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1.
Late Mesoproterozoic igneous rocks in the SW Yangtze Block are important for understanding the role of it in reconstruction of the Rodinia supercontinent.In the present study,we report new geochronological,geochemical,and Nd-Hf isotopic data for the Cuoke plagioclase amphibolites and granites in the SW Yangtze Block.Geochronological results show that the plagioclase amphibolites and granites have similar late Mesoproterozoic zircon U-Pb ages of 1168-1162 Ma,constituting a bimodal igneous assemblage.The plagioclase amphibolites have high and variable TiO2 contents(1.15-4.30 wt.%)and Mg#(34-66)values,similar to the tholeiitic series.They are characterized by enrichment in LREEs and LILEs,and have OIB-like affinities with positive Nb and Ta anomalies.The plagioclase amphibolites have positive whole-rockεNd(t)(+3.2 to+4.3)and zirconεHf(t)(+4.3 to+10.7)values,indicating that they were derived from an OIB-like asthenospheric mantle source.The granites belong to the reduced peralkaline A-type series and have negativeεNd(t)value of-6.0 andεHf(t)values of-5.8 to-13.8,indicating a derivation from the partial melting of ancient mafic lower crust.In combination with the~1.05-1.02 Ga bimodal igneous assemblage in the SW Yangtze Block,we propose that the Cuoke 1168-1162 Ma igneous rocks were likely formed in a continental rift basin and argue against the existance of Grenvillian Orogen in the SW Yangtze Block during the late Mesoproterozoic.  相似文献   

2.
In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at 3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.  相似文献   

3.
http://www.sciencedirect.com/science/article/pii/S1674987113000820   总被引：1，自引：1，他引：0  
The combined use of Hf,Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons.Sr and Nd seawater isotopes,although not as precise as εNd and εHf distributions,also record input from ocean ridge systems.Unlike detrital zircons where sources cannot be precisely located because of crustal recycling,both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data.Furthermore,primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly.Common to all of the isotopic studies are geographic sampling biases reflecting outcrop distributions,river system sampling,or geologists,and these may be responsible for most of the decorrelation observed between isotopic systems.Distributions between 3.5 and 2 Ga based on εHf median values of four detrital zircon databases as well as our compiled εNd database are noisy but uniformly distributed in time,whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations.Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens.Only after 1 Ga and before 3.5 Ga do we see wide variations and significant disagreement between databases,which may partially reflect variations in both the number of sample locations and the number of samples per location.External and internal orogens show similar patterns in εNd and εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions.However,both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode.Many secular changes in εHf and εNd distributions correlate with the supercontinent cycle.Although supercontinent breakup is correlated with short-lived decreasing εHf and εNd （≤ 100 Myr） for most supercontinents,there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna.Assembly of supercontinents by extroversion is recorded by decreasing εNd in granitoids and metasediments and decreasing εHf in zircons,attesting to the role of crustal reworking in external orogens in advancing mode.As expected,seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion.Pangea is the only supercontinent that has a clear isotopic record of introversion assembly,during which median εNd and εHf rise rapidly for ≤ 100 Myr.Although expected to increase,radiogenic seawater Sr decreases （and seawater Nd increases） during assembly of Pangea,a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode.The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime,as supported by Nd and Hf model ages.  相似文献   

4.
The East Kunlun Orogenic Belt(EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism.Here we report the petrology, bulk geochemistry, zircon Ue Pb dating and, Lue Hf and SreN d isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U-Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450 -430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their(~(87)Sr/~(86)Sr)ivalues range from 0.7059 to 0.7085, εNd(t) values from -1.6 to -6.0 and the zircon εHf(t) values show large variations from +9.1 to -8.6 with Hf model ages(T_(DM2)) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.  相似文献   

