Hf Isotopic Study Research Articles

Petrogenesis and Tectonic Setting of the Xishan Pluton in Nanling Range: Lithogeochemistry, Zircon U–Pb Chronology and Hf Isotope Evidence

ABSTRACTIn this article, a systematic LA‐ICP‐MS zircon U–Pb geochronology study was carried out on the Xishan pluton in Nanling Range. The results show that the formation ages of granitic porphyroclastic lava, medium‐fine‐grained porphyritic granite, and dacite are 154.0 ± 1.2–156.1 ± 2.0, 153.2 ± 0.9–157.3 ± 3.2, and 151.2 ± 3.1 Ma, which are determined to be the early Yanshanian. Geochemical data show that the Xishan pluton was characterised by rich silicone‐alkali and poor calcium–magnesium with K2O/Na2O ratios of 1.65–2.22, Al2O3 of 1.00–1.14, and FeO*/MgO ratios of 8.33–19.0 (with an average of 12.94). The rare earth content of granites ranges from 322.92 to 441.54 ppm, which is significantly higher than the world average. The rare earth element distribution curve is right‐leaning light rare earth enrichment type, with obvious negative europium anomaly, and the δEu values range from 0.06 to 0.24; These rocks are enriched in high‐field‐strength elements (HFSEs, e.g., Ga, Y, Nb, Zr, and Hf) and depleted elements such as Ba, Nd, Sr, P, Ti, etc, with Ga/Al ratios of 2.95 × 10−4 to 4.59 × 10−4 (with an average of 3.50 × 10−4); Zr + Nb + Ce + Y of 367.1–652.5 ppm (with an average of 441.95 ppm) and zircon saturation temperatures Tzr is 755.04°C–892.50°C (with an average of 810.04°C), similar to geochemical features of A‐type granites. The Hf isotope study shows that the εHf(t) of the Xishan pluton ranges from 2.11 to −8.77, and the petrographic, geochemical and available data confirm that it originates from the mixing of crustal and mantle‐derived magma, and the contribution of the latter is < 10%. The hafnium II model age of the Xishan pluton is 1251–1766 Ma, which confirms that its source rocks were detached from the mantle reservoir at the time of the Middle Proterozoic. Combined with the evolutionary history of the Nanling Range area, it is assumed that the Jiangnan Orogenic Zone was partially reactivated by the continuous lateral subduction of the ancient Pacific Plate in the Late Mesozoic, which triggered partial melting of the crust to form the A2‐type granite type, which is the origin of the Xishan pluton.

Isotope Evolution of the Depleted Mantle

The depleted mantle reservoir is that part of Earth's mantle from which crust has been extracted, leaving the remaining mantle depleted in incompatible elements. Knowing how and when it formed is essential for understanding the chemical evolution of Earth, including formation of continental crust. The best-constrained Hf isotope data presented here indicate that the mantle does not become significantly depleted until as late as 700 million years after Earth's accretion. This onset of mantle depletion coincides with the first appearance of substantial volumes of continental crust in the geological record. These data compel a revision to the reference depleted mantle parameters used in Hf isotope studies of planetary evolution. This new reference line follows chondritic evolution until 3.8 Ga and then describes a linear trajectory to a present-day depleted mid-ocean ridge basalt source mantle composition (εHf = +18). We infer that stabilization of continental crust only occurred in earnest on Earth after 3.8 Ga. ▪ Hf isotopes show that Earth's mantle does not become significantly depleted until 700 million years after planetary accretion. ▪ Most of Earth's oldest rocks formed from mantle sources that had radiogenic isotope compositions similar to those of chondritic meteorites. ▪ Isotope evidence shows that Hadean (>4.0-billion-year-old) crust was not essential for formation of younger crust in Archean terranes. ▪ Growth of Earth's continents only began in earnest after 3.8 Ga.

Timescales of pluton assembly and growth history of Colorado Plateau lithosphere: Geochronology (U-Pb, K-Ar, and Ar-Ar) and zircon Hf isotopic study of the Mount Hillers intrusive center (Henry Mountains, Utah, USA)

