Hf-O Isotopes Research Articles

The Chemical Evolution from Older (323–318 Ma) towards Younger Highly Evolved Tin Granites (315–314 Ma)—Sources and Metal Enrichment in Variscan Granites of the Western Erzgebirge (Central European Variscides, Germany)

The sources and critical enrichment processes for granite related tin ores are still not well understood. The Erzgebirge represents one of the classical regions for tin mineralization. We investigated the four largest plutons from the Western Erzgebirge (Germany) for the geochemistry of bulk rocks and autocrystic zircons and relate this information to their intrusion ages. The source rocks of the Variscan granites were identified as high-grade metamorphic rocks based on the comparison of Hf-O isotope data on zircons, the abundance of xenocrystic zircon ages as well as Nd and Hf model ages. Among these rocks, restite is the most likely candidate for later Variscan melts. Based on the evolution with time, we could reconstruct enrichment factors for tin and tungsten starting from the protoliths (575 Ma) that were later converted to high-grade metamorphic rocks (340 Ma) and served as sources for the older biotite granites (323–318 Ma) and the tin granites (315–314 Ma). This evolution involved a continuous enrichment of both tin and tungsten with an enrichment factor of ~15 for tin and ~7 for tungsten compared to the upper continental crust (UCC). Ore level concentrations (>10–100 times enrichment) were achieved only in the greisen bodies and dykes by subsequent hydrothermal processes.

Syn-exhumation magmatism during continental collision: Geochemical evidence from the early Paleozoic Fushui mafic rocks in the Qinling orogen, Central China

Mafic arc-like magmatism is rare during continental collision, but its products provide valuable information on the geochemical processes during plate convergence. Such rocks were found in the Fushui complex in the Qinling orogen, central China. In order to decipher their petrogenesis, we have carried an integrated study of geochronology and geochemistry. Zircon SIMS UPb dating yields consistent ages of 488 ± 4 to 490 ± 4 Ma for emplacement of mafic magmas. The ages are just slightly later than zircon UPb ages of 500–490 Ma for ultrahigh-pressure metamorphism of the continental crust in the North Qinling unit. Thus, the target rocks are the products of syn-exhumation magmatism during continental collision. These mafic igneous rocks exhibit arc-like trace-element patterns, such as enrichment in large ion lithophile elements (LILE), Pb, and light rare earth elements (LREE), and depletion in high field strength elements (HFSE). They show enriched whole-rock SrNd isotope compositions and variable zircon HfO isotope compositions, indicating involvement of the recycled oceanic basalt and terrigenous sediment in their mantle source. Such geochemical features suggest that the Fushui mafic magmas would originate from an enriched and fertile mantle source that was generated through the metasomatic reaction between the mantle wedge peridotite and the subducting Shangdan oceanic crust-derived liquid phase. With the tectonic transition from the lithospheric subduction for active compression to active extension for the crustal exhumation during the late Cambrian, the mantle source underwent decompression melting to produce mafic melts in synchronous with exhumation of the deeply subducted continental crust. Therefore, the syn-exhumation Fushui complex provides new geochemical insights into the nature of the orogenic lithospheric mantle and thus the tectonic evolution of this collisional orogen.

Geochemical evidence for the production of granitoids through reworking of the juvenile mafic arc crust in the Gangdese orogen, southern Tibet

Abstract Although continental crust is characterized by the widespread occurrence of granitoids, the causal relationship between continental crust growth and granitic magmatism still remains enigmatic. While fractional crystallization of basaltic magmas (with or without crustal contamination) and partial melting of mafic lower crust are two feasible mechanisms for the production of granitoids in continental arc regions, the problem has been encountered in discriminating between the two mechanisms by whole-rock geochemistry. This can be resolved by an integrated study of zircon U-Pb ages and Hf-O isotopes together with whole-rock major-trace elements and Sr-Nd-Pb isotopes, which is illustrated for Mesozoic granitoids from the Gangdese orogen in southern Tibet. The results provide geochemical evidence for prompt reworking of the juvenile mafic arc crust in the newly accreted continental margin. The target granitoids exhibit high contents of SiO2 (65.76–70.75 wt%) and Na2O + K2O (6.38–8.15 wt%) but low contents of MgO (0.19–0.98 wt%), Fe2O3 (0.88–3.13 wt%), CaO (2.00–3.82 wt%), Ni (<5.8 ppm), and Cr (≤10 ppm). They are enriched in large ion lithophile elements, Pb, and light rare earth elements but depleted in high field strength elements. The granitoids are relatively depleted in whole-rock Sr-Nd isotope compositions with low (87Sr/86Sr)i ratios of 0.7043–0.7048 and positive εNd(t) values of 0.5–2.6, and have relatively low 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Laser ablation–inductively coupled plasma–mass spectrometry and secondary ion mass spectrometry U-Pb dating on synmagmatic zircons yield ages of 77 ± 2–81 ± 1 Ma in the Late Cretaceous for their emplacement. Relict zircons have two groups of U-Pb ages in the late Mesozoic and the late Paleozoic, respectively. The whole-rock Sr-Nd isotopes in the granitoids are quite similar to those of Late Cretaceous mafic rocks in the Gangdese batholith. In addition, both synmagmatic zircons and relict zircons with Late Cretaceous U-Pb ages exhibit almost the same Hf-O isotope compositions to those of the slightly earlier mafic rocks. All these observations indicate that the granitoids were mainly derived from partial melting of the juvenile mafic arc crust. Therefore, reworking of the juvenile mafic arc crust is the mechanism for the origin of isotopically depleted granitoids in southern Tibet. It is this process that leads to differentiation of the juvenile mafic arc crust toward the felsic lithology in the continental arc. In this regard, the granitoids with depleted radiogenic isotope compositions do not necessarily contribute to the crustal growth at convergent plate boundaries.

