Protoliths Of Gneisses Research Articles

Zircon U-Pb Age and Hf-O Isotope Evidence of the Taihua Complex on the Southern Margin of the North China Craton: Implications for Its Magmatic Process During the Early Paleoproterozoic

The Xiaoqinling is the largest terrane of the Taihua Complex and records Paleoproterozoic silicic magmatism, which is one of the most potential areas for understanding secular changes in tectonic processes during the Tectono-Magmatic Lull (TML). In this study, we use zircon U–Pb dating, Lu–Hf and O isotopic analysis to constrain the age, provenance, and magmatic processes of the protoliths for the Paleoproterozoic granitic gneiss, tonalitic gneisses, and migmatitized tonalitic gneiss from the Xiaoqinling area. The isotopic composition of zircon provides a key proxy for querying the crustal record. Integration of our new U–Pb and Hf–O isotope data with available published data from other regions of the North China Craton (NCC), allows us to better assess the secular changes of the North China Craton during the magmatic lull at ∼2.3 Ga. This study shows that the protolith of the tonalitic gneisses in the study area have crust-like Hf–O isotopic compositions with slightly negative to positive εHf(t) (−3.0− +2.1) and low to moderately elevated δ18O values (2.71–7.87‰), indicating a mixed origin of continental crust assimilated by mantle-derived magmas.

The significance of Paleoarchean granitoids from the Saglek Block, Labrador, Canada

The Saglek Block in the Nain Province of northeast Canada is part of the North Atlantic Craton. It comprises Archean gneisses that record magmatic and metamorphic ages between ca. 3.9 and 2.5 Ga. In this study, a grey banded gneiss from Maidmonts Island records an age of 3.72 Ga, which is equated with Uivak I gneisses reported from across the Saglek Block, and contains 3.8 Ga xenocrysts. These rocks were deformed and metamorphosed prior to the intrusion of augen gneiss protoliths on both Maidmonts and Mentzel islands that record U-Pb zircon ages of 3.33 Ga. These rocks are composed of ferroan calc-alkaline granite and granodiorite and were likely generated by partial melting of pre-existing quartzo-feldspathic crust, as attested by the presence of ca. 3.8 Ga xenocrysts. Such augen gneiss was previously classified as ca. 3.6 Ga ‘Uivak II gneiss’, a term we argue should now be abandoned. The final magmatic events recorded on Maidmonts and Mentzel islands took place in the Neoproterozoic with the emplacement of granitic stocks at ca. 2.72 Ga, and sills and dykes at ca. 2.57 Ga. This sequence of magmatic events from the Eoarchean to Neoarchean is very similar to that recorded in the Itsaq Gneiss Complex of southwest Greenland, where, based on the Hf signature, it has been suggested that a change in tectonic environment resulted from the initiation of subduction at ca. 3.2 Ga. Although Lu-Hf and Sm-Nd isotopic signatures from the ca. 3.33 Ga gneisses have been interpreted in the literature as due to partial melting of Hadean mafic crust, alternatively, they can be generated by partial melting of Eoarchean continental crust in a continental arc setting. We argue this is more consistent with the non-TTG geochemistry of the augen gneisses.

Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit

Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39−2.28 Ga magmatism and 1.93−1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452−439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.

Zircon U-Pb-Hf constraints on the geological evolution of the Bihar Mica Belt, Central Indian Tectonic Zone, and its link to Columbia assembly

In this study, U-Pb-Hf isotope composition and trace element chemistry of zircon from basement gneisses and quartzites of the Bihar Mica Belt (BMB), a belt of supracrustal rocks within the Central Indian Tectonic Zone, is used to constrain crustal growth and reworking. The protoliths of BMB gneisses were emplaced at 1651 ± 5 Ma. The magmas were derived from juvenile and 1747 ± 7 Ma I-type long-lived felsic precursors that survive as xenocrystic cores. Zircon grains from quartzites display two distinct morphologies. Grains with little rounding of edges have internal structures comprising 1750 ± 5 Ma inherited cores mantled by 1657 ± 4 Ma igneous zones that are identical to those from the granite gneisses. The εHf (t) and trace element composition of both the xenocrystic cores and the igneous mantles in these detrital grains are identical to those of zircon in the gneisses. They constitute molasse-like sediments derived from the nearby basement. More rounded grains furnish age clusters between 1857 and 2848 Ma, suggestive of long-distance transport and derivation from multiple sources. In-situ formed metamorphic rims around zircons in gneisses and quartzites furnish 1476–1466 Ma ages, dating the earliest metamorphic overprint. The age of sedimentation is bracketed between 1657 Ma and 1466 Ma. The 1651 ± 5 Ma gneisses and its 1747 ± 7 Ma felsic precursor were I-type, and emplaced in an arc setting. They constitute part of the expansive suite of Paleoproterozoic gneisses of the Chhota Nagpur Gneiss Complex emplaced during protracted period of arc-magmatism between 1.75 Ga and 1.65 Ga. Strongly positive εHf(t) of 2.0–2.85 Ga detrital zircons conforms to the global trend of positive εHf(t) excursions and high dT/dP gradients, and may correspond to passive margin juvenile magmatism associated with the break-up of Archean supercratons prior to Columbia assembly. The 1.86 Ga and 1.75 Ga detrital and xenocrystic zircons from the BMB have enriched εHf(t), in accordance with negative excursions seen in global zircon between 2.0 Ga and 1.70 Ga, reflecting progressive assembly of Columbia through collisional orogenesis. An excursion to strongly positive εHf(t), coinciding with low dT/dP gradients at ca. 1.65 Ga, reflects wide-spread formation of arcs linked to accretionary growth of Columbia.