We studied the age, history of emplacement, and source of the Mount Hillers intrusive center (Henry Mountains, Colorado Plateau, Utah, USA) with in situ U-Pb and Lu-Hf isotopic analysis of the same zircon grains. Inherited cores in all of the zircon grains analyzed allowed us to document the Proterozoic history of the lithosphere of the Colorado Plateau. In addition, K-Ar age dating of feldspars from microgranular matrix and 40Ar/39Ar age dating of hornblende phenocrysts enabled us to compare zircon crystallization and cooling ages. U-Pb data from zircon rims from all of the samples provided consistent ages with a mean value of 25.97 ± 0.11 Ma. Our field mapping and structural data show that the sample set is representative of the entire history of the Mount Hillers intrusive center emplacement. We propose that the 220 k.y. covered by the analytical uncertainties is the minimum duration of emplacement. On the other hand, the maximum duration of 640 k.y. is given by ±2SD of the sample population. The distinct magmatic pulses identified in the field display similar ages within the analytical uncertainties. Given these durations, the averaged vertical displacement rate during emplacement of the Mount Hillers intrusive center was ∼0.3−0.9 cm/yr. The average magmatic flux was between 0.5 km3/yr and 1.6.10−4 km3/yr at the scale of both the entire Mount Hillers intrusive center and individual satellite intrusions. The U-Pb ages of the zircon cores range between 2.16 Ga and 1.05 Ga, with a major frequency peak between 1.45 Ga and 1.3 Ga, and several minor peaks between 1.75 Ga and 1.6 Ga. The 1.8−1.7 Ga Yavapai province and 1.7−1.6 Ga Mazatzal province age signatures are present beneath this part of the Colorado Plateau. Lu-Hf analyses of most of the zircon rims are consistent with those of the cores and show a lower crustal mafic source derived from a 1.8−1.3 Ga depleted-mantle reservoir, which incorporated variable amounts of recycled continental crust over time from 1.5 Ga. Even if both sources were partly mixed, they can still be observed in the Oligocene rims that crystallized during emplacement of the Mount Hillers intrusive center. With this core-rim duality, the data tell two stories that may seem totally disconnected: that of the Proterozoic lithosphere and that of the Oligocene magmatism of the Colorado Plateau. But the U-Pb and Lu-Hf data clearly show that these two stories are coupled. Our data document a magmatic legacy spanning 1.5 b.y. Our work unequivocally shows how important it is to combine robust geochronology based on careful sampling with relative chronology deduced from field-based structural mapping to correctly evaluate the emplacement duration and possibly to temporally discriminate between the different pulses in a pluton.

Age and sources of the kodar group rocks of the udokan complex (Aldan shield): results of geochemical, U–Th–Pb (LA-ICP-MS) geochronological and Nd–Hf isotopic studies

The paper presents the results of geochemical and Nd whole rock isotopic studies, as well as U–Th–Pb (LA-ICP-MS) geochronological and Hf isotope studies of detrital zircon from metaterrigenous rocks of the Kodar Group of the Udokan Complex, Aldan Shield. It has been established that the rocks of the Kodar Group have an age of 1.99–1.91 Ga, and the rocks of the Chinei and Kemen groups of the Udokan Complex are in the range of 1.90–1.87 Ga. This allows us to raise the question of identifying the Kodar Group as an independent stratigraphic unit. Archean igneous and metamorphic rocks of the Chara-Olekma Geoblock and, probably, the Kalar and Kurulta blocks of the Stanovoy suture zone, as well as Paleoproterozoic (2.04–1.99, 2.08, 2.20 and 2.30 Ga) complexes of active continental margins or ensialic island arcs in the western–northwestern and southern (in modern coordinates) framing of the Chara-Olekma Geoblock, not identified in the region on the modern erosion level, were the sources of terrigenous rocks of the Kodar Group. Erosion of rocks of the igneous arcs and the continental slope led to the accumulation of sediments of the Kodar Group in the retro-arc foreland basin setting, and the subsequent collapse of the orogen and the formation of an intracontinental extension basin led to the accumulation of terrigenous rocks of the Chinei and Kemen groups. Obtained data indicate widespread previously unidentified Paleoproterozoic continental crust formation at about 2.04–1.97 Ga in the western part of the Aldan Shield.

Petrogenesis of early Paleozoic syn-collisional granitoids and enclaves in Western Kunlun, Northwest China: Implications for the growth of continental crust

