Tectonothermal Events Research Articles

Extent and significance of the Upper Ordovician felsic volcanism in the Pyrenees and Mouthoumet Massifs, SW Europe

Abstract New geochronological (U–Pb isotope dilution thermal ionization mass spectrometry), geochemical and isotopic data from Upper Ordovician felsic volcanic rocks recorded in the Pyrenees and Mouthoumet massifs, SW Europe, suggest that this volcanic activity is more widely represented than previously accepted, and allows a better refinement of the age span involved in the Sardic Unconformity. This Sandbian volcanism represents the final pulse of the Sardic tectonothermal event, starting with the Floian–Darriwilian emplacement of voluminous plutonic rocks and the contemporaneous erosion of the uplifted pre–Upper Ordovician basement, and followed by a tholeiitic volcanism contemporaneous with extensional features and the opening of (half-)grabens finally sealed by Hirnantian glaciomarine deposits. The Sardic-related lithospheric extension may be linked to thermal doming originated by a superplume activity that caused, in turn, an extensive crustal melting responsible for the onset of the felsic (calc-alkaline-dominated), Floian–Darriwilian intrusive and Sandbian extrusive magmatism along the northern margin of Gondwana.

In situ titanite ages and Mesozoic metamorphism of Paleoproterozoic mafic granulites from the Wuhe metamorphic complex, southeastern North China Craton

The Wuhe Complex in the southern section of the Jiao-Liao mobile belt (JLMB) consists of an Archean–Paleoproterozoic crystalline basement and is essential for understanding the intricate tectonic evolution history of the southeastern North China Craton (NCC). Owing to the lack of Mesozoic metamorphic ages reported from the southern section of the JLMB, it is unclear whether the Wuhe Complex experienced Mesozoic tectonothermal overprint events. Titanites are common uranium-bearing minerals for U–Pb dating and are prone to metamorphic, tectonothermal, or hydrothermal processes. Thus, titanites were used in this study to unravel late metamorphic overprint events. Mesozoic titanites from the mafic granulites of the Wuhe Complex are believed to be of metamorphic origin and have experienced amphibolite-facies metamorphism, with a temperature range T = 677–723 ℃ and a pressure of P = 3.8 kbar. Laser ablation-inductively coupled plasma-mass spectroscopy U–Pb dating and in situ U–Pb dating of titanites yielded ages of 163–112 Ma and 123 Ma, respectively. This implies that the Wuhe Complex may have experienced a Mesozoic metamorphic overprint event during the tectonic transition period (from a subduction-related compressive setting to a tectonic stretch setting). Given the large number of Mesozoic intrusive rocks distributed across the NCC, it is important to rule out the effect of late tectonothermal events when studying Neoarchean or Paleoproterozoic metamorphism in the NCC.

U–Pb geochronology and metamorphic history of gneissic rocks from Sarwar‐Junia Fault Zone, Rajasthan, NW India: Implications for the tectonothermal evolution of the Aravalli‐Delhi Mobile Belt

The gneisses of the Sarwar‐Junia Fault Zone (SJFZ), in the Mangalwar Gneissic Complex, a part of the Banded Gneissic Complex (BGC‐II) of northwest India recorded two phases of metamorphism. The pelitic gneiss from SJFZ mainly consists of biotite, sillimanite, garnet, quartz, K‐feldspar and plagioclase. On the other hand, the granitic gneiss from SJFZ mainly consists of quartz, plagioclase, K‐feldspar and biotite. From zircon and monazite geochronology, the protolith of pelitic gneiss was deposited on basement granitic gneiss and the metamorphism occurred at ca. 950 Ma affecting both the granitic gneiss and the pelitic gneiss. Estimation of the P‐T conditions of metamorphism and pseudosection modelling of the pelitic gneiss revealed that the peak stage metamorphic condition of M2 culminated at ~850°C and ~7 kbar pressure followed by a near‐isobaric cooling. Large garnet porphyroblasts in contact with retrograde biotite shows Fe‐Mg resetting close to the core region, which implies a slow cooling. Therefore, the possible exhumation during the subduction‐collision tectonics was very slow, if not completely ceased, at ca. 950 Ma. The shared tectonothermal history of the Sandmata Metamorphic Complex and Mangalwar Metamorphic Complex in the BGC‐II is now evident from the present study of the SJFZ, and confirms the presence of Grenvillian‐aged tectonothermal events in the eastern part of BGC‐II. The abovementioned near‐isobarically cooled and hydrated crust of the SJFZ records subsequent event(s) of shallow crustal brittle faulting as evident by the presence of pseudotachylyte veins and cataclastic fabric observed in the studied gneisses. Brittle faulting possibly occurred between ca. 905 Ma and ca. 750 Ma, as interpreted from the present geochronological data.

