Paleomagnetism, geochronology and tectonic implications of the Cambrian-age Carion granite, Central Madagascar

International audienceThe Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1±5.2 Ma followed by relatively slow cooling as constrained by 40Ar/ 39Ar ages on hornblende and biotite. Four hornblende samples yielded a mean 40Ar/ 39Ar age of 512.7±1.3 Ma. A biotite sample yielded an age of 478.9±1.0 Ma. Paleomagnetic samples from the pluton and surrounding country rocks exhibit either SE-upwardly directed magnetizations (mean Dec=113°, Inc=-56°, k=106, α95=12°) or NW-downwardly directed magnetizations (mean Dec=270°, Inc=+64°, k=30, α95=11°) that pass a reversal test with a classification of 'C' and an angular difference of 14.4°. The 'normal' (negative inclinations) and 'reverse' (positive inclinations) directions also show a spatial bias within the pluton, suggesting a field transition from reverse to normal during cooling. The paleomagnetic pole calculated from the mean direction falls at 6.8°S, 001°E (dp=13°, dm=17°). Estimates of the blocking temperature for the magnetization are compared to the cooling history of the pluton and an age of 508.5±11.5 Ma is assigned to the pole. The Carion pole falls near similar-age poles from elsewhere in Gondwana, supporting the idea that the major orogenic events during Gondwana assembly were complete. A slight revision of the Gondwana apparent polar wander path (APWP) is proposed with rapid APW from 540 to 520 Ma; however, the proposed mechanisms to explain this rapid APW (including intertial-interchange true polar wander (TPW) or enhanced mantle driving forces) cannot fully explain all the data

Age and paleomagnetism of the 1210 Ma Gnowangerup–Fraser dyke swarm, Western Australia, and implications for late Mesoproterozoic paleogeography

Dolerite dykes of the Gnowangerup–Fraser Dyke Suite are subparallel to the southern and southeastern margins of the Yilgarn Craton. We collected oriented samples for paleomagnetic study from 19 dykes along the Phillips and Fitzgerald Rivers and north of Ravensthorpe. Alternating-field (AF) demagnetization revealed a stable two-polarity remanence in 14 dykes, and the primary nature of the magnetic directions is supported by a positive baked-contact test and by rock-magnetic evidence. U–Pb zircon and baddeleyite ages for two dykes confirm that the Gnowangerup–Fraser dykes are part of the 1210 Ma Marnda Moorn Large Igneous Province. The mean paleomagnetic pole, at 55.8° N, 323.9° E, A95 = 6.5°, is almost identical to the previously reported VGP of the 1212 Ma Fraser dyke, also supported by a positive baked-contact test. The combined robust paleopole places the West Australian Craton in a near-polar position at 1210 Ma. Comparison with coeval Laurentian paleopoles indicates that Laurentia and Australia were widely separated at that time. We present a paleomagnetically permissible drift model for these two continents between 1210 and 1070 Ma. One dyke yields a stable remanence with a VGP similar to the paleopole for the 755 Ma Mundine Well dykes indicating that this dyke may have been emplaced during the same event at c. 755 Ma. Differences in lengths and shapes of late Mesoproterozoic Apparent Polar Wander Paths of several continents suggests that a large supercontinent did not exist between about 1300 and 1050 Ma. This may have been a transitional time between the final breakup of Nuna and the assembly of Rodinia

Genesis of the 1.21 Ga Marnda Moorn large igneous province by plume–lithosphere interaction

The 1.21 Ga Marnda Moorn large igneous province (LIP) of the Yilgarn Craton is important for understanding the final breakup of the Nuna (Columbia) supercontinent. However, its petrogenesis is poorly understood owing to the lack of geochemical data. We conducted geochemical analyses of the Gnowangerup–Fraser Dyke Suite, a major part of the Marnda Moorn LIP, and report the first geochemical and Nd isotope data for this LIP. Results of a complementary paleomagnetic study of these dykes will be published elsewhere. Most of the studied dykes consist of predominately tholeiitic and OIB-like dolerite (Group 1) and one arc-like and more felsic dyke (Group 2). Group 1 samples have incompatible trace element compositions similar to those of tholeiitic Hawaiian plume-induced OIB and typical asthenospheric mantle-derived Nd isotopes with ɛNd(t) varying from +3.7 to +7.5, produced mainly within the spinel stability field (<75 km depth). Their source region most likely contains recycled oceanic crust. Samples from the Group 2 dyke are characterized by extremely unradiogenic Nd isotopes with ɛNd(t) of about −12, strong depletion of Nb–Ta–Zr–Hf–Ti, chondritic Nb/Ta ratios (20–18), oversaturated silica, and strong deficiencies in CaO, FeOt, TiO2, and Ni. This implies that the dyke was produced by partial melting of enriched sub-continental lithospheric mantle. The coexistence of OIB- and arc-like end-members but mainly Hawaiian OIB-like tholeiitic mafic dykes, interpreted large-scale asthenosphere upwelling in a very short time, and the large volume of mafic magma, favour a plume origin for the Marnda Moorn LIP. The geochemical and emplacement characteristics are attributed to relief of the lithosphere–asthenosphere boundary across the Yilgarn craton and a complex interplay between the plume, heated lithosphere, normal asthenosphere, and recycled components. We propose a two-stage melting model to explain the geochemical composition and emplacement of the Marnda Moorn LIP. Our plume-lithosphere interaction model is consistent with the occurrence of synchronous ultrahigh-temperature events in the Musgrave Province of central Australia