The Western Kunlun Orogenic Belt (WKOB), along the northwestern margin of the Qinghai–Xizang Plateau, was formed by the collision of Gondwana-derived terranes to the south and the Tarim Block to the north and was closely associated with closure of the Proto-Tethys Ocean during the late Neoproterozoic to early Paleozoic. We present a combined zircon UPb geochronology, whole-rock composition, and Sr–Nd–Hf isotopic study of syn-collisional granitoid plutons and mafic microgranular enclaves (MMEs) in the region. Zircon UPb dating yields ages of 443.8 ± 4.4, 451.9 ± 4.2, 462.9 ± 3.5, and 456 ± 4.2 Ma for the Tongayoupuagezi, Shanjie, and Pishigai plutons and MMEs from the Pishigai pluton, respectively. The granitoids are metaluminous to weakly peraluminous (A/CNK = 1.00–1.17) and belong to the high-K calc-alkaline series. They have (87Sr/86Sr)i ratios of 0.7058–0.7154, εNd(t) values of −8.78 to −0.93, and εHf(t) values of −19.72 to +6.87. The MMEs have variable SiO2 contents (45.7–60.2 wt%) and are more mafic than the host granitoids, but have similar Sr–Nd–Hf isotopic compositions to the host granitoids [(87Sr/86Sr)i = 0.7102–0.7110; εNd(t) = −6.57 to −3.56; εHf(t) = −7.17 to −1.81]. The MMEs are fragments of cumulates formed during the early stages of magma evolution. The granitoids were produced by the partial melting of a mélange source. The new data support the view that the Middle–Late Ordovician syn-collisional granitoids with MMEs distributed along the WKOB represent a magmatic response to terrane collision. This suggests that juvenile crustal growth in older orogenic systems, which occurs by arc addition, also involves some vertical addition during the final stage of orogenic collision. Our study suggests that mélange diaper melting is a key mechanism of crustal growth during the syn-collision stage in continental collision zones, associated with slab breakoff.

Zircon Chronology and Hf Isotope Study of Sanyanlong-Galazi Granite in the Southern Songpan-Garze Orogenic Belt

Zircon Chronology and Hf Isotope Study of Sanyanlong-Galazi Granite in the Southern Songpan-Garze Orogenic Belt

Age and Sources of Rocks of the Kodar Group of the Udokan Complex (Aldan Shield): Results of Geochemical, U–Th–Pb (LA-ICP-MS) Geochronological and Nd–Hf Isotopic Studies

Age and Sources of Rocks of the Kodar Group of the Udokan Complex (Aldan Shield): Results of Geochemical, U–Th–Pb (LA-ICP-MS) Geochronological and Nd–Hf Isotopic Studies

Petrogenesis and Geodynamic Mechanisms of Porphyry Copper Deposits in a Collisional Setting: A Case from an Oligocene Porphyry Cu (Au) Deposit in Western Yangtze Craton, SW China

The Xifanping deposit is a distinct Cenozoic porphyry Cu (Au) deposit located in the Sanjing porphyry metallogenic belt 100–150 km east of the JinshajFiang fault in the western Yangtze craton. We present new zircon U–Pb–Lu–Hf isotopic studies and geochemical data of the ore-bearing quartz monzonite porphyry from the Xifanping deposit to determine their petrogenesis and geodynamic mechanisms. LA–ICP–MS zircon U–Pb dating yielded precise emplacement ages of 31.87 ± 0.41 Ma (MSWD = 0.86) and 32.24 ± 0.61 Ma (MSWD = 1.8) for quartz monzonite porphyry intrusions, and 254.9 ± 5.1 Ma (MSWD = 1.7) for inherited zircons of the monzonite porphyry. The ore-bearing monzonite porphyry is characterized by high-K calc–alkaline to shoshonite and peraluminous series, relatively enriched in light over heavy REEs, with no distinct Eu anomalies, as well as enrichment in LILEs and depletion of HFSEs, with adakitic affinities. The zircon Lu–Hf isotope data ranged from εHf(t) values of −2.94 to +3.68 (average −0.47) with crustal model (TDM2) ages ranging from 0.88 to 1.30 Ga, whereas the inherited zircons displayed positive εHf(t) values ranging from +1.83 to +7.98 (average +5.82), with crustal model (TDM2) ages ranging from 0.77 to 1.17 Ga. Results suggest that the Xifanping porphyry Cu (Au) deposit is related to two periods of magmatic activities. Early magmas were generated from the Paleo-Tethys oceanic subduction during the Late Permian. The subsequent porphyry magma was likely formed by the remelting of previously subduction-modified arc lithosphere, triggered by the continental collision between the Indian and Asian plates in the Cenozoic. The deep magmas and late hydrothermal fluids took advantage of the early magma transport channels along tectonically weak zones during the transition from an extrusive to an extensional–tensional tectonic environment. Early dikes from remelted and assimilated crust contributed to the two age ranges observed in the porphyry intrusions from the Xifanping deposit. The juvenile lower crust materials of the early magmatic arc were potential sources of the Cenozoic porphyry magmas, which has significant implications for mineral exploration and the geological understanding of porphyry Cu deposits in this region.

