scholarly journals Shrimp and conventional U-Pb age, Sm-Nd isotopic characteristics and tectonic significance of the K-rich Itapuranga suite in Goiás, Central Brazil

2003 ◽  
Vol 75 (1) ◽  
pp. 97-108 ◽  
Author(s):  
MÁRCIO M. PIMENTEL ◽  
ELTON L. DANTAS ◽  
REINHARDT A. FUCK ◽  
RICHARD A. ARMSTRONG
Keyword(s):  
Magma Mixing ◽  
Regional Metamorphism ◽  
Quartz Syenite ◽  
Sialic Crust ◽  
Central Brazil ◽  
Tectonic Significance ◽  
Mafic Magmas ◽  
End Members ◽  
T Values ◽  
Tectonic Origin

The Itapuranga alkali granite and Uruana quartz syenite are large K-rich EW-elongated intrusions, in the central part of the Neoproterozoic Brasília Belt, central Brazil. They are associated with Pireneus lineaments, which cut the regional NNW-SSE structures of the southern part of the belt. SHRIMP and conventional U-Pb data for the Itapuranga and Uruana intrusions indicate crystallization ages of 624 ± 10 Ma and 618 ± 4 Ma, respectively. Three zircon cores from the Itapuranga granite yielded U-Pb ages between 1.79 and 1.49 Ga. Sm-Nd T DM ages for both intrusions are 1.44 Ga and epsilonNd(T) values are -5.1 and -5.7, suggesting the input of material derived from older (Paleo- to Mesoproterozoic) sialic crust in the origin of the parental magmas. Magma mixing structures indicate co-existence of mafic and felsic end-members. The felsic end-member of the intrusions is dominantly represented by crust-derived melts, formed in response to the invasion of Paleo/Mesoproterozoic sialic crust by alkali-rich mafic magmas at ca. 620 Ma. These intrusions are roughly contemporaneous with, or perhaps slightly younger than, the peak of regional metamorphism in the southern Brasília Belt. Their emplacement along the Pireneus lineament suggest a syn-tectonic origin for them, most probably in transtensional settings along these faults.

scholarly journals Latest Paleoproterozoic (ca. 1.8–1.6 Ga) extensional tectonic setting in the Dunhuang terrane, NW China: Evidence from geochronological and geochemical investigations on A-type granite and metamafic rock

Lithosphere ◽  
2019 ◽  
Vol 11 (6) ◽  
pp. 834-854 ◽  
Author(s):  
Yan Zhao ◽  
Wenhao Ao ◽  
Hong Zhang ◽  
Qian Wang ◽  
Mingguo Zhai ◽  
...  
Keyword(s):  
Mantle Source ◽  
Magma Mixing ◽  
Tectonic Setting ◽  
Regional Metamorphism ◽  
Global Scale ◽  
Mixing Processes ◽  
Tectonic Event ◽  
Depleted Mantle ◽  
Type Granite ◽  
T Values

