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1.
Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (εNd(t) range from + 3.1 to + 7.5 and 206Pb/204Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO2) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.  相似文献   

2.
The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100 km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417 ± 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive εNd(t) values (+ 4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ34S values of the sulfides from the deposit vary from − 1.24 to + 4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (206Pb/204Pb = 18.310–18.656, 207Pb/204Pb = 15.489–15.643 and 208Pb/204Pb = 37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ13CPDB and δ18OSMOW values of the associated hydrothermal calcite crystals vary from − 2.3‰ to + 0.27‰ and from + 14.6 to + 24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ13CPDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ18O values for the hydrothermal fluids, calculated from the measured values for quartz, are between − 2.1‰ and + 3.5‰. The corresponding δD values for the fluids range from − 59‰ to − 84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.  相似文献   

3.
The present study reports new zircon LA–ICP–MS U–Pb ages, trace element and Hf isotope data, and whole-rock major and trace element data from Cambrian metarhyolites from Zhaqian and Zhakang in the central Lhasa subterrane of southern Tibet. One sample from Zhakang provides a weighted mean 206Pb/238U age of 510.4 ± 4.0 Ma and two samples from Zhaqian yield weighted mean 206Pb/238U ages of 510.6 ± 2.6 Ma and 524.8 ± 2.9 Ma, indicating that the Zhaqian and Zhakang metarhyolites were contemporaneous. Both are characterized by high SiO2 and K2O and low Na2O. They are also primarily high-K calc-alkaline, are enriched in Th, U, and light rare earth elements (LREEs), and are depleted in Nb, Ta, Ti, and P. Thus, they are geochemically similar to typical arc volcanic rocks. Moreover, the Zhaqian metarhyolites exhibit varying zircon εHf(t) values (−3.8 to +0.3) that are comparable to those of the Zhakang metarhyolites (−4.9 to −1.0). Both metarhyolites are interpreted as resulting from partial melting of Proterozoic metasedimentary rocks with mantle-derived magma contributions. Contemporaneous magmatism in the early Paleozoic has also been recognized in other microcontinents along the Gondwana proto-Tethyan margin. The emplacement of these magmatic rocks and the development of a Cambro–Ordovician angular unconformity in the central Lhasa subterrane can be attributed to subduction of proto-Tethys Ocean lithosphere in a Andean-type magmatic arc setting following the assembly of various continental components within the Gondwana supercontinent.  相似文献   

4.
Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial 87Sr/86Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial 87Sr/86Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.  相似文献   

5.
The Bastar Craton of Central India has a thick sequence of volcano–sedimentary rocks preserved in Kotri–Dongargarh belt that developed on a tonalite-trondhjemite-granodiorite (TTG) basement followed upwards by the Amgaon, Bengpal, Bailadila, and Nandgaon Groups of rocks. Here, we report the U-Pb geochronology and Lu-Hf isotope systematics and whole rock geochemistry of volcanic rocks and associated granitoids belonging to the Pitepani basalts, Bijli rhyolites, and Dongargarh granite in the Nandgaon Group of the Kotri belt. The volcanic rocks of the Nandgaon Group are bimodal in nature in which the basalts exhibit intergranular, porphyritic to spherulitic texture composed of pyroxenes, plagioclase, tremolite, actinolite, and chlorite ± Fe oxides. The rhyolites display porphyritic texture consisting of K-feldspar, quartz, and plagioclase as phenocrysts. The associated porphyritic granitoids have K-feldspar, microcline, plagioclase, and biotite phenocrysts within a groundmass of similar composition. The bimodal suite displays LILE, LREE enrichment, and HFSE depletion with significant negative Nb-Ta anomalies combined with slightly fractionated REE patterns in the basalts and highly fractionated patterns and prominent negative Eu anomalies in the rhyolites endorsing their generation in an island-arc/back-arc tectonic setting. The geochemical features of the associated granitoids indicate that these are potassic and classify as within-plate A-type granites. Zircons from the basalts show clear oscillatory zoning in their CL images. They cluster as a coherent group with 207Pb/206Pb spot ages ranging from 2446 to 2522 Ma and weighted mean age of 2471 ± 7 Ma. Zircons from the rhyolite samples are subhedral to euhedral and show simple oscillatory zoning with some heterogeneous fractured domains. The data from two samples define upper intercept ages of 2479 ± 13 Ma and 2463 ± 14 Ma. Zircon grains in the granite show clear oscillatory zoning and their U-Pb data define an upper intercept age of 2506 ± 50 Ma. The Lu-Hf isotopic data on the zircons from the basalts show initial 176Hf/177Hf ratios from 0.280925 to 0.281018. Their εHf(t) values are in the range of − 10.0 to − 6.7. The Hf-depleted model ages (TDM) are between 3038 Ma and 3171 Ma, and Hf crustal model ages (TDMC) vary from 3387–3589 Ma. The zircons from the rhyolites show initial 176Hf/177Hf ratios from 0.280919 to 0.281020 and from 0.281000 to 0.281103, respectively, with εHf(t) values varying from − 10 to − 6.4 and from − 7.5 to − 3.9. Among these, one sample shows TDM between 3038 Ma and 3182 Ma, and TDMC varies from 3377 to 3596 Ma, whereas the other sample shows ages of 2925 Ma and 3072 Ma with TDMC varying from 3208 to 3432 Ma. The initial 176Hf/177Hf ratios of the granites range from 0.280937 to 0.281062 with εHf(t) values of − 8.8 to − 4.3. The TDM shows a range of 2979 Ma and 3170 Ma, and TDMC varies from 3269 to 3541 Ma. The predominant negative εHf(t) values of zircons from these rocks suggest that the source material was evolved from the Paleoarchean crust. The geological, geochemical, and geochronological evidence suggests coeval tectonic and magmatic episodes of volcanic and plutonic activity in an island-arc setting where the arc migrated toward the continental margin and played a significant role in the Neoarchean–Paleoproterozoic crustal growth of the Kotri belt of Central India.  相似文献   