Tin-polymetallic mineralization and granitic rocks in the Nanling daping mining area: Geochemical and geochronological constraints

The Daping polymetallic deposit in the Nanling is a recently discovered deposit with significant potential in the Dayishan area. High-precision geochronological studies indicate that the emplacement ages of two fine-grained porphyritic (containing tourmaline) biotite granite units are 153.2 ± 0.9 Ma and 152.3 ± 1.0 Ma, representing products of the fifth magmatic activity in the Dayishan intrusion. The cassiterite U-Pb ages from the residual skarn type and greisen-type tin ores are 153.4 ± 3.6 Ma and 150.22 ± 0.37 Ma, respectively, and they are consistent within the error range, both belonging to the Yanshanian Early period. Petrogeochemical analyses of the granite reveal characteristics of high silica content (73.56 ∼ 75.91 %), alkali (K2O = 4.04 ∼ 5.58 %, Na2O = 2.22 ∼ 3.67 %), low calcium (0.42 ∼ 0.80 %), low magnesium (0.075 ∼ 0.29 %), and peraluminous nature (A/CNK = 1.26 ∼ 1.35). The rare earth element (REE) contents range from 35.78 × 10-6 to 111.57 × 10-6, with δEu values between 0.006 and 0.026, LREE/HREE ratios of 2.23 ∼ 3.41, and (La/Yb)N values of 1.20 ∼ 2.42. The granite is enriched in elements such as Rb, K, Th, U, Nd, Hf, and depleted in Ba, Sr, P, Nb, Ti, indicating strong crystal differentiation in the area. Sr, Nd, and Hf isotope studies show that the granite samples have ISr values of 0.67612–0.70040, εNd(t) values of −5.83 ∼ -8.23, εHf(t) values of −6.1 ∼ -1.9, (Nd)TDM2 values ranging from 1361 to 1573 Ma, and (Hf)TDM2 values ranging from 1147 to 1408 Ma. It is proposed that the Dayishan intrusion originated from the mixing of a mid-Paleoproterozoic mafic rock and a metamorphic sedimentary source region, with the latter being predominant. The detachment of source rocks from the mantle reservoir occurred in the mid-Paleoproterozoic. Combining lithological, petrogeochemical, and comparative analyses, it is classified as a highly fractionated A-type granite formed in a local extensional setting related to the subduction of the Pacific Plate. Additionally, this paper constructs a ore-forming model for the Daping tin deposit, aiming to provide reference for regional rare metal ore exploration.

Final closure of the Paleo-Tethys Ocean: Insights from Triassic granitoids in the central Qiangtang area, northern Tibetan Plateau

Abstract The geodynamic evolution during the closure of the Paleo-Tethys Ocean in the Tibetan Plateau remains to be fully understood. The Longmu Co–Shuanghu suture zone in the northern Tibetan Plateau has usually been considered to represent the main ocean basin of the Paleo-Tethys Ocean, so it plays a key role in understanding the evolution of the Paleo-Tethys Ocean. In this study, we focused on the Gacuo and Bensong batholiths on the north and south sides of the Longmu Co–Shuanghu suture zone, respectively. We conducted detailed zircon geochronology and whole-rock geochemical and Sr-Nd isotopic analyses, as well as zircon Hf isotope studies. Zircon U-Pb dating indicates that the Gacuo batholith was formed ca. 223–209 Ma, and the age of the Bensong batholith is ca. 213–203 Ma. The Gacuo batholith is mainly composed of I-type granitoids, which are most likely attributed to partial melting of ancient sedimentary materials of the North Qiangtang terrane with a mixture of ~0%–30% amounts of mantle-derived components. In contrast, the Bensong batholith has granitoids of A-type affinity, and it was probably generated by partial melting of Mesoproterozoic crust of the South Qiangtang terrane with limited mantle contribution (<5%). Finally, we suggest that the Gacuo batholith was probably generated by the break-off of the oceanic slab beneath the North Qiangtang terrane, while the Bensong batholith was related to a possible lithospheric delamination process of the South Qiangtang terrane after continental collision. Therefore, the Gacuo and Bensong batholiths both developed in a postcollisional tectonic setting, and they recorded the evolutionary process of the subduction and closure of the Paleo-Tethys Ocean during the Late Triassic.