Metamorphosed Plagiogranite Veins In Salma Eclogites, Belomorian Eclogite Province

The Salma subduction eclogites of the Meso-Neoarchean Belomorian Eclogite Province have compositions of N-MORB oceanic crust protoliths and contain felsic veins of plagiogranite compositions. Felsic veins consist of metamorphic minerals sush as garnet, quartz and kyanite, the earliest high-pressure inclusion of phengite and zoisite in garnet. Veins have geochemistry characteristics that are typical for island arc granites or adakites. They are compositionally comparable with eclogite-bearing tonalite-trondhjemite-granodiorite (TTG) gneisses, modern adakite, and Archaean andesite association of the Vedlozero–Segozero greenstone belt. The origin of these felsic veins can be represented as a product of melting of fluid-modified oceanic mafic slab at PT-conditions of eclogite metamorphism.Geothermometry and pseudosection calculations of metamorphic conditions in felsic veins, for eclogitic mineral assemblage of pyropic garnet, kyanite, high-silica phengite and quartz gave high pressures conditions (ca. 16–22 kbar) at relatively low temperatures (ca. 600–650 °C); for secondary decompression plagioclase ± biotite coronas between quartz, kyanite and garnet gave 14–11 kbar at 650–700 °C. This estimate corresponds “warm” subduction, which is most consistent with the parameters of the modern subduction of the base of the Cascadia plate . Mineral assemblages in eclogites, that included the felsic veins, give 15–11 kbar at 750–700 °C. Thermodynamic modelling indicates that the eclogites hosting the felsic veins did not reach partial melting conditions under the estimated PT conditions: exposure temperatures to the mafic rocks must have been significantly higher. A probable explanation for high-pressure parageneses in felsic veins may be the formation of plagiogranites as a result of the melting of the mafic slab under higher temperature and pressure conditions than were measured in this work. Thus, the high-pressure paragenesis of garnet, phengite, zoisite, and kyanite reflects the path of rock exhumation through eclogitic conditions.Zircon from the felsic veins gave several Archean and Paleoproterozoic ages ∼ 2.87, 2.78, ∼2.70 and, ∼1.9 Ga. The oldest age gave the fluid-modified oceanic zircon (∼2.9 Ga) from Fe-Ti eclogites directly near the veins. The oldest igneous zircons in the veins are represented by a population with the age of ∼ 2.87 Ga and correspond to the igneous age of some TTG gneisses, which do not contradict a hypothesis of the plagiogranite formation during the melting of the mafic slab in the subduction zone. A Neoarchean zircon population of ∼ 2.78 Ga is often present in eclogitized mafic rocks and surrounding TTG gneisses which can be associated both with the formation of TTG gneiss protoliths in the subduction zone (?) and with the recrystallization of older zircon as a result of fluid or thermal impact. The ɛHf of the older population (∼2.87 Ga) is + 3 to + 4, indicating a juvenile origin; the ∼ 2.78 Ga population has similar Hf-isotope compositions, suggesting that their ages reflect resetting (non-zero Pb loss) of the older zircons. The Sm-Nd model TDM ages of Grt-Ky veins have Meso-Neoarchean values (3.0–2.75 Ga) with positive εNd = 1.8–3.8 that may indicate a juvenile nature of sources of the plagiogranite melt. The eclogitized rocks of the Belomorian province generally underwent the granulite facies overprint at ∼ 2.74–2.64 Ga ago that is coincides with the age of granulite facies metamorphism of regional and global rank. The Paleoproterozoic evolution of the Salma eclogites and the East European Craton as a whole: at 2.5–2.4 Ga and 2.0–1.9 Ga with formation of the Lapland–Mid-Russia–South Baltia intracontinental and Svecofennian accretionary orogens was terminated in Paleoproterozoic time at ∼ 1.87–1.7 Ga. These age values correspond to closure of Pb-Pb and Sm-Nd isotopic systems of rock-forming metamorphic minerals of the felsic veins.

