Zircon Hf Isotopic Compositions Research Articles

Crustal accretion and reworking processes of micro-continental massifs within orogenic belt: A case study of the Erguna Massif, NE China

This paper summarizes the geochronological, geochemical and zircon Hf isotopic data for Mesozoic granitoids within the Erguna Massif, NE China, and discusses the spatial-temporal variation of zircon Hf isotopic compositions, with the aim of constraining the accretion and reworking processes of continental crust within the Erguna Massif, and shedding light on the crustal evolution of the eastern segment of the Central Asian Orogenic Belt. Based on the zircon U-Pb dating results, the Mesozoic granitic magmatisms within the Erguna Massif can be subdivided into five stages: Early-Middle Triassic (249–237 Ma), Late Triassic (229–201 Ma), Early-Middle Jurassic (199–171 Ma), Late Jurassic (155–149 Ma), and Early Cretaceous (145–125 Ma). The Triassic to Early-Middle Jurassic granitoids are mainly I-type granites and minor adakitic rocks, whereas the Late Jurassic to Early Cretaceous granitoids are mainly A-type granites. This change in magmatism is consistent with the southward subduction of the Mongol-Okhotsk oceanic plate and subsequent collision and crustal thickening, followed by post-collision extension. Zircon Hf isotopic data indicate that crustal accretion of the Erguna Massif occurred in the Mesoproterozoic and Neoproterozoic. Zircon e Hf( t ) values increase gradually over time, whereas two-stage model ( T DM2) ages decrease throughout the Mesozoic. The latter result indicates a change in the source of granitic magmas from the melting of ancient crust to more juvenile crust. Zircon e Hf( t ) values also exhibit spatial variations, with values decreasing northwards, whereas TDM2 ages increase. This pattern suggests that, moving from south to north, there is an increasing component of ancient crustal material within the lower continental crust of the Erguna Massif. Even if at the same latitude, the zircon Hf isotopic compositions are also inconsistent. These results reveal lateral and vertical heterogeneities in the lower continental crust of the Erguna Massif during the Mesozoic, which we use as the basis of a structural and tectonic model for this region.

In situ U–Pb and Lu–Hf isotopic studies of zircons from the Sancheong–Hadong AMCG suite, Yeongnam Massif, Korea: Implications for the petrogenesis of ∼1.86 Ga massif-type anorthosite

Isotopic and geochemical characteristics of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite have long been used for tracing the mantle–crustal source and magmatic evolution. We analyzed Lu–Hf isotopic compositions of zircon from the Sancheong–Hadong AMCG complex, Yeongnam Massif, Korea, in order to understand tectonomagmatic evolution of the Paleoproterozoic AMCG suite occurring at the southeastern margin of the North China Craton (NCC). The anorthositic rocks in this complex, associated with charnockitic and granitic gneisses, were recrystallized to eradicate magmatic features. In situ SHRIMP (sensitive high-resolution ion microprobe) U–Pb analyses of zircon from a leuconorite and an oxide-bearing gabbroic dyke yielded weighted mean 207Pb/206Pb ages of 1870±2Ma and 1861±6Ma, respectively. Charnockitic, granitic, and porphyroblastic gneisses yielded weighted mean 207Pb/206Pb zircon ages of 1861±6Ma, 1872±6Ma, and 1873±4Ma, respectively. These crystallization ages, together with our previous geochronological data for anorthosites (1862±2Ma), are indicative of episodic AMCG magmatism over an ∼10Ma interval. Initial εHf(t) values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from +2.1 to −6.1 and −0.3 to −5.4, respectively. Zircon Hf isotopic data in combination with available whole rock Sr–Nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination. This crustal component is also reflected in charnockitic-granitic magmas produced primarily by the melting of lower crust. Taken together, the AMCG magmatism at 1.87–1.86Ga in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic NCC amalgamation tectonically linked to assembly of the Columbia supercontinent.

Crustal evolution in the Gyeongsang Arc, southeastern Korea: Geochronological, geochemical and Sr-Nd-Hf isotopic constraints from granitoid rocks

