Juvenile Mafic Crust Research Articles

Cassiterite and zircon U-Pb ages and compositions from ore-bearing and barren granites in Thailand: Constraints on the formation of tin deposits in Southeast Asia

The Southeast Asian Tin Province comprises western, central, and eastern belts and hosts significant granite-related Sn deposits. The genetic links between granites and Sn mineralization are still unclear. Most Sn deposits are in Thailand’s western and central belts, but their origin remains poorly elucidated due to the absence of direct dating of mineralization. Herein in-situ U-Pb age data of wolframite and cassiterite grains from nine representative Sn deposits in Thailand are obtained, which fall into two stages. Triassic deposits (224–210 Ma) are found in the Central belt, with Cretaceous deposits (78–67 Ma) in the Central belt being younger than those in the Western belt (84–74 Ma). However, ore-bearing granites, spanning two periods (227–205 Ma and 85–69 Ma), occur in central and western belts. Some Triassic ore-bearing granites exhibit significantly older ages than ore-forming ages. Newly identified ore-forming granites contain zircon grains with relatively low εHf(t) values (−29.5 to + 4.1; average = − 11.8), indicative of an origin from supracrustal sediments from the Sibumasu block. In contrast, barren granites have ages from 303–224 Ma and higher zircon εHf(t) values (−9.9 to + 13.2; average = +1.2), which suggests that they were derived from the juvenile mafic crust. Even after experiencing hydrothermal fluids exsolution, some low-fractionated ore-forming granites (D.I. < 90) still maintain remarkably high tin contents. Both ore-forming and barren granites crystallized under reducing conditions. Our study highlights the importance of Sn-rich sources of parental magmas in forming Sn deposits. The metasediment-rich basement of the Sibumasu block distributing along the continental margin is likely the Sn-rich source. These sources played a crucial role in forming two stages of tin deposits in distinct tectonic settings, that are syn-collisional crustal thickening in Paleo-Tethys and post-collisional extension-related settings in Neo-Tethys.

An Overview on the Composition and Age of Upper Crust of Proto‐Tethyan Lajishan Intra‐oceanic Arc, NE Tibet Plateau

AbstractIdentification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo‐ocean basin. This article summarizes the geological, geochemical, and geochronological characteristics of upper crust of Proto‐Tethyan Lajishan intra‐oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean. The intra‐oceanic arc volcanic rocks, including intermediate–mafic lava, breccia, tuff, and minor felsic rocks, are distributed along southern part of the Lajishan ophiolite belt. Geochemical and isotopic compositions indicate that the intermediate–mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids, whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra‐oceanic arc setting. Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma, suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra‐oceanic arc. Combined with the Cambrian forearc ophiolite and accretionary complex, we suggest that the Cambrian intra‐oceanic arc in the Lajishan ophiolite belt is belonging to the intra‐oceanic arc system which was generated by south‐directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.

Plagioclase as archive of the incremental assembly of the Quxu batholith, South Tibet: Implication for the nature of magma reservoir

The formation of magma reservoir and resultant construction of large batholith are increasingly recognized as long-term incremental processes. Here we employed back-scattered electron and cathodoluminescence images, in-situ major and trace elements, and Sr isotopes of plagioclase for five lithological groups from the Quxu complex batholith (south Tibet), which shed new light on the nature and incremental construction processes of batholith. Our results show that plagioclases display diverse textures and show systematic differences in geochemistry and Sr isotopes for different lithologies, indicating that their host rocks originated from multiple magma sources and/or underwent distinct magmatic processes. Plagioclases in Group I granodiorites and Group III mafic dykes and mafic magmatic enclaves exhibit comparable textural features (e.g., resorption and sieved textures) and share similar Sr (< 1500 ppm) and Ba (< 200 ppm) contents, and 87Sr/86Sr ratios (ISr) (0.7035–0.7050) at given lower An (20–50) values, suggesting that they were evolved from similar mafic magma sources. The significant variations in An (20–90), Sr and Ba contents of plagioclase in Group III indicate both fractional crystallization and magma mixing. Plagioclases in Group II monzogranites have a similar An ranges to Group I but higher Sr (900–2200 ppm) contents and ISr (mostly 0.7045 to 0.7066) values, suggesting that they formed from partial melting of juvenile mafic crust without significant mantle contribution. Plagioclases in Group IV granodiorites and Group V monzogranites exhibit higher Sr (500–3000 ppm) contents than other groups at given An values. Their ISr values fall within the range of Group II, indicating a similar magma source. Abundant plagioclases displaying normal and oscillatory zonings in them implies that fractional crystallization occurred, while the presence of typical glomerocrysts in Group V indicates mush disaggregation. The simulation results of the trace elements indicate that the different lithologies of the Quxu batholith represent different states of magma reservoirs, corresponding to crystal mushes with varying degrees of extracted melts. The incremental growth processes of lager batholith operate as an open system, in which magma convection, mixing and recharge, crystal accumulation, recycling and reaction with melts, and mush disaggregation commonly occur.

