Tectonothermal Events Research Articles

Origin of hydrothermal dolomitization in the Huize Zn–Pb ore district, SW China: Insights from in situ U–Pb dating, fluid inclusion, and C–O–Sr–Mg isotope analyses

The temporal and genetic association between Mississippi Valley-type (MVT) Zn–Pb mineralization and hydrothermal dolomitization remains controversial. To determine the origin of hydrothermal dolomite and its genetic links with the MVT ore deposit, detailed petrographic observations and geochemical analyses were conducted on various carbonates from the giant Huize MVT Zn–Pb ore district in SW China. The following paragenetic sequence of the carbonates (from the early to late stages) was established: host limestone (ML), early diagenetic micritic dolostone (D1), late diagenetic or pre-ore fine- to medium-grained ferroan dolostone (D2) and medium- to coarse-crystalline non-ferroan dolostone (D3), pre-ore reworked D3 dolomite (D3o), ore-related void-filling dolomite cement (DC), and calcite cement (CC) related to sulfide mineralization. D2, D3, DC, and CC exhibit higher homogenization temperatures for fluid inclusion than the burial temperature, indicating a hydrothermal origin. Geochemical data indicate that D2, D3, D3o, DC, and CC display oxygen isotope depletion and radiogenic Sr isotope enrichment signatures relative to D1. Their parent fluids have more positive δ18O values and similar or lower δ26Mg values relative to those of D1 and seawater. These geochemical proxies indicate that the pre-ore hydrothermal dolomites (D2 and D3) formed from modified seawater circulated in the underlying sandstone aquifers through fault-related thermal convection. DC and CC, related to Zn–Pb mineralization, were formed by the dissolution and reprecipitation of preexisting carbonates. Sphalerite shows higher temperatures and salinities compared with D2 and D3 dolostones, indicating that the ore-forming fluid, different from the hydrothermal dolomitizing fluid, originated from a deep-sourced brine. In situ U–Pb dating of D3o reveals that the pre-ore hydrothermal dolomitization occurred at 253.7 ± 8.7 Ma, and a late-stage hydrothermal imprint occurred at 203 ± 11 Ma, likely related to tectono-thermal events, including the Emeishan large igneous province and Indosinian Orogeny, respectively. These findings imply that the hydrothermal dolomitization and Zn–Pb mineralization in the Huize ore district are likely associated with the multistage basin and basement fluid flows driven by elevated geothermal gradient and tectonic compression, respectively. The void-filling DC and CC and their cathodoluminescence characteristics are useful indicators for MVT Zn–Pb ore exploration.

The Gulf of Suez rifting: implications from low-temperature thermochronology

ABSTRACT The Gulf of Suez is a continental rift basin that has been developed during the Oligocene-Miocene as a northern extension of the Red Sea rift system. The Oligo-Miocene rift-activated flanks are represented by exposures of the Neoproterozoic basement rocks of the Arabian-Nubian Shield. An additional thermal overprint (i.e. mantle plume) and the preceding tectono-thermal events that also affected the rifting dynamics and the uplift of its flanks remain uncertain. We here present thermochronological data for 20 basement samples collected from the Gabal Somr ElQaa area at the northwestern flank of the Gulf of Suez. The resulted zircon fission-track ages could be divided temporospatially into two groups: the Cambrian and the Silurian-Devonian with average ages of 503 ± 14 Ma and 407 ± 11 Ma, respectively. While apatite fission-track data reveal three different age groups of the Devonian-Carboniferous, the Permian-Triassic, and the Late Cretaceous, with average ages of 349 ± 10 Ma, 246 ± 6 Ma, and 81 ± 5 Ma, respectively. The modelled t-T histories indicate four cooling pulses during the Neoproterozoic, the Devonian-Carboniferous, the Cretaceous, and the Oligocene-Miocene. Integrating our findings with the regional tectonism and depositional records indicates that these cooling pulses could be attributed to synchronous activities of the post-accretion erosional event, the Variscan tectonic event, the Mid-Atlantic Ridge spreading, and the Gulf of Suez rifting. The Gulf of Suez is a passive rift, formed by a single and rapid rifting pulse within the framework of the Red Sea rift system. An additional east-southward far-field thermal overprint from the Arabian marginal plume partially affected the Suez rifting process. This thermal print affected only the rift’s southern portion of its eastern flank, causing syn-rift cooling ages, highly elevated rift flanks, and an increased heat flow. The western rift flank and the northern segment of the eastern flank are characterized by pre-rift cooling ages, modest rift flanks, and reduction in heat flow.

