Early Paleozoic Tectonothermal Event Research Articles

Microtexture, geochemistry and geochronology of monazite and zircon from the Jialu deposit in the Lesser Qinling: Implications for multi-stage magmatic and metamorphic events in the southern margin of the North China Craton

The Lesser Qinling area is the largest outcrop of the Precambrian basement in the southern margin of the North China Craton (S-NCC). Notably, many carbonatite-related polymetallic deposits are distributed in this area, including the Jialu deposit with light and heavy REE mineralization. Carbonatite and granite from the Jialu deposit contain diverse monazite and zircon grains, and deciphering the age distributions and chemical compositions of these monazites and zircons is crucial for comprehending the multi-stage magmatic and metamorphic events in the S-NCC. Type I monazite from banded REE ore veins yields a U-Pb age of 217.0 ± 5.0 Ma, representing the formation age of carbonatite. Type II monazite from granite yields consistent U-Pb and Sm-Nd ages of ∼1.95 Ga, representing the emplacement age of granite. Type III monazite is hydrothermal in origin, and yields two dominant age peaks at ∼550 Ma and ∼525 Ma. Types I, II and III zircons are xenocrysts from the Taihua Group and yield two main age peaks at ∼2.35 and ∼2.17 Ga. Of note, type III zircon also records an early Paleozoic tectonothermal event by a lower intercept age of 532 ± 59 Ma, which overlaps the ages recorded by hydrothermal monazite. The early Paleozoic tectonothermal events discovered may be a manifestation of the global Pan-African Orogeny in the S-NCC. This new finding further implies that the S-NCC was significantly affected by the assembly of the Gondwana supercontinent during the early Paleozoic.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Early Paleozoic magmatism in northern Kontum Massif, Central Vietnam: Insights into tectonic evolution of the eastern Indochina Block

The Chu Lai and Dai Loc granitic complexes are distributed on two sides of the early Paleozoic Tam Ky – Phuoc Son suture zone (TPSZ) in the northern Kontum Massif (KTM), central Vietnam. Studies of their petrogeneses and tectonic setting provide new insights into the tectonic evolution of the eastern Indochina Block and its affinity with the South China Block (SCB). Zircon U-Pb dating indicates that the Chu Lai and Dai Loc granites formed at 445–452 Ma and 415–420 Ma, respectively. The Chu Lai granites have high ASI, K2O/Na2O and Rb/Sr as well as strongly negative zircon εHf(t) (−14.6 to −2.9) and complex inherited zircon components, characteristics of typical S–type granites. The Chu Lai granites may be subdivided into two groups based on their geochemistry. However, both groups have similar zircon Hf isotopes and inherited–zircon age spectra, suggesting that they probably were derived from different degrees of partial melting of the same late Neoproterozoic meta–greywacke source at different crustal levels. The Dai Loc granites have variable major– and trace–element contents, ASI, K2O/Na2O, Rb/Sr, and zircon Hf–isotope compositions, indicating that most of them are S–type granites and some are I–type granites, suggesting derivation from heterogeneous sources. The petrology, geochemistry, geochronology and formation temperatures suggest that the Chu Lai granites formed in a syn–collisional setting between the KTM and Truong Son Belt (TSB) and the Dai Loc granites in a post–collision extensional setting triggered by the upwelling asthenosphere mantle. Similarities of age spectra and Hf isotope compositions of inherited zircons from the Chu Lai granites with detrital zircons from the Neoproterozoic sedimentary rocks in Western Cathaysia Block and Tethyan Himalaya suggest they are adjacent each other during the late Neoproterozoic. The ages and zircon Hf–isotope compositions of early Paleozoic granites between the KTM and TSB are consistent with those in the SCB, but different from many other microcontinents from the northern margin of East Gondwana, suggesting that the SCB was close to the KTM and TSB and experienced similar early Paleozoic tectonothermal events. It can be inferred that there was a concealed early Paleozoic subduction zone south of the SCB, connecting with the TPSZ in central Vietnam, and that the far–field effects of the subduction of an unknown oceanic plate triggered the early Paleozoic intracontinental orogeny in the SCB.

Two kinds of authigenic xenotime overgrowths in response to an Early Paleozoic tectonothermal event in South China

This study has documented two kinds of xenotime overgrowths from the Cryogenian-Ediacaran sedimentary rocks in South China. The identical tetragonal dipyramidal structures prompted one type of xenotime to overgrow detrital zircon grains, whereas the other type coexists with rutile crystals. SIMS Pb-Pb, NanoSIMS, EPMA, and TEM imaging analyses suggest these xenotime overgrowths, together with their rutile substrate could be of hydrothermal origin; formed by means of percolating fluids leaching ilmenite and REE(Y) bearing silicate minerals. Iron, Ti, and REE(Y) ions leached in this way were combined with O, S, and P anions to precipitate rutile, xenotime, iron oxides, and pyrite in other pore spaces. Such fluid induced activities were probably induced by the ca. 460–390 Ma Wuyi-Yunkai Orogeny in South China. The most important finding is that the rutile-xenotime assemblage was precipitated almost simultaneously, rendering a dissolution-reprecipitation process impossible. Absence of dissolution margins on detrital zircons suggest detrital Fe-Ti oxides are more prone to be leached by fluids than zircon grains, probably because the latter has more stable chemical properties.