A Long-Lived Accretionary Process during the Amalgamation of the North China Craton: Insights from Neoarchean–Paleoproterozoic Polyphase Magmatism in the Lüliang Complex

Abstract There has been a long debate regarding the timing of the final amalgamation of the North China Craton, which is considered to have occurred either during the Neoarchean or Paleoproterozoic era. One major point of contention is whether there existed a long-lived subduction lasting through the Neoarchean to Paleoproterozoic. The Lüliang Complex contains multiphases of magmatism and thus represents the most viable region to address this controversy. In this study, we carried geochronological and geochemical analysis on the representative granitoids. Secondary ion mass spectrometry U–Pb dating revealed four distinct granitoid groups emplaced at 2531 ± 4, 2189–2173, 2027 ± 25, and 1852 ± 41 Ma, respectively. Notably, the 2531 Ma granitic gneiss was identified for the first time in this region. Based on the geochemical characteristics, the granitoids can be divided into two types. The 2531 and 2027 Ma groups display I-type features, while the 2189–2173 and 1852 Ma groups exhibit A-type geochemical affinities. Both I-type groups exhibit enrichment in Rb, depletion in Nb, Ta, and Ti, moderate fractionated REE patterns, substantial negative Eu anomalies, low Sr/Y ratios, and positive εHf(t) (+3.51 to +5.53 and +5.59 to +7.32, respectively), indicating that they were generated from partial melting of the juvenile mafic crust. In contrast, the 2189–2173 Ma granitoids belong to A2-type and were most likely generated by the partial melting of felsic rocks in the back-arc region, while the 1852 Ma granitoids belong to A1-type and were most possibly the result of partial melting of mafic-intermediate rocks during the post-collisional stage. Based on the records of A-type granitic magmatism and the ~1950 Ma peak metamorphism throughout the Trans-North China Orogen, we propose that a long-lived subduction process (2531–1950 Ma) can mostly explain the existing geological phenomena. It is likely that the subduction between the Eastern and Western Blocks should have commenced at ~2531 Ma, followed by a long-lived subduction. The two blocks ultimately collided with each other to form the North China Craton at ~1950 Ma, which triggered post-collisional exhumation and partial melting at ~1852 Ma.

Secular change in the nature of mantle and tectonic evolution of northwestern margin of the Yangtze Block during Neoproterozoic: Constraints from the mafic intrusions and associated granitoids of the Hannan and Xiaomoling complexes

To reveal the Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block, a petrological and geochemical study was carried out for the mafic intrusions and associated granitoids from the Hannan and Xiaomoling Complexes. The ca. 900–880 Ma Xiaomoling diorite and granodiorite have high K2O contents (2.29–3.57 wt%) and belong to the high-K calc-alkaline series. They display depleted whole-rock Sr–Nd–Hf isotope compositions (0.70151–0.70398, +0.6 to +2.5 and +4.4 to +11.2, respectively) and positive zircon εHf(t) (+3.6 to +9.9) with slightly elevated bulk zircon δ18O values (6.4–6.8 ‰) and minor Paleoproterozoic (∼2.2 Ga) inherited zircons, indicative of derivation from remelting of juvenile mafic crust with crustal contamination in a continental arc setting. The ca. 866 Ma Hannan gabbro-diorites show enrichments in LILEs and LREEs and depletions in HFSEs and display depleted Sr–Nd–Hf isotope compositions (0.70267, +5.4 and +17.8, respectively), positive zircon εHf(t) values (+8.6 to +12.0) and mantle-like zircon δ18O values (5.0–6.2 ‰), suggesting that they were generated through flux melting of a depleted mantle wedge. The ca. 850–840 Ma Xiaomoling hornblende gabbro and diabase display depleted Sr–Nd–Hf isotope compositions (0.70279–0.70288, +2.7 to +3.2 and +9.8 to +10.5, respectively) and are characterized by high Nb/U ratios and less enrichment in LREEs and LILEs, indicating that they were formed by decompression melting of a depleted mantle wedge in a back-arc setting. However, the coeval high-Mg diorite displays enriched whole-rock Sr–Nd–Hf isotopes (0.70477–0.70701, −4.0 to −6.0 and −7.2 to −7.4, respectively), negative zircon εHf(t) values (−1.6 to −9.2) and elevated zircon δ18O values (5.3–7.5 ‰), reflecting that the mantle-derived melts have underwent various degrees of crustal contamination. The ca. 780 Ma Hannan gabbro is characterized by earlier crystallization of plagioclase than clinopyroxene, implying that their parental magma is anhydrous. Additionally, the relatively shallow emplacement depth and tholeiitic affinities imply that they were generated by mantle decompression melting probably associated with a continental rift environment in response to Rodinia breakup. Taken together, the secular changes in the geochemical and isotopic compositions of these mafic rocks in the northwestern margin of the Yangtze Block record the evolution from a continental arc at ca. 900–880 Ma to continental rifting at ca. 780 Ma.

