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1.
The development of Early Cretaceous mafic dikes in northern and southern Jiangxi allows an understanding of the geodynamic setting and characteristics of the mantle in southeast China in the Cretaceous. Geological and geochemical characteristics for the mafic dikes from the Wushan copper deposit and No. 640 uranium deposit are given in order to constrain the nature of source mantle, genesis and tectonic implications. According to the mineral composition,the mafic dikes in northern Jiangxi can be divided into spessartite and olive odinite types, which belong to slightly potassium-rich calc-alkaline lamprophyre characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), large depletion in high strength field elements (HSFE) and with negative Nb, Ta and Ti anomalies, as well as 87Sr/86Sr ratios varying from 0.7055 to 0.7095 and 143Nd/r44Nd ratios varying from 0.5119 to 0.5122.All features indicate that the magma responsible for the mafic dikes was derived mainly from metasomatic lithosphere mantle related to dehydration and/or upper crust melting during subduction. Differences in geochemical characteristics between the mafic dikes in northern Jiangxi and the Dajishan area, southern Jiangxi were also studied and they are attributed to differences in regional lithospheric mantle components and/or magma emplacement depth. Combining geological and geochemical characteristics with regional geological history, we argue that southeast China was dominated by an extensional tectonic setting in the Early Cretaceous, and the nature of the mantle source area was related to enrichment induced by asthenosphere upwelling and infiltration of upper crust-derived fluids responding to Pacific Plate subduction.  相似文献   

2.
Diabase dyke swarms are widespread in the East Tianshan and Beishan regions. LA-ICP-MS zircon U-Pb ages of these diabase vary from 305 Ma to 278 Ma, showing that these dykes were formed during Late Carboniferous-Early Permian magmatism. All diabase samples are subalkali calc-alkali, characterized by slight LREE and LILEs enrichment, and weak negative Ti, Nb and Ta anomalies. The diabase samples have positive εNd(t) values (>+3), high Sr isotopic compositions (initial 87Sr/86Sr values=0.7030-0.7097), and large variation of Pb isotopic compositions, indicating they were derived from a deplete mantle source. Regional geology and geochemistry evidences indicate that these diabase dyke swarms were generated in a lithosphere extensional setting and had the same magma sources. Initial magmas may be a mixture of depleted asthenosphere mantle and enriched lithospheric mantle during rapid magma ascending.  相似文献   

3.
The Kooh-Shah region located in a Tertiary volcanic-plutonic belt of the Lut Block in eastern Iran comprises several subvolcanic intermediate to acidic intrusive rocks, diorite to syenite in composition, which have intruded into volcanic rocks. The Kooh-Shah granitoid rocks are characterized by enrichment in large ion-lithophile elements (LILE: e.g. Sr, Ba, Rb) and depletion in high field-strength elements (HFSE: e.g. Nb, Ta, Ti). The chondrite-normalized REE patterns are characterized by moderate LREE enrichment (La/Yb)N=6.01-10.01, medium-heavy REE enrichment, and absence of Eu anomalies. The Kooh-Shah intrusive rocks are metaluminous, shoshonitic with calc-alkaline affinity and high values of magnetic susceptibility, and classified as the magnetite-series of oxidant I-type granitoids. The age of Kooh-Shah granitoid rocks based on zircon U-Pb age dating is 39.7±0.7 Ma (=Middle Eocene) and the ranges of their initial 87Sr/86Sr and 143Nd/144Nd ratios are from 0.704812 to 0.704920 and 0.512579 to 0.512644, respectively, when recalculated to an age of 39 Ma. The initial ?Nd isotope values for the Kooh-Shah intrusive rocks range from -0.18 to 1.09. This geochemical data indicates that the Kooh-Shah granitoid rocks formed from depleted mantle in an island arc setting. The geochemical signature of the studied granitoid rocks represents a characteristic guide for future exploration of copper-gold porphyry-type deposits in the Lut block.  相似文献   

