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1.
The Cretaceous-Paleogene granites of the Eastern Sikhote Alin volcanic belt (ESAVB) and Late Cretaceous granitoids of the Tatibin Series (Central Sikhote Alin) are subdivided into three groups according to their oxygen isotope composition: group I with δ18O from +5.5 to +6.5‰, group II with δ18O from +7.6 to +10.2‰, and group III with less than +4.5‰. Group I rocks are similar in oxygen isotope composition to that of oceanic basalts and can be derived by melting of basaltic crust. Group II (rocks of the Tatibin Series) have higher δ18O, which suggests that their parental melts were contaminated by sedimentary material. The low 18O composition of group III rocks can be explained by their derivation from 18O-depleted rocks or by subsolidus isotopic exchange with low-18O fluid or meteoric waters. The relatively low δ18O and 87Sr/86Sr in the granitoids of Primorye suggest their derivation from rocks with a short-lived crustal history and can result from the following: (1) melting of sedimentary rocks enriched in young volcanic material that was accumulated in the trench along the transform continental margin (granites of the Tatibin Series) and (2) melting of a mixture of abyssal sediments, ocean floor basalts, and upper mantle in the lithospheric plate that subsided beneath the continent in the subduction zone (granites of the ESAVB).  相似文献   

2.
Zircon U–Pb ages and geochemical and isotopic data for Late Ordovician granites in the Baoshan Block reveal the early Palaeozoic tectonic evolution of the margin of East Gondwana. The granites are high-K, calc-alkaline, metaluminous to strongly peraluminous rocks with A/CNK values of 0.93–1.18, are enriched in SiO2, K2O, and Rb, and depleted in Nb, P, Ti, Eu, and heavy rare earth elements, which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed at ca. 445 Ma. High initial 87Sr/86Sr ratios of 0.719761–0.726754, negative ?Nd(t) values of –6.6 to –8.3, and two-stage model ages of 1.52–1.64 Ga suggest a crustal origin, with the magmas derived from the partial melting of ancient metagreywacke at high temperature. A synthesis of data for the early Palaeozoic igneous rocks in the Baoshan Block and adjacent Tengchong Block indicates two stages of flare-up of granitic and mafic magmatism caused by different tectonic settings along the East Gondwana margin. Late Cambrian to Early Ordovician granitic rocks (ca. 490 Ma) were produced when underplated mafic magmas induced crustal melting along the margin of East Gondwana related to the break-off of subducted Proto-Tethyan oceanic slab. In addition, the cession of the mafic magmatism between late Cambrian-Early Ordovician and Late Ordovician could have been caused by the collision of the Baoshan Block and outward micro-continent along the margin of East Gondwana and crust and lithosphere thickening. The Late Ordovician granites in the Baoshan Block were produced in an extensional setting resulting from the delamination of an already thickened crust and lithospheric mantle followed by the injection of synchronous mafic magma.  相似文献   

3.
《Resource Geology》2018,68(3):227-243
As a newly discovered medium‐sized deposit (proven Pb + Zn resources of 0.23 Mt, 9.43% Pb and 8.73% Zn), the Dongzhongla skarn Pb–Zn deposit is located in the northern margin of the eastern Gangdese, central Lhasa block. Based on the geological conditions in this deposit of ore‐forming fluids, H, O, C, S, Pb, Sr, and noble gas isotopic compositions were analyzed. Results show that δ18OSMOW of quartz and calcite ranged from −9.85 to 4.17‰, and δDSMOW ranged from −124.7 to −99.6‰ (where SMOW is the standard mean ocean water), indicating magma fluids mixed with meteoric water in ore‐forming fluids. The δ13CPDB and δ18OSMOW values of calcite range from −1.4 to −1.1‰ and from 5.3 to 15.90‰, respectively, show compositions consistent with the carbonate limestone in the surrounding rocks, implying that the carbon was primarily sourced from the dissolution of carbonate strata in the Luobadui Formation. The ore δ34S composition varied in a narrow range of 2.8 to 5.7‰, mostly between 4‰ and 5‰. The total sulfur isotopic value δ34S was 4.7‰ with characteristics of magmatic sulfur. The 3He/4He values of pyrite and galena ranged from 0.101 to 5.7 Ra, lower than those of mantle‐derived fluids (6 ± 1 Ra), but higher than those of the crust (0.01–0.05 Ra), and therefore classified as a crust–mantle mixed source. The Pb isotopic composition for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ores were in the ranges of 18.628–18.746, 15.698–15.802, and 39.077–39.430, respectively, consistent with the Pb isotopic composition of magmatic rocks in the deposit, classified as upper‐crust lead. The ore lead was likely sourced partially from the crustal basement of the Lhasa Terrane. The initial (87Sr/86Sr)i value from five sulfide samples ranged from 0.71732 to 0.72767, and associated ore‐forming fluids were mainly sourced from the partial melting of the upper‐crust materials. Pb isotopic compositions of ore sulfides from the Dongzhongla deposit are similar to that of the Yuiguila and Mengya'a deposit, indicating that they have similar sources of metal‐rich ore‐forming solution. According to basic skarn mineralogy, the economic metals, and the origin of the ore‐forming fluids, the Dongzhongla deposit was classified as a skarn‐type Pb–Zn deposit.  相似文献   