Subsurface structural trends in Central-Western India: Inferences from magnetotelluric data using the complex MT apparent resistivity tensor

The central-western part of the Indian subcontinent exhibits significant geological diversity and possesses a rich record of different tectonothermal events in the earth's evolutionary history, spanning from the Archean to the present. Precambrian geology in the region is mostly obscured by Cretaceous-Tertiary/Paleocene volcanic eruptions and Proterozoic to Quaternary/recent sedimentary cover. Magnetotelluric (MT) impedance and tipper responses from 146 stations, covering central-western India in a grid fashion with a spacing of ∼55 km, were analyzed to evaluate the subsurface structural trends, dimensionalities, and qualitative characteristics of the electric lithosphere in the region. An advanced Complex Apparent Resistivity Tensor (CART) approach, along with the popular Phase Tensor (PT) approach and induction arrows are utilized to achieve the above goals. The analysis revealed a high degree of 3D induction effects in the data at almost every station and period. The directionality information retrieved from the tensor properties showed that the structural trends in the deep crust and upper mantle of the major geologic domains correlate with the known surface tectonic trend of the respective geologic domain. This study provides vital evidence to support the suspected bifurcation of the Precambrian Aravalli-Delhi Mobile Belt (ADMB) tectonic trend and its extensions into neighboring Kutch, Saurashtra, and Central Indian Tectonic Zone (CITZ) domains. The analysis results indicate a lithospheric scale enhanced conductivity beneath the Malwa plateau, which marks the first major phase of the Reunion mantle plume eruption in India. A critical finding of this study is that the Resistivity Phase Tensor better defines the subsurface resistivity structure and provides much clearer structural information than the conventional Phase Tensor.

Polymetamorphic evolution of the Vestfold Block in East Antarctica and implications for the amalgamation of terranes

The Vestfold Block, a typical polymetamorphic Archean terrane in East Antarctica, is a key area to understand amalgamations of Rodinia and East Gondwana continents. However, multiphase overprinting makes it difficult to determine the timing and nature of each tectonothermal event. In this study, we present P–T estimates, zircon, monazite U(–Th)–Pb and biotite/K–feldspar Rb–Sr isochron ages of paragneisses from the SE Vestfold Block. One paragneiss sample, which is assigned to the Chelnok Paragneiss, has experienced a protracted metamorphism from the Neoarchean to the early Paleoproterozoic. Phase equilibria modeling constrained the peak P–T conditions to 7.2–9.6 kbar and 850–880 ℃, and the post–peak metamorphism to 4.2–5.6 kbar and 720–790 ℃, respectively. On the other hand, a paragneiss sample close to the ice sheet documented a high–grade metamorphic event at 918 ± 23 Ma, with peak P–T conditions of 6.0–8.0 kbar and 860–880 ℃. Biotite/K–feldspar Rb–Sr dating for these two samples yields isochron ages of 474 ± 12 and 442 ± 7 Ma, respectively, representing the cooling ages of the Pan–African reworking. Collectively, an integrated application of diverse chronometers, combined with published data, indicates that the Vestfold Block may have experienced at least three major thermal events with variable intensities and extents. Initially, the supracrustal rocks in this region pervasively underwent a protracted high–grade thermal event from the Neoarchean to the early Paleoproterozoic, which formed the backbone of the block. Thereafter, the southern Vestfold Block experienced a Grenvillian granulite facies metamorphism, indicating that the Vestfold Block has been locally involved in the Rayner orogeny (i.e. the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and East Antarctica). Ultimately, the whole Vestfold Block may have been reworked under relatively low temperatures during the Pan–African Prydz tectonic event.

Source and age of the Ngaye banded iron formations, Adamawa Yadé Domain, Central Cameroon: Constraints from whole-rock geochemistry and U-Pb zircon geochronology

The Ngaye inlier of the Adamawa Yadé Domain is located in the Central African Fold Belt, adjacent to the Nyong Complex at the northern margin of the Congo craton, in central Cameroon. This area consists of metamorphosed granite-greenstone associations comprising amphibole- and pyroxene-rich banded iron formations (BIFs), amphibolites, and migmatitic gneisses. This study reports detailed petrographic, whole-rock geochemical, and LA-ICP-MS U-Pb zircon data for the Ngaye BIFs and associated amphibolites to better constrain their source, tectonic setting, and age. Amphibole-rich BIFs exhibit high REE-Y content and uncommon LREE-enriched patterns. In contrast, pyroxene-rich BIFs display combined low REE-Y content, seawater-like patterns, and positive Eu anomalies, suggesting a mixture of seawater and hydrothermal fluids during their deposition. The geochemical data of associated amphibolites suggest a back-arc setting for the Ngaye metavolcanosedimentary sequence, similar to that of the Nyong Complex. LA-ICP-MS U-Pb zircon dating of amphibole-rich BIFs indicate a maximum depositional age of ca. 2186 Ma and subsequent metamorphism at ca. 2038 Ma, overlapping with that of metavolcanosedimentary sequences from the Nyong Complex. Neoproterozoic age of ca. 598 Ma obtained for these BIFs is interpreted as the Pan-African metamorphic/hydrothermal imprint. This finding suggests that the post-depositional fluid overprint was probably related to the regional Pan-African tectono-thermal event.