Permian-Triassic tectonic evolution of the Ailaoshan orogenic belt in SW China: insights from magmatic records

ABSTRACT The Ailaoshan orogen in SW China records the evolution of the Ailaoshan Paleo-Tethyan ocean and the amalgamation history between the South China and Indochina blocks, an important process for the formation of Southeast Asia. However, its tectonic evolution remains ambiguous with various competing models. We have carried out an integrated geochronological–petrological–geochemical study for the granites, dacites-rhyolites, granodiorites and trachyandesites in this orogen to give new constraints on this issue. The 255–254 Ma granites and the 250–248 Ma dacites-rhyolites show similar geochemical features and are characterized by strongly peraluminous nature, continental crust-like trace element patterns and negative zircon εHf(t) values (−14.4 to −6.6), indicating they derived from partial melting of ancient crust. The 229 Ma hornblende-bearing granodiorites are metaluminous to weakly peraluminous (A/CNK = 0.96–1.01) and belong to high-K calc-alkaline I-type granites. They show depleted whole-rock Sr-Nd and zircon Hf isotope compositions (initial 87Sr/86Sr = 0.7027 to 0.7038, εNd(t) = 4.0 to 4.1, εHf(t) = 9.3 to 11.5), suggestive of derivation from partial melting of juvenile mafic crust. The 226 Ma trachyandesites are characterized by high MgO (4.1–4.6 wt.%), high Mg# (56) and arc-type geochemical affinities with enriched isotope compositions (initial 87Sr/86Sr = 0.7108 to 0.7111, εNd(t) = −7.7 to −7.5, εHf(t) = −6.3 to −3.5). These features indicate that their parental magmas derived from partial melting of enriched lithospheric mantle metasomatized by oceanic sediment-derived melts. These new data, in conjunction with spatial-temporal-temperature variations of igneous rocks, metamorphic records and stratigraphic changes in the region, suggest that the Ailaoshan Paleo-Tethyan ocean closed at the Early Triassic (ca. 250 Ma) which was followed by the Late Triassic postcollisional slab breakoff, and the ocean experienced westward unipolar subduction during its evolution with no assumed eastward subduction.

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 &amp;gt;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.

Petrogenesis and tectonic setting of Neoarchean tonalitic-trondhjemitic-granodioritic gneisses in the Xiwulanbulang area of the Yinshan block, North China craton

As one of the major Archean microcontinental blocks in the North China craton, the Yinshan block consists of the Guyang granite-greenstone belt and the Wuchuan high-grade complex, both of which preserve large volumes of Neoarchean tonalitic-trondhjemitic-granodioritic (TTG) gneisses. The petrogenesis and tectonic setting of these TTG gneisses are of great significance to understanding the Neoarchean crustal growth and evolution of the Yinshan block. Although extensive investigations have been carried out on the TTG gneisses in the Yinshan block, controversy remains regarding their petrogenesis and tectonic setting. In this study, we present new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the Neoarchean TTG gneisses in the Xiwulanbulang area from the Wuchuan high-grade complex. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating results from six TTG gneissic samples show that their igneous TTG plutons were emplaced in the period 2.61−2.48 Ga. The results from zircon Lu-Hf isotopes show that εHf(t) values (−3.2 to +7.5) and depleted mantle model ages (3270−2651 Ma) are like those of the oldest trondhjemite (ca. 2.7 Ga) exposed in the Xiwulanbulang area, indicating that it mainly resulted from partial melting of 2.7 Ga juvenile mafic crust. Geochemically, the Xiwulanbulang TTG gneisses contain high Sr (280−800 ppm) contents, low Y (1.05−9.54 ppm) and Yb (0.05−0.90 ppm) contents, and relatively high LaN/YbN (12.8−105) and Sr/Y (39.91−1392.6 ppm) ratios, with relative enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), showing the characteristics of adakitic rocks. In addition, their lower contents of MgO, Ni, Cr, and Mg# indicate that the Xiwulanbulang TTG rocks belong to adakite sourced from thickened continental lower crust. Also, the TTG gneisses show similar initial 87Sr/86Sr ratios (0.701659−0.702635) and εNd(t) values (−4.37 to + 3.55), with 206Pb/204Pb(t) = 13.910485−15.241164, 207Pb/204Pb(t) = 14.806723−15.259938, and 208Pb/204Pb(t) = 33.758621−35.592727, suggesting a depleted mantle source modified with some enriched components. Collectively, the Xiwulanbulang TTG rocks were most likely derived from the partial melting of thickened mafic crust between 2.61 and 2.48 Ga. Combining previous studies with data presented in this paper, we propose that the Yinshan block underwent two major igneous events during Neoarchean time, with the earlier one occurring ca. 2.7 Ga, forming a thickened mafic crust and minor TTG rocks, and the second igneous event at 2.61−2.48 Ga, resulting in the partial melting of the ca. 2.7 Ga continental lower (mafic) crust to form the TTG plutons. The geochemical data for the Xiwulanbulang TTG gneisses are inconsistent with a model for the partial melting of subducted slabs, but combined with other geologic considerations, a model in which the partial melting of a thickened continental lower (mafic) crust was heated by a mantle plume is favored for the formation of these Neoarchean TTG rocks in the Yinshan block.