Gold mineralization in the Jiangnan Orogenic Belt of South China: Geological, geochemical and geochronological characteristics, ore deposit-type and geodynamic setting

Located in the southeastern margin of the Yangtze Block and generally interpreted as the Neoproterozoic collisional product of the Yangtze with the Cathaysia Blocks of South China, the Jiangnan Orogenic Belt (JOB) contains a number of gold (Au) (-polymetallic) ore deposits and mineral showings, mostly hosted by Neoproterozoic low-grade metamorphic volcaniclastic and sedimentary rocks. The mineralization styles mainly include auriferous quartz veins and disseminated mineralization in altered mylonite and cataclasite that are developed along shear zones, fracture zones and inter- or intra-formational fault zones closely related to regional folding and shearing deformation. Three gold mineralizing epochs are recognized in the JOB. The ca. 423–397Ma mineralization was associated with the early Paleozoic tectonothermal event(s), which induced widespread emplacement of Silurian S-type granites, low-grade metamorphism and enrichment of gold in the Neoproterozoic rocks (i.e., forming Au source beds). The second Au mineralization epoch, occurring at ca. 176–170Ma (Jurassic), was related to the subduction of the Paleo-Pacific plate beneath the South China continental margin. The third and most important Au mineralization epoch took place at ca. 144–130Ma (early Cretaceous), when a Basin-and-Range tectonic pattern was developed, characterized by NE–NNE-trending strike-slip faults, granitic domes and metamorphic core complexes (MCC), and basins filled with red bed lithologies. C, H, O, He-Ar, S and Pb isotopic and fluid-inclusion data suggest that the ore fluids were predominantly metamorphic and/or magmatic, with variable input of mantle-derived fluids and the progressive involvement of meteoric waters in the later stages of mineralization. Ore materials were mostly contributed by the Neoproterozoic source beds, plus a possible contribution from mantle- or magma-derived components. The Au (-polymetallic) deposits in the JOB, particularly those formed in the early Cretaceous, share many geological and geochemical features with the orogenic-type and Carlin-type deposits. In the context of tectonic evolution of South China, the gold mineralization in the JOB may be considered an “intracontinental reactivation type”, characterized by synchronous development of Au–polymetallic mineralization, reactivation of stuctures developed in Neoproterozoic metamorphic rocks, and widespread granite emplacement in the late Mesozoic.

Pan-African and early Paleozoic tectonothermal events in the Nyainrong microcontinent: Constraints from geochronology and geochemistry

To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 206Pb/238U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P2O5 and Al2O3 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.

An early Paleozoic tectonothermal event in western Mongolia: Implications for regional extension of the Cambrian orogenic belt

An early Paleozoic tectonothermal event in western Mongolia: Implications for regional extension of the Cambrian orogenic belt

Tectonic Evolution of the Amdo Terrane, Central Tibet: Petrochemistry and Zircon U-Pb Geochronology

The formation and evolution of the basement rocks in the Tibetan Plateau remain poorly constrained. Here we report petrology, geochemistry, and zircon U-Pb geochronology of orthogneisses, metamafic intrusions, and metasedimentary rocks from the Amdo basement in central Tibet. The oldest Neoproterozoic (∼820 Ma) orthogneisses show chemical affinity to a volcanic arc setting. These rocks, together with the coeval arc-magmatic rocks, belong to an Andean-type setting traced from the Seychelles, Madagascar, western India, Lhasa, and South China blocks, following the Rodinia supercontinent assembly. The later early Paleozoic (∼500 Ma) bimodal intrusions in Amdo also show an affinity to arc-related igneous rocks and probably formed in an active continental margin associated with the subduction of the Proto-Tethys Ocean beneath the Gondwana supercontinent. In contrast to the adjacent Lhasa and Qiangtang terranes, the Amdo block experienced complex Neoproterozoic and early Paleozoic tectono-thermal events and coherent Jurassic (∼190 Ma) high-pressure granulite-facies metamorphism and associated magmatism, suggesting that the Amdo block was an isolated microcontinent in the Tethyan Ocean and underwent deep subduction before its collision with the Qiangtang or Lhasa terrane.