4.
紫木凼金矿床是黔西南卡林型金矿区一个重要的大型金矿床,其成矿物质来源尚不明确.对紫木凼金矿床不同类型矿石和赋矿围岩进行了S、C、O、Pb和Sr同位素组成对比研究.矿石中硫化物的δ34S值为-13.49‰~17.91‰（主要为-0.99‰~3.58‰）,赋矿围岩的δ34S值为-26.23‰~-19.63‰,矿床成矿期硫主要来源于岩浆,部分来源于赋矿地层中成矿前黄铁矿.热液期方解石的δ13C和δ18O分别为-9.10‰~0.59‰和15.65‰~23.82‰,与赋矿围岩、区域地层的碳、氧同位素组成差别较大,成矿流体的碳、氧部分来源于碳酸盐岩溶解,部分可能来源于岩浆.矿石中硫化物的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别为18.064~18.973、15.585~15.670和38.219~39.054,赋矿围岩的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别为18.136~18.650、15.574~15.656和38.423~38.812,矿石铅的来源较复杂,赋矿地层和岩浆可能都为其提供了部分铅.矿石中石英和方解石（87Sr/86Sr）i比值为0.707 26~0.708 11,赋矿围岩的（87Sr/86Sr）i比值为0.707 28~0.707 31,成矿流体中的锶主要来源于赋矿地层.紫木凼金矿床成矿物质具壳幔混合来源特征,成矿物质主要来自矿床深部隐伏岩浆岩,部分来自二叠系-三叠系赋矿地层.   相似文献   

5.
胶东金矿区矿田体系产出的脉岩是一套富含碳酸盐矿物相的中基性-中酸性脉岩，岩石类型主要有煌斑岩、安山玢岩类。对部分矿田体系产出脉岩的碳酸盐(方解石)碳、氧同位素及全岩氧同位素的研究表明该套脉岩的δ18OSMOW，方解石、δ13CPDB，方解石和δ  相似文献   

6.
The Cangyuan Pb-Zn-Ag polymetallic deposit is located in the Baoshan Block, southern Sanjiang Orogen. The orebodies are hosted in low-grade metamorphic rocks and skarn in contact with Cenozoic granitic rocks. Studies on fluid inclusions (FIs) of the deposit indicate that the ore-forming fluids are CO2-bearing, NaCl-H2O. The initial fluids evolved from high temperatures (462–498 °C) and high salinities (54.5–58.4 wt% NaCl equiv) during the skarn stage into mesothermal (260–397 °C) and low salinities (1.2–9.5 wt% NaCl equiv) during the sulfide stage. The oxygen and hydrogen isotopic compositions (δ18OH2O: 2.7–8.8‰; δD: −82 to −120‰) suggest that the ore-forming fluids are mixture of magmatic fluids and meteoric water. Sulfur isotopic compositions of the sulfides yield δ34S values of −2.3 to 3.2‰; lead isotopic compositions of ore sulfides are similar to those of granitic rocks, indicating that the sulfur and ore-metals are derived from the granitic magma. We propose that the Cangyuan Pb-Zn-Ag deposit formed from magmatic hydrothermal fluids. These Cenozoic deposits situated in the west of Lanping-Changdu Basin share many similarities with the Cangyuan in isotopic compositions, including the Laochang, Lanuoma and Jinman deposits. This reveals that the Cenozoic granites could have contributed to Pb-Zn-Cu mineralization in the Sanjiang region despite the abundance of Cenozoic Pb-Zn deposits in the region, such as the Jingding Pb-Zn deposit, that is thought to be of basin brine origin.  相似文献   