4.
The Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed2.3-2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the ArchaeanRangrim and Ryonggang Blocks of the northeastern Sino-Korean Craton. The Complex ismainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrologicaland geochemical studies show that the protoliths of the Complex are mainly assoctations ofbimodal volcanics and anorogenic granites. The Kuandian amphibolites are depleted in Nb, Ta,P and Ti, and enriched in LILE, e.g. K, Rb and Cs, with pronounced depletion of Sr relative toNd and Pb; La/Nb ratios are higher than 1(1.75 to 5.18). The trace element patterns of theamphibolites are similar to continental flood basalts formed by the Gondwana break-up, suchas those in South Karoo and Tasmania, which shows continental contamination. ε_(Nd) valuesranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yband La/Yb suggest that contamination of basaltic magma happened in the mantle, rather thanalong the conduit. Isotope ratios of ~(208)Pb/~(204)Pb, ~(207)Pb/ ~(204)Pb, ~(206)Pb/~(204)Pb, ~(143)Nd/~(144)Ndand ~(87)Sr/~(86)Sr indicate that the magma was derived from a contaminated mantle source likeDMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is thecase with some continental basalts in the south hemisphere, e.g. in South Karoo and Tasmania.Petrochemical modelling proposes that the Kuandian gneiss, granite, and amphibolite camefrom the same parental magma, being products of strong fractional crystallization. Protoliths ofthe Kuandian Complex were formed in extensional tectonic setting during the transition fromcontinental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3or 2.4 Ga ago tectonic evolution of the Sino-Korean craton was different from that of otherwell-studied Precambrian cratons, e.g. the North American shield, European platform andAustralian continent in that strong volcanic eruption resulted in its accretion. Besides, the con-taminated magma source with a Dupal anomaly for the Complex indicates that crust-mantleconvection whose scale was similar to that of the present plate tectonics had occurred at leastbefore the formation of the Kuandian Complex (2.3-2.4 Ga B.P).  相似文献   

5.
We conducted geochemical and isotopic studies on the Oligocene–Miocene Niyasar plutonic suite in the central Urumieh–Dokhtar magmatic belt, in order better to understand the magma sources and tectonic implications. The Niyasar plutonic suite comprises early Eocene microdiorite, early Oligocene dioritic sills, and middle Miocene tonalite + quartzdiorite and minor diorite assemblages. All samples show a medium-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of large-ion lithophile elements (LILEs, e.g. Rb, Ba, Sr), enrichment of light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, e.g. Nb, Ta, Ti, P). The chondrite-normalized rare earth element (REE) patterns of microdiorite and dioritic sills are slightly fractionated [(La/Yb)n = 1.1–4] and display weak Eu anomalies (Eu/Eu* = 0.72–1.1). Isotopic data for these mafic mantle-derived rocks display ISr = 0.70604–0.70813, ?Nd (microdiorite: 50 Ma and dioritic sills: 35 Ma, respectively) = +1.6 and ?0.4, TDM = 1.3 Ga, and lead isotopic ratios are (206Pb/204Pb) = 18.62–18.57, (207Pb/204Pb) = 15.61–15.66, and (208Pb/204Pb) = 38.65–38.69. The middle Miocene granitoids (18 Ma) are also characterized by relatively high REE and minor Eu anomalies (Eu/Eu* = 0.77–0.98) and have uniform initial 87Sr/86Sr (0.7065–0.7082), a range of initial Nd isotopic ratios [?Nd(T)] varying from ?2.3 to ?3.7, and Pb isotopic composition (206Pb/204Pb) = 18.67–18.94, (207Pb/204Pb) = 15.63–15.71, and (208Pb/204Pb) = 38.73–39.01. Geochemical and isotopic evidence for these Eocene–Ologocene mafic rocks suggests that the magmas originated from lithospheric mantle with a large involvement of EMII component during subduction of the Neotethyan ocean slab beneath the Central Iranian plate, and were significantly affected by crustal contamination. Geochemical and isotopic data of the middle Miocene granitoids rule out a purely crustal-derived magma genesis, and suggest a mixed mantle–crustal [MASH (melting, assimilation, storage, and homogenization)] origin in a post-collision extensional setting. Sr–Nd isotope modelling shows that the generation of these magmas involved ～60% to 70% of a lower crustal-derived melt and ～30% to 40% of subcontinental lithospheric mantle. All Niyasar plutons exhibit transitional geochemical features, indicating that involvement of an EMII component in the subcontinental mantle and also continental crust beneath the Urumieh–Dokhtar magmatic belt increased from early Eocene to middle Miocene time.  相似文献   

