Potassic Granitoids Research Articles

Mesoarchean synchronous emplacement of TTG gneisses and potassic granitoids in the Nyabessane granite-greenstone terranes, NW Congo Craton (southern Cameroon): Zircon U[sbnd]Pb geochronology, petrogenesis and tectonic implications

Archean granitoids played key roles in the generation and differentiation of the Archean continental crust and provide clues to understand crustal processes in the early Earth. Abundant Mesoarchean granitoids were emplaced in the Nyabessane granite-greenstone terrane (NGB), part of the Ntem Complex of the Northwest Congo Craton. They include charnockites, tonalite-trondhjemite-granodiorite (TTG), granitic and monzogranitic gneisses. Here, we present a geochemical and geochronological (zircon LA-ICP-SF-MS UPb) study of these granitoids to determine their petrogenesis and to better constrain the crustal evolution of the Ntem Complex. Field and petrographic observations indicate that most of these granitoids underwent extensive metamorphism and deformations, associated with anatexis. Zircon UPb dating results suggest that the charnockite, TTGs and granitic gneisses, and monzogranites have emplacement ages of 2910 ± 11 Ma, 2870–2865 Ma and 2852 ± 31 Ma, respectively. The charnockites have low SiO2 (55–58 wt%) and high Al2O3 (16–18 wt%), CaO (7–8 wt%) and MgO (∼ 4.5 wt%) content with Mg# ∼ 54, and exhibit magnesian, metaluminous characteristics of the Cordilleran granitoid-type formed in magmatic arc. The 2.87–2.86 Ga TTG gneisses are silica-rich (55–58 wt% SiO2), sodic (3–5 wt% Na2O, Na2O/K2O = 1–3), with HREE-depleted, and display the typical Archean medium- to low pressure TTG geochemical features groups. Their chemical compositions are characteristics of TTG-like melts derived from the partial melting of hydrated low- to high-K metabasic/thickened lower crust at various depths followed by magmatic differentiation during ascent. When compared to the TTG gneisses in the NGB, the ∼2.87 Ga granitic gneisses are K-rich, and high Gd/Yb, Th/Yb, Th/Nb, Sr/Y and La/Yb ratios but lower MgO, Yb, V, Y, Cr, Ni and Sr contents, matching typical Archean hybrid and potassic granitic rocks. We propose that the granitic gneisses were derived from the intra-crustal melting of a pre-existing felsic crust. The granitic and the TTG gneisses were generated contemporaneously through the same magmatic event at ∼2.87–2.86 Ga. The ∼2852 Ma monzogranitic gneisses are ferroan and metaluminous rocks, and show LREE enrichment with strongly fractionated REE patterns and positive Eu anomalies. Geochemical features, together with the presence of ∼2.9 Ga-old inherited zircon grains are consistent with the remelting of Mesoarchean granitoids. Considering the petrogenetic, regional geological and geochronological data, the Mesoarchean granitoid magmatism of the Ntem Complex was likely generated via complex transitional geodynamic regimes involving subduction and accretion processes.

Eocene–Miocene tectonic reworking of the southeastern Tibetan Plateau: Geochronological and geochemical records from granitoids along the Ailao Shan–Red River Shear Zone

The convergence of the Indian Plate and Eurasia shaped the Earth’s broadest and highest-elevation collisional system and topographic growth, which is not simply illustrated by ongoing contractional deformation. The Cenozoic magmatism along the Ailao Shan-Red River Shear Zone (ARSZ), linking surface geology with deep lithospheric processes, provides additional information regarding the age and progressive deformation of the India-Asia collision. In this study, we present new zircon U–Pb geochronological, elemental geochemical and zircon Hf isotopic data for the Eocene–Miocene granitoid to resolve the southeastern Tibetan Plateau deformation history. The Jinping granitoids in the southern ARSZ were dated at 37.5–33.6 Ma. They show similar geochemical characteristics (whole-rock major oxides, trace element, Sr–Nd–Pb isotopic and zircon in-situ Hf isotopic data) with respect to coeval potassic granitoids in NW Yunnan, suggesting their origination from partial melting of thickened lower crust with additional contribution from enriched mantle-derived magmas. Leucogranite samples within the ARSZ yielded distinctive zircon U–Pb ages of 28.3 Ma and 18.5 Ma. Geochemical features suggest that they were derived from a heterogeneous crustal source consisting of metasediments and metabasites. Our results, along with those previous studies, suggest that the widespread potassic magmatism in the central and southeastern Tibetan Plateau is related to lithospheric mantle upwelling, which resulted in the partial melting of the thickened lower crust and a short-lived crustal extension during a period of decreased convergence rate. Subsequently, the orogenic-scale sinistral strike-slip movement provided shearing heat for generating leucogranites within the shear zones during the late Oligocene–Miocene. The plateau-wide shift in stress reorganization and tectonic reworking led to the lateral extrusion of the SE Tibetan Plateau and its attainment of modern high elevation.