Late Mesoproterozoic passive continental margin in the northwestern Yangtze Block, South China: Insights from the Huodiya Group in the Hannan‐Micangshan Massif

Whether the Yangtze Block was involved in the global late Mesoproterozoic orogeny remains contentious. The Mesoproterozoic Huodiya Group exposed in the Hannan‐Micangshan Massif on the northwestern margin of the Yangtze Block could provide sufficient evidence to further clarify the regional tectonic affinity. However, quantitative insights into the depositional age, provenance and basin tectonic setting of the Huodiya Group still lack comprehensively constraining. This study presents a combined analysis of zircon U–Pb dating for meta‐sedimentary rocks of the Shangliang Formation of the upper Huodiya Group and intruded gabbroic dykes. Intergraded dating results constrain the late Mesoproterozoic maximum depositional age at ca. 1050 Ma. Ca. 900 Ma formation age for the intruded gabbroic dyke, plus the minimum formation age of the overlying Xixiang Group at ca. 950 Ma, further suggests its deposition should be prior to ca. 950–900 Ma. Concordant detrital zircon ages define pronounced age clusters of ca. 2229–1741 and 2950–2388 Ma and few age populations of ca. 3232–3082 and 1069–1033 Ma, consistent with the tectonothermal events in proximal domains at the northwestern and northern Yangtze Block. Detrital zircon age distribution patterns and cumulative curves, coupled with the stable carbonate platform and shallow‐marine facies sedimentation, suggest a passive continental margin setting for the Huodiya Group on the Yangtze Block margin at the late Mesoproterozoic. Comparative insights into the contemporaneous passive continental margin from north to southwest argue against the existence of a late Mesoproterozoic orogeny along the exterior periphery of the Yangtze Block.

Structural and chronological constraints for Longquan‐Badu ductile shear zone: Implication for Triassic intraplate orogeny in South China Block

The ductile shear deformation of Precambrian basement rocks in Wuyishan provides a crucial perspective on intraplate orogeny in the South China Block (SCB). This study focuses on the Longquan‐Badu ductile shear zone in southeastern Zhejiang, employing field observations, thin section analysis, quartz electron backscatter diffraction (EBSD), zircon U–Pb dating and 40Ar/39Ar geochronology. Two distinct phases of deformation, referred to as D1 and D2, have been identified. D1 is primarily characterized by a WNW–ESE striking foliation within a NE‐plunging lineation, indicating top‐to‐SSW shearing. The paragneiss within the Badu complex that experienced D1 deformation has been dated to 247–239 Ma through zircon U–Pb analysis, corresponding to the prevalent high‐pressure metamorphic age in the region. This correlation suggests that the D1 deformation event coincided with crustal thickening during the Early Triassic. D2 deformation exhibits folds, foliation, S‐C fabrics and mylonitic microstructures and is mainly characterized by striking NNE–SSW with steeply dip, demonstrating a dominant dextral strike–slip component. Quartz c‐axis orientations in mylonitic rocks indicate deformation temperatures between 350°C and 550°C with asymmetric girdle patterns suggesting simultaneous basal and prism slip. The plateau ages of muscovite from mylonitic rocks obtained through 40Ar/39Ar dating are approximately ~228 Ma implying that the D2 deformation occurred under retrograde amphibolite to greenschist facies metamorphic conditions during Middle Triassic. Collectively these data along with regional geological evidence signify two distinct intracontinental orogenic processes occurring in eastern SCB. Considering Early Mesozoic tectonothermal events in Cathaysia Block, it can be inferred that intraplate orogeny in Wuyishan resulted from plate‐margin collisions between SCB and peripheral plates following scissors closure of Palaeo‐Tethys from east to west.

Precambrian geological history of the Tarim Craton (NW China) involving the assembly and fragmentation of the Columbia and Rodinia supercontinents: review and synthesis

The Tarim Craton (NW China) is a significant archive of the tectonic events that occurred during the assembly and break-up of the Precambrian supercontinents Columbia and Rodinia. It provides a comprehensive record of crustal development during the Proterozoic. We review and synthesize the magmatic, metamorphic and stratigraphic records of the Precambrian Tarim Craton and delineate the geochronology of significant geological events that led to the formation of this major cratonic block. The extant geophysical and geological data show that the Tarim Craton consists mainly of the South and North Tarim blocks. The record of late Paleoproterozoic tectonothermal events is displayed well in and across the craton, involving the amalgamation of these two blocks to form its unified crystalline basement during the build-up of Columbia. However, the record of mid-Neoproterozoic events during the assembly of Rodinia is only exposed around its periphery. The Proterozoic record of the craton includes Mesoproterozoic–early Neoproterozoic low-grade metamorphic rock units and a late Neoproterozoic sedimentary cover. The early Mesoproterozoic stratigraphy in the SW, NE and SE of the Tarim Basin suggests a period of tectonic stability following the amalgamation of the North and South Tarim blocks in the late Paleoproterozoic, indicating the completion of its cratonic build-up.

