Zircon Hf Isotopic Compositions Research Articles

Using the magmatic record to constrain the growth of continental crust—The Eoarchean zircon Hf record of Greenland

Southern West Greenland contains some of the best-studied and best-preserved magmatic Eoarchean rocks on Earth, and these provide an excellent vantage point from which to view long-standing questions regarding the growth of the earliest continental crust. In order to address the questions surrounding early crustal growth and complementary mantle depletion, we present Laser Ablation Split Stream (LASS) analyses of the U–Pb and Hf isotope compositions of zircon from eleven samples of the least-altered meta-igneous rocks from the Itsaq (Amîtsoq) Gneisses of the Isukasia and Nuuk regions of southern West Greenland. This analytical technique allows a less ambiguous approach to determining the age and Hf isotope composition of complicated zircon. Results corroborate previous findings that Eoarchean zircon from the Itsaq Gneiss (∼3.85 Ga to ∼3.63 Ga) were derived from a broadly chondritic source. In contrast to the Sm–Nd whole rock isotope record for southern West Greenland, the zircon Lu–Hf isotope record provides no evidence for early mantle depletion, nor does it suggest the presence of crust older than ∼3.85 Ga in Greenland.Utilizing LASS U–Pb and Hf data from the Greenland zircons studied here, we demonstrate the importance of focusing on the magmatic (rather than detrital) zircon record to more confidently understand early crustal growth and mantle depletion. We compare the Greenland Hf isotope data with other Eoarchean magmatic complexes such as the Acasta Gneiss Complex, Nuvvuagittuq greenstone belt, and the gneissic complexes of southern Africa, and all lack zircons with suprachondritic Hf isotope compositions. In total, these data suggest only a very modest volume of crust was produced during (or survived from) the Hadean and earliest Eoarchean. There remains no record of planet-scale early Earth mantle depletion in the Hf isotope record prior to 3.8 Ga.

Identification of Eocene-Oligocene magmatic pulses associated with flare-up in east Iran: Timing and sources

The Sangan Magmatic Complex (SMC), at the northeastern edge of the Lut block, includes a thick pile of extrusive and pyroclastic rocks, intruded by younger granitoid stocks. New zircon U–Pb ages show subaerial eruptions at ~42–44Ma, followed by emplacement of granitoids at ~41–40Ma. The granitoids have high K2O (~3.6–5.9wt%), with SiO2 (~63.1 and 71.9wt%) contents. They are metaluminous to peraluminous, calc alkaline and I-type in composition. The SMC magmatic rocks have typical high–K and shoshonitic signatures, and are characterized by enrichment in large–ion lithophile elements (LILEs) and depletion in high–field–strength elements (HFSE). Zircon εHf(t) from the SMC magmatic rocks ranges from +0.45 to +3.5 for volcanic rocks, −1.6 to +2.5 for granitoids and −4.1 to −1.4 for ignimbrites. Zircon δ18O values for the SMC are variable from +6.1 to +8.1‰, significantly higher than those of mantle–derived melts. The whole–rock εNd(t) values range between −4.5 to −3.5 for granitoids, −4.6 to −3 for volcanic rocks and −5.3 to +0.7 for ignimbrites. The whole–rock Nd and zircon Hf crustal model ages (TDMC) for the SMC magmatic rocks range between 0.8 and 1.2Ga. All of the SMC magmatic rocks have quite similar trace element patterns, and slightly different whole–rock Nd and zircon Hf isotopic composition. High 207Pb/204Pb and 208Pb/204Pb values for the SMC rocks indicate the involvement of the thick continental crust during the formation of these rocks. Modeling of zircon O–Hf, bulk–rock trace elements, and Sr–Nd isotopes suggest the magmas were generated by interaction of mantle–derived melts with thick continental crust through AFC processes. Compiled geochronological and geochemical data from east Iranian magmatic belt and Urumieh-Dokhtar magmatic belt allow identification of Eocene-Oligocene flare-up associated with several magmatic pulses.

Petrogenesis of syn-orogenic rare metal pegmatites in the Chinese Altai: Evidences from geology, mineralogy, zircon U-Pb age and Hf isotope

