Pacific Margin Of Gondwana Research Articles

Validity of Myobradypterygius hauthali von Huene, 1927 (Ichthyosauria: Ophthalmosauria) from the Early Cretaceous of Chile and Argentina

Abstract Early Cretaceous ichthyosaurs were globally distributed pelagic marine reptiles, but many remains are fragmentary, creating a Northern Hemisphere diversity bias. A rich Hauterivian locality near the Tyndall Glacier inside Torres del Paine National Park in southern Chile yields important new data regarding ichthyosaurian diversity along the Pacific margin of Gondwana. These new data will contribute to clarifying questions regarding ichthyosaur taxonomy and the palaeobiogeographical relationships between the southern Gondwanan and Northern Hemisphere ichthyosaur groups during the Early Cretaceous. Here, we describe three new ichthyosaur specimens from this locality. Two of them are referred to Myobradypterygius hauthali, expanding the distribution of this species from the Barremian of Argentina to the Hauterivian of the Chilean Patagonia. This material shows that M. hauthali differs from Platypterygius platydactylus in forefin construction and scapular morphology, supporting its classification as a separate genus within Platypterygiinae. The third specimen is a large-bodied indeterminate ophthalmosaurine ichthyosaur. This record represents the southernmost record of Ophthalmosaurinae and the first occurrence of this group from the Cretaceous of the Southern Hemisphere. These discoveries show that ophthalmosaurines and platypterygiines continued to occur sympatrically in southernmost Gondwana during the Early Cretaceous, expanding the pattern documented in Europe to the Pacific region.

A New Reconstruction for Permian East Gondwana Based on Zircon Data From Ophiolite of the East Australian Great Serpentinite Belt

AbstractThe Great Serpentinite Belt of eastern Australia is a ∼1500 km long dismembered ophiolite assumed to be Cambrian based on studies of small (typically <50 m2) exotic meta‐igneous inclusions despite contrasting ages (Cambrian—Devonian) and complex P‐T histories. To overcome these issues, we studied a ∼18 km2 coherent block of dismembered ophiolite that provides robust geological context to sampling the ophiolite. Zircon U‐Pb‐Hf‐O isotope and trace analyses from three plagiogranite dykes cutting massive gabbro confirm ∼283–277 Ma ages and a mantle source. As a result, we argue older Cambrian to Devonian plagiogranite and subducted blocks were inherited from previous subduction events in eastern Australia. These findings allow us to match the Great Serpentinite Belt with the contemporary Dun Mountain ophiolite (New Zealand) and the Koh ophiolite (New Caledonia), thus supporting a new, integrated Pacific Gondwana margin paleogeography involving multiple arcs and subduction zones.

Timing and Metamorphic Evolution of the Ross Orogeny in and around the Mountaineer Range, Northern Victoria Land, Antarctica

The Ross(–Delamerian) Orogeny significantly impacted the formation of the tectonic structure of the Pacific Gondwana margin during the early Paleozoic era. Northern Victoria Land (NVL) in Antarctica preserves the aspect of the Ross Orogeny that led to the union of the Wilson (WT)–Bowers (BT)–Robertson Bay Terrane. The aspect of the Ross Orogeny in the NVL is characterized by subduction of oceanic domains toward the continental margin (continental arc) and the accretion of the associated marine–continental substances from 530–480 Ma. In the Mountaineer Range in NVL, the Ross Orogeny strain zone is identified at the WT/BT boundary regions. In these areas, fold and thrust shear zones are observed and aspects of them can be seen at Mt. Murchison, the Descent Unit and the Black Spider Greenschist zone. The Dessent Unit corresponds to a tectonic slice sheared between the WT and BT. The metamorphic evolution phase of the Dessent Unit is summarized in the peak pressure (M1), peak temperature (M2) and retrograde (M3). The sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb ages of 514.6 ± 2.0 Ma and 499.2 ± 3.4 Ma obtained from the Dessent Unit amphibolite are comparable to the M1 and M2 stages, respectively. The Dessent Unit underwent intermediate pressure (P)/temperature (T)-type metamorphism characterized by 10.0–10.5 kbar/~600 °C (M1) and ~7 kbar/~700 °C (M2) followed by 4.0–4.5 kbar/~450 °C (M3). Mafic to intermediate magmatism (497–501 Ma) within the WT/BT boundary region may have given rise to the M2 stage of the Dessent Unit, and this magmatism is synchronous with the migmatization period of Mt. Murchison (498.3 ± 3.4 Ma). This indicates that a continuous process of fold-shearing–magmatic intrusion–partial melting, which is typically associated with a continental arc orogeny, occurred before and after c. 500 Ma in the Mountaineer Range. During the Ross Orogeny, the Dessent unit was initially subducted underneath the WT at depth (10.0–10.5 kbar, ~35 km) and then thrust into the shallow (~7 kbar, ~23 km), hot (≥700 °C) magmatic arc docking with the Mt. Murchison terrain, where migmatization prevailed.