6.
The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro–diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the Xuhe mafic rocks are enriched in large ion lithophile elements(LITE), such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite-normalized REE patterns display LREE enrichment(LaN/YbN = 9.34–13.99) and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SIMS zircon U–Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low Mg O(basic rock: 3.11–7.21 wt%; pyroxene diorite: 0.89–1.21 wt%) and Mg#(0.20–0.49) for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low(5%–10%) and in garnetspinel transition facies. Sr-Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMII and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back-arc extension.  相似文献   

7.
赣西北基性岩脉的地质地球化学特征及其意义   总被引：24，自引：7，他引：24  
对赣西北三个地区基性岩脉的地质地球化学特征进行了研究，并初步讨论了基性脉岩的成岩方式和源区性质。基性岩脉包括闪斜煌斑岩，辉绿岩和橄榄录玢岩。微量元素相对富集大离子亲石元素，而亏损高场强元素，具有Nb,Ta和Ti异常。武山煌班岩稀土元素总量高，明显富集轻;益口辉绿岩稀土元素总量低，略微富集轻稀土元素，上高橄榄辉绿玢岩介于两者之间，三者都无明显的铕异常，结合基性脉岩的地球化学分析可知，三者的岩石化学变化受到部分熔融和分离结晶所控制，武山煌斑岩和上高橄榄辉绿玢岩明显发生分离结晶作用，而隘口辉绿岩主要受部分熔融作用影响。其源区可能为与俯冲流体有关的富集地幔，有待于进一步研究。  相似文献   

8.
The Baer ophiolitic massif is located in the northern sub-belt of the western segment of the Yarlung Zangbo Suture Zone (YZSZ) and mainly consists of a lherzolite-dominant mantle suite, dolerite intrusions and limited crustal outcrops. The dolerites show sub-ophitic texture and light rare earth element-depleted chondrite-normalized rare earth element patterns similar to normal-mid-ocean ridge basalts (N-MORB); though, they display enrichments in fluid-mobile elements (Rb, Ba, and Sr) and marked depletions in Th and Nb. The U–Pb ages of several magmatic zircon grains recovered from two dolerite samples indicate that the intrusion of the dikes into the Baer lherzolitic mantle occurred at 125.6–126.3 Ma, consistent with the widespread mafic magmatism between 120 and 130 Ma in the Yarlung Zangbo ophiolites. The dolerites have slightly more radiogenic 87Sr/86Sr ratios (0.7043–0.7054) in comparison to N-MORB, whereas they show 143Nd/144Nd values (0.513067–0.513114) similar to N-MORB and high zircon Hf-isotope compositions. They have a limited range of Nd-isotope (εNd(t) values: +8.2 to +9.1) and juvenile Hf-isotope compositions (εHf(t) values: +8.4 to +14.2 and +10.0 to +15.1) indicating derivation from mantle melts. The moderate spread in the εHf (t) values of zircons indicates derivation of the dolerites parental magma from a weakly contaminated spinel-bearing mantle source. This is also corroborated by the geochemical signatures of the Baer dolerites (enrichment in LILE and depletion in HFSE) suggesting minor slab input to the mantle source of the dike-filling melt. We suggest that the genesis of the dolerite dike-forming melt happened at a stage of subduction initiation in a sub-oceanic mantle domain mildly affected by fluids emanating from the downgoing slab. Our data combined with literature data allow us to presume that the intrusion of the dolerites into the Baer mantle corresponds to an early phase of subduction initiation beneath a developing forearc basin.  相似文献   