Neoarchaean and Palaeoproterozoic tectono-metamorphic events along the southern margin of the Zimbabwe craton: Insights from muscovite 40Ar/39Ar geochronology from rare-metal pegmatites, Zimbabwe

Muscovite 40Ar/39Ar geochronology on ten samples from complex-type, rare-metal (e.g., Li, Cs, Ta, Rb, Be, Nb, Sn and W) pegmatites from the Bikita and Mweza pegmatite fields in Zimbabwe yield ages that range from ca. 2622–1940 Ma. One pegmatite from the Bikita field yield a relict age of ca. 2622 Ma which is similar to the ca. 2.62 Ga emplacement age of the Main Bikita Pegmatite previously determined by laser ablation analyses of Ta–Nb–Sn oxides. Younger muscovite 40Ar/39Ar ages of ca. 2580–2539 Ma in the Bikita field may reflect post-pegmatite emplacement tectono-thermal events under fluid-mediated conditions. Data from the Mweza pegmatite district yield dates of ca. 2027–1940 Ma which reflect a Palaeoproterozoic overprint. Palaeoproterozoic 40Ar/39Ar ages are spatially restricted to near the Northern Marginal Zone–Zimbabwe Craton boundary and are absent towards the interior of the craton in the Bikita pegmatite field.

Studies on the 2.2 Ga monzogranite and its wall rocks in western part of North Qinling Orogen: Constraints on early Precambrian tectonic attributes and evolution of the southwestern margin of Ordos Block

Precambrian continent remnants in Phanerozoic orogenic belts contain vital information about not only the Precambrian evolution history of continent remnants but also the relationship between orogenic belts and adjacent blocks. This paper presents an integrated study of petrology, geochemistry and accessory mineral chronology on a monzogranite, its wall rocks biotite-quartz schist and an amphibolite interlayer in the schist in the Kuanping Group, from the western part of the North Qinling Orogen (NQO), adjacent to the southwestern margin of the Ordos Block (OB). The results reveal that the crystallization age of the monzogranite is 2200 ± 12 Ma and the protolith of the biotite-quartz schist deposited during 526–447 Ma, and they experienced similar multistage metamorphism during 450–400 Ma. The 2.2 Ga formation age of the monzogranite is significantly older than the protolith deposition age of the biotite-quartz schist, and there is almost no ca. 2.2 Ga detrital zircon age in the schist, indicating that the monzogranite is neither an intrusive body in the Kuanping Group, nor its provenance. Considering the ca. 2.2 Ga magmatism is widely distributed in North China Craton but never found in the NQO, and the monzogranite and host schist recorded similar metamorphic ages, we proposed that the monzogranite initially formed at the southwestern margin of the North China Craton and involved into the NQO during the closure of the Kuanping sedimentary basin at early Paleozoic. Combined with existing studies, three Paleoproterozoic magmatic events of ca. 2.5 Ga, 2.2–2.0 Ga and 1.8–1.75 Ga, and one ca. 1.9–1.85 Ga metamorphic event occurred at the southwestern margin of OB. These tectono-thermal events as well as their geological setting are very similar to the basement rocks of the central-southern part of the OB. Therefore, the early Precambrian rocks at the southwestern margin of the OB should belong to the basement of the central-southern part of OB, and the late tectonic events placed them at their present position.

Early Paleozoic tectonic evolution in the central Vietnam: evidence from geochronological and geochemical constraints

ABSTRACT Magmatic rocks distributed along the western part of the Po Ko fault, which documented tectono-thermal evolution of the Indochina block, when and how they formed are less constrained. Our new geochronological and geochemical data reveal that they were generated at 488–457 Ma and display a variety of SiO2 concentrations (SiO2 = 50.7–72.8 wt%). They also generally show slightly negative Eu anomalies (Eu/Eu* = 0.65–0.98) and enrich in LILEs (e.g. Rb and K) but deplete in HFSEs (e.g. Nb and Ta), consistent with arc-related signatures. Combined with their relatively high Mg# values (Mg# = 56–40) and initial87Sr/86Sr ratios (0.7085–0.7167) but negative εNd(t) values (−2.5 to −3.7), we consider those magmatic rocks originated from an enriched mantle source (e.g. EMII), which was metasomatized in the past. In comparing our data with tectono-thermal events in the southern Truong Son belt, we infer that there existed an ocean (a branch of the proto-Tethys Ocean) between the Truong Son belt and the Kontum massif during Late Cambrian, which subsequently underwent southeast subduction beneath the Kontum mass if initiated at ~488 Ma and terminated at ~457 Ma.

