Magmatic Ages Research Articles

Age of magmatism and alteration of basaltic rocks cored at the base of IODP Site U1513, Naturaliste Plateau, southwestern Australia

An 82.2 m-thick sequence of basaltic rocks was recovered from a deep-sea core on the eastern flank of the Naturaliste Plateau, offshore southwestern Australia during International Ocean Discovery Program (IODP) Expedition 369. The basaltic rocks were cored at the base of IODP Site U1513 and represent the acoustic basement of the Mentelle Basin. The recovered materials consist of subaerial to shallow-water basalt flows interbedded with volcaniclastic beds and cross-cutting dolerite dykes, all of which have been altered to some degree. Existing paleomagnetic data obtained from the overlying sedimentary sequences indicate that the basaltic sequence cored at Site U1513 was deposited before chron M10Nn.1n (which could be as young as 130.9 Ma or as old as 134.2 Ma, depending on the time scale used). Here, we present the results of 40Ar/39Ar step-heating experiments conducted on whole-rock samples of two basalt flows and six dolerite dykes to provide greater certainty of the timing of magmatism and subsequent alteration. Each sample was also characterised using optical and scanning electron microscopy together with X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. The dolerite dykes are the only rocks that were sampled that record a remnant estimate of the primary cooling age of the magmatic event at ca 134 Ma. This magmatism was contemporaneous with rifting that preceded the break-up of Greater India from Australia/Antarctica and is potentially associated with the Greater Kerguelen large igneous province. The basalt flows record younger apparent ages (between 118 and 110 Ma) that cannot represent extrusive ages given their stratigraphic context, as well as older ages obtained from cross-cutting dykes. We interpret the younger apparent ages to reflect a phase of hydrothermal alteration that reset the argon isotopic system. A tectonic reconstruction, which shows the distribution of magmatism associated with the Greater Kerguelen large igneous province, indicates that the earliest phases of magmatism occurred within an ellipse with a NW–SE-oriented long axis. The reconstruction also shows that the Naturaliste Plateau was approximately 500 km from a known source of magmatism at ca 115 Ma. This distance is within the radius of a mantle plume head and indicates that hydrothermal and/or magmatic activity responsible for resetting the argon isotopic system of the dated basalt flows may have been associated with a mantle plume or was otherwise driven by a local source of magmatism that remained active during the Aptian (126.3–113.0 Ma). KEY POINTS 40Ar/39Ar dating indicates Naturaliste Plateau magmatism occurred between 141.3 and 130.9 Ma. The basaltic rocks potentially record a phase of hydrothermal alteration at ca 118–110 Ma. Greater Kerguelen large igneous province magmatism occurred within an ellipse with a NW–SE oriented long axis.

Latest Mesoproterozoic provenance shift in the southwestern Yangtze Block, South China: Insights into tectonic evolution in the context of the supercontinent cycle

The Huili Group, one of the most representative late Mesoproterozoic strata in the Yangtze Block, bears key information for the tectonic history of the South China Block in the context of the supercontinent cycle. We present an integrated dataset involving field observation, petrology, detrital zircon U-Pb dating, and Hf isotope analyses for the Huili Group. Our new detrital zircon U-Pb ages, in combination with available magmatic age data, confirm that the Huili Group was mainly deposited at ca. 1.1–1.0 Ga. The lower Huili Group is characterized by subrounded to rounded detrital zircon grains with age intervals of 2.80–2.40 Ga (~21%), 2.34–1.72 Ga (~64%), and 1.68–1.19 Ga (~14%). In contrast, zircon grains from the upper Huili Group have variable morphologies and yield two pronounced age populations of 2.00–1.25 Ga (~50%; subrounded to rounded grains) and 1.2–1.0 Ga (~46%; euhedral to subhedral grains). The majority of zircon grains have εHf(t) values and depleted mantle two-stage (TDM2) model ages comparable to those of magmatic rocks or older sediments in the Yangtze Block. Multiple sources are suggested to have contributed older detrital zircon grains to the lower Huili Group, whereas the upper part received greater proportions of zircon grains with ages close to or equal to the depositional age from proximal magmatic sources. This observation indicates a drastic provenance shift most likely occurred within the Huili Group at ~1.04 Ga. Through reappraising our detrital zircon results and available geological datasets, we suggest that the terrigenous clastic and carbonate rocks of the lower-middle Huili Group were deposited on a passive continental margin, but the upper volcano-sedimentary sequences were formed in a back-arc extensional setting. We further propose that the Yangtze Block was spatially separated from (i.e., not yet part of) the Rodinia supercontinent during the sedimentation of the Huili Group, supporting that a global supercontinent did not exist yet in the late Mesoproterozoic.