Zircon from banded iron formation as a sensitive indicator of its polychronous background: a case study on the Kostomuksha greenstone belt, Karelian Craton, Fennoscandian Shield

ABSTRACT Zircon is not a common accessory mineral of banded iron formation (BIF), but it occurs in all BIF samples from the Kostomuksha Greenstone Belt (KGB) in the Karelian Craton of the Fennoscandian Shield. The Kostomuksha BIF is of Algoma type and is part of three variably old rock associations: BIF-1 is part of a sedimentary sequence in Mesoarchean (2.87–2.84 Ga) basaltic komatiites with dacites, BIF-2 is located in Meso-Neoarchean (2.8–2.79 Ga) felsic volcanics, and BIF-3 is part of a Neoarchean (2.76–2.74 Ga) greywacke unit with felsic volcanics. Analysis of zircon has revealed grains with cores and several generations of rims. The U-Th-Pb systems of zircon indicate that its isotopic age varies considerably from 2.98 to 1.89 Ga, forming up to 4 age clusters in each sample. Most samples contain single inherited grains. The bulk of zircon in BIF is younger than the host rocks and is interpreted as metamorphic. Most of metamorphic zircons display an oscillatory zoning, and their Th/U ratio varies from 0.01 to 4.7, i.e. their origin is hard to interpret reliably, based on the above characteristics. Paleoproterozoic (1.89–1.85 Ga) zircons from BIF-1 and 3, occurring as rims and individual grains, are related to local tectono-thermal processes simultaneous with the formation of the Svecofennian orogen and were described for the central Karelian Craton for the first time. Thus, both inherited and metamorphic zircons have been revealed in Meso-Neoarchean BIF metamorphosed repeatedly in the Neoarchean-Paleoproterozoic under up to amphibolite-facies conditions. All metamorphic generations of zircons from BIF are correlated with the tectono-thermal events in the Karelian Craton, including the effect of the Svecofennian orogeny.

Multiscale Structural Analysis of Ediacaran–Cambrian Rocks on the Northeastern Edge of the Saghro Inlier (Eastern Anti-Atlas): Relevance of Post-Cambrian Deformation

The Ediacaran–Cambrian rocks on the northeastern edge of the Saghro inlier experienced polycyclic tectono-thermal events, which are reported here based on a multiscale structural analysis, from field measurements to fluid inclusion planes. Three striking populations were identified, cutting across both the Ediacaran and Cambrian formations. These tectonic structures were generated during four tectonic events. (i) E-W-striking structures that host ore mineralized bodies (sulfide, oxide, quartz, and barite). They display a polyphase tectonic history, caused by a dextral movement in response to a NW–SE-oriented shortening, leading to the formation of quartz gashes and veins. This tectonic event took place during the Neovariscan. These E–W-striking structures were subsequently reactivated during the Mesozoic time under a sinistral strike-slip regime as a result of NE–SW shortening syn-kinematic with barite mineralization. (ii) NE–SW-striking strike-slip structures (dextral or sinistral) crosscut the E–W-striking veins. These faults are related to the NW–SE-oriented shortening that occurred during the Neogene. (iii) The last tectonic episode, related to the N–S shortening, took place during the late Neogene to the Quaternary period. It resulted in NW–SE to N–S-striking structures that were related to dextral and sinistral strike-slip movements, which crosscut the preexisting E–W structures.

Middle Devonian gold mineralization at the northern margin of the North China craton: Constraints from hydrothermal titanite, garnet, and zircon U-Pb geochronology of the Jinjiazhuang gold deposit

The northern margin of the North China craton (NCC) is adjacent to the Central Asian orogenic belt. The region underwent arc-continental collision during the Middle Paleozoic, oceanic plate subduction and closure during the Late Paleozoic to Early Mesozoic, and tectonic reactivation during the Late Mesozoic. The Late Paleozoic–Early Mesozoic and Late Mesozoic tectonothermal events formed numerous gold deposits along the northern margin of the NCC. However, until now, it was unclear whether any gold mineralization formed during the Middle Paleozoic arc-continental collision. The Jinjiazhuang gold deposit is located in the Zhang–Xuan district at the center of the northern margin of the NCC. Gold mineralization is structurally controlled by NWW-SEE-striking faults cutting the Xiaozhangjiakou ultramafic intrusion and Da’nanshan alkaline complex, which exhibit zircon U-Pb ages of 400 ± 5 Ma (2σ, MSWD = 0.3) and 389 ± 3 Ma (2σ, MSWD = 0.8), respectively. Petrographic and geochemical data for titanite, garnet, and zircon grains from Jinjiazhuang gold ores indicate that these minerals have a hydrothermal origin and coprecipitated with the gold. Laser ablation inductively coupled plasma mass spectrometry analyses of the hydrothermal titanite, garnet, and zircon grains yielded reproducible U-Pb dates of 383 ± 12 Ma (2σ, MSWD = 1.4), 392 ± 2 Ma (2σ, MSWD = 0.79), and 391 ± 3 Ma (2σ, MSWD = 1.4), respectively. These new data suggest that the Jinjiazhuang gold deposit formed in the Middle Devonian and was coeval with the emplacement of the Da’nanshan alkaline complex, the genesis of which is related to post-collisional extension across the Bainaimiao island arc and NCC. The recognition of Middle Devonian gold mineralization related to alkaline complexes is of great significance for targeting similar gold deposits along the Fengning–Longhua fault, along which numerous Middle Devonian alkaline intrusions are located.

