Petrogenesis and metallogenic implications for the Machang, Huangdaoshan, and Tuncang plutons in eastern Anhui: an integrated age, petrologic, and geochemical study
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Abstract:
The Tuncang–Chuzhou–Machang area (eastern Anhui province) is geologically located in the intersection between the Yangtze block and the Qinling–Dabie orogenic belt. Many Mesozoic plutons outcrop in this district that are Cu–Au prospective but inadequately studied. We report new LA-ICP-MS zircon U–Pb ages, petrologic, and whole rock geochemical data for three representative plutons at Machang, Huangdaoshan, and Tuncang. New dating results suggest that all the Machang (129.3 ± 1.6 Ma), Huangdaoshan (129 ± 1.7 Ma), and Tuncang (130.8 ± 1.9 Ma) plutons were emplaced in the Early Cretaceous, slightly older than other plutons in neighbourhood of the Zhangbaling uplift. The three plutons contain typical low-Mg adakitic affinities, in which the rocks contain SiO2 >56%, Al2O3 ≥15%, Mg# <53, elevated Sr, Ba, Cr, Ni, Sr/Y, and La/Yb, low Y and Yb and no discernible Eu anomaly. Their petrogenesis may have been related to the delamination and partial melting of the lower crust, which is different from the Chuzhou pluton, which was interpreted to have formed by partial melting of the subducted slabs. We suggest that this petrogenetic difference may explain why the pluton at Chuzhou is Cu–Au fertile, whereas those at Machang, Huangdaoshan, and Tuncang are largely barren. It is proposed that adakitic plutons formed by partial melting of the subducted slabs have high metallogenetic potentiality in the area.Keywords:
Petrogenesis
Outcrop
Adakite
The petrogenesis of adakites holds important clues to the formation of the continental crust and copper ± gold porphyry mineralization. However, it remains highly debated as to whether adakites form by slab melting, by partial melting of the lower continental crust, or by fractional crystallization of normal arc magmas. Here, we show that to form adakitic signature, partial melting of a subducting oceanic slab would require high pressure at depths of >50 km, whereas partial melting of the lower continental crust would require the presence of plagioclase and thus shallower depths and additional water. These two types of adakites can be discriminated using geochemical indexes. Compiled data show that adakites from circum-Pacific regions, which have close affinity to subduction of young hot oceanic plate, can be clearly discriminated from adakites from the Dabie Mountains and the Tibetan Plateau, which have been attributed to partial melting of continental crust, in Sr/Y-versus-La/Yb diagram. Given that oceanic crust has copper concentrations about two times higher than those in the continental crust, whereas the high oxygen fugacity in the subduction environment promotes the release of copper during partial melting, slab melting provides the most efficient mechanism to concentrate copper and gold; slab melts would be more than two times greater in copper (and also gold) concentrations than lower continental crust melts and normal arc magmas. Thus, identification of slab melt adakites is important for predicting exploration targets for copper- and gold-porphyry ore deposits. This explains the close association of ridge subduction with large porphyry copper deposits because ridge subduction is the most favorable place for slab melting.
Adakite
Fractional crystallization (geology)
Petrogenesis
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The timing and mechanism of the closure of the Palaeo‐Asian Ocean are problematic and controversial. To help resolve these problems, we report geochronological, geochemical, and isotopic data from mid‐Triassic adakitic intrusions in the Eastern Tianshan, NW China. U–Pb dating shows that the adakitic intrusions formed at 243–234 Ma. These mid‐Triassic adakitic intrusions are characterized by high Sr/Y and La/Yb ratios, low Yb and Y, and positive ε Nd (t) (+3.12 − +4.71) and low ( 87 Sr/ 86 Sr) i (0.703956–0.704487) values. However, they have high K and low A/NK values (1.16–1.71), relatively low MgO (0.43–1.81 wt%) contents, and Mg # (44–61), low abundances of compatible elements (Cr = 4.09–16.69 ppm, Ni = 2.02–8.03 ppm), which are different from the typical slab‐melting adakites. These features indicate that they were derived from the partial melting of the relatively depleted thickened lower crust. Our new geochronological, geochemical, and isotopic data integrated with the established amalgamation of nearby arcs lead us to conclude that the adakitic intrusions were most likely generated by partial melting of the tectonically thickened (>40 km) juvenile Dananhu intra‐oceanic arc and, therefore, the Kanguer branch of the Palaeo‐Asian Ocean closed since ca. 234 Ma.
Adakite
Island arc
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Neoproterozoic magmatism in the Hannan region at the northwestern margin of the Yangtze Block is characterized by numerous felsic plutons associated with minor mafic-ultramafic intrusions. The felsic plutons are either adakitic or normal-arc granitic in composition. The adakitic plutons are ∼735 Ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust. Among the normal-arc-related felsic plutons, the Tianpinghe pluton is the largest and has a SHRIMP zircon U-Pb age of 762 ± 4 Ma, older than the adakitic plutons in the region. Rocks from the Tianpinghe pluton have relatively high SiO (67.1–70.1 wt%) and KO + NaO (7.8–8.6 wt%) and relatively low MgO (0.7–1.3 wt%) and AlO contents (14.5–15.6 wt%), with AlO/(CaO + KO + NaO) (A/CNK) values ranging from 0.95 to 1.08. They have arc-affinity trace-element compositions that are characterized by enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with strong positive Pb and negative Ti anomalies. They have a narrow range of εNd values (+0.15 to -1.76) and relatively high zircon εHf values (+0.6 to +8.3). These geochemical features are typical of I-type granites. The rocks from the Tianpinghe pluton have relatively young single-stage and two-stage Hf model ages (1.01–1.31 and 1.31–2.01 Ga, respectively), suggesting that the pluton was generated by partial melting of newly formed basaltic rocks. On the basis of its arc-related geochemical affinity and its emplacement before voluminous adakitic magmatism but after mafic-ultramafic intrusions, the Tianpinghe pluton is considered to be Neoproterozoic arc granite formed during a period of crustal growth and reworking. Generation of the later adakitic plutons suggests that the crustal thickness increased to more than 50 km by mafic magma underplating.
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Adakite
Solidus
Volcanic arc
Underplating
Eclogitization
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Adakite
Amphibole
Island arc
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Adakite
Andesites
Protolith
Lile
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