Timing of Proterozoic magmatism in the Sunsas belt, Bolivian Precambrian Shield, SW Amazonian Craton

We present new U–Pb zircon and monazite ages from the Sunsas belt granitic magmatism in Bolivia, SW Amazonian Craton. The geochronological results revealed four major magmatic events recorded along the Sunsas belt domains. The older igneous event formed a granitic basement coeval to the Rio Apa Terrane (1.95 – 1.85 Ga) in the southern domain. The second magmatic episode is represented by 1.68 Ga granites associated to the Paraguá Terrane (1.69–1.66 Ga) in the northern domain. The 1.37–1.34 Ga granites related to San Ignacio orogeny represent the third and more pervasive magmatic event, recorded throughout the Sunsas belt. Moreover, magmatic ages of ~1.42 Ga revealed that the granitogenesis associated to the Santa Helena orogeny also affected the Sunsas belt, indicating that it was not restricted to the Jauru Terrane. Lastly, the 1.10–1.04 Ga youngest magmatism was developed during the Sunsas orogeny and represents the final magmatic evolution related to Rodinia assembly. Likewise, the 1.95–1.85 and 1.68 Ga inherited zircon cores obtained in the ~1.3 Ga and 1.0 Ga granite samples suggest strong partial melting of the Paleoproterozoic sources. The 1079 ± 14 Ma and 1018 ± 6 Ma monazite crystallization ages can be correlated to the collisional tectono-thermal event of the Sunsas orogeny, associated to reactions of medium- to high-grade metamorphism. Thus, the Sunsas belt was built by heterogeneous 1.95–1.85 Ga and 1.68 Ga crustal fragments that were reworked at 1.37–1.34 Ga and 1.10–1.04 Ga related to orogenic collages. Furthermore, the 1.01 Ga monazite age suggests that granites previously dated by zircon can bear evidence of a younger thermal history. Therefore, the geochronological evolution of the Sunsas belt may have been more complex than previously thought.

U–Pb Dating of Apatite, Titanite and Zircon of the Kingash Mafic–Ultramafic Massif, Kan Terrane, Siberia: from Rodinia Break-up to the Reunion with the Siberian Craton

Abstract The initial stage of Rodinia supercontinent break-up occurred at about 750 Ma. It preceded formation of the Irkutsk and Franklin large igneous provinces (LIPs) at 712 ± 2 to 739 ± 8 Ma. These LIPs were emplaced within the formerly connected Laurentian and Siberian cratons. The Kingash massif is located in the Precambrian Kan terrane in direct contact with the Siberian Craton at its southwestern boundary. It has been linked to an important suite of mafic–ultramafic intrusions that border the southern margin of the Siberian craton, and that have been inferred to belong to the Irkutsk LIP. The massif is also significant, because it hosts platinum group element (PGE)–Cu–Ni-rich mineralization and is the only large deposit in the region. However, despite numerous dating attempts, the age of the massif had not been resolved. A significant difficulty is post-magmatic recrystallization at amphibolite facies that affected the rocks of the massif. In this study we used U–Pb dating of zircon, titanite and apatite from rocks of the Kingash massif and cross-cutting granite and monzonite veins. The oldest igneous zircon grain of the Kingash massif analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields an age of c. 750 Ma, taken as a tentative age of magmatism. Dating of multiple grains of metamorphic zircon by chemical abrasion isotope dilution thermal ionization mass spectrometry yielded 564·8 ± 2·2 Ma, which is in agreement with LA-ICP-MS titanite ages of 557 ± 19, 565 ± 35 and 551 ± 17 Ma. Apatite of two different samples showed ages of 496·4 ± 7·9 and 497·0 ± 1·8 Ma (LA-ICP-MS), which are interpreted as the time when the terrane cooled below the closure temperature of apatite. Using our new data we suggest that at the time of the Irkutsk–Franklin LIP event the Kan terrane was a part of Rodinia, then it separated from either Siberia or Laurentia during the break-up of Rodinia and finally collided with Siberia at 560 Ma, the time of regional amphibole-facies metamorphism.

Antiquity and tectonic lineage of Japanese Islands: New discovery of Archean–Paleoproterozoic Complex

A new rock complex has been discovered from the Tsuwano area of Southwest Japan. This granitic rock complex namely, Tsuwano Complex contains foliated granitic gneiss, massive granitoids, and metaquartzite, forming a distinct unit at the westernmost end of the east-west elongate Maizuru Terrane. The above-mentioned litho-assemblage is characteristically different in appearance from the typical rocks of the Maizuru Terrane as well as the surrounding Jurassic and Permian accretionary rocks. We report here the zircon U-Pb LA-ICP-MS geochronological data of the rocks of this Tsuwano Complex. The comprehensive geochronological data indicate that this complex is much older than the surrounding rocks of the Maizuru Terrane. Zircon cores of the granitic gneiss yield possible magmatic age at ca. 2700 and 2500 Ma, while rim parts show a possible metamorphic age of ca. 1850 Ma. Associated dioritic to tonalitic granitoids also show a comparable age of crystallization at ca. 1850 Ma, indicating Neoarchean granitic rocks suffering a thermal overprinting and associated granitoid magmatism at Paleoproterozoic era. The exposed metaquartzite also yielded Neoarchean detrital zircon grains with the calculated maximum age of deposition at ca. 2430 Ma. While, all the other studied rocks including the grayish granitoids, felsic tuff, and metagabbro yield Paleozoic to Mesozoic ages. This is the first discovery of a Precambrian basement rock complex from the Japanese main islands. Based on the present geochronological dataset, we propose for a correlation and paleo-positioning of these basement rocks on the East Asian cratonic margin and a possible tectonic model of its evolution.