New ages from the Shackleton Glacier area and their context in the regional tectonomagmatic evolution of the Ross orogen of Antarctica

ABSTRACT The Ross orogenic belt in Antarctica is one of several Neoproterozoic-early Palaeozoic orogens that crisscrossed Gondwana and are associated with Gondwana’s assembly. We present new age data from the Queen Maud Mountains, Ross orogen, from areas that hitherto have lacked precise ages from the local plutonic rocks. The zircon U-Pb igneous crystallization ages (n = 7) and a hornblende 40Ar/39Ar cooling age (n = 1) constrain plutonism to primarily lie within the Cambrian to Ordovician. Cumulative zircon U-Pb crystallization age data yield polymodal age distributions (516 Ma, 506–502 Ma, and 488 Ma age peaks) that are similar to other areas of the Queen Maud-Horlick Mountains, consistent with regional magmatic flare-ups along the Pacific-Gondwana margin during these times. The ages of deformed plutons constrain deformation to the Cambrian (Series 2) to Ordovician (Lower), with some regions indicating a transition to post-tectonic magmatism and cooling at ~509-470 Ma. Collectively, the data indicate that the Queen Maud-Horlick Mountains share a similar petrotectonic history with other regions of the Pacific-Gondwana margin, providing new evidence that this tectonostratigraphic province is part of and not exotic to the larger igneous-sedimentary successions developed in the peri-Gondwana realm under a broadly convergent margin setting.

Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist, New Zealand

AbstractThe Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear. To investigate the timing of these processes, we used Lu–Hf garnet geochronology to date six samples of the Alpine Schist, which represents the metamorphic section of the former Zealandia margin. The garnet dates range from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Compositional zoning in garnet indicates that the spread in ages results from diachronous metamorphism in the upper plate at the Pacific–Gondwana margin, occurring concurrently with rifting of Zealandia from East Gondwana via opening of the Tasman Sea. Clear spatial trends in the timing of garnet growth throughout the Alpine Schist are absent, indicating that either regional age trends were offset by post‐metamorphic deformation, or that metamorphism did not result from a single regional heat source, and was instead driven by short‐duration, spatially dispersed processes such as episodic fluid‐fluxing or mechanical heating. Diachronous metamorphism of the Alpine Schist can be attributed to heat conduction from the rising upper mantle during widespread extension, progressive burial and heating of accretionary wedge sediments during ongoing horizontal shortening, or fluid‐fluxing sourced from a subducting and dehydrating Hikurangi Plateau. These results indicate that during separation of Zealandia from East Gondwana in the late Cretaceous, the crust at the Pacific–Gondwana margin remained hot, potentially facilitating the extensive thinning of the Zealandia lithosphere during this time.

A Late Jurassic plesiosaur in Antarctica: Evidence of the dispersion of marine fauna through the Trans-Erythraean Seaway?