Combined bulk-rock Lu-Hf and Sm-Nd isotopic study of Archean granitoids and mafic rocks from Sangmelima terranes (Ntem Complex, south Cameroon): Geodynamic implications

Hafnium and neodymium isotopes provide robust tool tracers for tracking the mechanisms of crustal generation and differentiation, from mantle extraction to the late geological processes undergone by the studied rocks. Though these two isotope systems commonly behave similarly and define a positive correlation between 143Nd/144Nd and 176Hf/177Hf referred to as the “terrestrial array”, they may differ in terrains having a complex geological history, such as Archean cratons. This study presents the first combined whole-rock Hf and Nd isotopic analyses of Archean mafic and felsic igneous rocks from the Sangmelima terranes in the Ntem Complex (located in the Northwest of Congo Craton (NW CC) in southern Cameroon). It mainly consists of Mesoarchean charnockites and tonalite-trondhjemite-granodiorite (TTG) suites, greenstone belts, mafic enclaves or dykes, and potassic granitoids. Mesoarchean charnockite and TTG suites have subchondritic initial 176Hf/177Hf and 143Nd/144Nd ratios with a mean initial (suffix (i)) ε176Hf(i) and ɛ143Nd(i) of –3.30 ± 1.40 and –3.85 ± 0.65 (2SD) respectively, implying the involvement of Eo- to Paleoarchean components in their petrogenesis. The bulk rock initial Hf and Nd isotope compositions of the gabbro and gabbronorites are suprachondritic (ε176Hf(i) = +3.4 to + 8.2; ε143Nd(i) = +0.9 to + 1.6), indicate that they were derived from a depleted mantle source. Coupled with pre-existing data from the literature, these new Hf-Nd isotopic data indicate that two episodes of mantle-derived mafic magmatism in the Ntem Complex occurred during the Mesoarchean period (i.e., at ∼ 3.1 and 2.86 billion years (Ga)). Neoarchean dolerite dykes show broadly chondritic to supra-chondritic ε176Hf(i) and ε143Nd(i) values of –0.28 to + 4.53 and TCHUR ages of 2.74 – 2.76 Ga, suggesting the derivation of doleritic magma from a depleted mantle source with either none or limited crustal residence time. Considering other Archean domains of the NW CC, there were multi-stage magmatism events and crustal growth events occurred at ∼3.75 – 3.31 Ga, ∼3.26 – 3.0 Ga, ∼2.92 – 2.85 Ga, and ∼2.75 – 2.72 Ga from Eoarchean to Neoarchean in the NW Congo Craton.

The Fundamental Role of H2O in the Generation of Coeval Sodic and Potassic Granitoids at Continental Arcs: An Example from the Yangtze Block, South China

AbstractThe bulk rock composition of granitoids reflects the composition of their source and the conditions of partial melting, which are functions of the geodynamic setting in which they formed. Granitoids in active continental margins (continental arcs) are dominated by calc-alkaline rocks with subordinate alkaline compositions, although how these different magma compositions formed is not well understood. Neoproterozoic magmatic rocks are widely distributed along the western margin of the Yangtze Block in South China to form the >1000-km long Panxi continental arc system, which is dominated by granitoids with minor mafic–ultramafic and intermediate plutons. The granitoids are subdivided into sodic and potassic variants that occur as belts along the western and eastern sides of the continental arc, respectively. Sodic granitoids from the western part consist of tonalite, granodiorite, and monzogranite with crystallisation ages ranging from 870 Ma to 740 Ma. They have low K2O/Na2O ratios (0.1–1.0) and high Na2O contents (3.5–6.7 wt%), high but variable SiO2 (61–75 wt%) concentrations, and negative to positive whole-rock εNd(t) values (−1.7 to +2.9). Zircon grains from the sodic granitoids have εHf(t) values ranging from +0.3 to +9.6 and δ18O from 3.90‰ to 7.71‰. The potassic granitoids from the eastern side consist of monzogranite and syenogranite with crystallisation ages from 820 Ma to 790 Ma. They have high K2O/Na2O ratios (0.6–2.2), K2O (2.6–5.9 wt%) and SiO2 contents (69–78 wt%), but whole-rock εNd(t) (−0.9 to +2.9) and zircon εHf(t) (+1.8 to +12.9), and δ18O values (2.98‰ to 6.41‰) similar to those of the sodic granitoids. The isotopic compositions of both the sodic and potassic granitoids are similar to those of spatially- and temporally-related mantle-derived (mafic to ultramafic) rocks, and are considered to have been derived from juvenile mafic continental crust. Phase equilibrium modelling shows that the H2O content of the granitoid source rocks played a key role in their petrogenesis, both in lowering solidus temperatures and in controlling the compositions of the derived partial melts. Our results indicate that calc-alkaline sodic granitoids can be formed by water-fluxed melting of juvenile mafic crust at 750–900°C and 9–12 kbar in which the required H2O was derived from the dewatering of underplating mafic arc magmas. By contrast, the potassic granitoids were generated by fluid-absent (H2O-undersaturated) partial melting of a similar juvenile mafic source at 725–900°C and 6–9 kbar. We conclude that the sodic granitoids were derived from partial melting of the newly-formed mafic lower crust in the continental arc, whereas the potassic granitoids were likely generated in the back-arc setting induced by upwelling of asthenospheric mantle.

