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Field, petrologic and geochemical data indicate that post-Archean A-type granites were emplaced in extensional, compressional or transpressional tectonic regimes, mostly in accretionary orogen (>75%) and continental rift settings (10-20%), with only minor examples in hotspot (≤ 8%) and collisional orogen (≤ 5%) settings. Only hotspot/rift-related A-type granites should be considered characteristic of within-plate or intracratonic settings since most A-type granites are orogen-related. The greatest proportion of post-Archean A-type granites was produced in orogens of two ages: 1.7-1.0 Ga in the Great Proterozoic Accretionary Orogen in Laurentia and Baltica, and at 300-100 Ma in East Asian orogens. The widespread production of A-type granite magma in orogens between 1700 and 1100 Ma is not consistent with a single-lid tectonic regime during the Mesoproterozoic. Age-frequency peaks in A-type granites occur at 2.7, 1.85, 1.45, 1.1, 0.6, 0.3 and 0.1 Ga. The geographic age distribution of these peaks is heterogeneous as recorded in Eastern Asia (300 and 100 Ma), Arabian-Nubian orogen (600 Ma), the Laurentian Grenville orogen (1100 Ma), Central/Southwest Laurentian orogens (1450 Ma), and in Brazilian orogens (1850 and 2700 Ma). Thus, none of these age peaks should be considered global based on our current database. During supercontinent assembly, orogen A-type granites dominate (chiefly in back-arc settings), and during breakup, hotspot/rift types increase in abundance but do not dominate until the breakup of Pangea in the last 200 Myr. A-type granite frequency peaks at 1.85, 1.1, 0.6 and 0.3 Ga comprise mostly orogenic back-arc types and correspond to a high frequency of orogens at these times and those at 1.85, 1.45, 1.1 and 0.6 Ga also coincide with peaks in plate speed. A remarkable correlation of six orogen A-type granite frequency peaks at 2.7, 1.85, 1.1, 0.6, 0.3 and 0.1 Ga with zircon and LIP age peaks suggests that back arc A-type granite frequency is related to secular mantle events such as global plume events or mantle overturn. • A-type granites are produced chiefly in orogens. • A-type granites should not be used as evidence of a single-lid tectonic regime. • Abundance of A-type granites at 1450 Ma is the result of sample bias. • Frequency peaks of A-type granites may reflect secular mantle events. [ABSTRACT FROM AUTHOR]

We present results of a paleomagnetic study from Carboniferous forearc basin rocks that occur at both limbs of the Texas Orocline (New England Orogen, eastern Australia). Using thermal and alternating field demagnetizations, two remanence components have been isolated from rocks sampled from the Emu Creek terrane, in the eastern limb of the orocline. A middle-temperature Component M is post-folding and was likely acquired during low-temperature oxidation at 65–35 Ma. A high-temperature Component H is pre-folding, but its comparison with the paleomagnetic data from coeval rocks in the northern Tamworth terrane on the other limb of Texas Orocline does not indicate rotations around a vertical axis, as expected from geological data. A likely explanation for this apparent discrepancy is that Component H postdates the oroclinal bending, but predates folding in late stages of the 265–230 Ma Hunter Bowen Orogeny. The post-Kiaman age of Component H is supported by the presence of an alternating paleomagnetic polarity in the studied rocks. A paleomagnetic study of volcanic and volcaniclastic rocks in the Boomi Creek area (northern Tamworth terrane) revealed a stable high-temperature pre-folding characteristic remanence, which is dated to c. 318 Ma using U–Pb zircon geochronology. The new paleopole (37.8°S, 182.7°E, A 95 = 16.2°) is consistent with previously published poles from coeval rocks from the northern Tamworth terrane. The combination of our new paleomagnetic and geochronological data with previously published results allows us to develop a revised kinematic model of the New England Orogen from 340 Ma to 270 Ma, which compared to the previous model, incorporates a different orientation of the northern Tamworth terrane at 340 Ma. [ABSTRACT FROM AUTHOR]