Recently, an increasing number of anatetic pegmatites have been confirmed. In contrast to the granitic pegmatite, the anatetic pegmatite has no relation to granite. It is therefore necessary to reacquaint the origin and classification of pegmatitic rocks. As one of the largest pegmatite provinces, the Chinese Altai provides a natural laboratory for pegmatite study. To date, the type, age, source and tectonic setting of the syn-orogenic pegmatites in the Chinese Altai remain unclear. In this work, the CMS (Chemical composition-Mineral assemblage-Structural geology) classification is primarily applied on the pegmatitic rocks in the Chinese Altai. Five types’ pegmatitic rocks, including metapegmatite, pegmatoid, pegmatite, pseudopegmatite and granite pegmatite, are identified by their CMS features. In addition, the geology, mineralogy, zircon U-Pb ages and Hf isotopes of six pegmatites are presented in this work. The pegmatites, numbered JMHB02, QBL02, TLT01, ALJK01, AMLG01 and TMLT01, are dated at 394.8 ± 4.0 Ma, 402.6 ± 5.5 Ma, 385.9 ± 3.5 Ma, 368.0 ± 4.0 Ma, 358.3 ± 4.6 Ma and 333.0 ± 6.0 Ma, respectively, indicating that they were formed during the syn-orogenic stage of the Altai orogenic process. They are exposed in the Qiongkuer domain in the southern Chinese Altai, juxtaposed with the late- and/or post-orogenic pegmatites in the same pegmatite field. No parental granite can be confirmed for all these pegmatites. Comparatively, the synorogenic pegmatites are scarce, and have lower mineralization grades than the late- and postorogenic pegmatites. Zircon Hf isotopic compositions of the syn-orogenic pegmatites have positive ɛHf(t) values of +2.06–+8.28 and old TDM2 ages of 1,234–860 Ma and are consistent with those of most of the syn-orogenic I- and S-granites, which indicates that the pegmatites and granites have a common immature crustal source from the early Paleozoic metasedimentary rocks. Compared to the post-orogenic pegmatites in the Central Altai domain, the syn-orogenic pegmatites in this work have larger positive ɛHf(t) values and younger TDM2 ages, indicating that more juvenile components were involved in their magma sources. Combining this study with previous works, we suggest that the anatexis of metasediments induced by decompression and injection of heat energy and mantle-derived materials under a forearc extensional setting led to the formation of the syn-orogenic pegmatites in the Chinese Altai.

Geochronology and geochemistry of Late Devonian-Carboniferous igneous rocks in the Songnen-Zhangguangcai Range Massif, NE China: Constraints on the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt

This paper presents new zircon U–Pb ages and Hf isotopic data, and whole–rock geochemical data for Late Devonian–Carboniferous igneous rocks from the Songnen–Zhangguangcai Range Massif (SRM), NE China, to constrain the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt and the nature of the crust beneath the SRM. LA–ICP–MS zircon U–Pb dating reveals two stages of magmatism in the Late Devonian (366Ma) and Carboniferous (325–313Ma). The Late Devonian magmatic event produced alkali feldspar granites with high SiO2 and total alkali contents, as well as pronounced negative Eu anomalies, similar to A-type granites. Based on these features, their high zircon εHf(t) values (+9.6 to +13.0), and the early Paleozoic tectonic evolution of the eastern SRM, we conclude that the primary magma for the Late Devonian granites was generated by partial melting of juvenile lower crust in a post-collisional extensional environment. The Carboniferous magmatic event produced a suite of intermediate–felsic igneous rocks. The intermediate rocks, including basaltic andesite, andesite, and quartz monzodiorites, have low SiO2 contents, high Mg#, high Cr, Co, and Ni contents, and display arc-like trace element characteristics. Based on these observations and their high zircon εHf(t) values (+5.8 to +10.7), we suggest their primary magma was derived from partial melting of depleted mantle, metasomatized by subduction-related fluids. The felsic rhyolites and monzogranites have high SiO2 contents, low Mg#, extremely low Cr, Co, and Ni contents, and zircon εHf(t) values of +10.5 to +13.7 and +4.2 to +6.9, respectively. These results indicate that the primary magma for the felsic rocks was derived from partial melting of heterogeneous juvenile crust. The Carboniferous igneous rock assemblages, together with coeval carbonate sedimentary formations, record an intra-plate extensional setting. The zircon εHf(t) data of Paleozoic igneous rocks from the eastern SRM define a temporal trend that is consistent with tectonic evolution. The increase in εHf(t) values during the early late Paleozoic age mirrors changes in magma source, corresponding to a switch from subduction–collision to post-collision tectonic regimes in the SRM. The zircon Hf isotopic compositions of the studied Late Devonian–Carboniferous igneous rocks record crustal accretion in the eastern SRM during the Neoproterozoic–early Paleozoic. Reworking of these crustal materials occurred in a post-collisional extensional environment during the late Paleozoic.