Syn-tectonic fluids decoding effects of tectono-metamorphic cycles on regional metallogenic evolution of the Chinese Altai, central Asia

Despite their close temporal and spatial relationships, the effects of tectono-thermal events on ore formation remain obscure. To better understand this process, a comprehensive geochemical investigation on paleofluids from syn-tectonic felsic and quartz veins associated with the Devonian subduction and Permian collision of the Chinese Altai was conducted. We found that the Devonian fluids were organic alkanes-CO2-S-Ca-Mg-rich saline fluids with variable CO2/CH4 (0.09−5.03) and lower F−/SO42− (0.02−0.14) and Al3+/Mg2+ (0−0.11) ratios, whereas the Permian fluids were immiscible fluids including CO2-C4H10-CO-rich oxidized gas bubbles and CH4-C3H8-C2H6-Ca-Na-K-Al-S-Cl-F-rich reduced saline fluids with lower CO2/CH4 (0−1.31, mostly <1) and higher F−/SO42− (0.21−0.76) and Al3+/Mg2+ (0.10−2.56) ratios. The Devonian and Permian fluids also have similar δ13C-CO2 values of −23.8‰ to −3.5‰ and −16.5‰ to −3.7‰, respectively. These data suggest that both fluids derived mainly from devolatilization and dehydration melting of metasediments; the Permian fluids likely involve more biotite melting in the deeper crust and more mantle-derived components, whereas the Devonian fluids contain more meteoric components. Base metal-dominated Devonian mineralization occurred as deep-sourced organic matter- and S-rich fluids promoted base metal migration, whereas the relatively oxidized fluid conditions inhibited the mineralization of many other metals. By contrast, the more reduced and F-rich Permian fluids with more mantle contributions facilitated the extraction of Au and uptakes of rare metals from reworked metasediments and promoted their mineralization. These findings provide a more complete picture of how tectono-thermal events fertilize the crust and demonstrate that syn-tectonic fluids can serve as proxies for metallogenic processes during orogenic cycles in general.

Paleo‐ to Mesoproterozoic Tectono‐Magmatic Events Recorded in the Huwan Complex from the Dabie Orogen, Central China: Evidence from Petrology and U‐Pb Geochronology

AbstractTo better understand the Paleo‐ to Mesoproterozoic tectonic evolution of the Dabie Orogen in the northern margin of Yangtze Block, we present geochronological data for metasedimentary and metavolcanic rocks in the Huwan complex. A total of 385 detrital zircon LA‐ICP‐MS analyses for metasedimentary rocks yielded three 207Pb/206Pb age populations: 1.50–1.80 Ga, 1.81–1.87 Ga and 1.93–2.0 Ga, providing a maximum depositional timing of ca. 1.50 Ga; while metafelsic volcanic gneisses yielded protolith U‐Pb ages of 1893 ± 54 Ma. The peak ages are remarkably consistent with the tectonothermal events that occurred in the northern Yangtze Block, indicating the presence of Paleo‐ to Mesoproterozoic magmatism in the Dabie Orogen. The age range of 1.93–2.0 Ga correlates with the Paleoproterozoic collision; the age range of 1.81–1.87 Ga coincides with the period of the post‐orogenic extension; and the age range of 1.50–1.80 Ga is interpreted to associate with an extensional regime. Zircon cores with age of 1732–1965 Ma have εHf(t) values ranging from –11.70 to –2.47, indicating that juvenile crust involved in their magma sources. Owing to the similar age spectra, we proposed that the nucleus of the Dabie Orogen was close to the Yangtze Block since the Paleoproterozoic. The Huwan complex has an intimate affiliation to the Yangtze Block, and implies multiple orogenic cycles. It was not only experienced the Paleo‐Tethys ocean subduction and collision, but also recorded Paleo‐ to Mesoproterozoic tectono‐magmatic events in the Dabie Orogen.

The geochronology of pegmatites in the Jiajika lithium deposit, western Sichuan, China: Implications for multi-stage magmatic-hydrothermal events in the Songpan-Ganze rare metal metallogenic belt