7.
We present new Fe and Si isotope ratio data for the Torres del Paine igneous complex in southern Chile. The multi-composition pluton consists of an approximately 1 km vertical exposure of homogenous granite overlying a contemporaneous 250-m-thick mafic gabbro suite. This first-of-its-kind spatially dependent Fe and Si isotope investigation of a convergent margin-related pluton aims to understand the nature of granite and silicic igneous rock formation. Results collected by MC-ICP-MS show a trend of increasing δ56Fe and δ30Si with increasing silica content as well as a systematic increase in δ56Fe away from the mafic base of the pluton. The marginal Torres del Paine granites have heavier Fe isotope signatures (δ56Fe = +0.25 ± 0.02 2se) compared to granites found in the interior pluton (δ56Fe = +0.17 ± 0.02 2se). Cerro Toro country rock values are isotopically light in both Fe and Si isotopic systems (δ56Fe = +0.05 ± 0.02 ‰; δ30Si = ?0.38 ± 0.07 ‰). The variations in the Fe and Si isotopic data cannot be accounted for by local assimilation of the wall rocks, in situ fractional crystallization, late-stage fluid exsolution or some combination of these processes. Instead, we conclude that thermal diffusion or source magma variation is the most likely process producing Fe isotope ratio variations in the Torres del Paine pluton.  相似文献   

8.
High‐precision calcium isotopic compositions of a set of geological reference materials from the IAG (OU‐6), ANRT (UB‐N), MPI‐DING, USGS and GSJ, relative to NIST SRM 915a, are reported here. Measurements were performed by thermal ionisation mass spectrometry (Triton instrument) using a 42Ca–43Ca double spike. δ44/40Ca values of selected reference materials, mainly felsic rocks, are reported for the first time. Felsic rock values of δ44/40Ca ranged from 0.13‰ to 1.17‰, probably implying Ca isotopic fractionation could occur during magma evolution. δ44/40Ca values of ultramafic rocks, ranging from 0.74‰ to 1.51‰, were positively correlated with MgO and negatively with CaO contents, possibly owing to Ca isotopic fractionation during partial melting. δ44/40Ca of intermediate‐mafic rocks were around 0.78‰ and displayed limited variation, suggesting Ca isotopic fractionation is insignificant during magma evolution processes. As expected, δ44/40Ca of sedimentary and metamorphic rocks varied widely due to complex geological processes.  相似文献   

9.
Post-collisional K-rich volcanic rocks (KVRs) can provide an opportunity to constrain the architecture of the lithosphere and the mechanisms of plateau uplift. However, their petrogenesis and geodynamic setting remain in dispute. Lithium concentrations and isotopic compositions of 87 potassic, ultrapotassic and Mg-rich potassic volcanic rocks (PVRs, UPVs, and MPRs, respectively) in SW Tibet, along with new Pb–Sr–Nd isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. These rocks are characterized by very similar δ7Li values: PVRs vary from −4.9‰ to +3.2‰, UPVs from −3.9‰ to +1.7‰, and MPRs from −1.2‰ to +3.5‰. They can be classified into two groups: Group I (19 out of 87 samples) with heavier δ7Li values (+1.0‰ to +3.5‰) similar to those reported for mid-ocean-ridge and ocean-island basalts (MORBs and OIBs, respectively), and Group II (68 out of 87 samples) with lighter values (−4.9‰ to +1.0‰) similar to those of Indian lower crust. These variable isotopic compositions may record the isotopic signature of the early-middle Miocene subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of isotopically light mantle domains beneath the Lhasa terrane, which were ascribed to the interaction with fluids/melts derived from the subducted Indian lower crust. The modeling curves of Indian lower crust with a metasomatized mantle composition fully account for compositional variations in the PVRs, UPVs, and MPRs. They were generated by the partial melting of SCLM, which was metasomatized by fluids/melts derived from the subducted Indian lower crust (ca. 4–14%, ca. 4–10%, and ca. 6–10% for the PVRs, UPVs, and MPRs, respectively). The Li isotopic data indicate that the Indian lower crust was subducted beneath the central Lhasa subterrane, and this sheds new light on the formation of the Tibet Plateau.  相似文献   