40Ar/39Ar and LA-ICP-MS U–Pb geochronology for the New England portion of the Early Cretaceous New England-Quebec igneous province: Implications for the postrift evolution of the eastern North American Margin

The Early Cretaceous New England-Quebec igneous province is a classic example of postrift magmatism along the eastern North American passive margin. Although a suite of ⁴⁰Ar/³⁹Ar ages has been available for the Monteregian Hills lobe in the Quebec portion of the New England-Quebec igneous province for many years, only a single high accuracy radiometric age has been published for the Burlington lobe and none for the Taconic lobe in the New England portion of the province. As a result, the timing of and driving mechanisms behind the magmatism have remained unresolved, and a hotspot origin for the entire province persists in the literature. We have dated four dikes and one pluton in the Burlington and Taconic lobes using ⁴⁰Ar/³⁹Ar and U–Pb geochronology to improve understanding of the age of magmatism in the New England portion of the province. In the Burlington lobe, ⁴⁰Ar/³⁹Ar plateau ages include a 137.55 ± 1.80 Ma biotite age and a 136.9 ± 4.2 Ma amphibole age for a lamprophyre dike from Charlotte, Vermont, and a 133.6 ± 2.2 Ma biotite age for a lamprophyre dike from Colchester, Vermont. In the Taconic lobe, ages include an ⁴⁰Ar/³⁹Ar plateau amphibole age of 107.09 ± 1.32 Ma for a lamprophyre dike from Castleton, Vermont, a 122 Ma minimum ⁴⁰Ar/³⁹Ar biotite age for a lamprophyre dike from Poultney, Vermont, and a 103.13 ± 0.53 Ma LA-ICP-MS U–Pb zircon age from the quartz syenite of the Cuttingsville complex. These results show that magmatism spanned at least 35 Ma, from ∼138 to 103 Ma, which is broadly consistent with the span of magmatism suggested by workers in the 1970s and 1980s based on K–Ar and Rb–Sr ages. This extended span of magmatism for the Burlington and Taconic lobes is in contrast to the brief 1 to 2 Ma episode of magmatism at ∼124 Ma inferred for the Monteregian Hills lobe. The New England-Quebec igneous province has traditionally been attributed to passage of the Great Meteor hotspot. However, given the close proximity of the Burlington and Taconic lobes, the magmatism in these lobes should span only a few Ma if the product of a hotspot. The age data are also difficult to reconcile with a more complex expression of hotspot magmatism in continental lithosphere related to either plume head magmatism or long-distance migration of plume material. Instead, the extended duration of Early Cretaceous New England-Quebec igneous province magmatism in New England may represent an expression of edge-driven convection, a process known to occur along passive margins and inferred to be operating beneath the eastern North American margin today.

Tectonic history of the Grenville-age Trenton Prong inlier, Central Appalachians, USA: evidence from SHRIMP U–Pb geochronology

New zircon U–Pb geochronologic data from the Grenville-age Trenton Prong provide information on the age of magmatism, timing of metamorphism, and post-metamorphic history of the inlier. Diorite gneiss (1318 ± 13 Ma) of the Colonial Lake Suite temporally correlates to magmatic arc sequences that formed along the eastern margin of Laurentia at <1.4 Ga. Metasedimentary gneisses yielded detrital zircon ages of ca. 1319–1133 Ma and ca. 1370–1207, consistent with sediment derived from a similar local source of Laurentian affinity. A small population of zircon (either detrital or igneous in origin) in one sample yielded ages of ca. 1074–1037 Ma. Possible interpretations for their formation are explored. Ca. 1060 Ma overgrowths on zircon in the northern part of the inlier constrain the timing of granulite-facies metamorphism to the Ottawan phase of the Grenvillian Orogeny. The undeformed Assunpink Creek Granite (1041 ± 6 Ma) intruded country rocks as small bodies of late-orogenic syenogranite. It provides a minimum age for amphibolite-facies metamorphism and Ottawan orogenesis elsewhere in the inlier. Regionally, zircon rim ages of ca. 1010–960 Ma record continued thermal activity during the Rigolet phase of the orogen that resulted in migmatization of paragneiss at ca. 1004 Ma and juxtaposition of upper- and mid-crustal rocks during orogenic collapse. The lithologic ages and tectonic history of the Trenton Prong correlate to those in other Appalachian Mesoproterozoic inliers and parts of the Canadian Grenville Province, confirming it is not an exotic terrane that was accreted to eastern Laurentia during Grenvillian orogenesis.