Paleogeographic affinity of the Alxa Block across the Archean–Proterozoic: Insights from metamorphosed Archean basement

The Alxa, Quanji, and Dunhuang blocks are microblocks that link the North China craton (NCC) and the Tarim craton (TC). Their paleogeographic affinities during the late Archean–early Paleoproterozoic remain poorly constrained due to a paucity of reliable paleomagnetic data and less exposed Archean basement, especially in the Alxa Block. This study presents new geochronological and Hf isotopic data from Archean granitic gneisses in the northern Alxa Block. LA-ICP-MS zircon U-Pb dating results indicate that the protoliths of these granitic gneisses were emplaced at 2.67–2.65 Ga and subsequently metamorphosed at ca. 2.5 Ga and 1.84–1.83 Ga. In addition, in-situ zircon Hf isotopic compositions suggest that the protoliths were likely derived from reworking of Mesoarchean crust. These newly obtained data, together with previous data across the Alxa Block, indicate that Mesoarchean crustal materials were principally reworked to generate the 2.7–2.3 Ga crystalline basements of the Alxa Block. Given the recently reported ca. 2.6–2.5 Ga orthogneisses elsewhere in the Alxa Block, there should have existed a Neoarchean basement in the block, which experienced two episodes of magmatism at 2.7–2.5 Ga and 2.3–2.0 Ga, along with a metamorphic event at ca. 2.5 Ga. A further review of field geology and data across the Alxa Block and its adjacent blocks constrain a late Archean–early Paleoproterozoic crustal evolutionary history of the Alxa Block that was analogous to those of the Kuruktag Block of TC, the Khondalite Belt of NCC and the Quanji Block, indicating a close paleogeographic affinity of these blocks at this time.

METATERRIGENIOUS ROCKS OF THE OLKHON TERRANE OF THE CENTRAL ASIAN OROGENIC BELT: U-Pb ZIRCON AGE, GEOCHEMICAL CHARACTERISTICS, AND FORMATION MODELS OF SEDIMENTARY PROTOLITHS

The paper presents a petrographic, geochemical, and Sm-Nd isotopic data on the gneisses from different tectonic zones of the Olkhon terrane of the Central Asian Orogenic Belt, as well as the composition of garnets and the age of zircons in these metaterrigenous rocks. The garnet-biotite gneisses of the Anga-Sakhurta zone, as well as the garnet-bearing and garnet-free gneisses (granulites) of the Chernorud zone may result from metamorphism of immature terrigenous rocks of polymictic or greywacke compositions similar in geochemical characteristics to rocks of continental arcs. At the same time, the gneisses of these zones show both similarities and some differences in geochemical and isotopic characteristics, including variations in ɛNd(T) values from –0.2 to –9.0, which may indicate different proportions of one or another source in their protoliths. The age of most detrital zircons in the gneisses of the Chernorud and Anga-Sakhurta zones corresponds to 530–1000 Ma, and the youngest detrital zircons have an age of 522–537 Ma. The geochemical and geochronological data on the gneisses of the Chernorud and Anga-Sakhyurta zones suggests that the protoliths of these gneisses could be formed from the same sources of predominantly Neoproterozoic age. We assumed that gneiss protoliths could initially be sediments of the continental slope of Neoproterozoic composite terrane assembled to the Siberian Platform at 600‒610 Ma. The transport of clastic material being gneiss protoliths from this composite terrane, took place towards the Paleo-Asian Ocean. The youngest zircons with an age of about 530 Ma could be generated from igneous complexes intruding the Neoproterozoic composite superterrane. The garnet-biotite gneisses of the Krestovskaya zone are similar in chemical composition to immature graywacke sandstones, The ɛNd(T) value in these gneisses is –3.7. Detrital zircons in the gneisses of the Krestovskaya zone form age peaks at 780–820 and 498–515 Ma. Based on geochemical and geochronological data we assume that these gneisses could be formed in an intracontinental basin formed at the orogenic stage during accretionary-collisional events at the amalgamation the Orso block and the Birkhin island arc into the Krestovskaya zone.All terrigenous rocks being gneiss protoliths were metamorphosed at 460–510 Ma under granulite or amphibolite facies associated with accretionary and collisional events, which led to the formation of the Early Paleozoic Olkhon composite terrane.

Geochemistry and Phase Equilibrium Modelling of Garnet-Biotite Gneiss from Mauranipur, Bundelkhand Craton, Northern India: Implication for Tectonic Setting and Metamorphism

The Mauranipur region is situated along the central part of the Bundelkhand Craton (BuC) in the northern Indian shield, which consists of garnet- biotite gneisses with various deformational structures in the form of folding, faulting, augen and tail structures. These deformation structures are tectonic imprints that reveal the tectonic nature of the garnet-biotite gneisses. The groundmass of Grt-Bt gneisses is characterized by presence of garnet, biotite, plagioclase, K-feldspar, quartz, and ilmenite. The phase equilibrium modelling and geochemical attributes depict the tectonic activity and metamorphic evolution of the studied rocks. The P-T pseudosection has been calculated in the NCKFMASHT system, which revealed that the peak mineral assemblage stabilized in the P-T range of 6.35–6.75 kbar and 755–780ºC, and it further goes to retrograde metamorphism under P-T condition ranging from 4.80–5.28 kbar and 718–735ºC. These gneisses represent a calc-alkaline to high-K calc-alkaline series of protolithic origin. The negative anomaly of Nb and Ti for all samples indicates that a subduction tectonic setting has occurred in the BuC. The (La/Lu)N ratio and differences in the trace elements indicate heterogeneous sources and large variation in the degree of partial melting. The Y vs Nb and (Y+Nb) vs Rb tectonic discrimination diagrams indicate that the Grt-Bt gneisses have an affinity towards the volcanic arc granite and developed during subduction setting. The geochemical interpretation provides significant evidence that protoliths of Grt-Bt gneisses were further metamorphosed by the continent-continent collision. Keywords: Garnet-biotite gneiss, Pseudosection, P-T condition, Geochemistry, Bundelkhand