The Songpan-Ganze orogenic belt is a large-scale rare metal metallogenic belt in western China. Several large-scale granitic pegmatite-type lithium deposits are located in the Songpan-Ganze orogenic belt, which include the Jiajika Li deposit, one of the largest pegmatite-type Li deposits in China. The geochronological constraints of pegmatites and granite in Jiajika are still inadequate for revealing the relationships between mineralization and tectonothermal events. In this study, we performed columbite-group minerals (CGM) and cassiterite U–Pb dating on seven different pegmatite veins with different occurrences and deformation styles in the Jiajika deposit. These pegmatites yield CGM U–Pb ages of 213 to 208 Ma and cassiterite U–Pb ages of 199 to 192 Ma, regardless of their occurrences and deformation. Columbite-group minerals exhibit low Ta# (=Ta/(Ta + Nb)) cores and high Ta# rims, which were formed during the magmatic stage and the hydrothermal stage, respectively. The appearance of cassitertite is closely related with greisenization. Therefore, the CGM U–Pb ages, which were obtained from the low Ta# cores of CGM, are interpreted as magmatic crystallization age, while the cassiterite ages reflect another younger stage of magmatic-hydrothermal event. According to field observation, we consider that the occurrences and deformation of pegmatite veins are controlled by the development of various fracture systems formed during the ascent of the granitic pluton. Our results suggest long-lasting multi-stage magmatic-hydrothermal events in the Jiajika region. Geochronological data of other rare metal deposits located in the Songpan-Ganze orogenic belt and the Western Kunlun orogenic belt are comparable with those of the Jiajika Li ore deposit, indicating that the entire Songpan-Ganze rare metal metallogenic belt experienced long-lasting and multiple magmatic and fluid activities. The overlaps of the geochronological results of granites and pegmatites in the Songpan-Ganze rare metal metallogenic belt suggest that regional magmatism is closely related to Li mineralization. The enrichment of Li in the Songpan-Ganze rare metal metallogenic belt is possibly related to its unique geodynamic setting and the long duration of magmatic-hydrothermal activities.

Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai: Recording a Devonian-Permian tectonic regime switch in the southwestern Central Asian Orogenic Belt

Recent studies revealed that the Chinese Altai may have experienced two major tectonothermal events in the Devonian and Permian but their mutual relationships remain poorly constrained. In this study, we carried out a comprehensive analysis of macro- and micro-structural features of the Ordovician schists that were subjected to Barrovian-type metamorphism in the Chonghuer area of the northwestern Chinese Altai. Field structural and microstructural analyses revealed that metamorphic foliation S1 was folded by the recumbent fold F2, forming the sub-horizontal foliation S2. The S2 fabric was later reworked by the D3 transpressional shearing along the narrow sillimanite-bearing zone to form the foliation S3. Monazite U–Pb ages obtained from the staurolite schist located close to the D3 shear zone, as well as the zircon U–Pb ages from both folded and unfolded pegmatites that cross-cut granite, indicate that the D3 deformation took place during the Permian period, approximately 257–250 Ma. The EBSD study of quartz in the matrix and staurolite porphyroblasts conducted along the NE-SW cross-section allowed us to: 1) constrain the medium-temperature D2 and high-temperature D3 regimes and, 2) reconstruct foliation intersection axis (FIA) patterns both inside and outside the D3 deformation zone. The FIA between Devonian D1 and D2 preserved in staurolite porphyroblast was initially oriented in a N–S direction, which was ultimately reoriented to a NW-SE direction after undergoing the D3 shear deformation. This result is further reinforced by the numerical stochastic modelling, which quantifies the influence of the Permian HT sinistral transpression on the reworking of the Devonian FIA fabrics. Micro-structural data of this study verifies the significant tectonic switch from the Devonian E-W contraction to the Permian NNW-SSE transpression. Quantitative EBSD-based FIA analysis proves to be an effective approach for the structural analysis of polycyclic orogenic systems.

Crustal and lithospheric mantle structure below the Indian shield based on 3-D constrained gravity inversion and Deep Neural Network approach: Geological implications

The Indian shield region comprises of several Archean cratons, Proterozoic mobile belts, Proterozoic-Phanerozoic rift basins, and volcanic provinces and has been significantly modified by various tectono-thermal events and the supercontinental cycles. These overprinting processes and evolution can be understood, to some extent by characterizing the lithosphere structure. In this study, we carried out seismically constrained 3-D nonlinear gravity inversion using tesseroids in the spherical coordinate system to delineate the Moho geometry for the Indian shield. Further, the supervised machine learning approach using Deep Neural Network (DNN) is implemented on the geopotential data to obtain Lithosphere-Asthenosphere Boundary (LAB) across the Indian shield. The inversion results show that the Moho depth varies between 33 and 60 km below the Indian shield with a mean difference of ∼ −0.3 km and a standard deviation of ∼4.9 km relative to the Moho from receiver function data. For the DNN model, the estimated LAB depth values range between 95 and 280 km below the Indian shield. The model gave rise to the coefficient of determination R2 of 0.93, Mean Absolute Error of ∼14 km, and Root Mean Square Error of ∼22 km between the observed and predicted LAB depths. Apart from the general agreement of Moho and LAB depth estimates with previous seismological studies, the present study provides better resolved information of lithosphere structure across the Indian shield. The modelled lithosphere structure therefore will be useful to integrate with the geochronological data in order to to understand the Proterozoic tectonic evolution of the Indian Shield.