6.
Ore-forming porphyries and barren granitoids from porphyry Cu deposits differ in many ways, particularly with respect to their adakitic affinity and calc-alkaline characteristics. In this study, zircon U–Pb and molybdenite Re–Os dating, whole rock geochemistry, whole rock Sr–Nd–Pb and zircon O–Hf isotopic analyses were carried out on the ore-forming granitoids from the Kounrad, Borly and Sayak deposits, and also on pre-ore and post-ore granitoids in adjacent regions of Central Kazakhstan. Geochronology results indicate that pre-ore magmatism occurred in the Late Devonian to Early Carboniferous (361.3–339.4 Ma), followed by large scale Cu mineralization (325.0–327.3 Ma at Kounrad, 311.4–315.2 Ma at Borly and 309.5–311.4 Ma at Sayak), and finally, emplacement of the Late Carboniferous post-ore barren granitoids (305.0 Ma). The geochemistry of these rocks is consistent with calc-alkaline arc magmatism characterized by strong depletions in Nb, Ta and Ti and enrichments in light rare earth elements and large ion lithophile elements, suggesting a supra-subduction zone setting. However, the ore-forming rocks at Kounrad and Sayak show adakitic characteristics with high Sr (517.5–785.3 ppm), Sr/Y (50.60–79.26), (La/Yb)N (9.37–19.62) but low Y (6.94–11.54 ppm) and Yb (0.57–1.07 ppm), whereas ore-forming rocks at Borly and barren rocks from northwest of Borly and Sayak have normal arc magma geochemical features. The Sr–Nd–Hf–O isotopic compositions show three different signatures: (1) Sayak granitoids have very young juvenile lower crust-derived compositions ((87Sr/86Sr)i = 0.70384 to 0.70451, ɛNd (t) = + 4.9 to + 6.0; TDM2 (Nd) = 580 to 670 Ma, ɛHf (t) = + 11.3 to + 15.5; TDMC (Hf) = 330 to 600 Ma, δ18O = 6.0 to 8.1‰), and were probably generated from depleted mantle-derived magma with 5–15% sediment melt addition in the magma source; (2) the Kt-1 granite from northwest of Sayak shows extremely enriched Sr–Nd isotopic compositions ((87Sr/86Sr)i = 0.71050, ɛNd (t) = − 7.8, TDM2 (Nd) = 1700 Ma), likely derived from partial melting of ancient continental crust; (3) other granitoids have transitional Sr–Nd compositions between the Sayak and Kt-1 samples, indicating a juvenile lower crust source with the addition of 10–30% of ancient crustal material. The pre-ore magmatism was probably related to partial melting of juvenile lower crust due to northward subduction of the Junggar–Balkhash Ocean, whereas the ore-forming adakitic rocks at Aktogai, Kounrad and Sayak formed by partial melting of thickened lower crust which subsequently delaminated. The ore-forming rocks at Borly, and the later post-ore barren granites, formed by partial melting of juvenile lower crust with normal thickness. This tectonic setting supports the existence of an Andean-type magmatic arc in the Devonian to the Late Carboniferous, resulting from the subduction of the Junggar–Balkhash oceanic plate. The link between whole rock geochemistry and scale of mineralization suggests a higher metallogenic potential for adakitic rocks than for normal arc magmatism.  相似文献   

