Detrital zircon ages from the Ross Supergroup,north Victoria Land,Antarctica: Implications for the tectonostratigraphic evolution of the Pacific-Gondwana margin

We present new U–Pb isotopic age data for detrital zircons from 16 deformed sandstones of the Ross Supergroup in north Victoria Land, Antarctica. Zircon U/Th ratios primarily point to dominantly igneous parent rocks with subordinate contributions from metamorphic sources. Comparative analysis of detrital zircon age populations indicates that inboard stratigraphic successions (Wilson Terrane) and those located outboard of the East Antarctic craton (the Bowers and Robertson Bay terranes) have similar ~ 1200–950 Ma (Mesoproterozoic–Neoproterozoic) and ~ 700–490 Ma (late Neoproterozoic–Cambrian, Furongian) age populations. The affinity of the age populations of the sandstones to each other, as well as Gondwana sources and Pacific-Gondwana marginal stratigraphic belts, challenges the notion that the outboard successions form exotic terranes that docked with Gondwana during the Ross orogeny and instead places the terranes in proximity to each other and within the peri-Gondwana realm during the late Neoproterozoic to Cambrian. The cumulative zircon age suite from north Victoria Land yields a polymodal age spectra with a younger, primary 700–480 Ma age population that peaks at ~ 580 Ma. Cumulative analysis of zircons with elevated U/Th ratios (> 20) indicating metamorphic heritage yield ~ 657–532 Ma age probability peaks, which overlap with the younger dominantly igneous zircon population. The data are interpreted to give important new evidence that is consistent with ongoing convergent arc magmatism by ~ 626 Ma, which provided the dominant zircon-rich igneous rocks and subordinate metamorphic rocks. Maximum depositional ages as young as ~ 493–481 Ma yielded by deformed sequences in the outboard Bowers and Robertson Bay terrane samples provide new support for late Cambrian to Ordovician deformation in outboard sectors of the orogen, consistent with tectonic models that call for cyclic phases of contraction along the north Victoria Land sector of the Ross–Delamerian orogen.     

Extensional collapse of the Gondwana orogen: Evidence from Cambrian mafic magmatism in the Trivandrum Block,southern India

The assembly of Late Neoproterozoice Cambrian supercontinent Gondwana involved prolonged subduction and accretion generating arc magmatic and accretionary complexes, culminating in collision and formation of high grade metamorphic orogens. Here we report evidence for mafic magmatism associated with post-collisional extension from a suite of gabbroic rocks in the Trivandrum Block of southern Indian Gondwana fragment. Our petrological and geochemical data on these gabbroic suite show that they are analogous to high Fe tholeiitic basalts with evolution of the parental melts dominantly controlled by fractional crystallization. They display enrichment of LILE and LREE and depletion of HFSE with negative anomalies at Zre Hf and Ti corresponding to subduction zone magmatic regime. The tectonic affinity of the gabbros coupled with their geochemical features endorse a heterogeneous mantle source with collective melt contributions from sub-slab asthenospheric mantle upwelling through slab break-off and arc-related metasomatized mantle wedge, with magma emplacement in subduction to post-collisional intraplate settings. The high Nb contents and positive Nbe Ta anomalies of the rocks are attributed to inflow of asthenospheric melts containing ancient recycled subducted slab components and/or fusion of subducted slab materials owing to upwelling of hot asthenosphere. Zircon grains from the gabbros show magmatic crystallization texture with low U and Pb content. The LA-ICPMS analyses show ²⁰⁶ Pb/²³⁸ U mean ages in the range of 507-494 Ma suggesting Cambrian mafic magmatism. The post-collisional mafic magmatism identified in our study provides new insights into mantle dynamics during the waning stage of the birth of a supercontinent.     