CAI and microtextures of low-grade metamorphosed conodonts related to lithological and geological controls

Textures, microtextures, and CAI of early Permian conodonts recovered from three sections of the La Cueva Limestone and the Mina México Formation from Sonora, northwestern Mexico, are herein reported. Nearly uniform values of CAI 5 (a few at CAI 5.5) indicate low-grade metamorphic conditions in the study area. Metamorphic effects in conodonts are demonstrated by the development of the granular microtexture (recrystallization). It is documented herein that facies of reduced porosity and permeability due to compaction and cementation yielded the most recrystallized conodonts. Observations have revealed a correlation between the external cast microtexture and the micrite crystals found in mudstone to packstone lithologies, with no exceptional requirements for sorting, porosity, and permeability for its development. Furthermore, it has been noted that the development of small and large apatite crystal microtextures primarily occurs in highly porous and permeable lithologies, such as packstone and grainstone. Additionally, most of the sparry calcite cements appear to precipitate after the formation of the apatite overgrowths microtextures during intermediate and deep burial diagenesis. These results on conodont microtextures are useful in the recognition of regional tectonothermal events.

Early Mesozoic orogenic gold mineralization in the North Qinling Terrane: Insights from rutile U-Pb, mica and K-feldspar 40Ar/39Ar, and H-O-S-Pb isotopes of the Yangxie gold deposit

The North Qinling Terrane is located between China’s second largest, Early Cretaceous gold province (southern North China Craton) to the northeast and the third largest, Triassic gold province (western South Qinling Terrane) to the southwest. However, only several small- to medium-sized Au deposits and occurrences have been found so far in the North Qinling Terrane, and their genesis and possible links with the gold mineralization in the above two large gold provinces remain unclear. In this study, we present a comprehensive geochronology, stable and radiogenic isotope geochemistry study of the Yangxie gold deposit in the North Qinling Terrane to provide new insights into gold genesis in the North Qinling Terrane and its genetic relations to the neighboring gold provinces. The Yangxie gold deposit consists of quartz-sulfides veins that are hosted by NW- and NE-striking brittle faults cutting amphibolite- to granulite-facies metamorphic rocks of the Paleoproterozoic to Neoproterozoic Qinling Group. Gold mineralization is represented by four paragenetic stages consisting of milky quartz-K-feldspar (I), light grey quartz-pyrite (II), light grey quartz-polymetallic sulfides (III), and quartz-calcite (IV). Gold was deposited mainly in stages II and III, forming abundant native gold grains that occur as inclusions in pyrite, microfracture infillings in sulfide minerals, or interstitial growths to other ore and gangue minerals. Ore-related alteration assemblages comprise mainly quartz, sericite, muscovite, K-feldspar, and calcite. Hydrothermal rutile is widely recognized in gold ores and texturally intergrown with gold-bearing pyrite and other alteration minerals, indicating synchronous precipitation of rutile and gold. Laser ablation ICP-MS U-Pb dating on two rutile samples yield reproducible dates of 214.6 ± 5.5 Ma (MSWD = 0.82, 2σ) and 210.1 ± 6.9 Ma (MSWD = 1.3, 2σ), which are interpreted as the time of gold deposition. 40Ar/39Ar dating of sericite, muscovite, and K-feldspar from the alteration assemblages yield more scattered and younger dates ranging from 199.8 ± 1.1 to 119.3 ± 0.6 Ma, which are considered as the cooling ages of the deposit related to later tectonothermal events. Oxygen and hydrogen isotopes of ore-related quartz and extracted fluid inclusion waters are 10.4 ∼ 15.0 and −87.7 ∼ -67.2 per mil, respectively. Pyrite and galena have variable δ34S values ranging from −9.4 to 0.5 per mil. These sulfides have lead isotopic compositions of 17.956–18.067 for 206Pb/204Pb, 15.565–15.621 for 207Pb/204Pb, and 38.045–38.301 for 208Pb/204Pb. The isotope data collectively suggest that the ore-forming fluids and metals were derived from the devolatilization of the early Paleozoic sedimentary rocks distributed in the North Qinling Terrane. The results presented here, combined with independent studies, show that the Yangxie deposit was a product of the Late Triassic orogenic gold mineralization event that formed the western South Qinling gold province. Recognition of the Late Triassic Yangxie gold deposit suggests that faults within and around the metamorphosed Paleozoic sedimentary rocks and subsidiary faults affiliated to the Shangdan Suture in the North Qinling Terrane could be favorable targets for further Triassic orogenic gold exploration.

Review of the thermo-tectonic evolution of the Central Zone of the Limpopo Complex with implications for conflicting published geodynamic models