10.
A peraluminous granite belt occurs along the connecting zone between the Turpan-Hami Precambrian block and the Upper Paleozoic island arc belt. Muscovite granite and twomica granite are the essential lithological components of that belt. All the potassium feldspars in these granites are microcline. Heavy minerals are dominated by magnetite. SiO2 contents of these granites are greater than 73% with most of the A/NKC values greater than 1.1, normative corundum values greater than 1. Plots of CIPW norms in the (Al-K-Na)-Ca-(Fe2++ Mg) diagram are mostly situated in the plagioclase-cordierite-muscovite region. The rocks are characterized by very low contents of minor elements and ΣREE with strong Eu depletion, δ18O values between 6.6‰ and 7.0‰, Rb-Sr isochron age of 260.2 ± 6.2 Ma and an initial87Sr/86 Sr ratio of 0.7052. These granites might have been produced by partial melting of moderately acidic volcanites and low-maturity sediments in the basement sequences and could be genetically connected with the southward A-type subduction of the Turpan-Hami block following the closure of the Middle Carboniferous back-arc basin.  相似文献   

11.
The Jiapigou gold belt (>150 t Au), one of the most important gold-producing districts in China, is located at the northeastern margin of the North China Craton. It is composed of 17 gold deposits with an average grade around 10 g/t Au. The deposits are hosted in Archean gneiss and TTG rocks, and are all in shear zones or fractures of varying orientations and magnitudes. The δ34S values of sulfide from ores are mainly between 2.7?‰ and 10?‰. The Pb isotope characteristics of ore sulfides are different from those of the Archean metamorphic rocks and Mesozoic granites and dikes, and indicate that they have different lead sources. The sulfur and lead isotope compositions imply that the ore-forming materials might originate from multiple, mainly deep sources. Fluid inclusions in pyrite have 3He/4He ratios of 0.6 to 2.5 Ra, whereas their 40Ar/36Ar ratios range from 1,444 to 9,805, indicating a dominantly mantle fluid with a negligible crustal component. δ18O values calculated from hydrothermal quartz are between ?0.2?‰ and +5.9?‰, and δD values of the fluids in the fluid inclusions in quartz are from ?70?‰ to ?96?‰. These ranges suggest dominantly magmatic water with a minor meteoric component. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore-forming fluids have a dominantly mantle source with minor crustal addition.  相似文献   

12.
The western Kunlun orogen occupies a key position along the tectonic junction between the Pan-Asian and Tethyan domains, reflecting Proto- and Palaeo-Tethys subduction and terrane collision during early Palaeozoic to early Mesozoic time. We present the first detailed zircon U–Pb chronology, major and trace element, and Sr–Nd–O–Hf isotope geochemistry of the Qiukesu pluton and its microgranular enclaves from this multiple orogenic belt. SHRIMP zircon U–Pb dating shows that the Qiukesu pluton was emplaced in the early Silurian (ca. 435 Ma). It consists of weakly peraluminous high-K calc-alkaline monzogranite and syenogranite, with initial 87Sr/86Sr ratios of 0.7131–0.7229, ?Nd(T) of –4.1 to –5.7, δ18O of 8.0–10.8‰, and ?Hf(T) (in situ zircon) of –4.9. Elemental and isotopic data suggest that the granites formed by partial melting of lower-crustal granulitized metasedimentary-igneous Precambrian basement triggered by underplating of coeval mantle-derived enclave-forming intermediate magmas. Fractional crystallization of these purely crustal melts may explain the more felsic end-member granitic rocks, whereas such crustal melts plus additional input from coeval enclave-forming intermediate magma could account for the less felsic granites. The enclaves are intermediate (SiO2 57.6–62.2 wt.%) with high K2O (1.8–3.6 wt.%). They have initial 87Sr/86Sr ratios of 0.7132–0.7226, ?Nd(T) of –5.0 to –6.0, δ18O of 6.9–9.9‰, and ?Hf(T) (in situ zircon) of –8.1. We interpret the enclave magmas as having been derived by partial melting of subduction-modified mantle in the P–T transition zone between the spinel and spinel-garnet stability fields. Our new data suggest that subduction of the Proto-Tethyan oceanic crust was continuous to the early Silurian (ca. 435 Ma); the final closure of the Proto-Tethys occurred in the middle Silurian.  相似文献   