A loosely assembled patchwork of Cretaceous to Paleogene granitoid plutons comprise the platform of the Gyeongsang Arc in southeastern Korea and represent a high-flux pulse of magmatism in Northeast Asia. The present study uses new and published zircon U-Pb ages, whole-rock geochemical and Sr-Nd isotopic data, and zircon Hf isotopic data of the plutons to decipher the crustal evolution in the Gyeongsang Arc and compare it with adjacent accretionary terranes. The ion microprobe zircon U-Pb ages range from 91 to 27 Ma, revealing the selective occurrence of Paleogene plutons in the eastern coastal area, to the east of the NNE-trending Yangsan fault system. Paleoproterozoic to Paleogene xenocrystic cores are occasionally observed in the zircon grains. The plutons are composed of dominantly magnesian, high- to medium-K calc-alkaline granites-granodiorites exhibiting typical geochemical characteristics of subduction zone magma, and less abundant ferroan alkaline granites. Their trace element patterns require residual amphibole and feldspar, but not garnet in the source. The whole-rock Sr-Nd and zircon Hf isotopic compositions of the granitoids (initial 87Sr/86Sr = 0.7043 to 0.7069; eNd(t) = –4.8 to +2.9; eHf(t) = –5.0 to +19.7) are distinctly more primitive than those of inland granitoids distributed outside of the Gyeongsang backarc basin. An important role of relatively young, most likely Paleozoic juvenile crust in the formation of the Late Cretaceous granitoids is suggested by the time-eHf trend of high-eHf zircons that converges toward data points of the Late Permian Yeongdeok adakite composing the arc basement. The asthenospheric mantle input is highlighted by significantly high (>+17) eHf(t) values of some zircons from the early Eocene alkaline plutons. Subsequent reworking of the rejuvenated crust yielded granitoid plutons possessing slightly but recognizably higher eNd(t) and eHf(t) values than the older plutons. These isotopic features demonstrate that the Cretaceous-Paleogene calc-alkaline granitoids in the Gyeongsang Arc are not “juvenile” (sensu stricto) despite their mostly positive epsilon Nd and Hf values, but are basically a product of crustal reworking. The Sr-Nd-Hf isotopic compositions of Late Cretaceous granitoids in the Gyeongsang Arc are comparatively more primitive than those in adjacent accretionary terranes such as Southwest Japan and Fujian province in southeastern China, reflecting differences in the formation age of the basement on which the arc system was built.

Early Cretaceous continental arc-related volcanic rocks in the Duobuzha area, northern Tibet: implications for evolution history of the Bangong–Nujiang Ocean

ABSTRACTNew whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon εHf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area.

Geochronology and geochemistry of the Early Jurassic Yeba Formation volcanic rocks in southern Tibet: Initiation of back-arc rifting and crustal accretion in the southern Lhasa Terrane

Understanding the geological history of the Lhasa Terrane prior to the India–Asia collision (~55±10Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~18–10Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183–174Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr–Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I–S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane.

Cogenetic origin of mafic microgranular enclaves in calc-alkaline granitoids: The Permian plutons in the northern North China Block

Mafic microgranular (microgranitoid) enclaves (MMEs) with fine-to medium-grain–size igneous microstructures are abundant in Permian granitoid plutons in the northern North China Block (NCB). They are mainly dioritic in composition with SiO 2 contents from 51.0 wt% to 62.7 wt% and are contiguous with their host granitoids (SiO 2 = 60.4–68.4 wt%). Their main mineral assemblage of plagioclase (oligoclase and andesine), hornblende, biotite, K-feldspar, and quartz is similar to that of their host granitoids but with different mineral proportions. Zircon laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb dating, hornblende-plagioclase thermobarometry, and Ti-in-zircon thermometry results show that the crystallization ages and pressure-temperature ( P-T ) conditions of the MMEs in each Permian granitoid pluton are very similar to those of their host granitoids, indicating simultaneous crystallization at the middle to upper crustal levels. Petrographical, geochemical, and Sr-Nd-Hf isotopic data show that the source areas of the Permian granitoid plutons in the northern NCB are temporally and spatially different and magma mixing was likely involved during formation of some granitoid plutons. However, the whole-rock Sr-Nd and zircon Hf isotopic compositions of the MMEs in each pluton are very similar to those of their host granitoids, indicating complete isotopic equilibration between the host granitoids and their enclaves. Combined with similar mineral assemblage and mineral chemistry between the MMEs and their host granitoids and absence of coeval mantle-derived rocks in or nearby the plutons, we confirm that the MMEs were crystallized from a coeval magma that gave rise to the host granitoids, and not by mixing and/or mingling between mantle-derived mafic and crustal-derived felsic magmas or by synplutonic injections of mafic magma into the host granitoid magma. We conclude that magma isotopic equilibration between host granitoids and their enclaves was achieved prior to emplacement and the magma mixing process during formation of some Permian granitoid plutons occurred mainly in the melting, assimilation, storage, and homogenization (MASH) zone in the lowermost crust or mantle-crust transition, not in the magma conduits or chambers at middle to upper crustal levels. A two-stage model that includes rapid cooling within the cogenetic host granitoid magma operating at pluton margins to form solid to sub-solid dioritic rocks and crystal accumulations of the host granitoid magma at the bottom of the pluton to form cumulates, and dioritic rocks and cumulates then fragmentated and interacted with progressive evolved host granitoid magma to change their shapes and orientations is proposed for origin of the MMEs in the Permian granitoid plutons in the northern NCB. This model may be more widely applicable to the generation of many MMEs in granitic rocks, especially where no direct evidence exists for the presence of mafic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves. Our new results on origin of the enclaves in the granitoid plutons in the northern NCB show that some MMEs previously used as evidence for magma mixing and/or mingling are “autoliths (cognate xenoliths)” crystallized from the same magma as their host granitoids. There is an essential need for caution in using the MMEs as evidence for crustal-mantle interaction or mixing and/or mingling between crustal-derived felsic and mantle-derived mafic magmas during formation of granitic rocks, especially where no independent evidence is found for such a process. The role of MMEs in evaluating the mixing and/or mingling processes between crustal-derived felsic and mantle-derived mafic magmas has been much more likely overestimated by some previous studies and should be reconsidered. Instead, the MMEs in this origin can provide valuable information on emplacement mechanism and incremental growth of granitoid plutons in continental crust.