Abstract This paper addresses the credibility of published data utilised to underpin conflicting models recently proposed for the geodynamic evolution of the Limpopo Complex (LC), Southern Africa, in the Neoarchaean and Palaeoproterozoic. We are mainly focused on the tectonic and metamorphic processes that affected the Central Zone (CZ) of the LC, but also consider the significance of the timing of the thermo-tectonic interaction of the Southern Marginal Zone (SMZ) of the LC with the granite-greenstone terrane of the Kaapvaal Craton (KVC) at the position of the steep north-dipping Hout River Shear Zone (HRSZ). HRSZ-linked tectonism at the contact with the KVC is expressed as a narrow “hot-iron zone” directly dated at 2.72 to 2.69 Ga and developed in the footwall of the north-dipping HRSZ. HRSZ-linked tectonic activity intermittently continued up to 2.65 to 2.62 Ga with no structural-metamorphic or geochronological evidence that the SMZ and the rest of the KVC were affected by regional thermo-tectonic (orogenic) activity after ca. 2.68 Ga. The complementary evolution of the CZ at 2.72 to 2.62 Ga prior to emplacement at 2.612 Ga of the Bulai granitic pluton is expressed by two thermo-tectonic events, at 2.72 to 2.66 Ga and 2.65 to 2.62 Ga, respectively. The early 2.72 to 2.66 Ga event was associated with near-vertical exhumation of the CZ from a rising crustal-scale granulite diapir, accompanied by emplacement of steeply-dipping isoclinal folded and granoblastic-textured CZ granulites at the mid-crustal level (20 km depth). A moderately (~45°) northeast-directed shear deformational event, accompanied by extensive granitic diapirism, controlled the final emplacement of the CZ in the Neoarchaean at 2.65 to 2.62 Ga prior to intrusion of the 2.612 Ga Bulai pluton. This second tectono-thermal event is expressed by major sheared structural features that include mega-closed folds, mega-north-south trending folds, and the 29 km-wide southwest-northeast-trending and moderately southeast-dipping and northeast-verging Tshipise Straightening Zone (TSZ) that bounds the CZ in the south. The CZ was finally exhumed and emplaced at the upper crustal level 600 Myr later (at ca. 2.02 Ga) during a regional high-temperature Palaeoproterozoic thermal event associated with major near-vertical strike-slip shear zones that overprint Neoarchaean oblique slip shear zones that bound the CZ. Thermo-tectonic activity in the CZ that is associated with this mainly thermal event is recognised as discrete steeply-dipping narrow fabric-parallel shear zones dated at ca. 2.02 Ga that overprint older structures. A gravity-driven crustal-scale diapiric model is utilised to explain the evolution of the SMZ and CZ of the LC at 2.72 to 2.62 Ga during the Limpopo Orogeny. Data presented and discussed contradict alternative published models that propose a continent-continent collisional orogeny at 2.65 to 2.62 Ga involving the SMZ and KVC, followed 600 Myr later at 2.02 Ga by a transpressional orogeny associated with near-horizontal thrust tectonics.

Grenvillian evolution of the Qaidam block and its position in Rodinia constrained by U–Pb–Hf composition of detrital zircons from the Altyn Tagh, Northern Tibet

The U–Pb ages and Hf isotopic composition of detrital zircons provide important constraints on basin evolution and paleogeography. The Qaidam block, a continental fragment along the northeastern margin of the Tibetan Plateau, has been suggested to have been either in an internal or a peripheral position in Rodinia. We present a systematic study of late Mesoproterozoic to Ediacaran sedimentary sequences from the Altyn Tagh belt, a part of the Qaidam block displaced by the Altyn Tagh fault, in an effort to establish its position within Rodinia. The (meta-)sedimentary rocks of the Altyn Complex of South Altyn can be correlated with the Taxidaban Group, the part of the basement(meta-)sedimentarysequence of Central Altyn, on the basis of lithological and geochronological similarities. Both of these were deposited in the late Mesoproterozoic to early Neoproterozoic. The early Neoproterozoic Suoerkuli Group of Central Altyn and its coeval magmatism formed in an extensional setting. This extensional episode facilitated thinning of the lithosphere during the late Tonian–Ediacaran when the sedimentary protoliths of the Hongliuquan Complex were deposited. Detrital zircon grains from late Mesoproterozoic to Ediacaran sedimentary sequences in the Altyn Tagh orogenic belt show a wide distribution of Meso- and Paleoproterozoic ages between 1800 and 1000 Ma, which match well with contemporaneous supracrustal sequences in northeastern Laurentia. In combination with the similarity in late Mesoproterozoic to Ediacaran tectono-thermal events, we propose the Qaidam block was located along the northeast Laurentian margin during the late Meso- to Neoproterozoic assembly of Rodinia.

FIRST LA-ICP-MS APATITE FISSION-TRACK AGES FROM THE SIBERIAN PLATFORM BASEMENT (NEPSKO-BOTUOBINSKAYA ANTECLISE)

We present the first results of LA-ICP-MS apatite fission-track dating (AFT) from nine core samples from wells, that exposed the top of the crystalline basement of the Siberian platform within the Nepsko-Botuobinskaya anteclise on the ~2 km depth. Obtained AFT ages belong to three clusters with mean values of 200, 140, and 60 Ma. A thermal event of ~200 Ma is widespread throughout almost the entire Siberian platform and reflects the stage of its Early Jurassic uplift. Reset of the fission-track geochronometer in apatite ~140 Ma correlates with tectono-thermal event marking the final stage of the Mongol-Okhotsk fold belt collision. The youngest AFT ages of ~60 Ma, on the one hand, may reflect the early (crypto) stage of the Baikal rifting, and, on the other hand, be a consequence of the high content of uranium in apatite.