5.
How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U–Pb dating by SHRIMP RG and LA–ICP–MS U–Pb, zircon SHRIMP SI oxygen and LA–MC–ICP–MS Hf isotope, and whole-rock Nd–O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U–Pb isotopic dating and measured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (εHf(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic εHf(t) extending down to −3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The εHf(t) values of the Neoarchean zircons from these gneisses overlap the 3.4–3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0–2.8 Ga zircon evolution field. However, taking all the εHf(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustal materials and the depleted mantle is also supported by zircon O isotopic data and Hf–O isotopic modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world.  相似文献   

6.
Potassium (K)-rich mafic rocks are viewed as being derived from partial melting of an enriched mantle source, but it is controversial about which processes cause the mantle enrichment. The Fushui intrusive complex is the largest early Paleozoic K-rich intrusive complex in the eastern Qinling orogen. Therefore, detailed studies on the Fushui complex can contribute not only to understanding of the petrogenesis of K-rich mafic rocks, but also to unraveling the Paleozoic evolution of the Qinling orogen. In this study, we provide an integrated investigation of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock major and trace elements, as well as Sr–Nd–Hf isotopes, for the Fushui intrusive complex. In situ zircon secondary ion mass spectrometry (SIMS) / laser ablation induction coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating reveals that different rock types of the Fushui complex have identical formation ages of 488–484 Ma. The Fushui complex belongs to the shoshonitic series, and is characterized by extreme large ion lithophile element (LILE, e.g., Ba, U, Th and Sr) and Pb enrichment and depletion of high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P and Ti). It shows high initial Sr isotopic ratios (0.7100–0.7151), negative whole-rock ε Nd(t) (?3.97 to ?5.68) and negative to slight positive whole-rock (?2.24 to 2.38) and zircon (?2.85 to 0.34) ε Hf(t) values, as well as high zircon δ18O values (6.86 ± 0.13 ‰). The Hf–Nd isotopic systems are decoupled with positive Δε Hf values (3.85–5.37). These geochemical features indicate that the mantle source has incorporated subducted zircon–barren oceanic sediments. A simple two-end-members mixing model constrains the amount of subducted sediments in the Fushui mantle source to 5–8 %. The Fushui complex originated from 1 to 6 % equilibrium melting of a phlogopite-bearing garnet lherzolite by non-modal melting. As shoshonitic magmas have been discovered in modern nascent arcs, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean, when it jumped from the northern to the southern boundary of the North Qinling microcontinent.  相似文献   

7.
The felsic volcanogenic tuffs named “green-bean rocks” (GBRs), characterized by a green or yellowish green color, are widely distributed in the western Yangtze platform and have a high lithium content (286–957 ppm). This paper studies the ages, origin and tectonic setting of the GBRs in the Sichuan basin on the western margin of the Yangtze platform through the whole-rock geochemistry and zircon trace elements by using U–Pb dating and Hf–O isotopes. The GBR samples from the Quxian and Beibei sections yielded zircon U–Pb ages of 245.5 ± 1.8 Ma and 244.8 ± 2.2 Ma. These samples can be used as the isochronous stratigraphic marker of the Early–Middle Triassic boundary (EMTB) for regional correlation. The whole-rock and zircon geochemistry, and zircon Hf–O isotopes exhibited S-type geochemical affinities with high positive δ18O values (9.28‰–11.98‰), low negative εHf(t) values (?13.87 to ? 6.79), and TDM2 ages of 2150–1703 Ma, indicating that the lithium-rich GBRs were generated by the remelting of the pre-existing ancient Paleoproterozoic layer without mantle source contamination in the arc-related/orogenic tectonic setting. The results of this study demonstrate that the lithium-rich GBRs in the western Yangtze platform were derived from arc volcanic eruptions along the Sanjiang orogen, triggered by the closure of the eastern Paleo-Tethys Ocean and the syn-collision between the continental Indochina and Yangtze blocks at ca. 247 Ma. This was marked by a major shift from I-type magmas with intermediate εHf(t) values to S-type magmas with low negative εHf(t) values. Collectively, our results provide new insights into the origin of the GBRs and decodes the closure of the eastern Paleo-Tethys.  相似文献   