Tectonic juxtaposition of plume and subduction derived magmatic sequences in the Bababudan greenstone terrane, western Dharwar Craton, India: Constraining crustal accretion processes in a Neoarchean subduction-collision orogeny

Archean greenstone belts in the Western Dharwar Craton preserve important clues on Early Earth geodynamics. Here we investigate the ultramafic-mafic metavolcanic rock sequences from the Santaveri Formation of Bababudan greenstone terrane to address the tectonic evolution, mantle dynamics and crustal growth processes of western Dharwar Craton. The ultramafic volcanic rocks are geochemically classified as komatiite-picrite-basanites, whereas the mafic volcanic rocks correspond to magnesian andesite -Niobium-enriched basaltic andesite association. The pristine mineralogy for these ultramafic volcanic rocks is overprinted by tremolite-actinolite-biotite assemblage marking greenschist facies of metamorphism. The komatiites include Al-undepleted, Al-depleted and Ti-enriched types and share overlapping geochemical characteristics with the picrites. The basanites are distinguished as alkaline ultramafic rocks with total alkali content >3 wt% and MgO > 18 wt%. The coherent association of komatiite-picrite-basanite suggests pervasive plume-lithosphere interaction and complies with polybaric melting of a chemically heterogeneous mantle plume through a subduction-modified sub-continental lithospheric mantle at a rifted cratonic margin. Our results substantiate the role of an Archean mantle refertilized by subduction-driven recycling of Mesoarchean supracrustals concurrent with the onset of subduction event at ≥3.2 Ga. The mafic rocks were derived through melting of a hot oceanic slab and mantle hybridization. The xenocrystic zircon grains from komatiite suggest lower crustal inheritance and reworking of basement gneisses by ascending ultramafic melts. The 3177 Ma magmatic age obtained from these zircon grains indicate a Mesoarchean magmatic event in the genesis of the basement gneisses. The emplacement of komatiite-picrite-basanite association and magnesian andesite -Niobium-enriched basaltic andesite lithologies envisages a post 3.1 Ga Neoarchean crustal growth over Mesoarchean protocontinent of WDC. This crust generation episode can be translated in terms of plume-driven bottom-up and subduction-driven top-down tectono-magmatic processes at continental rift and ocean-continent convergence set-up respectively. Geochemical characteristics and petrogenetic aspects suggest that the studied volcanic supracrustals of Bababudan greenstone terrane encapsulate a Neoarchean subduction-collision orogenic system comprising plume-derived komatiite-picrite-basanite and subduction-derived magnesian andesite -niobium-enriched basaltic andesite magmatic sequences tectonically juxtaposed during Neoarchaen (~2.7 Ga) subduction-accretion-collision event.

New geochemical, U–Pb SIMS geochronology and Lu–Hf isotopic data in zircon from Tandilia basement rocks, Río de la Plata craton, Argentina: Evidence of a sanukitoid precursor for some Paleoproterozoic granitoids

In this paper new geochemical, geochronological and isotopic Lu–Hf data from the Siempre Amigos Hills (SA), western Tandilia, Argentina, is presented. Outcrops in this small area include representative lithologies of the basement Río de la Plata craton (RPC) known as Tandilia Terrane. The new geochemical data characterize a heterogeneous Paleoproterozoic crust with a similar U–Pb SIMS magmatic age (~2.1 Ga) represented by a suite of mafic (amphibolites, charnockitic granulites), intermediate and felsic compositions (diorites, granodiorites and granitic gneisses), as well as intrusive granitic rocks (with Neoproterozoic age of ~652 Ma in zircon). Geochemical features suggest processes of magmatic differentiation from a mafic igneous protolith associated with the ‘Camboriu orogeny’ of the Trans-Amazonian–Eburnean cycle. Some amphibolites may represent sanukitoid magmas similar to those formed at the Archean-Proterozoic transition. This paper examines the hypothesis of a sanukitoid precursor for RPC Rhyacian magmatism. The potassic and calc-alkaline character of magmatic series, together with negative Nb–Ta–Ti anomalies and REE patterns, are compatible with classical modern subduction-related magmas. Precise Lu–Hf analyses of zircons collected from a granodiorite and a gneiss indicate a predominantly juvenile source (+εHf), with minor crustal recycling (-εHf). An inheritance age is recorded in a single Neoarchaean zircon core from a granodiorite (2771 ± 15 Ma), similar to calculated maximum Hf TDM (2.8 Ga). Considering the new available data from this region of the RPC, a sustainable petrogenetic model is proposed to explain the presence of geochemical and isotopic evidence of sanukitoid magmatism which may have occurred after 2.7 Ga, which seems to constitute a magmatic series which is a precursor to the development of the Rhyacian magmatic arc in the Tandilia Terrane. Medium-to high-grade metamorphic conditions probably resulted from Paleoproterozoic subduction followed by the intrusion of huge volumes of Rhyacian calc-alkaline magmatism. This model is compatible with RPC configuration as a collage of terranes, and also consistent with Trans-Amazonian–Eburnean amalgamation of various microcontinents in the formation of Atlantica.