Unraveling Precambrian cratonic roots beneath South America: A contribution from surface wave tomography

We examine some aspects of the tectonic evolution of Precambrian cratonic roots beneath South America based on lithospheric distribution of shear-wave velocities. We derive our model by inverting 26,984 fundamental mode Rayleigh wave group velocity dispersion curves, at periods of 9–180 s. We first regionalize our measurements and then invert the result for a 3D S-wave velocity model extending to 200 km depth. Fast velocities beneath the Amazonian and São Francisco cratons, and beneath buried cratonic units in the Parnaíba and Paraná basins, are long wavelength features consistent with previous tomography studies. For the Amazonian craton at 150 km depth, we find an increase of velocities with province age, except for the Maroni-Itacaiúnas province, where we hypothesize that K'Mudku intraplate tectono-thermal events at the middle-late Mesoproterozoic and emplacement of a large igneous province following the breakup of Pangea could have altered at least partially its lithosphere. Our results are consistent with a São Francisco paleocontinent whose borders extend beyond the surface limits of the present São Francisco craton into the neighboring Araçuaí and Brasília belts. Based on slow shear-wave velocities in the upper mantle beneath the Borborema province, consistent with lithospheric thinning, we argue that a possible cratonic root of the São Francisco Paleocontinent beneath this province has likely been eroded away. This analysis is further corroborated by tectonic events that led to the alteration of the Borborema mantle, including hydration in the Paleoproterozoic, rifting in the early-middle Tonian, reworking during Neoproterozoic Brasiliano events, and lithospheric stretching during the breakup of Pangea. Finally, we also image a fast shear-wave velocity structure in the region of the Río de la Plata craton, consistent with magnetotelluric studies.

Genesis of W mineralization in the Yangla Cu-polymetallic deposit (NW Yunnan, China): Constraints from scheelite microstructure, trace element, U-Pb dating and Sr isotope geochemistry

The Yangla Cu-polymetallic deposit is the largest skarn Cu deposit in the Jinshajiang suture zone. Recently, an new W orebody was identified in the depths of the Cu orebodies. However, the genetic relationship between W and Cu orebodies, along with the tungsten ore-forming processes, remains unclear. This study delves into the careful investigation of three types of scheelites focusing on microstructure, trace element compositions and Sr isotope geochemistry. The aim is to elucidate the source and evolution of ore-forming fluids and ore genesis in the multi-episodic metallogenic systems. Primary scheelite (Sch-I) generally displays oscillatory zoning in CL images with the lowest REE (3.8–142 ppm) and highest Sr (604–1480 ppm) contents. Secondary scheelite (Sch-III), commonly precipitated as overprinting rims on primary crystal, has the highest REE (145–1071 ppm) and lowest Sr (376–793 ppm) contents. In contrast, Sch-II represents a transitional stage from primary to secondary, with REE and Sr contents at intermediate levels. Sch-I is characterized by LREE enrichment and a positive Eu anomaly in the chondrite-normalized REE patterns, whereas Sch-III shows MREE-rich patterns with a weak positive Eu anomaly. Scheelite U-Pb dating (30.3 ± 1.5 Ma) indicates that the W mineralization is not genetically related to Triassic granitoids in the Yangla district. Instead, it is likely associated with magmatic activities and tectono-thermal events in the Jinshajiang fault zone during the Cenozoic. The high initial 87Sr/86Sr ratios (0.71771–0.72229) in the scheelite, significantly higher than those of Triassic granites and ore-hosting marble, suggest that the geochemical features of Sch-I are inherited from the parental ore-forming fluids. These fluids are likely derived from the mixed hydrothermal fluids involved by the Neoproterozoic strata and/or concealed magmatism.

Ultrahigh-temperature crustal anatexis and final cratonization in Eastern Hebei, North China Craton: Insights from ca. 2.46 Ga Taipingzhai enderbites