Convergent continental margin volcanic source for ash beds at the Permian-Triassic boundary, South China: Constraints from trace elements and Hf-isotopes

Volcanic activity around the time of the Permian-Triassic boundary (PTB) has been proposed as a trigger for the associated biocrisis. Multiple claystone beds are prominent near the PTB sections in South China. Twenty one PTB ash beds from three sections at Shangsi, Jianshi and Meishan in South China were sampled and analyzed. Volcanic ash geochemistry indicate dacite and rhyolite compositions. Zircons from the ash layers yield comparatively low Nb/Hf and high Th/Nb ratios, falling in the range of arc/orogenic-related settings. Zircon Hf-isotope compositions show that εHf(t) values vary from −11.7 to 1.8, indicating involvement of both juvenile and ancient crustal components. The ash beds (SS27a, JS129, JS130, JS133,MS25, MS26) near the biotic extinction horizon display a large variation in εHf (t) and relatively positive average values, indicating input of juvenile mantle or crust, and implying rapid transit through the older basement of the South China Craton. Spatial and temporal distribution of ash beds from thirty one PTB sections worldwide reveal that volcanic ash beds occur mainly in, or proximal to, the Tethys region and were sourced locally. Integration of Hf-isotope and trace-element compositions from magmatic zircons suggests that the PTB volcanism occurred along the convergent continent margin in, or near, southwestern South China as a result of the closure of the Paleo-Tethys Ocean.

Component variation in the late Neoproterozoic to Cambrian sedimentary rocks of SW China – NE Vietnam, and its tectonic significance

The Nanwenhe-Song Chay (NSC) area straddles the border of SW China and NE Vietnam, at the tectonic junction of the Yangtze, Cathaysia, and Indochina blocks. However, the lack of a systematic understanding of the basement rocks in this area has led to a debate about its tectonic affinity. This study presents integrated bulk geochemical and zircon U-Pb-Hf isotopic data for the basement rocks in the NSC area. Zircon U-Pb dating results suggest that the Mengdong Group in SW China and the Thac Ba Formation in NE Vietnam were deposited in the late Neoproterozoic (<619–592 Ma), while the Tianpeng Formation in SW China and the Ha Giang Formation in NE Vietnam were deposited in the Cambrian (<527–507 Ma). Bulk geochemical data and zircon Hf-isotope compositions indicate that the sources of these sedimentary rocks were dominated by felsic rocks. The sources of the late Neoproterozoic sedimentary rocks probably underwent weaker weathering under drier and colder conditions than those of the Cambrian ones. The late Neoproterozoic Mengdong Group and the Thac Ba Formation contain abundant 802–747 Ma zircons. Their age distributions and the Hf-isotope compositions are similar to those of Neoproterozoic sedimentary rocks in the southern Yangtze Block (Fanjingshan-Sibao area). In contrast, the Cambrian Tianpeng and Ha Giang formations are characterized by Grenvillian detrital zircons with an age peak at ∼980 Ma. The age spectra and Hf-isotope compositions of the detrital zircons from these rocks are similar to those in the Cathaysia Block (Nanling-Yunkai area) and/or Indochina Block.Integration of our data with published geological data suggests that the Precambrian basement under the NSC area is the part of the Yangtze Block. Thus, the western boundary between Yangtze and Cathaysia blocks must be located to the south or southeast and the boundary between the South China and Indochina blocks should be to the southwest of the NSC area.Numerous Pan-African (600–500 Ma) detrital zircons and the diagnostic age spectra of detrital zircons in the early Paleozoic sedimentary rocks indicate that the SCB was located at the northern margin of East Gondwana during the early Paleozoic. The dramatic change in the clastic components from the late Neoproterozoic to Cambrian sedimentary rocks suggests that the NSC area was probably affected by the Pan-African orogeny.

Geochemistry, geochronology and Hf isotope of granitoids in the Chinese Altai: Implications for Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The Chinese Altai in northwestern Xinjiang has numerous outcrops of granitoids which provide critical information on accretionary orogenic processes and crustal growth of the Central Asian Orogenic Belt. Zircon U–Pb ages, Hf-isotopic compositions and whole-rock geochemistry of monzogranite and granodiorites in the Qinghe County are employed to elucidate Paleozoic tectonics of the Chinese Altai. Granodiorites have crystallization ages of 424.6 ± 3.1 Ma (MSWD = 0.23) and 404.0 ± 3.4 Ma (MSWD = 0.18); monzogranite was emplaced in the early Permian with a crystallization age of 293.7 ± 4.6 Ma (MSWD = 1.06). Both granodiorites and monzogranite are I-type granites with A/CNK ratios of 0.92–0.97 and 1.03–1.06, respectively. They also show similar geochemical features of high HREE and Y contents, low Sr contents and Sr/Y ratios, as well as enrichment of Cs, Rb, Th and U, and depletion of Nb, Ta, P and Ti. These geochemical features indicate that the monzogranite and granodiorites were formed in an arc setting related to subduction. The gneissic monzogranites display high SiO2 and K2O contents, and belong to the high-K calc-alkaline series. In the chondrite normalized REE distribution pattern, the monzogranite samples exhibit enrichment of LREE with strong negative Eu anomalies (δEu = 0.44–0.53), zircon εHf(t) values from +7.24 to +12.63 and two-stage Hf model ages of 463–740 Ma. This suggests that the monzogranite was generated from the mixing of pelitic and mantle material. The granodiorite samples are calc-alkaline granites with lower contents of SiO2 and Na2O + K2O, higher contents of TiO2, Fe2O3t, MgO and CaO compared to the monzogranite samples. They also show enrichment of LREE and moderate negative Eu anomalies (δEu = 0.54–0.81), as well as slightly higher differentiation of LREE than that of HREE. The 425 Ma granodiorite has zircon εHf(t) values from −0.51 to +1.98 and two-stage Hf model ages of 1133–1240 Ma, whereas the 404 Ma granodiorite displays those of +2.52 to +7.50 and 816–1071 Ma. Geochemistry and zircon Hf isotopic compositions indicate that granodiorites were formed by partial melting of juvenile lower crust. Together with regional geology and previous data, the geochemical and geochronological data of the monzogranite and granodiorites from this study suggest long-lived subduction and accretion along the Altai Orogen during ca. 425–294 Ma.

