Tectonothermal Events Research Articles

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.

Late Cretaceous tectonothermal events of the Gangdese belt, southern Tibet

Abstract The Gangdese belt of the southern Lhasa terrane (southern Tibet) records a Chileantype accretionary orogeny driven by subduction of Neotethyan oceanic lithosphere, prior to Indo-Asian collision and formation of the Tibetan Plateau. We present detailed structural analysis of outcrops and a drill core in the Jiama copper ore district along with 40Ar-39Ar cooling ages from white mica, plagioclase, and potassium feldspar and zircon U-Pb geochronology of granitoids and sandstone. These data add new constraints to the formation of a major angular unconformity, deformation along and within the footwall of the Gangdese décollement, and the coupling between deformation and magmatism. Structural analysis indicates that top- to- the- south motion along the décollement produced southvergent folding and thrusting of Upper Jurassic to Cretaceous strata in the Gangdese back-arc basin. A synthesis of new and compiled age data reveals that the décollement and associated south-vergent deformation occurred between ca. 90 and 65 Ma, contemporaneous with the formation of a major ca. 85–69 Ma angular unconformity between the overlying Paleocene–Eocene Linzizong Formation and the underlying Upper Cretaceous Shexing Formation. We posit that this deformation in the Gangdese belt resulted from flat-slab subduction of the Neotethyan oceanic slab beneath the southern margin of the Asian continent. A flat-slab subduction geometry is consistent with previously documented synchronous thrusting in the forearc and back-arc basins as well as the observed arc magmatic lull of the Gangdese belt between ca. 80 and 65 Ma.

Relict Mesoarchean (2.99–2.94 Ga) metamorphism overprinted by late Neoarchean tectonothermal event(s) from the Sukma Group supracrustal rocks, Bastar craton, India: Evidence from new Lu-Hf and Sm-Nd garnet isochron and Th-U-total Pb monazite ages

Mesoarchean metasupracrustal successions can preserve evidence of tectonothermal reworking involving Paleoarchean felsic crust. Paleoarchean tonalite and trondhjemite of the Bastar craton, central India contain remnants of amphibolite-facies meta-supracrustal rocks of the Sukma Group, which likely represents the oldest Archean terrestrial sedimentary rocks. In order to constrain the time of metamorphic events combined garnet Lu-Hf and Sm-Nd geochronology and Th-U-total Pb dating of texturally preserved monazite from garnet-bearing metapsammite samples, representative of cover rocks overlying orthopyroxene-bearing tonalite basement, were conducted. Relics of an earlier garnet porphyroblast-forming metamorphic stage 1 (M1) developed at upper amphibolite-to granulite-facies conditions, are texturally distinct from the widespread metamorphic stage 2 (M2) marked by the garnet-breakdown to cordierite-anthophyllite symplectite. The M2-stage conditions, at ∼ 650–700 °C and ∼ 5.7 kbar, in the metasupracrustal remnants, are similar to those in garnet-amphibole-bearing quartzite and also as inferred from associated calc-silicate gneiss. Samarium-Nd chronology on whole-rock and porphyroblastic garnet fractions from three garnet-bearing quartzite samples yielded a weighted mean age of 2990 ± 10 Ma whereas Lu-Hf isotopic systematics on the same garnets from two of these quartzite samples yielded a weighted mean age of 2940 ± 31 Ma. Superposed Late Neoarchean M2 stage tectonothermal events include mylonitization of a ∼ 3.5 Ga basement tonaliteat 2525 ± 20 Ma and garnet formation within a Fe-rich quartzite at 2413 ± 21 Ma, as constrained from Sm-Nd isochron ages and similar to the in-situ weighted mean chemical age of 2367 ± 19 Ma of monazite included within M2-stage symplectitic cordierite around the older M1-stage garnet. The relict 2.99–2.94 Ga mid-crustal amphibolite-to granulite-facies M1 metamorphic assemblages stabilized within metasupracrustal rocks, formed in response to crustal thickening during Mesoarchean convergent tectonics, and were likely related to amalgamation of Paleoarchean crustal fragments.