Detrital zircon U–Pb and Lu–Hf constraints on the age, provenance and tectonic setting of arc-related high-grade units of the transition zone of the Araçuaí and Ribeira orogens (SE Brazil)

Sedimentary basins with epiclastic and volcanoclastic components are important geotectonic components for understanding the complex evolution of orogenic systems. The age, provenance and tectonic setting of ancient basins are, however, often hard to constrain, especially in the high-grade core of ancient mountain belts where original sedimentary and stratigraphic relations have been largely obliterated by deformation and metamorphism. In this case, analytical tools such as whole-rock geochemistry and U–Pb and Lu–Hf analyses on detrital zircon grains can be deployed to unravel the provenance and tectonic setting of high-grade metasedimentary and metavolcaniclastic units. Here, we apply these analytical tools on samples from gneissic paleosomes of migmatites found in the eastern transition zone between the Neoproterozoic–Eopaleozoic Araçuaí and Ribeira orogens (southeastern Brazil). The studied paleosomes comprise peraluminous paragneiss rich in biotite, garnet and sillimanite, metaluminous hornblende-bearing gneiss free of peraluminous silicates, and intermediate terms between these compositions, all of them with geochemical characteristics pointing to sources located in magmatic arc systems. Overall, U–Pb data from detrital zircon grains bracket a maximum sedimentation age for all samples between 600 and 627 Ma, while εHf(t): +5.7 to −18.7 and Hf TDM model ages of 1.0–2.2 Ga suggest moderately juvenile to evolved sources for the gneiss protoliths. However, despite the similarity in age range, the analyzed samples show contrasting isotope data indicating significant variability in the source areas. Sample 7 only has grains with negative εHf (−5.9 to −18.7), most of them ranging in age from 708 Ma to 758 Ma, with very few older (1021–1143 Ma) and younger (642–651 Ma) zircons grains, indicating evolved crustal sources of distinct ages (e.g., the Late Tonian – Early Cryogenian South Bahia Alkaline Province, the Early Cryogenian magmatism found in the West Congo Belt, the Stenian to Early Tonian Espinhaço and Matadi-Noqui igneous rocks, and magmatic rocks of the early Rio Doce arc). Conversely, sample 13 has twenty grains with positive εHf values and ages between 602 and 758 Ma, suggesting sourcing from (meta)igneous rocks of the Rio Negro arc and their remelting products, and forty grains with negative εHf values with ages between 591 and 646 Ma suggesting magmatic sources in the Rio Doce arc. Additionally, the two youngest spot ages (553 Ma and 571 Ma) have highly negative εHf values (−11.1 and −16.4) suggesting their origin from collisional anatexis. Similarly, sample 12 shows detrital grains within the consistent time interval (ca. 596 Ma – 798 Ma) of the Rio Negro arc and Rio Doce arcs, reinforcing that the fragments of these arcs, or zircons that were inherited from these terranes, are potential sources to the sedimentary protoliths of the paragneisses. The regional scenario together with our field and analytical data, compared with data available in the literature, suggest the sedimentary protoliths of the studied samples filled orogenic basins related to the Rio Doce magmatic arc. Most detrital zircon ages are close to the maximum depositional age, a classical signature of orogenic basins related to magmatic arcs. The studied paragneiss unit correlates with the Nova Venécia Complex, located on the intra-arc to back-arc zones of the Araçuaí orogen, and with the orogenic succession of the distal Andrelândia Group, shared by both the Araçuaí and Ribeira orogens. Furthermore, our data also suggest the juvenile Rio Negro - Serra da Prata arc system was either amalgamated with the Rio Doce arc or that fragments of these arcs were incorporated as unmelted xenoliths and xenocrystals within the Rio Doce arc rocks before ca. 614 Ma, providing key hints to unravel the evolution of the Araçuaí - Ribeira orogenic system (AROS) and its role in the Gondwana assembly.

Metasomatism and cyclic skarn growth along lithological contacts: Physical and geochemical evidence from a distal Pb[sbnd]Zn skarn