Abstract Latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatic rocks outcrop in the Dunhuang terrane, represented by A-type granites and mafic (basaltic) rocks that have metamorphosed into amphibolites. The A-type granites, emplaced at ca. 1.79–1.77 Ga, are geochemically characterized by high Na2O + K2O, Fe2O3T, Zr, Nb, and Ce contents, as well as high Fe2O3T/(Fe2O3T + MgO) and Ga/Al ratios. Furthermore, they have Nb/Ta, Y/Nb, Rb/Nb, and Sc/Nb ratios of 12.10–15.56, 1.45–1.79, 3.52–6.51, and 0.11–0.19, respectively, showing affinity to A2-type granite. The A-type granites have negative εNd(t) values (−5.4 to −4.8) with Neoarchean depleted mantle (TDM2) ages (2591–2494 Ma), corresponding to coupling between εHf(t) values (−4.85 to -0.92) and TDM2 ages (2817–2556 Ma) of zircons. Therefore, the A-type granite pluton was mostly generated by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex in a postcollisional tectonic setting following a late Paleoproterozoic continent-continent collisional event. The metamafic rocks have a protolith age of 1605 ± 45 Ma and metamorphic age of 317 ± 20 Ma, indicating a Paleozoic tectonic event. The metamafic rock samples are geochemically characterized by relatively high alkali (Na2O + K2O = 4.39–4.81 wt%) contents and low Nb/Y (0.63–0.66) ratios, and they show steep rare earth element (REE) patterns with light REE enrichment and insignificant Eu anomalies and Nb-Ta, Zr-Hf, and Ti anomalies, resembling subalkaline oceanic-island basalt affinity. They have positive εNd(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable εHf(t) values (-1.09 to +9.06) of zircons. Hence, the protolith of the metamafic rocks may have been produced by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source during the initial rifting stage in an extensional setting, completing the formation of the Precambrian Dunhuang Complex. Considering the ca. 1.85–1.80 Ga regional metamorphism in the Dunhuang terrane, the latest Paleoproterozoic (ca. 1.8–1.6 Ga) A2-type granitic magmatism and mafic magmatism documented the postorogenic to initial rifting processes following the global-scale late Paleoproterozoic collisional event, which is comparable with ca. 1.80–1.67 Ga postcollisional and ca. 1.60–1.53 Ga anorogenic magmatism in the North China craton, but different from that of the Tarim craton.

scholarly journals Sr and Nd isotopic characteristics of 1.77-1.58 Ga rift-related granites and volcanics of the Goiás tin province, central Brazil

2001 ◽  
Vol 73 (2) ◽  
pp. 263-276 ◽  
Author(s):  
MÁRCIO M. PIMENTEL ◽  
NILSON F. BOTELHO
Keyword(s):  
Continental Rift ◽  
Rift System ◽  
Isotopic Data ◽  
Sialic Crust ◽  
Central Brazil ◽  
T Values

Supracrustal rocks of the Araí Group, together with coeval A-type granites represent a ca. 1.77-1.58 Ga old continental rift in Brazil. Two granite families are identified: the older (1.77 Ga) group forms small undeformed plutons, and the younger granites (ca. 1.58 Ga) constitute larger, deformed plutons. Sr-Nd isotopic data for these rocks indicate that the magmatism is mostly product of re-melting of Paleoproterozoic sialic crust. Initial Sr ratios for both granite families are ca 0.726 and 0.720. Most TDM model ages are between 2.58 and 1.80 Ga. epsilonND(T) values are between +3.6 and -11.9. Araí volcanics are bimodal, with basalts and dacites/rhyolites interlayered with continental sediments. The felsic volcanics show Nd isotopic characteristics which are very similar to the granites, and are also interpreted as reworking of Paleoproterozoic crust. Detrital sediments of the Araí Group revealed T DM model ages between 2.4 and 2.16 Ga, indicating that they are the product of erosion of Paleoproterozoic crust. The data indicate that the Araí rift system was established on crust that had just become stable after the Paleoproterozoic orogeny.

Origin of syn-collisional granitoids in the Gangdese orogen: Reworking of the juvenile arc crust and the ancient continental crust

2021 ◽  
Author(s):  
Yu-Wei Tang ◽  
Long Chen ◽  
Zi-Fu Zhao ◽  
Yong-Fei Zheng
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Continental Collision ◽  
Late Eocene ◽  
Igneous Rocks ◽  
Early Eocene ◽  
Southern Tibet ◽  
Crustal Rocks ◽  
Mafic Magmas ◽  
T Values