During the Jurassic two main marine pathways might act as dispersion routes for vertebrates and invertebrates between Laurasia and Gondwana: the Caribbean Seaway (between North and South America) and the Trans-Erythraean Seaway (splitting Africa from India, Madagascar). The former has proven to be of relevance as a dispersion route for marine vertebrates and invertebrates between the Tethys and Pacific margin of Gondwana. Nevertheless, little is known about the role of the Trans-Erythraean Seaway as a vertebrate dispersion pathway. The Trans-Erythraean Seaway divides the eastern and western South of Gondwana landmasses in the so-called break-up of Gondwana and connects the Tethys Sea with the Palaeo-Pacific. We describe a newly recovered plesiosaur specimen from the Ameghino (=Nordensköld) Formation, Antarctic Peninsula, the first Jurassic plesiosaur from Antarctica. We discuss the importance of this record regarding the hypothesis of marine vertebrate dispersion through the Trans-Erythraean Seaway.

Paleotethys slab pull, self-lubricated weak lithospheric zones, poloidal and toroidal plate motions, and Gondwana tectonics

Abstract The Gondwana megacontinent was composed of different domains separated by self-lubricated weak lithospheric zones, two of which could have extended into Laurasia. Displacement vectors determined through three consecutive paleomagnetism-constrained paleogeographic reconstructions (Early Pennsylvanian–early Guadalupian, ca. 320–270 Ma; late Guadalupian–Middle Triassic, ca. 260–240 Ma; and Late Triassic–early Late Jurassic, ca. 230–160 Ma) show similar orientations to coeval tectonic stresses along Gondwana. Triggered by slab pull at the northern subduction margin of the Paleotethys Ocean, differential displacements between the Gondwana domains caused localized deformation along their borders, reactivating old weak lithospheric zones (e.g., Ventana fold belt south of Buenos Aries province, Argentina; basins such as Cuvette in central Africa; and Neuquén on the Pacific margin of Gondwana). We propose that the wide extent of these structures was possible due to the transmission of mantle toroidal flow induced by strike-slip movements along these focused self-lubricated weak lithospheric zones, along with the northward drift of Pangea. These processes occurred simultaneously with a major mantle reorganization from a huge cold downwelling to a hot upwelling event caused by thermal energy storage beneath Pangea.

A continental arc tempo discovered in the Pacific-Gondwana margin mudpile?

Abstract The integration of detrital zircon age and trace element analyses provides a powerful tool with which to reconstruct continental arc evolution. Detrital zircons from the Ross-Delamerian orogen along the Pacific-Gondwana margin in north Victoria Land in Antarctica yield a broad 700–500 Ma U-Pb age population that shows a prominent period of activity centered at ca. 630–550 Ma. This activity is well correlated with the highest zircon Th/U and U/Yb ratios, suggesting an increase in lithospheric contribution coincident with fluid input from oceanic slab subduction, respectively. A low Yb/Gd ratio over this same period also suggests crustal thickening. Determination of zircon parent rock types using trace element proxies reveals the presence of previously unrecognized distinct pulses of granitoid activity that occur over tens of millions of years. Lulls between granitoid flare-ups overlap with increases in mafic-carbonatite-alkaline magma production, suggesting an influx of mantle or lower crustal melts during syn-subduction extension. A concomitant increase in the number of metamorphic zircons (U/Th > 10) and 40Ar/39Ar white mica cooling ages found during these extensional episodes suggest that significant thermal perturbations of the crust coincided with orogenic cooling, which was possibly influenced by uplift and exhumation.