9.
The Great Xing′an and Lesser Xing′an ranges are characterized by immense volumes of Mesozoic granitoids. In this study, we present major and trace element geochemistry, U–Pb geochronology and systematic Sr–Nd–Hf isotopes for the representative samples, in order to constrain their petrogenesis and the tectonic evolution in NE China. The granitoids generally have high SiO2 (66.5–78.8 wt.%) and Na2O + K2O (7.0–8.9 wt.%) contents and belong to high‐K calc‐alkaline to shoshonitic series. All of them show enrichment in Rb, Th, U, Pb and light rare earth elements (LREE), and depletion in Nb, Ta, P and Ti. Zircon U–Pb dating suggests that there was continuous magmatism in both the Great Xing′an Range and the Lesser Xing′an Range during the Jurassic–Early Cretaceous interval. Seven Jurassic granitoids have (87Sr/86Sr)i values of 0.704351 to 0.707374, with ϵNd(t) values of −3.4 to 2.4 and ϵHf(t) values of 0.8 to 11.3, indicating that they originated from mixed sources involving depleted mantle and pre‐existing crustal components. One Early Cretaceous sample yields (87Sr/86Sr)i value of 0.706184, ϵNd(t) value of 0.6, and ϵHf(t) values of 7.0 to 8.2, which is in accordance with previous studies and indicates a major juvenile mantle source for the granitoids in this period. In the Jurassic, the magmatism in the Great Xing′an Range was induced by the subduction of the Mongol–Okhotsk Ocean, while the contemporaneous magmatism in the Lesser Xing′an Range was related to the subduction of the Palaeo‐Pacific Ocean. In the Early Cretaceous, extensive magmatism in NE China was probably attributed to large‐scale lithospheric delamination. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

10.
The aim of this article is to examine the geochemistry and geochronology of the Cadomian Mishu granites from northwest Iran, in order to elucidate petrogenesis and their role in the evolution of the Cadomian crust of Iran. The Mishu granites mainly consist of two-mica granites associated with scarce outcrops of tonalite, amphibole granodiorite, and diorite. Leucogranitic dikes locally crosscut the Mishu granites. Two-mica granites show S-type characteristics whereas amphibole granodiorite, tonalities, and diorites have I-type signatures. The I-type granites show enrichment in large-ion lithophile elements (e.g. Rb, Ba and K) and depletion in high field strength elements (e.g. Nb, Ti and Ta). These characteristics show that these granites have been formed along an ancient, fossilized subduction zone. The S-type granites have high K, Rb, Cs (and other large ion lithophile elements) contents, resembling collision-related granites. U–Pb zircon dating of the Mishu rocks yielded 238U/206Pb crystallization ages of ca. 550 Ma. Moreover, Rb–Sr errorchron shows an early Ediacaran age (547 ± 84 Ma) for the Mishu igneous rocks. The two-mica granites (S-type granites) show high 87Sr/86Sr(i) ratios, ranging from 0.7068 to 0.7095. Their ?Nd values change between ?4.2 and ?4.6. Amphibole granitoids and diorites (I-type granites) are characterized by relatively low 87Sr/86Sr(i) ratios (0.7048–0.7079) and higher values of ?Nd (?0.8 to ?4.2). Leucogranitic dikes have quite juvenile signature, with ?Nd values ranging from +1.1 to +1.4 and Nd model ages (TDM) from 1.1 to 1.2 Ga. The isotopic data suggests interaction of juvenile, mantle-derived melts with old continental crust to be the main factor for the generation of the Mishu granites. Interaction with older continental crust is also confirmed by the presence of abundant inherited zircon cores. The liquid-line of descend in the Harker diagrams suggests fractional crystallization was also a predominant mechanism during evolution of the Mishu I-type granites. The zircon U–Pb ages, whole rock trace elements, and Sr–Nd isotope data strongly indicate the similarities between the Mishu Cadomian granites with other late Neoproterozoic–early Cambrian (600–520 Ma) granites across Iran and the surrounding areas such as Turkey and Iberia. The generation of the Mishu I-type granites could be related to the subduction of the Proto-Tethyan Ocean during Cadomian orogeny, through interaction between juvenile melts and old (Mesoproterozoic or Archaean) continental crust. The S-type granites are related to the pooling of the basaltic melts within the middle–upper parts of the thick continental crust and then partial melting of that crust.  相似文献   