Early cretaceous ophiolites of the Yarlung Zangbo Suture Zone: insights from dolerites and peridotites from the Baer upper mantle suite, SW Tibet (China)

ABSTRACTThe Baer ophiolitic massif is located in the northern sub-belt of the western segment of the Yarlung Zangbo Suture Zone (YZSZ) and mainly consists of a lherzolite-dominant mantle suite, dolerite intrusions and limited crustal outcrops. The dolerites show sub-ophitic texture and light rare earth element-depleted chondrite-normalized rare earth element patterns similar to normal-mid-ocean ridge basalts (N-MORB); though, they display enrichments in fluid-mobile elements (Rb, Ba, and Sr) and marked depletions in Th and Nb. The U–Pb ages of several magmatic zircon grains recovered from two dolerite samples indicate that the intrusion of the dikes into the Baer lherzolitic mantle occurred at 125.6–126.3 Ma, consistent with the widespread mafic magmatism between 120 and 130 Ma in the Yarlung Zangbo ophiolites. The dolerites have slightly more radiogenic 87Sr/86Sr ratios (0.7043–0.7054) in comparison to N-MORB, whereas they show 143Nd/144Nd values (0.513067–0.513114) similar to N-MORB and high zircon Hf-isotope compositions. They have a limited range of Nd-isotope (εNd(t) values: +8.2 to +9.1) and juvenile Hf-isotope compositions (εHf(t) values: +8.4 to +14.2 and +10.0 to +15.1) indicating derivation from mantle melts. The moderate spread in the εHf (t) values of zircons indicates derivation of the dolerites parental magma from a weakly contaminated spinel-bearing mantle source. This is also corroborated by the geochemical signatures of the Baer dolerites (enrichment in LILE and depletion in HFSE) suggesting minor slab input to the mantle source of the dike-filling melt. We suggest that the genesis of the dolerite dike-forming melt happened at a stage of subduction initiation in a sub-oceanic mantle domain mildly affected by fluids emanating from the downgoing slab. Our data combined with literature data allow us to presume that the intrusion of the dolerites into the Baer mantle corresponds to an early phase of subduction initiation beneath a developing forearc basin.

The distribution, geochronology and geochemistry of early Paleozoic granitoid plutons in the North Altun orogenic belt, NW China: Implications for the petrogenesis and tectonic evolution

Abundant early Paleozoic granitoid plutons are widely distributed in the North Altun orogenic belt. These rocks provide clues to the tectonic evolution of the North Altun orogenic belt and adjacent areas. In this paper, we report an integrated study of petrological features, U–Pb zircon dating, in situ zircon Hf isotope and whole-rock geochemical compositions for the Abei, 4337 Highland and Kaladawan Plutons from north to south in the North Altun orogenic belt. The dating yielded magma crystallization ages of 514Ma for the Abei Pluton, 494Ma for the 4337 Highland Pluton and 480–460Ma for the Kaladawan Pluton, suggesting that they are all products of oceanic slab subduction because of the age constraint. The Abei monzogranites derived from the recycle of Paleoproterozoic continental crust under low-pressure and high-temperature conditions are products of subduction initiation. The 4337 Highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust. The Kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive εHf(t) values, and the Kaladawan monzogranite-syenogranite are derived from partial melting of Neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle. These results, together with existing data, provide significant information about the evolution history of oceanic crust subduction during the 520–460Ma. The initiation of subduction occurred during 520–500Ma with formation of Abei Pluton; subsequent transition from steep-angle to flat-slab subduction at ca.500Ma due to the arrival of buoyant oceanic plateaus, which induces the formation of 4337 Highland Pluton. With ongoing subduction, the steep-angle subduction system is reestablished to cause the formation of 480–460Ma Kaladawan Pluton. Meanwhile, it is this model that account for the temporal–spatial distribution of these early Paleozoic magmatic rocks in the North Altun orogenic belt.