Complexity of In-situ zircon U–Pb–Hf isotope systematics during arc magma genesis at the roots of a Cretaceous arc, Fiordland, New Zealand

Zircons from seventeen samples of Western Fiordland Orthogneiss (WFO) diorites and three samples of country rock (two schists and one Darran Suite diorite) from the lowermost exposed sections of the Median Batholith, Fiordland, New Zealand, were analysed for in-situ U–Pb and Hf-isotopes. The WFO represents the deeper levels of Early Cretaceous continental arc magmatism on the Pacific margin of Gondwana, marking the final stage of long-lived arc magmatism on the margin spanning the Palaeozoic. The WFO plutons were emplaced at high-P (mid to deep crust at c. 8–12kbar) between 124 and 114Ma. Minor very high-P (c. 18kbar) WFO eclogite and omphacite granulite facies orthogneiss (Breaksea Orthogneiss) are inferred to have crystallised in the base of thickened crust at c. 124Ma. Zircons from the Breaksea Orthogneiss are considered to be variably affected by Pb-loss due to emplacement of the adjacent (Malaspina) Pluton at c. 114Ma. By identifying Pb-loss, magmatic ages were able to be inferred in respect to apparent Pb-loss ages. Hf isotope data for the WFO define an excursion to less radiogenic Hf isotope ratios with time, reflecting increased recycling of an old source component. Peaks at c. 555, 770 and 2480Ma, determine the age spectra of inherited populations of zircons within the WFO. This contrasts with detrital zircon patterns in country rocks of the Takaka terrane, which include peaks at c. 465Ma, and 1250–900Ma that are absent in the WFO inheritance pattern. These results indicate a previously unrecognised Precambrian lower crustal component of New Zealand. Recycling of this lower crust became increasingly important as a source for the final stage or Mesozoic arc magmatism along this segment of the palaeo-Pacific margin of Gondwana.

Plate kinematics of the Rocas Verdes Basin and Patagonian orocline

The processes of orocline formation are a topic of debate in geosciences. The Patagonian orocline has been a case in point for over a century. Large anomalous paleomagnetic pole rotations show that the orocline started to form at the same time as mid-Cretaceous closure of the Rocas Verdes Basin, today known from ophiolitic and basin fill remnants in the Patagonian and Fuegian Andes. Some studies therefore present bending of the Andes and closure of the basin as shared consequences of rotation of a small plate that was driven by subduction-related forces at the Pacific margin of Gondwana. An alternative view of the orocline is as a product of Cretaceous to Paleogene-aged sinistral oblique convergence at the plate-boundary scale. Geological data from Tierra del Fuego have been interpreted in support of both views. Here, I test these suggestions by comparing the Rocas Verdes Basin's tectonostratigraphy to predictions of a plate kinematic model for fragmentation of the western interior of Gondwana. The model is sufficient to explain the known history of basin opening to a width of ~100–300km during the period 152–141Ma and later closure in oblique plate convergence. As this convergence occurred by motion around a distant Euler pole, it could not have produced the Patagonian orocline by rotation of a lithospheric plate on its Pacific flank. The large anomalous paleomagnetic rotations of Tierra del Fuego, instead, are likely to have occurred within the crust by rotation and deformation of regional strike-slip faults and the intervening rocks to accommodate oblique convergence of the South American and Antarctic plates between Albian and Paleocene times.

Proto-Pacific-margin source for the Ordovician turbidite submarine fan, Lachlan Orogen, southeast Australia: Geochemical constraints