11.
Mesozoic mafic dikes in the Gan-Hang tectonic belt (GHTB) provide an opportunity to explore both the nature of their mantle source(s) and the secular evolution of the underlying Mesozoic lithospheric mantle in the region. The geochronology and primary geochemical and Sr–Nd–Pb isotopic compositions of Group 1 (middle section of GHTB) and Group 2 (the rest of the section) dolerite dikes spanning the GHTB were investigated. K–Ar ages indicate that dikes of both groups were emplaced during the Cretaceous (131–69 Ma). The dikes are doleritic in composition and are enriched in both large ion lithophile elements (LILEs; e.g. Rb, Ba, and Pb) and light rare earth elements (LREEs), with a wide range of Eu anomalies, but are depleted in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti) and heavy rare earth elements (HREEs). Dikes sampled in the middle section of the GHTB (Group 1) show more pronounced REE differentiation and a greater contribution from crustal material than those from the east and west sections (Group 2) and are similar to GHTB volcanic rocks in exhibiting a slight enrichment in LREEs. The dolerites are further characterized by a wide range in 87Sr/86Sr i ?=?0.7041–0.7110, 143Nd/144Nd i ?=?0.511951–0.512758, ?Nd t ?=?–10.4 to?+5.6, and Pb isotopic ratios (206Pb/204Pb i ?=?18.1–18.3, 207Pb/204Pb i ≈ 15.6, and 208Pb/204Pb i ?=?38.2–38.7). The dikes have undergone fractional crystallization of olivine, clinopyroxene, plagioclase, and Ti-bearing phases, except for dikes from the Anding area, which possibly experienced fractionation of plagioclase. Geochemically, all the dike samples originated from mantle sources ranging in composition from depleted to enriched that contained a component of foundered lower crust; crustal contamination during the ascent of these magmas was negligible. In the context of the late Mesozoic lithospheric extension across South China, mafic dike magmatism was likely triggered by the reactivation of deep faults, which promoted foundering of the lower crust and subsequent mantle upwelling in the GHTB.  相似文献   

12.
The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ?Nd(T) values (+0.86 to?+2.65), whereas values for ?Sr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO2, K2O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SW Amazonian Craton – the Serra da Providência, Colorado, and Nova Brasilândia bimodal suites – indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.  相似文献   

13.
The Langdu high-K calc-alkaline intrusions are located in the Zhongdian area, which is the southern part of the Yidun island arc. These intrusive rocks consist mainly of monzonite porphyry, granodiorite, and diorite porphyry. The K2O content of majority of these rocks is greater than 3%, and, in the K2O-SiO2 diagram, all the samples fall into the high-K calc-alkaline to shoshonitic fields. They are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs; LaN/YbN = 14.3-21.2), and show slightly negative Eu anomalies (δEu = 0.77-1.00). These rocks have high K, Rb, Sr, and Ba contents; moderate to high enrichment of compatible elements (Cr = 36.7-79.9 ppm, Co = 9.6-16.4 ppm, and MgO = 2.2%-3.4%); low Nb, Ta, and Ti contents, and characteristic of low high field strength elements(HFSEs) versus incompatible elements ratios (Nb/Th = 0.75, Nb/La = 0.34) and incompatible elements ratios (Nb/U = 3.0 and Ce/Pb = 5.1, Ba/Rb = 12.0). These rocks exhibit restricted Sr and Nd isotopic compositions, with (87Sr/86Sr) i values ranging from 0.7044 to 0.7069 and εNd(t) values from -2.8 to -2.2. The Sr-Nd isotope systematic and specific trace element ratios suggest that Langdu high-K calc-alkaline intrusive rocks derived from a metasomatized mantle source. The unique geochemical feature of intrusive rocks can be modeled successfully using different members of a slightly enriched mantle, a slab-derived fluid, and terrigenous sediments. It can be inferred that the degree of partial melting and the presence of specific components are temporally related to the tectonic evolution of the Zhongdian island arc. Formation of these rocks can be explained by the various degrees of melting within an ascending region of the slightly enriched mantle, triggered by the subduction of the Garzê-Litang ocean, and an interaction between the slab-derived fluid and the terrigenous sediments.  相似文献   

14.
In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K2O contents (5.9–6.1 wt.%) and K2O/Na2O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al2O3 contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low εNd(t) and εHf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.  相似文献   