La-ICP-MS U–Pb and 40Ar/ 39Ar geochronology of the sheared metamorphic rocks in the Wuyishan: Constraints on the timing of Early Paleozoic and Early Mesozoic tectono-thermal events in SE China

Mylonite and ultramylonite have been well documented in Pre-Devonian meta-volcanic and meta-sedimentary rocks in the Wuyishan, SE China. Field observations and thin section analyses show dextral strike-slip shearing. Deformation temperature is estimated in 400 °C–500 °C and the pressure between 7 kbar and 10 kbar. La-ICP-MS U–Pb zircon and 40Ar/ 39Ar step-heating biotite–muscovite ages are used to constrain the timing of the dextral shear zone and its tectonothermal history. La-ICP-MS U–Pb dating of zircons from three gneissic granitic samples provides magmatic crystallization ages between 441 and 431 Ma, and dating of the growth rims of zircons from one ultramylonite sample provides metamorphic age of 433.8 ± 6.8 Ma. These dates provide additional constraint on the timing of the Early Paleozoic tectono-thermal event in the Wuyishan region. 40Ar/ 39Ar step-heating dating of biotite and muscovite separated from two mylonite samples yields ages of 235.3 ± 2.8 Ma and 238.5 ± 2.8 Ma, respectively; and an undeformed granitic dyke intruded into the shear zone was dated, 229.8 ± 2.2 Ma, by La-ICP-MS U–Pb zircon method. These dates reveal that metamorphic rocks in the Wuyishan suffered from dextral shearing at the time span between 230 Ma and 239 Ma, during the Triassic Indosinian tectono-thermal event.

Geological, geochronological and geochemical features of granulites in the Eastern Tianshan, NW China

This paper reports new results on the petrology, geochronology and geochemistry of two km-scale high-pressure granulite exposures, namely the Weiya and the Yushugou granulites, that occur in the central and southern Tianshan Paleozoic sub-belts, respectively. Petrological and geochemical results suggest that the Weiya and the Yushugou granulites are different in both the rock types and geological structures. The protolith of the Weiya granulite is basic to intermediate rocks, and that of the Yushugou granulite is basic rocks; the former is distributed as fault-bounded blocks within the Precambrian schist-gneiss, and the latter occurs as thrust slabs in the allochthonous ophiolitic mélange. Observation of deformation structures provides some evidence for two-phase ductile deformation of the Tianshan granulites, characterized by deformed garnet-pyroxene and deformed feldspar-quartz microstructures, corresponding to rheological conditions of the lower crustal level and of the middle crustal level, respectively. The deformation structures also indicate that the Weiya granulite was involved in the late stage retrograde metamorphism forming new amphibole, following a thrusting movement toward the north and ductile deformation of feldspar-quartz grains. The Yushugou granulite underwent strong ductile shearing during the Silurian–Devonian interval. Three stages of metamorphism are recorded by mineral assemblages of the Weiya granulite: M1=Pl+Qtz+Bt, M2=Cpx+Grt+Pl+Qtz+Atp±Opx, and M3=Amp+Ep+Pl+Bt+Qtz, in contrast to a two-stage metamorphic history of the Yushugou granulite: M1=Cpx+Grt+Pl±Hyp, M2=Amp+Pl+Qtz±Bt. The P–T condition estimates suggest that the Weiya granulite was formed between 910–1025 °C and 1.08–1.12 GPa, and underwent retrograde metamorphism at 650–670 °C, while the peak metamorphism of the Yushugou granulite took place between 800–870 °C and 0.88–1.13 GPa. Geochemical characteristics (major, REE and incompatible elements) of the Weiya and Yushugou granulites indicate that their protoliths were formed in a volcanic arc setting, implying that a late Precambrian active continental margin may have existed in the study area. The Sm–Nd isochron dating for the Weiya granulite suggests that the peak metamorphism occurred at 538±24 Ma, which was likely related to the ancient plate subduction. The 40Ar/39Ar dating on amphibole grains separated from the Weiya granulite yielded a plateau age of 432±1 Ma for the retrograde metamorphism, which represents an exhumation age, suggesting a significant early Paleozoic tectonothermal event.

Polyphase tectonothermal evolution of exotic caledonian nappes in Troms, Norway: Evidence from [formula omitted] mineral ages

40 Ar 39 Ar incremental-release ages have been determined for hornblende and muscovite from several high-level, exotic nappes in northern Troms County, Norway. Within the Nordmannvik Nappe and upper tectonic levels of the Lyngen Nappe Complex (Balsfjord Group) both minerals record plateau and isotope correlation ages between c. 427 and 417 Ma. These indicate relatively rapid post-metamorphic cooling which is interpreted to have occurred during Scandian translation of exotic tectonic elements to higher crustal levels along the Baltoscandian margin. Muscovite within various structural levels of the Tromsø Nappe Complex (Uppermost Allochthon) yeilds plateau ages of 427-410 Ma which also record post-Scandian metamorphic cooling. However, hornblende within the Tromsø Nappe Complex records significantly older isotopic correlation ages which range between c. 481 Ma and 432 Ma. The 40 Ar 39 Ar spectra display no evidence of partial Scandian rejuvenation, and the isotope correlation ages are therefore interpreted to date cooling following pre- Scandian tectonothermal activity. These results indicate that early Paleozoic tectonothermal events affected outboard exotic tectonic elements at approximately the same time as the “early” Caledonian orogenesis previously described for nappe units with initial Baltic palinspastic affinities (e.g., Seve Nappe Complex).