7.
Volcanic rocks preserved in the Lampang–Den Chai area in NW Thailand are important components of the giant Paleotethyan igneous belt. Constraining their age and petrogenesis is critical for better understanding their temporal-spatial relationship with the Lancangjiang igneous zone and the Paleotethyan tectonic evolution in SE Asia. The volcanic suite is constituted by intermediate to acid rocks with zircon U–Pb ages of 240.4 ± 1.7 Ma and 240.6 ± 1.9 Ma for the representative andesitic and rhyolitic samples, respectively. Volcanic sequence is dominated by calc-alkaline andesites, dacites and rhyolites. The andesitic and dacitic samples are characterized by high Mg# (37–57) and TiO2 (0.91–1.59 wt%), and can be classified as high-Mg series. They are enriched in LILEs and LREEs and depleted in HFSEs. Representative andesitic samples have 87Sr/86Sr (i) ratios of 0.70398–0.70567, εNd (t) values of +3.6–+3.9, zircon εHf (t) values of +2.8–+8.0 and δ18O values of 7.01–8.11‰, respectively. The rhyolitic samples are characterized by high Mg# (38–70) and low TiO2 (0.25–0.61 wt%). They are enriched in LILEs and LREEs, along with 87Sr/86Sr (i) = 0.70468–0.70645, εNd (t) = +2.0–+4.3 and zircon εHf (t) = +5.7–+13.6. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a newly modified mantle source by slab-derived fluids and recycled sediments, and rhyolitic samples were derived from juvenile mafic crust. It is proposed that the Middle Triassic high-Mg volcanic rocks in the Lampang–Den Chai area formed in response to slab roll-back during transition of tectonic regime from subduction to continental collision between the Sibumasu and Indochina blocks. These rocks constitute part of the Chiang Khong–Lampang–Tak igneous zone, and can northerly link with the Lancangjiang igneous zone and southerly extend to the Chanthaburi, Malaysia and Singapore areas.  相似文献   

8.
With the aim of constraining the Early Mesozoic tectonic evolution of the eastern section of the Central Asian Orogenic Belt (CAOB), we undertook zircon U–Pb dating and geochemical analyses (major and trace elements, Sr–Nd isotopes) of volcanic rocks of the Luoquanzhan Formation and Daxinggou Group in eastern Heilongjiang and Jilin provinces, China. The analyzed rocks consist mainly of dacite and rhyolite, with SiO2 contents of 68.52–76.65 wt%. Three samples from the Luoquanzhan Formation and one from the Daxinggou Group were analyzed using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon techniques. Three zircons with well-defined oscillatory zoning yielded weighted mean 206Pb/238U ages of 217 ± 1, 214 ± 2, and 208 ± 1 Ma, and one zircon with oscillatory zoning yielded a weighted mean 206Pb/238U age of 201 ± 1 Ma. These ages are interpreted to represent the timing of eruption of the volcanic rocks. The Triassic volcanic rocks are characterized by high SiO2 and low MgO concentrations, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), (87Sr/86Sr)i = 0.7040–0.7050 (Luoquanzhan Formation) and 0.7163–0.7381 (Daxinggou Group), and εNd (t) = 1.89–3.94 (Luoquanzhan Formation) and 3.42–3.68 (Daxinggou Group). These geochemical features indicate an origin involving the partial melting of juvenile lower crust (Nd model ages (TDM2) of 651–821 Ma) and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation. These volcanic rocks formed within an extensional environment following collision of the NCC and Jiamusi-Khanka Massif during the Late Paleozoic–Early Triassic.  相似文献   