15.
The North Qilian orogenic belt (NQOB) has been defined as a subduction-collision zone between the Alxa Block and the Qilian Block during the Early Paleozoic. To constrain the post-collisional tectonic evolution of the NQOB, analyses of zircon U-Pb-Hf isotopes, whole-rock major, trace element and Sr-Nd-Pb isotope compositions of the newly discovered Early Devonian lamprophyres and diorites dikes from the Longshoushan area in southwestern margin of the Alxa Block were conducted. Zircon U-Pb dating yields emplacement ages of 400 ± 4 Ma and 403 ± 6 Ma for two lamprophyre dikes and 391 ± 3 Ma for two diorite dikes. The lamprophyres dikes are shoshonitic-high-K (calc-alkaline) in nature, and are characterized by SiO2 contents of 53.6–56.3 wt %, (87Sr/86Sr)i ratios of 0.7064 to 0.7072, εNd(t) values of 0.1–1.0, and zircon εHf(t) values of −8.0 to −2.9. The diorite dikes are high-K (calc-alkaline), and are characterized by MgO contents of 6.32–6.98 wt %, (87Sr/86Sr)i ratios of 0.7089–0.7137, εNd(t) values of −3.8 to −3.5, and zircon εHf(t) values of −9.9–0.4. Both the lamprophyre and diorite dikes show parallel enrichments in LREEs and LILEs and depletions in HREEs and HFSEs and have similar ratios of (206Pb/204Pb)i (17.587–18.133), (207Pb/204Pb)i (15.518–15.584) and (208Pb/204Pb)i (37.676–38.058). Geochemical and isotopic data suggest that the lamprophyre and diorite dikes were derived from low-degrees melts of amphibole- and phlogopite-bearing lherzolite and phlogopite-bearing lherzolite, respectively, in the spinel-garnet transition facies. Their parental magmas both experienced extensive fractional crystallization in a deep magma chamber and negligible crustal contamination during their ascent. Regarding the Palaeozoic tectonic development of the North Qilian orogenic belt, we propose that the Early Devonian lamprophyres and diorites possibly are related to North Qilian orogen unrooting and collapse and marking the end of the North Qilian orogenic events.  相似文献   

16.
《Gondwana Research》2006,9(4):529-538
Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (87Sr/86Sr = 0.70330∼0.70555, 143Nd/144Nd = 0.51298∼0.51256) compared with MORB.207Pb/204Pb and 208Pb/204Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (206Pb/204Pb = 18.61∼19.12, 207Pb/204Pb = 15.54∼15.69, 208Pb/204Pb = 38.98∼39.72) are significantly more radiogenic than the rest (206Pb/204Pb = 17.72∼18.03, 207Pb/204Pb = 15.44∼15.58, 208Pb/204Pb = 37.77∼38.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea.  相似文献   

17.
SHRIMP zircon U–Pb geochronological, elemental and Sr–Nd isotopic data from Early Cretaceous mafic dykes in North Dabie orogenic belt elucidate a change of Mesozoic lithospheric mantle in eastern China. The dykes are predominantly dolerite with the major mineral assemblage clinopyroxene + hornblende + plagioclase and yield a SHRIMP zircon U–Pb age of 111.6 ± 5.3 Ma. They have a narrow range of SiO2 from 46.16% to 49.78%, and relative low concentrations of K2O (1.07−2.62%), Na2O (2.45−3.54%), Al2O3 (13.04−14.07%), and P2O5 (0.42−0.55%) but relatively high concentration of MgO (5.94–6.61%) with Mg# 52–54. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and high field strength elements (HFSE, e.g., Nb, Ti). (87Sr/86Sr)i ratios from 0.704 to 0.705, εNd values from 3.36 to 4.33 and mantle‐depletion Nd model ages (T2DM) in the range 0.56–0.64 Ga indicate that the magma of the Baiyashan mafic dykes was derived from a young depleted mantle source. This finding is different from previous research on mafic dykes in the age range 120–138 Ma that revealed enrichment of LILE and depletion of HFSE, high initial Sr isotopic ratios and negative εNd, value which represents an old enriched mantle source. Ours is the first report of the existence of Early Cretaceous depleted mantle in eastern China and it implies that changing of enriched mantle to depleted mantle occurred at ca. 112 Ma, associated with back‐arc extension which resulted from the subduction of the Palaeo‐Pacific Plate towards the Asian Continent. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