Crustal growth and evolution in convergent margin: Evidence from three Paleozoic granitic pulses in the junction zone between Qinling and Qilian orogenic belt

Paleozoic igneous rocks in the junction zone between the western Qinling and Qilian orogenic belts preserve important message about the deep crustal structure and their tectonic relationship. By means of zircon U Pb dating, Lu Hf isotope, bulk-rock geochemistry for a suite of Paleozoic granites from the western Qinling and Qilian orogenic belt, three main granite pulses were identified: 1) First pulse (~500 to 450 Ma) quartz diorite with typical island-arc affinity, i.e., intermediate SiO 2 (56.42 to 61.44 wt%) contents, variable Na 2 O/K 2 O ratios of 0.9 to 1.7, high MgO contents (3.14 to 4.40 wt%) with high Mg# values of 55.6 to 60, moderate Sr/Y (22 to 29) ratios and insignificant Eu*/Eu anomalies (0.77 to 0.84), zircons from the quartz diorite have positive ε Hf (t) values of +2.2 to +5.5; all these features indicate that the quartz diorite was formed by hydrous melting of depleted mantle wedge in subduction setting; 2) Second pulse (ca. 440 to 430 Ma), monzogranite (e.g., the Dangchuan and Caochuanpu monzogranite), they display higher SiO 2 (71.23 to 73.91 wt%) contents, K 2 O/Na 2 O (1.10 to 2.26) ratios, lower MgO (0.34 to 0.43 wt%) contents, significant negative Eu anomalies of 0.36 to 0.48, evolved Sr Nd isotopic compositions, and variable zircon Lu Hf (ε Hf (t) = −5.0 to +10.6) compositions, suggesting dehydration melting of heterogeneous crust; 3) Third pulse (ca. 420 to 410 Ma), biotite granite that display typical A-type affinity, i.e., high FeO T /MgO and Ga/Al (10,000 × Ga/Al = 2.7 to 3.0) ratios, high Zr (230 to 313 ppm), Nb (3.49 to 34.6 ppm) and Zr + Nb + Ce + Yb (351 to 512 ppm) contents, most zircons from these biotite granites have negative ε Hf (t) values of −2.7 to −26.6, in combination with their higher Zr saturation temperatures (822 to 842 °C), suggesting high temperature melting of matured continental crust in extensional setting. These results indicate that the western Qinling and Qilian orogenic belts experience a tectonic switch from subduction to extension in Early-Paleozoic (ca. 460 to 410 Ma). All these granites record the crustal evolution and switch of melting regime in convergent margin. • Three Paleozoic magmatic pulses in the junction zone between North Qinling and Qilian. • Hydrous melting of depleted mantle wedge formed the first pulse. • Dehydration melting of juvenile mafic crust formed the second pulse. • High temperature melting of heterogeneous crust in extensional setting formed the third pulse.

Revisiting the origin of the Carboniferous Oytag pluton in West Kunlun orogenic belt, northwest China

The Oytag pluton in the West Kunlun orogenic belt of China represents the rare exposed Carboniferous granitoids associated with the early-stage evolution of the Paleo-Tethys Ocean, and its origin is controversial. New zircon U-Pb data suggest an emplacement age of 327–324 Ma for trondhjemite, which slightly predated tonalite (∼314 Ma). These results, along with available published ages, indicate several episodes of magmatism to build the Oytag pluton. Both trondhjemite and tonalite have positive whole-rock Nd isotopic compositions (ε Nd (t) = +4.56 to +8.01 for trondhjemite; ε Nd (t) = +4.68 to +6.15 for tonalite), which positively correlate with whole-rock MgO values. Given the presence of Ordovician inherited zircon grains, variations of whole-rock Nd isotopic compositions are attributed to addition of <10% crustal contaminants during assimilation and fractional crystallization (AFC) processes. Interestingly, trondhjemite and tonalite follow distinct AFC trends, consistent with their different ages. Zircons from two trondhjemite samples have overlapping U-Pb ages but show distinctly different trace elements and internal structures (sector-zoned vs. oscillatory-zoned). The sector-zoned zircons have a less evolved signature of lower Hf contents, higher Eu/Eu* and Th/U ratios than the oscillatory-zoned ones, indicating significant crystal fractionation processes for their parental melts. The more variable Hf contents of the oscillatory-zoned zircons than those of the sector-zoned ones reflect a more dynamic environment where they crystallized. We propose that the sector-zoned zircons were crystallized from locked, interstitial melt in a mush and the oscillatory-zoned zircons from extracted melt. In situ crystal-liquid segregation in a mush chamber thereby could be an efficient approach to the differentiation of Oytag trondhjemite. On the other hand, zircons from tonalite have similar Hf contents to the sector-zoned zircons from trondhjemite, but display slightly higher Eu/Eu* ratios and much higher Th/U ratios. This indicates that tonalite should have experienced much different AFC processes from trondhjemite. Both the episodic magmatism feature and the distinct differentiation processes argue for rises of different batches of magmatism to generate trondhjemite and tonalite in Oytag. Along with the large variations of Hf isotopes (5–6 epsilon units) in zircons, we propose a partial melting of juvenile mafic crust model for the Oytag pluton. Heat from upwelling of asthenosphere may have triggered the partial melting of wet, mafic underplates during the opening of a Paleo-Tethyan back-arc basin. • The Oytag pluton comprises predominant trondhjemite with minor tonalite. • Oytag trondhjemites (327–324 Ma) formed earlier than tonalites (∼314 Ma). • The Oytag pluton was built by different batches of magmas via partial melting.