U–Pb ages and Hf isotopic composition of zircons and whole rock geochemistry of volcanic rocks from the Fangniugou area: Implications for early–middle Paleozoic tectonic evolution in Jilin Province, NE China

U–Pb ages and Hf isotopic composition of zircons and whole rock geochemistry of volcanic rocks from the Fangniugou area: Implications for early–middle Paleozoic tectonic evolution in Jilin Province, NE China

Genesis of the Jurassic Dongfengbeishan porphyry Mo deposit in Eastern Yanbian, NE China inferred from molybdenite Re–Os and zircon U–Pb ages, and whole-rock elemental and zircon Hf isotopic compositions

The Dongfengbeishan porphyry Mo deposit is located to the southeast of the Lesser Xing’an Range–Zhangguangcai Range metallogenic belt in NE China. LA–ICP–MS zircon U–Pb dating of the Beishan monzogranite yields an age of 194.75 ± 0.81 Ma. Five molybdenite samples define a Re–Os isochron age of 194.0 ± 2.0 Ma, suggesting that mineralization was associated with emplacement of the Beishan monzogranitic pluton. Monzogranite samples are classified as high-K calc-alkaline and are relatively enriched in light rare earth elements, depleted in heavy rare earth elements, Nb, Ba, P, Sr, Ti, and Zr, and yield negative Eu anomalies. These characteristics are interpreted to record fractional crystallization of the parental magmas. Zircon εHf(t) values for Beishan monzogranite samples range from +8.2 to +15.9, and two-stage model ages (TDM2) range from 217 to 711 Ma, indicating that the parental magma was produced during partial melting of juvenile lower crust. Combined with previous studies, our data suggest that the Dongfengbeishan porphyry Mo deposit was associated with westward subduction of the paleo-Pacific plate in the Early Jurassic.

Petrogenesis of the Neoarchean diorite-granite association in the Wangwushan area, southern North China Craton: Implications for continental crust evolution

The crustal evolutionary scenarios and geodynamic driving mechanisms of the North China Craton (NCC) during the late Neoarchean (∼2.5 Ga) are still lacking comprehensive understanding due to subsequent strong deformation and metamorphic overprinting events. The widespread ∼2.5 Ga tectono-thermal activities throughout the NCC can be studied to constrain the tectonic evolution at this period. The Wangwushan Neoarchean diorites and high-K granites in the southern NCC were formed at ∼2.52–2.51 Ga. The diorites have high Mg#, Th, Y, low Sr and Sr/Y ratios and variably positive zircon εHf(t) (+2.4 – +8.4) and whole rock εNd(t) values (−0.26 – +3.24), indicating a depleted mantle wedge source which was metasomatized by the melted subducted silicic sediments. The depleted Nb, Ti and enriched LILE and Pb features indicate that the diorites were derived from slightly metasomatized mantle wedge in a subduction-related setting. The high-K granites show a shoshonite affinity and peraluminous features. The rocks have low 10,000 Ga/Al ratios, Zr and Zr + Nb + Ce + Y concentrations, and calculated zircon saturation temperature (TZr = 686–837 °C, 774 °C on average), belong to fractionated I-type granite. Low Sr, Y, flat HREE, and negative Eu anomalies indicate the presence of Ca-rich plagioclase and absence of garnet in the residue during partial melting of the sources, thus further indicating a shallower source with a pressure of <10 kbar and a depth less than 35 km. Zircon Hf and whole-rock Nd isotopic compositions (εHf(t) = −2.7 to +7.4, εNd(t) = +1.25 to +3.09, TDM2 ages are 3.20–2.56 Ga and 3.20–2.65 Ga, respectively) are identical to those of the Neoarchean TTGs, amphibolites and diorites in the Wangwushan area, which likely represent plausible sources for the high-K granites. The partial melting was probably triggered by underplating of mantle-derived mafic magmas in a post-collision setting. Together with 2.57–2.52 Ga TTGs and amphibolites in this area, the magmatic spectrum of these igneous rocks are typical of subduction-related magmatism, involving multi-stage processes in a convergent plate margin, and likely record a transitional regime from early oceanic plate subduction to late post-collisional extension.