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.

Dating of Variscan magmatism in Kraishte and Balkan Mts

New age determinations by in-situ zircon and titanite LA-ICP-MS analysis for Variscan magmatic activity from Kraishte and Balkan Mts. are reported. The results confirm the presumed Variscan age of the magmatic rocks. For the Sedemte Prestola potassic granitoids, titanite dating reveal an age of 308.7±9.1 Ma. The zircon dating of potassic syenites, cropping out west of Shipka town gave less consistent results – 355±89 Ma. For potassic-alkaline syenite porphyries from the Lutskan pluton, Variscan 206Pb/238U ages cluster in 328–308 Ma interval. Zircons dating of potassic quartzdiorite porphyry from the Lutskan pluton display important inheritance with single 206Pb/238U age of 316 Ma, whereas results of titanite dating range between 350 and 315 Ma, defining poor isochrone of 301±91 Ma and MSWD of 5.6. Zircons from gabbro-diorite porphyries, cutting the Lutskan granitoids plotted on Terra-Wasserburg diagram yield Variscan age of 312±21 Ma and Concordia age of 296±4.6 Ma.

Chemical-Abrasion U-Pb zircon geochronology reveals 150 Myr of partial melting events in the Archean crust of the São Francisco Craton

Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic–ultramafic intrusions to the south of the BHC.

Eocene–Oligocene Crustal Thickening-Collapse of the Eastern Tibetan Plateau: Evidence from the Potassic Granitoids in SW China

Abstract The eastern Tibetan Plateau is a key part of the eastern India–Asia collisional zone, a region that records multiple overprinting tectonic and magmatic events. This study presents new geochronological, geochemical, and Sr–Nd–Hf–O isotopic data for Cenozoic potassic granitoids in eastern Tibet, southwestern China, which recorded the tectonic evolution of the eastern Tibetan Plateau. These potassic granitoids are formed between 37.6 and 32.9 Ma and are geochemically subdivided into the following: Group 1, adakite-like granites; Group 2, syenites; and Group 3, low-εNdt granitoids. The Group 1 samples are similar to high-silica adakites in that they have variable SiO2 contents (63.31–73.62 wt.%) and high Sr/Y and La/Yb ratios. These samples have εNdt and εHft values that range from −5.8 to −0.6 and from −4.3 to +5.2, respectively, with δ18O values of 6.78‰–7.36‰. The Group 2 samples are syenites, contain 56.36–63.86 wt.% SiO2 and high concentrations of Y and Yb, and have εNdt values from −8.4 to −2.4, εHft values from −6.1 to +1.1, and δ18O values of 6.37‰–6.89‰. The Group 3 samples have a narrow range of SiO2 concentrations (62.27–64.59 wt.%), high Sr/Y and La/Yb ratios, δ18O values of 6.31‰–6.82‰, and low εNdt and εHft values (−12.6 to −10.9 and −11.4 to −6.6, respectively) that are similar to the values obtained for the contemporaneous Yao’an lamprophyres. These data indicate that the Group 1 samples are formed from magmas sourced from a heterogeneous and thickened region of the lower crust containing an enriched mantle component. Group 2 magmas were most likely derived from contemporaneous mafic melts sourced from an ancient region of the lithospheric mantle previously modified by the incorporation of recycled components. The Group 3 samples have distinct Sr–Nd–Hf isotopic compositions that are indicative of derivation from magmas generated by the fractional crystallization of lower crustal melts sourced from ancient enriched mantle of the Yangtze Block. Combining these new data with the results of previous research suggests that the Cenozoic potassic igneous rocks of eastern Tibet were formed as a result of the thinning of the lithospheric mantle and an associated crustal collapse event, potentially representing a regional late Eocene to early Oligocene transition from compression to transtension in the eastern Tibetan Plateau. These potassic igneous rocks are contemporaneous with or are younger than igneous rocks in the Qiangtang Block, suggesting that the magmatic response to the India–Asia collisional event was initiated in the central Tibetan Plateau before propagating towards the eastern margin of this region.