13.
岩浆去气作用碳硫同位素效应   总被引：6，自引：0，他引：6       下载免费PDF全文
 根据开放体系条件下的瑞利分馏原理,并考虑岩浆中可能溶解的合碳和含硫组分,从理论上定量模式了岩浆去气作用对火成岩碳、硫同位素组成的影响。结果表明,岩浆CO2去气作用能够导致岩石中碳酸盐显着亏损13C,其δ13C值能够从原始-5‰变化到-20‰(PDB);岩浆CH4去气作用则导致岩石中碳酸盐相对富集13C,其δ13C值能够从原始-5‰变化到+4‰。岩浆SO2去气作用可以导致岩石中硫化物显着亏损34S,其δ34S值能够从0‰变化到-8‰(CDT);岩浆H2S去气作用则导致岩石中的硫化物相对富集4S,其δ34S值能够从0‰变化到+6‰。因此,除源岩原始同位素不均一性和地壳物质混染能引起火成岩的碳、硫同位素组成发生较大变化外,岩浆去气作用也是重要原因之一。  相似文献   

14.
ABSTRACT

Mesozoic granitoid rocks in the eastern Qinling terrane along the southern margin of the North China Craton are significant for understanding geodynamics processes related to lithospheric delamination in the Late Mesozoic. In addition, these rocks have close genetic association with the largest Mo-(Au-Ag) polymetallic mineralization belt in China. In the present study, zircon U-Pb ages and elemental and Sr-Nd-Pb isotopic geochemistry are reported for two representative granite plutons (Heyu and Lantian) exposed in the eastern Qinling terrane. Granitoid rocks from both plutons are metaluminous to weakly peraluminous and are classified as I-type granite with formation ages of 145 ~ 135 Ma and 150 Ma, respectively. These rocks are characterized by adakite-like affinity without significant Eu anomalies. Both plutons have relatively high initial 87Sr/86Sr ratios ranging between 0.7063 and 0.7109 and variable εNd(t) values of – 29.2 to – 6.5. Their Pb isotopic compositions are comparable to those of old high-grade metamorphic basement rocks from the North China Craton but are different from those of country rocks from the Xiong’er and Taihua groups. These features indicate genesis via reworking of the lower crust beneath North China and partial involvement of juvenile crustal material to different extents. Occurrences of ca. 1000-Ma-old inherited zircon in the Lantian granites imply the contribution of the Qinling Group in North Qinling to the magma source(s). This magmatism occurred intensively in a transitional setting from compression to extension and was likely induced by lithosphere thinning and asthenosphere upwelling beneath eastern China during the Cretaceous.  相似文献   

15.
Deciphering the contribution of crustal materials to A-type granites is critical to understanding their petrogenesis. Abundant alkaline syenitic and granitic intrusions distributed in Tarim Large Igneous Province, NW China, offer a good opportunity to address relevant issues. This paper presents new zircon Hf-O isotopic data and U-Pb dates on these intrusions, together with whole-rock geochemical compositions, to constrain crustal melting processes associated with a mantle plume. The ∼280 Ma Xiaohaizi quartz syenite porphyry and syenite exhibit identical zircon δ18O values of 4.40 ± 0.34‰ (2σ) and 4.48 ± 0.28‰ (2σ), respectively, corresponding to whole-rock δ18O values of 5.6‰ and 6.0‰, respectively. These values are similar to mantle value and suggest an origin of closed-system fractional crystallization from Tarim plume-derived melts. In contrast, the ∼275 Ma Halajun A-type granites have higher δ18O values (8.82–9.26‰) than the mantle. Together with their whole-rock εNd(t) (−2.0–+0.6) and zircon εHf(t) (−0.6–+1.5) values, they were derived from mixing between crust- and mantle-derived melts. These felsic rocks thus record crustal melting above the Tarim mantle plume. At ∼280–275 Ma, melts derived from decompression melting of Tarim mantle plume were emplaced into the crust, where fractional crystallization of a common parental magma generated mafic-ultramafic complex, syenite, and quartz syenite porphyry as exemplified in the Xiaohaizi region. Meanwhile, partial melting of upper crustal materials would occur in response to basaltic magma underplating. The resultant partial melts mixed with Tarim plume-derived basaltic magmas coupled with fractional crystallization led to formation of the Halajun A-type granites.  相似文献   