Subduction polarity of the Ailaoshan Ocean (eastern Paleotethys): Constraints from detrital zircon U-Pb and Hf-O isotopes for the Longtan Formation

Abstract The Paleotethys Ailaoshan Ocean separated the South China and Indochina blocks during the late Paleozoic. Uncertainty remains regarding subduction of this ocean—whether it was subducted eastward beneath the South China block or westward beneath the Indochina block. In this study, we present new detrital zircon U-Pb age, and Hf and O isotope data from the Longtan Formation, which was recognized to be deposited before the ocean closed. Our results show that the formation can be divided into three units: Unit 1 is distributed west of the suture and dominates the area; it contains major age peaks at 290–250 Ma and minor multiple old age peaks. Unit 2 consists of a minor distribution west of the suture, and it shows a dominant 250 Ma age peak; old zircons are very few or not present. Their Hf and O isotopic signatures are similar to those of unit 1. Unit 3 is distributed east of the suture and is characterized by a single distinct ca. 240 Ma age peak with almost no Precambrian zircons. We interpret that units 1 and 2 were likely deposited in a back-arc and forearc basin, respectively, and a volcanic arc developed on the eastern margin of the Indochina block, similar to the present-day northeastern Japan arc. Meanwhile, unit 3 was likely deposited in a forearc basin on the western margin of the South China block. Therefore, the Ailaoshan Ocean may undergone bipolar subduction both westward and eastward beneath the Indochina and South China blocks, respectively.

A Yilgarn seed to the Pilbara Craton (Australia)? Evidence from inherited zircons

Abstract Knowledge of the age and compositional architecture of Archean cratonic lithosphere is critical for models of geodynamics and continental growth on early Earth, but can be difficult to unravel from the exposed geology. We report the occurrence of numerous >3.7 Ga zircon crystals in 3.45 Ga rhyolites of the eastern Pilbara Craton (Western Australia), which preserve evidence for an Eoarchean meta-igneous component in the deep Pilbara crust. This inherited zircon population shares similar and distinctive age and Hf-O isotope characteristics with the oldest gneissic components of the Yilgarn Craton ∼500 km farther south, suggesting a common ca. 3.75 Ga felsic crustal nucleus to these two Archean granite-greenstone terranes. We infer a pivotal role for such ‘seeds’ in facilitating the growth and persistence of Archean continental lithosphere.

Eoarchean to Paleoproterozoic crustal evolution in the North China Craton: Evidence from U-Pb and Hf-O isotopes of zircons from deep-crustal xenoliths

Granulite-facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age, origin and history of the lower continental crust. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from the North China craton. The geochronological data demonstrates that the oldest lower crustal remnants formed at ∼3.82 Ga and related magmatism continued until ∼3.55 Ga. The Eo-Paleoarchean zircons, with exception of one analysis, have subchondritic initial Hf isotopes with negative εHf(t) and are characterized by normal mantle to elevated δ18O values (5.37–6.90‰), indicating a development of a low Lu/Hf reservoir and hydrosphere-crust interactions in the early Earth. Magmatic zircons from lower crustal xenoliths of North China define a strongly episodic distribution of ages at 3.82–3.55 Ga, ∼2.7 Ga, ∼2.5 Ga and 1.95–1.85 Ga and a non-linear Hf isotope-age array for nearly 2 Gyr, indicating episodic crustal generation and reworking through time. The lower crustal xenoliths record a change in zircon O isotope compositions from a restricted range at 3.8–2.6 Ga to more variable after ∼2.5 Ga. Both heavier (δ18O up to 10.25‰) and lighter (δ18O low to 2.16‰) O isotope compositions than the normal mantle are observed in ∼2.5 Ga magmatic zircons. The secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials (including high-δ18O sediments, weathered and altered rocks and low-δ18O altered oceanic crust) into magma sources at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time.