Origin and tectonic evolution of early Paleozoic arc terranes abutting the northern margin of North China Craton

The origin and tectonic evolution of the early Paleozoic arc terranes abutting the northern margin of the North China Craton (NCC) are widely debated. This paper presents detrital zircon U–Pb and Hf isotopic data of early Paleozoic strata in the Zhangjiatun arc terrane of central Jilin Province, northeast (NE) China, and compares them with the Bainaimiao and Jiangyu arc terranes abutting the northern margin of the NCC. Detrital zircons from early Paleozoic strata in three arc terranes exhibit comparable age groupings of 539–430, 1250–577, and 2800–1600 Ma. The Paleoproterozoic to Neoarchean ages and Hf isotopic composition of the detrital zircons imply the existence of the Precambrian fragments beneath the arc terranes. Given the evidences from geology, igneous rocks, and detrital zircons, we proposed that the early Paleozoic arc terranes abutting the northern margin of the NCC are a united arc terrane including the exotic Precambrian fragments, and these fragments shared a common evolutionary history from Neoproterozoic to early–middle Paleozoic.

Do Hf isotopes in magmatic zircons represent those of their host rocks?

Lu–Hf isotopic system in zircon is a powerful and widely used geochemical tracer in studying petrogenesis of magmatic rocks and crustal evolution, assuming that zircon Hf isotopes can represent initial Hf isotopes of their parental whole rock. However, this assumption may not always be valid. Disequilibrium partial melting of continental crust would preferentially melt out non-zircon minerals with high time-integrated Lu/Hf ratios and generate partial melts with Hf isotope compositions that are more radiogenic than those of its magma source. Dissolution experiments (with hotplate, bomb and sintering procedures) of zircon-bearing samples demonstrate this disequilibrium effect where partial dissolution yielded variable and more radiogenic Hf isotope compositions than fully dissolved samples. A case study from the Neoproterozoic Jiuling batholith in southern China shows that about half of the investigated samples show decoupled Hf isotopes between zircons and the bulk rocks. This decoupling could reflect complex and prolonged magmatic processes, such as crustal assimilation, magma mixing, and disequilibrium melting, which are consistent with the wide temperature spectrum from ∼630 °C to ∼900 °C by Ti-in-zircon thermometer. We suggest that magmatic zircons may only record the Hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their Hf isotopic compositions can represent the primary Hf isotopic compositions of the bulk magmas. In this regard, using zircon Hf isotopic compositions to trace crustal evolution may be biased since most of these could be originally from disequilibrium partial melts.

Long‐Term Geochemical and Geodynamic Segmentation of the Paleo‐Pacific Margin of Gondwana: Insight From the Antarctic and Adjacent Sectors

AbstractCombined zircon geochemistry and geochronology of Mesozoic volcaniclastic sediments of the central Transantarctic Mountains, Antarctica, yield a comprehensive record of both the timing and geochemical evolution of the magmatic arc along the Antarctic sector of the paleo‐Pacific margin of Gondwana. Zircon age populations at 266–183 Ma, 367–328 Ma, and 550–490 Ma correspond to episodic arc activity from the Ediacaran to the Jurassic. Zircon trace element geochemistry indicates a temporal shift from granitoid‐dominated source(s) during Ediacaran to Early Ordovician times to mafic sources in the Devonian through Early Jurassic. Zircon initial εHf shifts to more radiogenic Hf isotope compositions following the Ross Orogeny and is inferred to reflect juvenile crustal growth within an extensional arc system during progressive slab rollback. These new ages and Hf isotopic record are similar to those from the Australian sector, indicating that these regions constituted an ~3,000 km laterally continuous extensional arc from at least the Carboniferous to the Permian. Conversely, the South American sector records enriched zircon Hf isotopic compositions and compressional/advancing arc tectonics during the same time period. Our new data constrain the location of this profound along‐arc geochemical and geodynamic “switch” to the vicinity of the Thurston Island block of West Antarctica.