High-K granites between the Archean Gavião and Jequié blocks, São Francisco Craton, Brazil: Implications for cratonization and amalgamation of the Rhyacian Atlantica continent

Precambrian terrane boundaries are sites of diverse rock associations and structural complexity. Motions along the boundary zones are commonly related to the construction of ancient large continental masses. This seems to be the case in the São Francisco Craton, Brazil, where three major blocks (Gavião, Jequié and Serrinha) collided in the Paleoproterozoic to form the northern part of the craton. This manuscript presents zircon U-Pb ages and whole-rock geochemistry for rock associations on the western border of the Jequié Block, which is defined by the north-trending left-lateral 600-km long Contendas-Jacobina fault zone. The studied rocks are granulite to amphibolite facies Neoarchean orthopyroxene-hornblende granite gneisses of the Jequié block, which also occur tectonically imbricated within Paleoproterozoic metavolcanosedimentary rocks of the Contendas-Mirante belt. The rocks are 2.65-2.63 Ga high-K ferroan, metaluminous to slightly peraluminous syeno-monzogranite in composition, with negative ƐNd(2650) and Nd model ages of 3.05-2.88 Ga. They are depleted in Sr and Ti, enriched in Th, Ba and Pb enrichment, and have marked negative Eu/Eu*. This geochemical signature coupled with the Nd-isotope data indicate that the parental granitic magma was generated by crustal reworking of felsic Mesoarchean crust distinct from the western-sitting >3.3 Ga Gavião Block. The investigated granitic gneisses are similar to other potassic granitoids that occur in the São Francisco Craton, which mark the thermal stabilization of the Archean lithosphere. The westward thrusting of the Neoarchean hornblende granitic gneiss and associated high-Mg tremolitite (meta-basalts) onto Paleoproterozoic volcano-sedimentary sequences establishes field evidence of Paleoproterozoic nappe stacking before lateral displacement during block collision. The crustal amalgamation is related to the assembly of the São Francisco-Congo Craton, a major landmass established before the Columbia supercontinent.

Lu–Hf Isotope Composition of Zircon and Magma Sources of the Vendian–Early Paleozoic Granitoids in Tuva (by the Example of the Kaa-Khem and East Tannu-Ola Batholiths)

Abstract —We present results of geochemical and Sr–Nd isotope studies of rocks and of local dating and determination of the Lu–Hf isotope composition of zircons from late Vendian–early Cambrian and Cambrian–Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the εHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian–early Paleozoic intrusive associations formed at the island arc and accretion–collision stages. Late Vendian (572–562 Ma, Kopto and Buren massifs) and early Cambrian (522–518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian–early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro–monzodiorite–granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion–collision stage. Island arc complexes were the main source of Cambrian–Ordovician accretion–collision calc-alkalic plagiogranitoids (500–450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva–Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren’ massif (450 Ma), marking the completion of accretion–collision processes in this region. The isotope parameters of the Vendian–early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion–collision processes.

Archean basement components and metamorphic overprints of the Rangnim Massif in the northern part of the Korean Peninsula and tectonic implications for the Sino-Korean Craton

The geology of the Korean Peninsula’s northern part is little known to the international community. Even though this area is a critical link between various eastern Asia terranes, its tectonic architecture and geochronological framework are still gaps in knowledge. This study reports integrated field, geochemical and geochronological characteristics of the Precambrian gneissic rocks from this region, focusing on the Rangnim Massif. The studied samples are divisible into three groups according to their field occurrence as granitic gneiss, fine-grained biotite gneiss and leuco-veins. The essential mineralogy of all these rocks is biotite (Bt), muscovite (Mu), plagioclase (Pl), K-feldspar (Kf) and quartz. The analyzed samples show high SiO2 (65.92–77.63 wt%) and Al2O3 (10.23 to 17.56 wt%) contents, low MgO and CaO. Their K2O contents is mostly higher than Na2O. SIMS (Secondary Ion Mass Spectrometry) zircon U-Pb dating of the eleven samples yields magmatic protolith ages of 2454–2567 Ma and multiple metamorphic overprintings during the earliest Paleoproterozoic (~2.45 Ga), Paleoproterozoic (1.87–1.93 Ga) and Jurassic (160 Ma). The presence of these potassic granitoids in different locations indicate the high extent maturation of Neoarchean continental crust in the Rangnim Massif. Zircon Lu-Hf isotopic results of these samples show large variations, mostly plotting between the Depleted mantle linear model line and Chondritic Uniform Reservoir (CHUR) line, but some have negative εHf(t) values. Considering that their protoliths are all crust melting products, it can be concluded that the Neoarchean magmatism incorporated both juvenile crustal components as well as a reworking of older crust. This is supported by the detection of rare early Archean xenocrysts. Based on Juvenile crust having εHf values of ~0 before 3500–3600 Ma, it is concluded that the reworked components were Eoarchean in age, not Hadean. Magmatic as well as Precambrian metamorphic pulses of the Rangnim Massif show broad similarities with the eastern North China Craton, Gyeonggi and Yeongnam Massifs and southwest Japan, which seem to suggest that they might share a coherent Precambrian basement and evolutionary history, at least before the end of late Paleoproterozoic.