8.
《International Geology Review》2012,54(12):1445-1461
We present zircon U–Pb ages, Hf isotopes, and whole-rock geochemistry of the Xiaochuan gneissic granite intrusion, SE China, to constrain its petrogenesis and provide insights into early crustal evolution of the Cathaysia Block. LA-ICP-MS zircon U–Pb dating of a representative sample yields a weighted mean 206Pb/207Pb age of 1839 ±16 Ma, interpreted as the emplacement age of the Xiaochuan granite. Zircons have ?Hf(t) values ranging from –8.1 to 2.7 and T DM2 model ages from 2.23 to 3.03 Ga. The granites are strongly peraluminious (A/CNK = 1.14–1.41), with relatively high FeOt, TiO2, and CaO/Na2O, and low CaO, Al2O3/TiO2, and Rb/Sr values. In addition, they show strongly negative Ba, Sr, Nb, and Ta and positive Th and Pb anomalies in the primitive mantle-normalized spider diagram, similar to other Cathaysia Palaeoproterozoic S-type granites. The geochemical and Hf isotopic signatures suggest that the Xiaochuan gneissic granites were generated by partial melting of Archaean crustal materials in an intraplate extensional setting. Our results, combined with existing geochronological data, further demonstrate that the Wuyishan terrane is underlain by Palaeoproterozoic crystalline basement.  相似文献   

9.
Zircon from lower crustal xenoliths erupted in the Navajo volcanic field was analyzed for U–Pb and Lu–Hf isotopic compositions to characterize the lower crust beneath the Colorado Plateau and to determine whether it was affected by ∼1.4 Ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern Laurentia. Igneous zircon in felsic xenoliths crystallized at 1.73 and 1.65 Ga, and igneous zircon in mafic xenoliths crystallized at 1.43 Ga. Most igneous zircon has unradiogenic initial Hf isotopic compositions (ɛHf=+4.1–+7.8) and 1.7–1.6 Ga depleted mantle model ages, consistent with 1.7–1.6 Ga felsic protoliths being derived from “juvenile” Proterozoic crust and 1.4 Ga mafic protoliths having interacted with older crust. Metamorphic zircon grew in four pulses between 1.42 and 1.36 Ga, at least one of which was at granulite facies. Significant variability within and between xenoliths in metamorphic zircon initial Hf isotopic compositions (ɛHf=−0.7 to +13.6) indicates growth from different aged sources with diverse time-integrated Lu/Hf ratios. These results show a strong link between 1.4 Ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust.  相似文献   

10.
ABSTRACT

In this article we present zircon U–Pb ages, Hf isotopes, and whole-rock geochemistry of the Longzhu rhyolite porphyry from the Cathaysia Block, Southeast China to constrain its petrogenesis and provide insights into the early Precambrian tectonic evolution of the Cathaysia Block. LA-ICP-MS zircon U–Pb dating of a representative sample yields a weighted mean 206Pb/207Pb age of 1819 ± 16 Ma, interpreted as the crystallization age of the Longzhu rhyolite porphyry. Zircons from this sample have εHf(t) values ranging from – 8.4 to – 2.2 and THfDM2 model ages from 2.76 to 2.46 Ga. The whole-rock Nd isotopic data from the Longzhu rhyolite porphyries yield εNd(t) values spanning – 6.3 to – 4.7 and TNdDM2 model ages from 2.81 to 2.69 Ga. The rhyolite porphyries have geochemical features similar to those of the typical A-type granites (rhyolites), with high SiO2, total alkali contents and FeOt/MgO ratios, and low CaO and MgO contents. Additionally, the rhyolite porphyries have high total rare earth element concentrations (627 ~ 760 ppm), high (La/Yb)N values (14.5 ~ 26.9), strongly negative Eu anomalies (δEu = 0.28 ~ 0.41), and display enrichments of Rb, Ga, Th, and U and depletions of Sr, Nb, Ta, Eu, and Ti. The geochemical and Nd-Hf isotopic features suggest that the Palaeoproterozoic Longzhu rhyolite porphyries were generated by partial melting of source rocks similar to those of the Badu Complex in an intra-plate extensional setting. The results from this study, when combined with existing geochronological data, further demonstrate that the Palaeoproterozoic rocks of Wuyishan terrane probably represent a remnant of the Columbia supercontinent.  相似文献   