The Missing Link in the Genesis of the Lower Paleozoic Copper Deposits of the Anti-Atlas (Morocco): The Late Triassic Central Atlantic Magmatic Province Event

Copper mineralization in the Lower Paleozoic sedimentary cover of the Anti-Atlas (Morocco) is continually being revised not only to improve its mining capacity, but also to determine its origin, which remains a matter of debate. As evidenced by the various models proposed, the related research is fragmented, localized, and confusing. The origin of the Anti-Atlas Lower Paleozoic copper mineralization is shared between synergistic and epigenetic processes or a superposition of the two processes. Based on new tectono-magmatic data and a reinterpretation of the ore structural arrangement, we propose a link between the last concentration of copper deposits and the Late Triassic–Early Liassic CAMP (Central Atlantic Magmatic Province) tectono-thermal event, as evidenced by the significant concentration of copper mineralization in the three NE–SW corridors affected by extensional faults, some of which are filled with dolerite CAMP magma. The heat flow generated by the mafic dykes within these reactivated corridors causes mineralized fluids to up well into the sedimentary layers, depositing material rich in juvenile or leached copper, or even a mixture of the two. In some cases, these fluids are trapped by fracture systems that accompany passive folds initiated on normal faults. In other cases, these fluids can infiltrate bedding planes, and even karst caves, formed during carbonate exhumation. Notably, extensive NE–SW faults systematically cover the early Hercynian structures, suggesting that they belong to a post-Hercynian extensional episode. During the Late Triassic, the global fragmentation of the Pangaea supercontinent was manifested by the stretching of the continental crust at the margin of northwest Africa, with the simultaneous opening of the Central Atlantic Ocean and emplacement of CAMP magmatism. This last and often overlooked tectonothermal event must be considered in the remobilization and reconcentration of copper mineralization and other mineralization in Morocco.

Paleozoic remelting of carbonatite in Bayan Obo (China): Further insights into the formation of a giant REE deposit

Currently, it has been confirmed that the ore-hosting dolomite of the giant Bayan Obo deposit is the Mesoproterozoic igneous carbonatite, but the roles, that the Paleozoic tectonothermal events played, are still poorly understood. In this study, we present a comprehensive investigation on the geochemistry of various carbonates to understand the generation of the Bayan Obo carbonatite. Hydrothermal monazite Th-Pb dating (peaking at ∼ 420 Ma) identified an atypical coarse-grained dolomite, which occurring as veins, and cutting through typical fine dolomitic facies. Typical coarse dolomitic facies and atypical coarse-grained dolomite are significantly different in terms of MgO, FeO, MnO, Y, Al, Si, V, Ti, Ni, Sc, and ∑REE contents and the C-O-Sr isotopic compositions, and atypical coarse-grained dolomite has lower Ba and Sr contents, lower Co/Ni ratio, with strong Eu positive anomaly and more scattered element concentration, suggesting that it has a different origin from typical coarse dolomitic facies, crystallizing from a relatively low-temperature and oxidized environment. The pervasiveness of magma flow structure and magnetite mass in the contact zone between the granite plutons and the fine dolomitic facies indicates that, at ∼ 270 Ma, the heat flux from granitic magma triggered melting of the fine dolomitic facies, forming remelted calcitic carbonatite. In addition, Th-Pb ages of monazite ranging from 1350 Ma to 240 Ma and the REE-poor properties of both remelted carbonatites suggest that both Paleozoic thermal events only caused REE activation and redistribution, without introducing additionally exogenous ore-forming metals. Accordingly, we propose that the dolomite mass in Bayan Obo are Mesoproterozoic mantle-derived carbonatites (including ore-hosting dolomite and carbonatite dike), which underwent remelting at ∼ 440 Ma and ∼ 270 Ma, respectively.