9.
New field work, in addition to zircon geochronology, Nd isotopes and reconnaissance geochemical data allow the recognition of Paleoproterozoic volcanic and metavolcanic sequences in the São Luís Craton of northern Brazil. These sequences record at least five volcanic pulses occurring probably in three distinct epochs and in different tectonic settings. (1) The Pirocaua Formation of the Aurizona Group comprises early arc-related calc-alkaline metapyroclastic rocks of 2240 ± 5 Ma formed from juvenile protoliths in addition to minor older crustal components. (2) The Matará Formation of the Aurizona Group holds mafic tholeiitic and ultramafic metavolcanic rocks of back arc and/or island arc setting, which are likely coeval to the Pirocaua Formation. (3) The Serra do Jacaré volcanic unit is composed of tholeiitic basalts and predominantly metaluminous, normal- to high-K calc-alkaline andesites of 2164 ± 3 Ma formed in mature arc or active continental margin from juvenile protoliths along with subordinate older (Paleoproterozoic) materials and associated to the main calc-alkaline orogenic stage. (4) The Rio Diamante Formation consists of late-orogenic metaluminous, medium-K, calc-alkaline rhyolite to dacite and tuffs of 2160 ± 8 Ma formed in continental margin setting from reworked Paleoproterozoic crust (island arc) with incipient Archean contribution. (5) The Rosilha volcanic unit is composed of weakly peraluminous, medium-K, calc-alkaline dacite and tuff formed probably at about 2068 Ma from reworked crustal protoliths. As a whole the volcanic and metavolcanic rocks record and characterized better the previously proposed orogenic evolution of the São Luís Craton.  相似文献   

10.
The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, 206Pb/204Pb vs. Th/Nb, Th/Nb vs. δ18O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.  相似文献   

11.
LA-ICP-MS zircon U–Pb ages and geochemical data are presented for the Mesozoic volcanic rocks in northeast China, with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the Paleo-Asian Ocean, Mongol–Okhotsk, and circum-Pacific tectonic regimes. The new ages, together with other available age data from the literature, indicate that Mesozoic volcanism in NE China can be subdivided into six episodes: Late Triassic (228–201 Ma), Early–Middle Jurassic (190–173 Ma), Middle–Late Jurassic (166–155 Ma), early Early Cretaceous (145–138 Ma), late Early Cretaceous (133–106 Ma), and Late Cretaceous (97–88 Ma). The Late Triassic volcanic rocks occur in the Lesser Xing’an–Zhangguangcai Ranges, where the volcanic rocks are bimodal, and in the eastern Heilongjiang–Jilin provinces where the volcanics are A-type rhyolites, implying that they formed in an extensional environment after the final closure of the Paleo-Asian Ocean. The Early–Middle Jurassic (190–173 Ma) volcanic rocks, both in the Erguna Massif and the eastern Heilongjiang–Jilin provinces, belong chemically to the calc-alkaline series, implying an active continental margin setting. The volcanics in the Erguna Massif are related to the subduction of the Mongol–Okhotsk oceanic plate beneath the Massif, and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The coeval bimodal volcanic rocks in the Lesser Xing’an–Zhangguangcai Ranges were probably formed under an extensional environment similar to a backarc setting of double-direction subduction. Volcanic rocks of Middle–Late Jurassic (155–166 Ma) and early Early Cretaceous (145–138 Ma) age only occur in the Great Xing’an Range and the northern Hebei and western Liaoning provinces (limited to the west of the Songliao Basin), and they belong chemically to high-K calc-alkaline series and A-type rhyolites, respectively. Combined with the regional unconformity and thrust structures in the northern Hebei and western Liaoning provinces, we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the Mongol–Okhotsk suture belt. The late Early Cretaceous volcanic rocks, widely distributed in NE China, belong chemically to a low- to medium-K calc-alkaline series in the eastern Heilongjiang–Jilin provinces (i.e., the Eurasian continental margin), and to a bimodal volcanic rock association within both the Songliao Basin and the Great Xing’an Range. The volcanics in the eastern Heilongjiang–Jilin provinces formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust, or both. Late Cretaceous volcanics, limited to the eastern Heilongjiang–Jilin provinces and the eastern North China Craton (NCC), consist of calc-alkaline rocks in the eastern Heilongjiang–Jilin provinces and alkaline basalts in the eastern NCC, suggesting that the former originated during subduction of the Paleo-Pacific Plate beneath the Eurasian continent, whereas the latter formed in an extensional environment similar to a backarc setting. Taking all this into account, we conclude that (1) the transformation from the Paleo-Asian Ocean regime to the circum-Pacific tectonic regime happened during the Late Triassic to Early Jurassic; (2) the effect of the Mongol–Okhotsk suture belt on NE China was mainly in the Early Jurassic, Middle–Late Jurassic, and early Early Cretaceous; and (3) the late Early Cretaceous and Late Cretaceous volcanics can be attributed to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.  相似文献   