Chronological framework of Precambrian Dantazi Complex: Implications for the formation and evolution of the northern North China Craton

The Dantazi Complex in the North Hebei Terrane is located in the middle segment of the northern North China Craton, and plays a crucial role for understanding the early Precambrian crustal evolution of the craton. The Dantazi Complex is composed mainly of late Neoarchean tonalites-trondhjemites-granodiorites, potassic granites, sanukitoid granodiorites-monzodiorites and quartz syenites, which were emplaced by Paleoproterozoic gabbroic stocks. SHRIMP and LA–ICP–MS zircon dating analyses have revealed that the Neoarchean diversified granitoids of the Dantazi Complex were formed in ∼2.52–2.47 Ga, and preserved ∼2.60–2.53 Ga inherited/captured zircon ages. All these rocks experienced early metamorphism and deformation at ∼2.40 Ga, later granulite facies metamorphism at ∼1.80 Ga, and alterations at ∼1.73 Ga. Whole-rock chemical characteristics and Sm–Nd–Lu–Hf isotopes indicate that (1) The ∼2.52 Ga tonalites-trondhjemites-granodiorites were derived from partial melting of juvenile mafic crust; (2) the ∼2.51 Ga sanukitoid monzodiorite-granodiorite series came from slab-mantle interaction with different melt/rock ratios; (3) The ∼2.50–2.47 Ga potassic granites were generated by intracrustal recycling of metagreywacks that sourced from the mixture between juvenile and ancient crustal materials; and (4) The ∼2.50 Ga quartz syenites were originated from fractional crystallization of mantle-derived magmas. These petrogenetic processes reveal that the subduction-related crust-mantle interaction is the key reason for the diversity of late Neoarchean granitoids in the Dantazi Complex. Regional comparison indicates that the Precambrian North Hebei Terrane show distinct structure in the chronology and lithological assemblages from other basement terranes in the trans-North China Orogen and Western Block, implying that they were formed in different crust-mantle dynamic systems. Therefore, the Precambrian North Hebei Terrane was probably an independent micro-continental block at least until the early Paleoproterozoic. In the Archean crystalline basement range of the North Hebei–West Liaoning–East Hebei, from NW to SE, the magmatic crystallization ages gradually became older, and the granitoid assemblages transformed from sodium-rich to potassium-rich, the spatio-temporal evolution of chemical composition and lithological assemblages further indicates that the North Hebei Terrane was most likely developed in an arc environment with continuous retreat subduction caused by the slab roll-back.

Cambrian intra-oceanic subduction within the southern branch of the Proto-Tethyan Ocean: Constraints from rhyolites in the Lajishan suture, NE Tibetan Plateau

The South Qilian suture in the Qilian Orogen is an early Paleozoic belt that developed as a result of closure of the southern branch of the Proto-Tethyan Ocean. It is characterized by widespread Cambrian intermediate-basic volcanic rocks which record the early-stage evolution of the paleo-ocean basin. However, Cambrian evolution of the paleo-ocean basin remains controversial due to the lack of research on the age and tectonic affinity of these volcanic rocks. In this study, we identified rhyolites from the intermediate-basic volcanic rocks in the South Qilian suture. They occur as pebbles in the breccias, or as veins and interlayers within the intermediate-basic volcanic rocks. Zircon U-Pb datings yield ages of 514.0 ± 2.8 Ma, 521 ± 4 Ma, 523 ± 5 Ma, and 513 ± 9 Ma for four rhyolites, respectively. The Cambrian rhyolites show variable La/Sm ratios and Rb contents and positive zircon εHf(t) values (+6.8 to +14.6) and whole-rock εNd(t) values (+2.1 to +4.6), suggesting that they were derived from partial melting of juvenile mafic crust. They also have high Al2O3 and CaO contents, as well as low K2O and trace element concentrations, which are similar to those of oceanic arc rhyolite. These early Cambrian oceanic arc rhyolites, along with the SSZ-type ophiolite (ca. 530–520 Ma) to the north and late-stage granitoid intrusions (ca. 474–450 Ma), indicate that the southern branch of the Proto-Tethyan Ocean experienced southward intra-oceanic subduction during the early Cambrian-early Ordovician period.