Paleogeographic Reconstruction of Precambrian Terranes Reworked by Phanerozoic Orogens: An Example Based on Detrital Zircon REE From Lhasa Terrane in Southern Tibet

AbstractPaleogeographic reconstruction of Precambrian terranes reworked by Phanerozoic orogens (e.g., the Tibetan Plateau) results in complex lithotectonic relations due to intracrustal reworking by tectonothermal events. Detrital zircon rare earth element (REE) databases at global (global major river sands) and regional (the Gangdese Mountains, southern Tibet) scales reveal trends in and Eu/Eu* that effectively record the crustal evolution of the source, including crustal thickness and redox state of the magma that generated the zircons. Regional comparisons of these chemical markers provide a new approach for paleogeographic reconstructions that we apply to study the origin of the Lhasa terrane, southern Tibet. Using Precambrian to early Paleozoic sedimentary and igneous rocks in the Lhasa terrane and compiling detrital zircon analyses from the northern margin of Gondwana, we show that the Lhasa terrane had an African affinity in the Rodinia–Gondwana supercontinent cycles (ca. 1.4–0.4 Ga).

Paleoproterozoic thick-skinned tectonics in the Central Indian Tectonic Zone: implications on the tectonic reconstructions of cratonic nuclei

ABSTRACT The Satpura Mountain Belt, or the Central Indian Tectonic Zone (CITZ), is a Paleoproterozoic mountain belt of Central India, with Mesoproterozoic and Neoproterozoic rejuvenation history. Structural studies carried out in the southern margin of this ancient orogenic belt have identified a two-phase evolution. The documentation of mapped fold interference of Type 2 ‘boomerang-shaped’ pattern and the spread of early lineations over the hinge area of second-generation folds provide an estimate of ~40-43° angle between the regional axial directions of the folds of the two orogenic episodes. The map patterns indicate the presence of basement-involved recumbent fold nappes placed above the sedimentary cover sequence. The identified décollement structure on the surface provides information regarding the deformation of the upper crust in ductile manner. The patterns of the deep crustal reflectors mapped through previous deep seismic reflection study interpreted during the present investigation brought out the thick-skinned deformation in the region as well as identification of crustal low-velocity zones of lower strength and a domal geometry of the lower crust. The regional thrusts are found to deform deeper crustal layers, one thrust affecting the crust-mantle boundary. The behaviours of the upper and lower crust in terms of rheological layering have been used to interpret the regional structural style of the lithosphere. The structural features of the region do not support the argument regarding the status of the Central Indian Shear as a suture, but indicate it to be the outermost emergent thrust towards the foreland side. The patterns of dip of the thrusts and thrust transport directions indicate a northward dip of the South Indian Block during the subduction stage and the position of the collisional suture of the Satpura orogen to the north of the Sausar belt. A tectonic model has been proposed here to explain the formation of the Satpura Mountain Belt or CITZ involving subduction-collision at ~2250 Ma, followed by erosion, stretching and basin development between 2100 and 1500 Ma. A collisional event in the eastern and western craton margins of India, at ~1500 Ma, caused the second uplift of the Satpura Mountain Belt and its northern margin, developing a temporal unconformity of ~300 Ma gap between the Semri and Kaimur Groups (Vindhyan Supergroup). A third event in the region at ~950 Ma was similar to the second event. No continental amalgamation took place along the CITZ during the tectonothermal events at ~1500 Ma and ~950 Ma; these involved reactivation of old thrusts and basal accretion which were the effect of far-field stresses emanating from the orogenic events at the eastern margin of the Bastar Craton and western margin of the Aravalli Craton.