Formation of the Yandangshan volcanic–plutonic complex (SE China) by melt extraction and crystal accumulation

The association of volcanic and shallow plutonic rocks in caldera may provide important clues to the geochemical evolution of silicic magma systems. The Yandangshan caldera is a typical example of late Mesozoic volcanic–plutonic complex in SE China. It is composed of a series of rhyolitic extrusives and subvolcanic intrusions of porphyritic quartz syenites. In this work, we conducted petrological and geochemical studies, as well as zircon dating, on the coexisting volcanic and plutonic rocks from the Yandangshan caldera. The results of SHRIMP and LA-ICP-MS zircon U–Pb dating revealed that the crystallization of the rhyolitic extrusives and subvolcanic intrusions was contemporaneous within analytical errors and in a short period (104–98Ma). Geochemically, the volcanic rocks are characterized by high Rb/Sr and Rb/Ba ratios and depletion in Ba, Sr, P, Eu and Ti, while the shallow plutons show high K, Ba, Al, Fe and low Rb/Sr and Rb/Ba ratios with insignificant negative Eu anomalies. The volcanic and plutonic rocks have a similar range of zircon Hf isotopic compositions (εHf(t)=−10.0 to +1.5) and TDM2 model ages of 2.10–1.23Ga. They also have comparable whole-rock Sr and Nd isotopic compositions ((87Sr/86Sr)i=0.7084–0.7090; εNd(t)=−7.8 to −6.5) and zircon oxygen isotopic compositions (δ18O mainly=4.5 to 6.0‰). We argue that the volcanic–plutonic complex of the Yandangshan caldera was formed by reworking of Paleoproterozoic lower crusts in the eastern Cathaysia block, and that the complex could be linked by fractional crystallization and crystal accumulation in a shallow magma chamber. The volcanic rocks represent the highly fractionated end-member, whereas the subvolcanic intrusions of porphyritic quartz syenites could be the residual crystal mushes. This case study could have a general implication for the genetic relationship between volcanic and shallow plutonic rocks in calderas.

Late Jurassic adakitic granodiorite in the Dong Co area, northern Tibet: Implications for subduction of the Bangong–Nujiang oceanic lithosphere and related accretion of the Southern Qiangtang terrane

In this paper we present new major and trace element chemical data, zircon U–Pb age data, and zircon Hf isotopic compositions for the Dong Co granodiorites (DGs) that were emplaced directly in the Dong Co ophiolites (DOs) in the middle segment of the Bangong–Nujiang suture zone (BNSZ), northern Tibet. We use these new data to discuss the genesis of the DGs and implications for the evolution of the region. The DGs have high contents of SiO2, Al2O3, Na2O, and Sr, high values of Mg#, low contents of Yb and Y, and markedly high Sr/Y ratios, indicating an adakitic affinity, and we show that they were derived from the partial melting of subducted oceanic crust. The DGs contain large numbers of inherited zircons that have similar shapes and peak ages to the detrital zircons in the accretionary wedge graywackes that surround the DGs, suggesting that many of these sediments were assimilated during the formation of the DGs. Five DG samples yield the youngest zircon U–Pb ages of 155–160Ma, indicating that the DGs formed during the Late Jurassic. Based on the present results and regional geological data, we infer that the Bangong–Nujiang oceanic lithosphere was subducted northwards beneath the Southern Qiangtang terrane during the formation of a continental arc system during the Late Jurassic. The Bangong–Nujiang oceanic lithosphere retreated rapidly after its initial subduction, and the Southern Qiangtang terrane to the north of the Bangong–Nujiang Ocean acquired a massive amount of new oceanic material in the form of an accretionary wedge. This wedge, which included the DOs and the Mugagangri Group, became sufficiently large (both in size and width) for arc-type magmas (DGs) to develop within it during the Late Jurassic.

