Paleomagnetic study on mid-Paleoproterozoic rocks from the Rio de la Plata craton: Implications for Atlantica
Authors:
Rapalini et al
Abstract:
The first successful paleomagnetic study on middle Paleoproterozoic rocks from the Rio de la Plata craton is reported. Samples collected from the Soca and Isla Mala granitic bodies, located in southern Uruguay, provided characteristic remanences that were used to compute the first paleomagnetic poles for the craton for ca. 2.05–2.02 Ga. The poles were complemented by a virtual geomagnetic pole from the slightly older Marincho and Mahoma complexes. The paleomagnetic results suggest fast apparent polar wander at high paleolatitudes for the Rio de la Plata craton. Comparison with coeval poles from the Guiana, Congo–São Francisco and West African cratons indicates that a configuration of Atlantica that resembles their Western Gondwana fit is not supported by paleomagnetic data. The geologic similarities in these four cratons are supportive of a major crustal forming event between 2.2–2.0 Ga. A modified configuration for Atlantica is proposed that is consistent with our new (and older) paleomagnetic data. Atlantica was assembled at 2.1–2.05 Ga at polar latitudes and drifted towards the equator soon afterwards.
Proterozoic supercontinental restorations: Constraints from provenance studies of Mesoproterozoic to Cambrian clastic rocks, eastern Siberian Craton
Authors:
Khudoley et al
Abstract:
The Mesoproterozoic–Neoproterozoic sedimentary succession of the eastern part of the Siberian Craton consists of several unconformity-bounded, kilometer-scale siliciclastic-carbonate cycles. The overlying Lower Cambrian rocks are often compositionally similar to the uppermost units of the Neoproterozoic succession.
Twenty-nine samples were collected for U–Pb detrital zircon study and 27 samples were analyzed for whole-rock Sm–Nd isotopes. In total, 1491 detrital zircon grains were dated and 1148 grains were selected for provenance interpretation. Samples from the Uchur and Aimchan groups only contain detrital zircons of Paleoproterozoic and Archean age. Samples from the Kerpyl Group located on the Siberian Craton contain Paleoproterozoic and Archean grains as well, but samples from the Kerpyl Group in the Sette-Daban Ridge have significant numbers of Mesoproterozoic detrital zircons. Mesoproterozoic detrital zircons predominate in samples from the Uy Group. In the northern part of the study area, samples from the uppermost Neoproterozoic and Lower Cambrian strata contain numerous ca. 790–590 Ma detrital zircons, whereas in the southern part of the study area only Paleoproterozoic and Archean grains have been found. The whole-rock Sm–Nd isotopic values of clastic rocks show that most samples have isotopic signatures typical for the Siberian Craton basement, whereas some samples from the Kerpyl Group and younger rock units have isotopic signatures typical of the Grenville Orogen.
Most of the Archean and Paleoproterozoic detrital zircons were eroded from the basement of the Siberian Craton, although some ca. 2080–2030 Ma detrital zircons are likely to have a non-Siberian provenance. However, rocks younger than ca. 1700 Ma are not known in the Siberian Craton basement and all Mesoproterozoic and younger grains must therefore have a non-Siberian provenance.
The detrital zircon age distributions and whole-rock Nd isotopic signatures of many samples from the Kerpyl Group and younger units are very close to those of the Grenville Orogen in North America, suggesting that erosion of the latter contributed to clastic deposition along the Siberian margin. Three paleocontinental restorations proposed by Sears and Price (1978, 2003), Rainbird et al. (1998) and Pisarevsky and Natapov (2003) are invoked to explain the occurrence of Grenville-age detrital zircons in the Siberian sedimentary succession. The provenance of ca. 790–590 Ma detrital zircons is most likely to be located within the Central Taimyr accretionary belt formed along the northern margin of the Siberian Craton in the Neoproterozoic.