Contribution of an Eastern Indochina-derived fragment to the formation of island arc systems in the Philippine Mobile Belt

AbstractThe Philippine Mobile Belt is a complex plate boundary with multiple terranes in Southeast Asia, yet its early tectonic evolution is still not fully understood due to a scarcity of solid evidence. Here we report new whole rock geochemical, Sr-Nd isotopic, and zircon U-Pb-Hf isotopic data for Cretaceous-Miocene arc magmatic rocks from the Cebu and Bohol Islands, Philippine Mobile Belt. Bulk geochemical data display arc affinities with enriched large ion lithophile elements (e.g., Sr and Ba) and depleted high field strength elements (e.g., Nb, Ta, and Ti). The high positive εNd(t) (+4.6 to +9.1) values and low initial 87Sr/86Sr ratios (0.7032–0.7048) suggest that these igneous rocks were generated by partial melting of mantle wedge in an arc setting. U-Pb dating of zircons revealed Cretaceous (ca. 120–90 Ma), middle Eocene to early Oligocene (ca. 43–30 Ma), and middle Miocene (ca. 14 Ma) crystallization ages for the arc magmatism with abundant Permian-Triassic zircon xenocrysts clustering at ca. 250 Ma. The Permian-Triassic grains show dominantly negative εHf(t) values ranging from −16.2 to −6.6, which are similar to those of coeval rocks in Eastern Indochina. Combined with previous paleomagnetic studies, we propose that an Eastern Indochina-derived continental fragment was involved during the formation of arcs in the Cebu and Bohol Islands, which highlights the potential contribution of ancient continental materials in the formation of intra-oceanic arcs. This scenario does not support the previously proposed model that the Cretaceous arc in the Philippine Mobile Belt formed in the northern margin of the proto-Philippine Sea Plate and Australian margin.

Age and petrogenesis of the beryl-bearing granitic magmatism of the Velasco Pegmatite District, Pampeana Province, NW Argentina

Two porphyritic granitoids (Huaco and Sanagasta) in the Velasco district of the Pampeana Pegmatite Province in Northwestern Argentina are recognized. They are considered the fertile granites of the beryl-bearing pegmatites and can be described as post-orogenic and peraluminous A-type granites formed in an intraplate tectonic setting during an extensive regime, whose magma source is predominantly of cortical origin and to a lesser extent, mantle-derived. The pegmatites are classified as Rare Elements of the beryl type and beryl-columbite-phosphate subtype, while the entire district shows characteristics related to the NYF (Nb-Y-F) petrogenetic family. From rocks and cogenetic minerals of an individual ‘Ismiango’ pegmatite of the Velasco district, two Rb/Sr isochrons have been constructed. They define an age of 330.3 ± 8.3 and 331.7 ± 2.3 Ma and fall in the Lower Carboniferous period, consistent with the age of the parental and host-rock, the Huaco granite. As the Ismiango pegmatite has a similar composition and structure to the other beryl mineralized pegmatites of the Velasco district, the obtained age is attributed extensively for the entire district. According to the initial 87Sr/86Sr value obtained of 0.713, the pegmatite-magmatism of the Velasco District might be mainly derived from the crust with some minor participation of mantle materials.

How many extensional stages marked the Variscan gravitational collapse in the Bohemian Massif?

Tectonic development of the Variscan belt in Central Europe included, besides important compression, also an extensional phase related to gravitational collapse, which governed the origin of many sedimentary basins and magmatic bodies. One of these bodies is the Benešov pluton, featuring primary magmatic fabrics as well as deformational fabrics, related to subsequent extensional stages. Recognition of these fabrics and their links to other significant extension-induced structures in the Bohemicum and Moldanubicum not only sheds new light on the pluton itself but also extends a general knowledge of deformational stages, accompanying gravitational collapse of the Variscan orogen. The authors found that this pluton was strongly strained in a normal-faulting regime under brittle-ductile conditions. The age of deformation is constrained by a magmatic age of 347 ±3 Ma and by the age of Carboniferous sedimentary cover. New data indicate a three-stage extensional history during the phase of gravitational collapse: (1) Tournaisian extension (~350–345 Ma) within arc-related tonalitic intrusions; (2) late Viséan to Serpukhovian extension (~332–320 Ma), connected to the brittle-ductile unroofing and origin of a NE–SW basin system; and (3) Gzhelian to Cisuralian extension (~303–280 Ma), related to normal faulting and sedimentation in “Permo–Carboniferous” troughs, elongated NNE–SSW. Consequently, the gravitational collapse studied involved a complex succession of individual extensional stages, rather than a simple process.