11.
The timing and extent of cratonic destruction are crucial to understanding the crustal evolution of the North China Craton (NCC). Zircon U–Pb–Hf isotope data and the whole-rock major and trace element characteristics of the Huyu igneous rocks in northwestern Beijing, China, provide possible new evidence for the initial destruction of the NCC. The igneous rocks occur as several sills and dikes, including lamprophyre, monzonite porphyry, and aplite. The lamprophyres have high Mg# and K2O contents. The monzonite porphyries have high Mg#, high K2O contents, and negative εHf(t) values with zircon U–Pb ages of 225.5–227.7 Ma. These two types of rocks are both enriched in large ion lithosphere elements (LILEs) and light rare earth elements (LREEs) but are depleted in high field strength elements (HFSEs) and high rare earth elements (HREEs) and have almost no Eu anomalies and relatively high total rare earth element (ΣREE) contents. In contrast, the aplites exhibit high silica and K2O contents, low MgO contents, and more negative εHf(t) values with a zircon U–Pb age of 206.2 Ma. The aplites are also enriched in LILEs and LREEs but are depleted in HFSEs and HREEs, with strongly negative Eu, Ti, P, La, Ce, and Sr anomalies and relatively low ΣREE contents. These results indicate that the lamprophyres and monzonite porphyries represent a continuous cogenetic magma evolution series after melt derived from an enriched metasomatized lithospheric mantle experienced crust assimilation and fractional crystallization. The aplites were produced by the fractional crystallization of low-Mg parental magma derived from melting of the ancient Archaean crust. The occurrence of the Huyu intrusive rocks with many other plutons of similar ages on the northern margin of the NCC suggests that the northern NCC entered an intraplate extensional tectonic environment in the Late Triassic.  相似文献   

12.
In this paper, we present zircon U–Pb age and Hf isotope data to document the significance of magma mixing in the formation of Late Jurassic granitoid intrusions in the eastern Qinling Orogen, China. The Muhuguan granitoid pluton from this orogen consists of monzogranite and lesser biotite granite and granodiorite, all containing abundant hornblende-rich cumulates, dioritic xenoliths, and mafic magmatic enclaves (MMEs). The monzogranite and granodiorite are intruded by a number of lamprophyre dykes. Both a cumulate and a dioritic xenolith samples have concordant zircon U–Pb ages of ca. 161 ± 1 Ma, but possess contrasting Hf isotopic compositions. The cumulate has more radiogenic zircon Hf isotopes with negative ε Hf(t) values (?7.9 to ?2.5) and T DM1 ages of 0.9–1.1 Ga, indicating its derivation likely from basaltic rocks of the Neoproterozoic to early Paleozoic Kuanping Group in the area. The dioritic xenolith has much lower zircon ε Hf(t) values of ?19.5 to ?8.8 and T DM2 ages of 2.4–1.7 Ga, consistent with a juvenile Paleoproterozoic crust source presumably represented by the metabasic rocks of the Qinling Group in the area. Individual samples of the monzogranite, MME, and a lamprophyre dyke have U–Pb ages of 150 ± 1, 152 ± 1, and 152 ± 1 Ma, respectively, demonstrating coeval mafic and felsic magmatism in the Late Jurassic. The lamprophyre dyke has homogeneous, highly negative zircon ε Hf(t) values (?29.8 to ?24.8) and Archean T DM2 ages (3.0–2.7 Ga), and its genesis is interpreted as partial melting of an ancient enriched subcontinental mantle source. Zircons from the fine-grained MME show a large range of ε Hf(t) between ?29.1 and ?9.8, overlapping values of the monzogranite and lamprophyre dyke samples. Zircon U–Pb age and Hf isotopes of the MMEs are consistent with their formation by mixing of crustal- and enriched mantle-derived magmas. The main group of zircons from the monzogranite has ε Hf(t) values (?17.9 to ?9.3) and T DM2 ages (2.3–1.8 Ga) that are compatible with the dioritic xenoliths, indicating that the former was produced by partial melting of Paleoproterozoic crustal source with involvement of mantle-derived magmas. Mafic magmatism revealed from the Muhuguan pluton indicates that the eastern Qinling Orogen was dominated by lithospheric extension during the Late Jurassic. Compilation of existing geological and geochronological data suggests that this extensional event started in Late Jurassic (ca. 160 Ma) and persisted into the Early Cretaceous until ca. 110 Ma. The Jura-Cretaceous extension may have resulted from the late Mesozoic westward subduction of the Pacific plate beneath the East Asian continental margin.  相似文献   