The Neoproterozoic continental rift magmatism of the eastern Jiangnan orogen: new evidence from the mafic sheets in northern Zhejiang Province,South China

Massive mafic sheets were recently recognized intruding the Neoproterozoic strata in Fuyang area, eastern Jiangnan orogen. Geochronological, geochemical, and isotopic studies were carried out to understand their mantle source, crust–mantle interaction, and tectonic setting. LA-ICP-MS U-Pb zircon data indicate that the sheets were generated at 808 ± 7 Ma. The mafic sheets consist of two groups: high Ti and low Ti. They are enriched in light rare earth elements (LREE; 3.3–5.3 ppm) and show negligible Eu anomalies (δEu = 0.77–1.12). They also have strong large-ion lithophile element (LILE; Sr, K, Rb, Ba) enrichment, moderately strong high-field-strength element (HFSE) enrichment (except for Nb-Ta depletion), and positive εNd(t) (5.1–9.1). Geochemical and isotopic data indicate that the mafic sheets were generated from a depleted asthenospheric mantle source. The high-Ti mafic sheets have higher HFSE contents and less Nb-Ta depletion than the low Ti series, indicating a lower degree of partial melting and crustal contamination. The mafic sheets grew in a within-plate setting, concurrent with the ~820–750 Ma rifting events in the eastern Jiangnan orogen. They are likely related to the breakup of the Rodinia supercontinent.     

Extreme oxygen isotope signature of meteoric water in magmatic zircon from metagranite in the Sulu orogen, China: Implications for Neoproterozoic rift magmatism

Unusual ¹⁸O depletion, with δ¹⁸O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ¹⁸O zircons to be distinguished between magmatic and metamorphic origins, because the ¹⁸O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ¹⁸O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual ¹⁸O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ¹⁸O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ¹⁸O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ¹⁸O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U-Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ¹⁸O zircons are interpreted as crystallizing from negative δ¹⁸O magmas due to melting of meteoric-hydrothermally altered negative δ¹⁸O rocks in an active rift setting at about 780 Ma. The variation in zircon δ¹⁸O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu-Hf isotope analyses give positive ε_Hf(t) values of 1.6-4.1 and Hf model ages of 1.18-1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ¹⁸O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ¹⁸O zircons may register centers of low δ¹⁸O magmatism during the supercontinental rifting.     

Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings: Implications for Ediacaran taphonomic models

It is beyond doubt that the appearance of infaunal bioturbation and metazoan biomineralization across the Ediacaran–Cambrian transition irreversibly affected the nature of marine sediment architecture and biogeochemistry. Here we review those changes in relation to their likely effect upon the processes of fossil preservation, especially within siliciclastic sediments. Processes of soft-tissue preservation in siliciclastic settings from the Ediacaran Period, including microbes and microbial mats as well as Ediacaran macrofossils, are here reviewed within this context. Highlighted examples include the exceptional preservation of microbes found in association with wrinkle structures and Ediacaran macrofossils in England and Newfoundland (replicated by silicate minerals) and in the White Sea region of Russia (replicated by iron sulphide). These occurrences show that soft-tissue preservation in siliciclastic settings went well beyond that typical for Ediacaran macrofossils alone and also extended to similar modes of preservation in associated microbes. Using these new observations it can be argued that several existing explanations for Ediacaran fossil preservation can be united within a biogeochemical model that involves evolution of the sediment mixed layer across this transition.     

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen,Antarctica

The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.     

Early Neoproterozoic granulite facies metamorphism of mafic dykes from the Vestfold Block,east Antarctica

Proterozoic mafic dykes from the southwestern Vestfold Block experienced heterogeneous granulite facies metamorphism, characterized by spotted or fractured garnet‐bearing aggregates in garnet‐absent groundmass. The garnet‐absent groundmass typically preserves an ophitic texture composed of lathy plagioclase, intergranular clinopyroxene and Fe–Ti oxides. Garnet‐bearing domains consist mainly of a metamorphic assemblage of garnet, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, K‐feldspar, quartz and Fe–Ti oxides. Chemical compositions and textural relationships suggest that these metamorphic minerals reached local equilibrium in the centre of the garnet‐bearing domains. Pseudosection calculations in the model system NCFMASHTO (Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃) yield P–T estimates of 820–870 °C and 8.4–9.7 kbar. Ion microprobe U–Pb zircon dating reveals that the NW‐ and N‐trending mafic dykes were emplaced at 1764 ± 25 and 1232 ± 12 Ma, respectively, whereas their metamorphic ages cluster between 957 ± 7 and 938 ± 9 Ma. The identification of granulite facies mineral inclusions in metamorphic zircon domains is also consistent with early Neoproterozoic metamorphism. Therefore, the southwestern margin of the Vestfold Block is inferred to have been buried to depths of ~30–35 km beneath the Rayner orogen during the late stage of the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and east Antarctica (i.e. the Lambert Terrane or the Ruker craton including the Lambert Terrane). The lack of penetrative deformation and intensive fluid–rock interaction in the rigid Vestfold Block prevented the nucleation and growth of garnet and resulted in the heterogeneous granulite facies metamorphism of the mafic dykes.     