Geochemistry, Zircon U-Pb Geochronology and Hf-O Isotopes of the Banzhusi Granite Porphyry from the Xiong’ershan Area, East Qinling Orogen, China: Implications for Petrogenesis and Geodynamics

The Banzhusi granite porphyry is located in the Xiong’ershan area, East Qinling orogenic belt (EQOB). This study presents an integrated whole-rock geochemistry and zircon U-Pb-Hf-O isotope analysis of the Banzhusi granite porphyry. These rocks have metaluminous, high-K alkali-calcic and shoshonitic features and show significant enrichment in light rare earth elements (LREEs) over heavy rare earth elements (HREEs) with negative Eu anomalies. These samples are also greatly enriched in Rb, Ba, K, Pb, Th and U and depleted in Nb, Ta, P and Ti, and they mostly overlap the ranges of the Taihua Group tonalite–trondhjemite–granodiorite (TTG) gneiss. Magmatic zircons from three samples of the Banzhusi granite porphyry yield U-Pb ages of 125.1 ± 0.97 Ma, 128.1 ± 1.2 Ma and 128.2 ± 1.3 Ma. The Hf-O isotope features of zircons from the three samples are very similar (δ18Ozircon = 4.84‰ to 6.51‰, εHf(t) = −26.9 to −14.4). The co-variations of geochemical and isotopic data in these granite porphyries imply that the Banzhusi granite porphyry resulted from the mixing of the partially melted Taihua Group and mantle-derived material in a post-collisional setting from 128–125 Ma.

Zircon U-Pb age and Hf-O isotope insights into genesis of Permian Tarim felsic rocks, NW China: Implications for crustal melting in response to a mantle plume

Deciphering the contribution of crustal materials to A-type granites is critical to understanding their petrogenesis. Abundant alkaline syenitic and granitic intrusions distributed in Tarim Large Igneous Province, NW China, offer a good opportunity to address relevant issues. This paper presents new zircon Hf-O isotopic data and U-Pb dates on these intrusions, together with whole-rock geochemical compositions, to constrain crustal melting processes associated with a mantle plume. The ∼280 Ma Xiaohaizi quartz syenite porphyry and syenite exhibit identical zircon δ18O values of 4.40 ± 0.34‰ (2σ) and 4.48 ± 0.28‰ (2σ), respectively, corresponding to whole-rock δ18O values of 5.6‰ and 6.0‰, respectively. These values are similar to mantle value and suggest an origin of closed-system fractional crystallization from Tarim plume-derived melts. In contrast, the ∼275 Ma Halajun A-type granites have higher δ18O values (8.82–9.26‰) than the mantle. Together with their whole-rock εNd(t) (−2.0–+0.6) and zircon εHf(t) (−0.6–+1.5) values, they were derived from mixing between crust- and mantle-derived melts. These felsic rocks thus record crustal melting above the Tarim mantle plume. At ∼280–275 Ma, melts derived from decompression melting of Tarim mantle plume were emplaced into the crust, where fractional crystallization of a common parental magma generated mafic-ultramafic complex, syenite, and quartz syenite porphyry as exemplified in the Xiaohaizi region. Meanwhile, partial melting of upper crustal materials would occur in response to basaltic magma underplating. The resultant partial melts mixed with Tarim plume-derived basaltic magmas coupled with fractional crystallization led to formation of the Halajun A-type granites.

Lower crustal contribution to the magma formation of the Damiao massif-type anorthosite, North China Craton: Evidence from zircon Hf-O isotopes

The formation of massif-type anorthosites and the associated mangerite, charnockite and (rapakivi-) granite (AMCG suite) are mostly restricted in the “Earth’s Middle Age” (~1.70–0.75 Ga), and the magma sources of massif-type anorthosites are highly debated between the depleted mantle and the mafic lower crust. The ~1.7 Ga Damiao massif-type anorthosite in the North China Craton consists mainly of anorthosite and leuconorite with minor melanorite, mangerite, Fe-Ti-(P) ores and abundant ferrodioritic or gabbroic dykes. In-situ zircon Hf-O isotopic data from anorthosite reveal decoupling of mantle-like δ18O values (5.1–7.4) and ancient crustal-like εHf(T) values (−4.0 to −8.9). Since the significant 177Lu/176Hf reduction in the lower mafic crust, the mafic lower crustal materials derived from depleted mantle could display an ancient crustal medium to high negative εHf(T) values after long period of time (e.g., 0.5–1.0 Ga). In contrast, if no supracrustal materials were added, these mafic crustal materials would retain the mantle δ18O features. Thus, the decoupling of mantle-like δ18O and ancient crustal-like εHf(T) values suggest a lower crustal origin of the Damiao massif-type anorthosite. Compilation of Sr-Nd-O isotopic data of massif-type anorthosites reveal decoupling of εHf(T), initial 87Sr/86Sr and δ18O values, which signifies important lower crustal input in magmas sources of most massif-type anorthosites. We proposed that zircon U-Pb age of the Damiao massif-type anorthosite (~1.70 Ga) and regional age correlation suggest that the anorthosite was emplaced in an intraplate, anorogenic extensional setting after the Paleoproterozoic orogeny had ended.