Geochronology and geochemistry of granitoids from the Mongolian Altai

Granitoid magmatism is widespread in the Mongolian Altai, and it provides critical information to understand the crustal formation, evolution, and growth. This study reports newly investigated petrography, whole–rock geochemistry, and zircon U–Pb geochronology of granites and quartz syenites emplaced in the Mongolian Altai to investigate their sources and petrogenesis. Our results allow that granitoids, five petrological groups, and four geochronological stages from this study provide new information to understand the crustal formation and evolution of the Mongolian Altai. Geochemically, the group–I (Bt–Ms granite) and –II (Crd–Bt granite) have high–K calc–alkaline and peraluminous affinity, whereas group–III (Hbl–Bt granite) has calc–alkaline and metaluminous affinity. Group–IV (Bt quartz syenite) and –V (Kfs–porphyritic granite) show shoshonitic– to high–K calc–alkaline and metaluminous characteristics. The zircon U–Pb dating constraints the Devonian magmatic ages of 387–361 Ma for group–I, 369–353 Ma for group–II, and 366–356 Ma for group–III granites, whereas ~ 315 Ma for group–IV quartz syenite and 208–200 Ma for group–V granite. Groups–I and –II peraluminous granites have high–Th/Nb, and low–Ba/Th reflecting sources might be derived from sedimentary rocks, whereas group–III metaluminous granite has low–Th/Nb and high–Ba/Th reflecting a source that might be derived from gabbroic crustal material. Groups–I and –II peraluminous granites have similar geochemical characteristics, but their emplacement ages are different, which suggests they were formed from different magmas. Group–II peraluminous and group–III metaluminous granites were contemporaneous, and their Rb/Ba versus Rb/Sr correlation define a linear trend of the magma mixing process. It demonstrates that group–III metaluminous magma would carry heat to the crust, inducing partial melting of sedimentary rocks to produce group–II peraluminous granite. Finally, geochemical and geochronological data of groups–I, –II, and –III granites demonstrate that they were formed during the main orogenic activity in the Mongolian Altai, which is consistent with the timing of the main metamorphic event at Late Devonian to Early Carboniferous. Subordinated group–IV quartz syenite and group–V granite were probably formed in extensional to post–collisional environments after main orogenic activity in the Mongolian Altai.

Crystallization ages of the basic intrusive Ediacaran magmatism in the southeastern Dom Feliciano Belt, southernmost Brazil: Implications in the belt geodynamic evolution

The participation of mantle-derived magmas, especially hydrous basaltic liquids, in the evolution of granitic belts in both orogenic and post-collisional environments is crucial to evaluate both mantle and crustal evolution, as well as the material transference processes between these two reservoirs. Besides, this transition in the geodynamic context creates specific conditions, promoting the generation of voluminous magmatism. The Dom Feliciano Belt is a mobile belt in southernmost Brazil, generated during the Brasiliano-Pan-African Orogeny. It comprises an eastern unit, the Pelotas Terrane, essentially formed by Cryogenian and Ediacaran granitoids, with minor dioritic and gabbroic rocks in the southeast of the area. This paper presents geochronological studies to determine the crystallization ages (zircon U-Pb SHRIMP) of the most representative bodies of two associations of basic rocks, the Passo da Fabiana Gabbros (PFG), and the Alto Alegre Diorite (AAD). Besides, two granites in the PFG region were analyzed (zircon U-Pb LA-ICP-MS) to evaluate its emplacement, when compared to those of the basic magmas. The results show that PFG yield a crystallization age of 591.2 ± 3.5 Ma (MSWD = 1.3), while the AAD presented a crystallization age of 562 ± 3.7 Ma (MSWD = 0.73). In the PFG region, a biotite monzogranite, included in the Cerro Grande Suite, yielded an age of 612 ± 1 Ma (MSWD = 0.00083), and a leucosyenogranite of the Dom Feliciano Suite, showed crystallization age of 588 ± 2 Ma (MSWD = 0.00037). Considering the tectonic context of the Dom Feliciano Belt, it is necessary to highlight that an important mantle component, responsible for the generation of water-rich high-alumina calc-alkaline basaltic liquids, was involved in the belt late evolution (590-560 Ma). In this period, the belt was already in post-collisional settings, in a geodynamic context where compressive tensions were cessing, followed by an extensional regime. These conditions enabled mantle-partial melting, which was responsible for the basic rocks generation, with deep regional structures controlling its evolution and emplacement.