Origin of Early Eocene diabase dikes in the southern Lhasa terrane: Implications for the Neo-Tethyan slab breakoff

The geodynamic evolution of the Himalayan–Tibetan orogen during the transition from Neo-Tethyan oceanic to Indian continental subduction remains unclear. Therefore, we conducted a new geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study of post-collisional diabase dikes in the central segment of the southern Lhasa subterrane. The diabase dike samples yield zircon U–Pb ages of 51–49 Myr, consistent with the timing of the Gangdese magmatic flare-up (ca. 53–50 Myr). In addition, the diabase dikes exhibit a geochemical intraplate affinity and depleted Sr–Nd–Pb isotopic compositions. Their variable Nb contents allow, the dikes to be divided into low-Nb (<8 ppm) and high-Nb (>9 ppm) types, which represent the melting products of slab fluid-modified Gangdese lithospheric mantle and a mixed mantle source comprising Gangdese lithospheric and deep asthenospheric mantle, respectively. In addition, the wide variation in εHf(t) values (−10.0 to +12.3) of zircons from these diabase dikes is consistent with the Gangdese zircon Hf isotopic ‘pull-down’ at ca. 50 Ma. Integrating our new data with published results suggests that the zircon Hf isotopic ‘pull-down’ was a direct consequence of the involvement of subducted Indian continental components in the formation of the Gangdese continental lithosphere. The combined data further suggest that upwelling of the asthenosphere, triggered by breakoff of the Neo-Tethyan oceanic slab, resulted in extensive mantle and crustal melting in the Lhasa terrane and the Tethyan Himalaya in southern Tibet.

Petrogenesis and Tectonic Implications of the Qulihai Pluton in the Northern Margin of the Yili Block, NW China

Abstract Despite extensive research work that has been done, whether the northern margin of the Yili Block (YB) is still an active continental margin during the early Carboniferous period is still in debate. Herein, we conducted zircon U–Pb geochronology, geochemistry, and zircon Lu–Hf isotope studies on the Qulihai pluton in the northern margin of the YB to constrain the petrogenesis and dynamic process. The Qulihai pluton is a granitoid complex that is composed of quartz diorite, quartz monzonite, and syenogranite. The zircon U–Pb dating results revealed that the pluton was formed and emplaced between 346 and 342 Ma. The three different Qulihai pluton rock types had comparable εHf(t) values, ranging from +3 to +8. The corresponding two-stage model ages of 817–1182 Ma indicated their potential derivation from the Meso-Neoproterozoic juvenile crust. The Qulihai pluton typically features medium-to-high SiO2, K2O, and Al2O3 contents and low MgO and Fe2O3T contents. The quartz diorite and quartz monzonite samples had Mg# values of 43–47, indicating the input of mantle-derived melts in the source area. In contrast, the source region of syenogranite was purely crustal material. The Qulihai pluton is mainly characterized as strong metaluminous moderate-to-high-K cal-alkaline rocks of the I-type granite series, which are enriched in large-ion lithophile elements and depleted in high field strength elements while exhibiting active continental margin island arc magmatism. Our findings, combined with the comprehensive analysis of previous studies, suggest that the Qulihai pluton formation resulted from the subduction of the North Tianshan oceanic crust beneath the YB during the early Carboniferous period, contemporary with the tectonic regime transition from subduction advance to subduction retreat.

Petrogenesis and Tectonic Setting of Early Cretaceous A-Type Granite from the Southern Great Xing’an Range, Northeastern China: Geochronological, Geochemical, and Hf Isotopic Evidence

The southern Great Xing’an Range is located in the eastern Central Asian Orogenic Belt, where voluminous igneous rocks developed during the Late Mesozoic period. The east slope of the southern Great Xing’an Range has been the topic of numerous debates on the level of influence of the Mongol-Okhotsk and the Paleo-Pacific regimes in the Late Mesozoic period. Therefore, this area is a suitable region in which to study the temporal changes in magma sources and tectono-magmatic evolution. In this paper, whole-rock geochemical data, zircon U-Pb geochronology, and zircon Hf isotope studies were carried out on the granitoids in the east slope area of the southern Great Xing’an Range. LA-ICP-MS zircon U-Pb dating revealed the ages of four granitoid samples: 135.0 ± 0.6 Ma, 130.7 ± 1.4 Ma, 130.4 ± 1.0 Ma, and 127.6 ± 0.8 Ma, respectively. The Hf isotope values 176Hf/177Hf = 0.282751–0.283015, εHf (t) = +2.0~+11.5, and T2DM = 583~1442 Ma suggest that the magma was generated by partial melting of Meso- and Neoproterozoic accreted and thickened low crust. The whole-rock geochemical data implied that these granitoids are A-type granite and their formation is closely linked to the subduction of the Paleo-Pacific Ocean plate. These geochemical, isotopic, and geochronological data suggest that the Early Cretaceous magmatism in the east slope area of the southern Great Xing’an Range formed in an extensional back-arc tectonic setting associated with the slab roll-back of the Paleo-Pacific plate subduction.