Diorite enclaves and host granite of the early Miocene Gorontalo pluton in the North Sulawesi Arc, Indonesia: Implications for recycled oceanic crust and crust-mantle interaction

• Granites from the Early Miocene Gorontalo pluton were derived from a juvenile crust. • Diorite enclaves originated from mixing of depleted mantle- and juvenile crust-derived magmas . • Sub-normal mantle δ 18 O values suggest the involvement of recycled altered oceanic crust . • Early Miocene crust-mantle interaction occurred in the North Sulawesi Arc. Zircon Hf-O isotopes, combined with whole-rock Sr-Nd-Pb isotopes, can provide unambiguous evidence for crustal recycling and crust-mantle interaction processes. The North Sulawesi Arc (Indonesian) was traditionally regarded as an intra-oceanic arc, and the diorite enclaves and host granites of the Gorontalo pluton provide excellent opportunities for evaluating these issues, based on our zircon U-Pb geochronological and Hf-O isotopic, and whole-rock geochemical analyses. This study firstly reports the zircon U-Pb ages of ∼20.1–22.4 Ma for the host granites and ∼19.5–20.4 Ma for the diorite enclaves. The host granites have high SiO 2 , and low 10000*Ga/Al, A/CNK and Rb/Sr, classified as unfractionated I-type granites. They are characterized by high εNd (t) (from +4.8 to +5.7), high zircon εHf (t) (from +11.1 to +16.4), sub-normal mantle zircon δ 18 O (4.6–5.4‰), moderate ( 206 Pb/ 204 Pb) i (18.29–18.45), and young two-stage Hf model ages (52–394 Ma), deriving from partial melting of a juvenile mafic crust. The diorite enclaves show typical magmatic textures and contain abundant acicular apatite and plagioclase megacrysts with an obvious rim-core texture. They also have high εNd (t) (from +4.8 to +5.1), high zircon εHf (t) (from +10.7 to +15.2), sub-normal mantle zircon δ 18 O (4.1–5.1‰) and moderate ( 206 Pb/ 204 Pb) i (18.3), interpreted as the products of mixing of magmas derived from a depleted mantle and a juvenile crust. The diorite enclaves have sub-normal mantle δ 18 O values, positive εNd (t) and zircon εHf (t) , suggesting recycled altered oceanic crust into depleted mantle prior to magma generation. The presence of the host granites and diorite enclaves provides robust evidence for early Miocene (or prior to early Miocene) crust-mantle interaction in the North Sulawesi Arc.

3.63 Ga grey gneisses reveal the Eoarchaean history of the Zimbabwe craton

Abstract Grey gneisses from the Tokwe and Rhodesdale terrains of the Zimbabwe craton have zircon ages of ~3.63, 3.52, and 3.34 Ga, validating the existence of Eo- to Palaeoarchaean crust. In-situ zircon Hf isotope compositions reveal the interplay between episodes of juvenile magma addition, crustal thickening and crustal differentiation. Starting from juvenile mafic crust (with chondritic composition) at ~3.9 Ga, the oldest nucleus of the Zimbabwe craton developed into a stable crustal block by ~3.35 Ga, following a tectonic and mantle evolution that is mirrored by other ancient terrains.

Relicts of a Cambrian oceanic arc in the Lajishan suture, NE Tibetan Plateau: Evidence for early-stage subduction within the Proto-Tethyan Ocean

Lajishan suture in the Qilian Orogen records subduction process of the southern branch of the Proto-Tethyan Ocean. However, when and where the subduction started remain controversial issues. We present a study of the Cambrian volcanic rocks from the Lajishan suture. These volcanic rocks are coherent and composed of basalt, andesite, and volcanic breccia with minor interlayers or veins of dacite and rhyolite. Zircons from the dacite and rhyolite yield concordant Early-Middle Cambrian ages ranging from 521 to 510 Ma. Basalt and andesite exhibit subduction-related geochemical signatures with enrichment in large ion lithophile elements and depletion in high field strength elements. They have evolved REE, LREE, ε Nd (t) (+2.6 to +8.2), and (La/Sm) N ratios, suggesting derivation from depleted mantle wedge with addition of hydrous fluid or various amounts of sediment-derived melts. Dacite and rhyolite have positive zircon ε Hf (t) values (+6.1 to +11.6) and whole-rock ε Nd (t) values (+1.7 to +4.2). They were generated by partial melting of juvenile mafic crust resulting from magma underplating. These Cambrian volcanic rocks in the Lajishan suture are exposed to the south of the accretionary complex and therefore are likely to be relicts of Cambrian oceanic arc built on the oceanic crust during southward subduction of the South Qilian Ocean. Considering the Cambrian-Ordovician intra-oceanic trench-arc-basin system reconstructed in the North Qilian suture, it is suggested that both southern and northern branches of the Proto-Tethyan Ocean have experienced intra-oceanic subduction process which initiated at ca. 530 Ma. • Cambrian volcanic rocks are coherent and widely exposed in the Lajishan suture. • They represent the relicts of an oceanic arc within the South Qilian Ocean. • Early-stage subduction of the Proto-Tethyan Ocean initiated at ca. 530 Ma.