The Early Palaeozoic proto-Pacific Pacific margin of Gondwana was characterised by a huge turbidite submarine fan with abundant clastic detritus derived from unknown sources within Gondwana. These deposits are widespread in the Lachlan Orogen of southeast Australia and include the Ordovician Adaminaby Group. Here we show that the mudstones and sandstones of the Adaminaby Group have chemical compositions that indicate the detritus in them was derived from a felsic, continental source similar in composition to Post Archean Australian Shales (PAAS). Chondrite normalised REE patterns showing LREE enrichment, flat PAAS normalised patterns and elemental ratios La/Sc, Cr/Th, Cr/V, Th/Sc and Th/U, have been used to support this interpretation. The dominance of quartz, and to a lesser degree plagioclase and biotite in the sandstones, suggests that the source was mainly granodioritic to tonalitic in composition. Th/Yb and Ta/Yb ratios indicate that the source was probably calc-alkaline, continental and shoshonitic. In addition, the presence of detrital muscovite, low-grade metamorphic and felsic volcanic clasts, demonstrates that a low-grade metamorphic terrane and volcanic arc contributed to the detritus observed in the samples. The presence of well-rounded zircons and tourmalines, very high Zr contents, high Zr/Sc and higher Cr/V ratios in some samples particularly in the Shoalhaven River area, indicate that some of the detritus was recycled.SiO2 versus (Al2O3+K2O+Na2O) plots suggest the source areas experienced conditions varying from humid/semi-humid to semi-arid. Textural features and weathering trends of samples from all locations follow a curved pathway on Al2O3 – (CaO*+Na2O) – K2O (ACNK) diagrams, and indicate that the clays formed from weathering had been K-metasomatised prior to penetrative deformation. Chemical indices of alteration (CIA) reveal that even the freshest sandstones are altered and others are moderately to strongly altered.Discrimination diagrams involving major, trace and REE strongly support a collisional/continental volcanic arc setting that was substantially eroded to produce the plutonic detritus observed in the sandstones. The collisional setting accords with that proposed previously by other authors who suggested that it developed during the Delamerian Orogeny, resulting in the uplifted source areas providing detritus that inundated the backarc and forearc sites of the Macquarie Arc. Some of the detritus, however, may have been derived from a continental arc that existed in the late Cambrian along the margin of the Ross Orogen. Based on palaeocurrent analyses in previous studies and shoshonitic signature of the detritus, it is proposed that the Cambrian volcanics along the eastern active margin of Gondwana provided much of the detritus in the Adaminaby Group. Zircons with the Grenvillian signature suggest that some detritus were also derived from the Ross Orogen.

Age provinces in the Antarctic craton: Evidence from detrital zircons in Permian strata from the Beardmore Glacier region, Antarctica

The Antarctic Gondwana sequence crops out along the Transantarctic Mountains. Permian strata at Mt. Bowers and Mt. Weeks in the central part of the range were mainly derived from the ice-covered East Antarctic craton, for which the geology is inferred from coastal outcrops and the Miller Range in the Transantarctic Mountains. Analysis of detrital zircons from Permian sandstones at those two localities demonstrate a slowly changing zircon age-probability distribution for four samples from the lower part of the sequence, with an abrupt change for the upper Buckley Formation sandstone. The stratigraphically lowest sample, from a unit informally designated the Mt. Bowers sandstone, is dominated by zircons in the age range 610–540Ma, with minor components at ∼1.0Ga and∼2.7Ga. Upward, the Pagoda Formation sandstone is likewise dominated by zircons of 600–550Ma age together with a significant Grenville-age cluster (1.3–1.0Ga) and a minor 1.5–1.4Ga cluster. The Fairchild Formation sandstone has a preponderance of grains in the range 600–530Ma, with a scatter of older ages. Zircon grains in the lower Buckley Formation sandstone are principally in the range 575–520Ma with another cluster at 470–450Ma and a scattering of older ages. Most zircons in the upper Buckley Formation sandstone are Permian in age, but with a very small component in the range 650–475Ma and a trace of older ages. Archean to early Mesoproterozoic zircon grains are few in number and may have been recycled from distal outcrops. The 1.5–1.4Ga grains suggest an unexposed Mesoproterozoic source within the East Antarctic craton, which could have been linked to Laurentian North America. The Grenville-age source is considered to be, similarly, a sub-glacial belt in the interior of East Antarctica. Zircons with a late Neoproterozoic to Ordovician signature (600–460Ma) were derived from a Ross Orogen foreland sequence, and/or an early and inland extension of the orogen itself. Permian zircons in the upper Buckley sandstone reflect the development of a magmatic arc on the Gondwana Pacific margin.

Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist

This paper exposes the unique set of attributes explaining why precious opal has formed in such abundance in central Australia, and almost nowhere else on Earth. The Early Cretaceous history of the Great Artesian Basin is that of a high-latitude flexural foreland basin associated with a Cordillera Orogen built along the Pacific margin of Gondwana. The basin, flooded by the Eromanga Sea, acted as a sink for volcaniclastic sediments eroded from the Cordillera's volcanic arc. The Eromanga Sea was shallow, cold, poorly connected to the open ocean, muddy and stagnant, which explains the absence of significant carbonates. Iron-rich and organic matter-rich sediments contributed to the development of an anoxic sub-seafloor in which anaerobic, pyrite-producing bacteria thrived. Rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash, Lower Cretaceous lithologies have an exceptionally large acidification potential and pH neutralisation capacity. This makes Lower Cretaceous lithologies particularly reactive to oxidative weathering. From 97 to 60 Ma, Australia remained at high latitude, and a protracted period of uplift, erosion, denudation and crustal cooling unfolded. It is possibly during this period that the bulk of precious opal was formed via acidic oxidative weathering. When uplift stopped at ca 60 Ma, the opalised redox front was preserved by the widespread deposition of a veneer of Cenozoic sediments. On Earth, regional acidic weathering is rare. Interestingly, acidic oxidative weathering has been documented at the surface of Mars, which shares an intriguing set of attributes with the Great Artesian Basin including: (i) volcaniclastic lithologies; (ii) absence of significant carbonate; (iii) similar secondary assemblages including opaline silica; (iv) similar acidic oxidative weathering driven by very similar surface drying out; and, not surprisingly, (v) the same colour. This suggests that the Australian red centre could well be the best regional terrestrial analogue for the surface of the red planet.

Crustal structure of the Antarctic Peninsula sector of the Gondwana margin around Anvers Island from geophysical data

We investigated the crustal structure of the northern Antarctic Peninsula (AP) — a composite magmatic arc terrane at the Pacific margin of Gondwana. It was formed in Mesozoic–Cenozoic by collision/accretion of allochthonous terranes to the Gondwana margin during subduction and progressive ridge collision of former Phoenix Plate under the AP. The AP is distinguished by belts of high-amplitude, gravity and magnetic anomalies, inferred to indicate the presence of plutons of mafic composition comprising the huge AP batholith. For a better understanding of the lithosphere structure and tectonic processes of the AP continental margin we performed 2D gravity and magnetic modeling along line III-III, which crosses the margin south of Anvers Island. Our model incorporates data on seismic refraction (DSS-13) and reflection (line 8545-03) profiles and physical property data.Our modeling shows that the crust of the AP margin near Anvers Island is of continental type and comprises two crustal domains, which were welded together during mid-Cretaceous collision. On the east there is a thick (~40km), high-velocity/density continental crust of the Gondwana margin, which occupies the AP inner shelf and mainland, and Larsen Ice Shelf. The space between the Gondwanian block and oceanic crust of the Bellingshausen Sea is occupied by thinner (~26km) crust of lower densities of supposed accreted terrane, characterized by gravity anomalies caused probably by accretional tectonics as a result of Cretaceous subduction. Two belts of magnetic anomalies forming the PMA (Pacific Margin Anomaly) probably belong to different crustal domains: the western belt of the PMA relates to accretional block, while the eastern belt of the PMA corresponds to the Gondwana margin block. This may represent an evidence that the PMA relates to a tectono-magmatic event occurred after collision of Gondwana margin and accreted terrane (in mid-Cretaceous) and had been imprinted on already formed continental margin.