Crustal growth and reworking of the eastern North China Craton: Constraints from the age and geochemistry of the Neoarchean Taishan TTG gneisses

The Taishan Complex is located in the central part of the eastern North China Craton (NCC), and preserves extensive Neoarchean TTG gneisses and potassic granitoids. Their formation age, geochemical features and petrogenesis can provide important insights into understanding the crustal growth and reworking events involving in the evolution of the craton. Previous studies were mainly focused on geochronology of these Neoarchean rock assemblages, but lacked study on their petrogenesis and related tectonic regime. In this study, we carried out lithological, geochronological and geochemical analyses for the representative TTG gneisses of the Taishan Complex. U-Pb zircon age dating results reveal that the Taishan TTG gneisses were mainly emplaced at 2718-2548 Ma. Lu-Hf isotopic data show uniform positive zircon εHf(t) values (+2.2 to +6.6), with Hf model ages of 2949-2665 Ma, indicating that the TTG gneisses were mostly derived from juvenile crustal materials extracted from the depleted mantle during the Mesoarchean to Neoarchean. The δ18O values of zircons vary from +4.6 ‰ to +6.7 ‰, indicating that the source region was not significantly affected by the supracrustal materials. Geochemical data show that these TTG gneisses have high Na2O (3.81-4.83 wt%) and ratio of Zr/Sm (24-80), but show low Mg# (40-56), content of Sr (292-508 ppm), and ratios of Sr/Y (35.6-66.6) and Nb/Ta (10.1-15.9). These TTG gneisses were probably derived from partial melting of juvenile crustal materials under a relatively low-pressure environment, which were equilibrated with a certain amount of amphibole and plagioclase and minor garnet in the residue. It indicates that a large-scale magmatic underplating could be a most possible tectonic regime operating in this region during the Neoarchean. Our new results, combined with the available data from the adjacent areas, reveal a significant crustal growth event in the eastern NCC at ca. 2.94-2.70 Ga, which was followed by the pervasive crustal reworking event at ca. 2.70-2.55 Ga and regional metamorphism at ca. 2.55-2.45 Ga.

Mechanism of Paleoarchean continental crust formation as archived in granitoids from the northern part of Singhbhum Craton, eastern India

Abstract Many Paleoarchean cratons display a gradual change from early sodic tonalite–trondhjemite–granodiorite magmatism to late K-rich granitoid magmatism; the geodynamic significance of this change is debatable though. This contribution presents field, geochemical and zircon U–Pb age and Hf isotope results of four different 3.32–3.25 Ga granitoid bodies from the northern part of Singhbhum Craton to investigate their petrogenesis and role in crustal evolution. The granitoids range in composition from tonalites to trondhjemites, derived from intracrustal melting at low- to medium-pressure conditions. The source was mainly low-K mafic rock. The granitoids show intrasuite fractional crystallization. These sodic granitoids represent the last stage of granitoid magmatism in the Singhbhum Craton which formed contemporaneously with K-rich granitoids occurring in other parts of the craton. This fact suggests that, contrary to the popular notion (of only potassic granitoids), both sodic and potassic granitoids could form at the terminal phase of cratonization, implying reworking of heterogeneous (mafic to tonalite) crust. A combination of evidence from geochemical data, secular change in granitoid composition, structural pattern and rock association of the Singhbhum Craton reflects that recurring mantle plume-related mafic–ultramafic magma emplacement in an oceanic plateau setting and attendant crustal melting can explain the Paleoarchean crustal evolution pattern.

Crustal growth of the Eastern Dharwar Craton: a Neoarchean collisional orogeny?