Distal skarns form by the metasomatic reactions of a host rock induced by far-traveled hydrothermal fluids. Physical and structural characteristics and geochemical patterns of distal PbZn skarn bodies were studied at the Petrovitsa deposit in southern Bulgaria. Skarn bodies formed from the interaction of hydrothermal fluids with reactive host lithologies (marble and gneiss). These fluids were transported along sub-vertical feeder structures and lithological contacts. Epidote skarn developed in gneiss protolith, and pyroxene (johannsenite) skarn developed in marble. Detailed geological mapping, complimented by measurements of the internal structure of the skarn body using pyroxene growth versors, quantifies the propagation direction of the skarn body: 1) away from the major local fluid conduit (feeder structure), and 2) away from lithological contacts between aluminosilicate rock and marble. Such growth suggests that fluid flow was generally orthogonal to the skarn front propagation direction in the pyroxene skarn.Textural, mineralogical and geochemical data from skarn samples reveal multiple growth generations of major skarn calc-silicates epidote and pyroxene. The epidote skarn is characterized by limited spatial distribution and fine-grained epidote/clinozoisite growth associated with massive replacement and sulfide mineralization. The pyroxene skarn consists of acicular clinopyroxene crystals which form spheroidal aggregates with discrete growth banding. These bands are the physical representation of the cyclic fluid pulses which resulted in rhythmic skarn growth marked by geochemical banding. In situ geochemical analyses in the epidote skarn reveal early Al-rich epidote overprinted by Fe-rich epidote associated with higher Mn and Sr contents and irregular compositional banding. Clinopyroxene (Jo60–95) shows general increase in Na, Al, Mn, and REE + Y with distance from the feeder structure and lithologic contacts. These elements correlate with the distance traveled by the hydrothermal fluid from the feeder to the site of skarnification, which we define using a proxy based on the Al content of pyroxene crystals. This reflects an increasing degree of fluid “contamination” by interaction with the aluminosilicate host rocks and functions as a proxy for fluid transport distance. The spatial distribution of trace-elements in pyroxene on an outcrop scale is indicative of discrete pulses of hydrothermal fluid resulting in precipitation of skarn calc-silicates along the increasingly tortuous fluid pathway between the feeder structure and the skarn front, resulting in both the macro- and micro-scale chemical and textural variability of the skarn body.

Petrogenesis of (meta-) basalts from the North Qilian Orogenic Belt, NW China: implications for the Palaeoproterozoic–Mesoproterozoic tectonic evolution of the North Qilian Block

AbstractThe North Qilian Orogenic Belt is surrounded by the Tarim Craton to the NW and the North China Craton to the NE. The Precambrian continental crust remnants that are distributed in the North Qilian Orogenic Belt are termed the North Qilian Block (NQB), and their tectonic evolution has profound implications for the evolution of the Columbia Supercontinent. Here we present major- and trace-element and Sr–Nd–Hf isotope data for (meta-) basalts from the Beidahe Group (BDHG) and Zhulongguan Group (ZLGG) in the western North Qilian Orogenic Belt, to investigate the tectonic evolution of the NQB during the Proterozoic Eon. The protoliths of Palaeoproterozoic amphibole gneisses and plagioclase amphibolites from the BDHG are calc-alkaline series basalts. These metabasalts show island-arc-basalt affinities with variable Nd and Hf isotopes (ϵNd(t) = −5.0–0.6 and 2.7–4.3;ϵHf(t) = −14.2–2.0 and 6.9–8.8) and were generated by partial melting of the asthenospheric mantle that was metasomatized by aqueous fluid and sediment melt in a continental-arc setting. The early Mesoproterozoic ZLGG basalts show features of shoshonite-series basalts and are geochemically similar to ocean-island basalts. These basalts show variable (87Sr/86Sr)i,ϵNd(t) andϵHf(t) values of 0.70464–0.70699, −1–2.6 and −1.5–5.7, and are products of mantle plume magmatism that participated with subducted oceanic crust in an intracontinental rift setting. This study suggests that the NQB underwent tectonic evolution from palaeo-oceanic subduction to intracontinental rifting during the Palaeoproterozoic–Mesoproterozoic eras. Furthermore, the above tectonomagmatic events were in response to convergence–splitting events of the Columbia Supercontinent during the Palaeoproterozoic–Mesoproterozoic eras.

Metamorphic history and Neoarchean–Paleoproterozoic crustal growth of the central Trans-North China Orogen: Evidence from granulite- to amphibolite-facies rocks of the Hengshan complex

The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional orogen exposed within the North China Craton (NCC). The orogen has undergone a complex tectonic evolution, but its role in the growth of continental crust is contentious. High-grade metamorphic rocks may carry key information required to clarify these issues. Here, using petrography, mineral chemistry, and zircon U–Pb–Hf isotopes, we investigate a suite of Neoarchean–Paleoproterozoic high-grade metamorphic rocks in the Hengshan Complex within the central TNCO. Garnet-bearing mafic granulites and amphibolites occur as lenses or dikes within tonalite–trondhjemite–granodiorite (TTG) gneisses. Three stages of metamorphic mineral assemblages are identified in a mafic granulite from the northern Hengshan Complex: (1) the prograde assemblage is preserved as small plagioclase and apatite inclusions within garnet porphyroblasts; (2) the peak mineral assemblage is recorded by a high-Ca garnet core coexisting with clinopyroxene, plagioclase, hornblende, biotite, and quartz; and (3) the retrograde assemblage is represented mainly by vermicular symplectites of hornblende + plagioclase and plagioclase coronae surrounding low-Ca garnet rims. Phase equilibria modeling and conventional geothermobarometry suggest that the mafic granulite records a clockwise P–T path, including decompression and cooling from the granulite-facies peak (12.5–14 kbar and >750 °C) to amphibolite-facies conditions during retrogression (7.3–8.3 kbar and 615–721 °C). In contrast, the amphibolite in the southern Hengshan Complex preserves only amphibolite-facies assemblages (hornblende + plagioclase + quartz), consistent with the metamorphic pressures of <7.1 kbar and temperatures below 720 °C.Zircon U--Pb dating of magmatic zircons shows that the protoliths of the TTG gneisses and amphibolite were emplaced at ca. 2.5 and 2.1 Ga, respectively. The metamorphic zircons reveal ages of 1.92–1.80 Ga for the mafic granulite (weighted mean of ca. 1.87 Ga) and 1.92–1.83 Ga for the amphibolites (weighted mean of ca. 1.88 Ga). These 1.88–1.87 Ga metamorphic ages are interpreted to represent the time when the rocks cooled to the solidus. By comparing Paleoproterozoic metamorphic ages across the TNCO, we infer that the subduction–collision–amalgamation between the Western and Eastern Blocks of the NCC lasted from 1.95 to 1.80 Ga at least, marking the final cratonization of the NCC. The ca. 2.5 Ga magmatic zircons within the TTG gneisses show positive εHf(t) values (+0.49 to +7.12) with model ages of 2.8–2.7 Ga, suggesting that the protoliths were derived from the partial melting of Neoarchean juvenile mafic crust. Zircon Hf model ages (TDM = 2.1–2.0 Ga) of the mafic granulite may represent the approximate time at which the mafic magma was generated in the mantle. Thus, the Hengshan Complex within the central TNCO records two episodes of crustal growth at 2.8–2.7 and 2.1–2.0 Ga, respectively. To further understand the processes involved in crustal evolution, we reconstructed a layered crust model for the Hengshan Complex, and the layered crust was strongly influenced by Paleoproterozoic thermal events.

