The source of A-type magmas in two contrasting settings: U–Pb, Lu–Hf and Re–Os isotopic constraints

The sources of post-orogenic A-type magmas from two distinct geodynamic settings are compared. The end of the ca. 514–480 Ma Delamerian Orogeny, southeastern South Australia, was marked by ~ 10 Myr of bimodal A-type magmatism, driven by convective removal of thickened lithosphere. Initial Os and Hf isotope ratios record a heterogeneous lithospheric mantle source, with some input from aesthenospheric mantle. Mafic parental melts fractionated to produce the granites. In contrast, initial Os isotope ratios of the A-type magmas that comprise the ca. 1598–1583 Ma Mesoproterozoic Gawler Felsic Large Igneous Province, central South Australia, record a dominant evolved lower crust component. However, initial Hf isotope ratios from these samples are depleted, indicating a mantle source for lithophile elements. This voluminous, bimodal magmatism lasted for ~ 15 Myr, and ended the Wartakan Orogeny. In both cases the homogenisation of chemical (rheological) heterogeneities, inherited from terrain amalgamation and orogenic thickening, strengthened the lithosphere. The contemporaneous fusion of heterogeneous mantle ± crust may represent a common, stabilising influence on the lithospheric column regardless of tectono-magmatic setting

Provenance of the Early Mesoproterozoic Radium Creek Group in the northern Mount Painter Inlier: Correlating isotopic signatures to inform tectonic reconstructions

New in situ zircon LA-ICPMS geochronologic and Hf-isotope data from the Radium Creek Group within the Mount Painter Inlier provide important temporal constraints on the Early Mesoproterozoic palaeogeography of eastern Proterozoic Australia. The entire Radium Creek Group was deposited in a single basin forming phase, and has a maximum depositional age of 1595. ±. 3.7. Ma. Detrital zircon from these metasedimentary rocks have U-Pb age populations at ca. 1595. Ma, 1660-1680. Ma, 1710-1780. Ma, ca. 1850. Ma and ca. 2500. Ma. These grains are characterised by isotopically diverse and evolved sources, and have crystallised within predominantly felsic igneous host-rocks. The relative age spectra and isotopic character has more similarity with the Gawler Craton than the Arunta Block, Curnamona Province or the Mount Isa Inlier. These observations suggest that the Mount Painter Province was adjacent to the Gawler Craton in the Early Mesoproterozoic. Our data supports a coherent South Australian Craton at ca. 1595. Ma and a contiguous continental mass that included the North and South Australian cratons. The Mount Painter Inlier occupied a complex plate tectonic setting in the overriding plate of two convergent margins. © 2014 Elsevier B.V

U-Pb zircon geochronology from Haag Nunataks, Coats Land and Shackleton Range (Antarctica): constraining the extent of juvenile Late Mesoproterozoic arc terranes

Understanding the accretionary stages of Rodinia evolution and the arrangement of cratons and arc terranes is dependent upon high-precision geochronology from key piercing points of Mesoproterozoic rocks. U-Pb zircon dating is presented here from the Mesoproterozoic Haag Nunataks gneiss complex of West Antarctica where the dominant granodiorite protolith was emplaced at 1238 ± 4 Ma, aplite/pegmatite sheets were intruded at 1064 ± 4 Ma and the final intrusive phase of microgranite sheets were emplaced at 1056 ± 8 Ma. A separate magmatic event at ∼1170 Ma is recorded as inherited zircons in the later stage intrusions. Based on field relationships, the main phase of deformation at Haag Nunataks is thought to have developed prior to the emplacement of the microgranite sheet at ∼1056 Ma but after the ∼1064 Ma aplite/pegmatite intrusive phase. Potentially correlative units from the Shackleton Range and Coats Land of East Antarctica are also dated to test supposed correlations with arc terranes and crustal blocks at the margins of Laurentia and the proto-Kalahari craton. An ice-transported granite pegmatite sample recovered from the Brunt Ice Shelf is used as a partial proxy for unexposed rocks of the ice-covered Coats Land block and has been dated at ∼1100 Ma. A diorite gneiss from the Shackleton Range was also analysed as it forms part of a magnetic domain shared with the Haag Nunataks crustal block. Core zircon ages of ∼2470 Ma were determined, and the age of migmatisation is interpreted at ∼1740 Ma and rules out any potential correlation with the Haag Nunataks gneiss complex. The magmatic precursors of the Haag Nunataks orthogneisses were emplaced in a juvenile arc setting. We argue that this arc was located in the Natal Embayment region, contiguous with the Namaqua-Natal-Maud belt of arc terranes typified by enhanced magmatism at ∼1240 Ma and ∼1170 Ma not associated with any significant deformation events. The later magmatic events at Haag Nunataks at ∼1060 Ma are more closely associated with collision of Laurentia with the proto-Kalahari craton and the associated deformation is correlated with the Ottawan phase of the Grenville orogeny

Age and tectonic significance of the Lassiter Coast Intrusive Suite, Eastern Ellsworth Land, Antarctic Peninsula

Depleted mantle model ages derived from granitoids of the Lassiter Coast Intrusive Suite, sampled over a wide geographical area in eastern Ellsworth Land, Antarctica, cluster between 1000 Ma and 1200 Ma and suggest involvement of Proterozoic crust in the petrogenesis of the suite. Ion-microprobe U–Pb zircon analyses from a small intrusion at Mount Harry, situated at the English Coast, yield a concordant age of 105.2 ± 1.1 Ma, consistent with published ages from other parts of the Lassiter Coast Intrusive Suite. Significant variation in the Sr and Nd isotope composition of the granitoids, along the extrapolation of the Eastern Palmer Land Shear Zone (a proposed terrane boundary) located close to the English Coast, is not evident. However, the isotope signature at the English Coast is more homogeneous than the Lassiter Coast; this variation may relate to geographical proximity to the Pacific margin during intrusion, may reflect subtle changes in basement with a broadly similar character across the proposed terrane boundary, or suggest that any major fault structure is located further to the north, with implications for the kinematics of regional mid-Cretaceous transpression

A revised geochronology of Thurston Island, West Antarctica, and correlations along the proto-Pacific margin of Gondwana.

The continental margin of Gondwana preserves a record of long-lived magmatism from the Andean Cordillera to Australia. The crustal blocks of West Antarctica form part of this margin, with Palaeozoic–Mesozoic magmatism particularly well preserved in the Antarctic Peninsula and Marie Byrd Land. Magmatic events on the intervening Thurston Island crustal block are poorly defined, which has hindered accurate correlations along the margin. Six samples are dated here using U-Pb geochronology and cover the geological history on Thurston Island. The basement gneisses from Morgan Inlet have a protolith age of 349±2 Ma and correlate closely with the Devonian–Carboniferous magmatism of Marie Byrd Land and New Zealand. Triassic (240–220 Ma) magmatism is identified at two sites on Thurston Island, with Hf isotopes indicating magma extraction from Mesoproterozoic-age lower crust. Several sites on Thurston Island preserve rhyolitic tuffs that have been dated at 182 Ma and are likely to correlate with the successions in the Antarctic Peninsula, particularly given the pre-break-up position of the Thurston Island crustal block. Silicic volcanism was widespread in Patagonia and the Antarctic Peninsula at ~ 183 Ma forming the extensive Chon Aike Province. The most extensive episode of magmatism along the active margin took place during the mid-Cretaceous. This Cordillera ‘flare-up’ event of the Gondwana margin is also developed on Thurston Island with granitoid magmatism dated in the interval 110–100 Ma