12.
The composite Zhaheba ophiolite complex, exposed in Eastern Junggar in the Southern Altaids, records an unusually long record of oceanic crust and magmatic arc evolution. The Zhaheba ophiolite complex consists of ultramafic rocks, gabbro, diorite, basalt and chert intruded by diabase dikes and diorite porphyry. These rocks are overlain by a several-km-thick section of tuffaceous rocks, volcaniclastic sedimentary rocks, and intermediate volcanic rocks. The igneous rocks of the ophiolite complex show negative Nb and Ta anomalies and LREE enrichment relative to HREE, suggesting the influence of fluids derived from a subducting oceanic slab. The LA-ICPMS U–Pb age of zircons from gabbro is 495.1 ± 3.5 Ma. Zircon ages from diorite and basalt are 458.3 ± 7.2 Ma and 446.6 ± 6.0 Ma, respectively. The basalt is locally overlain by bedded chert. Diabase dikes and diorite porphyry yield the U–Pb ages of 421.5 ± 4.1 Ma and 423.7 ± 6.5 Ma, respectively. The age of stratigraphically lower part of the overlying volcanic–volcaniclastic section is constrained to be about 410 Ma, the maximum depositional age of the tuffaceous sandstone from U–Pb detrital zircon ages. Late rhyolite at the top of the stratigraphic section yielded a U–Pb zircon age of 280.3 ± 3.7 Ma. The age and stratigraphic relationships for the Zhaheba ophiolite complex and related rocks suggest that the period of ~ 70 Ma of initial supra-subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 Ma. Many other ophiolites in the southern Altaids appear to record similar relationships, and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra-subduction zone. The occurrence of the Zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic Tethyan SSZ ophiolites but similar to some ophiolites in North America. Although the Zhaheba ophiolite belt is flanked by the Dulate arc in the north and Yemaquan arc in the south, it cannot stand a suture between two arcs. It is suggested that Devonian–Carboniferous Dulate arc was built on the late Cambrian–middle Ordovician Zhaheba supra-subduction oceanic crust. The late Carboniferous rocks and early Permian rocks in Dulate arc are interpreted to form in the extensional process within Zhaheba–Dulate arc composite system.  相似文献   

13.
In this paper we present geochemical, zircon U–Pb and Hf isotopic data on the late Paleozoic volcanic rocks of the Dashizhai Formation, which are exposed along the northwestern margin of the Songnen terrane in eastern Inner Mongolia. Our aim is to constrain the petrogenesis and tectonic setting of the volcanic rocks and to unravel the late Paleozoic tectonic evolution of the northwestern part of the Songnen terrane, along the eastern segment of the Central Asian Orogenic Belt. Lithologically, the Dashizhai Formation is composed mainly of rhyolitic tuff, rhyolite, dacite, andesite, basaltic andesite and basalt, with minor basaltic trachyandesite. The zircons separated from these rocks are euhedral–subhedral, have high Th/U ratios (0.2–1.6), and display broad oscillatory growth zoning, indicating a magmatic origin. The results of zircon U–Pb dating indicate the volcanic rocks formed during the early Permian (295–283 Ma). Geochemically, these volcanic rocks belong to the mid-K to high-K calc-alkaline series and are characterized by an enrichment in large ion lithophile elements (LILEs) and a depletion in high field strength elements (HFSEs, such as Nb, Ta, and Ti), similar to igneous rocks that form in active continental margin settings. Most magmatic zircons of the rhyolites show positive εHf(t) values (+ 3.65 to + 13.0) and two-stage model ages (TDM2) of 1396–551 Ma. These geochemical characteristics indicate that the acidic volcanic rocks of the Dashizhai Formation were most likely derived from the partial melting of dominantly juvenile crustal components with a possible addition of “old” materials. In contrast, the basic to intermediate volcanic rocks were derived from the partial melting of a depleted lithospheric mantle that had been metasomatized by fluids derived from a subducted slab. These data, together with regional geological investigations, suggest that the generation of the early Permian volcanic rocks of the Dashizhai Formation was related to the southward subduction of the Paleo–Asian oceanic plate beneath the Songnen terrane. This also implies that the terminal collision between the Songnen and Xing'an terranes did not occur before the early Permian.  相似文献   