Late Mesoproterozoic–early Neoproterozoic quartzite–schist sequences of the Aktau–Mointy terrane (Central Kazakhstan): Provenance, crustal evolution, and implications for paleotectonic reconstruction

Metasedimentary complexes represented by quartzite–schist Kiik Group quartzite–schists play a significant role in the Aktau–Mointy terrane structure, which is one of the largest Precambrian blocks in the western Central Asian Orogenic Belt (CAOB). U-Pb LA-ICP-MS dating of detrital zircons from quartzites in different Kiik Group areas and related magmatic age data establish that accumulation occurred in the late Mesoproterozoic to early Neoproterozoic (from 1150 to 920 Ma). Concordant age estimates of detrital zircons from Kiik Group quartzites predominately fell within intervals of 1149–2105 and 2272–2850 Ma with the prominent peaks at 1217, 1332, 1466, 1582, 1621, 1748, 1849, 2517 and 2689 Ma, and minor peaks at 1992, 2282, 2791 and 2823 Ma. Hf-in-zircon and Nd whole rock data suggest that the primary stages of continental crustal growth in the Aktau–Mointy terrane were associated with the tectono-magmatic processes of the Neoarchean, Paleoproterozoic, and Mesoproterozoic. Juvenile continental crust production occurred during the Neoarchean and Paleoproterozoic, with older complex tectono-magmatic reworking, and juvenile crust production finally prevailed in the mid-Mesoproterozoic. Based on the age patterns and Hf isotopic compositions of detrital zircons from the Aktau–Mointy terrane and other terranes in the western CAOB, the crustal basement of the Aktau–Mointy terrane exhibits certain similarities with those of the Northern Kazakhstan and Issyk–Kul terranes. They all exhibit a close tectonic affinity with the Chinese Central Tian Shan terranes, Yili Block, North and China Craton, while differing from the Tarim Craton. In the Rodinia supercontinent structure, the Aktau–Mointy terrane was probably located near the Sveconorwegian Orogen in the western Baltica with other Kazakhstan and Northern Tian Shan Precambrian blocks.

Lower Permian basaltic agglomerate from the Tsengel River valley, Mongolian Altai

A new occurrence of Permian volcanic and volcaniclastic rocks in the Mongolian Altai south of the Main Mongolian Lineament was described between soums of Tugrug and Tseel in Gobi-Altai aimag. Studied vitrophyric pyroxene basalt lies in a layer of agglomerate and amygdaloidal lavas, which is a part of NE–SW trending subvertical sequence of varicolored siltstones and volcaniclastic rocks in the Tsengel River valley. This high-Mg basalt is enriched in large ion lithophile elements, Pb and Sr and depleted in Nb and Ta. LA-ICP-MS dating on 44 spots reveals several concordia clusters. The whole rock geochemistry of sample fits volcanic arc characteristic in the geotectonic discrimination diagrams. Dominant zircon data yield Upper Carboniferous and Permian magmatic ages 304.4 ± 2.3 and 288.6 ± 1.9 Ma. Two smaller clusters of Upper Devonian (376 ± 4.7 Ma) to Lower Carboniferous ages (351.9 ± 3.5 Ma) indicate probably contamination of ascending magmatic material. Youngest Triassic age found in three morphologically differing grains reflects probably lead loss. Described high-Mg basalt lava represents sub-aerial volcanism in volcanic arc environment developed over the N dipping subduction zone in the southwestern Mongolia in the time span from Uppermost Carboniferous to Permian during terminal stage of its activity.

Age and associated stress field of middle Miocene back‐arc basalt magmatism in Northeast Japan

AbstractA large volume of middle Miocene basaltic rocks is widely distributed across the back‐arc region of Northeast Japan, including around the Dewa Mountains. Petrological research has shown that basaltic rocks of the Aosawa Formation around the Dewa Mountains were generated as a result of the opening of the Sea of Japan. To determine the precise ages of the middle Miocene basaltic magmatism, we conducted U–Pb and fission‐track (FT) dating of a rhyolite lava that constitutes the uppermost part of the Aosawa Formation. In addition, we estimated the paleostress field of the volcanism using data from a basaltic dike swarm in the same formation. The rhyolite lava yields a U–Pb age of 10.73 ±0.22 Ma (2σ) and a FT age of 10.6 ±1.6 Ma (2σ), and the paleostress analysis suggests a normal‐faulting stress regime with a NW–SE‐trending σ3‐axis, a relatively high stress ratio, and a relatively high magma pressure. Our results show that the late Aosawa magmatism occurred under NW–SE extensional stress and ended at ~ 11 Ma.