Paleoproterozoic juvenile crust formation and stabilisation in the south-eastern West African Craton (Ghana); New insights from U-Pb-Hf zircon data and geochemistry

The crust of the Paleoproterozoic West African Craton in northern Ghana is dominantly made of Tonalite-Trondhjemite-Granodiorite (TTG) suites, low-K to high-K calc-alkaline granites, with minor LILE-enriched diorites, and chemically related lavas. Results of zircon U-Pb dating indicate that all these different rock types were formed together over a prolonged period of ca. 100Ma, between 2.21 and 2.11Ga. Zircon Hf isotope compositions of five samples are supra-chondritic (εHft from +1 to +6) indicating that the granitoids derive from the reworking of juvenile crustal components as old as 2.60Ga. Geochemical data show that the TTGs were derived from the reworking of a low-K mafic crust, while biotite granites formed by the reworking of older felsic crustal rocks, and diorites derived from the melting of LILE-enriched mantle sources.The combined datasets support a three-step model for the formation of the crust of the West African craton in northern Ghana: (1) a juvenile, mafic proto-crust was extracted from the depleted mantle between 2.60 and 2.30Ga, in an oceanic environment remote from any pre-existing continental nucleus; (2) the mafic proto-crust was reworked in several primitive magmatic arcs or in accreted oceanic plateaux. Crustal reworking initially produced minor felsic magmatism between 2.35 and 2.21Ga, followed by the 2.21–2.15Ga emplacement of low- to medium-P TTG and K-rich calc-alkaline granitoid suites, and was coeval with the formation of an enriched mantle source. (3) Various proto-continental terranes collided during the Eburnean orogeny between 2.14 and 2.11Ga, triggering intense reworking of the composite crust and mantle reservoirs formed in the previous two steps, i.e. remnants of the juvenile mafic crust, older TTGs and biotite granites, and previously enriched mantle sources. The emplacement of high-P TTGs, granites and LILE-enriched diorites resulted from lower crustal heating in response to the delamination of the lower crust and lithospheric mantle. The Eburnean collision therefore marks the final stages of a 200–500Ma long period of juvenile continental crust formation and stabilisation. The datasets further reveal that the Paleoproterozoic West African Craton comprises a minimum of two distinct crustal blocks, with north-western Ghana lying at their interface.

The source and tectonic implications of late Carboniferous–early Permian A-type granites and dikes from the eastern Alataw Mountains, Xinjiang: geochemical and Sr–Nd–Hf isotopic constraints

ABSTRACTZircon U–Pb ages, and geochemical, Sr–Nd and zircon Hf isotopic compositions are reported for the A-type granites and dikes in the Alataw Mountains of the northwestern Tianshan Orogenic Belt (NTOB), with the aim of investigating the sources and genesis of A-type granites and dikes. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of A-type granites yielded a concordant weighted mean 206Pb/238U age of 297.4 ± 1.5 and 300.6 ± 0.9 Ma, respectively, defining a late Carboniferous–early Permian magmatic event. Geochemically, the granitic intrusions and dikes are characterized by high SiO2 and total alkalies (K2O + Na2O), high Zr, Nb, Ta content, and Ga/Al ratio with prominent negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate that the granitic intrusions and dikes in the eastern Alataw Mountains are of an A-type affinity. The depleted Nd isotope compositions of the granitic intrusions and dikes are consistent with those of the Carboniferous volcanic rocks in the Alataw Mountains, especially Carboniferous adakites (εNd(t) = +3.6 to +6.6), suggesting that they were likely generated by partial melting of less evolved crustal materials, such as oceanic crust stored in the middle and/or lower crust or Carboniferous volcanic arc crust. The widespread late Carboniferous–early Permian magmatism in the NTOB may have been related to a ridge subduction accompanied by slab roll-back of the subducting plate of the North Tianshan Ocean.

Late Triassic syn-exhumation magmatism in central Qiangtang, Tibet: Evidence from the Sangehu adakitic rocks