Granitoids at convergent plate boundaries can be produced either by partial melting of crustal rocks (either continental or oceanic) or by fractional crystallization of mantle-derived mafic magmas. Whereas granitoid formation through partial melting of the continental crust results in reworking of the pre-existing continental crust, granitoid formation through either partial melting of the oceanic crust or fractional crystallization of the mafic magmas leads to growth of the continental crust. This category is primarily based on the radiogenic Nd isotope compositions of crustal rocks; positive εNd(t) values indicate juvenile crust whereas negative εNd(t) values indicate ancient crust. Positive εNd(t) values are common for syn-collisional granitoids in southern Tibet, which leads to the hypothesis that continental collision zones are important sites for the net growth of continental crust. This hypothesis is examined through an integrated study of in situ zircon U-Pb ages and Hf isotopes, whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen in southern Tibet. The results show that these rocks can be divided into two groups according to their emplacement ages and geochemical features. The first group is less granitic with lower SiO2 contents of 59.82−64.41 wt%, and it was emplaced at 50−48 Ma in the early Eocene. The second group is more granitic with higher SiO2 contents of 63.93−68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early Eocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotope compositions with low (87Sr/86Sr)i ratios of 0.7044−0.7048, positive εNd(t) values of 0.6−3.9, εHf(t) values of 6.5−10.5, zircon εHf(t) values of 1.6−12.1, and zircon δ18O values of 5.28−6.26‰. These isotopic characteristics are quite similar to those of Late Cretaceous mafic arc igneous rocks in the Gangdese orogen, which indicates their derivation from partial melting of the juvenile mafic arc crust. In comparison, the late Eocene granitoids have relatively lower MgO, Fe2O3, Al2O3, and heavy rare earth element (HREE) contents but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb)N ratios. They also exhibit more enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7070−0.7085, negative εNd(t) values of −5.2 to −3.9 and neutral εHf(t) values of 0.9−2.3, and relatively lower zircon εHf(t) values of −2.8−8.0 and slightly higher zircon δ18O values of 6.25−6.68‰. An integrated interpretation of these geochemical features is that both the juvenile arc crust and the ancient continental crust partially melted to produce the late Eocene granitoids. In this regard, the compositional evolution of syn-collisional granitoids from the early to late Eocene indicates a temporal change of their magma sources from the complete juvenile arc crust to a mixture of the juvenile and ancient crust. In either case, the syn-collisional granitoids in the Gangdese orogen are the reworking products of the pre-existing continental crust. Therefore, they do not contribute to crustal growth in the continental collision zone.

scholarly journals Paleoproterozoic Multiple Tectonothermal Events in the Longshoushan Area, Western North China Craton and Their Geological Implication: Evidence from Geochemistry, Zircon U–Pb Geochronology and Hf Isotopes

Minerals ◽  
2018 ◽  
Vol 8 (9) ◽  
pp. 361 ◽  
Author(s):  
Renyu Zeng ◽  
Jianqing Lai ◽  
Xiancheng Mao ◽  
Bin Li ◽  
Jiandong Zhang ◽  
...  
Keyword(s):  
North China Craton ◽  
Tectonic Evolution ◽  
North China ◽  
Regional Metamorphism ◽  
Hf Isotope ◽  
Crustal Growth ◽  
Quartz Syenite ◽  
Isotope Data ◽  
The North ◽  
Rock Types