Geochemistry, in-situ Sr-Nd-Hf-O isotopes, and mineralogical constraints on origin and magmatic-hydrothermal evolution of the Yulong porphyry Cu[sbnd]Mo deposit, Eastern Tibet

The giant Yulong porphyry CuMo deposit was formed in postsubduction setting in eastern Tibet. Origin of the ore-related Yulong intrusion remains a matter of debate. This study presents new whole-rock major and trace element geochemistry, in-situ apatite SrNd and zircon HfO isotopes, and mineralogical chemistry of the Yulong intrusion. Least-altered samples from the Yulong intrusion have high SiO2 (66.3–69.5 wt%) and Al2O3 (14.9–15.5 wt%) contents, high La/Yb (36.4–68.0) and Sr/Y (46.0–76.3) ratios, and low MgO (0.63–1.24 wt%) and Cr (<30 ppm) contents, similar to adakitic rocks deriving from thick juvenile lower crust. They are enriched in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and show listric REE patterns. In-situ apatite SrNd isotopes show limited variations ((87Sr/86Sr)i = 0.7060–0.7068, εNd(t) = −4.8–0.2), which plot between Paleo-Tethys ocean-related arc magmas and the ancient crust in eastern Tibet. Zircon grains from this study and published studies have mostly positive yet variable εHf(t) values (−20.6 to +12.2) and young Hf model ages that overlap those of the Paleo-Tethys ocean-related arc magmas. The above Sr-Nd-Hf isotopes, together with the elevated zircon δ18O values (6.4 to 9.3‰) and arc-like trace element patterns, collectively suggest that the Yulong intrusion may have originated from partial melting of juvenile lower arc crust related to the subduction of the Paleo-Tethys ocean, with incorporation of a small amount of ancient crustal materials.Two generations of amphibole were recognized at Yulong. Their compositions are used to calculate crystallization depths, magmatic oxidation states, and water contents. The caculated results show that the early-stage euhedral high-Al (5.87–8.51 wt%) amphibole phenocrysts may have crystallized in the underlying magma chamber (7.1–12.5 km in depth), whereas the late-stage xenomorphic low-Al (3.47–4.87 wt%) amphibole grains may have crystallized in the porphyritic stock (4.0–5.6 km). Magmatic water contents decrease from early- (3.5–4.6 wt%) to late-stage (2.8–3.5 wt%) amphibole, which is interpreted to indicate fluid exsolution from the magma chamber during emplacement of the Yulong intrusion. Caculated oxidation states increase from early- (ΔNNO = 0.6–1.5) to late-stage (ΔNNO = 1.9–2.3) amphibole. Plagioclase phenocrysts show periodic or reverse core-to-rim zonation of An contents (range up to 25 mol%), which are coupled by FeO contents, probably suggesting magma recharge events. Collectively, we propose that the magma chamber beneath Yulong was recharged by a less evolved magma, and was saturated in fluids to produce intensive alteration and mineralization. The relatively high oxidation states allow the metals to be enriched in the evolving magma, and to be deposited in the hydrothermal alteration stage.

Neoarchean growth and Paleoproterozoic metamorphism of an Archean ophiolite mélange in the North China Craton

The North China Craton (NCC) preserves important records of Neoarchean crustal growth and Paleoproterozoic metamorphism. Here we investigate a suite of metamorphic rocks belonging to the Hongqiyingzi Group from the northern part of the Trans-North China Orogen (TNCO). These rocks are dominantly composed of amphibole plagioclase gneisses with minor retrograded eclogites and serpentinited peridotites. We employ zircon U-Pb geochronology and Hf-O isotopes on the amphibole plagioclase gneisses to unravel the formation age and metamorphic history. The magmatic zircon cores yield upper intercept ages of 2664 Ma and 2518 Ma, suggesting that the protoliths of the Hongqiyingzi gneisses formed in the Neoarchean. The metamorphic rims with lower U and Th concentration yield upper intercept ages of 1840 Ma and 1847 Ma, constraining the timing of metamorphism as ca. 1.85 Ga. The εHf(t) values of the magmatic zircon grains are dominantly positive, whereas those of the metamorphic grains are negative. Both magmatic and metamorphic zircon grains display a restricted range of δ18O (6.11–7.44‰), slightly higher than the normal mantle value. These isotopic features indicate that the younger metamorphic event did not modify the Hf-O isotopic compositions. The P-T computations yield temperatures of 700–760 °C and pressures to be 5–7 Kbar, suggesting the rocks have been retrograded to amphibolites facies. The retrograded eclogites and serpentinized peridotites of the Hongqiyingzi Group, exposed as tectonic lenses within the amphibole plagioclase gneisses, are interpreted as parts of an ancient oceanic lithosphere, whereas the amphibole plagioclase gneisses represent meta-volcano-sedimentary units. We thus interpret that the Hongqiyingzi Group represents a tectonically dismembered Neoarchean ophiolitic mélange that formed on the continental margin and metamorphosed to eclogite facies during deep subduction in Paleoproterozoic and exhumed during Phanerozoic.