The geodynamic setting of Late Triassic magmatic activity along the Longmu Co–Shuanghu suture zone (LSSZ) in central Qiangtang, Tibet is a matter of debate. This paper presents zircon LA–ICP–MS U–Pb ages, zircon Hf isotopic compositions, and whole-rock geochemical data for the Sangehu (SGH) granitic intrusion in central Qiangtang, and addresses the petrogenesis of Late Triassic magmatism, and the history of collision between the northern and southern Qiangtang terranes. The SGH pluton consists mainly of biotite adamellite with mafic microgranular enclaves (MMEs), and small amounts of K-feldspar granite. The biotite adamellite, MMEs, and K-feldspar granite give ages of 207.8±3.0Ma, 212.4±31Ma, and 211.6±3.8Ma, respectively. The MMEs show magmatic textures and acicular apatite, and are coeval with the host biotite adamellite, suggesting they were produced by magma mixing. All samples from the SGH pluton show high Sr and low Y contents, and positive Eu anomalies, similar to adakitic rocks. The high K2O contents and low Mg#, Cr, and Ni contents, and enriched Hf isotopic characteristics of the zircons indicate that these magmas were derived from the partial melting of thickened crust. However, the whole-rock geochemical data and zircon Hf isotopic compositions also reveal heterogeneity at the source. The combined magmatic and metamorphic records suggest that Triassic magmatic activity in central Qiangtang was closely related to the collision of the northern and southern Qiangtang terranes. The large-scale Late Triassic (225–200Ma) magmatic event in central Qiangtang may have resulted from the breakoff of the Longmu Co–Shuanghu Tethys Ocean lithospheric slab in the early Late Triassic (236–230Ma). The Late Triassic magmatic rocks, including adakitic rocks, are coeval with retrograde high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in central Qiangtang, and show characteristics of syn-exhumation magmatism. The early adakitic rocks (>220Ma) were generated by melting of the edge of the slab, which was heated by upwelling asthenospheric mantle after subduction slab breakoff, and the late adakitic rocks (<220Ma) were generated by partial melting of the thickened crust.

Discovery of the early Jurassic Gajia mélange in the Bangong–Nujiang suture zone: Southward subduction of the Bangong–Nujiang Ocean?

Melange records a series of geological processes associated with oceanic subduction and continental collision. This paper reports for the first time the presence of Early Jurassic melange from NW Nagqu in the southern margin of the Bangong–Nujiang suture zone, termed as the Gajia melange. It shows typically blocks-in-matrix structure with matrix of black shale and siliceous mudstone, and several centimeters to several meters sized blocks of sandstone, silicalite, limestone and basalt. The sandstone blocks consist of homologous sandstone and two types of exotic sandstone, with different modal compositions. The Group 1 of exotic sandstone blocks consists of mainly of feldspar and quartz, whereas the Group 2 is rich in volcanic detritus. The Group 3 of homologous sandstone blocks is rich in feldspar and volcanic detritus with rare occurrence of quartz. U–Pb age data and in situ Hf isotopic compositions of detrital zircons from sandstone blocks are similar to those from the Lhasa terrane, suggesting that the sandstone blocks in the Gajia melange most probably came from the Lhasa terrane. The YC1σ(2+) age of homologous sandstone blocks is 177 ± 2.4 Ma, suggesting an Early Jurassic depositional age for the sandstones within the Gajia melange. The Gajia melange likely records the southward subduction of the Bangong–Nujiang Ocean during the Early Jurassic.

Precambrian tectonothermal evolution of South Qinling and its affinity to the Yangtze Block: Evidence from zircon ages and Hf-Nd isotopic compositions of basement rocks

South Qinling is a tectonic block in the Mesozoic Qinling orogenic belt, central China, which is generally believed to have had tight affinity with the Yangtze Block during Precambrian times. In recent years, it has been recognized that the South Qinling Block hosts several Precambrian basement complexes. Here we report U-Pb ages and Hf isotopic data for zircons and Sr-Nd isotopic data for whole-rocks of the Douling complex, the oldest rock unit exposed in South Qinling, to reveal the early evolution of this basement. The Douling complex mainly consists of paragneisses and granitic orthogneisses. Zircons of magmatic origin from three orthogneiss samples give U-Pb ages of c. 2.5Ga, suggesting an early Paleoproterozoic magmatic activity. Zircon Hf isotopic compositions imply the growth of juvenile crust since c. 3.0Ga. The magmatic activity at c. 2.5Ga points to differences in the Precambrian history between South Qinling and the Archean Kongling terrene, the oldest basement of the Yangtze Block. Three paragneiss samples contain complex detrital zircons of different ages and origins. Their U-Pb ages mainly cluster around 2.5Ga, 2.0Ga, 1.5Ga and 800Ma. Zircon grains with ages of 2.0Ga and 800Ma have low Th/U ratios and no oscillatory zoning, implying two episodes of metamorphic overprint. Such Paleoproterozoic metamorphic event has also been traced in the Yangtze Block, confirming its close relationship with South Qinling. The record of Neoproterozoic amphibolite-facies metamorphism in detrital zircons suggest that the tectonic transition from convergent to extensional environments in the Yangtze Block must have occurred later than 800Ma ago.