New constraints on the age, geochemistry, and environmental impact of High Arctic Large Igneous Province magmatism: Tracing the extension of the Alpha Ridge onto Ellesmere Island, Canada

Abstract The High Arctic Large Igneous Province (HALIP) represents extensive Cretaceous magmatism throughout the circum-Arctic borderlands and within the Arctic Ocean (e.g., the Alpha-Mendeleev Ridge). Recent aeromagnetic data shows anomalies that extend from the Alpha Ridge onto the northern coast of Ellesmere Island, Nunavut, Canada. To test this linkage we present new bulk rock major and trace element geochemistry, and mineral compositions for clinopyroxene, plagioclase, and olivine of basaltic dykes and sheets and rhyolitic lavas for the stratotype section at Hansen Point, which coincides geographically with the magnetic anomaly at northern Ellesmere Island. New U-Pb chronology is also presented. The basaltic and basaltic-andesite dykes and sheets at Hansen Point are all evolved with 5.5–2.5 wt% MgO, 48.3–57.0 wt% SiO2, and have light rare-earth element enriched patterns. They classify as tholeiites and in Th/Yb vs. Nb/Yb space they define a trend extending from the mantle array toward upper continental crust. This trend, also including a rhyolite lava, can be modeled successfully by assimilation and fractional crystallization. The U-Pb data for a dacite sample, that is cut by basaltic dykes at Hansen Point, yields a crystallization age of 95.5 ± 1.0 Ma, and also shows crustal inheritance. The chronology and the geochemistry of the Hansen Point samples are correlative with the basaltic lavas, sills, and dykes of the Strand Fiord Formation on Axel Heiberg Island, Nunavut, Canada. In contrast, a new U-Pb age for an alkaline syenite at Audhild Bay is significantly younger at 79.5 ± 0.5 Ma, and correlative to alkaline basalts and rhyolites from other locations of northern Ellesmere Island (Audhild Bay, Philips Inlet, and Yelverton Bay West; 83–73 Ma). We propose these volcanic occurrences be referred to collectively as the Audhild Bay alkaline suite (ABAS). In this revised nomenclature, the rocks of Hansen Point stratotype and other tholeiitic rocks are ascribed to the Hansen Point tholeiitic suite (HPTS) that was emplaced at 97–93 Ma. We suggest this subdivision into suites replace the collective term Hansen Point volcanic complex. The few dredge samples of alkali basalt available from the top of the Alpha Ridge are akin to ABAS in terms of geochemistry. Our revised dates also suggest that the HPTS and Strand Fiord Formation volcanic rocks may be the hypothesized subaerial large igneous province eruption that drove the Cretaceous Ocean Anoxic Event 2.

Synkinematic interplay between felsic dykes and host rock mylonitization: how magmatism assists the formation of ductile narrow shear zones in the Sierra Chica de Córdoba, Argentina

This study provides new insights about mechanisms of synkinematic interplay between dyking emplacement and ductile narrow shear zones formation in the Sierra Chica de Córdoba. A detailed analysis of structural, microstructural, geometric-scaling and geochronologic data is performed in order to demonstrate the important role that minor and meso-scale felsic dykes played in the formation of paired shear zones symmetrically from dyke contacts. This magmatism and associated deformation conspicuously overprint medium-to high grade metamorphic and igneous rocks of Pampean age. Internally, the shear zones are formed by mylonites with mineral paragenesis in the amphibolite facies and foliation planes totally concordant to dyke contacts. Low rheological contrast between igneous intrusions and host rocks, together with fast rates of magma channeling and emplacement, was responsible for a brittle mechanical regime at moderate to high temperatures (500–700 °C). Melt-filled shear fractures rapidly propagated while the host rock was being mylonitized at ductile conditions. Shear structures controlling dyke construction and mylonite formation adjust to a strike-slip dominated dextral transtensional regime. The predicted axes of finite maximum shortening (Z) and maximum extension (X) have NNE and NNW directions, respectively. Ar/Ar muscovite cooling ages determined for pegmatite dykes, yielded minimum crystallization ages between 451 ± 1 and 435 ± 3 My. These geochronological data are further supported by field structural relationships and the regional knowledge, pointing to a Famatinian age for magmatism and related narrow shear zones. This paper constitutes a new contribution reporting the occurrence of magmatism and strike-slip-dominated dextral transtension deformation in the inner foreland of the Famatinian accretionary orogen, developed in the Lower Paleozoic along the southwestern Gondwana margin.

Finding forms and Micro- and Nanoscale Assemblages of Gold as Indicators of Formation Conditions, Distribution, and Typification of Orogenic Deposits of Uzbekistan (South Tien Shan)

Abstract—Gold deposits of the Kyzylkum and Nurata regions of Uzbekistan are confined to the South Tien Shan orogenic belt. They are hosted in black shales (Muruntau and Myutenbai); carbonate, terrigenous, and volcanogenic rocks (Kokpatas and Balpantau); and in intrusive units (Zarmitan zone). The age of gold mineralization is 280–290 Ma, which corresponds to the age of postcollisional granitoid magmatism. Gold is observed as micro- and nanoscopic particles and is a part of various minerals (Au2Bi, AuTe2, AuAg2Te3, AuAgS, AuAg2Se3, AuSb2, etc.), which form inclusions in pyrite, arsenopyrite, antimonite, and quartz. These minerals form regular micro- and nanoscopic assemblages, which are direct signs of prospecting and evaluating gold ores of the distinguished mineral-geochemical types.