Zircon from Orogenic Peridotite: An Ideal Indicator for Mantle-Crust Interaction in Subduction Zones

The orogenic peridotites can be subdivided into crust- and mantle-derived types. They record complex geological processes in subduction and collision zones. The crust-derived peridotites are derived from cumulates crystallized from ultramafic-mafic magmas in deep continental crust, an early mantle-crust interaction, prior to subduction. The mantle-derived orogenic peridotites are originated from subcontinental lithospheric mantle (SCLM) wedge and other mantle domains, and are later involved in the subduction channel and orogenic system. The mantle-derived peridotites usually record complex metasomatism, ultra-high pressure (UHP) metamorphism and mantle-crust interaction during the orogenic processes. Zircons are rarely found in orogenic peridotites. These zircons in orogenic peridotites are generally formed during metasomatism, they can be divided into old zircons (mainly the cores of residual magmatic and recrystallized) and newly grown zircons. Three key factors for zircon formation in orogenic peridotites are that: (1) zircon has strong crystallization ability, and Zr is easier to exchange Si in zircon crystal structure than other elements in the mantle; (2) metamorphic destruction of Zr-bearing minerals and precipitation of intergranular melts during the high-grade metamorphism can nucleate zircon under sub-solidus conditions; (3) the melts/fluids released from the subducted crust can metasomatize the mantle wedge to form zircons. In-situ studies on zircons and zircon inclusions enclosed in mantle minerals indicate that zircon can be an ideal indicator for mantle- crust interaction in subduction zones. The inclusions in zircons and Hf-O isotope of zircons are effective to reflect the composition of the melts/fluids, source properties, and the physical and chemical conditions. Dating of the zircons has been widely used in the studies of lithospheric evolution and crust-mantle interaction. During the complex processes of plate convergence, the orogenic peridotites can be subjected to the melt/fluid metasomatism, modifying the mineral and elemental compositions of peridotites. Thus, zircon is very useful to unravel the history of specific lithospheric mantle and the relationship between continental cratonic cores and their margins.

Crystal fractionation of contaminated melts and re-melting of newly underplated basaltic crust generated Late Triassic andesitic and dioritic intrusions in the southern Yidun Terrane, SW China

The genesis of andesitic and dioritic rocks in continental arcs is hotly debated, and proposed models include slab melting, juvenile mafic lower-crustal melting, mafic and felsic magma mixing, and mantle-derived melts that underwent a MASH (melting, assimilation, storage, and homogenization) process. The controversy comes from the possibility of their polygenesis and the ambiguity of bulk-rock isotope tracers, not to mention the processes of magma evolution. Here, we report on our investigations in the southern Yidun Terrane (a Triassic continental arc in SW China) into the petrogenesis of two stages of the dioritic Xiuwacu intrusions (phases 1 and 2), the mafic microgranular enclaves (MMEs) hosted by the phase 1 intrusion, as well as the andesitic Hongshan intrusions. We used U-Pb zircon dating, in situ zircon Hf-O isotope analyses, bulk-rock Sr-Nd isotope analyses, and trace element analyses of the zircons from different stages of the intrusions and the MMEs to shed light on their sources and mantle–crust interactions involved in their petrogenesis. Our results suggest that the Xiuwacu MMEs from the phase 1 intrusion crystallized from a melt. Their variable zircon Hf-O isotopic compositions (εHf(t) = +0.4 to +5.3 and δ18O = 4.80‰ to 7.79‰) indicate they were formed by the mixing of mantle-derived basaltic melts and felsic melts. The phase 1 intrusion have an origin similar to that of the MMEs, as indicated by their homogeneous zircon Hf-O isotopic compositions (εHf(t) = +1.3 to +3.9 and δ18O = 6.58‰ to 7.31‰), which fall within the range of the isotopic compositions of the MMEs. The discovery of several inherited zircons and a small recrystallized felsic microgranular enclave hosted by an MME reveals that the MMEs contain relatively more felsic components. Even so, the MMEs still have lower silica contents (62.1–63.4 wt%) than those of the phase 1 intrusion (63.4–70.5 wt%), and this indicates that direct magma mixing of basaltic and felsic melts generated the andesitic MMEs, whereas a crystal fractionation process is required for the dioritic phase 1 intrusion. The porphyritic phase 2 and Hongshan intrusions formed at ~216 Ma, after the phase 1 intrusion, and they had higher oxygen fugacities and crystallization temperatures than the phase 1 intrusion. Given the conclusions of previous researchers that slab breakoff probably occurred at ~216 Ma, we propose that the Xiuwacu phase 2 and Hongshan intrusions formed during the slab breakoff process, the former by partial melting of the newly underplated basaltic lower crust in the MASH zone, and the latter by differentiation of mantle-derived basaltic melts at relatively high pressures.