A newly identified Precambrian terrane at the Pamir Plateau: The Archean basement and Neoproterozoic granitic intrusions

A number of Precambrian terranes are distributed in the Tibetan Plateau, including the Lhasa, Sibumasu, Qiangtang, Qaidam, Qilian and Tianshuihai terranes. These terranes exhibit distinct features in rock associations and tectonothermal history. They are important for understanding the evolution of the Tethys Ocean and the assemblage and breakup of the Precambrian supercontinents. In this contribution, we identify a Precambrian terrane at the southeastern section of the Pamir Syntax and we name it as Mazar terrane. Systematic zircon U-Pb dating shows that the basement, mainly composed of metamorphic bimodal volcanic rocks and gneiss/schist, deposited at ca.2.5 Ga and was overprinted by ca.2.0 Ga amphibolite-facies metamorphism. The Neoproterozoic granitic intrusions into the basement, emplaced at 840–835 Ma. Geochemistry of the granitic intrusions defines two distinct types of granitic rocks, including the trondhjemite-tonalites and granodiorites. The sodic trondhjemite- tonalites show TTG-like or adakitic signatures such as enrichment of LREE and LILEs, depletion of HREE and HFSE, resulting in their high Sr/Y and (La/Yb)N ratios. The granodiorites exhibit typical signatures of the calc-alkaline I-type granites. Both trondhjemite-tonalites and granodiorites have coupled unradiogenic whole-rock Nd and zircon Hf isotope compositions. εNd(T) and εHf(T) values of the trondhjemite- tonalites range from −9.5 to −8.7 and from −14 to −10, respectively. The granodiorites have slightly more radiogenic Nd-Hf isotope compositions with εNd(T) values ranging from −5.5 to −4.4 and εHf(T) values ranging from −10 to −7. According to elemental and isotope signatures of the granitic intrusions, the trondhjemite-tonalites were derived from partial melting of a mafic crust at depth ≥30 km with a garnet amphibolite residue whereas the granodiorite was formed by partial melting of a mafic crust at a shallower level and mantle-derived magma could be involved in the genesis of the granodiorite. In line with their geochemistry and the fact that the granodiorite is slightly younger than that of the trondhjemite-tonalites, we propose that those granitic intrusions were most likely formed in an extensional environment, and that they could be part of the earliest igneous activity related to the initial breakup of the Rodinia supercontinent. Our study, in combination with previous works, suggests that the Mazar terrane, as well as the Qilian, Qaidam, Tianshuihai terranes, is most likely continental fragments drifted from the Yangtze Block during the breakup of the Rodinia supercontinent.

Petrogenesis of Cretaceous volcanic-intrusive complex from the giant Yanbei tin deposit, South China: Implication for multiple magma sources, tin mineralization, and geodynamic setting

The giant Yanbei tin ore deposit is the largest porphyry-type tin deposit in South China. The orebodies are hosted by the granite porphyry in the central part of the Yanbei volcanic basin in southern Jiangxi Province. The Yanbei volcanic-intrusive complex mainly consists of dacitic-rhyolitic volcanic rocks, granite, granite porphyry and diabase dikes. In previous papers, the granite porphyry was considered as subvolcanic rocks, which came from the same single magma chamber with the volcanic rocks. In this study, zircon U-Pb ages and Hf isotope data, as well as whole-rock geochemical and Sr-Nd isotopic compositions of different magmatic units in the Yanbei complex are reported. Geochronologic results show that various magmatic units have different formation ages. The dacite yielded a zircon U-Pb age of 143±1Ma, and the granite porphyry has the emplacement age of 138±1Ma. Diabase dikes which represented the final stage of magmatism, yielded a zircon U-Pb age of 128±1Ma. Distinctive whole rock Sr-Nd and zircon Hf isotopic compositions suggest that these magmatic units were derived from different magma sources. The volcanic rocks were mainly derived from the partial melting of Paleoproterozoic metasedimentary rocks without additions of mantle-derived magma. The granite porphyry has an A-type geochemical affinity, and was derived from remelting of Paleo-Mesoproterozoic crustal source with involvement of a subordinate mantle-derived magma. The granite porphyry is also a typical stanniferous granite with high F (4070–6090ppm) and Sn (7–39ppm) contents. It underwent strongly crystal fractionation of plagioclase, K-feldspar, and accessory minerals (like apatite, Fe-Ti oxides), which may contribute to the tin mineralization. The diabase was derived by partial melting of enriched lithospheric mantle which had been metasomatised by slab-derived fluids. The change of magmatic sources reflected an increasing extensional tectonic environment, perhaps induced by slab rollback of subducted paleo-Pacific plate.