FEATURES OF THE STRUCTURE AND EVOLUTION OF THE LOWER PARTS OF THE CONTINENTAL CRUST OF THE YAKUTIAN DIAMONDIFEROUS PROVINCE IN THE AREA OF THE UPPER-MUNA KIMBERLITE FIELD

Upper Muna kimberlite field) showed that the crust at different levels is composed of pyroxene, garnet-pyroxene crystalline schists and garnet-pyroxene gneisses. Exsolution textures in pyroxenes and amphiboles, granat rims around grains of ilmenite and pyroxenes indicate that the mineral associations of crystalline schists were formed during cooling at constant pressure. Р–Т equilibrium parameters indicate that garnet-pyroxene crystalline schists are present in the middle crust (P = 7–8 kbar), while garnet-pyroxene gneisses can be considered as rocks of the lower crust (P = 9–10.1 kbar). For the first time, sodalite was found in xenoliths of crystalline schists, which indicates the presence of brines with a high concentration of NaCl at the final stages of rock cooling. The determination of the U–Pb age of zircons testifies to the Neoarchean (2.7 Ma) tectono-thermal event, accompanied by the melting of the crust. In the garnet–pyroxene gneisses, the 1.9 stage is weakly manifistated. The obtained data confirm the earlier conclusion about the vertical and lateral heterogeneity of the crust of the Yakutsk diamondiferous province and the absence of dependence between the degree of crust reworking and spatial location relative to the main collision zones of the Siberian craton.

Constraining the timing of deep magmatic pulses from diamondiferous kimberlite and related rocks in the South China Continent and implications for diamond exploration

Mantle-derived magmas provide windows to the deep earth, and precise dating of the deep magmatic rocks such as kimberlite and related lithologies is crucial to attaining a better understanding of the mantle evolution and mantle-crust interaction in conjunction with global geodynamic cycle. Here we report a geochronology investigation of the diamondiferous kimberlite and related rocks from the southeastern side of the Yangtze Craton in China with the aim to constrain the deep mantle magma emplacement patterns and mantle evolution through time. Ultrabasic magmatic events occurred intermittently between 492 and 441 Ma, as inferred from apatite U-Pb dating and phlogopite 40Ar-39Ar dating. Combined with the previous data, two pulses of diamondiferous magmatism (ca. 488 Ma and ca. 450 Ma) are identified. The emplacement ages and patterns of these deep-seated diamondiferous magmas are unrelated to the reported Pan-African tectonothermal event (peak at ca. 522 Ma), Kwangsian Orogeny (peak at ca. 435 Ma), and Emeishan plume event (peak at ca. 260 Ma) in the South China Continent. Instead, we infer that the kimberlite and associated rocks in the South China Continent may be related to the upwellings from the edges of the Large Low Shear Velocity Province located at the core-mantle boundary at that time. The lack of magmatic event during 760 to 460 Ma and the existence of the thick lithosphere (>175 km) beneath the South China Continent are suitable for lithospheric diamond formation during the Early Paleozoic. These data suggest that future diamond exploration should be focused on the kimberlites emplaced during the Cambrian–Ordovician transition on the southeastern side of the former Yangzte Craton.

Turonian-coniacian definition of the Caribbean plate: tectonic and metamorphic record in the Median Belt, Dominican Republic

ABSTRACT Uplift and unroofing of Jurassic-Cretaceous, mantle and crust, arc- and plume-related rock units in the Median Belt of the Dominican Republic exposed basement rocks with a protracted record of tectono-thermal events delineating the evolution of the northern edge of the Caribbean plate. In this article we focus on crustal rock units in the north-eastern half of the Median Belt. First 40Ar/39Ar dating of metamorphic ferri-winchite (86.4 ± 2.5 Ma; crystallization date) and albite (82.3 ± 5.8 and 79.8 ± 1.6 Ma; cooling dates) in metabasites of boninitic photolith from the Aptian-Albian Maimón Formation in the Ozama shear zone points to a tectono-metamorphic event in the Upper Cretaceous. Beltwide, this event caused syn-metamorphic N- to NNE-directed, simple-shear dominated, mylonitic and phyllonitic deformation of the Maimón Formation at peak metamorphic conditions of 8.2 kbar and 380°C. Such conditions are consistent with subduction of a coherent portion of forearc (represented by the Maimón Formation) to depths of ~25-29 km. The tectono-metamorphic event dated here overlaps with the inception of Turonian-Coniacian SW-dipping subduction and metamorphic sole formation in a back-arc position recorded in the Moa-Baracoa ophiolite complex in neighboring Eastern Cuba. Contemporaneity between the subduction inception of forearc and back-arc portions of the Caribbean arc and the main pulse of plume activity recorded in the Caribbean Large Igneous Province (CLIP) suggests that plume activity promoted general plate instability leading to a regional-scale plate reorganization. This mantle-plume-induced plate margin reorganization was coeval with the inception of the NE-dipping subduction of the Farallón plate beneath Central America leading to the definition of the Caribbean Plate by double-verging subduction zones along its northern and southwestern margins.