Paleo-position of the North China craton within the supercontinent Columbia: Constraints from new paleomagnetic results
Authors:
Xu et al
Abstract:
Several new paleomagnetic and geological studies focused on the reconstruction of the North China Craton (NCC) within the Paleo-Mesoproterozoic Columbia supercontinent. In spite of these new data, there are still widely divergent opinions regarding supercontinental reconstructions. In addition to qualitative correlations of orogenic belts, rift basin ages, stratigraphy and distribution of igneous provinces, paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe. In this paper, we report a detailed paleomagnetic study on the coeval ∼1780 Ma mafic dyke swarm and Xiong’er volcanic province, which extended for more than one thousand kilometers in the central NCC. Rock magnetic studies, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples, indicate that the dominant magnetic carriers are PSD magnetite. Stepwise thermal demagnetization isolated higher-temperature components directed to NNE/SSW with shallow inclinations from 37 sampling sites (16 sites in Yinshan area, 13 sites in Taihang area and 8 sites in Xiaoshan area). A baked contact test conducted on two Yinshan dykes intruded by a younger dyke demonstrates the magnetization in the Yinshan dykes pre-dates 1320 Ma. The existence of dual-polarity magnetizations in both Taihang and Xiong’er areas support our contention that the ChRM was acquired during the cooling of the magma. The primary origin of the ChRM is also supported by a positive fold test on the Xiong’er data, and a coherent regional test between the results from the Taihang and Xiong’er areas. Two different site-mean directions were compiled from these new results along with previous publications. The first direction, from the Taihang and Xiong’er areas, yields Declination (D)/Inclination (I) = 12.4°/−3.7° (κ = 20.5, α95 = 4.3°, N = 57 sites). The second, from the Yinshan area is at (D) 36.7°/(I)−12.4° (κ = 86.8, α95 = 2.7°, N = 32 sites). We argue that the difference is due to Mesozoic and/or Cenozoic vertical-axis rotation of the Taihang and Xiong’er areas with respect to the fixed Yinshan-Ordos basin. The corresponding paleopoles for the Yinshan dykes falls at 245.2°E/35.5°N (A95 = 2.4°). A comparison between the NCC, Laurentia, Siberia and Baltica is consistent with possible links between these four blocks in a perhaps, even larger, continent. The proximity of Australia and India to the NCC is also evaluated.
Palaeoproterozoic ancestry of Pan-African high-grade granitoids in southernmost India: Implications for Gondwana reconstructions
Authors:
Kroner et al
Abstract:
SHRIMP dating of magmatic zircons from granitoid gneisses) and charnockites of the Trivandrum and Nagercoil Blocks in the granulite terrane of southernmost India yielded well-defined protolith emplacement ages between 1765 and ca. 2100 Ma and also document variable recrystallization and/or lead-loss during the late Neoproterozoic Pan-African event at around 540 Ma. Hf-in-zircon and whole rock Nd isotopic data suggest that the granitoid host rocks were derived from mixed crustal sources, and Hf-Nd model ages vary between 2.2 and 2.8 Ga. A gabbroic dyke, emplaced into a charnockite protolith and deformed together with it, only contained metamorphic zircon whose mean age of 542.3 ± 4.0 Ma reflects the peak of granulite-facies metamorphism during the Pan-African high-graqde event. The Sm-Nd whole-rock isotopic system of several granitoid samples dated in this study was significantly disturbed during granulite-facies metamorphism, most likely due to a CO2-rich fluid phase. Whole-rock Nd model ages are consistently older than zircon-derived Hf model ages.
The Trivandrum and Nagercoil Blocks are reinterpreted as a single tectono-metamorphic terrane predominantly consisting of Palaeoproterozoic granitoids interlayered with supracrustal rocks that must be older than ca. 2100 Ma. Ductile deformation, migmatization and anatexis have obliterated the original rock relationships. These blocks probably have their counterpart in the Highland Complex of neighbouring Sri Lanka and in the high-grade Palaeoprotertopzoic terrane of southern Madagascar. We speculate that the southern Indian khondalites may have their counterparts in the khondalite belt of the North China Craton.