Crustal Evolution of NW Iran: Cadomian Arcs, Archean Fragments and the Cenozoic Magmatic Flare-Up

The Cadomian orogen of NW Iran includes a series of metamorphic rocks with zircon U-Pb ages between ca 562 and 505 Ma (Ediacaran to middle Cambrian). The Ediacaran-Cambrian basement is intruded by a series of Late Eocene-Late Oligocene I-type granitic rocks. U-Pb geochronology, integrated with geochemical and isotopic data for the basement rocks in NW Iran, provides further evidence of a Cadomian (562-505 Ma) arc-related magmatic event lasting ∼ 60 Myr. Cadomian magmatism in Iran was a part of a ∼ 100 Myr long episode of subduction-related arc magmatism at the northern margin of Gondwana. Zircon Hf-isotope compositions show that during Cadomian magmatic arc activity, juvenile arc magmas interacted with reworked Archean crust to generate the Ediacaran-Cambrian igneous rocks. Our results document both inheritance of old zircons and the presence of zircons with juvenile signatures in NW Iran, suggesting that the geotectonic setting for the Cadomian rocks was an Ediacaran continental magmatic arc and probably a neighboring back-arc basin. The occurrence of Ediacaran ophiolitic slices in NW Iran may provide evidence of back-arc basin opening at that time. Cenozoic plutonism in NW Iran is part of an Eocene-Oligocene magmatic ‘flare-up’ along the Urumieh-Dokhtar Magmatic Belt in central Iran, which lasted for ca 30 Myr. The melts responsible for the formation of these rocks had an essentially juvenile signature with minor contamination by Archean to Cadomian middle-lower continental crust. Continuous convergence between Arabia and Iran was accompanied by the transition of SW Eurasia from a compressional to an extensional convergent plate margin in Eocene-Oligocene times, leading to orogenic collapse, core-complex formation, exhumation of Cadomian crust and a major increase in arc magmatism.

Petrogenesis and metallogenic implications of Late Mesozoic intrusive rocks in the Tongling region, eastern China: a case study and perspective review

ABSTRACTLarge-scale Cu–Au mineralization is associated with Late Mesozoic intrusive rocks in the Tongling region of eastern China, which mainly comprise pyroxene monzodiorite, quartz monzodiorite, and granodiorite. To constrain the petrogenesis of the intrusive rocks and Cu–Au mineralization, detailed analyses of the geochronology, apatite in situ geochemistry, whole-rock geochemistry, and zircon Hf isotopic compositions were performed. Magmatic zircons from pyroxene monzodiorites, quartz monzodiorites, and granodiorites yield U–Pb ages of 136–149 Ma, 136–146 Ma, and 138–152 Ma, respectively, indicating that their formation ages are contemporaneous. Quartz monzodiorites and granodiorites (SiO2 = 57.9–69.5 wt.%) are highly potassic calc-alkaline rocks with adakitic affinity and have low MgO and Y contents, low zircon εHf(t) values (−11.7 to −39.0), high apatite Cl contents (>0.2 wt.%), and log fO2 values (−23.2 to −8.23), indicating that they may have formed when metasomatized mantle-derived magmas mixed with slab-derived magmas before undergoing crustal assimilation and fractional crystallization. Pyroxene monzodiorites (SiO2 = 48.4–53.0 wt.%) are shoshonitic and record high MgO, P2O5, and Y contents, high zircon εHf(t) values (1.55 to −7.87), high oxygen fugacity, low Nb and Ta contents, and low apatite Cl contents (mainly <0.2 wt.%), suggesting that they were primarily derived from a metasomatized lithospheric mantle-derived magma that experienced the assimilation of lower crustal materials. The results indicate that the intrusive rocks and associated large-scale Cu–Au mineralization of the Tongling region resulted from the partial melting of the subducted oceanic slab in an oxidizing environment.

Formation of the Permian Taipinggou igneous rocks, north of Luobei (Northeast China): implications for the subduction of the Mudanjiang Ocean beneath the Bureya–Jiamusi Massif