Shoshonitic pluton and an associated base-metal deposit in the Tarom–Hashtjin metallogenic zone, NW Iran: implication for tectono-magmatic evolution and metallogenic considerations

Petrological, geochemical and zircon U–Pb and Hf isotope studies were carried out on the Hajseyran pluton, along the Tarom–Hashtjin metallogenic zone, as part of the Alborz–Azerbaijan magmatic belt, which is located in the central part of the Alpine–Himalayan orogenic belt. The Hajseyran pluton, which is made up of syenite, monzonite, quartz-monzonite and granite, yields a zircon U–Pb age of 39.20 ± 0.56 Ma (for a quartz-monzonite sample) with shoshonitic features. Enrichment in large ion lithophile elements, depletion in high-field-strength elements and negative anomalies of Nb, Ta and Ti in Chomalou samples suggest a subduction-related arc magmatic setting. The Chomalou epithermal Pb–Zn–Cu deposit occurs close to the Hajseyran pluton with the quartz–base metal sulfide veins predominantly hosted by Eocene volcanic rocks. The ore-forming processes include chalcopyrite and sphalerite–galena formation, with the latter the major mineralisation in this region. Microthermometric results from the liquid–vapour fluid-inclusion assemblages show that the homogenisation temperatures in the first-stage quartz range from 230 to 310 °C, in the sphalerite and second-stage quartz from 161 to 293 °C and in the late-stage quartz from 175 to 262 °C. The salinities of fluids decrease from early-stage quartz to late-stage quartz. The calculated stable isotope values of ore–fluid components (δ18Owater = +3.28‰ to −3.68‰; δ34SH2S = +5.5‰ to +7.8‰) show that magmatic fluids have played a significant role during mineralisation. The Chomalou deposit is interpreted as a volcanic-hosted intermediate-sulfidation type of epithermal mineralisation formed through shallow-level emplacement of the Hajseyran pluton, which formed via partial melting of phlogopite–amphibole-bearing sub-continental lithospheric mantle. The Neotethyan slab roll-back and concomitant asthenospheric upwelling led to the decompression melting of metasomatised sub-continental lithospheric mantle and the formation of several K-rich plutons and associated epithermal deposits in the Alborz–Azerbaijan magmatic belt.

Isotopic and geochemical characteristics of mid- to-late-Miocene continental and oceanic arc volcanic and intrusive rocks, central North Island and offshore, New Zealand

Abstract Mid- to late-Miocene continental arc volcanism on the North Island of New Zealand is found in the Coromandel Peninsula, the Kiwitahi volcanic chain and the Taranaki Basin (Kora Volcano) offshore the western margin of the North Island. Coeval oceanic arc volcanism is also found along the offshore Colville Ridge/Kermadec Ridge north of New Zealand. This Pb–Sr–Nd–Hf isotopic study aims to evaluate mantle sources and potential crustal contaminants along these sections of the Miocene arc system. The Colville/Kermadec Ridge and Kora lavas have the lowest Sr (0.7029–0.7045) and highest Nd (0.51305–0.51292) ratios; the Coromandel and Kiwitahi lavas overlap (Sr = 0.704–0.706; Nd = 0.51268–0.51296). The Colville/Kermadec Ridge, Kora, and Coromandel/Kiwitahi rocks form three distinct arrays on Pb–Pb plots, all above the Northern Hemisphere Reference Line, but none trend towards local Mesozoic basement greywackes. Isotopic and trace element ratio variations suggest that subducted sediments are a component in Coromandel/Kiwitahi mafic lava sources. The younger, southern Kiwitahi lavas have a more depleted mantle source than that for the older, northern Kiwitahi chain. Evolved lavas commonly have interacted with Waipapa basement rocks. Kora rocks have compositions similar to those of back-arc lavas and have been emplaced as sills in a rift environment above a distinct subduction-modified mantle.