The Alxa block is located in the southwestern margin of the North China Craton. The Paleoproterozoic tectonic evolution, crustal growth and tectonic affinity of the block remain unknown or controversial. The Longshoushan (LS) area is one of the few areas that outcrop Paleoproterozoic to crystalline basement rocks in the Alxa Block. In this study, we preset whole-rock geochemistry, zircon U–Pb geochronology and Lu–Hf isotope data from metagabbro, metadiorite, quartz syenite, granitic leucosome and pegmatoid leucosome in the LS area. These rocks all are enriched in LREE and LILE, and depleted in HREE and HFSE. Eight new LA-ICP-MS zircon U–Pb ages yielded three magmatic ages of 2044 Ma, 2029 Ma and 1940 Ma, and three metamorphic ages of 1891 Ma, 1848 Ma and 1812 Ma. Lu–Hf analyses reveal that the magmatic zircons and anatectic/metamorphic zircons from all the rock types are characterized by positive εHf(t) (−0.16 to 10.89) and variable εHf(t) (−11.21 to 6.24), respectively. Based on the previous studies and our new data, we conclude that the LS area experienced three magmatic events (2.5–2.45 Ga, ~2.1–2.0 Ga and ~1.95–1.91 Ga) and three regional metamorphism/anataxis events (~1.93–1.89 Ga, ~1.86–1.84 Ga and ~1.81 Ga) in Paleoproterozoic. The age–Hf isotope data establishes two main crustal growth events at ~2.9–2.5 Ga and ~2.2–2.0 Ga in the LS area. These data indicate that the LS area experienced intraplate extensional setting in the middle Paleoproterozoic, and continental subduction, collision and exhumation in the late Paleoproterozoic. Combining the geochronological framework and tectonic evolution, we suggest that the Alxa Block is part of the Khondalite Belt.

scholarly journals U-Pb SHRIMP and Sm-Nd geochronology of the Silvânia Volcanics and Jurubatuba Granite: juvenile Paleoproterozoic crust in the basement of the Neoproterozoic Brasília Belt, Goiás, central Brazil

2001 ◽  
Vol 73 (3) ◽  
pp. 445-460 ◽  
Author(s):  
DANIELLE P. FISCHEL ◽  
MÁRCIO M. PIMENTEL ◽  
REINHARDT A. FUCK ◽  
RICHARD ARMSTRONG
Keyword(s):  
Central Brazil ◽  
Metavolcanic Rocks ◽  
Isotopic Analyses ◽  
T Values ◽  
Magmatic Event

U-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic metavolcanic rocks from the Silvânia Sequence and Jurubatuba Granite in the central part of the Brasília Belt. Zircon grains from a metavolcanic sample yielded 2115 ± 23 Ma and from the granite yielded 2089 ± 14 Ma, interpreted as crystallization ages of these rocks. Six metavolcanic samples of the Silvânia Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 ± 110 Ma and positive epsilonNd(T) = +3.0 interpreted as a juvenile magmatic event. Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic T DM model ages between 2.30 and 2.42 Ga and epsilonNd(T) values vary between -0.22 and -0.58. The oldest T DM value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvânia volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasília Belt that implies the presence of arc/suture hidden in reworked basement of the Brasília Belt.

Eocene post-collisional magmatism in Himalaya orogenic belt: evidence from petrography, zircon U-Pb age and Sr-Nd-Hf isotope of the Mayum alkaline complex, southern Lhasa subterrane

2020 ◽  
Author(s):  
Xiaoshuang Chen ◽  
Haijin Xu
Keyword(s):  
Oceanic Lithosphere ◽  
Hf Isotope ◽  
Alkaline Magmatism ◽  
Early Eocene ◽  
Alkaline Complex ◽  
Southern Tibet ◽  
Quartz Syenite ◽  
Alkaline Rocks ◽  
Slab Breakoff ◽  
T Values