大别山鲁家寨花岗岩地球化学、锆石年代学和Hf同位素特征:扬子克拉通北东缘新元古代岩浆活动证据

对大别山造山带的鲁家寨花岗岩进行了锆石U-Pb年代学、锆石Hf同位素和岩石地球化学研究.锆石LA-ICP-MS U-Pb定年结果表明鲁家寨花岗岩形成于新元古代((816±17)Ma).鲁家寨花岗岩总体具有高硅(SiO2 69.13%～75.47%)、准铝-弱过铝(A/CNK=0.98～1.01)的化学组成特征.稀土元素...     

Evidence for Palaeozoic magmatism recorded in the Late Neoproterozoic Marlborough ophiolite,New England Fold Belt,central Queensland

The ultramafic‐mafic complex in the Marlborough terrane of the northern New England Fold Belt is dominated by members of a Neoproterozoic (ca 560 Ma) ophiolite (V1). The ophiolite has been intruded by the products of three Palaeozoic tectonomagmatic episodes (V2, V3 and V4). The V2 magmatic episode is represented by tholeiitic and calc‐alkaline basalts and gabbros of island‐arc affinities. Sm/Nd isotopes give a whole‐rock isochron age of 380 ± 19 Ma (2σ) to this episode, some 180 million years younger than the V1 ophiolitic rocks. The V3 magmatic episode includes tholeiitic and alkali basalts with enriched geochemical signatures characteristic of intraplate volcanism. A whole‐rock Sm/Nd isochron age of 293 ± 35 Ma is obtained for this event. A fourth magmatic event (V4) is represented by basaltic andesites and siliceous intrusives with geochemical features similar to modern adakites. This event has its type locality in the Percy Isles. These data provide tectonic and geochronological constraints for the previously enigmatic Marlborough terrane and as such contribute to the ever‐evolving understanding of New England Fold Belt development.     

U-Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of ²⁰⁶Pb/²³⁸U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ¹⁸O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε_Hf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between ²⁰⁶Pb/²³⁸U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.     

Triassic arc mafic magmatism in North Qiangtang: Implications for tectonic reconstruction and mineral exploration

The North Qiangtang continental block in central Tibet is a critical piece of the Pangea puzzle. This paper uses integrated geochronological and geochemical data for selected mafic dykes and dioritic enclaves in this block to evaluate its tectonic evolution in the Triassic. Zircons from two mafic dykes and the dioritic enclaves of a large arc granodiorite pluton in eastern North Qiangtang yield indistinguishable U-Pb ages from 248 ± 2 to 251 ± 3 Ma, contemporaneous with widespread arc basaltic andesites and crust-derived rhyolites in the region. The mafic dykes and coeval arc basaltic andesites have almost identical Sr-Nd isotopes (initial ⁸⁷Sr/⁸⁶Sr = 0.707 to 0.708, ε_Nd = −4.4 to −3.6), and are all characterized by light REE enrichments and pronounced negative Nb-Ta anomalies. The dioritic enclaves and the hosts have indistinguishable zircon U-Pb ages, almost identical Sr-Nd isotopes (initial ⁸⁷Sr/⁸⁶Sr = 0.709 to 0.711, ε_Nd = −7.4 to −5.9), and similar zircon ε_Hf (−13.7 to −5.7), but contrasting chondrite-normalized REE patterns due to hornblende fractionation. The Sr-Nd isotope data indicate that the dioritic enclaves formed from the hybrid melts produced by mixing at depth between the arc basaltic andesites and the crust-derived rhyolites. We propose that the Early Triassic arc igneous suites are related to the northward subduction of the southern Paleo-Tethys beneath the North Qiangtang block from Early to Middle Triassic. The occurrence of several Late Triassic porphyry Cu deposits plus a VMS Ag-Pb-Zn deposit in the Yidun arc, which is the product of the southward subduction of the northern Paleo-Tethys beneath the North Qiangtang block in the Late Triassic, indicates that the arc magmas generated during the subduction of the Paleo-Tethys are fertile in ore metals. Therefore, exploration for Early–Middle Triassic porphyry Cu and VMS deposits in the southern part of the North Qiangtang block is warranted.     