Amalgamation of South China into Rodinia during the Grenvillian accretionary orogeny: Geochemical evidence from Early Neoproterozoic igneous rocks in the northern margin of the South China Block

In order to gain insights into the amalgamation process of South China into Rodinia, a combined study of whole-rock geochemistry, zircon U-Pb ages and Hf-O isotopes was carried out for igneous rocks from the Mianlue ophiolite in the northern margin of the South China Block. In-situ zircon U-Pb dating yields 206Pb/238U ages of 905 ± 8 Ma for plagiogranite, 936 ± 6 Ma for andesite and 950 ± 5 Ma for gabbro-diorite. The plagiogranite is characterized by high Na2O contents of 6.6–7.0 wt%, low TiO2 contents of 0.2–0.4 wt%, and enrichment in LREE and LILE but depletion in Nb, Ta and Ti. The zircons have positive εHf(t)values of 8.9 to 14.9 and slightly elevated δ18Ovalues of 4.6 to 6.8‰. The whole-rock yields low (87Sr/86Sr)i ratios of 0.7016–0.7031, positive εNd(t) values of 5.9 to 6.9 and positive εHf(t) values of 11.1 to 11.2. This indicates that the plagiogranite was produced by partial melting of juvenile oceanic crust that experienced seawater-hydrothermal alteration. The age of 905 ± 8 Ma for the plagiogranite marks the obduction of the ophiolite on continental margin during Rodinia assembly. The andesite and gabbro-diorite are enriched in LILE and LREE but depleted in HFSE. The andesite shows highly positive zircon εHf(t) values of 8.6 to 15.8 and mantle-like zircon δ18O values of 4.1 to 5.9‰. In addition, both andesite and gabbro-diorite whole-rocks are depleted in Sr-Nd-Hf isotopes, with positive εNd(t) values of 4.1 to 5.5 and highly positive εHf(t) values of 12.1 to 16.0. Such geochemical characteristics indicate that these mafic igneous rocks were derived from partial melting of arc-type mantle sources. In this regard, the ages of 936 ± 6 Ma and 950 ± 5 Ma for the andesite and gabbro-diorite date mafic arc magmatism above a Grenvillian oceanic subduction zone. Taken altogether, the subduction of the Grenvillian oceanic crust was continuous till the obduction of ophiolite at ∼900 Ma. Therefore, the Grenvillian oceanic subduction system was developed along the northern margin of the Yangtze Craton in the Early Neoproterozoic. This implies that South China was assembled into Rodinia through a Grenvillian accretionary orogeny.

Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet

Postcollisional potassium (K)-rich magmatic rocks have important implications for unveiling the deep geodynamic processes active during the evolution of orogenic belts and the surface uplift of plateaus. However, their petrogenesis remains highly controversial. Here we report on zircon U-Pb and mineral Ar-40-Ar-39 age, zircon Hf-O isotope, mineral composition, whole-rock major and trace element composition, and Nd-Sr isotope data for the postcollisional K-rich lavas in the Gemuchaka area of the central Qiangtang Block (central Tibet). Age dating suggests that these lavas were generated in the late Early Oligocene (ca. 30 Ma). Two series of rocks were recognized: silica-undersaturated and -saturated lavas. Both series are geochemically characterized by continuous variation trends. All rocks are enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs) and depleted in Ta-Nb-Ti, with negligible Sr and Eu and obviously positive Pb anomalies. The rocks have higher Th/ La (0.2-0.8) and Nb/Ta (18.6-19.8) values than asthenospheric mantle-derived rocks but slightly variable values of epsilon(Nd) (t) (-6.0 to -4.6), (Sr-87/Sr-86)(i) (0.7076-0.7083), and zircon epsilon(HF)(t) (-7.2 to +0.8) and delta O-18 (7.1-8.5 parts per thousand). We suggest that the parental magmas of these K-rich lavas were likely derived by variable partial melting of lithospheric mantle metaso-matized by subducting continental sediment-derived melts with the addition of minor asthenospheric components. Element modeling results suggest that the basic end members were likely generated by fractionation processes dominated by olivine and minor clinopyroxene from parental magmas, and some other evolved rocks were generated by crystal (olivine, clinopyroxene, Fe-Ti oxides) fractionation from basic magmas. Combining geological and Cenozoic magmatic rock data in the Qiangtang Block, we propose that the Gemuchaka magmatism was most likely triggered by the delamination of lithospheric mantle thickened by early (Paleocene- Eocene) intracontinental subduction and subsequent asthenospheric upwelling.

