Zircon U-Pb Ages and Geochemical Characteristics of the Laoyaquan A-type Granites in East Junggar,North Xinjiang,China
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Laoyaquan granitic pluton is the largest pluton containing tin deposit in the Beilekuduk tin metallogenic belt.It is mainly composed of biotite alkali-feldspar granites.LA-ICP-MS U-Pb dating for zircons from two samples yielded the 206Pb/238U ages of 301±2 Ma(2σ,MSWD=1.03) and 300±5 Ma(2σ,MSWD=5.90),suggesting that the emplacement of the pluton occurred at the end of Late Carboniferous.The Laoyaquan alkali-feldspar granite is enriched in silica,alkalis,REE,Rb,K,Th,U,Nd,Hf and depleted in Ba,Sr,P,and Ti,with a strong Eu negative anomaly,showing the features of A-type granites.Both the geochemical characteristics and the zircon U-Pb ages indicate that the formation of the pluton is related to the post-collision and the magmatic activity during Late Carboniferous in northern Xinjiang.Cite
CL imaging,LA-ICP-MS U-Pb dating and trace element analysis were carried out for the zircons from two representative K-feldspar granites in the Lianghe region,western Yunnan.The results show that the zircons display oscillatory zoning which suggests a magmatic origin.Zircons from two samples show similar REE patterns,which are high in Th,U,and REE,relative enrichment of HREE,and significant Ce positive anomalies and Eu negative anomalies.These are the characteristics of magmatic zircon of crustal origin.The weighted average 206Pb/238U ages for zircons from two samples are(60.7±0.6)Ma and(115.8±1.0)Ma,respectively,which show that the granites were formed in Paleocene and Early Cretaceous.Combined with regional geology,the multi-phases of magmatism in the Tengchong block took place in the late Mesozoic-early Cenozoic,which may be considered as magmatism response to the Tethys evolution in the Sanjiang region,western Yunnan.This offers constraints on the geological evolution in the eastern edge of Qinghai-Tibet plateau.
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Diorite
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Constraining the duration of magmatism is of vital importance to the understanding of the magmatic-hydrothermal mineral system. The Bozhushan batholith, located in the middle section of the southeastern Yunnan ore district, mainly consists of biotite monzogranite and monzogranite. Many Sn–W–polymetallic deposits are developed around the Bozhushan batholith, but their temporal and genetic relationships remain controversial. LA-ICP-MS U–Pb zircon and monazite dating were respectively conducted on the same two samples, yielding weighted mean 206Pb/238U zircon ages of 85.1 ± 0.7 and 85.6 ± 0.9 Ma, and weighted mean 206Pb/238U monazite ages of 87.1 ± 0.9 and 88.1 ± 1.1 Ma. The crystallization ages of S-type granites obtained from the zircon U–Th–Pb system and monazite U–Th–Pb system are consistent within the analytical errors. After combining the new ages obtained in this study with recently published U–Pb zircon and cassiterite ages from the giant Baniuchang Ag–Sn–Pb–Zn deposit in the north, and U–Pb zircon and Re-Os molybdenite ages from the large Guanfang W deposit in the south, a temporal framework of magmatism-mineralization in the Bozhushan region has been established. The duration of magmatic activity at Bozhushan is about 7 Ma, with W mineralization occurring at ca. 92 Ma and Sn mineralization at 88–87 Ma.
Batholith
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Cassiterite
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Molybdenite
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The Guandimao and Wawutang plutons are located at the center of Hunan, South China. The former is mainly composed of biotite monzonitic granites/granodiorites and two-mica monzonitic granites, but the latter only consists of biotite monzonitic granites. The zircon ages of 203.0±1.6 Ma (biotite monzonitic granites) and 208.0±3.2 Ma (two-mica monzonitic granites) for the Guandimao pluton and 204±3 Ma for the Wawutang pluton obtained with the LA-ICP-MS U-Pb dating indicate that they were formed during the late Indosinian. In consideration of other geochronological data from Indosinian rocks of South China and adjacent regions, it is inferred that the two plutons were derived from crustal materials by decompressional melting in a post-collisional tectonic setting during spontaneous thinning of the thickened curst. Moreover, the inherited zircon age of 1273±57 Ma from the Wawutang pluton indicates that the source of the two plutons is related to the early Proterozoic crustal basement.