16.
The Nage Cu-Pb deposit,a new found ore deposit in the southeast Guizhou province,southwest China,is located on the southwestern margin of the Jiangnan Orogenic Belt.Ore bodies are hosted in slate and phyllite of Neoproterozoic Jialu and Wuye Formations,and are structurally controlled by EW-trending fault.It contains Cu and Pb metals about 0.12 million tonnes with grades of 0.2 wt% to 3.4 wt% Cu and 1.1 wt% to 9.27 wt% Pb.Massive and disseminated Cu-Pb ores from the Nage deposit occur as either veinlets or disseminations in silicified rocks.The ore minerals include chalcopyrite,galena and pyrite,and gangue minerals are quartz,sericite and chlorite.The H-O isotopic compositions of quartz,S-Cu-Pb isotopic compositions of sulfide minerals,Pb isotopic compositions of whole rocks and ores have been analyzed to trace the sources of ore-forming fluids and metals for the Nage Cu-Pb deposit.The δ65CuNBS values of chalcopyrite range from-0.09% to +0.33‰,similar to basic igneous rocks and chalcopyrite from magmatic deposits.δ65CuNBS values of chalcopyrite from the early,middle and final mineralization stages show an increasing trend due to63Cu prior migrated in gas phase when fluids exsolution from magma.δ34SCDT values of sulfide minerals range from 2.7‰ to +2.8‰,similar to mantle-derived sulfur(0±3‰).The positive correlation between δ65CuNBS and δ34SCDT values of chalcopyrite indicates that a common source of copper metal and sulfur from magma.δDH2OSMOW and δ18OH2O-SMOW values of water in fluid inclusions of quartz range from 60.7‰ to 44.4‰ and +7.9‰ to +9.0‰(T=260°C),respectively and fall in the field for magmatic and metamorphic waters,implicating that mixed sources for H2O in hydrothermal fluids.Ores and sulfide minerals have a small range of Pb isotopic compositions(208Pb/204Pb=38.152 to 38.384,207Pb/204Pb=15.656 to 17.708 and 206Pb/204Pb=17.991 to 18.049) that are close to orogenic belt and upper crust Pb evolution curve,and similar to Neoproterozoic host rocks(208Pb/204Pb=38.201 to 38.6373,207Pb/204Pb=15.648 to 15.673 and 206Pb/204Pb=17.820 to 18.258),but higher than diabase(208Pb/204Pb=37.830 to 38.012,207Pb/204Pb=15.620 to 15.635 and206Pb/204Pb=17.808 to 17.902).These results imply that the Pb metal originated mainly from host rocks.The H-O-S-Cu-Pb isotopes tegather with geology,indicating that the ore genesis of the Nage Cu-Pb deposit is post-magmatic hydrothermal type.  相似文献   

17.
Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ26Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (87Sr/86Sr)i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (143Nd/144Nd)i from 0.511888 to 0.512040 and 0.511953 to 0.512076, 207Pb/204Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, 208Pb/204Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and 206Pb/204Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.  相似文献   

18.
《International Geology Review》2012,54(11):1054-1067
China is one of the richest countries in the world in terms of fluorite resources. On the basis of host rocks and mineralization patterns, the fluorite deposits in China have been classified into three types: (1) those occurring in Mesozoic volcanic regions, mainly consisting of veinfilling deposits of the quartz-fluorite association (Type I); (2) those occurring in granite areas, chiefly belonging to veinfilling deposits of the quartz-fluorite association or veinfilling or altered-rock deposits associated with Pb, Zn, W, Sn, etc. (Type II); and (3) those occurring in carbonate rocks as bedded deposits of quartz-fluorite, sulfide-fluorite, and quartz-barite-calcite-fluorite assemblages (Type III).

This paper summarizes the characteristics of fluorite deposits in China on the basis of strontium-, hydrogen-, and oxygen-isotopic and geochronological data, as well as geological investigations of deposits from 22 mining districts. δ18O and δD values of ore-forming fluids are ?10.2 to +3.7‰ and ?77.9 to 41.0‰, respectively, for Type-I deposits, ?13.1 to ?4.9‰ and ?65.5 to ?41.7‰ for Type-II deposits, and -5.6 to +4.3‰ and ?80 to ?29‰ for Type-Ill deposits. Study of the isotopic water-rock exchange indicates that the mineralizing fluids for these types of deposits have been derived mostly from circulating geothermal water that originated from Mesozoic meteoric water. The differences in the isotopic characteristics of hydrothermal systems for various types of deposits depend mainly on isotopic exchange between water and rocks, the water/rock values being 0.05 to 2.0 for Type-I deposits, generally more than 2.0 for Type-II deposits, and 0.5 to 3.0 for Type-Ill deposits.