Late Permian ultrapotassic rhyolites in SE Thailand: evidence for a Palaeotethyan continental rift basin

The volcanic rocks in the Chanthaburi zone are rarely reported, but are important in investigating the tectonic evolution of the Palaeotethyan Ocean in SE Thailand. Four rhyolitic samples from Ko Chang Island yield zircon ages of 258–254 Ma, confirming the presence of Late Permian volcanic rocks in SE Thailand. These rocks consist of Group 1 rhyolites and Group 2 rhyolitic ignimbrites and have high K 2 O contents of 4.92–7.10 wt% and A/CNK values of 1.10–1.69. They are enriched in the light rare earth elements, Rb, Th, U, Zr and Y and show negative anomalies of Ba, Sr, Nb, Ta and Ti, with clear Eu anomalies. Their whole-rock ε Nd ( t ) values range from −1.7 to −3.1. Zircon in situ ε Hf ( t ) and δ 18 O values range from 0.0 to +5.6 and from 8.2 to 9.6‰, respectively. They belong to the group of peraluminous, ultrapotassic A-type rhyolites and were derived from partial melting of a mixed source of Mesoproterozoic metasedimentary rocks with a component of juvenile mafic crust. These ultrapotassic rhyolites formed in a continental rift setting in response to the rollback of a subducted Palaeotethyan oceanic slab beneath the Indochina Block. Combining these results with previous geological observations, we propose that there were some sporadically distributed continental rift basins along the Eastern Palaeotethyan domain during the Permian. Supplementary material: Major oxides, trace element analytical and Nd isotopic data, LA–ICP–MS and SIMS zircon U–Pb ages and Zircon in-situ Hf–O isotopic compositions are available at https://doi.org/10.6084/m9.figshare.c.5635390

Paleo-Mesoarchean crustal growth and reworking in the western Dharwar Craton, southwestern India: Evidence from trondhjemitic gneiss and granitic gneiss

Paleo-Mesoarchean felsic orthogneisses are widely exposed in the southern part of the western Dharwar Craton (WDC), southern India shield. The felsic orthogneisses studied in this paper from the WDC can be divided into two groups based on their geological, geochronological, and geochemical data: the 3.4–3.3 Ga trondhjemitic gneiss (including trondhjemitic veins) and the ∼3.1–3.0 Ga granitic gneiss. The 3.4–3.3 Ga trondhjemitic gneiss is characterized by high contents of SiO2 (70.4–73.1 wt%), Na2O (5.49–5.91 wt%) and Sr (647–740 ppm), high ratios of Na2O/K2O (4.7–7.7) and Sr/Y (53–106), but relatively lower Mg# values (32–39), and Cr (9.5–28.7 ppm) and Ni (1.34–5.33 ppm) concentrations, as well as insignificant Eu anomalies, fractionated REE patterns and mostly positive zircon εHf(t) values (+0.8-+5.9). These data indicate that the trondhjemitic gneiss was derived from partial melting of juvenile mafic crust in a relatively moderate-shallow depth, with residual mineral assemblage of garnet, amphibole and clinopyroxene, implying crustal growth event at 3.4–3.3 Ga. By contrast, the ∼3.1–3.0 Ga granitic gneiss is characterized by relatively lower contents of Na2O (3.82–4.63 wt%) and Sr (96.9–291 ppm), low ratios of Na2O/K2O (1.4–1.7) and Sr/Y (3.02–8.65), as well as significantly negative Eu anomalies and negative to positive zircon εHf(t) values (-5.6-+4.5). Therefore, the granitic gneiss was probably derived from partial melting of pre-existing crustal rocks, suggesting a prominent crustal reworking event at ∼3.1–3.0 Ga. This study emphasizes that the western Dharwar Craton is dominated by Paleoarchean (3.4–3.2 Ga) crustal growth event and a strong crustal reworking event in early Mesoarchean (∼3.1–3.0 Ga), though the Mesoarchean crustal growth event is discernible, evidenced by regional ∼3.0 Ga TTG rocks. The early Mesoarchean (∼3.1–3.0 Ga) crustal reworking event, represented by series of potassium-rich granites, is also reported in other Archean cratons, and thus it may have global significances.