Charnockitic rocks are a suite of granulite-facies plutonic rocks that include dominantly granitic−tonalitic and partly dioritic rocks. The Na-rich endmembers of the charnockite series, including dioritic to tonalitic rocks, are also termed enderbites. Charnockitic rocks are the main component of the cratonic-type lower continental crust in Precambrian cratons worldwide. These rocks are generally considered to be products of the anatexis of the lower crust under high- to ultrahigh-temperature conditions and play a key role in stratification between upper and lower crustal layers as well as the cratonic stabilization (cratonization) of Precambrian continents, although further study is required to gather detailed information about these rocks. In this study, a group of igneous enderbites (dioritic−tonalitic charnockites) from Eastern Hebei, North China Craton, is investigated. Zircon U-Pb dating reveals that the enderbites formed at ca. 2.46 Ga, which is coeval with the regional granulite-facies metamorphic overprinting. The enderbites are primarily composed of clinopyroxene, orthopyroxene, plagioclase, and quartz, with minor amphibole, biotite, K-feldspar, and Fe-Ti oxides. The rocks are characterized by high Fe2O3T + MgO (9.80−15.9 wt%), Cr (71.0−292 ppm), and Ni (41.2−107 ppm) contents, as well as low Al2O3 (13.9−16.6 wt%) and K2O (1.07−2.43 wt%) contents, with high Na2O/K2O ratios (1.51−4.43) and low Sr/Y (24.5−49.5) ratios. Moreover, these rocks are enriched in light rare earth elements (LREEs), (La/Yb)N = 8.06−17.8, and yield weak Eu anomalies, (Eu/Eu* = 0.80−1.18), with negative Th, U, Ta, Nb, and Ti anomalies. Various mineral thermobarometers, oxybarometers, and hydrometers are used to constrain the crystallized P-T-ƒO2-H2O conditions of the enderbites. These rocks crystallized at high temperature (860−1000 °C), crystallization pressure (8.0 ± 1.0 kbar), and H2O-poor (1.5−2.4 wt%) conditions, with oxygen fugacities (ΔQFM) of 0.0−3.0, which suggests “hot” (high-temperature) and “dry” (water-poor) crystallization conditions. The enderbites also have heterogeneous in situ zircon Hf-O isotopic compositions: εHf(t) = 2.4−7.5; δ18O = 5.78‰−7.74‰. These new data, combined with trace element characteristics, suggest that the enderbites were derived from the partial melting of metabasites, and that assimilation and fractional crystallization controlled the compositional variation in the enderbites. Further thermodynamic and geochemical modeling suggests that the anatexis of Mg-Fe−rich metabasite under ultrahigh-temperature (>1000 °C) and H2O-poor (1.0−1.5 wt%) conditions at a low crustal depth (∼9.0 kbar) could yield a melt composition comparable to that of the observed enderbites. Postcollisional lithospheric extension and mafic magma underplating prompted the partial melting of lower crustal metabasite at ultrahigh temperatures and normal lower crustal depths, resulting in the formation of enderbites. This study demonstrates that the enderbites could be formed by ultrahigh-temperature anatexis of metabasite with amphibole dehydration melting (Pl + Amp → Opx + Cpx + melt) and offers robust evidence of the genetic links between the ultrahigh-temperature anatexis of basic rocks and the generation of enderbites. In addition, the occurrence of ca. 2.46 Ga enderbites may mark the final cratonization of the North China Craton, and the ca. 2.50−2.45 Ga tectono-thermal event was an ultrahigh-temperature metamorphic-anatexis process rather than simple regional granulite metamorphic overprinting. Therefore, the generation and emplacement of enderbites involved not only a magmatic process but also an element redistribution process in the lower crust, which has important implications for stabilization of the North China Craton at ca. 2.5 Ga.

Initiation of the Unazuki Belt, Southwest Japan, during the Carboniferous as an island arc system along the North China Craton

The Unazuki Belt, Southwest Japan, is a part of the Hida Belt, which is characterized as a plutono-metamorphic complex with a continental affinity formed between the late Paleozoic and early Mesozoic. The Unazuki Belt is known to be an important tectonic unit for the tectonic correlation between the proto-Japan and East Asian continents as it records Permo–Triassic tectono-thermal events, however, comprehensive chronology of the Unazuki Belt including the timing of the initiation of this Belt is yet unclear. The present study reveals certain Carboniferous magmatic and sedimentation events from the Unazuki Belt and their tectonic implications for the first time as follows. Zircon U–Pb age dating results and whole-rock geochemical compositions show that the protoliths of metagranite and metatrachyandesite in the Unazuki Belt formed at 328.2 ± 4.4 Ma and 332.8 ± 2.2 Ma, respectively, in an arc tectonic setting. Most metasedimentary rocks of the Unazuki Belt have the youngest detrital zircon grains of ~ 300 Ma with the Carboniferous single cluster at ~ 340–300 Ma without any Pre-360 Ma detrital zircon grains. However, one metasedimentary rock with Precambrian detrital zircons (~ 20%) has the youngest detrital zircon age of ~ 252 Ma, and ~ 66% of detrital zircons show a Permian age. Most of the εHf(t) values of zircon grains from all the studied Unazuki Belt samples, including the metasedimentary and metaigneous rocks with Carboniferous ages (~ 360–300 Ma) are positive (+ 6– + 18), whereas those of the zircon grains with Permian ages (~ 280–260 Ma) show wide variations between + 16 and -23. The whole-rock geochemical compositions of the ~ 300 Ma metasedimentary rocks of the Unazuki Belt show an island arc tectonic setting, whereas those of ~ 252 Ma metasedimentary rock have a continental arc affinity. These new data suggest that (1) igneous and sedimentary rocks in the Unazuki Belt formed in the island arc tectonic setting separated from the margin of continental crust during the Carboniferous but shifted to the continental arc tectonic setting during the latest Permian, (2) the collision between the island arc and continental arc may have caused the intermediate-P/T metamorphism during the Permian in the Unazuki Belt. The Carboniferous island arc tectonic setting in the Unazuki Belt indicates that the Hida Belt, including the Unazuki Belt, formed not at the margin of the South China Craton where Carboniferous subduction is absent but on the continuous subduction zone located along the eastern margin of the North China Craton, including the eastern margin of the northern Korean Peninsula where Carboniferous subduction occurred.Graphical

The mafic–ultramafic roots of a Mesoarchean microblock in southern India: Insights from petrochemistry and isotope geochronology of the Coorg Block

