| Huichang basin of Jiangxi province was located in the middle of Wuyi mountain which belong to a part of the Southeast China Mesozoic volcanic-intrusive rocks belt. The trachyte in Huichang basin of Jiangxi province was belong to a part of the Southeast China Mesozoic volcanic-intrusive rocks belt. Based on a systematic studies on geological background, spatial distributeion, chronology, petrology, petrochemical and Sr、Nd、Pb、O isotopic characteristic of the trachyte in Huichang basin, this paper carried out the geniesis of the trachyte.Huichang is one of the most developed red sedimentary basin with volcanic rocks, the trachyte are mainly distributed in the area of red sedimentary of Dongbei village,on the other side the basalt are mainly distributed in the area of red sedimentary of Zhantang village. The zircon SHRIMP U-Pb dating results of the trachyte showed that the apparent ages of 14 zircon crystals range from 114 Ma to 108 Ma,with a weighted average of 110.7±1.0Ma,MSWD=0.47. The zircon SHRIMP U-Pb dating results of the basalt showed that the apparent ages of 12 zircon crystals range from 94.2 Ma to 101.6 Ma, with a weighted average of 99.0±1.0Ma, MSWD=1.14.These results which represents the emplacement age of the trachyte and the basalt was formed in the late Early Cretaceous.The trachyte in Huichang basin are dense massive structure which in a colour of grayish to green ash and the fracture surface are rough. The trachyte display a higher Si O2 range from 59.0%~64.6%(the average is 61.94%),and enriched in Alkali and sodium, the content of total Alkali(Na2O+K2O) range from 7.19%~10.1%(average 9.70%), Na2 O range from 6.50%~7.70%(average 7.03%), K2 O range from 0.69%~3.23%(average 1.84%),and the ratio of K2O/Na2 O ranging from 0.11 to 0.47(average 0.26). In the diagram of Si O2-(Na2O+K2O), all samples fall in the field of alkali series trachyte or trachydacite except one; The CIPW result showed, the Ab=57.2%~65.7(average 60.5%),Q=6.07%~14.7%(average 9.95%),less than 20%,belongs to trachyte. The content of Ti O2 varying from 0.84% to 0.94%(mean 0.90%), Mg O varying from 0.28% to 0.92%(mean 0.55%), Ca O varying from 0.87% to 6.38%(mean 2.65), higher content of Al2O3 varying from 15.5% to 18.9(mean 17.7%), the ratio of A/CNK ranging from 0.63 to1.24( average 1.00), display peraluminous characteristics. The Huichang trachyte has low REE contents, ranges from 114×10-6 to 168×10-6, total LREE contents ranges from 101×10-6 to 148×10-6,total HREE contents varying from 14.9×10-6 to 19.6×10-6 and is significantly enriched in LREE and strongly depleted in HREE, with ∑LREE/∑HREE(L/H) ratios varying from 6.54 to 7.52, and(La/Yb)N ratios varying from 24.9 to 34.2(average 27.4),negative Eu anomaly is not obvious. δEu ranges from 0.81 to 1.10(average 9.95%),chondrite-normalized REE pattern suggest that the trachyte has relatively high pressure forming condition. A lower Y content ranges from 8.99×10-6~11.7×10-6 and Yb ranges from 0.55×10-6~0.82×10-6(average 0.7×10-6), a higher Sr content ranges from 329×10-6~836×10-6(average 498×10-6) and Sr/Y ratio ranges from 36.5~71.8(average 48.2),with the characteristics of adakite; Compared with primitive mantle,the lithophile elements(Cs、Rb、Ba、K)enriched significantly,and high field strength elements as Ta and Nd are depleted enormously which shows a characteristic of derived from crust.Trachyte samples from Huichang basin show a relatively large range of Sr–Nd–Pb isotope compositions with(87Sr/86Sr)i ranging from 0.7074 to 0.7082(mean 0.512494),(143Nd/144Nd)i ranging from 0.512491 to 0.512497(mean 0.512494),(206Pb/204Pb)i ranging from 18.10 to18.19(mean 18.14),(207Pb/204Pb)i ranging from15.76 to15.77(mean 15.77),(208Pb/204Pb)i ranging from38.52 to 38.53(mean 38.54),higher εNd(t) varying from-1.99~-2.60(average-2.29),the singlestage Nd model ages(TDM2) of all samples vary from 1071 to 1120 Ma(mean1094 Ma). All above of the trachyte suggested that it should be originated from partial melting of the young lower crust while a relatively high pressure condition. The feature all trachyte samples fall in the field between the DMM and EMⅡmember in the diagrams of(87Sr/86Sr)i-(206Pb/204Pb)i、(143Nd/144Nd)i-(206Pb/ 204Pb)i and εNd(t)-(87Sr/86Sr)i,shows that the lower crust source rock was the admixture of DMM and EM Ⅱ type mantle material. Based on the above geochemical characteristics, the proeesses from deep source magma underplating to the eruption of magma by partial melting of the young lower crust which consist with the admixture of DMM and EMⅡtype mantle material maybe the way the trachyte formed in Huichang basin. |
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