13.
We present results of combined in situ U–Pb dating of detrital zircons and zircon Hf and whole-rock Nd isotopic compositions for high-grade clastic metasedimentary rocks of the Slyudyansky Complex in eastern Siberia. This complex is located southwest of Lake Baikal and is part of an early Paleozoic metamorphic terrane in the eastern part of the Central Asian Orogenic Belt (CAOB). Our new zircon ages and Hf isotopic data as well as whole-rock Nd isotopic compositions provide important constraints on the time of deposition and provenance of early Paleozoic high-grade metasedimentary rocks as well as models of crustal growth in Central Asia. Ages of 0.49–0.90 Ga for detrital zircons from early Paleozoic high-grade clastic sediments indicate that deposition occurred in the late Neoproterozoic and early Paleozoic, between ca. 0.62–0.69 and 0.49–0.54 Ga. Hf isotopic data of 0.82–0.69 Ga zircons suggest Archean and Paleoproterozoic (ca. 2.7–2.8 and 2.2–2.3 Ga; Hfc = 2.5–3.9 Ga) sources that were affected by juvenile 0.69–0.82 Ga Neoproterozoic magmatism. An additional protolith was also identified. Its zircons yielded ages of 2.6–2.7 Ga, and showed high positive εHf(t) values of +4.1 to +8.0, and Hf model ages tHf(DM) = tHfc = 2.6–2.8 Ga, which is nearly identical to the crystallization ages. These isotopic characteristics suggest that the protolith was quite juvenile. The whole-rock Nd isotopic data indicate that at least part of the Slyudyansky Complex metasediments was derived from “non-Siberian” provenances. The crustal development in the eastern CAOB was characterized by reworking of the early Precambrian continental crust in the early Neoproterozoic and the late Neoproterozoic–early Paleozoic juvenile crust formation.  相似文献   