<p>Alkaline magmatism is commonly generated in extensional settings, playing an important role in constraining the timing of slab breakoff. Eocene post-collisional magmatism is widely distributed along the Gangdese belt of southern Tibet. However, few Eocene post-collisional alkaline magmatism has been identified. Here, we present a comprehensive study of whole-rock geochemistry, zircon U-Pb ages and Sr-Nd-Hf isotopes of the Mayum alkaline complex from the Southern Lhasa Subterrane, providing an insight into the timing of breakoff of the Neo-Tethyan slab. The alkaline complex is composed of amphibolite syenite, quartz syenite and alkaline granite. The mafic microgranular enclaves are ubiquitous in the syenites. Zircon U-Pb analyses indicates that the alkaline rocks were generated in Early Eocene (ca. 53-50 Ma). These ages suggest that the alkaline rocks emplaced shortly (10-15Ma) after the continental collision between the Indian and Eurasian plates. The alkaline rocks have high SiO<sub>2 </sub>(64.32-77.36 wt.%), Na<sub>2</sub>O + K<sub>2</sub>O (6.63-9.03 wt.%) contents, low MgO (0.14-2.52 wt.%) contents. These rocks show obvious arc-like geochemical features in trace elements, i.e., enrichment in LILEs (e.g., Rb, K), LREEs, Th and U, and depletion in HFSEs (e.g., Nb, Ta, Ti), HREEs with strongly to moderately negative Eu anomalies (δEu=0.28–0.72). These features together with high FeO<sup>T</sup>/MgO, Ga/Al, Ce/Nb and Y/Nb values, and low Ba, Sr contents, suggesting that the Mayum alkaline rocks belong to an A2-type granitoids. Besides, the alkaline rocks have homogeneous initial <sup>87</sup>Sr/<sup>86</sup>Sr ratios (0.7052-0.7059) and negative ε<sub>Nd</sub>(t) values (-2.1 to -0.9) for whole-rock, and positive zircon ε<sub>Hf</sub>(t) values (+0.73 to +11.16). Nd-Hf isotope decoupling suggests that the alkaline was likely produced by mixing of mantle- and crust-derived magmas under a post-collisional extensional setting. Combined with previous published results, we propose that the slab breakoff of the subducting Neo-Tethyan oceanic lithosphere at least prior to Early Eocene (ca. 53Ma). The Eocene Mayum alkaline complex might be related to asthenosphere upwelling trigged by the slab breakoff.</p>

scholarly journals The Role of Magma Mixing in Generating Granodioritic Intrusions Related to Cu–W Mineralization: A Case Study from Qiaomaishan Deposit, Eastern China

Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 171 ◽  
Author(s):  
Huasheng Qi ◽  
Sanming Lu ◽  
Xiaoyong Yang ◽  
Jianghong Deng ◽  
Yuzhang Zhou ◽  
...  
Keyword(s):  
Magma Mixing ◽  
Eastern China ◽  
Mafic Magma ◽  
Enriched Mantle ◽  
Microgranular Enclaves ◽  
Intrusive Rocks ◽  
Granodiorite Porphyry ◽  
Shallow Crust ◽  
T Values ◽  
Host Rocks

The newly exploited Qiaomaishan Cu−W deposit, located in the Xuancheng ore district in the MLYRB, is a middle-sized Cu–W skarn-type polymetallic deposit. As Cu–W mineralization is a rare and uncommon type in the Middle-Lower Yangtze River Belt (MLYRB), few studies have been carried out, and the geochemical characteristics and petrogenesis of Qiaomaishan intrusive rocks related to Cu–W mineralization are not well documented. We studied two types of ore-bearing intrusive rocks in the Qiaomaishan region, i.e., pure granodiorite porphyry and granodiorite porphyry with mafic microgranular enclaves (MMEs). Age characterization using zircon LA–ICP–MS showed that they were formed almost simultaneously, around 134.9 to 135.1 Ma. Granodiorite porphyries are high Mg# adakites, characterized by high-K calc-alkaline and metaluminous features that are enriched in LILEs (e.g., Sr and Ba) and LREEs, but depleted in HFSEs (e.g., Nb, Ta, and Ti) and HREEs. Moreover, they have enriched Sr–Nd–Hf isotopic compositions (with whole-rock (87Sr/86Sr)i ratios (0.706666−0.706714), negative εNd(t) values of −9.1 to −8.6, negative zircon εHf(t) values of −12.2 to −6.7, and two-stage Hf model ages (TDM2) between 1.5 and 2.0 Ga). However, compared to host rocks, the granodiorite porphyry with MMEs shows variable geochemical compositions, e.g., high Mg#, Cr, Ni, and V contents and enriched with LILEs. In addition, they have more depleted ISr, εNd(t), and εHf(t) values (0.706025 to 0.706269, −6.4 to −7.4, and −10.6 to −5.7, respectively), overlapping with regions of Early Cretaceous mafic rocks derived from enriched lithospheric mantle in the MLYRB. Coupled with significant disequilibrium textures and geochemical features of host rocks and MMEs, we propose that those rocks have resulted from mixing the felsic lower crust-derived magma and the mafic magma generated from the enriched mantle. The mixed magmas subsequently rose to shallow crust to form the ore-bearing rocks and facilitate Cu–W mineralization.

