U-Pb Zircon Geochronology And Geochemistry Of Mesozoic Volcanic Rocks From Dasijiazi Area At Zhangwu, West Liaoning Province

The Mesozoic or Cenozoic lithospheric thinning in eastern China is a warmly discussed topic. Recent research on intermediate-acidic volcanic rocks during the Yanshanian in eastern China reveals that many samples have similar geochemical characteristics of adakites which are also defined as adakitic rocks or C-type adakites. Because this type of rocks usually form in high pressure(>1000MPa) and they may provide some information about important plutonic process like as the partial melting of the slab or the partial melting of the underplated basaltic lower crust or delaminated lower continental crust etc, so they maybe have an important geological signifinance in discussion on Mesozoic or Cenozoic lithospheric thinning in eastern China.Adakite is first introduced by Defant and Drummond from their research on volcanic or intrusive rocks in Aleutian islands. These rocks are characterized by SiO₂≥56%, Al₂O₃≥15% (rarely lower), usually MgO<3% (rarely above 6% MgO), low Y and HREE relative to ADRS(for example Y and Yb≤18μg/g and≤1.9μg/g, respectively), high Sr relative to island arc ADR (rarely <400μg/g), high LREE, no negative Eu anomaly (or have slight negative Eu anomaly), low Y and Yb suggest that garnet were stable during the process of partial melting. High Sr and no negative Eu anomaly (relative to Ce and Nd) suggest that plagioclase is unstable during the process of partial melting. It was initially believed that adakite only occurs in island arc which is produced by partial melting of young (<25Ma) and hot subducted oceanic slab, different from andesite-dacitic rocks has replaced by fluid in the mantle. Nevertheless, alternative models for the petrogenesis of adakites have also been proposed by melting at the base of tectonically thickened crust, assimilation-fraction crystallization (AFC) processes involving a basaltic magma, and partial melting of the underplated basaltic lower crust or delaminated low continental crust or subducted low continental crust. Experimental studies have also suggested that melting of mafic material produces adakitic magmas at 1.2GPa pressure, with the residual phase containing garnet but no plagioclase. Although most well-knowm adakites are distributed in intraoceanic island arc settings and continental arcs, it is still controversial whether partial melting of the low continental crust can produce adakitic magma along continental margin or continent-continent collisional tectonic settings.In this paper, we studied intermediate-acidic volcanic rocks from Dasijiazi area at Zhangwu, west Liaoning province. We present detailed researches about geochronology, geochemistry and mineralogy on intermediate-acidic volcanic rocks which occur in the eastern Zhangwu area. Combined with the publishing data by Huang et al, we derived the following conclusion:(1) The Yixian volcanic rocks from Dasijiazi area at Zhangwu, west Liaoning province mainly consist of high-Mg intermediate-acidic volcanic rocks and rhyolites and most intermediate-acidic volcanic rocks have similar geochemical characteristics of high-Mg adakites.(2) Zircon U-Pb dating by LA-ICPMS reveals that age of Mesozoic volcanic rocks from Dasijiazi area at eastern Zhangwu, west Liaoning province is 122.4±0.4Ma which is early Cretaceous . But zircon U-Pb dating by Huang et al reveals that age of Mesozoic volcanic rocks from western Zhangwu is 253±5Ma, 172±2Ma,126±2Ma, respectively. Although these zircon U-Pb ages are also early Cretaceous, our age is younger than those obtained by Huang et al.(3) Geochemical compositions of 23 volcanic samples are studied. The results show that most samples except three rhyolites have similar geochemical characteristics of high-Mg adakite(SiO₂ = 56.46%～65.14%, Al₂O₃=14.60%～17.19%, Mg^# = 50～59, Sr=501～700μg/g, Yb = 1.04～1.54μg/g, Y=12.0～17.5μg/g, Eu/Eu^*=0.85～0.97, Sr/Y=29～46, La_N/Yb_N=13～28) . They also have high initial ⁸⁷Sr/⁸⁶Sr (0.70639～0.70647) and lowε_Nd (122Ma) values(-6.43～-12.26) that are inconsistent with the origin from slab melting.(4) There exist some reversely zoned orthopyroxene phenocryst and clinopyroxene phenocrysts and oscillatory zoned clinopyroxene phenocrysts from Zhangwu intermediate-acidic rocks by electron microprobe and back-scattered electron image. One oscillatory zoned clinopyroxene phenocryst shows that rare earth element concentrations decrease from pyroxene core to rim with negative Eu anomalies (Eu/Eu^*=0.64～0.76).(5) There are some obvious differences between the geochemistry of our samples and Huang et al, although all these samples come from Zhangwu area, Liaoning province. In major element, the samples by Huang et al are intermediate-basic rocks, but our samples are intermediate-acidic. In trace element, the samples from Huang et al show higher REE concentrations than our samples and also show obvious positive Sr anomalies. In Sr-Nd isotopic composition, the initial ⁸⁷Sr/⁸⁶Sr (122Ma) from the samples by Huang et al are higher than our samples, but the initial ¹⁴³Nd/¹⁴⁴Nd (122Ma) andε_Nd (122Ma) are similar to our samples; In mineralogy, the Al₂O₃ concentrations of clinopyroxene from the samples by Huang et al are variable, the Al₂O₃ concentrations in the core of clinopyroxene can reach 5% which indicate that the samples from Huang et al form at the mantle depth, but the all Al₂O₃ concentrations of clinopyroxene from our samples are less than 2.3% and show negative Eu anomalies which indicate that our samples form at the crust depth. We conclude that our samples may come from the upper part and the samples from Huang et al may come from the lower part of the Yixian Formation. They may have different petrogenesis.(6) Combined with previous studies on volcanic rocks from the low part of Yixian Formation in Zhangwu area, we propose that our samples from the Zhangwu area resulted from delamination and magma mixing. A crustally derived felsic magma resulted from partial melting of the low crust heated by asthenosphere upwelling and subsequently mixed with a mantle-derived high-Mg adakitic melt during magma storage or ascent to surface to form the high-Mg adakites in the Zhangwu area.