Establishing the occurrence of late Neoarchaean – earliest Palaeoproterozoic magmatism in the Daqingshan area, northwestern North China Craton: SIMS U–Pb zircon dating, Lu–Hf and Sm–Nd isotopes and whole-rock geochemistry

AbstractDaqingshan is located in the northwestern North China Craton where late Neoarchaean supracrustal rocks occur widely, but where magmatic zircon ages have rarely been reported for plutonic rocks. In this study, we report SIMS U–Pb zircon ages and Hf isotope, whole-rock element and Nd isotope compositions for 12 magmatic samples, including TTG, quartz monzonitic and monzogranitic gneisses, and meta-gabbroic and dioritic rocks. They have magmatic zircon ages of 2530–2469 Ma; some samples have ages of <2.48 Ga likely influenced by late Palaeoproterozoic tectonothermal events, making their ages less reliable. TTG gneisses have low Sr/Y and La/Yb ratios, with whole-rock ϵNd(t) andin situmagmatic zircon ϵHf(t) values of +1.2 to +2.4 and −1.1 to +6.2, respectively. Quartz monzonite and monzogranite gneisses and gabbroic to dioritic rocks have similar Nd–Hf isotope compositions to the TTG gneisses. The absence of zircon >2.6 Ga in the early Precambrian rocks suggests that the Sanggan Group may have formed in an oceanic environment, whereas the TTG rocks formed as a result of partial melting of the basaltic rocks of the Sanggan Group under relatively low-pressure conditions. Combined with previous studies, the main conclusions are that in the Daqingshan area, late Neoarchaean magmatism was widespread, the late Mesoarchaean – early Neoarchaean was an important period of juvenile continental crustal growth, and the late Neoarchaean supracrustal and plutonic rocks most likely formed in an arc environment. These are common signatures for Neoarchaean crustal evolution throughout much of the North China Craton, and also globally.

The geochronology of the rare metal pegmatite deposits: A case study in Nanping No. 31 pegmatite vein in northeastern Cathaysian block, China

Accurate dating of the pegmatite is a challenge due to the uncertainty and controversy of crystallization temperature, fractionation model, and cooling mechanism. In this study, the sizable Nanping No.31 pegmatite vein (southeast China) contain five structural zones that is considered as an ideal example for assessing credible dating method on rare metals pegmatite deposit. The CMS (Chemical composition-Mineral assemblage-Structural geology) classification is applied on the Nanping pegmatite. The No. 31 vein is identified as pegmatite by CMS features. New U–Pb dating CGMs (columbite-tantalite group minerals) in five zones have a consistent age (380 ± 10 Ma). Whilst U–Pb dating of apatite and cassiterite from different structural zones show remarkable two group isotopic ages (370–390 Ma and 145–160 Ma). The much younger age indicated the pegmatite vein has experienced a severe Yanshanian tectono-thermal event at Eastern China. By comparison with apatite, cassiterite and zircon, CGMs is the most suitable mineral for accurate dating of the pegmatite because the dating results of the minerals are much stable, even having metamictization caused by high U content or overprinting by late hydrothermal activity. This study provided new insight into the dating methods for rare metals pegmatites to accurate restrict its metallogenetic geodynamical setting.