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Geochronology
Quartz monzonite
Petrogenesis
Isotope Geochemistry
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Triassic A-type granites in eastern South China Block (SCB) are abundant in the Wuyi–Yunkai tectonic domain and provide an important opportunity to explore the early Mesozoic evolution of continental crust of the SE part of the SCB. We carried out U–Pb zircon dating, Lu–Hf isotope analyses of zircon, and whole-rock geochemical analyses for two granitic plutons, the Guiyantou (GYT) and Luoguyan (LGY) granites, from northwestern Fujian Province. LA–ICP–MS U–Pb zircon analyses yielded ages of 232 ± 4 to 231 ± 7 Ma and 221 ± 5 Ma (Middle-Late Triassic) for the GYT and LGY granites. These two granites belong to metaluminous to weakly peraluminous high K calc-alkaline A-type granite that are enriched in K, Al, light rare earth element and Rb, Th, U, and Pb, and depleted in Nb, Ta, P, and Ti. Their rare earth element patterns are highly fractionated with (La/Yb)N ratios of 2–21 and strong negative Eu anomalies (Eu/Eu* = 0.02–0.31). In situ Hf isotopic analysis of zircon from the GYT and LGY granites yielded εHf(t) values ranging from –11.5 to –1.1, with corresponding two-stage Hf model ages from 1.98 to 1.33 Ga, from which it is inferred that the GYT and LGY magmas formed by partial melting of Proterozoic metasedimentary rock in the Cathaysia block. The two granites were emplaced at 232 and 221 Ma and together with Triassic A-type granites in coastal region of the SCB, which is in agreement with an extensional tectonic setting in the Middle-Late Triassic. We suggest that the Middle-Late Triassic A-type granites in eastern SCB were probably formed in an intracontinental, post-orogenic extensional regime that collision was between the SCB and an 'unknown block' or the eastern extension of Indochina block.
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猴岩岩体位于佛冈复式岩体北东缘，总体呈南北向展布，出露面积约30 km2，岩性多为中细粒黑云母二长花岗岩、花岗闪长岩。本文对猴岩岩体花岗闪长岩样品进行LA-ICP-MS锆石U-Pb测年，获得的同位素年龄为(155.5 ± 4.2) Ma (MSWD = 2.0)，表明猴岩岩体侵位时代为晚侏罗世中期，是燕山第三幕岩浆活动之产物。通过对比，猴岩岩体与佛冈主体花岗岩的侵位时代相一致, 表明二者形成于同期构造–岩浆活动，猴岩岩体是佛冈主体花岗岩分异结晶之产物。猴岩岩体的花岗闪长岩是岩浆分异结晶时捕掳地层残留体，导致岩石成分产生差异而形成，为岩体的边缘相。佛冈复式岩体形成于古太平洋板块向欧亚大陆板块俯冲的构造背景下，其主体花岗岩的地球化学特征表明其物质主要来源于地壳，属陆壳改造型(即S型)花岗岩。 Located at the northeast of Fogang composite pluton, Houyan pluton, which is mainly composed of intermediate-fine grained biotite monzogranites and granodiorites, generally presents a north- south directional distribution and covers an area of 30 km2. In this paper, the LA-ICP-MS zircon U-Pb age of granodiorites of the Houyan pluton is (155.5 ± 4.2) Ma (MSWD = 2.0). The result shows that the emplacement age of Houyan pluton belonged to the middle stage of Late Jurassic, and it was the product of the third magma activity of Yanshan. As its age is the same as the main granite of Fogang composite pluton, they were in the same period of tectono-magmatic activity, and Houyan pluton was crystallized from magmas of main granite of Fogang composite pluton. As the crystallization differentiation of magmas captured the remained stratums, it caused differences in rock composition, and formed the marginal facies which are composed of granodiorites. Fogang composite pluton was formed in the process of subduction of the Pacific plate under Eurasian plate. The geochemical characteristics of its main granite indicate that its source mainly derived from the crust, which belongs to crustal transformation type (S-type) granite.
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Geochronology
Molybdenite
Isochron dating
Radiometric dating
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The Bashierxi pluton is located in the western segment of the eastern Kunlun,between Baiganhu fault and Aerjin fault,this pluton mainly consists of monzonitic granite and K-feldspar granite,and the monzonitic granite is closely related with the ore-forming process of the Baiganhu tungsten-tin deposit.The internal structures,cathodeluminescence(CL) images and Th/U ratios of the zircon grains from the monzonitic granite reveal that these zircon grains have typical characteristics of magmatic zircon.The LA-ICP-MS analyses show that the zircon grains from the monzonitic granite have concordant U-Pb ages of(458.0±9.0)Ma,on the other hand,the zircon grains from the K-feldspar granite have U-Pb ages of(432.3±0.8)Ma,these data indicated that the Bashierxi pluton is a intrusion formed from Late Ordovician to Early Silurian.Major and trace element data of the monzonitic granite show that Bashierxi granite is weakly metaluminous or peraluminous A-type granite.These rocks were considered as melts derived from crustal source during back-arc basin extension in eastern Kunlun,and the early Baiganhu tungsten-tin deposit era is represented by the ages of monzonitic granite.
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U–Pb zircon geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzogranite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded a concordant weighted mean 206Pb/238U age of 304 ± 5 Ma (n = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO2, FeOt/MgO, total alkalies (K2O + Na2O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P2O5, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial 87Sr/86Sr = 0.7014–0.7022, ϵNd(t) = +5.6–+7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.
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Fractional crystallization (geology)
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