Magnetic susceptibilities of rocks of the Antarctic Peninsula: Implications for the redox state of the batholith and the extent of metamorphic zones

A combination of 1555 magnetic susceptibility measurements from the Antarctic Peninsula between 65°S to 72°S (for which the raw data are made available), and petrological, structural and metamorphic data, has been used to improve understanding of: (1) magma sources for the Antarctic Peninsula batholith; (2) genetic relationships between granitoids and metamorphic rocks, and; (3) to provide a semi-quantitative measure of the extent and grade of metamorphic zones. The redox state of Jurassic plutonic rocks indicates that these had continental magma sources in the tectonostratigraphic terranes of the Central and Eastern domains. Triassic and Cretaceous plutons, on the other hand, had subduction-related sources in the Central Domain of the western Antarctic Peninsula and continental sources in the Gondwana-margin Eastern Domain. Circum-Pacific comparisons of magnetite–ilmenite belt geometries identify the Antarctic Peninsula belt as a natural continuation of comparable South American belts. Susceptibility trends are generally retained by orthogneisses, suggesting that they formed by deformation of plutonic rocks mostly at or below upper amphibolite-facies conditions. Metasedimentary rocks are weakly magnetic across the peninsula, and are spatially associated with continentally derived or crustally contaminated plutons (ilmenite-series). Orthogneisses and metabasites from strongly magnetic plutonic environments show areas of very low magnetic susceptibilities that may be linked to fluid-enhanced, tectonometamorphism-induced reduction of magnetic susceptibility. The distribution of reduced magnetic susceptibility areas follows the surface traces of major shear and fault zones in Palmer Land. Our results are consistent with tectonic interpretations of oblique compression and uplift of an arc block on the Pacific margin of Gondwana during the Palmer Land Event.

Crustal thickening along the West Antarctic Gondwana margin during mid-Cretaceous deformation of the Triassic intra-oceanic Dyer Arc

Subduction-related Mesozoic mafic dykes in eastern Palmer Land, Antarctic Peninsula, record the development of an intra-oceanic arc terrane, the Dyer Arc, probably of late Permian–Triassic age, represented by a tholeiitic dyke group. Arc rocks were deformed in late Triassic–early Jurassic and mid-Cretaceous times on the Gondwana margin. Eruption of syn- to post-mid-Cretaceous orogenesis magmatism is represented by a calc–alkaline group of dykes. The tholeiitic dykes intruded immature granitic crust of the Eastern Zone sub-terrane of the Central Domain, one of the magmatic terranes of the Pacific margin of Gondwana. The calc–alkaline dykes straddle the major tectonic boundary between the Central Domain and the continental margin Eastern Domain. 132 dykes were studied, 82 of which are calc–alkaline, 48 tholeiitic and two shoshonitic, across an area of approximately 4000km2. Tholeiitic dykes strike broadly NNW–SSE, transposed into parallelism with the strike of the tectonic boundary during Cretaceous orogenesis whereas calc–alkaline dykes strike tightly ESE–WNW at a high angle to the boundary. The tholeiites pre-date late Triassic deformation and metamorphism, and are interpreted as dominantly Triassic in age, whereas Ar–Ar dated calc–alkaline dykes are younger (~100Ma and ~97Ma) and field relations indicate that they overlap with the waning phase of the mid-Cretaceous Palmer Land Event. The tholeiites have trace element abundances similar to, but more depleted than, those of modern intra-oceanic arcs, as recorded by Zr/Hf ratios. Nb/Yb versus TiO2*/Yb and Sm/Yb versus La/Sm plots are used to model depths of partial melting. The tholeiites were mostly generated at shallow depths corresponding to 2.5 and certainly less than 3.0GPa in largely garnet-free mantle. The calc–alkaline magmas were generated at pressures greater than 3.0 to possibly >3.5GPa, in the garnet zone. The deepening of the mantle source, and onset of calc–alkaline magmatism are interpreted to have been caused by deformation of the intra-oceanic arc terrane on the Gondwana margin, and lithospheric thickening during the Palmer Land Event.