[Display omitted] • A global tectono-magmatic lull (TML) at 2365–2235 Ma is verified by new data. • There is a global decrease in number of zircon and large igneous province ages during the TML. • Paleomagnetic data imply the existence of plate tectonics during the TML. • Average minimal plate speed decreased during the TML. There is a 87–86% drop in frequency in both detrital and igneous zircon U/Pb ages during the Tectono-Magmatic Lull (TML, 2365–2235 Ma) and a significant decrease in frequency of granitoids, but no recognized age gap. The TML is transferred in detrital zircon ages through all younger depositional time windows, indicating it is a globally robust feature. This seems to require a decrease in production rate of felsic magma during the TML, and possibly also recycling of crust of this age into the mantle. However, εHf zircon and εNd whole-rock felsic igneous data show that juvenile continental crust continued to be produced and that the ratio of juvenile/reworked crust remained about the same or increased relative to the ratio before and after the lull. During the TML there is a large decrease in the frequency of LIPs, with only four that fall in the 2365–2235 Ma time frame, and there is a global LIP age gap at 2340–2260 Ma. Komatiite frequency in greenstones decreases rapidly at 2600–2000 Ma and enrichment in incompatible elements in greenstone basalts at 2700–2400 Ma reflects increasing enriched components in mantle sources. Although 13 orogens, all with convergent margin characteristics, are recognized between 2400 and 2200 Ma, only four are known with major deformation in the TML. Paleomagnetic data indicate that the relative positions of the Superior, Kaapvaal and India cratons changed significantly between 2435 and 2175 Ma, implying the existence of plate tectonics during this time interval. Also, paleomagnetic positions from several cratons confirm that average minimal plate speeds are relatively low during the TML (<8 cm/yr) and that there are no fast plates (>10 cm/yr) between 2.35 and 2.25 Ga. We propose a new testable model for the TML related to a mantle overturn event at 2.7 Ga that initiated widespread subduction. Sinking slabs covered the core-mantle boundary decreasing the rate of mantle convection so that oceanic lithosphere was consumed faster than it was produced. This led to a slow-down in plate speeds some 400 Myr later during the TML, with consequent decreases in magma production, mantle plume generation and orogenic activity. [ABSTRACT FROM AUTHOR]

The Neoproterozoic-Early Cambrian evolution of peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Cadomia) along the northern (Amazonia, West Africa) margin of Gondwana provides insights into the amalgamation of West Gondwana. The main phase of tectonothermal activity occurred between ca. 640-540 Ma and produced voluminous arc-related igneous and sedimentary successions related to subduction beneath the northern Gondwana margin. Subduction was not terminated by continental collision so that these terranes continued to face an open ocean into the Cambrian. Prior to the main phase of tectonothermal activity, Sm-Nd isotopic studies suggest that the basement of Avalonia, Carolinia and part of Cadomia was juvenile lithosphere generated between 0.8 and 1.1 Ga within the peri-Rodinian (Mirovoi) ocean. Vestiges of primitive 760-670 Ma arcs developed upon this lithosphere are preserved. Juvenile lithosphere generated between 0.8 and 1.1 Ga also underlies arcs formed in the Brazilide Ocean between the converging Congo/São Francisco and West Africa/Amazonia cratons (e.g. the Tocantins province of Brazil). Together, these juvenile arc assemblages with similar isotopic characteristics may reflect subduction in the Mirovoi and Brazilide oceans as a compensation for the ongoing breakup of Rodinia and the generation of the Paleopacific. Unlike the peri-Gondwanan terranes, however, arc magmatism in the Brazilide Ocean was terminated by continent-continent collisions and the resulting orogens became located within the interior of an amalgamated West Gondwana. Accretion of juvenile peri-Gondwanan terranes to the northern Gondwanan margin occurred in a piecemeal fashion between 650 and 600 Ma, after which subduction stepped outboard to produce the relatively mature and voluminous main arc phase along the periphery of West Gondwana. This accretionary event may be a far-field response to the breakup of Rodinia. The geodynamic relationship between the closure of the Brazilide Ocean, the collision between the Congo/São Francisco and Amazonia/West Africa cratons, and the tectonic evolution of the peri-Gondwanan terranes may be broadly analogous to the Mesozoic-Cenozoic closure of the Tethys Ocean, the collision between India and Asia beginning at ca. 50 Ma, and the tectonic evolution of the western Pacific Ocean. [ABSTRACT FROM AUTHOR]

Periodic assembly and dispersal of continental fragments has been a characteristic of the solid Earth for much of its history. Geodynamic drivers of this cyclic activity are inferred to be either top-down processes related to near surface lithospheric stresses at plate boundaries or bottom-up processes related to mantle convection and, in particular, mantle plumes, or some combination of the two. Analysis of the geological history of Rodinian crustal blocks suggests that internal rifting and breakup of the supercontinent were linked to the initiation of subduction and development of accretionary orogens around its periphery. Thus, breakup was a top-down instigated process. The locus of convergence was initially around north-eastern and northern Laurentia in the early Neoproterozoic before extending to outboard of Amazonia and Africa, including Avalonia–Cadomia, and arcs outboard of Siberia and eastern to northern Baltica in the mid-Neoproterozoic (∼760 Ma). The duration of subduction around the periphery of Rodinia coincides with the interval of lithospheric extension within the supercontinent, including the opening of the proto-Pacific at ca. 760 Ma and the commencement of rifting in east Laurentia. Final development of passive margin successions around Laurentia, Baltica and Siberia was not completed until the late Neoproterozoic to early Paleozoic (ca. 570–530 Ma), which corresponds with the termination of convergent plate interactions that gave rise to Gondwana and the consequent relocation of subduction zones to the periphery of this supercontinent. The temporal link between external subduction and internal extension suggests that breakup was initiated by a top-down process driven by accretionary tectonics along the periphery of the supercontinent. Plume-related magmatism may be present at specific times and in specific places during breakup but is not the prime driving force. Comparison of the Rodinia record of continental assembly and dispersal with that for Nuna, Gondwana and Pangea suggests grouping into two supercycles in which Nuna and Gondwana underwent only partial or no break-up phase prior to their incorporation into Rodinia and Pangea respectively. It was only after this final phase of assembly that the supercontinents then underwent full dispersal. [ABSTRACT FROM AUTHOR]