Formation of columnar/tabular and vermicular kaolinite in the crystal tonsteins within the Late Carboniferous Coal in the Datong Coalfield, Shanxi Province, North China

The Datong Coalfield, Shanxi Province, North China, is rich in high-grade coal-measure kaolin, primarily occurring as partings within the Late Carboniferous–Early Permian coal seams. There is a type of kaolinite-dominant coarse-grained parting similar to sandstone in appearance, whereas the phanerocrystalline minerals are columnar/tabular and vermicular kaolinite rather than terrigenous detrital minerals. The origins of the coarse-grained partings and the formation mechanism of the columnar/tabular and vermicular kaolinite remain ambiguous. Herein, petrographical, mineralogical, whole-rock geochemical, and zircon U–Pb–Hf isotopic studies of the coarse-grained partings were conducted to explore these issues. The zircons in the coarse-grained partings show typical features of magmatic origin and yield a single age group of very latest Carboniferous at ca. 300 Ma, indicating they were transformed from air-fall volcanic ash. Therefore, the coarse-grained partings are essentially crystal tonsteins. The wide range of negative εHf(t) values and arc-related tectonic affinity of the zircons genetically link the primary volcanic ash to the Late Paleozoic continental arc volcanoes that existed in the Inner Mongolia Paleo-Uplift, related to the southward subduction of the Paleo-Asian oceanic plate beneath the North China Block. Two orthogonal cleavages are generally observed on the columnar/tabular and vermicular kaolinite in the crystal tonsteins, contradicting the crystalline structure of kaolinite that belongs to a 1:1 type phyllosilicate with only one cleavage along its (001) lattice plane. These columnar/tabular and vermicular kaolinite are considered to be pseudomorphic kaolinite after plagioclase in the crystal-rich volcanic ash. When the crystal-rich volcanic ash fell into open and acidic peat-forming environments (mires), the low-pH water containing abundant organic acids and good water circulation conditions facilitate dissolution of plagioclase along the chemically unstable (001) lattice planes and leaching out of Na+, Ca2+, and much of silica. Simultaneously, the Al3+ or hydroxides of aluminum and silica recrystallized along the (001) lattice planes of plagioclase as the substrate, constraining layer-by-layer growth of large kaolinite platelets to eventually form pseudomorphic kaolinite. The cleavages perpendicular to the exclusive (001) cleavage of pseudomorphic kaolinite should be inherited from the twin planes and (010) cleavage cracks (if present) of plagioclase.

Petrogenesis of the ~1.94 Ga Meta‐gabbronorites in Liangcheng: Implications for Tectonic Evolution of the Khondalite Belt, North China Craton

AbstractWe investigated the meta‐gabbronorites in Liangcheng and used detailed petrography, geochemistry, zircon geochronological and in‐situ Hf isotopic studies to clarify their formation and metamorphic ages, petrogenesis, tectonic setting and provide constraints on the tectonic evolution of Khondalite Belt (KB). The zircon U‐Pb dating results show that the meta‐gabbronorites crystallized at ~1.94 Ga and were metamorphosed at ~1.91–1.89 Ga. They can be subdivided into the low‐Mg and high‐Mg groups. The low‐Mg meta‐gabbronorites contain relatively lower MgO and higher SiO2 contents than high‐Mg meta‐gabbronorites. They are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and exhibit positive (high‐Mg meta‐gabbronorites) and negative (low‐Mg meta‐gabbronorites) Sr and Eu anomalies. The zircon in‐situ εHf(t) of meta‐gabbronorites is 0.07–4.12, with Hf model ages (TDM) of 2169–2400 Ma. The meta‐gabbronorites in Liangcheng originated from the asthenospheric mantle and experienced fractional crystallization of olivine, orthopyroxene, clinopyroxene, and plagioclase. They were contaminated by the crustal rocks (mainly khondalite series) during ascent, especially for low‐Mg gabbronorites. The ridge subduction is the most plausible tectonic setting for meta‐gabbronorites, indicating the eastern segment of KB was in a ridge subduction setting at ~1.94 Ga following an orogenic thickening event during a prolonged orogenic process.

Age and petrogenesis of mafic granulites from central Madurai block, south India: implications on regional tectonics

AbstractThe Precambrian Southern Granulite Terrane (SGT) of south India is well-known for the preservation of high- to ultrahigh-temperature (HT-UHT) granulites, prominently exposed in its central part forming a linear belt referred to as the Kambam UHT belt. This belt also hosts widespread occurrences of mafic granulites that are observed in close spatial association with the HT-UHT granulites. This study presents detailed petrology, geochemistry and geochronology of representative mafic granulites from the area to understand their petrogenesis and tectonic setting. The results demonstrate that mafic granulites are low- to medium-K tholeiites, with continental arc affinity, formed by the partial melting of a subduction-modified enriched mantle source. The composition of the parent mantle source is modelled with a spinel/garnet lherzolite contribution ratio between 100/0 and 70/30, suggesting the mixing of spinel and garnet bearing melts during asthenosphere upwelling. Zircon U–Pb geochronology of mafic granulites constrains their emplacement between 612 Ma and 625 Ma, that subsequently underwent metamorphism between 581 Ma and 531 Ma. This overlaps with the timing of HT-UHT metamorphism in the Kambam UHT belt bracketed between 593 and 532 Ma. Zircon Hf isotopic studies reveal parent magma generation from reworked melting sources involving Archean and Proterozoic components. These results propose an alternative heat source for the formation of HT-UHT granulites in the Kambam UHT belt which can be designated as a major terrane boundary within the SGT.