The Coorg Block in southern India is an archive of Paleo-Mesoarchean arc magmatism and continental building in the early Earth. Here we investigate a suite of mafic and ultramafic rocks and associated lithologies to characterize the roots of this microcontinent. We present petrologic, geochemical, as well as U-Pb and Lu-Hf data on zircon and U-Pb data on monazite from these rocks which provide insights into the petrogenesis of the rocks as well as the timings of magmatism and subsequent metamorphism. The geochemical features of the mafic granulites indicate that the rock suite was derived from the fractionation of a sub-alkaline tholeiite magma. Most of the rocks correspond to arc setting in a subduction-related tectonic regime. The rocks exhibit relative enrichment in LILE, Pb, and LREE with depletion in HFSE such as Nb and Ta suggesting partial melting of a metasomatized mantle wedge. Their high LILE/HFSE and LREE/HFSE ratios indicate slab dehydration and mantle wedge melting in typical island arc settings. The overall trace element and REE patterns of the rocks indicate that the source magma of the ultramafic and mafic rocks might have been derived from a refractory mantle source followed by the metasomatic enrichment of lithospheric mantle wedge by incompatible elements through the influx of subduction-derived fluids/melts. We present comprehensive zircon U-Pb data and Lu-Hf data on a suite of mafic granulite, garnet-bearing granulite, tremolite actinolite schist, charnockite, and anorthositic gabbro as well as monazite U-Pb data from a charnockite sample. Most zircon grains show cores with magmatic crystallization features and many grains are mantled by metamorphic rims. The age data show a prominent peak at 3.1 Ga for most of the rocks suggesting that the major crustal building in the Coorg Block occurred during the Mesoarchean. Rocks on the margins of the block were affected by the younger (Neoarchean) tectonothermal event in the surrounding blocks. The Lu-Hf isotopic data on zircon grains with ca. 3.1 Ga ages show εHf(t) values ranging from −4.4 to + 3.7 (average + 0.3). In the εHf(t) versus 207Pb/206Pb age diagram, the plots are distributed close to the CHUR line suggesting Paleo-Eoarchean juvenile magma sources. The voluminous mafic lower crust at the root of the Coorg Block is consistent with addition of mafic magmas through slab melting induced by high mantle potential temperatures in the Archean, as well as the interaction of fluids/melts with the mantle wedge.

Late Neoproterozoic to early Cambrian high-grade metamorphism from Mikir Hills (Assam-Meghalaya gneissic Complex, northeast India): Implications for eastern Gondwana assembly

Mikir Hills region, which represents the eastern segment of the Assam-Meghalaya Gneissic Complex (AMGC) in northeast India, constitutes part of the Eastern Gondwana. The Mikir Hills preserves multiple metamorphic and magmatic events ranging from Early Mesoproterozoic to Early Cambrian. Out of these events, documenting the late Neoproterozoic to early Cambrian tectonothermal events is helpful in correlating the continental blocks of Eastern Gondwana. We present an integrated study involving field relations, petrology, P–T history and zircon-monazite geochronology of hitherto poorly studied pelitic and quartzo-feldspathic gneisses from the Mikir Hills region. These gneisses have experienced at least three deformation events (D1, D2 and D3) with dominant foliation indicated by ENE–WSW striking and shallow-moderately dipping (<40°) S2 gneissic foliation. The peak metamorphism in pelitic and quartzo-feldspathic gneisses is characterized by garnet(core)–K-feldspar–sillimanite–plagioclase–biotite–rutile–quartz–ilmenite–melt and garnet–plagioclase–K-feldspar–biotite–quartz–ilmenite–melt assemblages, respectively. The application of thermobarometric methods constrains the peak P–T conditions of 7.5–8.4 kbar at 674–778 °C and 6.7–7.4 kbar at 601–618 °C for pelitic and quartzo-feldspathic gneisses, respectively. These results are consistent with the values estimated using phase equilibria modelling and melt reintegration approach. The results of pseudosection modelling suggests a clockwiseP–Tpath for pelitic gneisses involving migmatisation during peak metamorphism followed by near isothermal decompression from 8.0 to 8.6 kbar at 768–780 °C to 4.0–5.0 kbar at 720–765 °C. In contrast, quartzo-feldspathic gneisses preserved slightly lower peak P–T conditions at 3.8–4.6 kbar and 590–650 °C. The U–Pb zircon dating of migmatised pelitic and quartzo-feldspathic gneisses yielded concordant ages of1647 ± 11 Ma and 1590 ± 7 Ma, respectively. These dates represent the inherited igneous protolith components, possibly equivalent to the Mesoproterozoic granulite facies metamorphism in the western AMGC. The rarely preserved cores of monazite in pelitic gneisses yielded an older population of1058 ± 35 Ma, most likely representing a weak tectonic imprint associated with the amalgamation of India with Western Australia and East Antarctica in the Rodinia assembly. However, the majority of monazite grains in pelitic and quartzo-feldspathic gneisses show high Th/U ratios with ages between 496 ± 7 Ma and 467 ± 16 Ma, indicating the timing of migmatisation that is contemporary with voluminous ∼ 500 Ma granite magmatism in and around the Mikir Hills. The similarities inP–T–thistoriesestimated in this study (eastern AMGC) and those obtained from the Sonapahar-Umpretha region (central AMGC) confirm that these domains experienced common tectonometamorphic history during Pan-Africanorogeny. The dominance of Late Neoproterozoic migmatisation and magmatism in the Mikir Hills region indicate that the eastern AMGC represent an active convergent margin with Western Australia and East Antarctica and evolved as a hot orogen during the assembly of Western and Eastern Gondwana continental fragments.