Abstract The Eastern Dharwar Craton (EDC) is predominantly made of Neoarchean potassic granitoids with subordinate linear greenstone belts. Available geochemical and isotopic systematics of these granitoids suggest variations in the source and petrogenetic mechanisms. By compiling the available geochemical data, these granitoids can be classified into four groups, namely: TTGs (tonalite–trondhjemite–granodiorite); sanukitoids; biotite and two-mica granites; and hybrid granites. This classification scheme is in line with the global classification of Neoarchean granites, and enables the sources and petrogenetic mechanisms of these variants to be distinguished. Available geochemical, isotopic and geochronological datasets of these granitoids are integrated and the existing tectonic models for the Neoarchean EDC are reviewed. The variability of the EDC granitoids is ascribed to crustal reworking associated with the collision of two continental blocks. The tectonomagmatic evolution of the EDC is analogous to the development of the Himalayan Orogeny. Based on the evolutionary history of the Dharwar Craton, it can be concluded that convergent margin tectonics were operational in the Indian Shield from at least c. 3.3 Ga and continued into the Phanerozoic. However, the nature and style of plate tectonics could be different with time.

Geochronological evolution of the Pitangui greenstone belt, southern São Francisco Craton, Brazil: Constraints from U-Pb zircon age, geochemistry and field relationships

The Pitangui greenstone belt (PGB) – located in the southern São Francisco Craton, (Brazil) – is a volcano-sedimentary assemblage that is tectonically imbricated over the orthogneiss and granitoid rocks collectively termed the Divinópolis Complex. One diorite gneiss sample from the northern Divinópolis Complex yields an igneous zircon age of 2878 ± 5.7 Ma, constraining the time of crystallization. The basal sequence of PGB predominantly comprise metabasalt with restricted bodies of metagabbro/metadiorite, metaultramafic volcanic rocks and BIF. The upper sequences of the PGB exhibit a maturation of the basin, grading to more felsic and sedimentary lithologies, such as clastic metasedimentary rocks, intermediate metavolcanic rocks, and metapyroclastic rocks. The first recognized deformation phase in the PGB (Dn) is associated with greenschist facies conditions and the development of the main tectonic foliation. The second deformation phase (Dn+1) is associated with amphibolite facies metamorphism, while a later (Dn+2) phase may be related to younger Proterozoic deformation (ca. 2.0 Ga). PGB metabasalts yield a crystallization age of 2740.2 ± 12 Ma and are characterized by a tholeiitic affinity and flat LREE patterns. Based on mixed MORB- and arc-affinities, a back-arc basin setting is suggested for the formation of PGB basalt. This mafic magmatism (2740 Ma) is slightly younger than the ages reported for the Rio das Velhas greenstone belt, with which the PGB is generally correlated. The metasedimentary rocks of the PGB have age intervals spanning from 2690 to 3250 Ma. One mica-quartz phyllite has a maximum depositional age of 2769.8 ± 8.9 Ma while a chlorite phyllite records and age of 2693.2 ± 7.9 Ma. The metasedimentary rocks are derived from a source area of mafic to felsic composition that is similar to TTGs, cratonic Archean shales and E-MORB. The predominant sources were likely the sialic Archean crust of the granite-gneiss complexes (e.g. the Divinópolis Complex and potassic granitoids of the Mamona Event) and the mafic sequences of the arc (e.g. PGB back-arc basalt). One late potassic pluton (Casquilho monzogranite) crosscutting the fabrics of the volcano-sedimentary assemblage and yields an intrusion age of 2694.7 ± 9 Ma, constraining the minimum sedimentation age assembly of the package. Collectively, a Neoarchean primitive island arc or back-arc basin is proposed to have generated the PGB, which is here interpreted as distinct from the Rio das Velhas greenstone belt. We suggest that PGB developed during an accretion-collision event in the southern São Francisco Craton before cratonic stabilization of the crust by the end of the potassic 2760-2680 Ma Mamona Event.

Late Neoarchean synchronous TTG gneisses and potassic granitoids in southwestern Liaoning Province, North China Craton: Zircon U-Pb-Hf isotopes, geochemistry and tectonic implications

Abundant late Neoarchean granitoids occur in southwestern Liaoning Province, part of the Eastern Ancient Terrane of the North China Craton. These rocks include intermediate gneiss, TTG gneisses and potassic granitoids, and we report on the geochemistry and zircon SHRIMP ages as well as Hf-in-zircon isotopes of these granitoids in order to determine their petrogenesis. Field relationships suggest that most of these granitoids experienced widespread metamorphism and deformation, associated with anatexis at some localities. The intermediate gneisses, TTG gneisses and potassic granitoids were all emplaced at the end of the Neoarchean (2.50–2.53 Ga), and CL images document widespread recrystallization in the zircons. The intermediate and TTG gneisses yielded similar Hf isotopic systematics (εHf(t) = −3.73 to +6.42) as the associated potassic granitoids (εHf(t) = −2.44 to +7.80), and both rock types yielded mean Hf crustal model ages of 2.8–2.9 Ga. Combined with the geochemistry, we propose that the formation of the intermediate and TTG gneisses was related to partial melting of mafic rocks at different depth, whereas the potassic granitoids have variable petrogenesis. The nearly coeval TTG gneisses and potassic granitoids and their widespread metamorphism, deformation and zircon recrystallization suggest that a large-scale heat source must have been present at or near the base of the crust in southwestern Liaoning Province at the end of the Neoarchean. We propose that collision and post-collisional extension is the most likely tectonic environment for generation of the above granitoids, and the formation of widespread potassic granitoids played an important role in the maturation of continental crust in the North China Craton.