ABSTRACTControversy has long surrounded the tectonic framework and evolution of the Mudanjiang Ocean between the Bureya–Jiamusi–Khanka Massif and Songnen–Zhangguangcai Range Massif, which are located in the easternmost segment of the Central Asian Orogenic Belt. To address these issues, we present zircon U-Pb ages, geochemical data, and zircon Hf isotopic compositions of the Taipinggou amphibolite and metagabbro exposed along the boundary area of Bureya–Jiamusi Massif and Songnen–Zhangguangcai Range Massif. Magmatic zircons from the amphibolite and metagabbro yield 206Pb/238U ages of 267 ± 2 Ma and 264 ± 2 Ma, respectively, which are interpreted as protolith ages. The geochemical data of the amphibolite samples show transitional characteristics of calcalkaline to tholeiitic series, with high MgO concentrations (9.44–10.48 wt.%) and Mg-numbers (73–75). These samples are enriched in large ion lithophile elements (e.g. Rb, Ba, and K) and light rare earth elements and are depleted in high-field-strength elements (e.g. Nb, Ta, and Ti) and heavy rare earth elements, with εHf(t) values of −6.63 to −3.26. It is inferred that the parental magma originated from an enriched lithospheric mantle that had been metasomatized by fluids derived from subducted oceanic slab. During magma evolution, the magma that formed the amphibolite mainly experienced accumulation with a shallow-level evolutionary process involving fractional crystallization. The Taipinggou metagabbro samples are subalkaline series and also characterized by enrichment in large ion lithophile elements (e.g. Rb, Ba, and K) and light rare earth elements and by depletion in Nb–Ta–P–Ti, with εHf(t) values of −3.09 to +1.16. The Taipinggou metagabbro and amphibolite have similar geochemical and Hf isotopic compositions, indicating a common parental magma source but with different degrees of magmatic differentiation. Based on the new geochronological and geochemical data presented in this study, we propose that both the Taipinggou metagabbro and amphibolite formed in a Middle Permian continental arc setting, closely related to eastward subduction beneath the Bureya–Jiamusi Massif. Combined with previous studies and regional geological observations, we suggest that a double-side subduction model is favoured for the Late Palaeozoic–Early Mesozoic geodynamic processes along the boundary area of Bureya–Jiamusi–Khanka Massif and Songnen–Zhangguangcai Range Massif.

Petrogenesis of the Baishan granite stock, Eastern Tianshan, NW China: Geodynamic setting and implications for potential mineralization

Located in a region rich in Cu-Ni and Mo mineralization, the Baishan granitic stock is barren for reasons that remain enigmatic. Whole rock elemental and Sr-Nd isotope analysis, major element analysis of a number of minerals, and zircon trace element, U-Pb and Hf isotope analysis were undertaken in order to reveal the petrogenesis of the granites. All granites show typical I-type characteristics including metaluminous to slightly peraluminous, calc-alkaline signatures with a strong depletion of Nb, Ta, Ti and P, enrichment of light rare earth elements and large ion lithophile elements (e.g., Cs, Rb, Th, U, K). In addition, a strong depletion in Ti and P, highly fractionated light rare earth element patterns and less fractionated heavy rare earth element patterns, and negative correlations between SiO2 and TiO2, Al2O3, MgO, FeOT, P2O5, Zr and Hf suggest significant fractional crystallization of amphibole, apatite, zircon and Ti-bearing minerals. Whole rock Sr-Nd and zircon Hf isotopic compositions show wide variations with (87Sr/86Sr)i values of 0.70358 to 0.70505, εNd (t) of 3.8 to 7.2, and εHf (t) of 2.4 to 12.2 indicating derivation from partial melting of juvenile lower crust with obvious addition of ancient crust. Zircon U-Pb ages indicate a formation age of ~292Ma, significantly older than the ore-forming granite porphyry and slightly older than the regional mafic-ultramafic, A-type and diabase magmatism of Eastern Tianshan. The granite stocks were likely derived during heating of ascending asthenospheric mantle above a mantle plume in the Early Permian. Mineral chemistry, saturation thermometry, mineral species and whole rock Fe2O3/FeO ratios indicate a crystallization temperature of >980 to 665°C, pressure of ~1.6kbar and oxygen fugacity of ≤NNO for the granite stock. Comparing the geochemistry, magma source and crystallization environment for the Early Permian barren granite and Late Triassic ore-related granite porphyry, the low ratios of Sr/Y and low (La/Yb)N, and reduced oxidation state (≤NNO) in the granitic stock are signatures of infertility for the Early Permian granite. This study implies high Mo mineralization potential for granitic rocks with high Sr/Y, (La/Yb)N and highly oxidized conditions.

Aulacogen Formation in Response to Opening the Ailaoshan Ocean: Origin of the Qin-Fang Trough, South China

AbstractThe Qin-Fang Trough, South China, trends northeast-southwest, orthogonal to the adjoining southern margin of the craton. The Devonian strata within the trough are unconformable on late Neoproterozoic units of the Yunkai Massif, indicating that strata within the trough are autochthonous. U-Pb ages and Hf isotope compositions of detrital zircons from the Silurian to Devonian succession are consistent with derivation from the massif. In comparisons of our data with those from equivalent units in the Ailaoshan Belt and Hainan Island, detrital zircons from the Silurian strata show similar age distributions and Hf isotope compositions, indicating that the three areas shared a common source and were adjacent to each other during the Silurian. In contrast, the age distributions of detrital zircons preserved in Devonian strata in the Qin-Fang area are different from those of equivalent units in the Ailaoshan Belt. This, along with the absence of Devonian strata on Hainan Island, suggests that the Qin-Fang ...