U-Pb ages, Hf-O isotopes and trace elements of zircons from the ore-bearing and ore-barren adakitic rocks in the Handan-Xingtai district: Implications for petrogenesis and iron mineralization

The petrogenesis of the Early Cretaceous adakitic intrusions in the Handan-Xingtai district, central North China Craton (NCC), and their genetic association with iron mineralization still remain controversial. Here, we present an integrated study of zircon U-Pb ages and Hf-O isotopes, as well as their trace elements for the ore-bearing and ore-barren complexes in the Handan-Xingtai district. Both of the ore-bearing and ore-barren complexes were emplaced contemporaneously (Early Cretaceous). Zircon εHf(t) and δ18O values of ore-barren complex range from −16.0 to −13.3 and from 6.3‰ to 7.1‰, respectively, whereas those of ore-bearing complexes have variable δ18O (5.7‰ to 8.3‰) and much lower εHf(t) values (−24.3 to −18.3). Furthermore, the ore-bearing complexes also have lower Ti-in-zircon temperatures but higher water contents and oxygen fugacity than those of ore-barren complexes. The zircon Hf-O isotopes, as well as their trace element data, indicate that the parental magma of ore-bearing complexes was a mixture of melts that derived from the enriched lithospheric mantle and ancient lower continental crust, whereas the ore-barren complex was most likely generated by partial melting of the enriched lithospheric mantle. We proposed that the high oxidation states and high water contents of magma, as well as the lower crustal materials may have played key roles in iron mineralization of Handan-Xingtai district.

Insights into the origin of coexisting A1- and A2-type granites: Implications from zircon Hf-O isotopes of the Huayuangong intrusion in the Lower Yangtze River Belt, eastern China

The origin of A-type granites has been the subject of great debate, especially the enigmatic synchronous A1- and A2-type granites. Cretaceous (~125 Ma) A1- and A2-type granites are common throughout the Lower Yangtze River Belt (LYRB), eastern China. However, their genesis still remains unclear. In this study, in-situ zircon Oisotopic data and chemical compositions of the Huayuangong (HYG) A-type granites in Anhui province, provide new insights into the origin and evolution of A-type granites in the LYRB, as well as the genetic link for synchronous A1- and A2-type granites. The HYG granites include syenogranite (75.9 wt%–76.6 wt% SiO2) and quartz syenite (66.1 wt%–66.9 wt% SiO2). Both are metaluminous and belong to ferroan series. They are characterized by high alkalis (K2O + Na2O = 8.36 wt%–8.55 wt% and 11.7 wt%–11.9 wt%), high field strength elements (Zr + Nb + Ce + Y = 909 ppm–1269 ppm and 1092 ppm–1329 ppm) and high Ga/Al ratios (10,000 ∗ Ga/Al = 4.91–4.96 and 2.64–2.68). The zircon saturation thermometer results indicate high magmatic temperatures (896–964 °C and 860–882 °C). All those geochemical features show an A-type granite affinity. They can be further classified into A1-and A2-type granites, corresponding to reduced and oxidized A-type granites, respectively. Additionally, the in-situ zircon O-Hf isotope compositions are also distinctly different, with δ18O = 4.7‰–6.0‰ and εHf(t) = −1.5 to −3.6 for A1-type granites, and δ18O = 7.0‰–7.8‰ and εHf(t) = −3.3 to −6.9 for A2-type granites. The geochemical signatures and newly discovered δ18O and εHf(t)values of the two A-type granite subgroups, indicate that they were derived from different source components and under disparate physicochemical conditions (e.g.,temperature, redox state and water contents). Lithospheric mantle-like isotopic data from zircons of A1-type granites suggest fractional crystallization of reduced, anhydrous basaltic magmas resulting in the formation of A1-type granites. In contrast, A2-type granites with relatively high δ18O and negative εHf values were generated from partial melting of the lithospheric mantle which was metasomatized by slab-derived melts/fluids. The coexisting A1- and A2-type granites were formed under the extensional setting where lithospheric thinning and asthenosphere upwelling occurred.

Continental crust growth induced by slab breakoff in collisional orogens: Evidence from the Eocene Gangdese granitoids and their mafic enclaves, South Tibet