Neoarchean reworking of TTG-like crust in the southernmost portion of the São Francisco Craton: U-Pb zircon dating and geochemical evidence from the São Tiago Batholith

Field, petrographic and geochemical data combined with in situ zircon U-Pb LA-ICP-MS ages are documented for the São Tiago Batholith (southernmost portion of the São Francisco Craton) to understand its origin and magmatic evolution. The geologic relations indicate that the batholith is composed of granitic to granodioritic orthogneisses (L2) with tonalitic xenoliths (L1) intruded by pegmatite (L3) and metagranite (L4). L1 consists of two facies of tonalitic orthogneiss, one biotite-rich, and the other biotite-poor. The geochemical evidence, including high K2O with mantle-like chemical signature, suggests that the Bt-rich tonalitic gneiss (2816 ± 30 Ma) was derived from contamination of mafic magmas by crustal-derived components. The Bt-poor tonalitic gneiss, of TTG affinity, was generated by partial melting of LILE-enriched mafic rocks, possibly from oceanic plateus in a subduction environment. L2 includes two distinct types of rocks: (i) granodioritic orthogneiss, chemically ranging from medium-pressure TTGs to potassic granitoids originated via partial melting of previous TTG crust, including L1 Bt-poor; and (ii) granitic gneiss (2664 ± 4 Ma), geochemically similar to crustal-derived granites, produced by melting of the L1 Bt-rich tonalitic gneiss or mixed TTG/metasedimentary sources. L3 pegmatite (2657 ± 23 Ma) results from melting of L2, whereas L4 metagranite (dikes and stocks) shows petrogenesis similar to that of the L2 granitic gneiss. Related orthogneisses occur near the São Tiago Batholith: (i) a hornblende-bearing tonalitic gneiss, and (ii) a hybrid hornblende-bearing granitic gneiss (2614 ± 13 Ma), whose genesis is linked with interaction of sanukitoid and felsic potassic melts, representing the last Archean magmatic pulse of the region. The Minas strata along the Jeceaba-Bom Sucesso lineament near our study region encircle the São Tiago Archean crust, representing an irregular paleo-coastline or a micro-terrane amalgamation with the São Francisco Proto-craton, with possible subsequent dome-and-keel deformational processes. Our petrological and geochronological data reevaluate nebulous concepts in the literature about the SFC, revealing (i) a chemically and compositionally diverse crustal segment generated at the Late Archean in diverse geodynamic scenarios, and (ii) a more complex lineament than previously thought in terms of the paleogeography of the southern São Francisco Craton.

Evaluate best-mixing model for estimating thermal conductivity for granitoids from mineralogy: A case study for the granitoids of the Bundelkhand craton, central India

Granitoid is an important part of the upper continental crust, and therefore its thermal conductivity (TC) plays an important role in understanding the lithospheric thermal structure in a region and for geotechnical or geoengineering purposes. In above context, due to the lack of TC data or absence of proper sample for its measurements, TC values are assumed which can lead to erroneous results. In such scenario, when direct measurements are not possible, TC can be estimated by indirect methods with proper precautions. An attempt is made here to arrive at the best mixing model for granitic rocks by using TC of the individual minerals and compare the deviation between the measured and calculated values. The considered mean models are: arithmetic, geometric, harmonic, effective, Voigt-Reuss-Hill and Hashin-Shtrikman along with its lower and upper bound. Studied rocks are potassic granitoid (PG), biotite granitoid (BG), sodic granitoid (GG) and gneisses (BnG) from the Bundelkhand craton, central India. Measurements of TC are done in the laboratory on 21 samples using steady-state method. Data show wide variations in TC values for granitoids (PG: 2.7 − 3.2, BG: 2.6 − 2.9, GG: 2.9 − 3.0 Wm−1 K−1) and gneisses (2.9 − 3.7 Wm−1 K−1). Modal mineralogy of the rocks are determined using petrological and geochemical data through modal analysis and normative (CIPW-NORM) methods. The calculated TC arrived by both the methods provide a satisfactory agreement for the harmonic mean model, showing deviation from −10.9 to 17.6% for modal analysis and −16.1 to 11.5% for NORM method. Deviations from the above methods decrease further (−23.3 to 2.8% and −27.7 to −3.1%, respectively) using minimum mineral TC. Therefore, we suggest that, in the case of non-availability of the proper sample for direct measurement, the TC of very low porous granitoids could be satisfactorily determined by assessing their modal mineralogy and considering the harmonic mean model.