Geochemistry and petrogenesis of Soltan Maidan basalts (E Alborz,Iran): Implications for asthenosphere-lithosphere interaction and rifting along the N margin of Gondwana

Soltan Maidan Basaltic Complex with thickness up to about 1300 m is located in the eastern Alborz zone, north of Iran. This complex is dominantly composed of transitional to mildly alkaline basaltic lava flows, agglomerates and tuffs, together with a few thin sedimentary interlayers. Field geological evidence and study of palynomorph assemblages in the shale interlayer show Late Ordovician to Early Late Silurian ages. Chondrite- and primitive-mantle normalized multi-element patterns of Soltan Maidan basalts demonstrate enrichment in highly incompatible elements relative to less incompatible ones and their patterns are most similar to OIB. Trace elemental and Sr-Nd isotopic compositions indicate interaction and mixing of asthenospheric mantle source (OIB-type) with enriched subcontinental lithospheric mantle components (EM1-type). This asthenosphere-lithosphere interaction occurred in an extensional continental setting, which resulted in opening of the Paleotethys Ocean in the north of Gondwana during the Late Silurian to Middle Devonian.     

Fluid characteristics of retrogressed eclogites and mafic granulites from the Cambrian Gondwana suture zone in southern India

Eclogite-facies rocks and high-pressure granulites provide windows to the deeper parts of subduction zones and the root of mountain chains, carrying potential records of fluids associated with subduction-accretion-collision tectonics. Here, we report petrological and fluid inclusion data on retrogressed eclogite and high-pressure granulite samples from Sittampundi, Kanji Malai and Perundarai in southern India. These rocks occur within the trace of the Cambrian collisional suture which marks the final phase of amalgamation of the Gondwana supercontinent. The garnet–clinopyroxene assemblage in the eclogites preserves relict omphacite, whereas the high-pressure granulites are characterized by an assemblage of garnet and clinopyroxene in the absence of omphacite and with minor plagioclase, orthopyroxene, and quartz. Phase relations computed for the eclogite assemblage yield peak P–T conditions of 19 kbar and 1,010°C. The mafic granulites also preserve the memory of high to ultrahigh-temperature metamorphism followed by an isothermal decompression. Systematic fluid inclusion optical, microthermometric and laser Raman spectroscopic studies were conducted in garnet and plagioclase from the eclogite–high pressure granulite suite. The results suggest that the early fluids were a mixture of CO₂, CH₄ and N₂ probably derived from decarbonation and devolatilization reactions in a subduction setting during the prograde stage. The later generation inclusions, which constitute the dominant category in all the samples studied, are characterized by a near-pure CO₂ composition with moderate to high densities (up to 1.154 g/cm³). The highest density fluid inclusions recorded in this study occur within the mafic granulites from Sittampundi (0.968–1.154 g/cm³) and Kanji Malai (1.092–1.116 g/cm³). In some cases, carbonate minerals such as dolomite and calcite are associated with the CO₂-rich fluid inclusions. The composition and densities of the later generation fluids closely match with those of the CO₂-bearing fluid inclusions reported from ultrahigh-temperature granulites occurring proximal to the eclogite–high pressure granulite suite within this suture zone, and suggest a common tectonic link for the fluid regime. We evaluate the fluid characteristics associated with convergent plate margin processes and propose that the early aqueous fluids probably associated with the eclogites were consumed during the formation of the retrograde hydrous mineral assemblages, whereas the fluid regime of the high-pressure and ultrahigh-temperature granulites was mostly CO₂-dominated. The tectonic setting of the rocks along a collisional suture marking the trace along which crustal blocks were welded through subduction–collision process is in favor of a model involving the derivation of CO₂ from sub-lithospheric sources such as a carbonated tectosphere invaded by hot asthenosphere, or underplated mafic magmas.     