Emplacement, metamorphism, deformation and affiliation of mid-Cretaceous orthogneiss from the Paparoa Metamorphic Core Complex lower-plate, Charleston, New Zealand

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating and whole-rock geochemistry of plutonic rocks from coastal exposures of the Paparoa Metamorphic Core Complex (PMCC) lower-plate has revealed their suite affinity and magmatic and metamorphic ages. Emplacement and metamorphism of the Charleston Orthogneiss occurred at 118±2 Ma and 107±2 Ma, respectively. Metamorphism of the Charleston Orthogneiss was coeval with Buckland Granite intrusion, suggesting a link between the two. Field relations between the post-metamorphic Doctor Bay dike (DBD) and Charleston Orthogneiss indicate intrusion of the dike at 105±2 Ma was syn-tectonic. Deformation of the Charleston Orthogneiss occurred during mid-crustal continental extension and PMCC formation in the New Zealand region of the Pacific Gondwana margin. Fabric development in the Charleston Orthogneiss may therefore be related to mid-crustal deformation associated with extension. The Charleston Orthogneiss and DBD are assigned to the I-/S-type Rahu Suite on the basis of their geochemistry, mid-Cretaceous ages, inherited zircon populations and inboard location.

Detrital zircons from late Paleozoic accretionary complexes in north-central Chile (28°–32°S): Possible fingerprints of the Chilenia terrane

Abstract During the Paleozoic the Andean basement of central Chile and Argentina grew westwards by the amalgamation of diverse tectonostratigraphic terranes some of them derived from Laurentia. The last to be accreted, in the Devonian, corresponds to the hypothetical Chilenia terrane. However, direct evidences about the nature of its basement are scarce because volcanics and intrusives associated to a Late Paleozoic arc and the Choiyoi Large Igneous Province concealed almost all older geological units. Indirect evidences about the nature of Chilenia can be obtained from the examination of the detrital zircon age populations in late Paleozoic accretionary prisms formed after its collision along the Pacific margin of Gondwana which may have incorporated sediments derived from the erosion of the Chilenia basement. Zircon populations from three of these accretionary complexes, El Transito, Huasco and Choapa (north-central Chile, 28–32°S) include Ordovician (Famatinian), Cambrian (Pampean), Neoproterozoic (Brasiliano) and Mesoproterozoic (Grenvillian) zircons whose sources can be tracked to Gondwana. Nevertheless, the three complexes also include a very large subpopulation of zircons that cannot easily be traced to well-known Gondwana sources and that are derived from the erosion of late Neoproterozoic to Early Cambrian (580–530 Ma) magmatic/metamorphic sources, that possibly form a significant component of the Chilenia microcontinental basement.

U-Th-Pb zircon and monazite geochronology of Western Province gneissic rocks, central-south Westland, New Zealand

Ion-microprobe (Sensitive High Resolution Ion MicroProbe or SHRIMP) zircon and monazite U-Th-Pb geochronology of amphibolite-facies orthogneiss and paragneiss units, mostly from the Fraser and Granite Hill Complexes of Westland, New Zealand, has constrained the ages of protolith rocks and metamorphic overprints. Paragneiss zircon (6 samples, 175 analyses) comprise detrital grains of Paleozoic to Archean age and match patterns from metasedimentary (Ordovician) Greenland Group rocks of the Western Province, which are characteristic of early Paleozoic flysch units around the southwest Pacific margin of Gondwana. Orthogneiss zircon (9 samples, 138 analyses) reflect intense Devonian and Cretaceous magmatism associated with emplacement of the Karamea-Paparoa and Hohonu Batholiths within the Western Province. Both paragneiss and orthogneiss monazite analyses (10 samples, 116 analyses) are dominated by Devonian and Cretaceous ages, reflecting thermal pulses related to regional and localised granitoid intrusions. These results indicate that the Fraser and Granite Hill Complex gneisses are metamorphosed parts of the Western Province Buller Terrane with Karamea-Paparoa and Hohonu Batholith intrusive components.

Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

SUMMARY The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.