Mesoproterozoic tectonic-thermal events in the Oulongbuluke Block, NW China: Constraints on the transition from supercontinent Columbia to Rodinia

The Oulongbuluke Block, which is one of several ancient micro-continents on the southeast margin of Tarim in NW China, has been considered a nearly extinct fragment of the Tarim Craton. Here, zircon U-Pb, Lu-Hf isotope, petrologic and geochemical studies were carried out on the migmatites, grey felsic gneisses, mafic granulites and augen granitic gneisses from Wulan County on the eastern margin of the Oulongbuluke Block. Macroscopic and microscopic observations show that the felsic migmatite is mainly composed of plagioclase-rich leucosomes and melanosomes. The plagioclase-rich leucosomes with positive Eu and Sr anomalies occur as layers and veins, showing in situ partial melting features. Zircon U-Pb dating and Hf isotope analyses on the melanosome, grey felsic gneisses, mafic granulites and augen granitic gneisses from the Oulongbuluke Block yield three age populations at 1500–1496 Ma, 1454–1443 Ma and 1113–1104 Ma. The ca. 1.5 Ga magmatic zircon cores of the grey felsic gneiss and melanosome possess εHf(t) values between −3.5 and 3.9, with two-stage Hf model ages (TDMC) between 2.4 Ga and 2.0 Ga. The ca. 1.1 Ga zircons from samples of augen granitic gneiss have εHf(t) values of −5.2 to 0.5, and the corresponding TDMC values range from 2.2 to 1.9 Ga. Petrological, geochemical and geochronological data suggest that ca. 1.5 Ga magmatic protoliths of grey felsic gneisses were produced by dehydration melting in predominant lower crust setting and experienced metamorphism and anatexis at 1.45 Ga in a rift setting. The precursor magmas of augen granitic gneisses were derived from partial melting of mafic crustal sources in an active continental margin environment.Comparing the tectonothermal events recorded in the Oulongbuluke Block with those in other cratons, the Oulongbuluke Block, Tarim Craton and North China Craton have similar evolutionary histories before the late Mesoproterozoic, suggesting that they were linked together and involved in the assembly and break-up of the Columbia supercontinent. However, the South China Craton, Qilian Block, Oulongbuluke Block, North Qaidam and Tarim Craton were likely temporarily connected during the late Mesoproterozoic-early Neoproterozoic (1.1–0.9 Ga), which corresponded to the assembly of the Rodinia supercontinent.

Tectonothermal evolution of a collisional orogen in the Khammam region, southeastern India: insights from structures, phase equilibria modeling and U–Th–(total) Pb monazite geochronology

The Khammam Schist Belt (KSB), southeastern India, represents a part of the collision zone that is sandwiched between the Eastern Dharwar Craton to the west and the Eastern Ghats Mobile Belt (EGMB) to the east. Quartzofeldspathic gneisses of the EGMB and garnet–kyanite metapelites of the KSB have been investigated to characterize P–T–t evolution of the Khammam region. Pseudosection modeling reveals that the melt-bearing quartzofeldspathic gneiss experienced peak and post-peak metamorphism at P–T conditions of 7.9–8.1 kbar/790–810 °C and ~ 7.0 kbar/740–750 °C, respectively. In contrast, metapelite witnessed peak metamorphism at 7.4–7.8 kbar/600–640 °C, followed by post-peak retrogression at 6.1–6.6 kbar/590–625 °C. U–Th–(total) Pb monazite ages from the gneiss constrain the peak and retrograde metamorphic episodes at 1.63–1.53 Ga and 1.48–1.38 Ga, while those in the metapelites were determined at 1.25–1.20 Ga and 1.18–1.10 Ga, respectively. These P–T–t estimates indicate that the KSB, Vinjamuru, and Ongole domains evolved distinctly during Late Paleoproterozoic–Late Mesoproterozoic. The younger ages (0.90–0.81 Ga) were ascribed to the formation of the Eastern Indian Tectonic Zone, implying its extension beyond the western margin of the EGMB. Besides, the distinct Neoarchean ages (2.79–2.45 Ga) are related to the Archean protolith of the quartzofeldspathic gneisses, which were likely derived from the Archean crust of the Napier–Rayner Complex. This tectonothermal restoration is new and characterizes the Khammam region as the hot and composite collision zone with protracted geological history. (250 words)