Buried but preserved: The Proterozoic Arubiddy Ophiolite, Madura Province, Western Australia

We describe a previously unidentified Proterozoic ophiolite complex situated in the Madura Province in southeastern Western Australia. The Madura Province is almost entirely covered by Mesozoic to Cenozoic basin rocks but new basement drillcores have revealed oceanic crustal assemblages that record continental marginal basin formation followed by oceanic subduction and basin closure. The Pinto Basalt has E-MORB/OIB chemical affinity and ?Nd(1600 Ma)from 2.54 to +3.3. It formed by mantle upwelling beneath extending crust in an ocean-continent transition zone and must be considerably older than c. 1389 Ma adakite that intrudes it. The Sleeper Camp Formation comprises mafic metavolcaniclastic schist intruded by metadolerite and plagiogranite veins. Zircon crystals from the metavolcaniclastic schist yield a dominant age component and maximum depositional age of 1536 ± 13 Ma. The metadolerite and plagiogranite veins have zircon crystallization ages of 1479 ± 8 Ma and 1471 ± 5 Ma, respectively. Interlayered basalt and sediments of the Malcolm Metamorphics have a maximum depositional age of c. 1470 Ma and were metamorphosed at 1315 ± 11 Ma. The mafic rocks from both units are tholeiitic, with MORB-like HFSE ratios that point to a depleted mantle source similar to N-MORB, but with trace element patterns that indicate subduction enrichment. V/Ti ratio trends suggest the Sleeper Camp Formation marks oceanic subduction initiation at c. 1479 Ma. The progression to oceanic arc formation is recorded by the Malcolm Metamorphics after c. 1470 Ma, and the Haig Cave Supersuite from 1415 to 1389 Ma. The majority of these rocks occur within the hanging wall of the Rodona Shear Zone and were structurally emplaced above the continental margin of the West Australian Craton between 1389 and 1330 Ma during oceanic arc–continent collision, forming the Arubiddy Ophiolite Complex. The occurrence of oceanic crustal assemblages behind the accreted ophiolite complex frozen by the emplacement of voluminous 1192–1125 Ma Moodini Supersuite ferrogabbros and granites demonstrates that continental collision did not occur between the West Australian and South Australian Cratons. The preservation of rocks of oceanic affinity behind ophiolites could be a hallmark of other Proterozoic terranes that have escaped full continent-continent collision

Tracking India within precambrian supercontinent cycles

The term supercontinent generally implies grouping of formerly dispersed continents and/or their fragments in a close packing accounting for about 75% of earth’s landmass in a given interval of geologic time. The assembly and disruption of supercontinents rely on plate tectonic processes, and therefore, much speculation is involved particularly considering the debates surrounding the applicability of differential plate motion, the key to plate tectonics during the early Precambrian. The presence of Precambrian orogenic belts in all major continents is often considered as the marker of ancient collisional or accretionary sutures, which provide us clues to the history of periodic assembly of ancient supercontinents. Testing of any model assembly/breakup depends on precise age data and paleomagnetic pole reconstruction. The record of dispersal of the continents and release of enormous stress lie in extensional geological features, such as rift valleys, regionally extensive flood basalts, granite-rhyolite terrane, anorthosite complexes, mafic dyke swarms, and remnants of ancient mid-oceanic ridges. Indian shield with extensive Precambrian rock records is known to bear signatures of the past supercontinents in a fragmentary manner. Vast tracts of Precambrian rocks exposed in peninsular India and in the Lesser Himalaya and the Shillong plateau further north and east provide valuable clues to global tectonic reconstructions and the geodynamics of the respective periods. The Indian shield is a mosaic of Archean cratonic nuclei surrounded by Proterozoic orogenic belts, which preserve the records of geologic events since the Paleoarchean/Eoarchean. Here we discuss the sojourn of the Indian plate from the Archean through Proterozoic, in light of available models for supercontinent assembly and breakup in the Precambrian. We also discuss the issues in constraining the configuration, which is mainly due to scanty exposures, lack of reliable paleomagnetic poles from different cratons, and their time of formation or amalgamation. In this chapter, we briefly review Precambrian geology of India to track her participation in the making of the supercontinents through time.Sarbani Patranabis-Deb, Dilip Saha, and M. Santos