14.
Porphyry Cu deposits occurred in the southern West Junggar of Xinjiang, NW China and are represented by the Baogutu and newly-discovered Jiamantieliek porphyry Cu deposits. Petrographical and geochemical studies show that both Jiamantieliek and Baogutu ore-bearing intrusions comprise main-stage diorite stock and minor late-stage diorite porphyry dikes and are the calc-alkaline intermediate intrusions. Based on U–Pb zircon SHRIMP analyses, the Jiamantieliek intrusion formed in 313 ± 4 Ma and 310 ± 5 Ma, while, based on U–Pb zircon SIMS analyses, the Baogutu intrusion formed in 313 ± 2 Ma and 312 ± 2 Ma. Rocks in the Jiamantieliek intrusion are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) with negative Nb anomaly. Their isotopic compositions (εNd(t) = +1.6 to +3.4, (87Sr/86Sr)i = 0.70369–0.70401, (207Pb/204Pb)i = 15.31–5.41) suggest a mixing origin from depleted to enriched mantle sources. In the Baogutu intrusion, the rocks are similar to those of the Jiamantieliek intrusion. Their Sr-Nd-Pb isotopic composition (εNd(t) = +4.4 to +6.0, (87Sr/86Sr)i = 0.70368–0.70385, (207Pb/204Pb)i = 15.34–5.42) shows a more depleted mantle source. These features suggest generation in an island arc. The Jiamantieliek and Baogutu intrusions have similar characteristics, indicating that a relatively uniform and integrated source region has existed in the southern West Junggar since the Palaeozoic. A larger contribution of calc-alkaline magma would be required to generate the Jiamantieliek intrusion, which may reflect the development of magma arc maturation towards the western section of the southern West Junggar.  相似文献   

15.
The Kanggur gold deposit is located in the southern margin of the Central Asia Orogenic Belt and in the western segment of the Kanggur–Huangshan ductile shear belt in Eastern Tianshan, northwestern China. The orebodies of this deposit are hosted in the Lower Carboniferous volcanic rocks of the Aqishan Formation and mainly consist of andesite, dacite and pyroclastic rocks. The SHRIMP zircon U–Pb age data of the andesite indicate that the volcanism in the Kanggur area might have occurred at ca. 339 Ma in the Early Carboniferous, and that the mineralization age of the Kanggur gold deposit was later than the age of volcanic rocks in the area. Geochemically, the andesite rocks of the Aqishan Formation belong to low-tholeiite and calc-alkaline series and display relative depletions in high field strength elements (HFSEs; i.e. Nb, Ta and Ti). The δ18Ow and δDw values vary from − 9.1‰ to + 3.8‰ and − 66.0‰ to − 33.9‰, respectively, indicating that the ore-forming fluids were mixtures of metamorphic and meteoric waters. The δ30Si values of 13 quartz samples range from − 0.3‰ to + 0.1‰ with an average of − 0.15‰, and the δ34S values of 18 sulphide samples range from − 0.9‰ to + 2.2‰ with an average of + 0.54‰. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 10 sulphide samples range from 18.166 to 18.880, 15.553 to 15.635 and 38.050 to 38.813, respectively, showing similarities to orogenic Pb; these values are consistent with those of the andesite from the Kanggur area, suggesting a common lead source. All of the silicon, sulphur and lead isotopic systems indicate that the ore-forming fluids and materials were mainly derived from the Aqishan Formation, and that the host volcanic rocks of the Aqishan Formation probably played a significant role in the Kanggur gold mineralization. Integrating the data obtained from studies on geology, geochronology, petro-geochemistry and H–O–Si–S–Pb isotope systematics, we suggest that the Kanggur gold deposit is an orogenic-type deposit formed in Eastern Tianshan orogenic belt during the Permian post-collisional tectonism.  相似文献   