Quartz and cristobalite ballen in impact melt rocks from the Ries impact structure, Germany, formed by dehydration of shock‐generated amorphous phases

AbstractQuartz and cristobalite ballen aggregates surrounded by dendritic cristobalite in gneiss clasts of impact melt rocks from the Ries impact structure are analyzed by Raman spectroscopy, microscopy, and electron backscattered diffraction to elucidate the development of the characteristic polycrystalline ballen that are defined by curved interfaces between each other. We suggest that the investigated ballen aggregates represent former fluid inclusion‐rich quartz grains from the granitic gneiss protolith. Upon shock loading, they transformed into an amorphous phase that partly retained information on the precursor structure. Volatiles from inclusions dissolved into the amorphous phase. During decompression and cooling, dehydration takes place and causes fracturing of the amorphous phase and disintegration into small globular ballen, with the fluid being expelled along the fractures. A similar formation of small globules due to dehydration of silica‐rich glass is known for perlitic structures of volcanic rocks. Remnants of the precursor structure are present in the amorphous phase and enabled topotactic crystallization of quartz, leading to a crystallographic preferred orientation. Crystallization of more distorted parts of the amorphous phase led to random orientations of the quartz crystals. Ballen comprised of cristobalite formed from a dehydrated amorphous phase with no structural memory of the precursor. Dendritic cristobalite exclusively occurring at the rim of quartz ballen aggregate is interpreted to have crystallized directly from a melt enriched in fluids that were expelled during dehydration of the amorphous phase.

Early Paleozoic Protoliths of Gneisses and Granitic Gneisses in the Eastern South Urals: Results of U–Th–Pb (SIMS) Geochronological Studies

The Early Paleozoic age of the protolith for gneisses in the East Uralian megazone (South Urals) is proved by zircon dating. Two metamorphic complexes have been identified in the eastern framing of the Middle–Late Paleozoic Suunduk granitoid pluton. The Zelenodolsk complex is represented by granitic gneisses, while the Kusakan complex (Kusakan Formation) is represented by metavolcanic gneisses, crystalline schists, and quartzites. U–Th–Pb analyses (SHRIMP II) of accessory magmatic zircons from the Zelenodolsk granitic gneiss yielded dates of 478 ± 5 Ma. The protolith age of the Kusakan gneiss (andesitic metatuff) is 529 ± 6 Ma. The origin of volcanic, volcanosedimentary rocks, and granitoids in the East Uralian megazone is associated with the evolution of the volcanoplutonic belt in the active continental margin.

Geochemical and zircon U-Pb-Hf isotopic study of metasedimentary rocks from the Huangyuan Group of the Central Qilian block (NW China): Implications for paleogeographic reconstruction of Rodinia

We present a systematic study of micaschists and felsic gneisses from the Huangyuan Group of the Central Qilian block in NW China, with aims to unravel the connection with the Rodinia supercontinent. The micaschists have detrital zircon ages of 2895–928 Ma that peaking at 1.80–1.40 Ga. They show strongly increasing zircon εHf(t) values of −8.1 to +12.1 from 1.6 Ga to 1.4 Ga. Detrital zircon ages from the felsic gneisses are dominantly 960–913 Ma with εHf(t) values of −0.1 to −10.7. The micaschists have a wide range of whole-rock major element compositions, and the felsic gneisses have higher SiO2 contents, combined with lower other major element contents than those of the micaschists. All samples have trace element compositions consistent with upper continental crustal origin. The protoliths of the micaschists are dominantly shales and minor wackes with maximum depositional ages from ca. 1317 to 928 Ma. The protoliths of the felsic gneisses are mostly wackes with a maximum depositional age of ca. 927 Ma. The source materials for these metasedimentary rocks originated from intermediate to felsic igneous rocks. The variable maximum depositional ages of the metasedimentary rocks in the Huangyuan Group indicate that their protoliths constituted a sedimentary series with a long history of deposition starting at ca. 1317 Ma in an oceanic island arc-related basin that developed through a transitional continental arc-related basin into an active continental marginal basin at ca. 927 Ma. It is inferred that the 1795–1321 Ma detritus was sourced from juvenile arc crust at the margin of the Indian or the Western Australian craton. The source rocks for 1317–913 Ma detritus were arc magmatic rocks formed during assembly of Rodinia. A sequence of initial intra-oceanic subduction (ca. 1317–967 Ma) and continuous oceanic crust-continent subduction with formation of a mature continental arc (ca. 967–896 Ma) at the margin of Rodinia during the formation of the Central Qilian block is suggested.