The Early Cretaceous tectonic evolution of the southern Great Xing’an Range, northeastern China: new constraints from A2-type granite and monzodiorite

The widespread Early Cretaceous plutons intruding along the southern Great Xing’an Range (SGXR) provide evidence for tectonic evolution of the region. Petrological, geochemical, zircon U–Pb geochronology, and zircon Hf isotopic studies are conducted on intrusions from Bianjiadayuan and Hongling areas. These suites classify as A2-type granites and monzodiorites, respectively. The 138–133 Ma A2-type granites originated from partial melting of continental crustal materials at high temperatures and shallow depths with significant addition of juvenile mafic lower crust sourced from a metasomatized mantle. The 136–134 Ma monzodiorites originated from the partial melting of an enriched mantle that was modified by melts of a previously subducted slab coupled with crustal contamination. The Early Cretaceous magmatism in the SGXR occurred in two periods: ∼145–136 Ma (peak at ∼139 Ma; εHf (t) = 5 to 10) and ∼136–130 Ma (peak at ∼131 Ma; εHf (t) = –10 to 15). The Early Cretaceous granite–monzodiorite suite in the SGXR suggests a bimodal magmatism in an extensional setting. The ∼145–130 Ma magmatism may have been triggered by asthenospheric upwelling induced by the Mongol–Okhotsk oceanic slab breakoff and large-scale lithospheric delamination resulting from post-orogenic extension. The variation of subduction direction of the Paleo-Pacific Ocean likely triggered a change in stress regime at ca. 136 Ma and likely promoted the lithospheric delamination beneath the SGXR resulting in intense magmatism originating from various sources. As such, the Paleo-Pacific Oceanic subduction likely played an important role in the Early Cretaceous magmatism in the SGXR.

Early Cretaceous subduction in NW Kalimantan: Geochronological and geochemical constraints from the Raya and Mensibau igneous rocks

The Kuching zone in NW Kalimantan and Malaysia Sarawak might be the southerly-extending segment of the paleo-Pacific subduction that is developed along the Coastal South China and SE Vietnam, and separated the Sundaland “old-land” from the Cenozoic Sibu zone. However, establishing this link is hindered by the lack of data on the Mesozoic orogenic history in NW Kalimantan. This paper presents zircon U-Pb geochronological and Hf-O isotopic data as well as whole-rock elemental and Sr-Nd-Pb isotopic results for the Raya Volcanics and their equivalents, as well as the Mensibau batholith in NW Kalimantan. The Raya Volcanics and Mensibau granitoids formed at 130–144 Ma, synchronous with the Menunuk Volcanics, but significantly older than the Pakong-Serabang mafic rocks and Serian Volcanics (~77–98 Ma) in the Sarawak Kuching zone. The Raya Volcanics and their equivalents belong compositionally to tholeiitic to low-K calc-alkaline series, and exhibit “spiky” PM-normalized patterns with Nb/La = 0.16–0.32, (87Sr/86Sr)i = 0.70187–0.70671 and εNd(t) = +0.9 to +3.6. Their (206Pb/204Pb)i, (208Pb/204Pb)i and (207Pb/204Pb)i range from 18.49 to 18.83, 38.39 to 39.19 and 15.57 to 15.70, and zircon εHf(t)-δ18O values from +4.1 to +16.1, and 5.21‰ to 6.05‰, respectively. They are derived from mantle wedge modified by recycled sediments. The Mensibau batholith is dominated by I-type granodiorite and granite. They share similar elemental ratios and Sr-Nd-Pb-Hf-O isotopic compositions with the Raya Volcanics, and originated from wedge-derived juvenile mafic crust. The temporal-spatial and geochemical accordance for the Raya Volcanics and Mensibau batholith document the extensive development of the early Cretaceous igneous rocks in NW Kalimantan. The Kuching zone is a northeasterly-propagated Cretaceous accretionary system that developed in response to the subduction of the paleo-Pacific plate.

Origin, Accretion, and Reworking of Continents

AbstractThe continental crust is unique to the Earth in the solar system, and controversies remain regarding its origin, accretion and reworking of continents. The plate tectonics theory has been significantly challenged in explaining the origin of Archean (especially pre‐3.0 Ga) continents as they rarely preserve hallmarks of plate tectonics. In contrast, growing evidence emerges to support oceanic plateau models that better explain characteristics of Archean continents, including the bimodal volcanics and nearly coeval emplacement of tonalite‐trondjhemite‐granodiorite (TTG) rocks, presence of ∼1600°C komatiites and dominant dome structures, and lack of ultra‐high‐pressure rocks, paired metamorphic belts and ophiolites. On the other hand, the theory of plate tectonics has been successfully applied to interpret the accretion of continents along subduction zones since the late Archean (3.0–2.5 Ga). During subduction processes, the new mafic crust is generated at the base of continents through partial melting of mantle wedge with the addition of H2O‐dominant fluids from subducted oceanic slabs and partial melting of the juvenile mafic crust results in the generation of new felsic crusts. This eventually leads to the outgrowth of continents. Subduction processes also cause softening, thinning, and recycling of continental lithosphere due to the vigorous infiltration of volatile‐rich fluids and melts, especially along weak belts/layers, leading to widespread continental reworking and even craton destruction. Reworking of continents also occurs in continental interiors due to either plate boundary processes or plume‐lithosphere interactions. The effects of plumes have proven to be less significant and cause lower degrees of lithospheric modification than subduction‐induced craton destruction.