14.
The Tibetan Plateau is one of the most significant Cu poly-metallic mineralization regions in the world and preserves important information related to subductional and collisional porphyry Cu mineralization. This study investigates a new occurrence of Cu mineralization-related andesitic porphyries in the western domain of the Gangdese magmatic belt and assesses its petrologic, zircon U-Pb geochronology, whole-rock chemistry, and Sr-Nd-Hf-Pb isotope data. Zircon U-Pb dating of three ore-related porphyries yields crystallization ages of 212–211 Ma. These ages are consistent with previous molybdenite Re-Os dating, indicating a late Triassic magmatic and Cu mineralization event in the western Gangdese magmatic belt. Nb, Ta, and Ti depletion, Th and LREE enrichment, and high La/Yb and Th/Yb ratios in addition to high U/Yb ratios from zircons suggest that the magma was generated in an active continental arc setting. The porphyries have radiogenic isotopic compositions with (87Sr/86Sr)i 0.70431–0.70473, εNd(t) +1.1 to +3.8, (207Pb/204Pb)i 15.601–15.622, and (208Pb/204Pb)i 38.450–38.693, as well as high positive zircon εHf(t) values from +6.2 to +10.6 (mean value 8.3), corresponding to model ages (TDM) ranging from 509 Ma to 819 Ma (mean 646 Ma). This suggests that the andesitic magmatism was dominantly sourced from depleted mantle materials that were modified by subducted oceanic sediment-derived melts during the subduction of the Neo-Tethys Ocean. The mineralization-related porphyries contain amphibole and epidote, as well as high whole-rock Fe2O3/FeO and zircon Ce4+/Ce3+ ratios, suggesting hydrous and highly oxidized parent magmas. Considering the existing Cu mineralization and highly oxidized magma of the well-preserved Triassic andesitic igneous rocks in the western Gangdese belt, the subduction-related continental arc magma system is favorable for subduction-related porphyry Cu deposits. The existence of Luerma porphyry mineralization demonstrates that there are at least five generations of porphyry Cu-(Mo-Au) mineralization in the Gangdese magmatic belt, which advances the timeframe of porphyry mineralization to the late Triassic.  相似文献   

15.
The Late Paleozoic magmatism in central Inner Mongolia provides important insights on the tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB), which formed due to the closure of the Paleo-Asian Ocean (PAO). This paper presents new zircon UPb ages and Hf isotopic compositions as well as whole-rock geochemical data on a suite of volcanic rocks from the Late Paleozoic Baoligaomiao Formation and coeval intrusions in the Baiyinwula region of the Mongolian Arc. This study revealed that the magmatic sequences evolution includes: (1) early andesites (317–311 Ma) with enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HSFEs), and positive zircon εHf (t) values from +9.0 to +15.5, indicating a derivation from enriched mantle; (2) felsic rocks emplaced from 306 Ma to 292 Ma, with relatively lower εHf (t) values from +6.3 to +11.3, implying juvenile crust as the primary magma source; and (3) A-type igneous rocks (280–278 Ma). The comparison of palaeontological, lithostratigraphical, and magmatic evolution in Late Paleozoic between different tectonic units in the eastern part of CAOB has displayed that the subduction of Paleo-Asian oceanic crust caused the opening of the Hegenshan Ocean along the southern margin of Mongolian Arc in Devonian; and the Baoligaomiao Formation volcanic rocks and coeval intrusions have recorded early northwards subduction and subsequent slab rollback of Hegenshan oceanic crust.  相似文献   

16.
The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044?0.7047, ?Nd(T) values of –1.5 to?+0.6, and ?Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.  相似文献   

17.
We constrain the origin and tectonic setting of the giant Duolong porphyry–epithermal Cu–Au deposit in the South Qiangtang Terrane of northern Tibet, based on new zircon U–Pb ages and Hf isotopic data, as well as whole-rock major and trace element data from poorly studied ore-associated intrusions in the Duolong area. The LA–ICP–MS zircon U–Pb dating indicates that the ore-associated rocks formed between 121 and 126 Ma. These ore-associated rocks are geochemically similar to low-K tholeiitic M-type granitoids and to mid- to high-K, calc-alkaline I-type granitoids. They have variable and predominantly positive zircon εHf(t) values (− 1.4 to + 15.6) and variable crustal model ages (TCDM(Hf); 176–1122 Ma). Taking into account previous data and the regional geology of the study area, we propose that the ore-associated rocks originated from fractional crystallization of mantle-derived mafic melts and magma mixing of mantle-derived mafic and hybrid lower crust-derived felsic melts, and the hybrid lower crust included a mix of juvenile and older continental material. The Duolong porphyry–epithermal Cu–Au deposit formed within an ‘ensialic forearc’ of an active continental margin as a result of the northwards subduction of the Bangong–Nujiang Ocean crust beneath the South Qiangtang Terrane.  相似文献   