Establishing Provenance from Highly Impoverished Heavy Mineral Suites: Detrital Apatite and Zircon Geochronology of Central North Sea Triassic Sandstones

A study of Triassic sandstones in the central North Sea, UK, has shown that combined detrital zircon and apatite geochronology and apatite trace element analysis is a powerful tool for reconstructing provenance for sandstones with diagenetically impoverished heavy mineral suites. Sandstones in the earlier part of the succession (Bunter Sandstone Member and Judy Sandstone Member) have characteristics that indicate derivation from Moinian–Dalradian metasediments affected by Caledonian tectonothermal events, in conjunction with a Palaeoproterozoic-Archaean source unaffected by Caledonian metamorphism. Palaeogeographic reconstructions indicate that the sediment cannot have been input directly from either of these cratonic areas. This, in conjunction with the presence of common rounded apatite, indicates that recycling is the most likely possibility. The zircon-apatite association in the younger Joanne Sandstone Member sandstones indicates derivation from lithologies with mid-Proterozoic zircons (either crystalline basement or metasediments in the Caledonian Nappes), subjected to Caledonian metamorphism to generate early Palaeozoic apatites. This combination is compatible with a source region in southern and western Norway. The low degree of textural maturity associated with the detrital apatite, together with the unimodal Caledonian age grouping, indicates the Joanne sandstones have a strong first-cycle component.

Geochronology and Petrogenesis of the Gol Mod Massif: Implications for the Geodynamic Evolution of the Orkhon-Selenge Belt, Northwestern Mongolia

The Orkhon-Selenge Belt is a Late Permian to Early Triassic volcanic plutonic belt located in northern Mongolia and is part of the Central Asian Orogenic Belt. The Selenge Complex, which is a part of the Orkhon-Selenge Belt, is a key area for studying the tectonic and magmatic evolution of the Central Asian Orogenic Belt. This study aims to contribute to understanding of the geodynamic evolution of the Orkhon-Selenge Belt by investigating the petrology, geochemistry and geochronology of the rocks in the region. Our results indicate that intrusive rocks were characterized as high-K, Calc-alkaline series and metaluminous to weakly peraluminous I-type granite affinities and their geochemical characteristics are indicating as arc-like geochemical signatures with depleted in elements such as Nb, Ta, Ti and Y and enriched in elements such as Rb, Cs, Th, K and light rare earth elements. Using zircon U-Pb dating, we determined an age of 257.3±0.73 Ma for the alkali granite, suggesting that south-western part of the Orkhon-Selenge Belt formed during the Late Permian time. The Selenge pluton, which is closely related to Erdenet-Ovoo porphyry type mineralization, is a composite intrusion. However, the zircon grains display magmatic and low oxygen fugacity conditions, which characteristics are likely the effect of weak mineralization of magma ascent with Late Permian tectonothermal event in the south-west part of the Orkhon-Selenge Belt. The results of this study will provide insights into the formation and evolution of the north-western segment of the Mongol-Okhotsk Belt, and will have implications for our understanding of the tectonic history of this region.

Detrital zircon U–Pb ages and geochemistry of Devonian–Carboniferous sandstones and volcanic rocks of the Hida Gaien belt, Southwest Japan: Provenance reveals a Gondwanan lineage for the early Paleozoic tectonic evolution of proto-Japan

Devonian–Carboniferous strata in the Hida Gaien belt, Southwest Japan, are characterized by a change in the dominant lithology from clastic rocks and limestones to volcanic rocks. The sandstone compositions and detrital zircon U–Pb ages indicate that the Devonian sedimentary rocks were derived from various basement rocks, including granite and basalt, that were uplifted, exposed, and eroded during the Early–Middle Devonian (398–383 Ma). The source of sediment to these strata changed to basalts and rhyolites in the early Carboniferous (∼346 Ma). Early Carboniferous bimodal volcanic rocks are present in the Hida Gaien and South Kitakami belts, the southeastern margin of the South China block (Hainan), and the Jilin area (NE China) of the eastern Central Asian Orogenic Belt (CAOB). The Cambrian–Silurian zircon grains (540–480 Ma and 460–420 Ma) in the Hida Gaien belt were derived from arc basement of a similar age to the South Kitakami and Kurosegawa belts, the Jiamusi–Khanka–Bureya (NE China and the Russian Far East) and Songliao–Xilinhot (Inner Mongolia) blocks in the eastern CAOB, and the eastern South China and Indochina blocks. The Devonian strata also contain Precambrian zircon grains derived from Mesoarchean basement (∼3000 Ma) and the Grenvillian tectonothermal event (∼1000 Ma), suggesting sediment supply from NE Gondwana (the Thomson and Lachlan orogens of eastern Australia) or related continental remnants. Detrital zircon U–Pb age spectra similar to those of the Devonian strata of the Hida Gaien belt are also observed in Silurian–Devonian metasedimentary rocks in the Jilin area. During the early Paleozoic, the Hida Gaien belt, the eastern margin of the South China block, and the magmatic arcs and microcontinents in the eastern CAOB were located close to NE Gondwana along the western margin of the Paleo-Pacific Ocean.