Tectono-magmatic evolution of the Chihuahua-Sinaloa border region in northern Mexico: Insights from zircon-apatite U–Pb geochronology, zircon Hf isotope composition and geochemistry of granodiorite intrusions

We present the whole-rock geochemistry, LA–ICP–MS zircon-apatite U–Pb ages and zircon Hf isotope composition of the granodioritic plutons at the southwestern boundary of Chihuahua with the states of Sinaloa and Sonora. These granodiorites are exposed in the north and south of the Rio El Fuerte in southwest Chihuahua and northern Sinaloa. The magmatism spans over a time period of 37Ma from 90 to 53Ma. Zircons are exclusively magmatic with strong oscillatory zoning. No inheritance of any age has been observed. Our new U–Pb dating (~250 analyses) does not support the involvement of older basement lithologies in the generation of the granitic magmas. The U–Pb apatite ages from granodiorites in southwest Chihuahua vary from 52 to 70Ma. These apatite ages are 1 to 20Ma younger than the corresponding zircon U–Pb crystallization ages, suggesting variable cooling rates from very fast to 15°C/Ma (~800°C to 500°C) and shallow to moderate emplacement depths. In contrast, U–Pb apatite ages from the Sinaloa batholith are restricted from 64 to 61Ma and are indistinguishable from the zircon U–Pb ages range from 67 to 60Ma within the error, indicating rapid cooling and very shallow emplacement. However, one sample from El Realito showed a larger difference of ~20Ma in zircon–apatite age pair: zircon 80±0.8Ma and apatite 60.6±4Ma, suggesting a slower cooling rate of ~15°C/Ma. The weighted mean initial εHf (t) isotope composition (2σ) of granodiorites varies from +1.8 to +5.2. The radiogenic Hf isotope composition coupled with previous Sr–Nd isotope data demonstrates a significant shift from multiple crustal sources in the Sonoran batholithic belt to the predominant contribution of the mantle-derived magmas in the southwest Chihuahua and northern Sinaloa. Based on U–Pb ages, the absence of inheritance, typical high Th/U ratio and radiogenic Hf isotope composition, we suggest that the Late Cretaceous–Paleogene magmatic rocks in this region are not derived from melting of a felsic older crust, neither evolved North American Proterozoic basement nor Jurassic metasedimentary rocks of the Guerrero terrane. Instead, the magma was primarily derived from partial melting of mantle related sources, possibly in the subcontinental mantle wedge above the Farallon plate. Our younger U–Pb zircon–apatite ages are well correlated with the late-stage cogenetic porphyry Cu–Au mineralization at 57 to 59Ma. If the origin of ore mineralization is related to the magma sources of host batholiths, then a minimal input from crustal melt is suggested in the genesis of Cu–Au porphyry system.

Neoproterozoic granitoids in the eastern Himalayan orogen and their tectonic implications

The Precambrian component and evolutionary history of the Greater Himalayan Sequence (GHS), forming the core of the Himalayan orogen, remain a subject of debate. Here, we report new geochronological and petrochemical data of the Neoproterozoic granitoids that occur as the orthogneisses of the GHS in Cona area, the eastern Himalaya orogen. The zircon U–Pb dating results reveal that these granitoids have crystallization ages of Neoproterozoic of 817–838Ma, and metamorphic ages of ca. 31Ma. The petrochemical data show that the granitoids are medium to high-K, peraluminous granodiorite and granite, display geochemical affinities with volcanic arc granitoids. The zircon Hf isotopic compositions show that the granitoids were probably derived from the partial melting of Paleoproterozoic crustal materials. These data, combined with previous results, indicate that an extensive Neoproterozoic magmatic belt may have existed in the Himalayan orogen, and that the Neoproterozoic granitoids resulted from the Andean-type orogeny that formed along the northwestern margin of Rodinia supercontinent, and experienced the Cenozoic high-grade metamorphism during the Himalayan orogeny.

Formation of porphyry Mo deposit in a deep fault zone, example from the Dabaoshan porphyry Mo deposit in northern Guangdong, South China