Cambro-Ordovician magmatism in the Delamerian orogeny: Implications for tectonic development of the southern Gondwanan margin

The Delamerian Orogen formed at the final stages of assembly of the Gondwana supercontinent. This system marks the initiation of subduction of the Pacific oceanic lithosphere along a prior rifted and extended passive margin. This paper explores the magmatic consequences following the early Cambrian initiation at the palaeo-Pacific margin in South Australia (SA) and western Victoria. Our data reveal a 50 Ma syn- to post-Delamerian tectono-magmatic history. Sampled from drill core from beneath the eastern Murray Basin cover in eastern SA, boninitic high Mg andesite from drill hole KTH12 and 516.1 ± 2 Ma quartz diorite suggest that first subduction established a volcanic arc within easternmost SA. Pacific-ward trench retreat then resulted in arc migration to reach the Mt Stavely Belt and Stawell Zones in western Victoria by ~510 Ma where boninitic arc magmatism continued until ~490 Ma. In the SA foreland of the Delamerian Orogen, early (522 ± 4 Ma) alkali basalt gave way to intrusion and extrusion of MORB-like tholeiites of back-arc basalt character.Through much of the middle and late Cambrian the SA Delamerian was in the back-arc and under extension but with periodic compression resulting from periodic Pacific-Australian plate coupling beneath the forearc in western Victoria.In SA syn-tectonic I- and S-type granites reflect interaction of MORB-like back-arc magmas and their transported heat with continental-derived sediment of the Kanmantoo Group. The termination of the Delamerian orogeny at ~490 Ma was accompanied by buoyancy-controlled, exhumation and erosion. This was driven by delamination of a mafic, crustal underplate, whose re-melting at 1.5 to 2 GPa and 1050 °C generated the unique 495 ± 1 Ma Kinchina/Monarto adakite. Delamination resulted in lithospheric mantle thinning and local convective overturn allowing upwelling of the asthenosphere to drive the post-kinematic magmatic phase of the Delamerian, yielding voluminous 490 Ma–470 Ma A-type granites.     

Neoproterozoic A-type granitoids of the central and southern Appalachians: intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765–680 and 620–550 Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeO_t/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U–Pb zircon ages obtained from the Polly Wright Cove (706±4 Ma) and Suck Mountain (680±4 Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U–Pb zircon ages for the Pound Ridge Granite Gneiss (562±5 Ma) and Yonkers Gneiss (563±2 Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous granitoids of the supersuite likely were derived through melting of lower crustal sources. Mildly peralkaline rocks of the Robertson River batholith and Irish Creek pluton may be derived from more chemically primitive sources similar in composition to ocean–island basalts. Blue Ridge granitoids define a plutonic episode that occurred during an unsuccessful pulse of crustal extension which predated opening of Iapetus by more than 100 million years. Granitoid gneisses in New York were emplaced during an extension-related, dominantly mafic magmatic episode that ultimately led to development of Iapetus.     

Geochemical evolution of intraplate magmatism in the Paleo-Asian Ocean from the Late Neoproterozoic to the Early Cambrian

A group of oceanic islands and/or seamounts (hereafter, paleoseamounts) was produced by oceanic hot-spot magmatism in the Late Proterozoic-Early Cambrian in the southwestern margin of the Paleo-Asian Ocean. They were accreted to the Kuznetsk-Altai island arc in the Late Cambrian and were subsequently incorporated during the closing of the paleocean into the accretionary complexes of the western part of the Altai-Sayan area (southwestern Siberia, Russia). The major-and trace-element compositions and Sr and Nd isotopic systematics of pillow lavas and basalt flows from the Kurai (600 Ma) and Katun’ (550–530 Ma) paleoseamounts of Gorny Altai characterize the evolution of Hawaiian-type magmatism in the Paleo-Asian Ocean during that period. The obtained data show a significant change in lava composition between 600 and 550–530 Ma. The tholeiitic basalts of the Kurai Paleoseamount (600 Ma) from the southern part of Gorny Altai have lower incompatible element contents and higher ¹⁴⁷Sm/¹⁴⁴Nd values compared with the younger tholeiitic and alkali basalts of the Katun’ Paleoseamount (550–530 Ma), whose rocks are exposed in northern Gorny Altai. The trace-element compositions of the Katun’ lavas are similar to those of the Hawaiian tholeiites, and their ¹⁴⁷Sm/¹⁴⁴Nd ratios are lower than those of the Kurai basalts. It was suggested that the older Kurai Paleoseamount was formed above a thinner oceanic lithosphere, i.e., closer to a paleospreading axis compared with the younger Katun’ Paleoseamount. The observed temporal variations in the chemical and isotopic characteristics of lavas are probably related to differences in the degree of melting of the heterogeneous mantle owing to the different thickness of the oceanic lithosphere above which the Kurai and Katun’ paleoseamounts were formed. During the Ediacaran, a plume developed beneath the younger and, consequently, thinner lithosphere of the Paleo-Asian Ocean. The higher degree of melting in the mantle column resulted in a more considerable contribution from the refractory depleted material of the upper mantle. After 50–70 Ma, i.e., in the Early Cambrian, the plume affected a thicker lithosphere, its mantle column became shorter, and the degree of melting was lower. Owing to this, the basaltic melt was more contributed by incompatible element enriched less refractory material of the lower mantle.     