16.
《Chemical Geology》2007,236(1-2):42-64
Carboniferous volcanic rocks in the Alataw area, Northern Tianshan Range (Xinjiang), consist of early Carboniferous (ca. 320 Ma) adakites and Nb-enriched arc basalts and basaltic andesites (NEBs), and late Carboniferous (ca. 306–310 Ma) mainly high-K calc-alkaline andesites, dacites and rhyolites. The adakites are calc-alkaline, and characterized by high Na2O/K2O (1.52–3.32) ratios, negligible to positive Eu anomalies, strong depletion of heavy rare earth elements (e.g., Yb = 0.74–1.47 ppm) and Y (6.7–14.9 ppm), positive Sr and Ba but negative Nb and Ti anomalies, and relatively constant εNd(T) values (+ 3.4–+ 6.6) and (87Sr/86Sr)i ratios (0.7035–0.7042). Some andesitic and dacitic adakite samples exhibit high MgO contents similar to magnesian andesites. The NEBs are sodium-rich (Na2O/K2O = 2.03–8.06), and differ from the vast majority of arc basalts in their higher Nb, Zr, TiO2 and P2O5 contents and Nb/Th, Nb/La and Nb/U ratios, and minor negative to positive anomalies in Ba, Nb, Sr, Zr and Ti. They have the highest εNd(T) values (+ 6.4–+ 11.6) but varying (87Sr/86Sr)i ratios (0.7007–0.7063). The high-K calc-alkaline suite is similar to typical ‘normal’ arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative Eu, Nb, Sr and Ti anomalies. They have εNd(T) values (+ 1.2–+ 6.4) and (87Sr/86Sr)i ratios (0.7018–0.7059). Geochemically, they are similar to coeval I-type granitoids in the Alataw area. Given the presence of early Carboniferous ophiolites in the Northern Tianshan Range, and the isotopically inappropriate compositions of Proterozoic metamorphic basement in the Alataw area, we argue that the Alataw adakites were most probably related to the melting of young subducted crust of the Northern Tianshan Ocean. The NEBs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab-derived fluids. The later high-K calc alkaline suite was generated by AFC processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids. The larger range of isotopic compositions exhibited by both the NEB and high-K suite, relative to the adakites, suggests that the mantle wedge was heterogeneous prior to slab- or fluid-mediated metasomatism.Continental crustal growth of the Central Asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 Ma and possibly afterwards. The results of this study suggest that other Carboniferous Nb-enriched basalts in the Tianshan Range were generated by subduction processes rather than by intraplate tectonics as previously proposed.  相似文献   

17.
Numerous small dismembered ophiolite fragments occur in South Mongolia, but they are very poorly studied. The lack of age data and geochemical analysis hampers our understanding of the Paleozoic tectonic evolution of the region. We conducted detailed studies on the Manlay ophiolitic complex and Huree volcanic rocks south of the Main Mongolian Lineament (MML) to provide some constraints on these rocks. The Manlay ophiolite consists of dunite, harzburgite, pyroxenite, gabbro, plagiogranite, basalt and chert, locally with chromite mineralization in dunite. The gabbro and plagiogranite yielded SHRIMP zircon weighted mean 206Pb/238U ages of 509 ± 5 Ma and 482 ± 4 Ma, respectively. The basalt and dolerite samples of this complex show enrichment in LREE and LILE and negative Nb, Ta and Ti anomalies, and the chrome spinel from the chromitite lens in the dunite is characterized by high Cr# and low TiO2 contents. These features suggest a supra-subduction zone (SSZ) origin for the ophiolitic complex. The Huree volcanic rocks, ranging from basalt to dacite, display enrichment in LREE and LILE, weak Eu anomalies and distinctly negative Nb, Ta and Ti anomalies, consistent with those of typical magmas in a subduction environment. An andesite sample from this arc yielded a SHRIMP 206Pb/238U zircon age of 487 ± 5 Ma, which is the oldest reliable age for an island arc in South Mongolia. Recognition of an Early Paleozoic ophiolitic complex and a coeval island arc indicates that South Mongolia underwent a period of active volcanism during Late Cambrian to Ordovician. Additionally, the tuff overlying the ophiolitic complex and a granite intruding the ophiolite have SHRIMP zircon U–Pb ages of 391 ± 5 Ma and 304 ± 4 Ma, respectively. Combining the available data, we propose that the Early Paleozoic subduction–accretionary complexes likely constitute the basement of the Late-Paleozoic arc formations and correlate with the Lake Zone in western Mongolia.  相似文献   