Geoquímica e geocronologia da Suíte Paciência: implicações para o fim da evolução riaciana do Bloco Itacambira-Monte Azu

Diversos corpos plutônicos alcalinos de alto potássio paleoproterozoicos ocorrem no embasamento do cráton São Francisco, bem como no embasamento dos orógenos que o bordejam. Localizado no bordo oeste do cráton São Francisco, o bloco Itacambira-Monte Azul integra o embasamento do orógeno Araçuaí, em partes retrabalhado pelo evento tectonometamórfico Brasiliano. Nesse bloco, os plutonitos Paciência, Morro do Quilombo e Serra Branca, constituídos de rochas alcalinas, são englobados na Suíte Paciência. Os resultados petrográficos para a Suíte Paciência mostram o predomínio de monzonitos e sienitos, equigranulares a porfiríticos, com ocorrência de enclaves máficos. As análises químicas indicam altas concentrações de álcalis, bem como o enriquecimento de óxidos ferromagnesianos e alto Mg# (número de magnésio). Essas rochas são dominantemente metaluminosas, alcalinas a alcalino-cálcicas e magnesianas, enriquecidas em Ba, Sr e Rb e depletadas em Nb, P, Ti e em elementos incompatíveis. Mostram altos conteúdos em elementos terras raras (ETR) com fracionamento dos ETR leves sobre os pesados e anomalias de Eu que variam de negativas a fracamente positivas. As características geoquímicas dessas rochas são similares às de granitoides potássicos originados tardiamente em eventos orogênicos associados à subducção, com fonte mantélica metassomatizada e enriquecida em elementos incompatíveis e participação de crosta arqueana na geração desse magma. A idade de cristalização em 2053 ± 6.3 Ma indica que as rochas da Suíte Paciência se relacionam aos estágios tardios da orogênese Transamazônica, possivelmente associados com a evolução de um extenso arco magmático desenvolvido ao final do Riaciano. Dessa forma, a Suíte Paciência e também as demais suítes alcalinas paleoproterozoicas presentes no embasamento do orógeno Araçuaí registram um período de estabilização do paleocontinente São Francisco-Congo na transição Riaciano-Orosiriano.

The southwestern extension of the Jiao-Liao-Ji belt in the North China Craton: Geochronological and geochemical evidence from the Wuhe Group in the Bengbu area

The Wuhe complex is located at the southeastern margin of the North China Craton. The complex consists of metamorphosed Paleoproterozoic potassic granitoids and supracrustal rocks, of which the latter include the Fengyang and Wuhe groups. Meta-mafic rocks from the lower Wuhe Group have igneous zircon U-Pb ages of 2126 ± 37 Ma with εHf(t) values of −6.22 to +8.38, and xenocrystic zircons of 2.39–2.36 Ga, 2.55–2.54 Ga and 2.77–2.69 Ga. Geochemically, the meta-mafic rocks can be classified into two groups. Group 1 island arc tholeiites display flat to slightly right declined REE patterns and moderately negative Nb, Ta, Zr, and Ti anomalies. Group 2 mature arc calcalkaline basalts display strongly fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. These meta-mafic rocks formed by partial melting of sub-arc depleted mantle wedge which had been modified by slab-derived melts at an active continental margin. Depositional age of the group can be constrained in the period of 2.16–2.10 Ga based on ages of the youngest detrital zircons and latter intrusions. U-Pb ages of detrital zircons yield major age peaks of 2.69 Ga and 2.52 Ga, with minor peaks at 2.88 Ga, 2.78 Ga, 2.35 Ga and 2.17 Ga, most of which are derived from the late Mesoarchean to early Paleoproterozoic granitoids in the Wuhe complex and the Jiaodong Terrane. Metamorphic zircons in the marbles coexisting with garnet amphibolites or granulites occur as either single grains or overgrowth (or recrystallization) rims surrounding magmatic zircon cores and yield ages of 1882 ± 19 Ma to 1844 ± 15 Ma. The comparable ca. 2.1 Ga potassic granites with A-type granite affinity, the ca. 2.1 Ga meta-mafic rocks with arc-like geochemical features, the 2.1–1.9 Ga meta-sedimentary units and the 1.9–1.8 Ga subduction- and collision-related granulite-facies metamorphism suggest that the Wuhe complex and the Jiao-Liao-Ji Belt share the same late Paleoproterozoic tectonic evolution process and the former is the southwestern extension of the latter.