A refined Archean-Paleoproterozoic magmatic framework of the Cuoke Complex, SW China, and its implications for early Precambrian evolution of the Yangtze Block

New age data from the Cuoke Complex in southwestern Yangtze Block, integrated with published data from this region, indicate an Archean to early Paleoproterozoic magmatic history distinct from the northern Yangtze Block. Zircon cores and rims from eleven granitoids of the Cuoke Complex were analyzed by LA-ICP-MS and SHRIMP and show magmatic U-Pb ages ranging from 2850 to 2194 Ma. Combined U-Pb zircon igneous age data from the Kunming-Hainan Domain in the southwestern Yangtze Block display major magmatism ranging 3.10–3.06 Ga with a peak at ca. 3.08 Ga, 2.92–2.77 Ga with a peak at ca. 2.85 Ga and 2.4–2.1 Ga with peaks at 2.36–2.32 Ga and 2.22 Ga. This age distribution contrasts with the zircon U-Pb magmatic age data from the igneous rocks of the Kongling Domain in the northern Yangtze Block. A gap in magmatic activity in the northern domain between 2.4 and 2.2 Ga overlaps with the major peak in magmatism in the southwestern domain. Furthermore, potassic granitoids, which mark the cratonization of the two domains are more than 200 Ma years earlier in the Kongling Domain than that in the Kunming-Hainan Domain. Hf isotopic compositions of zircons from igneous rocks of the southwestern domain indicates significant juvenile crustal generation during the Mesoarchean era peaked at ca. 3.08 Ga, whereas the early Paleoproterozoic was characterized by crustal reworking and remelting. Zircon Hf isotopic compositions also reveal differences in crustal growth between the two domains. The northern domain shows a wider two-stage Hf model age range, with a peak at ca. 3.17 Ga and a subsidiary peak at ca. 3.78 Ga. Therefore, the northern domain also shows significant crustal growth in the Mesoarchean, though the age peak is about 100 Ma older compared to the southwestern domain. The Eoarchean crustal growth event recorded in the northern domain is absent in the southwestern domain. Therefore, the southwestern and northern Yangtze blocks display independent magmatic histories during the Archean and early Paleoproterozoic. The common records of late Paleo- and Mesoproterozoic tectonothermal events indicate the two domains were juxtaposed at this time, probably in association with assembly of the Nuna supercontinent.

Triassic amphibolite-facies metamorphism in northeastern Hainan Island, South China