Proterozoic basins of the Bundelkhand Craton, India: Correlations and significance in understanding the tectonic evolution

The Bundelkhand Craton and associated basins of Peninsular India have received interest for debates related to research on their records of biological evolution, particularly reported occurrences of materials akin to triploblastic animals. Data related to geochronology and key tectono-stratigraphic markers of the block have been critically analysed and a model of evolution during the Proterozoic Eon is presented. The basement block, the Bundelkhand Granite Complex, evolved in three phases: Phase I (3551-3190 Ma), Phase II (2780-2550 Ma) and Phase III (2450-2250 Ma), followed by tectono-thermal events between the phases, representing temporal unconformities of ∼400 Ma and ∼100 Ma. The last phase of evolution was terminated with the amalgamation of the North Indian Block with the South Indian Block, forming the Satpura Orogen (phase IA) at ∼2250 Ma, which included the Singhbhum Orogenic belt. Orogenic collapse/extension at ∼2150 Ma developed a series of sedimentary basins running parallel to the orogenic belt. These Paleoproterozoic basins include the Mahakoshal, Bijawar, Sonrai and Gwalior basins around the Bundelkhand Granite Complex. The adjacent cratons also developed contemporaneous basins for the deposition of the Aravalli Supergroup (in the Western Indian Block), Cuddapah Supergroup (in the South Indian Block) and a proto-ocean at the site of the Eastern Ghats, extending towards the northern margin of the Singhbhum Block. All these basins had an inversion phase of 1950-1800 Ma, ending with the development of the Aravalli Orogen as well as Satpura Orogen (phase IB) and closing of sedimentation in the Paleoproterozoic basins. A new phase of extension at ∼1750 Ma initiated deposition of the Semri Group and Delhi Supergroup in the Northern and Western Indian blocks. The event was associated with the amalgamation of East Antarctic Block with the South Indian Block forming the Eastern Ghats (phase I)-Dalma Orogen. The later stage of Eastern Ghats (phase I)-Dalma Orogeny ∼1500 Ma was contemporaneous with the Delhi Orogeny (North Delhi Orogeny). The principal compressions acting opposite to each other during the Delhi Orogeny (with southeastward tectonic transport) and Eastern Ghats (phase I)-Dalma Orogeny (with northwestward tectonic transport) caused the second uplift of the Satpura Orogen (Satpura Orogeny II), as well as the uplift of the North Indian Block, developing a first-order spatio-temporal unconformity of ∼300 Ma gap above the Semri Group, till the extensional stress regime of ∼1200 Ma initiated deposition of the Kaimur Group. The closing of the Kaimur Basin took place during the Grenvillian Orogeny of ∼1100 Ma, which developed the Eastern Ghats (phase II) – Rayner Orogen on the eastern margin of Peninsular India. The far-field stress of the Eastern Ghats Orogeny (phase II) developed inversion/uplift in the Satpura Orogen and deposition of the Rewa Group in the North Indian Block. A second uplift of the Eastern Ghats (Eastern Ghats Orogeny phase II) and the Delhi Orogen (South Delhi Orogeny) at ∼1000-950 Ma closed the deposition of the Rewa Group and initiated deposition of the Bhander Group in the North Indian Block and the Sirohi Group in the Western Indian Block. The Pindwara Orogeny of ∼850 Ma in the Western Indian Block caused uplift of the North Indian Block and closed the deposition of the Bhander Group. The orogenic movements at ∼1500 Ma and ∼1100-950 Ma were associated with continental amalgamations along the western and southeastern margin of India, but did not involve any amalgamation in the central part of India, which was affected by reactivation/uplift of an older structural fabric.

Zircon U-Pb, whole-rock Rb-Sr and K-Ar ages of metamorphosed and metasomatized paleosol at the base of the Paleoproterozoic Aravalli Supergroup, NW India: A two-billion-year record of tectono-thermal events

The Precambrian paleosols are good proxies for deciphering paleoredox conditions. They are also good indicators of thermal events that affect the metamorphic terrane. The Rb-Sr isochron ages of the Precambrian paleosols are good indicators of the timing of alkali metasomatism. This work presents zircon U-Pb, whole-rock Rb-Sr isochron, and K-Ar ages of a metamorphosed and metasomatized paleosol occurring at the base of the Paleoproterozoic Aravalli Supergroup, Northwestern India, to constrain the time of major thermal events. The Aravalli paleosols (sericite schists) have witnessed a complex history of metamorphism and metasomatism. The LA-ICP-MS U-Pb ages of zircons from the two paleosol samples are 2586 ± 45 Ma and 2793 ± 21 Ma, similar to the zircon ages for the parent Ahar River Granite. Five paleosol samples define an Rb-Sr isochron age of 1394 ± 22 Ma with (87Sr/86Sr)i of 0.7493 ± 0.0034 (2σ) and an MSWD of 3.9, pointing to the timing of the latest alkali metasomatism affecting the paleosols. The whole-rock K-Ar ages of sericite-rich paleosol samples range from 940 ± 30 to 873 ± 25 Ma, corresponding with the emplacement of Erinpura Granite. Thus, the Aravalli paleosol recorded more than two billion years of thermal events in its multi-geochronometers. The correlation between Rb-Sr isotope compositions and REE contents in paleosol samples suggests the REE mobility during K-metasomatism.