scholarly journals Geochronology and Geochemistry of the Zengudi and Tuobake Granite Porphyries in the Sanjiang Region, SW China: Petrogenesis and Tectonic Significance

Minerals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 404
Author(s):  
Mao-Jun Tian ◽  
Huan Li ◽  
Landry Soh Tamehe ◽  
Zhen Xi
Keyword(s):  
Basaltic Magma ◽  
Volcanic Rocks ◽  
Isotopic Data ◽  
Tectonic Significance ◽  
Gondwana Margin ◽  
Granite Porphyries ◽  
Tethys Ocean ◽  
T Values ◽  
Crust Shortening ◽  
Representative Samples

The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N–S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Zengudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206Pb/238U ages of 247–254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon ԐHf(t) values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87Sr/86Sr ratios of 0.708 to 0.716. The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −6.55, ԐNd(t) values of −10.9 to −9.41, and initial 87Sr/86Sr ratios of 0.716 to 0.731. Both the Zengudi and Tuobake granite porphyries exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K2O + Na2O + K2O) ratios (1.07–1.86 and 0.83–1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern Lancangjiang area was involved in the continental collision after the subduction of the Paleo-Tethys Ocean before 254 Ma.

scholarly journals Decoupling of Sr-Nd Isotopic Composition Induced by Potassic Alteration in the Shapinggou Porphyry Mo Deposit of the Qinling–Dabie Orogenic Belt, China

Minerals ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 910
Author(s):  
Jun He ◽  
Xiaochun Xu ◽  
Zhongyang Fu ◽  
Yuhua An ◽  
Tianhu Chen ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Orogenic Belt ◽  
Granite Porphyry ◽  
Quartz Syenite ◽  
Porphyry Cu ◽  
Material Sources ◽  
Dabie Orogenic Belt ◽  
T Values ◽  
Host Rocks ◽  
Nd Isotopic Composition

In our previous study on petrogenesis of quartz syenite and granite porphyry, the host rocks of the Late Mesozoic Shapinggou Mo deposit in the Qinling–Dabie orogenic belt, we found that the initial Sr isotopic composition of the host rocks is strongly affected by the degree of K-alteration. Here, we provide further isotopic evidence of the host rocks and their minerals to investigate the geochemical behaviour of trace elements and isotopes during the alteration and to explain the phenomenon of decoupling of Sr–Nd isotopic composition. The quartz syenite and granite porphyry are altered by K-alteration in varying degrees and have high K2O and Rb contents and low Na2O, CaO, Sr, and Ba contents. Rock samples of both quartz syenite and granite porphyry have variable Rb/Sr ratios and initial 87Sr/86Sr values (even < 0.70) but contain quite homogeneous εNd(t) values (−12.8 to −14.8). Minerals from the rocks of moderate to intense K-alteration have very low initial 87Sr/86Sr values (even < −17), while those from the weakly altered rocks have 87Sr/86Sr(t) values of 0.7044 to 0.7084. The same phenomenon of the decoupling in Sr–Nd isotopic composition can be observed from several Mo deposits within the eastern Qinling–Dabie orogenic belt. This fact suggests similar hydrothermal features and a comparable origin for both the magmatic rocks and hydrothermal fluids in this belt. A comparison between porphyry Mo and porphyry Cu deposits shows that elements and the Rb–Sr isotope system have different behaviours during the K-alteration, implying distinct material sources and igneous rocks for porphyry Mo and porphyry Cu deposits, respectively.

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