The analysis of εHf(t) time series data from zircon reveals notable discrepancies based on sample type (igneous versus detrital), statistical weighting methodology, and geographic sampling bias. These differences warrant caution when interpreting data in the context of tectonic settings and the history of supercontinents. In terms of tectonic setting, accretionary orogens dominate in both sedimentary basins that host detrital zircons and in igneous zircon sources. Because of differences between the various time series, emphasis in this study is on peaks, valleys and secular trends and not on absolute εHf(t) values. Specifically, the igneous time series for εHf(t) in zircons shows more peaks and valleys than corresponding detrital time series for both weighted and unweighted data (weighting corrects for disproportionate geographic sampling). Also, sample-based (each sample considered separately) and site-based (samples grouped by geographic location and age) results align closely to the igneous time series, whereas the site-based detrital series displays more negative εHf(t) values. Regardless of the type of time series, the failure to compensate for disproportionate geographic sampling increases the prospects of producing an unrepresentative time-series. Nine zircon age peaks (both detrital and igneous) have corresponding εHf(t) peaks (3200, 2700, 2500, 2150, 1500, 1100, 750 Ma) and two have corresponding age valleys (1800–2000, 550 Ma). With exception of a geographically widespread 1500 Ma peak, most of the εHf(t) peaks and valleys are controlled by specific geographic regions and are likely not be global in extent. Two distinct periods (200–0 and 1800–1600 Ma) display εHf(t) signatures that rise steadily for 100–200 Myr, coinciding with the final stages of supercontinent assembly and the transition to the retreat of exterior orogens. An εHf(t) peak at 750 Ma and a high at 1400–1100 Ma partly overlap with supercontinent breakup and valleys at 550 Ma and 900 Ma with supercontinent assembly. A large εHf(t) valley at 2000–1800 Ma corresponds with the onset of craton collisions that led to the final assembly of Columbia at 1800–1600 Ma. The steep rise in εHf(t) in the last 200 Myr in both igneous and detrital zircons is controlled by sites in Circum-Pacific orogens in North and South America and Southwest Asia, and it parallels the breakup of Pangea. The general increase in zircon εHf(t) in the last 500 Myr in both detrital and igneous data reflects an increase in the proportion of isotopically juvenile components in accretionary orogens. [Display omitted] • Geographic sampling biases produce non-representative εHf(t) time series. • Nine global zircon age peaks have corresponding εHf(t) peaks. • Rises in εHf(t) at 300–200 and 1800–1600 Ma correspond to supercontinent assemblies. • Zircon sources of all ages are chiefly accretionary orogens. [ABSTRACT FROM AUTHOR]

Orogenic curvatures (oroclines) are common in modern and ancient orogens, but the geodynamic driving forces of many oroclines remain controversial. Here we focus on the New England oroclines of eastern Australia, the formation of which had been previously broadly constrained to the Early–Middle Permian. This time interval encompasses periods of both back-arc extension (at ~ 300–280 Ma) and subsequent contractional deformation (Hunter–Bowen Orogeny) that commenced at ~ 270 Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at ~ 272 Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during back-arc extension, and that a major contribution to the orogenic curvature was driven by trench retreat. [ABSTRACT FROM AUTHOR]

The supercontinent cycle of episodic assembly and breakup of almost all continents on Earth is commonly considered the longest period variation to affect mantle convection. However, global zircon Hf isotopic signatures and seawater Sr isotope ratios suggest the existence of a longer-term variation trend twice the duration of the supercontinent cycle. Here we propose that since ~2 Ga the superocean surrounding a supercontinent, as well as the circum-supercontinent subduction girdle, survive every second supercontinent cycle. This interpretation is in agreement with global palaeogeography and is supported by variations in passive margin, orogen, and mineral deposit records that each exhibits both ~500–700 Myr periodic signal and a 1000–1500 Myr variation trend. We suggest that the ca. 600 Myr supercontinent cycle is modulated by a ca. 1.2 Ga superocean/subduction girdle cycle, featuring a supercontinent assembly that alternates between dominantly extroversion after a more complete breakup, and dominantly introversion after an incomplete breakup of the previous supercontinent. [ABSTRACT FROM AUTHOR]