Neoproterozoic Collision Granitoids in the Southwestern Margin of the Siberian Craton: Chemical Composition, U−Pb Age, and Formation Conditions of the Gusyanka Massif

The paper provides evidence that collisional magmatism related to the Neoproterozoic (880−860 Ma) orogenic event occurred in the southwest of the Siberian Craton. Newly obtained data are presented on the major-component and trace-element composition, U−Pb (SHRIMP II) zircon age, and Sm−Nd isotope composition for rocks of the Gusyanka granitoid massif in the Yenisei fault zone of the Yenisei Ridge. The concordant U−Pb zircon age of the Gusyanka massif is 871 ± 11 Ma indicates that its rocks were formed in the mid-Early Neoproterozoic, simultaneously with the rocks of the Kalama and Eruda massifs in the Tatarka−Ishimba fault system, during the same stage of the collisional events at approximately 880–860 Ma. The calc-alkaline granites, granodiorites, and leucogranites of the Gusyanka massif are classified, on the basis of their high alumina content and trace element composition, as S-type and were derived from a metapelitic source. Many trace-element parameters of rocks of the Kalama and Eruda massifs correspond to those of low-potassium I-type granites, which were most likely derived from mafic rocks and tonalites. The granitoids of the Gusyanka massif, on the one hand, and the Kalama and Middle Tyrada massifs, on the other, differ contrastingly in Nd isotope composition. The source of the former was either metapelites of the Tungusik Group or metasedimentary rocks of the Sukhoi Pit Group, with the involvement of juvenile material. The melts of granites of the Kalama and Middle Tyrada massifs might have been derived from a source with the involvement of an older, possibly Paleoproterozoic, crustal material and a juvenile mafic source. Thus, the orogenic events at 880−860 Ma led to the generation of melts at different levels of the Paleo- to Mesoproterozoic crust of the trans-Angara region of the Yenisei Ridge. The geodynamic history of the region is correlated with the synchronous successions and similar style of tectono-thermal events along the peripheries of the large Precambrian cratons of Laurentia and Baltica, and this is consistent with paleocontinental reconstructions of the close spatiotemporal relations between these cratons, Siberia, and their incorporation into Rodinia.

Formation of the Huayangchuan (Central China) carbonatite-associated REE-Nb-U polymetallic deposit constrained from monazite mineral chemistry and isotope systematics: HREE enrichment in late-stage monazite

The process of enrichment of middle and heavy rare earth elements (MREE and HREE = Sm-Lu plus Y) in magmatic-hydrothermal systems, especially the carbonatite system, remains unclear. Here, we performed in-situ monazite element and isotopic analysis to investigate the genesis of the Huayangchuan REE-Nb-U polymetallic deposit in central China and the enrichment process of MREE and HREE during the evolution of the magmatic hydrothermal system. The Huayangchuan deposit is spatially associated with HREE-rich calcite carbonatites, which exist as two mineralization stages: the ores of the first stage are hosted by carbonatite itself; in the second stage, the ores are present as veins, dominantly by wall rocks around the carbonatite. The results of secondary ion mass spectrometry (SIMS) monazite U-Pb dating on the thin sections yielded Tera-Wasserburg lower intercept ages of 207 ± 4 Ma and 206 ± 5 Ma for carbonatite-hosted monazite and vein-type monazite, respectively. However, some monazite analysis points provide a wide Yanshanian age range (112–182 Ma). The lower intercept age of 206–207 Ma is explained as the mineralization age of the deposit, whereas the wide younger age with higher 238U/206Pb and lower 208Pb/232Th ratios may be ascribed to the loss of radioactive Pb, as the varying degrees of reworking resulted from the later Yanshanian tectono-thermal event. The monazite laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Sr-Nd isotopes from both carbonatite-hosted and wall-rock-hosted vein-type ores indicate that the Huayangchuan deposit is closely related to enriched mantle-derived (EM1) calcite carbonatite but locally affected by late fluid activation. Combined with the integrated age spectra, such reworking processes associated with new weak mineralization widely exist in other carbonatite-related deposits in the Lesser Qinling, but are not important for carbonatite-related mineralization in this area. Controlled by element behaviors, MREE and HREE are more likely to dissolve in hydrothermal fluids, enabling their long-distance migration and deposition, than light REE (LREE) in some carbonatite systems, resulting in much higher MREE and HREE contents of the late monazite in the vein-type ores than those of the early carbonatite-hosted monazite. Therefore, it could be expected that there might be MREE and HREE enrichment and exploration potential in the periphery of some large carbonatite-related LREE-dominated deposits.