The Dabaoshan porphyry Mo deposit is located in the Wuchuan-Sihui fault zone, a deep fault system in South China. The Dabaoshan Mo mineralization occurs as disseminations and veinlets in the Dabaoshan porphyries and skarn-greisen associated with the Chuandu porphyritic monzogranite. Here we report the zircon LA-ICP-MS U-Pb ages of the porphyries mentioned above. We also present the major and trace elements, whole-rock Sr-Nd and the zircon Hf isotopic compositions of the Dabaoshan porphyries. The Dabaoshan porphyries are composed of weakly to strongly peraluminous monzogranite porphyry and granite porphyry. They yield ages of 166.3±2.0Ma (MSWD=1.9) and 166.2±2.7Ma (MSWD=2.7), respectively, slightly older than the Chuandu porphyritic monzogranite pluton which crystallized at 162.1±1.6Ma (MSWD=2.7). This is indicative of two distinct magmatic pulses in this region. The Dabaoshan porphyries have 64.42%–75.41% SiO2 and show an affinity of high-K calc-alkaline and shoshonite in the plot of Th vs. Co. They have [La/Yb]N of 14.6–35.0 and εNd(t) values ranging from −8.2 to −6.6. The εNd(t) values of the rocks are more negative than those for the Middle Jurassic mafic rocks (εNd(t)=−2.7 to +7.9) in the region, but similar to those for Proterozoic crust in the Cathaysia Block at ~165Ma. Zircon from the Dabaoshan porphyries has εHf(t) values ranging from −13.2 to −7.5 and two-stages Hf model ages (TDM2) from 1.7 to 2.0Ga, close to the TDM2 (~1.7Ga) of the basement in the western Cathaysia Block. The Dabaoshan porphyries have averaged Th/U of 5.0, similar to those for the Proterozoic crystalline basement (Th/U=5.3–5.4) in the Cathaysia Block, but higher than those for arc magmas (Th/U=1.5–3.0). Moreover, Nb/Ta ratios of the porphyries (average 11.8) are also similar to those of the Cathaysia Proterozoic crystalline basement (average 12.4). These geochemical features indicate that the porphyries were derived from high-degree partial melting of the crystalline basement in the Cathaysia Block. Based on the available geochemical data of the Dabaoshan porphyries, together with that of porphyry Mo deposits distributed along the Wuchuan-Sihui deep fault zone, it is concluded that the Dabaoshan porphyry Mo deposit is genetically related to the movements of the Wuchuan-Sihui deep fault induced by the subduction of the paleo-Pacific plate during the Jurassic.

The geochemistry, U-Pb and Re-Os geochronology, and Hf isotopic constraints on the genesis of the Huangjiagou Mo deposit and related granite in the Dabie region, Hubei Province, China

In recent years, a number of Mo deposits have been discovered in the Dabie region of the Qinling-Dabie orogenic belt. These deposits have a genetic relationship with Late Jurassic to Early Cretaceous granites. In this study, we present a geochemical and geochronologic study on the recently found Huangjiagou Mo deposit, which is the first Mo deposit found in Hubei Province from the Dabie region. The Huangjiagou deposit is currently under exploration, and a middle-sized Mo deposit with 100,000t of proven Mo metal reserves has already been reached. Mo mineralization mainly occurs as small quartz-molybdenite veins and disseminations within the country rocks next to the Huangjiagou granite. Re-Os isotope dating of molybdenites from the ore veins yields an age of 137.0±8.1Ma, which is consistent with the LA-ICP-MS zircon U-Pb age of the Huangjiagou granite (137.6±1.6Ma), suggesting a coeval and causative relation between the granite and mineralization. Geochemical analysis indicates that the Huangjiagou granite is high-K calc-alkalic to shoshonitic and weakly peraluminous with relatively low Mg contents. The rocks are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs). The (La/Yb)N ratios vary from 24.39 to 40.37, and no significant Eu anomalies occur for these samples. The rocks also show enrichment in large ion lithophile elements (LILEs, e.g., Pb, Ba) and depletion in high-field-strength elements (HFSEs, e.g., Nb, Ti). The Hf isotopic compositions of zircons from the granite indicate εHf(t) variations from −17.3 to −22.1 (average −19.8±0.5) with two-stage model ages (TDM2) from 2.26 to 2.56Ga (average 2.42Ga). These geochemical and isotopic features imply that the Huangjiagou granite was likely derived from partial melting of a thickened lower crust that mainly contained garnet and clinopyroxene without feldspar. The abundance of Re in the molybdenites also indicates that the Mo may be derived from the thickened crust with possible minor mantle material mixing. Considering all of the available data for the Mo deposits and related granites in the entire Dabie region, the Mesozoic Mo mineralization can be divided into two stages. The early-stage Mo mineralization (>130Ma) is mainly related to granites that resulted from partial melting of the thickened lower crust (including eclogite) with few mantle components added, whereas the late-stage Mo mineralization (<130Ma) is related predominantly to continental crust melts, as eclogite had already been removed at ca. 130Ma in the region.

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircon. The weak positive correlation between 176Lu/177Hf and 176Hf/177Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values (from +9.8 to –3.3) and model age TC 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive εHf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower εHf and higher TC values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.