Tectonic evolution of circum-Rodinia subduction: Evidence from Neoproterozoic A-type granitic magmatism in the Central Tianshan Block, northwest China

Neoproterozoic igneous rocks associated with the assembly and configuration of the Rodinia supercontinent are widely distributed in the Central Tianshan Block, southwestern Central Asian Orogenic Belt (CAOB), northwest China. The Central Tianshan Block is an important geological unit in tectonic reconstructions of the CAOB and Rodinia. However, the early Neoproterozoic tectonic framework and evolution of the Central Tianshan Block are poorly understood. Here we present a geochronological, mineral and whole-rock geochemical, and Nd–Hf isotopic study of early Neoproterozoic augen granites from the Bingdaban area. In situ zircon U–Pb dating shows that the augen granites have crystallization ages of 1002–992 Ma. The augen granites have high SiO2 (70.46–76.24 wt%) and K2O (3.36–4.96 wt%) contents, and FeOT/(FeOT + MgO) ratios (0.76–0.84), low Mg# values (29–39), slightly enriched light rare earth element patterns, enriched large-ion lithophile elements, depleted high-field-strength elements (e.g., Nb, Ta, and Ti), and high 10,000 × Ga/Al ratios (2.3–8.3) and Zr + Nb + Ce + Y contents (256–408 ppm). The augen granites have geochemical features typical of A2-type granites, variable εNd(t) values of –1.64 to +6.87 and Nd model ages of 1.78–1.08 Ga, and zircon εHf(t) values of −1.96 to +6.42 with Hf model ages of 1.98–1.48 Ga. These data indicate the granites had a juvenile crustal-dominated source, with subordinate input of ancient crustal components, and formed in a subduction-related extensional setting. The extension might have been related to slab break-off beneath Rodinia. The early Neoproterozoic igneous rocks in the Central Tianshan and adjacent microcontinental blocks in the southwestern CAOB formed at or near an active continental margin, possibly part of the circum-Rodinian subduction zone during or after the assembly of Rodinia. Similar Neoproterozoic magmatism in the Central Tianshan and Yili blocks suggests that they had a comparable crustal evolution and a close tectonic and paleogeographic relationship during the Neoproterozoic. However, significant age and isotopic differences characterize the Central Tianshan–Yili blocks and Tarim Craton, suggesting that the Central Tianshan–Yili blocks were not separated from the Tarim Craton during the early Neoproterozoic.

Detrital zircons from high-pressure trench sediments (Qilian Orogen): Constraints on continental-arc accretion, subduction initiation and polarity of the Proto-Tethys Ocean

The Qilian Orogen in the northern Tibetan plateau records the tectonic history of the Proto-Tethys Ocean from its initial spreading, subduction to final closure. However, the timing of subduction initiation, subduction polarity and arc volcanic processes of the Proto-Tethys Ocean remain controversial. In this contribution, we present geochronological and Hf isotopic studies for 812 detrital zircons from blueschist- to eclogite-facies metamorphic sedimentary rocks from the trench and accretionary wedge in the Proto-Tethys subduction zone in the North Qilian suture zone. Out of all the U-Pb age data, the Neoproterozoic to Early Paleozoic zircons reveal the timing of opening and closure of the Proto-Tethys Ocean. We show that ∼50 % of the original sediments were derived from the arc-related magmas, with ages ranging from 535 Ma to 441 Ma. This range represents the time from subduction initiation to closure of the Proto-Tethys Ocean in the Qilian Orogen. Based on the closer resemblance of statistical ages to the crust to the north (Alxa) than the south (Central Qilian), we believe the initial subduction polarity of the Proto-Tethys Ocean is northward. The corresponding εHf(t) values (−11 ∼ +15) shows characteristics of continental arc formed by subduction process. The massive intrusion of arc magma during subduction led to a significant crustal growth, and the crustal thickness estimates based on Eu anomalies indicate that the thickness of the continental margin varies from ∼60 km at 500–490 Ma to ∼45 km at ca. 450 Ma, suggesting an evolution process from compression to back-arc extension.