18.
《地学前缘(英文版)》2020,11(5):1533-1548
The Chinese North Tianshan(CNTS) in the southern part of the Central Asian Orogenic Belt(CAOB) has undergone multistage accretion-collision processes during Paleozoic time,which remain controversial.This study addresses this issue by tracing the provenance of Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS through U-Pb dating and Lu-Hf isotopic analyses of detrital zircons.New detrital zircon U-Pb ages(N=519) from seven samples range from 261±4 Ma to 2827±32 Ma.The most prominent age peak is at 313 Ma and subordinate ages vary from 441 Ma to 601 Ma,with some Precambrian detrital zircon ages(～7%) lasting from 694 Ma to 1024 Ma.The youngest age components in each sample yielded weighted mean ages ranging from 272±9 Ma to 288±5 Ma,representing the maximum depositional ages.These and literature data indicate that some previously-assumed "Carboniferous"strata in the Bogda area were deposited in the Early Permian,including the Qijiaojing,Julideneng,Shaleisaierke,Yangbulake,Shamaershayi,Liushugou,Qijiagou,and Aoertu formations.The low maturity of the sandstones,zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East Junggar Arc and the Harlik-Dananhu Arc in the CNTS.The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc.Zircon E_(Hf)(t) values have increased since ～408 Ma,probably reflecting a tectonic transition from regional compression to extension.This event might correspond to the opening of the Bogda intraarc/back arc rift basin,possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean.A decrease of zircon ε_(Hf)(t) values at ～300 Ma was likely caused by the cessation of oceanic subduction and subsequent collision,which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous.  相似文献   

19.
The Central Hebei Basin （CHB） is one of the largest sedimentary basins in the North China Craton, extending in a northeast-southwest direction with an area of 〉350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi- mentary rocks recovered from drill holes that penetrated into the basement of the CHB, Two samples of gneissic granodiorite （XG1-1） and gneissic quartz diorite 048-1） have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41-2.51 and ～2.5 Ga, respec- tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher REE contents and lower Eu/Eu＊ and （La/Yb）n values. Two metasedimentary samples （MG1, H5） mainly contain ～2,5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf （2.5 Ga） values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2,9 Ga, respectively, Therefore, ～2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.  相似文献   

20.
In this study, a combined study of zircon U–Pb and Hf–O isotopes, as well as whole-rock major and trace elements and Nd isotopes has been conducted for Yangjia gneissic granite from the southern Wuyishan terrane, Southeast China, to constrain its petrogenesis and provide a new window for investigating the tectonic evolution of the Cathaysia basement. U–Pb dating for magmatic zircons yields a 207Pb/206Pb age of ca. 1.80 Ga, interpreted as the emplacement age of the Yangjia granite. The granites have relatively high K2O, Rb, Ga, Zr, Nb, Y, and Ce contents and show low Al2O3, CaO, and Ba concentrations. Their 10,000*Ga/Al ratios range between 2.8 and 3.2. Zircons from the granite have εHf(t) values ranging from ?13.2 to ?7.2, corresponding to THfDM2 model ages of 2.99 Ga to 2.72 Ga. The zircon δ18O values range between 6.7‰ and 9.1‰ with an average of 7.7‰. In addition, the whole-rock εNd(t) values of the granites range from ?6.5 to ?5.4 and the TNdDM2 model ages from 2.73 Ga to 2.82 Ga. All these geochemical and Nd–Hf–O isotopic signatures suggest an A-type affinity for the Yangjia granites, and they were likely generated by partial melting of Palaeoproterozoic parametamorphic rocks of the Wuyishan terrane in a post-collisional extensional setting. When our data is combined with existing geochronological data, it provides further evidence for the Palaeoproterozoic basement in the southern Wuyishan terrane, which records a rapid tectonic transition from post-collision to intraplate extension (1.80–1.77 Ga) related to the break-up of the supercontinent Columbia.  相似文献   

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