The ore-forming ages for Type-I and Type-II deposits are roughly separated into three groups—230 to 180 Ma, 120 Ma, and 90 to 60 Ma. These ages successively decrease from northwestern China to southeastern coastal areas.

87Sr/86Sr values of 0.7306 to 0.7710 (mean 0.7513) from fluorite in the early stage (main mineralization) for Type-I deposits are higher than those from host rocks (0.7081 to 0.7260) during the mineralizing event, and mostly fall in the range of 87Sr/86Sr values from basement metamorphic rocks (from 0.7455 to 0.9094) during the epoch of mineralization. But the 87Sr/86Sr values of 0.7102 to 0.7137 (mean 0.7122) for late-stage fluorite and calcite are similar to those of surrounding host rocks. This indicates that the mineralizing materials of early stages in the formation of deposits (Type-I) originated mostly from Precambrian basement metamorphic rocks; those of later stages (Type II) were derived chiefly from host rocks. Type-II and Type-Ill deposits from different mining areas exhibit great variations in 87Sr/86Sr values, but are quite similar to their host rocks, indicating that the mineralizing materials in Type-II and Type-Ill deposits were derived chiefly from their host rocks.  相似文献   

19.
Seventy samples of Hercynian peraluminous granites (Guéret, Millevaches and Saint Sylvestre massifs) and metamorphic units of the Limousin area were analysed for Rb–Sr and Sm–Nd. The source rocks of the peraluminous granites can be found in the metamorphic rocks of Limousin, among them meta-igneous rocks were largely predominent over meta-sedimentary rocks in the source of the three granites. Millevaches and Guéret granites were generated by the partial melting of rocks comprising meta-volcanics and meta-sediments, whereas the Saint Sylvestre granite was produced exclusively by the melting of late Precambrian granites. This leads to confusing T DM Nd values, the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites, which leads to dramatic fractionation in Sm/Nd ratios. The data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates. Late Precambrian time seems, however, to have been a period of extensive crustal generation in Western Europe.  相似文献   

20.
Xiaolonghe is a poorly studied greisen-type tin deposit that is hosted by biotite granite in the western Yunnan tin belt. The mineralisation-related metaluminous and weak peraluminous granite is characterised by high Si, Al and K and low Mg, Fe and Ca, with an average A/CNK of 1.02. The granite is enriched in LILEs (K and Rb), LREEs and HFSEs (Zr, Hf, Th, U and Ce) and depleted in Ba, Nb, Sr, P, and Ti, with zircon εHf(t) = − 10.8 to − 7.5 (TDM2 = 1.61–1.82 Ga). These characteristics indicate that the magma was generated by the partial melting of a thickened ancient crust. LA-ICP-MS U–Pb dating of igneous zircon and hydrothermal cassiterite yield ages of 71.4 ± 0.4 Ma and 71.6 ± 4.8 Ma, respectively. The igneous biotite and hydrothermal muscovite samples show Ar–Ar plateau ages of 72.3 ± 0.4 Ma and 70.6 ± 0.2 Ma, respectively. The close temporal relationship between the igneous emplacement and hydrothermal activity suggests that the tin mineralisation was closely linked to the igneous emplacement. The δ18O and δD values for the deposit range from + 3.11‰ to − 4.5‰ and from − 127.3‰ to − 94.7‰, respectively. The hydrothermal calcite C and O isotopic data show a wide range of δ13CPDB values from − 5.7‰ to − 4.4‰, and the δ18OSMOW values range from + 1.4‰ to + 11.2‰. The δ34SV-CDT data range from + 4.8‰ to + 8.9‰ for pyrite, and the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios range from 18.708 to 18.760, from 15.728 to 15.754 and from 39.237 to 39.341, respectively. The stable isotopic (C–H–O–S–Pb) compositions are all similar to those of magmatic and mantle-derived fluids, which indicate that the ore-forming fluids and materials were mainly derived from magmatic sources that were accompanied by meteoric water. The tin mineralisation in the Xiaolonghe district was closely associated with the Late Cretaceous crustal-melting S-type granites that formed during the subduction of the Neo-Tethys oceanic lithosphere. Combined with the tin deposits in the Southeast Asian tin belt, Tengchong block and Central Lhasa, we interpreted that a giant intermittent tin mineralisation belt should be present along the Asian Neo-Tethys margin.  相似文献   

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