Geochemistry and Zircon U-Pb-Hf Isotopes of Metamorphic Rocks from the Kaiyuan and Hulan Tectonic Mélanges, NE China: Implications for the Tectonic Evolution of the Paleo-Asian and Mudanjiang Oceans

The Late Paleozoic–Early Mesozoic tectonic evolution of the Changchun-Yanji suture (CYS) was mainly associated with the Paleo-Asian and Mudanjiang tectonic regimes. However, the spatial and temporal overprinting and variations of these two regimes remains are still dispute. In order to evaluate this issue, in this contribution, we present new zircon U-Pb ages and a whole-rock geochemical and zircon Hf isotopic dataset on a suite of metamorphic rocks, including gneisses, actinolite schist, leptynites, and biotite schists, from tectonic mélanges in northern Liaoning and central Jilin provinces, NE China. Based on zircon LA-ICP-MS U-Pb dating results, protoliths show wide ranges of aging spectrum, including Paleoproterozoic (2441 Ma), Early Permian (281 Ma), Late Permian (254 Ma), and Late Triassic (230 Ma). The Permian protoliths of leptynites from the Hulan Tectonic Mélange (HLTM) and gneisses from the Kaiyuan Tectonic Mélange (KYTM) exhibit arc-related geochemical signatures, implying that the Paleo-Asian Ocean (PAO) did not close prior to the Late Permian. The Late Triassic protoliths of gneisses from the KYTM, in combination with previously reported coeval igneous rocks along the CYS, comprises a typical bimodal igneous suite in an E–W-trending belt, suggesting a post-orogenic extensional environment. Consequently, we infer that the final closure of the PAO took place during the Early–Middle Triassic. The Early Permian protoliths of biotite schists from the HLTM are alkali basaltic rocks and contain multiple older inherited zircons, which, in conjunction with the geochemical features of the rocks, indicate that they were generated in a continental rift related to the initial opening of the Mudanjiang Ocean (MO). Data from this contribution and previous studies lead us to conclude that the MO probably opened during the Middle Triassic, due to the north–south trending compression caused by the final closure of the PAO.

Composition and Structure of Zircon from Hydrothermal Uranium Occurrences of the Litsa Ore Area (Kola Region, Russia)

Zircon grains from various metagranitoids (plagio- and monzo-granites, gneisses, metasomatic rocks, and pegmatoid veins) from the Skal’noe and Dikoe sites of the Litsa uranium ore area (Kola Region, Russia) were studied in order to reconstruct the sequence and timing of events in the area and to observe effects of hydrothermal process related to uranium mineralization on structure and composition of zircon. Individual zircon grains were studied by means of laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS), ion microprobe and Raman spectroscopy. Isotopic LA–ICPMS data for the Skal’noye and Dikoe ore occurrences suggest the following age sequence of events in the area: intrusion of plagiogranites—2829 ± 12 Ma, formation of magmatic protolith of gneisses—2781 ± 17 Ma, metamorphism of plagiogranites—2636 ± 34 Ma; intrusion of monzogranites and pegmatoid veins—2549–2526 Ma, hydrothermal event with uranium input—2276 ± 21 Ma, last metamorphism of plagio- and monzo-granites—1892–1696 Ma. Ore-bearing rocks in the area are pegmatoid veins and quartz–feldspar metasomatites which contain uraninite. During a 2.3 Ga hydrothermal process, newly formed zircon rims grew simultaneously with the precipitation of uraninite in the veins and metasomatites. These rims are characterized by high U and rare earth elements (REE) contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U ratios (0.1–0.007) and a flat LREE-enriched pattern, in some cases inherited from minerals, dissolved during a hydrothermal event (magmatic plagioclase and probably monazite). Hydrothermal zircon rims grew with partial dissolution of the magmatic zircon, as evidenced by the rounded and curved shapes of zircon cores. The degree of alteration caused by hydrothermal events depends on the uranium content in the pre-existing zircon. The effects of zircon alteration and newly formed zircon composition reflect the redistribution of uranium in rocks.

Dating the Sedimentary Protolith of the Daldyn Group Quartzite, Anabar Shield, Russia: New Detrital Zircon Constraints

Quartzites and paragneisses of the Archean granulite series of the Anabar Shield (Siberian Craton, Russia) are described geochemically. The Sm-Nd isotope systematics of the rocks and the U-Pb age (SHRIMP II) and geochemistry of zircons from quartzites and paragneisses are studied. Newly formed zircons from quartzites display geochemical characteristics of the magmatic type and were produced by rock anatexis upon granulite-facies metamorphism. The Paleoproterozoic age of the latest detrital zircons, 2250 ± 24 Ma, constrains the maximum age of sedimentary rock deposition. The anatectic rims around detrital zircons were formed ca. 2000 ± 9 Ma ago. The time of deposition of the sedimentary protolith of gneisses and quartzites falls within the age interval of the above-mentioned dates and is tentatively accepted as 2.1 Ga. The presence of Paleoproterozoic metasedimentary rocks in the Daldyn Group implies the tectonic heterogeneity of the series and the existence of Paleoproterozoic rock bodies among the predominant Archean rock sequences.