The Gondwanan margin in West Antarctica: Insights from Late Triassic magmatism of the Antarctic Peninsula

Triassic orthogneisses of the Antarctic Peninsula provide evidence for the Palaeozoic and Mesozoic geological evolution of southern Gondwana within Pangaea. These rocks are sporadically exposed in southeastern Graham Land and northwestern Palmer Land, although reliable geochronological, geochemical and isotopic data are sparse. We combine new geochronological (LA-ICP-MS zircon UPb), geochemical, and zircon (Hf, O) and whole rock isotopic (Nd, Sr and Pb) data to constrain the age and Triassic – Palaeozoic tectonic setting of these rocks. Zircon cores record Palaeozoic arc magmatism between 253 ± 2 and 528 ± 6 Ma, which was mainly located to the west of the Eastern Palmer Land Shear Zone (Central Domain; Vaughan and Storey, 2000). The arc is considered to be an extension of contemporaneous Palaeozoic arcs that have been identified along the Pacific margin of South America and the Thurston Island Block. Regions to the east of the Palmer Land Shear Zone (Eastern Domain, Vaughan and Storey, 2000) were located distal from the Terra Australis Margin, and possibly resided within Sunsas-aged belts within Pangaea. Triassic continental arc, calc-alkaline magmatism during 223–203 Ma modified the crust of the Antarctic Peninsula on both sides of the Eastern Palmer Land Shear Zone. Magmatic sources included igneous and sedimentary crustal material, which formed by crustal reworking during Sunsas- and Braziliano-aged orogenesis, and Palaeozoic arc magmatism. Arc magmatism accompanied sinistral extension which brought both domains into the arc and resulted in steady oceanward migration of the Triassic arc during the Middle – Late Triassic. We propose that sinistral displacement occurred along the Eastern Palmer Land Shear Zone, and this structure was active as early as the Triassic. Finally, we conclude that both the Eastern and Central Domains are autochthonous to Gondwana.     

First evidence for Cambrian rift-related magmatism in the West African Craton margin: The Derraman Peralkaline Felsic Complex

West of the southern, Archean, part of the Reguibat Rise of the West African Craton the Oulad Dlim Massif consists of metamorphic nappes stacked during the Mauritanides (Variscan) orogeny. In the Derraman region, about 12 km west of the nappes, we have found strongly deformed hypersolvus aegirine-riebeckite A1-type granites with SHRIMP zircon U–Pb ages of ca. 525 ± 3 Ma, ε(Nd)_525Ma (− 5.2 to − 6.8.) and Nd model ages T_CR ≈ 1.85 Ga. These granites define two km-sized bodies and a few smaller satellites. One body is emplaced within a 3.12 Ga leucocratic gneiss. The other body and its satellites are emplaced within an Archean low-grade metasedimentary sequence with detrital zircons that have ages that peak at 2.84 Ga, 2.91 Ga, and 3.15 Ga. These Archean gneisses and metapelite rocks define a tectonic unit, hereafter called the Derraman-Bulautad-Leglat (DBL) unit, which was formed from the Reguibat basement at the very margin of the WAC. The ~ 525 Ma Derraman granites are the oldest post-Archean rocks in this unit and were generated in an intraplate rifting environment from melting of crustal fenites during the ubiquitous Cambrian rifting event that affected this part of northern Gondwana. At the present level of knowledge, however, we cannot decide whether the “old” Nd isotope signature of Derraman granites resulted from melting of an old (Paleoproterozoic) fenite source or reflects the signature of the mantle-derived metasomatising fluids. The just-discovered Derraman granites are strikingly similar to other rift-related Cambrian–Ordovician hypersolvus aegirine–riebeckite granites widespread in North Gondwana. Understanding the potential connections between them would help to understand the Cambrian–Ordovician breakdown of northern Gondwana.