SHRIMP U-Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia
Authors:
Reid et al
Abstract:
The Mulgathing Complex within the Gawler Craton, South Australia, preserves evidence for magmatism, sedimentation and metamorphism spanning the transition between the Neoarchean and Paleoproterozoic (ca. 2555–2410 Ma). Prior to this study, limited data were available to constrain the timing of these tectonothermal events. We report SHRIMP zircon U-Pb dating of metamorphosed sedimentary and magmatic rocks from the Mulgathing Complex that shows metasedimentary gneisses (Christie Gneiss) have maximum depositional ages ca. 2480 Ma, in contrast to previous studies that have suggested deposition had occurred ca. 2510 Ma. The oldest metamorphic zircons in our data are ca. 2465 Ma, thus indicating there was a time interval of less than 15 Myr between the cessation of sedimentation and the occurrence of high-grade metamorphism. Metamorphic zircons have a range of ages, from ca. 2465 to ca. 2415 Ma, consistent with a period of ∼50 Myr during which high-grade metamorphism occurred. Mafic and felsic intrusions have ages that range from ca. 2520 Ma to 2460 Ma, indicating magmatism occurred during sedimentation and continued during the early stages of metamorphism and deformation of these rocks. The presence of bimodal magmatism with continental arc affinity (Devils Playground Volcanics) together with mafic intrusions showing temporal overlap with sedimentation within the Mulgathing Complex suggests that the overall tectonic regime prior to deformation likely involved lithospheric extension, possibly related to a continental magmatic arc. The Mulgathing Complex shows similarities in the nature and timing of these tectonothermal processes with several other Neoarchean - Paleoproterozoic terranes, in particular the Terra Adelie Craton in Antarctica, the Sask Craton, Canada, and regions within the North China Craton and the North Australian Craton. It is possible that similarities in process could imply along-strike relationships between these terranes, for example as laterally continuous terranes at the margin of a former continental domain (or domains). Basement rocks with ages of ca. 3250–3150 Ma, ca. 2950–2800 Ma and 2600–2550 Ma within the Gawler Craton, as inferred from magmatic and inherited zircons, are temporally equivalent to some of the main basement units within the Pilbara Craton, and basement rocks in both the Gawler and Pilbara cratons are overlain by sedimentary rocks at ca. 2480 Ma that include banded iron formations. It is possible to envisage the Gawler Craton as representing an extended continental margin adjacent to a larger, more intact continental domain, such as the Pilbara.
Geophysical characterization of the Azimuth 125° lineament with aeromagnetic data: contributions to the geology of central Brazil
Authors:
Rocha et al
Abstract:
This study analyzes a portion of the Azimuth 125° lineament (Az 125°), located in the Brazilian states of Goiás and Minas Gerais, based on its magnetic signature and previous studies. In particular, the determination of a chronology of events that shaped the Az 125° lineament is the focus of this contribution. Geological provinces with highly economically valuable mineralizations occur along Az 125, including the most important carbonatite and kimberlite complexes in Brazil. Az 125° is the main structural feature associated with these complexes, consisting of an extensive set of NW-SE oriented faults (approximately 850 km long and 70 km wide), however it is not mapped on geological maps at a regional scale as a continuous structural feature, because it is not consistently visible on the surface or in satellite images. Magnetic signature characterizes the lineament as a set of linear features with regional continuity in the subsurface. It is characterized by a higher magnetic susceptibility response that contrasts with the response exhibited by surrounding host rocks. Field data support geophysical data: occasional outcrops resulting from active erosion reveal dikes formed by gabbroic rocks and diabase and define the Az 125° lineament as a set of dikes of different ages. Other magnetic anomalies that occur along Az 125° are associated with intrusive rocks from the Goiás and Alto Paranaíba alkaline provinces. Based on magnetic signatures and mapped igneous rocks chronology, the Az 125° is partitioned, within the study area, into three main systems (called L1, L2, and L3). This subdivision, in conjunction with the dates assigned to various basic dikes within the azimuth, reveals that the evolution of the azimuth can be associated with three events. The first event occurred during the Brasiliano (950 Ma to 520 Ma). The second event occurred during the Gondwana fragmentation (starting circa 180 Ma). The third event is related to the tectonomagmatic activity of The Trindade plume (90 Ma to 80 Ma). We posit that the L1 system includes the older dikes, and the L3 system includes the youngest dikes; the L2 system which is intersected by L3, is therefore, of intermediate age. The determination of a chronology of events that characterizes the segmented structure of the Az 125° lineament, here defined as L-systems (L1, L2, and L3), based on both geological and geophysical data, and not yet reported in earlier scientific articles, is the main contribution of this study.
U-Pb age and Lu-Hf isotopic data of detrital zircons from the Neoproterozoic Damara Sequence: Implications for Congo and Kalahari before Gondwana
Authors:
Foster et al
Abstract:
The proximity of the Congo and Kalahari cratons during the Neoproterozoic breakup of the supercontinent Rodinia and during subsequent assembly of Gondwana is unclear. Neoproterozoic metasedimentary rocks from the rifted margins of Congo and Kalahari in the Damara Orogen yield distinctive detrital zircon U-Pb age distributions that correspond to the ages of prominent crustal components within the respective cratons. The most abundant zircons from Neoproterozoic strata deposited on the Congo margin give ages of 1150-1000 and 800-600 Ma, whereas, the most abundant zircons from the Kalahari margin strata range from 1350-1100 Ma. A 1350-1200 Ma detrital zircon population in the Kalahari margin strata is absent in the Damara-Congo strata. A prominent c. 1050-1000 Ma detrital zircon age population from Damara-Congo strata is nearly absent from the Damara-Kalahari strata, even though orogenic events of this age are found on both cratons. Damara strata on the Kalahari margin also lack detrital zircons with U-Pb ages of 900-600 Ma. The differences in detrital zircon age distributions are robust when comparing strata of the same age on both cratons, and remains so, even when younger, deeper water facies are excluded, which could have been biased by other sediment sources. These data suggest that the Congo and Kalahari cratons were not proximal in Rodinia, and did not establish their current relative positions until the end of the Neoproterozoic when they were sutured together during the collisional orogenies that formed Gondwana.