Geochemistry of the Winneba-Mankoadze pegmatites in Southern Ghana: A clue to the petrogenesis of the pegmatites

The Birimian Supergroup (2195–2135 Ma) of Ghana was affected by the Eburnean Tectono-thermal Event at approximately 2195 Ma. The latest stage of this event resulted in pegmatitic veining in the Birimian Supergroup of Ghana at 2072 Ma. Deposits of pegmatites have been reported by many researchers in the south-western coast of Ghana. However, their mineralogy and economic potential remain fully unexplored. This study determined the geochemical characteristics and mineralogical composition of the pegmatites in the Winneba-Mankoadze area using field observations, petrographic studies, and major and trace element data. From the field work and petrographic data, the pegmatites can be classified as rare-element type, LCT family. The modal composition of the major minerals are spodumene (0.0–30.0 %), muscovite (0.0–20.0 %), garnet (0.0–5.0 %), microcline (0.0–20.0 %), plagioclase (6.0–70.0 %), and quartz (20.0–85.0 %). The geochemical data indicates that the pegmatites are enriched in Rb (2.8–3465 ppm), Cr (22–1803 ppm), Sr (1.2–314 ppm), Ba (13–501 ppm) and the light rare-earth elements (LREE). The pegmatites are highly fractionated accounting for the enrichment in the LREE than the MREE and HREE. They are also depleted in K and Ti. The pegmatites are peraluminous with calc-alkaline trend and are late-orogenic to post-orogenic. Therefore, the source of the pegmatites is the Birimian meta-sedimentary rocks, and the source lithology is upper-to-middle crust supracrustal rocks.

EARLY NEOPROTEROZOIC GRANITOIDS IN THE RYAZANOVSKY MASSIF OF THE YENISEI RIDGE AS INDICATORS OF THE GRENVILLE OROGENY AT THE WESTERN MARGIN OF THE SIBERIAN CRATON

Studies of the geological history of the Yenisei Ridge are important not only for understanding the tectonic evolution of mobile belts at the boundaries of ancient cratons but also for problem solving whether the Siberian craton was a part of the Rodinia supercontinent. The mineralogical-petrological, geochemical and isotope-geochronological studies yielded new data on the petrogeochemical composition, petrogenesis features, U-Pb age of zircon, and Sr and 147Sm-143Nd isotopic parameters for the rocks of the Ryazanovsky granitoid massif located near the Yenisei fault zone of the Yenisei Ridge. These rocks are represented by high-ferruginous peraluminous varieties and are comparable to A-granites or highly differentiated I-granites. Their composition evolves from normal to subalkaline granites and leucogranites, characterized by increased concentrations of highly charged and radioactive elements. Isotopic (Sr, Nd) characteristics of the rocks indicate generation from an ancient crustal substrate, the average age of which corresponds to the Paleoproterozoic. The formation of these granites at the Meso-Neoproterozoic boundary (1013±9.9 Ma) corresponds to the early stage of the Grenville orogeny and the formation time of the structure of the Rodinia supercontinent. This episode of regional crustal evolution is correlated with the synchronous successions and similar style of tectonothermal events on the periphery of large Precambrian cratons (Laurentia and Baltica), thus confirming the reliability of the proposed paleocontinental reconstructions of incorporation of the Siberian craton into the Rodinia.

Extensive Resetting of Feldspar Pb Isotopic Composition in Archean–Paleoproterozoic Granitic Rocks from the Kongling Terrane, South China: Implications for Tracing Thermal Evolutionary History

AbstractFeldspar Pb isotopes have been widely used to trace magmatic formation and evolution processes. However, it remains unclear whether post‐magmatic thermal events can affect feldspar Pb isotopic ratios. Here, the in situ Pb isotopic composition of feldspar hosted in granitic rocks (thirteen Archean and one Paleoproterozoic) from the northern Kongling terrane, Yangtze Craton, South China, is analyzed. The samples reveal a substantial variation in their Pb isotopic composition, spanning the gap between the 1.9 Ga and present‐day geochrons, which indicates extensive resetting by later tectonothermal events. This resetting was interpreted to have likely resulted from Paleoproterozoic and Neoproterozoic tectonothermal events related to the assembly and breakup of the Columbia and Rodinia supercontinents. These results suggest that Pb isotopes should be used cautiously when tracing magma sources and petrogenesis in magmatic rocks that have experienced post‐magmatic reworking. However, the in situ Pb isotopic composition of feldspar in ancient granitoids may also potentially be used to reveal later tectonothermal events. The extensive resetting of the Pb isotopic composition in feldspar by regional thermal events may also provide new insights into our understanding of the Pb isotope paradox.