18.
The time of final closure of the Palaeo-Tethys and the Sibumasu-Indochina collision in Southeast Asia represents a major unresolved geologic problem. Here, we present zircon chronology, whole-rock elemental, Sr–Nd, and zircon Hf isotopic geochemistry for newly discovered mafic dikes from the northern segment of the Sibumasu terrane, to provide constraints on this issue. Zircon U–Pb data indicate that the dikes were emplaced at 240 ± 3 Ma. These are the earliest Mesozoic magmatic rocks reported so far in the Sibumasu terrane, the late Palaeozoic passive margin of the Palaeo-Tethys. They are subalkaline tholeiites, showing geochemical characteristics similar to those of enriched mid-ocean ridge basalts (E-MORBs). They have 87Sr/86Sr(t) ratios of 0.703161–0.703826, ?Nd(t) of +4.8 to +7.5, and zircon ?Hf(t) of +9.2 to +13.3, implying strong mantle depletion. They were derived by partial melting of asthenospheric mantle and underwent subsequent fractional crystallization and lithospheric assimilation. The geologic–petrologic evidence suggests that the mafic dikes were generated in a collisional setting, when suturing of the Baoshan and Simao subterranes (the two subterranes are part of the Sibumasu and Indochina terranes, respectively) occurred. These early Middle Triassic mafic dikes provide an upper limit for Sibumasu–Indochina collision. In conjunction with previous work, we conclude that the final closure of the Palaeo-Tethys and collision of the Sibumasu and Indochina terranes took place during the late Permian to Early Triassic.  相似文献   

19.
The Duolong porphyry Cu–Au deposit (5.4 Mt at 0.72% Cu, 41 t at 0.23 g/t Au) was recently discovered in the southern Qiangtang terrane, central Tibet. Here, new whole‐rock elemental and Sr–Nd–Pb isotope and zircon Hf isotopic data of syn‐ and post‐ore volcanic rocks and barren and ore‐bearing granodiorite porphyries are presented for a reconstruction of magmas associated with Cu–Au mineralization. LA–ICP–MS zircon U–Pb dating yields mean ages of 117.0 ± 2.0 and 120.9 ± 1.7 Ma for ore‐bearing granodiorite porphyry and 105.2 ± 1.3 Ma for post‐ore basaltic andesite. All the samples show high‐K calc‐alkaline compositions, with enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE: Cs and Rb) and depletion of high field strength elements (HFSE: Nb and Ti), consistent with the geochemical characteristics of arc‐type magmas. Syn‐ and post‐ore volcanic rocks show initial Sr ratios of 0.7045–0.7055, εNd(t) values of −0.8 to 3.6, (206Pb/204Pb)t ratios of 18.408–18.642, (207Pb/204Pb)t of 15.584–15.672 and positive zircon εHf(t) values of 1.3–10.5, likely suggesting they dominantly were derived from metasomatized mantle wedge and contaminated by southern Qiangtang crust. Compared to mafic volcanic rocks, barren and ore‐bearing granodiorite porphyries have relatively high initial Sr isotopic ratios (0.7054–0.7072), low εNd(t) values (−1.7 to −4.0), similar Pb and enriched zircon Hf isotopic compositions [εHf(t) of 1.5–9.7], possibly suggesting more contribution from southern Qiangtang crust. Duolong volcanic rocks and granodiorite porphyries likely formed in a continental arc setting during northward subduction of the Bangong–Nujiang ocean and evolved at the base of the lower crust by MASH (melting, assimilation, storage and homogenization) processes. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

20.
拉萨地体南缘汤白地区广泛分布新生代的辉绿岩脉。为探讨该辉绿岩脉形成时代、岩石成因和地质意义,对其开展了详细的岩相学、地球化学、锆石U-Pb年代学及Hf同位素研究。LA-ICP-MS锆石U-Pb定年结果显示辉绿岩脉的结晶年龄为(54±1)Ma,表明其形成于早始新世。微量元素地球化学特征显示富集大离子亲石元素(LILEs:如Rb、Sr和Ba),亏损高场强元素(HFSEs:如Nb、Ta和Ti)。与典型的弧岩浆岩、区域上叶巴组和桑日岩群中的玄武岩相比具有较高的Nb、TiO_2和Zr含量,在微量元素构造环境判别图解中显示出板内玄武岩地球化学属性。微量元素地球化学特征结合锆石Hf同位素表明岩浆源区除被俯冲板片释放的流体交代的岩石圈富集地幔外,还有软流圈亏损地幔物质加入。汤白辉绿岩脉侵入年龄与区域上林子宗群火山活动峰期接近(52 Ma)。同时结合岩石成因及构造背景,作者认为汤白辉绿岩脉是54～52 Ma新特提斯洋壳断离诱发岩浆作用的产物。根据最新大陆碰撞及板片断离的三维数值模型,暗示了印度板块与欧亚大陆碰撞的起始时间为65 Ma或者更早。  相似文献   

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