40Ar/39Ar geochronology and the paleoposition of Christmas Island (Australia), Northeast Indian Ocean
Authors:
Taneja et al
Abstract:
The Christmas Island seamount province is an extensive zone of volcanism in the Northeast Indian Ocean, consisting of numerous submerged seamounts and flat-topped guyots. Within this region lies two subaerial island groups, Christmas Island, and the Cocos Keeling archipelago. Christmas Island has experienced multiple episodes of volcanism that are exposed sporadically along its coastline. Here, we dated these volcanics using 40Ar/39Ar geochronology and analysed them for paleomagnetism. The oldest exposed volcanism occurred in the Eocene between 43 - 37 Ma. This is followed by a time gap of ~ 33 million years, before the eruption of a younger episode of Pliocene age (4.32 ± 0.17 Ma). It has, however, been suggested by previous workers there is a much older Late Cretaceous event beneath the limestone which is unexposed. In addition, this study conducted the first paleomagnetic analysis of samples from Christmas Island to determine its paleoposition and the paleomagnetic polarity of the sampled sites. Two normal and two reversal magnetic events have been recorded, that agree with the geomagnetic reversal timescales. Late Eocene (38-39 Ma) palaeomagnetic data suggest a palaeolatitude of − 43.5°− 11.2°+ 9.0°, which is further south than palaeolatitudes (around 30° S) derived from existing plate reconstruction models for the Australian plate. However, the Late Eocene palaeomagnetic data are limited (only two sites) and secular variation may not have been averaged out. During the Pliocene (ca. 4 Ma) we estimate a palaeolatitude of approximately 13° S. The presence of the Late Eocene ages at Christmas Island correlates well with the cessation of spreading of the Wharton Ridge (~ 43 Ma), the initiation of spreading along the South East Indian Ridge, and the transit of Christmas Island over a broad low velocity zone in the upper mantle. This suggests that changes in stress regimes following the tectonic reorganisation of the region (prior to ~ 43 Ma) may have allowed deeper-origin mantle melts to rise. Similarly, changes in the plate's stress regime at the flexural bulge of the Sunda-Java subduction zone may be implicated in renewed melting at ~ 4 Ma, suggesting tectonic stresses have exerted a first-order effect on the timing and emplacement of volcanism Christmas Island.
Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic
Authors:
Medig et al
Abstract:
Two supercontinents have been proposed for the latter half of the Precambrian: Columbia (or Nuna) from ca. 1.9-1.3 Ga, and Rodinia from ca. 1.1-0.75 Ga. In both supercontinents, Laurentia and Australia are regarded as probable neighbours, although their relative positions are contentious. Here we use detrital zircons ages from unit PR1 of the lower Fifteenmile group in Yukon, Canada, to demonstrate that northeastern Australia and northwestern Laurentia were firmly connected in the Mesoproterozoic. The zircon ages define a near-unimodal population with a peak at 1499 ± 3 Ma, which lies in an interval of magmatic quiescence on Laurentia, known as the North American magmatic gap (NAMG), and abundant magmatism in Australia. Sediment compositions and textures suggest the sediment was derived from a proximal metaplutonic source. We suggest that the Williams and Naraku batholiths in the Mt. Isa inlier in northeastern Australia, with crystallization ages ranging from 1493 ± 8 Ma to 1508 ± 4 Ma, are the most probable sources of sediment for the PR1 basin. The plutons were exhumed between 1460 and 1420 Ma, and likely formed an active, eroding highland in the Australian part of Columbia. Sediment derived from these plutons was carried eastward by a short, direct river system and deposited into the PR1 marine basin. Formation of the PR1 basin coincides with the formation of the southern Cordilleran Belt-Purcell, Hess Canyon, and Trampas basins. These basins, formed on the western margin of Laurentia, also have detrital zircon populations that fall into the NAMG, suggesting that sediment was derived from a non-Laurentian westerly source. The PR1 basin is herein correlated with the Belt-Purcell, Hess Canyon, and Trampas basins to the south, and together these basins record the onset of Columbia breakup along the length of the western margin of Laurentia from as far north as Yukon to as far south as Arizona.