18.
《Gondwana Research》2014,25(1):338-357
Four isolated metamorphic complexes located within post-collisional granitoids occupying up to 70% of the total area, were distinguished in Sinai (Egypt) and Elat area (southern Israel), the northernmost part of the Arabian–Nubian Shield. The metamorphic rocks include metasediments, felsic and mafic metavolcanic rocks intruded by granitic, dioritic, and gabbroic plutons, all subjected to penetrative deformation.We present new SIMS U–Pb dating of zircons from 13 rock units comprising metasediments, volcanic rocks, gneisses and plutons from three metamorphic complexes (Sa'al, Feiran–Solaf, and Kid). In addition we present a SIMS U–Pb titanite age of a granitic gneiss previously dated using zircon. On the basis of the new and published U–Pb data, three successive Meso- to Neoproterozoic island arcs formed during a period of ca. 500 My are recognized. The Sa'al arc (represented by the oldest arc rocks in the ANS) evolved from 1.03 to 0.93 Ga (100 My); the Feiran–Elat arc developed from ca. 870 to 740 Ma (130 My), and the Kid arc formed from ca. 640 to 620 Ma (20 My). Evidence for an older, ca. 1.1 Ga, pre-Sa'al island arc was established from the zircon xenocryst population, though no exposures of rocks of this age were found. In the Sa'al and Kid arcs both volcanic and sedimentary rocks are preserved, whereas in the Elat–Feiran arc volcanic rocks are missing. We suggest that at ~ 700 Ma the Elat−Feiran arc was subjected to rifting that resulted in separating of the Qenaia block and its movement to the SE.  相似文献   

19.
There is ongoing debate as to the subduction direction of the Bangong–Nujiang Ocean during the Mesozoic (northward, southward or bidirectional subduction). Arc-related intermediate to felsic intrusions could mark the location of the subduction zone and, more importantly, elucidate the dominant geodynamic processes. We report whole rock geochemical and zircon U–Pb and Hf isotopic data for granitoids from the west central Lhasa subterrane (E80° to E86°). All rocks show metaluminous to peraluminous, calc-alkaline signatures, with strong depletion of Nb, Ta and Ti, enrichment of large ion lithophile elements (e.g., Cs, Rb, K), a negative correlation between SiO2 and P2O5, and a positive correlation between Rb and Th. All these features are indicative of I-type arc magmatism. New zircon U–Pb results, together with data from the literature, indicate continuous magmatism from the Late Jurassic to the Early Cretaceous (160 to 130 Ma). Zircon U–Pb ages for samples from the northern part of the west central Lhasa subterrane (E80° to E82°30′) yielded formation ages of 165 to 150 Ma, whereas ages of 142 to 130 Ma were obtained on samples from the south. This suggests flat or low-angle subduction of the Bangong–Nujiang Ocean, consistent with a slight southward decrease in zircon εHf(t) values for Late Jurassic rocks. Considering the crustal shortening, the distance from the Bangong–Nujiang suture zone, and a typical subduction zone melting depth of ~ 100 km, the subduction angle was less than 14° for Late Jurassic magmatism in the central Lhasa interior, consistent with flat or low-angle subduction. Compared with Late Jurassic rocks (main εHf(t) values of − 16 to − 7), Early Cretaceous rocks (145 to 130 Ma) show markedly higher εHf(t) values (mainly − 8 to 0), possibly indicating slab roll-back, likely caused by slab foundering or break-off. Combined with previously published works on arc magmatism in the central Lhasa and west part of the southern Qiangtang subterranes, our results support the bidirectional subduction of the Bangong–Nujiang Ocean along the Bangong–Nujiang Suture Zone, and indicates flat or low-angle southward subduction (165 to 145 Ma) followed by slab roll-back (145 to 130 Ma).  相似文献   

20.
The footwall volcanic rocks of the Ordovician Tanjianshan Group in the world-class Xitieshan Pb–Zn deposit have experienced prolonged arc volcanism followed by strong metamorphism and deformation. This has resulted in a complex thermal history and led to ambiguity in interpretation of zircon geochronological results. An integrated study involving textural characterization, CL imaging, trace element analysis, Ti-in-zircon thermometry and LA-ICPMS U–Pb dating has provided tight constraints on the age and genesis of the zircon groups in the volcanic rocks. The temperature of metamorphism and deformation indicated by metacryst minerals and micro-structures in the volcanic rocks ranges from 550 to 650 °C, which partially overlaps with the lower temperature range of zircon crystallization (600–750 °C) calculated using the Ti-in-zircon thermometer. Cathodoluminescence images and trace element compositions confirm a magmatic origin for the zircons, which have also been variably altered by metamorphic fluids. Two ranges of U–Pb ages, 475–470 Ma and 460–450 Ma, have been obtained on typical magmatic zircons and are interpreted to represent pre-mineralization arc volcanism in the Xitieshan deposit. A younger age group of 440–430 Ma for the fluid-modified zircons is considered to record post-ore metamorphism during the North Qadaim Orogeny. Thus, we propose that the original exhalative ores at the Xitieshan Pb–Zn deposit formed at 450–440 Ma.  相似文献   

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