Collisional orogens are traditionally regarded as main regions related to crustal reworking with limited or without crust growth. However, our study on the granitoids and their mafic enclaves of the Ringqênzê plutonic complex, a representative syn-collisional intrusive complex of Gangdese batholith, shows huge mantle contributions and considerable continental crust growth. Zircon U-Pb dating yields Eocene ages of 50.2 ± 0.5 Ma, 46.4 ± 0.5 Ma, 50.6 ± 0.6 Ma and 50.4 ± 0.5 Ma for the quartz diorite, hornblende granodiorite, host pyroxene-bearing granodiorite and its dioritic enclave, respectively. Reverse zoning of plagioclase (spike zone), pyroxene and plagioclase relicts in both of the dioritic enclaves and the granitoids, as well as the mixing trends on plots of major and trace elements, indicate a petrogenesis of magmatic mixing. The relatively low SiO2 (53.43–56.23 wt%) contents, high Mg# values (48–56) and mineral compositions further suggest that precursor magmas of the enclaves are mantle-derived. The dioritic enclaves and granitoids have indistinguishable (87Sr/86Sr)i ratios (0.70460–0.70480) but slightly different εNd(t) values (+1.8 to +3.0 for the dioritic enclaves and +0.2 to +0.6 for the granitoids). The (87Sr/86Sr)i ratios and εNd(t) values plot on the mixing line defined by depleted mantle (DM) and lower crust xenolith from the Southern Lhasa subterrane and further simulation reveals a contribution of >55% from mantle. In the zircon εHf(t) versus δ18O diagram, the dioritic enclaves (εHf(t) = +3.7–+8.5, δ18O = 4.56–7.00‰) and the granitoids (εHf(t) = +1.2–+8.5, δ18O = 5.59–7.21‰) plot around the mixing line defined by zircons from granulite xenolith and a relatively depleted zircon from the dioritic enclaves. Meanwhile, the simple binary mixing calculation based on Hf-O isotopes suggests that the mantle contributions are not <30% (55–65% in average) to the magma of these granitoids. In addition, the synthesized data show the majority of the syn-collisional Gangdese granitoids were also generated by magma mixing processes with similar large quantity of mantle contributions to the Ringqênzê granitoids. Therefore, the extensive and significant mantle contributions in the magma of the collision-related Gangdese granitoids demonstrate considerable continental crust growth during the syn-collisional period. Furthermore, our data combined with previously published data suggest that the ca. 50 Ma samples have highest Ti-in-zircon temperatures (up to 900 °C) and high pyroxene crystallizing temperatures (>1000 °C), whereas other samples show lower Ti-in-zircon temperatures mostly below 750 °C. Because the Gangdese granitoids with unusual high temperatures had been produced dominantly at a very short time (~50 Ma) and the high temperature was related to underplating of mantle materials, we suggest that a slab breakoff was most likely responsible for the formation of the high-temperature magmas and the continental crust growth at the syn-collisional period in collisional orogens.

Petrogenesis and tectonic implications of Early Cretaceous volcanic rocks from Lingshan Island in the Sulu Orogenic Belt

The Dabie-Sulu orogenic belt in eastern China marks the boundary between the Yangtze Block and the North China Block. Here we investigate a suite of volcanic rocks from Lingshan Island in the Sulu belt comprising rhyolite, trachyte, trachyandesite and basaltic trachyandesite. We present petrological, geochemical and zircon UPb ages and HfO isotope data with a view to gain insights on the petrogenesis and tectonic implications. SHRIMP II analyses of zircon grains from the rhyolite yield 206Pb/238U age of 127.6 ± 1.3 Ma and LA-MC-ICP-MS dating show 126.3 ± 1.2 Ma and 127.3 ± 1.1 Ma, together constraining the eruption time as Early Cretaceous. LA-MC-ICP-MS analyses of zircon grains from the andesitic rocks yield 206Pb/238U ages of 129.0 ± 1.6 Ma, 129.8 ± 1.5 Ma and 130.9 ± 1.0 Ma. Geochemically, the rhyolite shows shoshonitic features with low MgO and Cr, but high Na2O + K2O. The zircon grains from these rocks yield negative εHf(t) values and low δ18O values, and these together with the presence of Neoproterozoic inherited zircons suggest that the magma source involved melting of the Yangtze crust. The andesitic rocks, including basaltic trachyandesite, trachyandesite and trachyte, show a wide range of SiO2, Mg# values, and Cr, enriched in LILE and LREE, depleted in HFSE (Nb, Ta and Ti), and have significantly negative zircon εHf(t) values, suggesting derivation from subcontinental lithosphere mantle that was metasomatized by felsic melts. Our results, integrated with those from previous studies suggest heterogeneous magma involving the mixing of mantle and crustal sources within an extensional setting in the Early Cretaceous.

Petrogenesis and tectonic setting of Early Cretaceous granodioritic porphyry from the giant Rongna porphyry Cu deposit, central Tibet

The Rongna Cu-(Au) deposit is a recently discovered giant low-grade composite porphyry-high sulfidation epithermal system in the Duolong ore district of central Tibet. We present zircon U-Pb ages, bulk-rock geochemical and Sr-Nd isotope data, as well as zircon Hf-O isotope data to constrain the petrogenesis of the Rongna granodioritic porphyry, on which the ore system is centered. Secondary ion mass spectrometry (SIMS) zircon U-Pb data for two samples yielded ages of 118.7 ± 0.9 Ma and 117.1 ± 0.9 Ma, respectively. The granodioritic porphyry is of high-K calc-alkaline and magnetite-series affinity, with enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (e.g., Rb, Pb, and K), and depletion of high-field strength elements (e.g., Ta, Nb, and Ti), typical of the geochemical signature of arc magmas. The Rongna granodioritic porphyry has initial Sr isotopic ratios of 0.7060–0.7099, and εNd(t) values of −5.7 to −1.7 with TDM (Nd) of 983–1900 Ma. The zircons have relatively narrow Hf-O isotopic compositions (εHf(t) = +3.30– +6.43; δ18O = +5.94– +6.96‰), indicating the involvement of old crustal materials. Our data suggest that the Rongna granodioritic porphyry likely evolved by fractional crystallization from a hybrid magma of mantle-derived mafic melts and lower crustal felsic melts during a period of magmatic flare-up following flat-slab subduction.