The Permian-Triassic tectonic evolution of Southeast Asia has long been a subject of considerable debate. Hainan Island in China is located at the junction of the South China and Indochina blocks adjacent to the Pacific (Philippine) Plate. Studies of metamorphic complexes in Hainan Island may shed lights on the Permian-Triassic tectono-thermal events of the eastern Paleo-Tethys. Among these complexes, the newly recognized Mulantou Metamorphic Complex is sporadically exposed in northeastern Hainan Island provides evidence for above-mentioned issues. In this paper, we present the result of a combined petrological and geochronological study of paragneisses in this complex. The paragneisses consist mainly of biotite, plagioclase, quartz, and rutile/ilmenite, with or without garnet, sillimanite, and muscovite. Textural relationships, mineral compositions, and thermobarometric calculations show that these paragneisses record peak metamorphic P − T conditions of 720–770 ℃ and 4.4–5.8 kbar. Zircon and monazite U − Pb dating by laser ablation–inductively coupled plasma–mass spectrometry indicates that metamorphism occurred at 250–235 Ma. Integrating published geochronological, geochemical, and structural data of Hainan Island and along the Jinshajiang − Ailaoshan − Song Ma suture zone, we infer that the Triassic amphibolite-facies metamorphism of the Mulantou Metamorphic Complex resulted from collision between the South China and Indochina blocks after closure of the Paleo-Tethys.

Metallogeny of the state of Oaxaca, Mexico

ABSTRACT The state of Oaxaca in southern Mexico includes copious metallic deposits of distinct types that were formed in several tectonothermal events since the Proterozoic. This work integrates a large database of mineral occurrences and deposits in Oaxaca. The 1:850,000 scale metallogenetic map depicts the location of deposits, mining regions, and mining districts on a lithological base, along with a summarized metallogenic history of the state and the distribution of the tectonostratigraphic terranes. We briefly describe the most relevant features of each mineralized area and the characteristics of the metallogenic epochs of Oaxaca. All the geolocation information was organized in GIS format.

Archean records in the Kontum Complex (Central Vietnam) and implications for Precambrian crustal evolution in Indochina

Zircon U-Pb age-clusters of ∼3.5 Ga and ∼3.2 Ga with εHf(t) = +1.0 ∼−13.5 and THfDM = ∼3.4–4.0 Ga are documented from plagioclase amphibolites in the Kham Duc rock-unit of the Kontum Complex, Indochina. These data provide evidence for a Paleoarchean crustal nucleus that is in part synchronous with, as well as older than, the Yangtze basement of the South China Block. In addition, the presence of ∼2.2 Ga, ∼1.85 Ga and ∼1.42 Ga age-records in the complex correlate with those in the southern Yangtze, suggesting their connection at the time of Nuna assembly and breakup. These data reveal a similar Paleoarchean-Mesoproterozoic crustal evolution among Indochina, Yangtze, Australia and NW Laurentia. The Ordovician-Silurian and Triassic tectonothermal events in the Kontum Complex link to the breakup of the Indochina Block from the northern margin of Gondwana prior to accretion to Asia.

Timing of shear deformation in the Singhbhum Shear Zone, India: implications for shear zone-hosted polymetallic mineralization

AbstractThe Singhbhum Shear Zone in eastern India hosts several Fe oxide–Cu–Au (IOCG)-type polymetallic deposits, mined primarily for U, Cu and apatite, with elevated concentrations of rare earth elements, Ni, Co, Mo, Te and Au in association with low-Ti magnetite. Although the main stages of hydrothermal U, Cu and rare earth element mineralization are known to be Palaeoproterozoic in age, the age of shear deformation in the host shear zone has hitherto not been constrained. Here, we report Re–Os ages of syn-shearing massive molybdenite occurring along shear surfaces transecting the uranium ores in the Jaduguda uranium deposit. Integrating the obtained Re–Os age of c. 1.64–1.59 Ga of molybdenite, the known ages of mineralization and the known tectonothermal events in the adjoining Proterozoic Mobile Belt, we propose that the main stages of polymetallic hydrothermal mineralization pre-dated the pervasive shear deformation event in the Singhbhum Shear Zone. We further suggest that the shear zone was not the principal foci of the hydrothermal mineralization of the main stages. Instead, the shear zone was localized during the Palaeoproterozoic to Mesoproterozoic transition (c. 1.64–1.59 Ga) along pre-existing crustal-scale extensional faults which had earlier been the foci of hydrothermal alteration and mineralization in Palaeoproterozoic time (c. 1.9–1.8 Ga). Shear deformation and metamorphism have reconstituted/redistributed existing mineral/metal inventories with/without neo-mineralization.