4 (quartz-diorite) and ca. −1 (granodiorite) and ϵSr141 from ca. −1 (quartz-diorite) to ca. +22 (granodiorite). This pluton contains both mantle and crustal components. We suggest that the quartz-diorite facies fractionated from a similar parent magma to that of the Moore Point gabbro, whilst the granodiorite fractionated from a gabbro-quartz-diorite magma during continued crustal assimilation. Geochemical variations within the Creswick Gap pluton cannot be generated by AFC models. Some of the granodiorite samples have chemical characteristics which typify partial melts of amphibole ± garnet-bearing crust. We conclude that the pluton comprises fractionates of basaltic magma and crustal partial melt. The Burns Bluff tonalite-granodiorite sheets have ϵNd141 between ca. +4 and ca. −2 and ϵSr141 from ca. +5 to ca. +31. Their isotopic and chemical compositions suggest that some are fractionates of basalt-gabbro magma, others are predominantly partial melts of amphibole ± garnet-bearing crust, and some are mixtures of these two magma types. The chemical and isotopic compositions of the Burns Bluff plutons constituent tonalite sheets and its dyke-like internal structure suggest that it grew incrementally via the addition of melt batches from a variety of crustal and mantle sources. The growth of this pluton, via dyking, may reflect magma pulsing associated with transtensional movement along the Creswick Gap shear zone. Moreover, the Burns Bluff pluton may be a ‘frozen’ conduit system for structurally higher plutons in the Antarctic Peninsula batholith.